11 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

11.1 Oxidation processes

11.1.1 Gas phase oxidation An example of this is the ammonia oxidation process processes of propylene using air and ammonia, which quickly replaced the previous process based on the reaction Introduction between acetylene and HCN, both because of the lower Selective oxidation processes, in particular those that raw material cost and the reduced safety issues. This make use of solid catalysts (heterogeneous oxidation made it possible to produce acrylonitrile with a processes), play a fundamental role in the petrochemical significant reduction in costs, which resulted in a rapid industry. About 50% of the principal chemical products expansion in the market for it between 1960-80. On the and over 80% of monomers are synthesized by means of other hand, the success of this product stimulated the at least one stage of selective heterogeneous catalytic development of research into the catalysts being used oxidation. Table 1 contains a list of the main selective (mixed Bi and Mo based oxides), resulting in their oxidation processes for hydrocarbons using solid gradual improvement. The first generation catalysts,

catalysts, with an indication of the conversion and based on supported Bi9PMo12O52, gave a yield of 55%, selectivity values obtained. In many commercial selective which increased to 65% with the development of second oxidation processes there is still scope for a significant generation systems containing iron as the redox element margin of improvement in performance. For example, the and to about 75% with the development of third potential increase in selectivity in the two main processes generation multi-component catalysts. The current of selective oxidation (ethylene to ethylene oxide and fourth generation catalysts, containing up to 25 propylene to acrylonitrile) could result in annual savings elements, allow yields in excess of 80% to be obtained. in reagent costs of around 800 million of euro. The development of new catalysts has brought about a The action of solid catalysts in oxidation processes comparable evolution in the type of catalytic reactors had already been noted by the beginning of the used, initially fixed bed, then ‘bubbling’ fluid bed and Nineteenth century, but it was only towards the middle finally ‘braked’ fluid bed. of the Twentieth century that a systematic study of In the period from 1990-2005 development and selective oxidation processes using solid catalysts and innovation in the sector was instead driven by the of their industrial applications was begun. The first growing importance attached to environmental and processes to be developed industrially were: safety issues. However, in the last decade of that period oxidation and ammonia oxidation (oxidation in the the introduction of new processes was heavily influenced presence of ammonia) of propylene to produce by the reduction of investment in petrochemicals acrolein and acrylonitrile respectively, oxidation of resulting from the restructuring taking place in ethylene to ethylene oxide and the oxidation of businesses throughout the sector. aromatics to form anhydrides (maleic and phthalic Below is a summary of the principal lines of anhydrides). The development of these processes, development during that time (Centi and Perathoner, which was also driven by the growing demand for 2003b). these types of products, led to the development of Use of new raw materials and alternative oxidizing fundamental research, with a synergistic effect on both agents. There has been an increasingly wider use of the development of new applications and the alkanes as raw materials, instead of aromatics and improvement of those already on the market. alkenes; for example, the synthesis of acrylonitrile from

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Table 1. Principal processes of selective oxidation of hydrocarbons using solid catalysts and typical results obtained (Arpentinier et al., 2001; Centi et al., 2002)

Conversion* Selectivity* REAGENT Principal product Types of catalysts (%) (%)

Methane/O2/NH3 HCN Lattice of Pt-Rh 100 60-70 CH4 or (CH2)x /O2 Syngas (CO/H2) Supported Rh or Ni 99 90-95

Methanol/air Formaldehyde Ag on a-Al2O3, or Fe-Mo oxides 97-99 91-98

** Ethylene/O2/acetic acid Vinyl acetate Pd-Cu-K on a-Al2O3 8-12 92 ** Ethylene/O2 Ethylene oxide Ag-K-Cl on a-Al2O3 13-18 72-76

Ethylene/air or O2/HCl 1,2-dichloroethane Oxychlorides of Cu-Mg(K) on g-Al2O3 95 93-96 ** Ethanol/O2 Acetaldehyde Ag, Cu 45-50 94-96 Propylene/air Acrolein Bi-Mo-Fe-Co-K supported oxides 92-97 80-88

Propylene/air/NH3 Acrylonitrile Bi-Mo-Fe-Co-K supported oxides 98-100 75-83 Acrolein/air Acrylic acid V-Mo-W oxides 95 90-95 n-butane/air Maleic anhydride V-P oxides 75-80 67-72 n-butane/air Butenes/butadiene Bi-Mo-P oxides 55-65 93-95 tert-butyl alcohol Methacrolein Bi-Mo-Fe-Co-K oxides 99 85-90 Isobutene/air Methacrolein Bi-Mo-Fe-Co-K oxides 97 85-90 Methacrolein/air Methacrylic acid V-Mo-W oxides 97-99 95-98 Benzene/air Maleic anhydride V-Mo oxides 98 75

o-xylene/air Phthalic anhydride Oxides of V-P-Cs-Sb on TiO2 98-100 81-87

Naphthalene/air Phthalic anhydride Oxides of V-K on SiO2 100 84

* Conversion of the reagents and selectivity of the products compared with the hydrocarbon ** In the processes in which the operation includes recycling of the unconverted reagent, the conversion figure is for a single pass

propane instead of propylene and the synthesis of maleic catalysts are increasingly replacing the homogeneous anhydride from n-butane instead of benzene, aimed at type, in order to reduce separation costs and the reducing costs and/or improving the eco-sustainability of environmental impact and/or to use new raw materials, the process. New processes that use alternative oxidants for example in the direct synthesis of acetic acid from are being researched. An example is the direct synthesis ethane. The processes for oxidative dehydrogenation of of phenol from benzene (instead of the multi-stage alkanes are increasingly more competitive than those for processing of benzene with cumene as an intermediate), dehydrogenation of alkenes. New processes are also

using N2O as the oxidizing agent instead of O2. This is in being investigated which will enable the reduction or order to reduce the complexity and the risks associated elimination of the formation of co-products and/or the with the process, to avoid the co-production of acetone formation of toxic or dangerous intermediates. An

and to make use of a by-product such as N2O (thereby example is the synthesis of methacrylic acid through also reducing its disposal costs). direct isobutane oxidation, as an alternative to the Development of new classes of catalysts and commercial acetone cyanohydrin process, which uses processes. The processes that use solid (heterogeneous) HCN as a reagent and co-produces ammonium sulphate.

618 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Conversion of processes based on the use of air into their corresponding olefins; selective oxidation of processes based on the feeding of pure oxygen. These alkanes such as synthesis of phthalic anhydride and processes enable a reduction in polluting emissions; as maleic anhydride from n-pentane, of acrylic acid from examples there are the synthesis of formaldehyde from propane and of methacrolein or methacrylic acid from methanol, the epoxidation of ethylene and the isobutane; and ammonia oxidation of propane into oxychlorination of ethylene to 1,2-dichloroethane. acrylonitrile. Improvement of the productivity of the processes. The catalysts used for these reactions can be This is the result of the development of new generation classified on the basis of their characteristic reaction catalysts with improved properties and/or mechanisms. improvements in the engineering of the reactors (for Allylic oxidation. For these reactions catalysts based example, the introduction of a monolithic reactor in the on mixed oxides of transition metals are used. These synthesis of formaldehyde, or of structured-bed reactors catalysts are capable of selectively extracting a hydrogen in the synthesis of phthalic anhydride). Moreover, atom by breaking a CH bond in the allyl position and during the period from 2000-05 there was a significant if necessary replacing it with an oxygen atom. Industrial increase of interest in the development of new reactor catalysts are generally multi-component (for example, technologies (such as, for example, membrane Bi-Mo oxides, used in the synthesis of acrylonitrile from reactors), which made it possible to achieve savings in propylene, contain various promoters such as Fe, Cu, W, processing even for small-medium scale production Te, Sb and K), but typically a principal phase can be (scale-down of the processes; Centi and Perathoner, identified (Bi-molybdate) which is able to catalyse 2003a). The goal was to decentralize production and different reactions, such as: the synthesis of acrolein reduce its environmental impact, in contrast with the from propylene, the ammonia oxidation of propylene to trend typical of the Twentieth century of achieving acrylonitrile, the dimerization of propylene to savings in processing costs through increases in scale cyclohexene and the oxidative dehydrogenation of and high integration in large petrochemical facilities. butenes to butadiene. These reactions are characterized This came about due to the high environmental impact by a common first stage of allylic oxidation (Fig. 1), and the strong public opposition to the latter approach, where the extraction of a hydrogen atom in the allyl as well as due to problems linked to a sluggish market position gives rise to a chemisorbed p-allylic complex with large fluctuations in demand. on the transition metal. The nature of the subsequent Selective catalytic oxidation processes can be divided stages determines the type of reaction and product that is into three categories. The first relates to oxidation of obtained. Oxidation and ammonia oxidation of a side inorganic molecules (for example, oxidation of ammonia chain of alkyl aromatics (for example, the oxidation of

to NO and of H2S by sulphur). The second class relates toluene to benzaldehyde or benzonitrile respectively) in to synthesis of basic chemical products (for example, principle follow a similar reaction mechanism, but the ammonia oxidation of methane by HCN or the partial interaction of the aromatic ring with the surface is

oxidation of methane by syngas; CO/H2 mixtures). different and therefore different types of catalysts are Finally, the third category relates to conversion of used, such as vanadium oxides supported on TiO2 or hydrocarbons by processing in the liquid phase catalysts based on molybdate of Fe-(V, P, K). (principally in the homogeneous phase even if there is a Nucleophilic oxidation to the CO group (oxidative growing interest in the use of heterogeneous catalysts) dehydrogenation of alcohols and oxidation of aldehydes and processing in the gas phase, which is the most to acids). Although this type of reaction has similarities commonly used industrially (see again Table 1). It to the mechanism previously described, there are various should be pointed out that this last class of processes types of substrates such as alcohols (methanol) or

uses air or O2 as the oxidant (other than the cited process aldehydes (acrolein or methacrolein) which interact too of direct hydroxylation of benzene by phenol with N2O), strongly with the surface of the catalyst when catalysts while in the liquid phase processes, in addition to O2, belonging to the first category are used. In the extensive use is also made of other oxidizing agents such conversion of methanol into formaldehyde, the catalyst

as alkyl peroxides and H2O2 (Centi and Perathoner, most often used on an industrial level is iron molybdate 2003b). (which also contains other components in small The different categories of gas phase selective quantities), while multi-component catalysts, based on oxidation processes (over solid catalysts) and the related Mo-V oxides or heteropolyacids of P-Mo-V, are used for principal industrial reactions are summarized in Table 2 the conversion of aldehydes into their corresponding (Arpentinier et al., 2001; Centi et al., 2002). Some acids. important classes of reactions, which are not mentioned Electrophilic insertion of an oxygen atom. The in the table, since they are not yet used commercially, catalysts for this category of reaction are highly specific.

include: oxidative dehydrogenation of C2-C5 alkanes to Examples are the systems based on Ag/a-Al2O3 for the

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H H H H CC H H C H CC H H C C C H C H H H H H H O H

MeO MeO MeO MeO Me O Me Me O Me O anionic vacancy

H H OH OH O OO O O O O

Bi Mo Bi Mo Bi Mo OO OO O O

p-allylic complex

Fig. 1. General outline of the allylic oxidation mechanism and example of the oxidation of propylene on bismuth molybdate to obtain acrolein.

synthesis of ethylene oxide from ethylene (this catalyst, hydrogen atom by a surface Lewis site (a transition for example, when applied to the synthesis of propylene metal) and of a second hydrogen atom by a base site oxide from propylene, is not selective) and Fe/ZSM-5 for (oxygen atoms) to give an alkene, which is immediately

the hydroxylation of phenol with N2O as the oxidant. converted into an oxygenated product through oxidation Oxidation (or ammonia oxidation) of alkanes. In this or allylic ammonia oxidation mechanisms. Catalysts case the slow stage is the initial selective activation of with properties which differ from those of catalysts the alkane, for example for the concerted extraction of a belonging to the first reaction category are necessary

Table 2. Different classes of gas phase selective oxidation processes (on solid catalysts) and the relative industrial reactions (Arpentinier et al., 2001; Centi et al., 2002)

Type of reaction Examples

Allylic oxidation – propylene to acrolein or acrylic acid – isobutene to methacrolein or methacrylic acid Synthesis of the acids can be carried out in a single stage from the alkene, but commercially it is preferred to use two stages for the best possible selectivities Oxidative dehydrogenation – butenes to butadiene and isopentenes to isoprenes – methanol to formaldehyde – isobutyric acid to methacrylic acid

Electrophilic insertion of an oxygen atom – epoxidation of ethylene to ethylene oxide with O2 – direct synthesis of phenol from benzene with N2O Acetoxylation synthesis of vinyl acetate from ethylene and acetic acid

Oxychlorination synthesis of 1,2-dichloroethane from ethylene and HCl in the presence of O2 Ammonia oxidation – propylene to acrylonitrile – isobutene to methacrylonitrile – a-methylstyrene to atroponitrile Synthesis of anhydrides – n-butane to maleic anhydride – o-xylene to phthalic anhydride

620 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

because of the weak interaction of the substrate with the the catalyst, but rather of the structural oxygen of the surface, and the activation mechanism. For example, catalyst (typically mixed oxides, see again Table 1). The catalysts based on vanadyl pyrophosphate are used for O2 oxygen ion removes the hydrogen atoms from the the oxidation of n-butane to maleic anhydride, or those hydrocarbon with the subsequent formation of water or, based on vanadium antimonates are used for the if inserted into the molecular structure of the reagent, it ammonia oxidation of propane. In the latter case, gives rise to the formation of oxygenated compounds antimony oxide is active in the ammonia oxidation of (see again Fig. 1). Instead, the gaseous oxygen propylene, but is not able to activate the propane intervenes in the reoxidation mechanism of the reduced molecule; the addition of V gives the system the catalyst, known as the Mars-van Krevelen mechanism capability of oxidizing the alkane. (Fig. 2).

Wacker-type oxidation mechanism. Vinyl acetate is The oxidation of the catalyst by O2 comes about produced through acetoxylation of ethylene with acetic through the formation of intermediate oxygen species acid in the presence of oxygen, with catalysts based on such as O2 and O , which have electrophilic supported Pd/Au. Pd supported on V2O5/Al2O3 or characteristics and tend to make an addition to the V2O5/TiO2 is selective in the gas phase synthesis of acetaldehyde from ethylene or of methylethylketone from 1-butene, with a similar reaction mechanism. H2O oxidized catalyst oxidized Oxychlorination. 1,2-dichloroethane is produced product commercially from ethylene, HC1 and O on supported 2 O 2 m n copper chloride based catalysts. The mechanism consists M2 M1 of a direct addition of chlorine atoms by the catalyst onto hydrocarbon the olefin, rather than in oxidation of hydrochloric acid reduced catalyst to molecular chlorine, followed by chlorination of the double bond. oxide (catalyst) Addition of oxygen to the aromatic nucleus, with ring opening. The electrophilic attack of oxygen on Fig. 2. Mars-van Krevelen mechanism of selective oxidation hydrocarbon substrates typically leads to the formation of hydrocarbons on oxide based catalysts. of carbon oxides, however in the case of the oxidation of benzene, selective oxidation to maleic anhydride is obtained. This process, which employs catalysts based O2 on mixed vanadium and molybdenum oxides, has been 2 2 partially replaced by the synthesis by oxidation of O O O2 O2 n-butane. Similar catalysts are used in the selective 2 (n1) n 2 n 2 oxidation of polyaromatic compounds. O M M O M O Non-classic oxidation mechanisms. Ethylbenzene 2 2 2 can be oxidatively dehydrogenated, with high selectivity, O O O to styrene on various catalysts such as oxides and O2 Mn O2 M(n1) Mn O2 phosphates, but the active phase is constituted by the formation of a thin surface layer of carbon containing A the active sites of the reaction. Recently even some types of carbon and carbon nanotubes have shown high selectivity in oxidative dehydrogenation of ethylbenzene electrophilic oxygen species to styrene. Another example is the ammoximation of products with rupture (O2 , O , ...) cyclohexanone (to cyclohexanone oxime) over of C C bond and C formation of CO amorphous silica. x O2 activation C C nucleophilic Characteristics of gas phase oxidation processes H products of selective oxygen C oxidation species (e.g. aldehydes) General aspects 2 B (O ) Although the petrochemical industry uses selective oxidation processes both in the gas phase and in the Fig. 3. A, schematic mechanism of the incorporation of liquid phase, those in gas phase are more widespread. oxygen into oxide based catalysts; B, outline of different Typically, oxygen (or air) is used as the oxidizing agent, types of attack on the hydrocarbon by nucleophilic and although the species involved in the selective oxidation electrophilic species of oxygen reaction are usually made up, not of adsorbed oxygen on (Centi et al., 2002).

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unsaturated molecule, breaking the double bond and ending with the formation of carbon oxides; in contrast, n-butane CO , H O the structural oxygen of the catalyst (O2) has x 2 nucleophilic characteristics (Fig. 3). concerted abstraction Hence, in order to be selective, a catalyst must not of 2 H atoms only possess activation sites for the hydrocarbon and for isomerization selective insertion of the oxygen on the substrate, but allylic H abstraction must also be rapidly reoxidizable. This is so as to prevent the non-selective chemisorbed oxygen species from having a lifetime long enough to allow combustion 1,4-oxygen insertion reactions to take place. This mechanism is generally allylic dehydrogenation accepted for the oxidation of alkenes on mixed oxides, and/or allylic oxygen but there are doubts about its validity in the case of insertion oxidation of other substrates, such as alkanes. oxygen insertion A general characteristic of selective oxidation maleic processes of hydrocarbons is the complexity of the anhydride reactions involved. For example, the oxidation of O O O n-butane to maleic anhydride is a reaction which involves 14 electrons, the removal of 8 hydrogen Fig. 4. Outline of the reaction mechanism in the selective atoms and the insertion of 3 oxygen atoms on the oxidation of n-butane to maleic anhydride on catalysts based

substrate, with the involvement of another 4 oxygen on (VO)2P2O7, showing the multifunctional character of the atoms of the catalyst to form 4 molecules of water. catalyst. Notwithstanding the complexity of the transformation, the reaction takes place without the formation of products with intermediate levels of oxidation; stages of interaction of the catalyst with the hydrocarbon selectivity of between 70 and 85% is achieved, and with oxygen) have led to the development of new depending on the reaction conditions. Therefore the classes of catalysts. The two aspects, involving the catalyst, made up of a mixed oxide of V and P with the development of the catalyst and the engineering of the

composition (VO)2P2O7, possesses characteristics such reactor, are therefore closely correlated. as to avoid both the desorption of the reaction Industrial reactors used in the petrochemical intermediates and their non-selective transformation industry for highly exothermic reactions, such as those into carbon oxides. for selective oxidation, are typically either of the Finally, another characteristic of selective oxidation multi-tubular fixed-bed or fluid-bed type. catalysts is their multi-functionality, which is necessary Nevertheless, there is a growing interest in the for the transformation of the hydrocarbon into the final development of new reactor solutions, such as for product; in fact, to bring about the complex mechanism example, the circulating fluid-bed reactor, recently described above, it is necessary that the catalyst be applied by DuPont to the synthesis of maleic capable of actuating different types of transformations anhydride from n-butane. The ‘uncoupling’ of the two on the substrate (Arpentinier et al., 2001). Moreover, it is redox reactions, of oxidation of the hydrocarbon by the necessary for the different stages involved in the catalyst and the reoxidation of the latter by oxygen transformation to have similar rates. Different relative (see again Fig. 2), makes it possible to increase the rates could lead to the desorption of intermediate selectivity towards maleic anhydride compared with products or an increase in the rate of parallel reactions, the reaction carried out in the simultaneous presence with a reduction in the selectivity for the desired of both the hydrocarbon and oxygen. Other advantages product. This is well illustrated in the selective oxidation of this type of reactor are isothermicity and a of n-butane (Fig. 4). reduction of the risk of explosion. Nevertheless, a limiting factor is its low productivity; in fact it is Combined design of the catalyst and the reactor necessary to circulate large quantities of the catalyst Optimizing the yield, productivity and selectivity of (equal to about 1 kg per g of maleic anhydride selective oxidation reactions requires not only a detailed produced) between the two reactor vessels, each of knowledge of the nature of the catalyst and the which is adapted to one of the reaction stages. mechanism of the interaction of the reagents and Another example of a new reactor configuration, products with the catalyst itself, but also the optimization adopted for petrochemical processes, is the monolithic of the reactor used. Recently, new reactor solutions type of reactor; these reactors combine the advantages of (which enable, for example, the separation of the two the possibility of autothermic conduction of the reaction

622 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

and a reduction in the loss of pressure. The new A new reactor technique being developed consists of generations of processes for oxidation of methanol to systems in which the flow is periodically reversed; this formaldehyde use a final adiabatic stage (post-reactor), makes it possible to make the activity and temperature with a catalyst structured in the form of a monolith. profile in the reactor more uniform, even though there Very interesting results have been obtained using are still some significant problems involving the reactor with extremely short contact times (on the difficulty of managing non-stationary operations and order of milliseconds, compared with times measured their potential danger. Also in this case, the design of the in seconds in conventional reactor), where the catalyst catalyst is different from that for operations in stationary configuration is also of a non-conventional type (for conditions. example, in a grid form). Given the high spatial rates used (that is the high ratio between the input rate of Use of air and pure oxygen as oxidizing agents the reagents and the quantity of the catalysts) and the Currently, air is the most widely used reagent in type of mechanism involved, it is possible to avoid gas phase oxidation processes, but there is a growing

subsequent oxidations and therefore to obtain high interest in the use of pure O2 as a means of increasing selectivity of the intermediate products (for example, the productivity and reducing pollutant emissions and in the oxidative dehydrogenation of alkanes into energy consumption. Table 3 illustrates an example of alkenes). the emissions from the process of oxychlorination of Finally, it is worth remembering the developments in ethylene, where air and oxygen are used as the the field of catalytic membrane reactors, which allow the oxidizing reagents. The significant reduction in the continuous removal of one of the products or the environmental impact of the second type of process differential addition along the catalytic bed of one of the can be seen.

reagents (for example, oxygen). This makes it possible to The following gas phase processes use pure O2, or maintain the optimum hydrocarbon/O2 ratio along the air enriched with oxygen, as an alternative to air: entire profile, to limit the formation of hot spots and to a) partial oxidation (to syngas) of heavy fractions from control the state of oxidation of the catalyst. the distillation of petroleum; b) oxidation of methanol Nevertheless, one of the current limiting factors is its to formaldehyde (air or enriched air); c) oxidation of low productivity, apart from the high cost of the ethylene to ethylene oxide (air or oxygen, the latter membrane itself. particularly in new plants); d) oxychlorination of Even conventional fixed-bed reactors can be ethylene to 1,2-dichloroethane (air or oxygen, the improved through greater integration of the design of the latter particularly in new plants); e) acetoxylation of catalyst and that of the reactor. In the process of the ethylene to vinyl acetate (oxygen); f ) oxidation of synthesis of phthalic anhydride from o-xylene, n-butane to acetic acid (air or oxygen); g) oxidation of specialized catalytic beds are used, that is, containing ethylene to acetaldehyde (air or oxygen); h) oxidation different layers of catalysts each having a different of acetaldehyde to acetic anhydride (air or oxygen); composition, in order to optimize the axial activity and and i) ammonia oxidation of propylene to acrylonitrile selectivity profile of the catalyst itself. (oxygen enriched air).

Table 3. Composition of the emissions from the process of oxychlorination of ethylene where air and oxygen are used (Arpentinier et al., 2001). DCE: 1,2-dichloroethane; VCM: vinyl chloride monomer

Process using air Process using O Component 2 Content (vol%); flow (m3/h) O2+Ar 4-8; 400-2,400 0.1-2.5; 25 Ethylene 0.1-0.8; 10-24 2-5; 50 COx (CO2/CO=3-4/1) 1-3; 100-900 15-30; 300 DCE and chlorinated compounds 0.02-0.2; 2-60 0.5-1; 10

N2 remainder remainder Waste (m3/h)* 10,000-30,000 1,000

*Approximately 300-900 m3 per t of VCM produced

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Selective oxidation catalysts for hydrocarbons thermodynamically favoured), a support is needed with a surface area which is not too large. In this way the rate of Characteristics of oxidation catalysts the undesired secondary reactions, which are also Oxidation catalysts belong to a wider class of dependent on the time needed by the product to diffuse materials having redox or oxidoreductive type from the active centre into the gas phase, is limited. A characteristics; systems which catalyse reactions of further task of the support is that of providing the hydrogenation, dehydrogenation, halogenation and resistance of the active phase to phenomena which can dehalogenation also belong to this class. The most cause abrasion or disintegration, especially for those important catalysts in the field of petrochemicals for the applications which involve particular mechanical oxidation of hydrocarbons for processes carried out in stresses on the catalyst (for example, in fluidized-bed the gas phase are listed in Table 1. In addition to these, it reactors), in addition to avoiding powdering during the is worth mentioning catalysts used for the oxidation of loading of the catalyst into packed fixed-bed reactors. inorganic compounds, such as those employed for the Finally, in some cases the support serves to alter the

oxidation of SO2 to SO3 (based on supported vanadium characteristics of the intrinsic chemical reactivity of the oxide), of ammonia to NO (based on Pt/Rh) and of active phase, through the effects of the interaction hydrogen chloride to molecular chlorine (based on between the latter and the support itself. This comes supported copper chloride). about when the support presents functional groups on its Below is a list of the principal characteristics of surface which can lead to the formation of chemical oxidation catalysts for gas phase reactions. bonds with the elements of the active phase, or it takes Presence of a transition metal as the principal active place as a result of particular crystallographic component (V, Mo, Cu, Fe, Pd, Pt, Rh, Ag). Often in similarities between the surface and the support. These these cases, a second element is also present which can interactive effects can be positive for the reactivity of the be transition or post transition (for example, P, Sb or Bi), catalyst itself, altering its oxidoreductive characteristics which contributes to establishing the reactive or reducing its volatility; the undesirable effects of loss characteristics of the catalyst. This effect can be by sublimation of components of the active phase are explained by the formation of a ‘mixed oxide’ (that is, of thus reduced.

a specific compound, such as for example Bi2Mo2O9, The most suitable combination of the type and possibly only on the surface of another oxide, of a solid number of active phases (including the promoters) and solution or of an oxide doped with the other element), the type of support is dependent on the characteristics of with reactive characteristics different from those of the the reaction and the type of reactors used. In particular, single elements, if present in distinct phases. In some below are listed the factors that have the greatest cases the element is initially present in a metallic form, influence on the formulation and the morphology of the but under reaction conditions it can generate the catalyst used for oxidation reactions. corresponding oxide (or chlorides or oxychlorides). Type of chemical transformation involved and Presence of small quantities of ‘promoter’(or mechanism through which it takes place. With increasing ‘doping’) elements. The purpose of these elements is to complexity of the transformation the composition of the optimize the performance of the principal active catalyst also becomes the more complex, in terms of the elements. The nature of the promoters can vary and they number of elements making up the active phase, or of can therefore play different roles in the transformation of the structural complexity (the formation of crystalline the reagents. The active elements and the promoter phases having multi-functional characteristics). For elements, constitute the active phase, that is the phase example, catalysts used for oxidation or allylic ammonia directly involved in the transformation of the reagents oxidation always contain Mo as the principal element for into products. the active phase, while catalysts for the synthesis of Presence of a support (usually silica, alumina or anhydrides or of acids almost always contain V. titanium oxide). This support in the catalyst’s Optimization of the redox characteristics or of the formulation can fulfil a variety of tasks. A primary task acidity or basicity properties of the catalyst. The is that of dispersing the active elements, conferring a promoters (or doping agents) can play a fundamental larger surface area to the active phase compared with role in the control of these properties. Promoters with what would have existed in the absence of the support. It base-type characteristics (alkaline or alkaline earth metal is clear, therefore, that the support must have surface oxides) can reduce the surface acidity of the active area characteristics suitable for the reaction of interest. phase, with a consequent improvement of selectivity In selective oxidation, where the selectivity in the through the suppression of the acid-catalysed reactions formation of the partially oxidized product is heavily (cracking, formation of oligomers of unsaturated dependent on the subsequent reactions to undesired compounds). Promoters with acid-type characteristics products (for example, carbon oxides, which are can reduce the interaction between the active phase and

624 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

intermediates of the reaction which have acid-type Oxidation catalyst mechanisms in gas phase characteristics, thus favouring their desorption into gas The principal catalytic oxidation mechanisms are phase and limiting the contribution of the subsequent listed in Table 2. The redox type mechanism is the one undesired reactions. Other promoters can optimize the that is used in the majority of oxidation reactions. It oxidoreductive properties of the active phase, by a works through a series of successive stages, which modification of the overall electronic properties of the include: the adsorption of the reagent (the substrate to be solid. oxidized) on the active centre; the transfer of electrons Reaction scheme. The presence of consecutive from the reagent to the active centre and the reactions (typically, combustion reactions of the desired simultaneous transfer of oxygen ions from this to the product, or reactions which lead from the reagent to the reagent (the oxygen is incorporated into the substrate, or desired product through the formation of intermediate alternatively returns in the formation of co-produced products with an increasing state of oxidation) involves water); and finally, the desorption of the product. The the use of a catalyst with characteristics such as to limit same sequence of stages involves the molecule of (or, alternatively, to favour) the contribution of these oxygen for the catalyst reoxidation stage: co-ordination reactions. This can be achieved not only by control of the at the metallic centre; transfer of electrons (up to 4 for intrinsic activity of the catalyst, but also by a each oxygen molecule); dissociation of the molecule into modification of the porosity of the active phase (and two atomic species in ionic form; finally incorporation therefore of the support, if present). High surface area of the oxygen in its ionic form within the active phase. and porosity values entail effective intra-particle One or more active centres may be involved for each residential times which are much higher that those reagent molecule, depending on the following factors: calculable from the feeding capacity of the reactor, and a) the overall number of electrons transferred and therefore a significant contribution from the consecutive therefore of oxygen ions involved in the oxidoreductive reactions for a given conversion of the reagent. This can process; b) the ionic and electronic conduction capacity have a considerable influence on the selectivity of the of the solid, and therefore of the surface active phase desired product. under reaction conditions; c) the level of cover of the Reaction heat levels. Highly exothermic reactions active phase by the adsorbed molecules (reagents and involve the need both for fluidized-bed catalytic reactors, products); d) the surface mobility of the reaction which are more efficient in removing heat than multi- intermediates; and e) the number of active centres close tubular reactors, and for catalysts capable of operating in to the one in which the activation of the hydrocarbon conditions of high mechanical stress. In these cases took place. fluidizable supports are used, which feature particles On the basis of the redox model, the selectivity of with an average diameter of between 50 and 150 mm, the process, that is the relationship between the quantity resistant to abrasion and with an appropriate density. For of the product formed and the total quantity of reagent medium-low heat levels, tubular or tube-bundle transformed, can be traced back to two different (multi-tubular) reactors can be used. In these cases the situations. First and foremost the selectivity depends on catalysts have a characteristic morphology for these the nature of the oxygen ions present as species applications and are produced in the form of extrusions adsorbed on the active phase and on the interaction (or pellets). When possible, supports with a high heat between them and the reagent or the reaction conduction capacity are used, such as SiC, in order to intermediates. As previously stated, the O2 species, assist the dissipation of the heat from the reaction. incorporated in the lattice of the oxide, is considered to Spatial rates in the reactor. High spatial rates in be the selective species, while the O2 and O species packed catalytic beds can lead to a high loss of pressure, have electrophilic characteristics and are considered to and therefore to the need for heavy compression of the be non-selective species. Since the formation of the first flow upstream of the reactor. It is possible to minimize species comes about by the intermediate formation of the loss of pressure by increasing the vacuum level in the the electrophilic species, it is clear that the catalytic bed, through the use of special structures of the transformation rate of each of them and their reactivity catalyst particles. This can be instrumental in with the reaction intermediates obtained through conditioning the performance of the process, as in the activation of the substrate determine the selectivity of case of oxidative dehydrogenation of methanol to the process (Bielanski and Haber, 1991). formaldehyde. In this case, the use of cylindrical pellets Moreover, the selectivity of the oxidation process is with an axial hole enables the loss of pressure to be traceable to the concentration of O2 species reduced and therefore, the linear rate in the reactor to be incorporated in the metal oxide lattice, and therefore increased for a given rate of feeding. This involves directly to the average state of oxidation of the catalyst shorter contact times, better reaction temperature control (Grasselli, 2002). A strongly oxidized catalyst has a high and reduced catalyst deactivation effects. density of active centres capable of receiving electrons

VOLUME II / REFINING AND PETROCHEMICALS 625 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

from the substrate and of releasing O2 ions, and coldest parts of the reactor. This induces, not just the therefore it is able to transform that substrate into deactivation of the catalyst, but also a progressive molecules with a high state of oxidation (for example, increase in pressure loss. into combustion products). In contrast, a catalyst made Reaction temperatures are typically within the range up of a partially reduced oxide has a modest oxidizing of 310-340°C, with conversions in excess of 98% and capacity, and therefore is potentially more selective for selectivity equal to 92-95%. Multi-tubular fixed-bed the partially oxidized products. According to the redox reactors are generally used. A recent development has model, the state of oxidation of a metal oxide in a seen the introduction of a final (post-reactor) adiabatic stationary state is dependent on the conditions of the stage. reaction; this implies that the selectivity in its turn is In dehydrogenation combined with partial

dependent on the operating parameters, such as the combustion of H2 (the overall process turns out to be composition of the feed (that is the relationship between partially exothermic) an understoichiometric oxygen the substrate to be oxidized and the oxidizing agent) or current is fed in, to operate in the upper region at the the reaction temperature. limit of flammability. Due to the thermodynamic limits Both models have been experimentally verified for of dehydrogenation, it is necessary to operate at higher different oxidation reactions, and still remain valid today reaction temperatures than those for oxidative for the explanation of the selectivity of oxidation dehydrogenation. In this process use is made of Ag processes involving reactions with redox type mechanisms. based catalysts supported on alumina with a low surface area, typically in spherical form with a diameter of Principal industrial processes and relevant 1-5 mm. If operating at temperatures in excess of 600°C applications (particularly 680-720°C), it is possible to obtain an almost total conversion of the methanol, while at lower Oxidative dehydrogenation of methanol temperatures (500-550°C) the conversion is less efficient to formaldehyde (65-75%) and it is necessary to recycle the methanol Formaldehyde (HCHO) is among the top twenty which failed to react. Moreover, it is necessary to use chemical compounds produced on a world scale, and is short contact times in order to avoid decomposition of used in the synthesis of various resins (urea- the formaldehyde. formaldehyde, phenol-formaldehyde, and polyacetals) Selectivity to formaldehyde of 98-99% is obtained, which find applications in the construction, automotive, with the formation of the following by-products:

textile and paper sectors. Methanol can be converted into dimethylether ((CH3)2O), whose formation is due to the formaldehyde both by direct oxidative dehydrogenation: presence of acidic sites in the catalyst; methyl formate (HCOOCH ) obtained through disproportionation of the CH OH0.5O HCHOH O 3 3 2 2 formaldehyde on basic sites; carbon oxides, derived ∆H°=155 kJ/mol from both parallel and serial reactions. To limit the

or by dehydrogenation combined with oxidation of the H2 formation of carbon oxides, rapid cooling of the reaction product: products is necessary when they leave the catalyst bed. High selectivity is achieved through the optimization of CH OH HCHOH ∆H°= 84 kJ/mol 3 2 the acid-base properties of the catalyst, limitation of the ∆ H2 0.5O2 H2O H°= 238 kJ/mol oxidation of the formaldehyde to formic acid (a product which decomposes easily) and control of the redox The two processes differ in their operating conditions properties of the catalyst. and type of catalysts. In the first process low Fig. 5 illustrates the reaction mechanism in the case concentrations of methanol are used in the feed, in order of direct oxidation of methanol on oxide based catalysts. to avoid the formation of explosive mixtures and to The methoxy species is the first chemisorbed species control the temperature of the reaction. Commercial that is formed by the contact of methanol with the catalysts are based on iron molybdate, but also contain catalyst; its subsequent transformation depends both on an excess of molybdenum (Fe2(MoO4)3 MoO3), since the reaction conditions and on the properties of the the presence of molybdenum oxide is a necessary catalyst. If the concentration of methanol is high and the condition for high selectivity. Typically a ratio of Mo/Fe rate of the subsequent oxidation of the methoxy species within the range of 1.5-3.0 is used; occasionally oxides is low, a condensation reaction takes place that leads to of Co and Cr are added as promoters. The excess of dimethylether (typical of the acidic oxides containing molybdenum is also necessary because the sublimation non-reducible cations, such as alumina). Oxidation of of the oxide (particularly at the points of greatest the methoxy species (extraction of an atom of H and overheating) cause the progressive depletion of the Mo transfer of an electron) leads to co-ordinated

in the catalyst and the condensation of MoO3 in the formaldehyde, which is in equilibrium with the

626 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

CH3OCH3H2O dimethoxymethane OCH CH OH 3 CH3OH3 CH2O CH2 H OCH3 CH2 CH2 formaldehyde CH2 2CH OH OH OOHO O O O OH 3

[H] metoxy[H] dioxymethylene methylformate H2O O disproportionation formic acid HCOOH H COCH3 H2O

CH CH3OH COx OO

formate oxygen vacancy

Fig. 5. Reaction mechanism of the oxidation of methanol on oxide based catalysts (Centi et al., 2002).

dioxymethylene species. The reaction requires a from 1-2 years), it is necessary to gradually increase the nucleophilic attack by the catalyst’s structural oxygen. If temperature of the reactor in order to keep the the MeO bond is of a covalent type, the equilibrium productivity at a constant level. The formaldehyde yield between the dioxymethylene and the co-ordinated is about 95-96%. The oldest processes operate with a formaldehyde is shifted towards the latter species, which feed of 6% of methanol in air (a concentration which is in its turn is in equilibrium with the gas phase less than the lower limit of flammability), but in this case formaldehyde. Ionic oxides, instead, favour the productivity is low, the purity of the formaldehyde is formation of the dioxymethylene species. This gives rise poor because of the formation of formic acid, the to the methoxy and formate species by Cannizzaro catalyst’s lifespan is limited and it is necessary to operate disproportionation; the formate can also form through with high volumes of inert gas. For this reason, about direct oxidative dehydrogenation from the half of the plants have been converted to a feed with dioxymethylene. By reactions with methanol, reduced levels of oxygen (10%), and higher dioxymethylene forms dimethoxymethane, which can concentrations of methanol (8.2%). On account of the desorb into gas phase. The formate species can also react high amount of heat generated, it is necessary to dilute with methanol to give methyl formate. The formation of the catalyst and improve the efficiency of the reactor’s this product requires that the desorption rate of the cooling system. Some plants also use steam as a diluent. formaldehyde be low and that the When the gas leaves the reactor vessel, it undergoes oxidation/disproportionation rate be relatively high. Both a heat recovery stage, after which it is sent to an absorber these reaction pathways contribute to the formation of column, where water is used as a solvent. The the formate species. Their relative rates depend on the formaldehyde solution, from the bottom of the absorber degree of surface coverage of the dioxymethylene column, has a concentration of from 50-60%; it is then species and on the reaction conditions. Nevertheless, the sent to an ionic exchange column for removal of the formate species can also be converted and therefore the formic acid. On the other hand, the gas from the top of selective formation of methyl formate requires that the the column is recycled; the waste stream keeps the conversion rate of the formate species be low and that composition in the reactor constant. the concentration of methanol be relatively high. The process using supported Ag as the catalyst (see A simplified outline of the commercial processes for again Fig. 6 B) has the advantage, in comparison with converting methanol to formaldehyde using iron the direct oxidation method, of producing a waste stream molybdate and supported Ag as the catalysts is shown in which can be sent directly for incineration, because it

Fig. 6. contains H2, methanol and formaldehyde, as well as Oxidative dehydrogenation of methanol on iron small quantities of N2 and CO2. The combustion of this molybdate based catalysts is carried out in a cooled flow produces the majority of the steam used in the multi-tubular reactor (see again Fig. 6 A). Due to the process. The outline of the process is similar to that progressive deactivation of the catalyst (the lifespan is discussed previously, however a purer methanol feed is

VOLUME II / REFINING AND PETROCHEMICALS 627 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

recycle using chlorohydrin, which is no longer used, and direct vent oxidation of ethylene with oxygen over supported Ag H2O based catalysts

cooling CH2 CH2 0.5O2 H2C CH2 water O ∆H°105 kJ/mol

CsCl and BaCl2 are used as promoters (the concentration of these doping agents is of 0-10 ppm), since both the alkaline metals and the chloride ions air promote the selectivity.

In addition to CO2, the by-products are acetaldehyde methanol and formaldehyde, but in concentrations no greater than formaldehyde 0.1%. Although there are still conflicting opinions on the solution matter, a significant body of data indicates that the active AA catalysing species is made up of AgO, with electrophilic characteristics, while oxygen bridging atoms recycle between the silver atoms (AgOAg) have a nucleophilic vent character and are non-selective, contrary to what happens catalyst in the case of selective oxidation on oxides (see again Fig. 3 B). In the past it was held that the selective species steam in epoxidation consisted of AgO , which after the H O 2 2 insertion of oxygen into the ethylene left an adsorbed O methanol atom and AgO on the surface, which in turn was responsible for the non-selective oxidation of ethylene to air CO2 and H2O. On the basis of this model, a maximum selectivity of 85.7% was hypothesized, whereas current commercial processes operate with greater selectivity (at H O 2 low conversion), in the range of 88-94%. waste The reaction scheme is of a ‘triangular’ type, with formaldehyde two parallel reactions of the transformation of ethylene B solution B into ethylene oxide and CO2, and a consecutive reaction of ethylene oxide to CO2. The principal factors Fig. 6. Simplified outline of the process of the synthesis of influencing the reaction kinetics and hence the choice of formaldehyde from methanol: A, direct oxidative dehydrogenation with catalysts based on iron molybdate; the optimal reaction conditions are summarized here. B, a process using catalysts based on supported Ag The ethylene oxide formation rate increases as the (Arpentinier et al., 2001). partial pressure of the oxygen increases, while a maximum rate is reached with respect to the concentration of ethylene, as a result of the competition required (there must be no iron carbonyls or sulphur between the ethylene and the ethylene oxide for the same compounds which would contaminate the catalyst), and catalytic sites. The ethylene oxide formation rate a heat exchanger is required to heat the methanol. decreases on increasing the concentration of the chloride Furthermore, the reactor must operate with very short ions used as a doping agent (nevertheless, above a contact times and have a rapid cooling system (cooling certain value traces of chlorinated compounds such as times for the discharge flow from the catalyst bed of less vinyl chloride and dichloroethylene form as by-products)

than 0.02 s), in order to avoid consecutive reactions of and the concentration of CO2. From a practical point of the formaldehyde, and finally two absorber columns in view, the concentration of ethylene is determined series should be used. essentially by the limits of flammability. The ratio of the rate of the two parallel reactions of formation of ethylene Epoxidation of ethylene to ethylene oxide oxide and combustion, that is the initial selectivity, rises Ethylene oxide is an intermediate product that is with the concentration of chloride ions, with the content synthesized on a large scale, being used in the of alkaline metal promoters and with the partial pressure production of ethylene glycol and polyglycols, of of the ethylene. ethanolamine and non-ionic detergents, and of esters of The processes employed for oxidation of ethylene ethylene glycol. There are two technologies: a process operate with air or with pure oxygen. The former are still

628 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Fig. 7. Simplified outline recycle ethylene of the process of the vent oxide synthesis of ethylene oxide from ethylene: A, processing with air; light ends B, processing with pure oxygen (Arpentinier et al., 2001).

ethylene heavy ends

air

air

steam

A

recycle CO2 vent

purge

inert steam

ethylene oxide

light ends ethylene

O 2 steam

B heavy ends

very widespread, but new plants mainly use pure type of diluent gas and lower reactor and equipment oxygen, which offers advantages in terms of higher costs. On the other hand, the process using pure oxygen yields and productivity, greater selectivity (due to the incurs greater costs due to its use and the need to

greater partial pressure of the ethylene in the reactor), separate the CO2 produced. The simplified outline of the lower volumes of waste gas, the ability to choose the two processes is illustrated in Fig. 7. In the process using

VOLUME II / REFINING AND PETROCHEMICALS 629 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

air, the ethylene is oxidized in a series of tubular reactors The use of an excess of ethylene, which has better (two, or three in the bigger plants), in order to increase thermal conduction properties than nitrogen, offers the conversion and manage the heat from the reaction advantages such as a more uniform temperature profile more effectively (in the first reactor the conversion is in the reactor (which results in better selectivity due to around 40%). The outflow from the reactor is cooled and less combustion and reduced formation of secondary then sent to an absorber column with water, for the products such as trichloroethane), better conversion of recovery of the ethylene oxide. Part of the gas the HC1 and a longer catalyst life (thanks to a lower discharged from the absorber column is recycled, while sublimation of the active phase and a reduced formation another part is sent to a second reactor where the of charcoal on the catalyst’s surface). Since the conversion reaches 80% (95% where there are three productivity of fixed-bed reactor is limited by the reactors in series). Selectivity is about 70% in the first capacity for transferring the heat to the cooling fluid, a reactor, but is lower in the subsequent ones. gas phase with higher conductivity results in an increase The process with pure oxygen (purity 97%) is in productivity.

single stage and uses multi-tubular reactors. The The principal problem related to the use of O2 is ethylene/O2 ratio is typically 3.0-3.5, while the O2 represented by its higher cost compared with air and the concentration is kept below 9% to avoid the formation of need for more complex systems for operating under safe inflammable mixtures. Conversion through the passage conditions. Nevertheless, many plants using air have been of ethylene is in the range 10-15%, while overall it is satisfactorily modified to operate with oxygen. The most greater than 97%. Selectivity typically is greater than widely used reactor is the multi-tubular fixed bed, because 80%. After cooling, the gases leaving the reactor are sent of the higher productivity attainable; nevertheless, the to the ethylene oxide absorber column, while the gases higher investment needed (corrosion resistant steel has to coming out of the column are compressed and recycled. be used) leads to a preference for fluidized-bed

Part of the gas is sent to a column to eliminate CO2 technology in the construction of new plants. through hot absorption in an aqueous solution of In Fig. 8 a simplified outline is portrayed of an potassium carbonate. A small fraction of the gases (less air-fed fixed-bed process. Three multi-tubular reactors than 1%) is discharged, to prevent an accumulation of in series are used; the catalyst is diluted with graphite, inert gases. In some processes a diluent, such as methane in order to reduce the thermal gradients in the reactor;

or ethane, is also used. Although CO2 itself can be an and a fourth reactor is used for direct chlorination of effective diluent, it can contaminate the catalyst and the residual ethylene with Cl2. The gases are cooled to therefore it is necessary to keep its concentration condense the DCE (1,2-dichloroethane), which is then level low. sent to the purification columns: the first removes the water, the second the light products (ethylene chloride, Oxychlorination of ethylene vinyl chloride, 1,1-dichloroethane and The oxychlorination (oxidation in the presence of dichloroethylene) and the third the heavy products HC1) reaction of ethylene to 1,2-dichloroethane (trichloroethane, perchloroethane and (DCE) perchloroethylene). To optimize the local ethylene/O ratio and to obtain CH CH 2HCl0.5O CH ClCH Cl 2 2 2 2 2 2 better control over the temperature in the reactors and ∆H°=238 kJ/mol hence of the selectivity, ethylene and HC1 are fed into is the basis for the production of the vinyl chloride the first reactor, while the air or oxygen feed is monomer (CH2 CHCl), used in the production of distributed over the three reactors in series. This also homo- and copolymers in PVC. makes it possible to operate outside the limits of

Three different processing options are possible, flammability, to reduce the formation of CO2, to have a depending on the technology used and the operating more homogeneous temperature profile and finally to conditions: the reactors can be based on fixed or increase the lifespan of the catalyst. The oxygen and

fluidized beds; either air or pure O2 can be used; and a ethylene are fed at slightly higher levels with respect to stoichiometric value or an excess of ethylene may be the stoichiometric quantities, in order to achieve a employed. Generally, pure oxygen is used when conversion of the HC1 greater than 99.5%. On the other operating in a fixed-bed reactor, with a large excess of hand, in processes that operate with oxygen and a large ethylene (compared with the stoichiometric value) where excess of ethylene, the unconverted hydrocarbon is the unconverted ethylene is recycled, or when using separated and recycled. stoichiometric ratios, where high conversion levels of The catalysts are based on copper chloride supported ethylene are achieved, but the gas flow is in any case on alumina and the promoters, made up of alkaline or recycled so as to reduce the environmental impact of the alkaline-earth metals, play a fundamental role. The process (see again Table 3). reaction temperatures are between 220 and 250°C, with

630 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Cl2

ethylene NaOH HCl H2O

air

water light ends 1,2-dichloroethane (to purification)

vent

heavy ends

Fig. 8. Simplified outline of the process of the oxychlorination of ethylene in a fixed-bed reactor (air based process) (Arpentinier et al., 2001).

pressures up to 5 bar. Selectivity of the DCE product is 10-20% acetic acid and 7-8% O2, as well as inert between 93 and 97%. ingredients (CO2, ethane, Ar, N2 and H2O). After the reaction, the gases are cooled causing condensation of the Acetoxylation of ethylene acetic acid, the water and the majority of the vinyl acetate. Acetoxylation (oxidation in the presence of acetic The liquid stream is split through azeotropic distillation to

acid) in the gas phase of ethylene with acetic acid and O2 recover the acetic acid and the vinyl acetate. Nevertheless is the primary process for producing vinyl acetate additional purification columns are necessary to achieve (CH3C(O)OCH CH2). Pd supported on silica is used, the required purity of the vinyl acetate, in particular to promoted by gold and alkaline metals (potassium lower the concentration of ethyl acetate below acetate). The gold allows the reduction of the secondary 150 ppm.The gas phase of the first condensation column reaction that forms ethyl acetate. Processing takes place is sent to an absorber column into which acetic acid is fed

with pure O2 at a temperature of around 150°C, with to remove the vinyl acetate, and then to a second column pressures in the range of 8-10 bar. In these conditions the fed with a solution of NaOH to remove the CO2. The reaction takes place in a liquid film, formed through gases are then recycled to the main reactor. capillary condensation in the pores of the catalyst. The reaction mechanism involves the reduction of Oxidation of propylene to acrolein and oxidation of the Pd acetate to metallic Pd through reaction with acrolein to acrylic acid ethylene and reoxidation of the Pd0 by the oxygen. Oxidation of propylene to acrylic acid (used in the Selectivity levels can reach 98%, although in industrial production of acrylic esters) is achieved through the processes they are typically between 92 and 95%. intermediate formation of acrolein: Conversion per pass is around 10%. C H O CH CHCHOH O After pre-heating, the cool ethylene, mixed with the 3 6 2 2 2 ∆H°=339 kJ/mol recycled ethylene and with acetic acid, is sent to the reactor containing the catalyst, previously mixed with O2. CH2 CHCHO 0.5O2 CH2 ∆ The typical composition is as follows: 40-55% ethylene, CHCOOH H2O H°= 255 kJ/mol

VOLUME II / REFINING AND PETROCHEMICALS 631 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

By-products of the reaction are carbon oxides, acetic presence of methanol) is the most widely used for the acid, propionic acid, formaldehyde, maleic acid, synthesis of methyl methacrylate and has many acetaldehyde and acetone. For the first reaction disadvantages, linked to the toxicity of HCN and the multi-component catalysts based on bismuth molybdates co-formation of high quantities of ammonium sulphate,

are used (for example, Mo12BiFe3Co4.5Ni2.5Sn0.5K0.1Ox), which is generated in the ratio of 2:1 with methyl while for the second reaction the catalysts are based on methacrylate. The alternative process is direct oxidation molybdenum and vanadium oxides (for example, of isobutene in the gas phase (with subsequent or

Mo12V3Cu2.5Fe1.25Mn0.1Mg0.1P0.1Ox). It is possible to integrated stages of esterification); however, this process carry out the synthesis in a single stage but, because of gives yields and selectivity which are too low to be the strong exothermicity of the reaction the lifespan of competitive. the catalyst is reduced. Moreover, the overall selectivity Alternative methods of synthesis are: a) direct is greater in a two stage process, as it is possible to oxidation of isobutane (a process which is still in the optimize each independently. In the first stage the research phase), which has the advantage of lower raw selectivity of acrolein is typically greater than 85% and material costs and lower environmental impact; b) the conversion of propylene exceeds 90%, while in the oxidation of isobutyric aldehyde to isobutyric acid, second stage the selectivity of acrylic acid is greater than which is then converted into methacrylic acid by 95%, with yields of between 90 and 96%. oxidative dehydrogenation (Mitsubishi Kasei/Asahi Multi-tubular fixed-bed reactors are used in series. method); c) oxidation of tert-butyl alcohol to The first reactor operates with a temperature in the range methacrolein, followed by oxidation to methacrylic acid of 330-400°C, and a spatial rate of 1,300-2,600 h1 (the and esterification; and d) hydroformylation of ethylene pressure is 2-2.5 bar), while the second operates at lower to propionic aldehyde, which is then condensed with temperatures (250-300°C), higher spatial rates (1,800- formaldehyde to give methacrolein, which finally is 3,600 h1) and lower pressures (due primarily to the loss oxidized to methacrylic acid and esterified (BASF of pressure in the first reactor). process). Among these alternatives, direct synthesis of The concentration of propylene entering the first isobutane is the most interesting; nevertheless, the reactor is 5-8% in air. Recirculating gas and/or steam are selectivity obtained and the stability of the catalysts are used as diluents, in order to operate outside the limits of not sufficient for it to be developed industrially. explosion. The use of steam also allows the reactions in The average composition of the catalysts for the last

homogeneous phase to be reduced, the thermal transfer of the above applications is as follows: (HmY0.2-1.5) to be improved and selectivity to be increased, favouring (P1-1.2Mo12-nX0.4-1.5Ox), where Y is the ion of an alkaline the desorption of acrolein and acrylic acid. Nevertheless, metal and X is an element such as V, As and Cu; excessive concentrations of steam lower the moreover, various other additives are present. It is concentration of the acrylic acid solution. necessary to use high concentrations of isobutane and The gases leaving the reactor, after cooling and heat steam (up to 65%) to obtain good selectivity and recovery, are sent to an absorber column with water. An stability of the catalysts. inhibitor is added to avoid polymerization of the acrylic acid. The discharge gases are then sent for incineration Synthesis of acrylonitrile by ammonia oxidation of and in part are recycled, after the elimination of propylene condensable compounds. The solution of acrylic acid is Acrylonitrile is produced on a large scale (over 5 sent to the purification section, which consists of a series million tons per year) by the process of catalysed of azeotropic distillation columns (with ammonia oxidation (oxidation in the presence of methylethylketone as the third component). In the case ammonia) of gas phase propylene: of diluted solutions it is possible, as an alternative, to CH2 CH CH3 NH3 1,5O2 carry out a separation by extraction, using ethyl acetate CH CHCN +3H O ∆H°=515 kJ/mol or aromatic compounds as solvents. The various 2 2 industrial processes differ in the composition of the Acrylonitrile finds applications in the synthesis of catalyst and in the separation section. various homo- and copolymers used as fibres, resins and elastomers; it is also an intermediate in the production of Synthesis of methyl methacrylate adiponitrile and acrylamide.

Methyl ester of methacrylic acid, By-products of the reaction are HCN, acetonitrile, N2 CH2 C(CH3)COOCH3, is used in the production of (from combustion of the ammonia) and carbon oxides. vinyl polymers. The acetone cyanohydrin process (which The reaction is strongly exothermic and to control the consists of a reaction between acetone and HCN, temperature most plants use fluid-bed reactors. The followed by a reaction of acetone cyanohydrin with commercial catalysts are multi-component, based on sulphuric acid and a final hydrolysis of the adduct in the bismuth molybdate and supported on silica (for example,

632 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

(K, Cs)0.1(Ni, Mg, Mn)7.5(Fe, Cr)2.3Bi0.5Mo12Ox/SiO2). concentrated for azeotropic distillation. The acrylonitrile It is not necessary to recycle the propylene, since is purified in two columns in series, to recover the conversion rates are as high as 95%, while maintaining a hydrogen cyanide and the impurities (acetone, high selectivity to acrylonitrile (above 80%). The fluid- acetaldehyde, propionaldehyde and acrolein). A final bed reactor contains a high quantity of catalyst, up to 70- stage of vacuum distillation is used to obtain 80 tons, in the form of spherical particles with an acrylonitrile with purity above 99.4%. average diameter of 40-50 mm, with a view to allowing Recently a number of studies have been devoted to an efficient fluidization. The purity of the feed must be the use of propane as an alternative reagent to propylene, very high (90% for the propylene and 99.5% for the by virtue of its lower cost. New classes of catalysts (for

NH3). The ratio of the ammonia/propylene feed is equal example those developed by Mitsubishi, composed of to 1.05-1.2, and the ratio of O2/propylene is within the quaternary oxides of molybdenum, vanadium, tellurium range 10-15. The reaction temperature is between 420 and niobium, MoV0.3Te 0.23Nb0.12Ox) give yields of and 450°C and the pressure between 1.5 and 3 bar. acrylonitrile above 50%. This value could be sufficient Pressures above 1 bar have a negative effect on the to justify development of a new process starting with selectivity to acrylonitrile, but are necessary in order to propane, although a further increase in yield and stability obtain good fluidization and to increase the productivity. of the catalyst would be hoped for. The reagents are fed into the reactor separately, to minimize reactions in the homogeneous phase and Ammonia oxidation of alkyl aromatics prevent the formation of explosive mixtures before Numerous aromatic nitriles such as benzonitrile, reaching the catalytic bed; the composition of the phthalonitrile, isophthalonitrile, terephthalonitrile and mixture in the reactor is within the limits of nicotinonitrile find applications in the synthesis of flammability, but the presence of the fluid bed inhibits products for fine chemicals. For example, nicotinonitrile the propagation of radical reactions thereby blocking any can be hydrolysed to the corresponding amine or to flame front. The fluid-bed reactor contains various coils nicotinic acid, used for the synthesis of vitamin B. and systems to minimize the formation of slugs and to Isophthalonitrile is used in the synthesis of herbicides reduce the phenomena of retro-mixing of the fluid. The and fungicides. Phthalonitrile is an intermediate for top of the reactor has a larger cross section, in order to pigments based on phthalocyanine. A number of reduce the velocity of the gas and to diminish the catalysts are active in the reaction: vanadium oxide

occurrences of pneumatic transmission and elution of supported on TiO2 (preferably in the anatase crystalline the catalyst. Appropriate cyclones allow the recovery of structure), doped with Cs, P and W; multi-component the catalyst particles and their reintroduction into the catalysts based on molybdates; vanadium antimonates reactor. doped with Bi and Fe; and supported heteropolyacids

The effluents from the reactor are sent to an absorber (PV3Mo12Ox on silica). Apart from maximizing the column with water, while the unconverted ammonia is selectivity to nitrile, it is also important to minimize the

neutralized with sulphuric acid. The gases leaving this oxidizing reaction of the ammonia to N2. Selectivity column, containing N2, carbon oxides and small levels above 90% are generally possible for conversions quantities of propylene, are sent for incineration. The which are between 50 and 80%, even though the results acetonitrile/acrylonitrile mixture forms an azeotrope vary considerably according to the type of substrate. with water that on separation gives rise to an aqueous One of the principal problems in the industrial phase (recycled to the absorber) and an organic phase, application of the process is the need to carry out rich in acrylonitrile and HCN, that is sent for successive production runs with different types of alkyl purification. The aqueous solution of acetonitrile is aromatics, as the market demand for these products is

Fig. 9. Outline of the synthesis of phthalimide by catalytic ammonia O oxidation of o-xylene in the gas phase. phthalimide NH O CH3 O2, NH3 CH3 O2, NH3 CO V O /TiO V O /TiO x CH3 2 x 2 CN 2 x 2 tolunitrile CN

phthalonitrile CN

VOLUME II / REFINING AND PETROCHEMICALS 633 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

insufficient to justify the use of the plant for a single from the reactor are cooled to 200°C, filtered to remove reaction. The aromatic imides can also be obtained by the finest particles and sent to the maleic anhydride means of catalytic ammonia oxidation (Fig. 9), for recovery section. The latter is based on absorption in example feeding o-xylene and using a catalyst based on cycloaliphatic solvents. The gases are sent for supported vanadium oxide on titanium oxide. incineration for the production of high temperature steam, while the solution is sent to a stripping unit. The Maleic anhydride from n-butane maleic anhydride is then further purified to remove the Maleic anhydride is used as an additive in the light products (which are sent for incineration), while the synthesis of various polymers, in the synthesis of solvent is recycled. chemical products for agriculture and of malic and The process gives high productivity, production of fumaric acids, as well as being an intermediate for the high temperature steam, reduced quantity of waste, synthesis of g-butyrolactone and of tetrahydrofuran. low formation of fumaric acid and heavy products. Today, most plants for production of maleic anhydride The production of steam contributes to the economy of use n-butane as the feed; this process has replaced that the process. of benzene, by virtue of the smaller number of The DuPont process is characterized by the use of by-products, the better atomic efficiency and the lower an innovative reactor for the selective oxidation sector costs, and the non-toxicity of the reagent. The derived from the catalytic cracking, that permits by-products obtained in the oxidation of n-butane are separate contact of the hydrocarbon and oxygen with carbon oxides and acetic acid, whereas numerous the catalyst. This brings about a significant increase in by-products are formed by benzene, or when using the selectivity, but involves the need for higher butenes or butadiene as the raw material. recirculation of the catalyst between the two reactors The various commercial processes differ in their (the riser reactor for contact with the hydrocarbon and reactor technology (fixed-bed, or fluid or circulating-bed the fluid-bed reactor for reoxidation of the catalyst). reactor), in the percentage of n-butane in the input (less Furthermore it calls for the catalyst to have high than 2% with fixed-bed reactors, between 2.6 and 5% mechanical resistance. The DuPont process for for processes with fluid-bed reactors, and more than synthesis of maleic anhydride is integrated with the 10% in the DuPont process, with circulating-bed downstream hydrogenation section, for the production reactors, similar to that used in catalytic cracking) and in of tetrahydrofuran. The absorption of the maleic the method of recovery and purification of the maleic anhydride is, in this case, performed with water. anhydride (use of organic or aqueous solvents in the recovery and purification method). In all processes the Phthalic anhydride from o-xylene catalyst consists of pyrophosphate of vanadyl Phthalic anhydride is used in the preparation of

(VO)2P2O7, where necessary promoted with doping diesters (plasticizers for PVC), alkylic resins, polyesters elements, although the methods of preparation, and colourings. The original process used naphthalene as activation and formation and the dimensions of the the raw material, but today most plants use o-xylene, by particles can vary. virtue of the reduced environmental and safety The feed composition is directly related to the choice problems, as well as the greater purity of the product. of reactor as shown above. An increase in the The principal by-products of o-xylene are concentration of n-butane increases the productivity, but o-tolualdehyde and phthalide, small quantities of maleic calls for special precautions for dispersal of the reaction anhydride, benzoic acid, toluic acid, as well as carbon heat and to reduce the risks of explosion. The oxides. The formation of phthalide is a critical aspect of unconverted n-butane is not recycled, but is used in the the process, in that, for applications in the polymer production of high temperature steam, since the value of sector, the concentration of this compound in phthalic this hydrocarbon is close to that of fuels. anhydride must be very low. In a multi-tubular reactor, The conversion rate of n-butane is between 80 and about a third of the catalytic bed enables over 90% 90% and the yield in maleic anhydride between 55 and conversion of the o-xylene to be obtained, while the 65%. The working temperature is 400-450°C. The remaining two thirds of the bed serves to reduce the principal processes are marketed by Denka/Scientific concentration of phthalide. Design, Amoco, BP-UCB, Lonza/Lummus, Mitsubishi The catalysts are based on vanadium oxide supported

Kasei, Mitsui Toatsu, Monsanto and DuPont. on TiO2 (in the octahedral crystalline form), with a Lonza’s ALMA process uses a fluid-bed reactor, relatively low surface area (around 10-20 m2/g). K, Cs, while the DuPont process uses a circulating-bed reactor. Sb, Nb, and P are used as promoters. The latest Fig. 10 shows a simplified outline of the two processes. generation of reactors are loaded with two or three layers In the ALMA process, after the separation of the of catalyst whose compositions differ from one another catalyst by means of two cyclones in series, the effluents (above all in terms of type and quantity of promoter

634 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

vent recovery/purification sections. Air is used as an oxidant, dry although the use of air enriched with oxygen allows filter higher productivity. The concentration of o-xylene is not greater than 1.2% in air, so as to avoid the formation of maleic flammable mixtures and to achieve good control of the anhydride heat of the reaction. Recovery of the phthalic anhydride is carried out by desublimation and then by absorption in oil. The fixed-bed reactor operates at temperatures of between 350-390°C and with low spatial rates, while the process in the fluid-bed reactor operates at higher temperatures (450-550°C) and with lower contact times. The o-xylene and the air, after preheating, are sent to a multi-tubular reactor operating at about 380°C and cooled with molten salts; these salts are cooled externally in a heat exchanger, producing high temperature steam. or t fresh solvent A yield of phthalic anhydride above 80% is obtained eac

r and a conversion of o-xylene greater than 99%. The effluents from the reactor are cooled, producing low n-butane steam pressure vapour, and then sent to an absorber column with water. The solution is first sent to a under vacuum air system to decompose the impurities (polymerisation A inhibitors are added) and then to a distillation column (also under vacuum) to separate the maleic anhydride and the benzoic and toluic acids from the top. The

by-products solution collected at the bottom of the distillation column is sent to a second column, to separate high vent purity phthalic anhydride (99.5% by weight). e l uran f ecyc r ro 2 d References y steam H

h H2 etra Arpentinier P. et al. (2001) The technology of catalytic t fluid-bed oxidations, Paris, Technip. regeneration aqueous Bielanski A., Haber J. (1991) Oxygen in catalysis, New York, air reactor solution of maleic Marcel Dekker. acid Centi G., Perathoner S. (2003a) Catalysis and sustainable green chemistry, «Catalysis Today», 77, 287-297. BB n-butane Centi G., Perathoner S. (2003b) Selective oxidation. Section n-butane vent recycle E (Industrial processes and relevant engineering issues), in: Horvath I.T. (editor in chief) Encyclopedia of catalysis,New Fig. 10. Simplified outline of the process of the synthesis York, John Wiley, 6v. of maleic anhydride from n-butane: A, the ALMA process; Centi G. et al. (2002) Selective oxidation by heterogeneous B, the DuPont process. catalysis, New York, Kluwer Academic-Plenum. Grasselli R.K. (2002) Fundamentals principles of selective heterogeneous oxidation catalysis, «Topics in Catalysis», elements); in this way it is possible to obtain maximum 21, 79-88. yield and selectivity, by optimizing the activity profile Fabrizio Cavani and minimizing ‘hot spots’ along the reactor. Promoters Dipartimento di Chimica Industriale e dei Materiali in the gas phase, such as SO2 also have a positive effect on performance, although they are no longer used today. Università degli Studi di Bologna Various processes exist, among which the principal Bologna, Italy ones are those developed by Wacker, BASF, Gabriele Centi Lonza-Alusuisse, Atochem and Nippon Shokubai. Dipartimento di Chimica Industriale e The processes differ in the type of reactor (fixed or fluid di Ingegneria dei Materiali bed, although the former is used in the majority of cases), Università degli Studi di Messina in the composition of the catalyst and in the Messina, Italy

VOLUME II / REFINING AND PETROCHEMICALS 635 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

11.1.2 Oxidation processes Liquid phase homogeneous in liquid phase with oxygen metal–catalysed oxidations by molecular oxygen Introduction The oxidation of organic substrates: Liquid phase oxidation is widely applied in the substrate + oxygen donor oxidized substrate + chemical industry for the synthesis of intermediates both reduced donor for the petrochemical industry and for the production of has considerable industrial importance and plays specialty chemicals and pharmaceuticals, as well as for fundamental roles in biological processes. Oxo wastewater decontamination. transfer oxygenations of organic substrates (mainly Liquid phase oxidation is chosen rather than alkanes and alkenes) are utilized in some processes heterogeneous gas phase catalysis in the following cases: and are the subject of several investigations. Different when the products are thermally unstable (i.e. in the oxygen donors may be used: molecular oxygen, production of hydroperoxides and carboxylic acids, with peracids, alkylhydroperoxides, hydrogen peroxide, the exception of b-unsaturated compounds); when the iodosylarenes, amine N-oxides, hypochlorite and products are so reactive that at high temperature they can ammonium persulfate. Different catalysts are be further oxidized (i.e. epoxides, aldehydes and employed, such as metalloporphyrins, ketones, with the exception of b-unsaturated metallophthalocyanines, soluble transition metal salts compounds, ethylene oxide and formaldehyde); and, in with chelating ligands and Schiff base complexes. fine chemical production where liquid phase oxidation is Polyoxometallates are among the most interesting especially suitable due to the thermal instability and/or systems that have been receiving considerable reactivity of the reagents themselves (i.e. in the oxidation attention in recent years. of polyhydroxy alcohols). Oxidation of a substrate with molecular oxygen In addition to being the largest application of may involve two electrons (with the involvement of homogeneous catalysis, liquid phase oxidation is more only one oxygen atom), or four electrons (with the important than heterogeneous gas phase oxidation involvement of both oxygen atoms). Coordination with processes, on the basis of tonnage and variety of the metal modifies the features of oxygen (i.e. its products (Prengle and Barona, 1970a; Lyons, 1980; basicity and radical character), making it more Sheldon and Kochi, 1981). In homogeneous catalysis, susceptible to reaction with organic substrates. A the main technological issues also include selectivity, summary of the possible complexes between removal of the heat of reaction and safety considerations. molecular oxygen and metal ions is presented in Liquid phase oxidations can be subdivided into the Table 1 (Sheldon and Kochi, 1981). following five types of catalytic processes, based on In addition to the superoxo and peroxo species, oxo the mechanism involved and the relative catalysts: a) metal species that occur via the transfer of two electrons free-radical, chain oxidation (with and without from the metal to the oxygen atom will be addressed. catalyst), with molecular oxygen as the oxidizing Hydroperoxo species can also form via hydrolysis agent; b) redox mechanism with Pd or Cu complexes, of peroxo complexes, or by molecular oxygen and molecular oxygen; c) catalytic oxygen transfer, insertion in metal hydrides: with either alkylhydroperoxide or H O as the 2 2 M H M O OH oxidizing agent, and with either homogeneous or heterogeneous catalysts; d) oxidative dehydrogenation O O with molecular oxygen and with supported, transition metal–based catalysts; e) photocatalytic processes. MOOMH MOOH M The first class is the most important from an M H O2 M O OH industrial point of view and, therefore, the most important technological aspects of this class of The formation of oxo metal species takes place if reaction will be examined in detail. the metal is able to furnish two electrons; such metals

636 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

liquid phase homogeneous oxidations in mild Table 1. Summary of the possible species conditions, the generation of oxo metal species when formed by complexation of oxygen to metal ions using molecular oxygen requires the presence of a reducing agent, which is necessary to furnish electrons n (n 1) M O2 M O2 (superoxo) to the system. In addition, the catalytic action of the (n 1) n (n 1) 2 oxo metal species in the oxygenation of the C H M O2 M M (O O) M(n1)(m-peroxo) bond must always compete with very fast autoxidation processes, thus with free radical chain reactions that M(n 1) (OO)2 M(n 1) 2M(n 2) O2 (oxo) are largely unselective and indiscriminate. Therefore, M(n 2) O2 Mn M(n 1) O2 M(n 1) (m-oxo) the objective is to develop systems that can operate in the absence of other reducing agents and that MOH O M(OH)OOH (metal peracid) 2 2 kinetically compete with radical-initiated autoxidations. M(OH) O OH H2O M (peroxo) Examples of reductants that reduce molecular (n 1) M O2 peroxo oxygen to H2O2 are anthraquinol derivatives, NADH and BH 4 . The metal-peroxo complexes n (n2) (n2) 2 include the redox couples M /M : Co(I)/Co(III), M O2 can react with water or an acid, leading Ir(I)/Ir(III), Pd(0)/Pd(II), Pt(0)/Pt(II). Metals in higher to the formation of H2O2 or of M OOH species: oxidation states (III and IV) can not transfer electrons (n 2) (n 2) 2 M 2e O2 M O2 (peroxo complex) to molecular oxygen and do not form stable adducts. (n 2) 2 (n 2) M O2 2 H M H2O2 In this mechanism, the formation of high–valent oxo M(n 2) O2 H M(n 2) OOH metal species occurs through the OO bond scission 2 of an intermediate m-peroxo dinuclear complex, with Similarly, metal-O2 superoxo adducts (i.e. where electron transfer from the metal and with spin pairing only one-electron charge transfer from the metal to in the molecular oxygen. A more suitable molecular oxygen has occurred) can be reduced to representation would take into account the two metal peroxo species in the presence of a one–electron resonance structures: reducing agent (sacrificial reductant):

(n1) (n1) 2 2 M(IV) O M(II) O M O O e M O2 (peroxo complex) The most usual representation of this moiety is MO. Different classes of metal-catalysed oxidations by The second step is the oxidation of metal O2 can be distinguished, according to the nature of the complexes to oxo metal species with H2O2, which can complex formed with the oxygen and the metal proceed as follows: oxidation states formed (i.e. to the role played by the M(II)(H O) H O M(IV)O 2H O metal atom in the oxygen activation; Sheldon and 2 2 2 2 Kochi, 1981; Drago and Beer, 1992). Particular or through ligand substitution and formation of the attention will be paid to the oxygenations by oxo dinuclear m-peroxo complex, which then decomposes metals, which are oxidations of relevant importance in to the oxo species. the functionality of organic substrates. The above is the same as the well-known reaction between hydrogen peroxide (or alkylhydroperoxides) Oxo metal species formed by interaction with metal ions, with oxidation of the metal ion and 2 with peroxides and molecular oxygen reduction of H2O2, catalysed by ions such as Fe or The mechanism for the formation of the MO Cu: species can be divided into two steps: the reaction for 2 3 2M 2H H2O2 2M 2H2O the formation of the peroxide and that of the peroxide 2M3 H O 2M2 O 2H with the metal complex to form the oxo metal species. 2 2 2 The metal-catalysed oxidation does not occur as a In this case, however, molecular oxygen is also consequence of the activation of molecular oxygen by generated, since the metal ion with higher oxidation the metal, since the metal characteristics do not allow state formed by the reaction catalyses the oxidation of

the electron transfer to oxygen. The presence of a H2O2 to O2. This occurs because the metal ion has two reductant enables the conversion of molecular oxygen stable oxidation states – that is M(II)/M(III) – and into H2O2 or into an alkylhydroperoxide (which are hydrogen peroxide dismutation is preferred rather than stronger oxidizing agents than oxygen) to be carried the formation of an even higher oxidation state M(IV). out; the latter may react with the metal complex to The formation of the oxo metal species yield the high-valent oxo metal species. In fact, in theoretically may involve either a homolytic or

VOLUME II / REFINING AND PETROCHEMICALS 637 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

heterolytic rupture of the OO bond in the [M(n 1) OH R] M(n 1) ROH hydroperoxide. The heterolytic process involves a two-electron transfer from the metal (increasing in Under mild reaction conditions, this mechanism is oxidation state from Mn a M(n2)), while the generally accepted as the operating one in liquid homolytic process would evidently imply oxidation to phase alkane hydroxylations by oxo metal species. M(n1). In this case, a radical HO (or RO in the case • H abstraction, where an ion pair is formed, of alkylhydroperoxides) is also generated, which [M(n2)OH R], which successively evolves would be responsible for undesired side reactions due to the ROH product and to M(n2). to its high intrinsic reactivity, as occurs in • Electron transfer with formation of a radical ion metal-initiated autoxidations. pair [M(n1)] [RH]. The formation of highly valent oxo metal species has • H abstraction, with the formation of an ion pair been proposed to occur in monooxygenase enzymes, [(MOH) R]. among which the cytochrome P-450 is the most widely • Concerted oxygen atom insertion, involving CO

studied and characterized. Such enzymes catalyse bond formation, which resembles a SN 2 transition reactions in which one atom of oxygen is incorporated state, where the MO bond interacts into an organic substrate, while the second atom of perpendicular to the CH bond. (n1) oxygen is reduced and forms water (Hayaishi, 1974): The radical pair [LxM OH R ] (L ligand) of the first mechanism can evolve differently (Hill, RH O 2e 2H ROH H O 2 2 1989). Aside from the possibility of a Fe-porphyrin complexes, analogues of cytochrome contemporaneous breaking of the MO bond and the P-450 (biomimetic systems), form Fe oxo species via formation of the CO bond (with formation of the preliminary reduction of molecular oxygen by means alcohol product), a possible fate of the radical pair is of a sacrificial co-reductant, such as NaBH4, LiBH4, conversion to an hydroxy-organometallic intermediate: n ascorbic acid, or Pt/H2 (Groves, 1985; Ortiz de (HO)(Lx)M R (Kochi, 1973), which may evolve to (n2) Montellano, 1985; Dawson, 1988; Mansuy and the ROH and LxM via reductive elimination. Battioni, 1989; Shilov, 1989). The mechanism involves Another possible evolution of the radical pair the reduction of molecular oxygen and the formation intermediate is conversion into an ion pair by electron of a Fe(III)-peroxo complex, which decomposes to transfer: [L M(n2)OH R]. Finally, as pointed out x (P)Fe(IV)O (Pporphyrin), and which, in turn, acts by Hill (1989), formation of the alcohol by hydrolysis as the active oxidizing agent. After oxidation, the of an intermediate ester may also occur, the ester being Fe(III) formed is reduced back to Fe(II). The use of formed by the attack of a freely diffusing radical at the

H2O2, instead of O2, renders the sacrificial reductant pendant oxygen. Diffusion of the radicals out of the useless, since the hydrogen peroxide directly forms the pair is possible especially in those cases where the oxo metal species (in enzymes, this is referred to as radical is particularly resistant towards oxidation (i.e. the hydrogen peroxide shunt). in primary radicals). Therefore, the fate of the Oxo metal centres have been claimed to be the intermediate radical pair is a function of the nature of { } active oxidizing site in Fe(TFPP)N3, Fe(TFPP) 2O the complex and the type of the C H bond, which is and Mn(TFPP)N3 catalysts (TFPP meso- to be oxyfunctionalized. tetraphenylporphyrinato) for the oxidation of Theoretically, an organometallic intermediate isobutane to t-butyl alcohol with molecular oxygen could be obtained via an electrophilic process (Ellis and Lyons, 1989a e 1989b). It is claimed that the involving the contemporaneous formation of the characteristics of the complexes used make the CMn bond and the OH bond by cleavage of the presence of the co-reductant unnecessary. CH bond in the substrate. However, no evidence In the reaction of CH bond hydroxylation in exists for such a mechanism in CH oxygenation by alkanes or in other substrates containing CH bonds, oxo metal species. Instead, this mechanism has been different mechanisms are possible (Hill, 1989), claimed in the case of the oxidation of CH2 depending on the nature of the intermediate species that groups to the corresponding ketones catalysed by the is formed. The type of reaction pathway is generally a so-called Gif(III) system, and the subsequently function of the electron acceptor properties of the oxo developed Gif(IV), GO, GoAgg(I) and GoAgg(III) metal bond. The following mechanisms are possible: systems (Barton et al., 1989; Sheu et al., 1990; Barton • A mechanism of H abstraction: and Doller, 1991). The ketone can be produced with very high selectivity. Mn O2 RH [M(n 1) OH R] These systems are essentially constituted of Fe(II), with the formation of a radical pair, which then or Fe(III), pyridine and acetic acid, aside from the evolves to: substrate to be oxidized, while the oxidizing agent may

638 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

be either a peroxide or molecular oxygen (with a the metal-catalysed radical chain reaction, including the reducing agent). The highly–valent Fe(V)O species well-known Haber-Weiss mechanism of ROOH is generated, which inserts into the CH bond decomposition, is the following: (Barton and Doller, 1991): M(II)ROOH M(III)OHRO(reduction step) Fe(III)OFe(V)O RCH R 2 M(III)OH ROOH M(II) ROO H2O Fe(III)OFe(V)(OH)CRRH (oxidation step) followed by evolution through several steps to the and the chain reaction is propagated: formation of the ketone and to minor amounts of the ROROOH ROH ROO secondary alcohol. 2ROO 2RO O2 ROORH ROOH R Metal peroxo species As mentioned above, free-radical mechanisms are In the free radical autoxidation process, metal very important in the metal-catalysed oxidation of catalysis involves the formation of chain-initiating organic substrates, and often they are kinetically the radicals via reaction with ROOH; therefore, the metal most important process in competition with oxo metal ion is more an initiator than a catalyst. transfer reactions. Usually, these processes are When the metal compound has two oxidation states indiscriminate and necessarily lead to poor of comparable stability, the reduction and the selectivities when the substrate has different sites oxidation reactions occur concurrently. Cobalt, iron, susceptible to attack. Even so, in some cases, these copper and manganese complexes are generally processes have industrial applications. effective compounds for the homolysis of peroxidic Reactions of peroxides (i.e. hydrogen peroxide, compounds. Correspondingly, Mn(II)/Mn(III) and alkyl hydroperoxides) with organic substrates, catalysed Fe(II)/Fe(III) are effective catalysts for the

by metal ions, can be divided into two classes: decomposition of H2O2, where the metal acts as both homolytic, one-electron processes, which involve the reductant and oxidant. formation of free radical intermediates, and heterolytic, Since alkylperoxy radicals are strong oxidants, two-electrons processes, where the metal forms metal they oxidize the reduced form of the metal: hydroperoxide or metal alkylperoxide complexes, able M(II) ROO M(III)OOR to attack the organic substrate. Therefore, chain propagation can be inhibited by Homolytic processes the formation of this metal alkylperoxo compound, Reaction between the metal species and the organic which corresponds to an induction period that is substrate generates the free radical R: observed in metal-catalysed autoxidations, especially in media of low polarity. The induction period can be Mn +RH M(n 1) +RH eliminated by the addition of small amounts of an RH B RBH , B base alkylhydroperoxide to the reaction medium. The formation of R by abstraction of H by the metal, Termination of the chain reaction occurs through with the formation of a hydride species, can be the following reaction: disregarded due to the fact that R–H bonds are 2ROO ROOOOR O RCO generally stronger than MH bonds. 2 2 The free radical R can then evolve either to the and again by the above-mentioned reaction, at high formation of a carbocation (electron transfer process): metal concentration, the prevailing mechanism of termination is M(II) ROO M(III) O O R. n R M(n 1) R M The activating effect of some metal ions on the

or via a ligand transfer mechanism: oxidizing activity of H2O2 is well known, as in Fenton’s reagent: RMn X RX Mn Fe(II) H O Fe(III)OH OH The prevailing mechanism is a function of the nature 2 2 OHH O H O HO of the ligand in the metal complex, and both 2 2 2 2

mechanisms can indeed operate at the same time. (undesired reaction of unproductive H2O2 The reactivity of alkylperoxides and hydroperoxides decomposition, which lowers the yield to oxidized with metal ions is of the above-mentioned type. Metal products) ions, such as Co(III), decompose ROOH, generating Fe(III)OH HO Fe(II) O H O alkylperoxy radicals, ROO, which initiate autoxidation 2 2 2 reactions of organic substrates. A general scheme for The OH produced generates R by reaction with

VOLUME II / REFINING AND PETROCHEMICALS 639 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

organic substrates, finally converted to oxidized compounds: Table 2. Oxidizability of some organic substances (Howard, 1972) RH OH R H2O RFe3 Fe2 R products Substrate 3 2 3 RFe Fe R 10 kpr 2kter R H RH Benzaldehyde 290 Several examples of oxidation reactions performed by 2,3-dimethyl-2-butene 3.2 peroxides in the presence of metal ions are described Cyclohexene 2.3 in the book of Sheldon and Kochi (1981). Cumene 1.5 Some of the most important industrial liquid phase Ethylbenzene 0.2 oxidation processes proceed via an autoxidation p-xylene 0.005 mechanism in the presence of molecular oxygen, such as: oxidation of cyclohexane to adipic acid, oxidation Toluene 0.01 of p-xylene to terephthalic acid, oxidation of toluene to benzoic acid, oxidation of cumene to phenol via intermediate cumylhydroperoxide, and oxidation of Table 3. CH bond energies linear aldehydes to linear carboxylic acids. Autoxidation processes are indiscriminate and usually Compound Energy (kcal/mol) low selectivities are achieved. Nevertheless, with CH H molecules that contain only one reactive CH bond, 3 103 it is possible to obtain high selectivity, especially if C3H7 H(C1) 99 low conversions are maintained. C3H7 H(C2) 94 In industrial practice, usually an initiator (In) is i-C H H(C ) 90 added, necessary to start the chain reaction and to 4 9 3 form ROOH: CH2 CH H 105 CH CHCH H 85 In 2In (by thermal decomposition) 2 2 2 InRH InH R C6H5 H 103 C H CH H 85 In the presence of molecular oxygen, the radical 6 5 2 species is transformed to a peroxy radical, which then evolves propagating the chain (rate-determining step ROOH (bond strength 90 kcal/mol; Benson, 1965), under usual reaction conditions, at a partial pressure of is stronger than the broken bond RH. molecular oxygen higher than 0.13 bar): The overall mechanism in the case of the oxidation of alkylbenzenes under oxygen pressure, in acetic acid, R O2 RO2 and with Co(III) (CH3COO)3 as the catalyst (and also RORH RO H R 2 2 traces of an activator, to reconvert Co(II) to Co(III), The reaction is terminated by the coupling of the avoiding long induction periods) is shown in Table 4. RO2 radical. Therefore, hydroperoxides are the The last reaction is inhibited by the high concentration primary products of the reaction network. of Co2, which traps the benzylperoxy radical. Table 2 reports the oxidizability of various organic substrates (Howard, 1972), while Table 3 presents Nucleophilic attack on peroxo complexes some CH bond energies (Kerr, 1966; Benson and Transition metals such as vanadium, molybdenum, Shaw, 1970). The oxidizability of organic substrates to tungsten, selenium and titanium in their high oxidation undergo autoxidation reactions is expressed in [10] state promote the heterolysis of the OO bond in

(see below) by the ratio between the rate constant H2O2 and in alkyl hydroperoxides; these metals have relative to the propagation reaction (kpr in Table 2), the role of improving the electrophilicity of the and the square root of twice the rate constant relative peroxide complex. An attack on a coordinated peroxo to the termination reaction via the mutual coupling of or alkylperoxo ligand by the substrate occurs, usually

two alkylperoxy radicals (kter in Table 2). At high with high specificity. The most important example is substrate conversion, the formation of secondary the industrial production of propylene oxide from products (aldehydes and ketones) considerably lowers propylene and alkyl hydroperoxide, catalysed in both the selectivity, and thus the conversion is usually the homogeneous and heterogeneous phase. maintained lower than 20%. Oxidation is rapid if the The above-mentioned metal oxides catalyse the

bond that will be formed in the rate determining step, H2O2 reactions through the formation of inorganic

640 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Fig. 2 reports the mechanism proposed by Clerici Table 4. Aerobic oxidation of alkylbenzenes and Ingallina (1993b) for the epoxidation of alkenes III catalyzed by a Co complex by H2O2 over Ti-silicalite catalysts. A five-membered ring species is the intermediate compound formed by 3 2 ArCH3 Co ArCH3 Co O2 interaction between the Ti sites in silicalite and H2O2, also including the interaction with the solvent ROH. ArCH ArCHH 3 2 The epoxidation rate was found to be the highest with 3 2 ArCH2 Co ArCH2 Co (in the absence of O2) methanol as the solvent, and lowest with the bulky t-butanol, due to the decreased electrophilicity and ArCH AcOHArCH OAcH 2 2 increased steric constraints of the cycle (Clerici, 1991;

Clerici et al., 1991; Bellussi et al., 1992). An ArCH2 O2 ArCH2O2 (in the presence of O2) alternative mechanism (Mimoun, 1982) involves the 2 2 ArCH2O2 Co ArCH2O2Co addition of the alkene double bond to form a 2 3 ArCH2O2Co ArCHO HOCo HOCo3 HOAcAcOCo3 H O 2 O H OH ArCH OArCH ArCH OOHArCH 2 2 3 2 2 MO epoxide O OR peracids, which undergo heterolytic cleavage of the R OO bond in the presence of nucleophiles; H OO O O MO3 H2O2 HO M O OH MOR M epoxide ROH O more generally:

MOROOH HOMOOR

In olefin epoxidation: O O

RCH CHR HO M O OR RCH(O)CHR M OR M epoxide HOMOR O OR HOMORROOH HOMOOR ROH

alkyl hydroperoxides are preferred to H2O2 due to the Fig. 1. Proposed mechanisms for olefin epoxidation, superior activity. The retarding effect of H2O2 has been involving direct oxygen transfer by the alkyl attributed to the presence of water. A peculiar feature hydroperoxide, in ROH solvent (adapted from Sheldon of this reaction is its stereoselectivity for cis and trans and Kochi, 1981). epoxides from the corresponding alkenes. Asymmetric epoxidation of prochiral allylic alcohols can be carried out with optically active diethyltartrate catalysts and R t-BuOOH, with more than 90% enantiomeric excess 0 SiO SiO SiO O H (Finn and Sharpless, 1985). The reason why d ROH, H2O2 transition metals are specific for this reaction has been Ti SiO Ti O H attributed to the fact that interaction between the SiO SiO SiO O

oxygen atom and vacant t2g orbitals of the metal ion provides a low energy route for the formation of the R R CO bond from the peroxometallocycle adduct SiO O H O H (Purcell, 1985). alkene H SiO Ti O Ti Different mechanisms have been proposed for C SiO O O the oxygen transfer in the epoxidation of olefins. C H One mechanism involves nucleophilic attack of epoxide the double bond on the alkyl hydroperoxide, by one of the different activated complexes, as shown Fig. 2. Proposed mechanism for olefin epoxidation with in Fig. 1 (Sheldon, 1973; Chong and Sharpless, H2O2 catalyzed by Ti-silicalite, in ROH solvent (Clerici and 1977). Ingallina, 1993).

VOLUME II / REFINING AND PETROCHEMICALS 641 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

peroxometallocycle, thereafter releasing the epoxide R R R (Fig. 3). Another mechanism involves the oxo metal O O O moiety, which is converted to a metal peroxo (Mimoun M M MOR O M O et al., 1970) complex – the active site. O O

Metal-centred oxidation of coordinated substrates Fig. 3. Mechanism for olefin epoxidation involving a This class of reactions involves the oxidation of a peroxometallocycle intermediate coordinated substrate by a metal ion. Examples (Mimoun, 1982). include the Pd(II)-catalysed oxidation of olefins (Wacker process), and the synthesis of ketones by the oxidehydrogenation of alcohols. The role of molecular Cl oxygen is to regenerate the high oxidation state of the 2 H2O PdCl4 C2H4 H2O ClPd OH2 metal. The substrate is activated towards nucleophilic H attack by the metal through the formation of a p-complex with the metal. The overall reaction is an Cl Cl Cl oxidative nucleophilic substitution of hydrogen with Cl two-electron reduction of the metal; therefore, the ClPd OH2 Pd OH2 H Pd OH2

process is heterolytic. Group VIII metal salts, such as CH2CH2OH CH2CH2OH Pt(II), Ir(III), Ru(III) and Rh(III), oxidize olefins in OH the same way: 0 CH3CHO Pd HCl H2O Pd(II)X2 RCH CH2 H2O RCOCH3 Pd(0) 2 HX Nucleophilic attack of water on a Pd(II)-ethylene p Fig. 4. Mechanism of oxidation of ethylene to acetaldehyde complex forms ethanol, which remains coordinated by (Wacker process). a PdC s bond (Fig. 4; Drago and Beer, 1992). The dissociation of a Cl ligand from the complex and the elimination of hydride from the ethanol form a vinyl deceleration (controlled by termination reactions). The alcohol p complex with Pd(II). The formation of the induction period is due to the difficulty in breaking the aldehyde is obtained by tautomerization of the CH bond and can be overcome by the addition of dissociated vinyl alcohol from the complex and small quantities of initiators, which readily decompose reduction of Pd(II) to Pd(0). Reoxidation of palladium in the reaction medium. is achieved through a Cu(II) co-catalyst, which is In a typical air-sparged system, in the zone near the reduced to Cu(I), and then finally reoxidized by sparger, the liquid contains higher concentrations of molecular oxygen to Cu(II). molecular oxygen and the latter can rapidly scavenge radicals via the reaction: Kinetics of liquid phase oxidation RO ROO of hydrocarbons 2 Therefore, the overall rate is controlled by the Analysis of the kinetics of liquid phase oxidation chemical reaction. Under these conditions, the reaction can be made according to the nature of the reaction is zero order with respect to molecular oxygen, and the mechanism: reactions that involve the homolytic apparent activation energy is typical of reactions under scission of the CH bond in the organic substrate, chemical reaction control. The concentration of thus involving free radicals (autoxidation), where the molecular oxygen in the bubbles progressively metal catalyst acts more like an initiator, and reactions decreases, going from the bottom to the top of the that involve the heterolytic scission of the CH bond, reactor, and the mass transfer of molecular oxygen where the substrate coordinates to a metal ion catalyst. from the gas to the liquid phase can become rate Autoxidation processes are of fundamental limiting, since the reaction between an alkyl radical importance in the chemical industry, and a great and oxygen is extremely fast. Under these conditions, number of reactions imply this kind of mechanism, as the supply of oxygen can become first order with

shown in Table 5 (Sheldon and Kochi, 1976; Carrà and respect to O2 partial pressure. In this zone, the Santacesaria, 1980; Santacesaria and Pimpinelli, apparent activation energy falls to low values, 1986). 5 kcal/mol. The process is autocatalytic and exhibits an The main reactions involved in the process of induction period, an acceleration (controlled by the homolytic oxidation are summarized in Table 6. When propagation and branching reactions) and a molecular oxygen is present in relatively high

642 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Table 5. Most important industrial processes involving liquid-phase oxidation of hydrocarbons through homolytic mechanisms

Reactant Product Catalyst

n-butane Acetic acid Co acetate/acetic acid Isobutane t-butylhydroperoxide – Naphtha Acetic acid Co acetate/acetic acid

Waxes (C18-C30) Fatty acids Mn salts/acetic acid Cyclohexane Cyclohexanol/cyclohexanone Co naphthenate Cyclohexanol/o Adipic acid Mn-Cu acetate Toluene Benzoic acid Mn or Co bromide/acetic Benzoic acid Phenol Cu-Mg benzoate p-xylene Terephthalic acid Mn or Co bromide/acetic Cumene Cumylhydroperoxide – Acetaldehyde Acetic acid Mn acetate/acetic acid Acetaldehyde Acetic anhydride Cu–Co acetate/ethyl

0.5 concentrations (for oxygen pressures usually higher 14421d[O2] 1242rin 1rin [5] k [RH] than 0.13 atm in the gas phase), the rate determining dt pr 2k 2 4 5 ter step is the propagation reaction, kox 10 10 kpr (Table 6). The RO2 species is thus the predominant one The term relative to the rate of initiation can be and mainly contributes to termination reactions. When neglected, and the final general relationship is the contribution of branching reactions is negligible, obtained for the rate of oxygen (or hydrocarbon) because ROOH is not decomposed under the reaction depletion: conditions (thus its contribution to the generation of 0.5 14421d[O2] 1242rin radicals is negligible and it is the main product of [6] k [RH] dt pr 2k reaction), under these conditions the rate of oxygen ter transformation may be expressed as the balance This expression is independent of oxygen between oxygen consumption in oxidation and oxygen concentration, and is thus generally valid under formation in termination: conditions of high oxygen concentration. A more general expression has been proposed (Twigg, 1962; 14421d[O2] [1] r r k [R][O ]k [RO]2 Franz and Sheldon, 1991), which considers all of the dt ox ter ox 2 ter 2 more important steps in the reaction scheme (reactions On the other hand, under stationary conditions, the 3 and 4 in Table 6 for propagation, reactions 7 and 9 rate of R formation (initiation plus propagation) is for termination, with corresponding kinetic constants equal to the rate of R disappearance (oxidation), kpr3, kpr4, kter7 and kter9): provided that termination reactions involving the R 14421d[O2] species can be neglected. Therefore: [7] dt [2] k [R][O ] k [RO ][RH]r ox 2 pr 2 in r0.5 k k [RH][O ] 12111111111111124341142in pr4 pr3 2 Under stationary conditions, the rate of initiation {k k 0.5 [RH] k k 0.5 [O ]k 0.5 k 0.5 r0.5} equals the rate of termination, and therefore: pr4 ter7 pr4 ter9 2 ter9 ter7 in Two limit situations can be considered. The first [3] r 2k [RO]2 in ter 2 regards oxygen concentrations approaching zero. which gives: Under these conditions, the rate of reaction is r 0.5 proportional to the oxygen concentration in the liquid 14421in [4] [RO2] phase, since the term at the denominator containing 2kter [O2] can be neglected. If, instead, the oxygen By replacing the corresponding terms in the concentration is very high, the rate of reaction expression for oxygen disappearance, it is possible to becomes independent of oxygen concentration and the obtain: relationship of [6] is valid, where kpr kpr4 and

VOLUME II / REFINING AND PETROCHEMICALS 643 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

OH radicals, which can lead to very fast reactions: Table 6. Reaction scheme for autoxidation RORH ROH R of organic substrates OH RH H2O R

1 In2 2 In (initiation) The prevailing termination reaction again involves the RO species, since the RO and OH radicals 2 2 In RH R InH (initiation) (generated by ROOH decomposition) are readily 3 R O2 RO2 (propagation by oxidation) converted. Under these conditions, the rate expression for RH transformation becomes: 4 RO RH ROOHR (propagation) 2 2 2 d[RH] 3kpr[RH] 144231 144114441 5 2ROOH RO RO2 H2O [9] (branching by bimolecular decomposition) dt kter ROOH ROOH (branching which takes into account that RH is consumed through 6 by unimolecolar hydroperoxide decomposition) reaction with RO2, RO and OH , and that three molecules of RH are consumed for each molecule of 7 2R non radical products (termination) ROOH that has decomposed. This expression 8 R RO2 non radical products (termination) represents a limit situation, and hence gives the

maximum rate that can be achieved. More generally: 9 2RO2 O2 non radical products (termination) d[RH] nk 144231 1 4411341pr [10] 0.5 [RH] dt f (2kter) 2kter kter9. On the other hand, since the value of kpr4 depends to a large extent on the nature of the organic where n is the number of radicals produced for each substrate, when it is very high (e.g. in the case of ROOH decomposed and f is the fraction of RH aldehydes), the first term in the denominator of the consumed via attack by RO2. Once again, the term 0.5 general equation is dominant and, therefore, the [kpr /(2kter) ], representing the relative importance of equation becomes: propagation and termination reactions, plays an important role in determining the rate of substrate d[O ] 144212 0.5 0.5 transformation. [8] rin kpr3[O2]kter7 dt The presence of a metal ion acting as a catalyst The rate of the reaction is thus proportional to the may increase the activity of the autoxidation process. oxygen concentration. The value of oxygen The catalytic action is related to the redox property of concentration at which the passage from first-order the metal ion, that is, to the redox potential for the dependence to zero-order dependence occurs is couple Mn/M(n1). One of the most important therefore a function of the nature of the organic effects is to favour the decomposition of the substrate. For aldehyde oxidation, the reaction depends hydroperoxides: on oxygen even at relatively high partial pressures, ROOH M(n 1) ROOH Mn while for cyclohexane oxidation, the reaction depends ROOH Mn ROH M(n 1) on oxygen partial pressure up to 0.2-0.3 atm, and for 2 cumene oxidation up to 1.5-1.7 atm (Ladhaboy and Clearly, only those metals having two oxidation Sharma, 1969; Manor and Schmitz, 1984; Andrigo et states of comparable stability (i.e. cobalt and al., 1992). manganese ions) can perform this catalytic cycle 0.5 The ratio [kpr /(2kter) ] represents the (Sheldon and Kochi, 1976). From the kinetic point of susceptibility of the substrate towards oxidation (see view, [10] can be used to calculate the maximum rate again Table 2; Sheldon and Kochi, 1976). The of reaction, assuming that n1 and f2/3. In this case, propagation rate is affected by the CH bond energy where the catalyst is only involved in the (the homolytic scission of this bond is involved and it decomposition of ROOH, n/f1.5. is necessary that the ROOH bond, which is going to The metal can also interact directly with the be formed, is at least energetically equivalent to the substrate: C H bond to be broken) and also by the reactivity of n RH M(n 1) H R alkylperoxo radicals (Howard, 1973; Sheldon and M R O2 RO2 Kochi, 1976; Gates et al., 1979). The latter are more M(n 1) ROOH Mn ROOH reactive in the presence of an electron-withdrawing substituent. Also in this case, the kinetic law obeyed is the Under conditions at which the hydroperoxide is not same as for the previous case, with n2 and f1/3. In stable, decomposition of the latter generates RO and all of these cases, therefore, the metal acts as an

644 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

initiator, decreasing the induction period and vapour pressure of the liquid), against an average 5 eventually favouring the oxidation process at lower mol% in the case of air as the oxidizing agent, thus temperatures. improving the transfer of molecular oxygen as well as When the bivalent oxidation state of the catalyst is the rate of reaction. As a consequence, similar only involved in ROOH decomposition and the productivities as with the air-based process can be trivalent state in the reaction with RH, then n/f6. achieved at significantly lower temperatures and Intermediate values of the n/f ratio, between 1.5 and 6, pressures. This may lead to a considerable indicate that the catalyst not only takes part in the improvement in selectivity, when the reaction network redox reaction with peroxidic compounds, but also consists of consecutive reactions of oxidative reacts with RH through the trivalent oxidation state. degradation (usually characterized by higher activation In industrial practice, small amounts of substances energy than the selective reaction) to the desired highly prone to autoxidation are added to the reaction product. medium, which will readily give rise to a co-oxidation Lower temperatures also entail less solvent loss. It effect and provide the hydroperoxides for maintaining may also happen that an increase in the oxygen partial the chain, thus leading to an increase in the reaction pressure, as a consequence of feeding oxygen instead rate. In this way, it is possible to avoid undesired co- of air, leads to a considerable improvement in the oxidation on the desired products of partial oxidation oxygen transfer rate, while the rate of reaction is not (i.e. aldehydes, ketones), which are more easily much affected by the variation in oxygen subjected to autoxidation than the reactant. concentration. Under these conditions, a passage from The kinetic expressions described above are often a mass-transfer limited process (as is usually the case in good agreement with the experimental limiting rate. for the oxidation of liquid hydrocarbons) to a However, this is not the case for the oxidation of kinetically controlled process can occur, and thus the alkylaromatics catalysed by cobalt acetate, where the reaction is transferred from the film surrounding the reaction is first order upon the hydrocarbon and also bubble (at the interface between the gas and the liquid depends on the catalyst concentration (Chester et al., phase) to the continuous liquid phase. This may also 1977). This is due to the different reaction mechanism change the temperature distribution from a involved. considerable temperature gradient in the film in the Regarding oxidation reactions occurring via former case (where the heat of reaction develops in the heterolytic dissociations of the CH bond in the thin film) to a lower, more uniform temperature in the substrate, they are catalysed by V, W, Mo and Ti ions latter case. The above may considerably improve the through the transfer of two electrons of the substrate selectivity. coordinated to the metallic complex. In other cases, The presence of a high oxygen concentration in the such as the Wacker process, the action of two redox reaction zone is also important for favouring the couples – Pd(II)/Pd(0) and Cu(II)/Cu(I) – is necessary oxidation reactions against the reactions between for the catalytic cycle. In the mechanism, the rate intermediates, leading to undesired high-molecular limiting step is the conversion of the p-complex weight compounds. A high oxygen concentration in 2 between the olefin and PdCl4 into a s complex, the the reaction zone is obtained when the process is other stages being at equilibrium. The rate expression kinetically controlled or when, under mass-transfer for ethylene disappearance, which better describes the limiting conditions, the rate of reaction is not very experimental results, is the following (Moiseev et al., much higher than the rate of diffusion (therefore, 1974): concentration gradients in the film are not too d[C H ] [PdCl2][C H ] pronounced). Higher oxygen concentrations may thus 14422312 4 14411144114444 2 14 [11] k lead to a higher oxygen concentration in the reaction dt [Cl ][H3O ] zone. Another advantage in the use of oxygen instead of air is the reduction in total gas throughput, with a Air vs. oxygen in liquid phase oxidations consequent reduction in the energy required for gas The use of molecular oxygen in liquid phase compression as well as in the amount of vent gas oxidations may provide a significant improvement in (which is also much more concentrated in the organic the reaction kinetics when the overall process is contaminants, due to the absence of diluent nitrogen) controlled by the reaction and the rate depends on the to be treated, with large gas treatment equipment. oxygen partial pressure over a wide interval of oxygen Therefore, the nitrogen favours stripping of the solvent concentration (though not often the case, as described and volatile organic compounds from the solution, above). Indeed, the oxygen concentration in the compounds which have to be removed to conform with bubbles (in addition to the continuous gaseous phase the regulations for environmental protection. at the top of the reactor) is 100% (neglecting the

VOLUME II / REFINING AND PETROCHEMICALS 645 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Free-radical chain oxidations • Production of hydroperoxides: isobutane to t-butylhydroperoxide (oxidizing agents for Industrial processes epoxidation of olefins); ethylbenzene to Free-radical chain oxidation consists in the ethylbenzene hydroperoxide (oxidizing agents for oxidation of organic substrates through the interaction epoxidation of olefins); cumene to of molecular oxygen with radicals formed by cumylhydroperoxide (and then to phenol and homolytic cleavage of CH bonds (which, in fact, acetone); p-diisopropylbenzene to the are referred to as ‘homolytic oxidation reactions’; corresponding dihydroperoxide (and then to Prengle and Barona, 1970a; Sheldon and Van Doom, hydroquinone). 1973, 1981; Lyons, 1980; Emanuel and Gal, 1986). • Production of acids: p-xylene to terephthalic acid; Three types of industrial, radical chain n-butane to acetic acid; toluene to benzoic acid oxidation applications can be distinguished: (and then to phenol); higher paraffins to high autoxidation, a spontaneous oxidation (without a molecular weight acids and alcohols; cyclohexane catalyst) of alkanes and alkylaromatics to produce to cyclohexanone and cyclohexanol (and then to hydroperoxides or of aldehydes to produce adipic acid); cyclododecane to cyclododecanol and peracids; catalysed autoxidation, where the catalyst cyclododecanone (and then to cyclododecan superimposes and/or partially replaces the dicarboxylic acid); acetaldehyde to acetic acid; mechanism of autoxidation, which is involved in pseudocumene to trimellitic acid; m-xylene to the production of acids, alcohols and aldehydes; isophthalic acid; 2,6-dimethyl naphthalene to acid decomposition of hydroperoxides, which 2,6-naphthendicarboxylic acid; n-butyraldehyde to serves to obtain alcohols and ketones. n-butyric acid; paraffins to fatty acids. The common first step in all of the reaction • Production of ketones, alcohols, anhydrides and mechanisms is the introduction of an OOH group quinones: fluorene to fluorenone; acetaldehyde to into an organic substrate (formation of hydroperoxides acetic anhydride; naphthalene to naphthoquinone; or of peracids), which evolves further in the presence anthracene to anthraquinone; cymene of catalysts to more stable products, depending on the (p-methylisopropylbenzene) to cresol; long-chain type and amount of catalyst, the structure of the n-alkanes to secondary alcohols. organic molecule containing the OOH group, and The operating conditions and performance of the the reaction conditions and reactor type. A further most important industrial processes of autoxidation of differentiation of the processes is related to the organic substrates are summarized in Table 7. OOH group decomposition step, which can be carried out either in situ or in a second catalytic step. The mechanism of autoxidation and kinetic The free-radical chain oxidation is involved in the considerations industrial production of the following compounds: The autoxidation of organic substrates occurs hydroperoxides, carboxylic and aromatic acids, through three stages or periods: an induction period, a alcohols and ketones (only for some special steady chain-propagation period and a termination applications) and hydrogen peroxide. period. The main industrial processes can be classified as The reaction mechanism (see above: Kinetics of follows: liquid phase oxidation of hydrocarbons) involves

Table 7. Reaction conditions of industrial liquid-phase, radical-chain oxidation process

Per passage Selectivity Substrate Oxidant T (°C) P Catalyst (atm) conversion (%) (%) n-butane O2/air 160-180 50-60 90 50-65 Co, Mn salts

Isobutane O2 110-140 25-35 25-40 50-60 no Cyclohexane air 150-170 8-10 4-10 75-85 Co salts

Toluene O2/air 140-180 2-10 20-40 91-93 Co salts Ethylbenzene air 120-140 3-5 10-20 84-87 no p-xylene air 195-205 15-30 95 90-95 Co/Mn/Br

Cumene O2/air 90-130 1-8 25-35 90-97 Co, Mn salts

Acetaldehyde O2/air 60-80 2-10 91-98 90-95 Co, Mn salts

646 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

initiation, propagation, branching and termination rate depends on the structure of the radical and also on reactions. the oxygen partial pressure. In all industrial processes, Initiation reactions. The initiator reacts with the at the end of the reactor, the partial pressure of oxygen substrate RH, generating radicals R: is maintained practically nil, mainly for safety reasons. Therefore, radicals accumulate and termination occurs In 2In 2 by their recombination. The presence of foreign InRH RInH molecules (impurities) that interact with radicals can or where the organic substrate is itself an initiator: also be the source for termination reactions. The formulation of kinetic expressions for the RH RH autoxidation of organic substrates must not only take RHO RHO 2 2 account of the reactions occuring in the liquid phase, 2RHO 2RH O 2 2 2 but also of the transfer of molecular oxygen from the The initiation can be brought about by the gaseous to the liquid phase (Prengle and Barona, introduction of thermally unstable compounds, such as 1970b; Hobbs et al., 1972a; Astarita et al., 1983; peroxides or azocompounds, generating radicals or Doraiswamy and Sharma, 1984). When the entire metal ions, which autoxidize more readily than RH. process is either mass-transfer limited or reaction The initiation step requires a finite time to develop a limited, more simple rate expressions can be used, sufficient concentration of radicals to support the which are representative of the rate-determining step. propagation step. Usually, the decomposition of the This is usually the case for liquid phase autoxidation hydroperoxide formed with the reaction is the source processes, where the diffusion of oxygen is very quick of radicals (i.e. branching reactions, see below). In this for oxygen partial pressures above 0.1-1.0 atm (the case, the reaction temperature is related to the limit value depending mostly on reaction conditions), temperature of decomposition of the hydroperoxide. while it becomes rate-controlling when the oxygen For this reason, in industrial processes, usually the partial pressure is lower than these values (this product is partially recycled to the first reactor in order typically occurs in continuous stirred reactors; Uri, to furnish the necessary concentration of initiator. 1961; Bamford and Tipper, 1980). Propagation reactions. The radicals generated However, general rate expressions are necessary react with molecular oxygen forming RO2, which in when the rate determining-step changes in the same turn reacts with RH, generating other R: reactor. For instance, in bubble reactors where the oxygen partial pressure progressively diminishes along RO RO 2 2 the reactor, one can move from a reaction-limited RORH RROOH 2 region (where the oxygen partial pressure is high) to a Branching reactions. These reactions are due to the region where the process is oxygen-transfer-limited decomposition of alkylhydroperoxy species to (Hobbs et al., 1972a; Jacobi and Baerns, 1983). In generate RO, ROO, OH radicals. The nature of the correspondence, one may move from a region where products formed in the branching reactions depends on the apparent activation energy is higher than 20 the type of substrate. kcal/mol to a region where it is very low, typical of diffusional phenomena (less than 5 kcal/mol). ROOH ROOH The situation becomes even more complex when 2ROOH ROOROH O 2 taking into account the very complex reaction RORH RROH mechanism, with the formation of many by-products OHRH RH O 2 and the participation of several intermediate chemical ROOH ROH 2ROH O 2 species (i.e. radical compounds). However, it is usually Termination reactions. The radicals generate possible to adopt simplified kinetic models, where the non-radical products: concentrations of radical intermediates are not taken into consideration (i.e. lumped models). This type of 2R R 2 approach has been utilized for several different RRO ROOR 2 examples of liquid phase catalytic oxidation reactions 2RO ROOR O 2 2 (Cavalieri d’Oro et al., 1980; Chen et al., 1985; ROR ROR Krzysztoforski et al., 1986; Morbidelli et al., 1986; The recombination of radicals to terminate the Cao et al., 1994).

chain reaction occurs more easily with large radicals, The direct reaction between RH and O2 to yield whose lifetime is longer than for small radicals. In ROOH is thermally neutral and is characterized by a fact, large radicals are less reactive because they high activation energy (35 kcal/mol). Therefore, it is dissipate energy by atomic vibrations. The termination very slow at temperatures below 150°C. The cleavage

VOLUME II / REFINING AND PETROCHEMICALS 647 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

of RH by reaction with RO is 104 to 106 times slower film, and thus [O ]*= P /H = P /H (where H is the 2 2 O2* O2 than the reaction between R and O , and thus the Henry constant for molecular oxygen and P is the 2 O2 former reaction is usually the rate-determining step. partial pressure of molecular oxygen in the gas phase), The rate expression then becomes the following expression is finally obtained (Jacobi (in a chemically-controlled regime): and Baerns, 1983): d[RH] d[O ] P 1 k 1 144231 1442312 12O2 3 133 11211ter [12] rpr kpr[RO ][RH] [19] dt dt H akL koxkpr[RH] Under these conditions, the rate expression does Higher rates of hydroperoxide formation are not depend on oxygen partial pressure. Since the observed from substrates, which present either more reaction between the R species and oxygen is very labile CH bonds or form more stable radicals. In quick, often total molecular oxygen consumption is the alkane series, the rate is higher for tertiary CH already achieved in the liquid film. bonds, followed by bonds in secondary and primary The reaction between O2 and R becomes carbon atoms. In arylalkanes, the order is the rate-determining only when the partial pressure of following: tertiary secondary primary oxygen is very low (below approximately 0.15 atm); benzylic. This order is due more to the instability of the rate of reaction then becomes dependent upon the radicals formed than to the lability of the CH molecular oxygen and is strongly limited by oxygen bonds (which is the reason for the lower oxidizability transfer into the liquid phase: of toluene). d[O ] 1442312 The role of catalyst [13] rox kox[R ][O2] dt Two types of catalysts are used in the industrial Therefore, both propagation reactions and processes of autoxidation. The first type consists of oxidation reactions must be taken into account when catalysts that superimpose their action on that of the developing the rate expression. Termination occurs classical autoxidation by favouring the decomposition mainly by combination of ROO and R; the of OOH groups and by addressing the corresponding rate expression is: decomposition profile selectively towards the formation of different, more stable products. These [14] r k [R][ROO] ter ter catalysts are either transition element ions or acid/base By applying the steady-state approximation to systems. The second type includes catalysts that are radical compounds ROO and R, the following able not only to decompose the OOH group, but expression can be obtained (for low oxygen partial also to directly activate the substrate. Therefore, these pressure): catalysts also participate in the initiation step. They are d[O ] k k Co(III) in acetic acid (unique amongst the transition 1442312 1313ox pr elements), the MIC (Amoco Mid Century) catalyst [15] [O2][RH] dt kter based on Co/Mn/Br ions in acetic acid, and Co(III) Under these steady-state conditions, the with a co-oxidant. mass-transfer rate of molecular oxygen (OTR): Redox catalysts active in the decomposition of [16] OTR ak ([O ]*[O ]) L 2 2 hydroperoxide is equal to the rate of oxygen consumption in the Salts of elements of the first row of the transition liquid phase, where a is the specific interfacial area, kL series, which present one-electron redox couples such is the liquid film mass-transfer coefficient for as Co(II)/Co(III), Mn(II)/Mn(III), Fe(II)/Fe(III),

molecular oxygen and [O2]* is the oxygen Cu(I)/Cu(II) (Co salts are the most commonly used), concentration at the gas-liquid inter-phase. A general or metal chelates, which are soluble in the reaction rate expression that takes into account both the medium, are used to accelerate the rate of chemical and diffusional steps is achieved by decomposition of hydroperoxides through the combining the two expressions: Haber-Weiss mechanism: k k (n 1) n 144231ox pr M ROOH RO2 H M [17] [O2][RH] akL([O2]* [O2]) Mn ROOH ROOH M(n 1) kter d[O ] k k The total reaction is: 1442312 1313ox pr [18] [O2][RH] dt k ter 2ROOH ROO H2O RO and by assuming a negligible gradient in the gaseous In the first reaction, the electron transfer occurs

648 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

between the hydroperoxide and the Co(III) species, cleavage of a C CH3 bond in a tertiary radical through complexation of the hydroperoxide, which (Lyons, 1980). The CC cleavage reaction occurs weakens the OO bond and facilitates the when methylenic groups are near the carbon atom, as breakdown of the complex and the formation of the in 3-methylpentane (Lyons, 1980). An important role RO2 radical. The second reaction is a reduction by is also played by the transition metal in directing the Co(II) with the formation of a RO radical. The second decomposition of the hydroperoxide to different step is very rapid, due to the high oxidation power of products. hydroperoxides, while the first step has a very slow The decomposition product of hydroperoxides or rate of reaction since hydroperoxides are weakly peracids can be further transformed to different types reducing agents. Therefore, optimal catalysts are those of compounds, depending on the type of transition which contain ions with high oxidation power in order metal used. to increase the rate of the first step. In fact, Co and Mn ions, which are present in the best catalytic systems, Acid and basic catalysts active in the decomposition have a very high redox potential: 1.82 eV for of the hydroperoxide Co(III)e Co(II); 1.51 eV for Mn(III)e Mn(II). Acid can also cause the catalytic decomposition of Only one-electron redox couples are to be used in the hydroperoxide. This is the case with phenol and order to avoid side reactions. The transition element acetone production by acid decomposition of must be used in an organic solvent; in order to increase cumylhydroperoxide. For this reason, an emulsion the solubility of the catalyst, carboxylic acids or with a base is usually used as the reacting medium in naftenic acid must be used as the solvent. the production of the hydroperoxide, in order to avoid In the oxidation of hydrocarbons, the concentration its decomposition due to the occasional presence of of metal ranges from 1 to 500 ppm, while in the case acidity. of aldehyde oxidations, it ranges from 0.001 to 0.1 ppm. Higher concentrations of metal ions have a Catalysts for the direct activation of the organic negative effect, since the catalyst decomposes all of substrate the hydroperoxide and the rate of oxidation decreases Co(III) in acetic acid acts as an initiating species because there are not enough radicals for the initiation reacting with the hydrocarbon and forming, through a step. Therefore, the role of the transition element is to one electron exchange, a radical carbocation, which decrease the activation energy relative to the successively releases a proton forming a radical decomposition of the hydroperoxide, thus allowing the species. Co(III) is the unique element able to activate a reaction to occur at lower temperatures and with high hydrocarbon, especially benzylic hydrogens and enough rates for industrial application. Moreover, by CH bonds in aldehydes. Through this mechanism is operating at lower temperatures, the selectivity can be possible to carry out an autoxidation reaction at lower better controlled. temperatures, as compared to the classical ones. The type of products obtained in the catalytic A second mechanism of activation by Co(III) is decomposition of hydroperoxides depends mainly on indirect and operates through a co-oxidant, thus in the their structure, but also on the type of catalyst used presence of Mn and Br ions. In this type of catalytic and the reaction conditions. For example, system (MIC catalyst), the role of Co(III) is to oxidize t-butylhydroperoxide is decomposed at 45°C in the manganese; the latter oxidizes Br to Br and is able presence of Co(II) octanoate to yield 88% t-butyl to abstract H from less reactive CH bonds, such as alcohol and only 1% acetone (besides molecular methyl groups in polymethylated aromatics. The oxygen and the dimer of the t-butoxy radical; Hiatt et advantages of the MIC catalyst with respect to al., 1968). A similar distribution of products is classical Co-based catalysts include the rate of obtained by decomposition of cumylhydroperoxide, reaction is higher, and therefore it is possible to use with the formation of the tertiary alcohol (95%) and a smaller amounts of catalyst or to oxidize groups that product of ketonic cleavage (5%). Under the same present lower reactivity, as well as many aromatic conditions, sec-butylhydroperoxide decomposes to acids, and it precipitates in acetic acid, making the 61% sec-butylalcohol and 36% methylethylketone, separation of the products easier. Disadvantages while n-butylhydroperoxide yields 67% n-butanol and include the decarboxylation of acetic acid at high 32% butyraldehyde. For all hydroperoxides, alcohol is temperature and the problems tied to material the main product of decomposition, but other corrosion, which make the use of lined apparatus by-products form depending on the nature of the necessary. An alternative synthesis consists in the use hydroperoxide. The cleavage of a CH bond in a of an organic co-oxidant, as in the case of n-butane secondary or primary alkoxy radical to yield the oxidation to acetic acid, where methylethylketone corresponding carbonyl compound is easier than the (obtained as a by-product of the reaction and recycled

VOLUME II / REFINING AND PETROCHEMICALS 649 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

to the oxidation reactor) is the co-oxidant. The latter is gas-spraying devices, or through rapid circulation of first activated by cobalt, and then the radical formed the reactor content by pumping, as well as efficient activates n-butane at low temperatures and with high heat removal for strongly exothermal reactions, selectivity. realized either via heat exchangers inside the reactor Co-based catalysts can compete with the MIC or by the recirculation of the reacting mixture after catalyst, provided that higher amounts of catalyst and a external refrigeration and the evaporation of the co-oxidant (either acetaldehyde or methylethylketone) substrate, the product or the solvent. are used. In all of the processes operating in continuous stirred apparatus or in semi-batch reactors, the oxygen Reaction conditions and reactor types diffusion is the rate-limiting step, due to the All reactions are carried out in the liquid phase at concentration of oxygen in the gas phase, which is kept temperatures between 75 and 200°C, and at a pressure low for safety reasons (at the expense of selectivity). ranging from 3 to 70 atm (to keep oxygen dissolved in The rate of oxygen diffusion is a function of its partial the liquid phase). The pressure must be high enough to pressure in the bubble phase and of the interfacial area maintain the boiling temperature of the liquid phase (i.e. of bubble size). Operation under conditions of higher than the reaction temperature, since high mass-transfer resistance may be more favourable, since vapour concentrations in the bubble phase decrease the the rate of reaction is practically unaffected by solubility of oxygen. variations in the temperature (the diffusional constant When the substrate is a gas or a low-boiling is characterized by a very low activation energy; Hobbs liquid, high-boiling solvents are used, which in some et al., 1972a). Instead, under conditions of chemical cases can be the product itself. Acetic acid is often control, fluctuations in temperature can lead to the used as the solvent because it is not easily oxidizable, extinction of the reaction. Complete mass-transfer and it has both an optimal boiling temperature control, on the other hand, may cause a lack of oxygen (116-118°C) and freezing temperature, which makes in the liquid phase, favouring bimolecular reactions it easy to handle. It is easily available, low in cost and between radicals and the termination of the chain it dissolves the majority of aromatics at low reaction, thus decreasing the process efficiency. In temperature. In some cases, mixtures of two solvents continuous columns, a stable situation consists in are used. In the oxidation of m-xylene to isophthalic having a region where the rate is chemically controlled acid, a mixture of acetic acid and dichlorobenzene is and another where the process is under mass-transfer the solvent for the reaction. With this solvent control. This ensures stable operation, being that the mixture, an increase in selectivity is achieved, overall system shows very little dependency on ranging from 89 to 91%. temperature (Hobbs et al., 1972a). The reactors used can be either towers or stirred It is necessary to avoid coalescence of the bubbles, tanks. The choice essentially depends on the rate- which can be induced by high, turbulent flow rates and determining step of the reaction. Often in-series heavy agitation. The gas must be introduced through reactors are set up, in order to limit the heat of reaction large-area devices. The reactor surface-to-volume ratio evolved or to minimize the overall volume necessary has to be minimized in order to reduce reactions of for achieving a defined conversion. Sometimes, batch radical combination at the wall. operation is preferred over continuous operation in Safety considerations include good control of heat stirred reactors, since backmixing phenomena can removal to prevent thermal decomposition of the have negative effects on selectivity, favouring hydroperoxide and operation at low conversion, so undesired overoxidation reactions. On the other hand, limiting the concentration of hydroperoxide and the a certain extent of backmixing can be necessary to extent of consecutive reaction of decomposition. This keep the correct concentration of radicals in the can be realized by using staged reactors, with reaction medium. A compromize can be reached by intermediate feeding of oxygen. In addition, one must stirred reactors in series (Prengle and Barona, 1970b). ensure that in the gas phase, the composition is always An advantage arising from the absence of above the upper flammability limit of the organic diffusional limitations roots in the non-necessity of vapour by inertizing with nitrogen and with water stirring inside the reactor in order to improve the vapour. Particular care must be taken in the start-up, gas-liquid interfacial area and hence the mass-transfer when no oxygen is consumed along the reactor, and rate; therefore, less costly reactors sometimes during the shutdown of the reactor. can be used. Particular attention must be paid in the design The control of selectivity of these reactors to achieve efficient gas-liquid contact, When the hydroperoxide is the final product, the realized either through mechanical stirring or by lowering of selectivity is due to its decomposition

650 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

(with formation of alcohols and ketones). Therefore, lower entrainment of organic vapours in the waste gas. low conversions and low temperatures allow higher Air is used for its low cost and its safe handling. selectivity to be obtained. Usually, when choosing Air can be advantageous when the reaction is under experimental conditions, a compromise between purity chemical control and the partial pressure of oxygen of the hydroperoxide and productivity (conversion) bears no influence on the yields. Further advantages must be reached. An addition of bases can increase the are related to the stirring effect of nitrogen as well as selectivity by decreasing the extent of hydroperoxide the more favoured dissipation of the heat of reaction. decomposition, which is catalysed by acids present in When air is used under pressure, energy can be the reaction medium. recuperated in a turbine before venting from When the hydroperoxide is not the final product pressurized nitrogen. but only an intermediate, the presence of the catalyst A good example of the importance in choosing an determines the selectivity. Also in this case, the oxidant roots in the processes for the synthesis of conversion can be a key factor to control the hydroperoxides starting from isobutane and from selectivity for those products which are more easily ethylbenzene. The corresponding hydroperoxides are oxidizable than the substrate. This is the case with the used as oxidizing agents in propylene epoxidation. The oxidation of cyclohexane to cyclohexanol and main difference between the two oxidation processes is cyclohexanone. Higher conversion and selectivity can that in the case of isobutene, molecular oxygen is be reached by the addition of boric acid, which preferred and the process operates at high pressure, interacts with cyclohexanol, forming an ester and while in the oxidation of ethylbenzene, air is the preventing its further oxidation. The ester is finally oxidant of choice and the process operates at low hydrolysed to recover cyclohexanol. pressure. In the latter case, the rate is chemically A change from a chemically limited rate to a controlled and, therefore, it is not influenced by the mass-transfer limited rate, which may occur partial pressure of oxygen. In isobutane oxidation, occasionally due to an increase in the reaction instead, the rate-limiting step is oxygen diffusion. temperature, can lead to significant changes in the distribution of products. For instance, in the oxidation Main industrial processes of liquid phase of methylethylketone to acetic acid, this leads to a oxidation change in the average oxidation state of the cobalt catalyst, which in the absence of dissolved oxygen is In this section, some of the most important reduced, and to an increase in the formation of CO, industrial processes of liquid phase oxidation of methanol and formic acid, due to the following organic substrates are examined, which well represent reactions starting from the acetyl radical under the chemistry and technology involved in this class of molecular oxygen starvation (Hobbs et al., 1972a): reactions. Moreover, the industrial technology of the synthesis of propylene oxide by oxidation is compared CH3CO CH3 CO with the processes of propylene epoxidation under CH3 OH CH3OH study or development, in order to exemplify the CH3OH 1/2O2 HCOOH H2O current trends and future developments in the field of liquid-phase selective oxidation. Molecular oxygen or air as the oxidizing agent Molecular oxygen, oxygen-enriched air or air are Autoxidation processes used as the oxidants. The choice is dictated by both Autoxidation processes have a great importance for safety and economic considerations (Trifirò and the chemical industry. Despite the usual, low Cavani, 1994). selectivity obtained in radical-like processes, industrial Pure molecular oxygen can not be used; a small applications have widely developed for those reactions fraction of nitrogen must be present, in a concentration involving the use of organic substrates that possess of higher than 3%, in order to keep the gas phase over only one site of preferential attack. the reacting liquid always outside the higher explosion limit of the organic vapour. The use of molecular The oxidation of cumene to cumylhydroperoxide oxygen, or oxygen-enriched air, is advantageous when The oxidation of cumene to cumylhydroperoxide: the reaction is mass-transfer controlled, and thus the rate depends on the partial pressure of oxygen and on O2 OOH efficient mass transfer. Further advantages in the use ∆H°∆ H31° kcal/mol 31 kcal/mol of oxygen include the non-necessity for separation of large amounts of inert (i.e. nitrogen) before recycling, is carried out at 80-120°C, either at 6 atm of pressure the use of smaller and less expensive apparatus and (Hercules process; Fleming et al., 1976), in an

VOLUME II / REFINING AND PETROCHEMICALS 651 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

homogeneous solution containing a sodium carbonate obtain acceptable rates of cumene oxidation. The buffer (removed by washing with water before hydroperoxide contributes to initiation, decomposing distillation; Hercules process), or at atmospheric into RO and OH species. In fact, long induction pressure (Allied process; Sifniades et al., 1982), periods are necessary when the reaction is started without the addition of a base (pH3-5). In the latter without the presence of the hydroperoxide. Therefore, case, however, the stream is washed with caustic water the hydroperoxide is in part recycled (together with before recycling. Per pass cumene conversion is in the unconverted cumene) to the first reactor, which range of 25 to 35%. operates at 8-10 wt% hydroperoxide. The maximum The reaction mechanism is a typical free-radical concentration of hydroperoxide in the last reactor is 30 chain, with initiation (usually accelerated by wt%; higher concentrations, in fact, do not lead to decomposition of the hydroperoxide itself), further increases in the reaction rate, but rather cause propagation and termination steps. Main by-products an increase in the side reactions of include acetophenone, dimethylphenylcarbinol cumylhydroperoxide decomposition. (a, a-dimethylbenzyl-alcohol) and a-methylstyrene. The selectivity to cumylhydroperoxide ranges from 90 The oxidation of cyclohexane to cyclohexanol to 96%. The reaction steps responsible for the and cyclohexanone formation of by-products are presented in Table 8 Cyclohexanol and cyclohexanone (the so-called ol (chain-branching reactions). one mixture) are produced by oxidation of In the buffered solution (pH 7-8), the organic acids cyclohexane: formed (i.e. formic acid), which are responsible for the in situ decomposition of the hydroperoxide with 2 3/2O2 OH O H2O formation of phenol (inhibitor of cumene oxidation), ∆H∆H°° 7070 kcal/mol kcal/mol are destroyed. The reaction is carried out in several (typically and the products are then used for the synthesis of four) in-series reactors. Air is the preferred oxidizing adipic acid. agent, for both economic and safety considerations. The most accepted reaction pattern for the Fresh air is pumped into each reactor. The lower and transformation of cyclohexane to cyclohexanol/ higher flammability limits for cumene/air mixtures are /cyclohexanone mixtures consists in the oxidation of 0.8 and 8.8%, respectively. The concentration of the cycloalkane to cyclohexylhydroperoxide, molecular oxygen at the exit of each reactor is kept followed by its decomposition to the alcohol and to lower than 4%, in order to avoid the formation of the ketone. Cyclohexanone is also formed by the flammable compositions with cumene (Fleming et al., consecutive reaction of cyclohexanol oxidation. The 1976). The vent gas also contains organic compounds, ketone is then oxidized to adipic acid and to other which are condensed by refrigeration and adsorbed on by-products. active charcoal. The alcohol and the ketone are more reactive than The rate equation for cumene oxidation in the cyclohexane; therefore, they are easily converted to conditions which are industrially applied can be several by-products. High selectivity to alcohol expressed as follows: 11111 ketone can be reached only at low conversion. The d[O ]2k [ROOH] selectivity is around 90% at very low cyclohexane 1442312 141111in [20] kpr[RH] conversion (1-2%), while already at 4-5% conversion, dt kter the selectivity drops to 77-85%. Therefore, all

where kin, kpr and kter are the rate constants for chain processes operate at low per–pass cyclohexane initiation, propagation and termination, respectively; conversion, with recycle of unconverted reactant. RH is cumene and ROOH is the hydroperoxide. The A detailed kinetic study has shown that the rate of reaction is therefore not dependent on oxygen reaction rate is initially zero order in oxygen partial concentration (Melville and Richards, 1954). However, pressure, and then becomes first order with decreasing this expression is only approximate; detailed kinetic oxygen partial pressure in a batch reactor (Suresh et studies indeed have demonstrated that the reaction al., 1988). does depend on oxygen up to a partial pressure of The main by-product is adipic acid, whose approximately 0.3 atm at 130°C; at 100°C, the rate selectivity reaches high values only at low does not depend on oxygen partial pressure for P temperatures (T100°C). The other by-products, such O2 higher than approximately 0.1 atm (Hendry and Amer, as glutaric acid or the esters formed by condensation 1967; Hattori et al., 1970; Andrigo et al., 1992). between the acids and cyclohexanol are derived from Moreover, the rate expression shows that a certain the decarboxylation of adipic acid. Although adipic concentration of the hydroperoxide is necessary to acid is the final desired product, its formation must be

652 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

avoided at this stage, since it is accompanied by several other by-products. Table 8. Reactions yielding by-products in the oxidation Different processes of oxidation of cyclohexane of cumene to cumylhydroperoxide (R C6H5C(CH3)2 ) have been proposed, which yield either a mixture of cyclohexanone and cycloexanol, or adipic acid RO2 RO2 RO RO O2 directly. Only those processes which produce ROC H C(O)CH CH (acetophenone) cyclohexanol/cyclohexanone have been 6 5 3 3 commercialized. Common aspects of the various CH3 RH CH4 R processes include the very low residence times CHROOHCH ROO (around 40 min), necessary to limit the consecutive 3 4 reactions that lead to the formation of by-products, the CH3 O2 CH3OO reaction temperature, which must be high enough CH OORHCH OOHR (around 150°C) to kinetically favour the 3 3 decomposition of the intermediate hydroperoxide and CH3OO ROO CH2O ROH O2 the pressure, which must be such as to keep the (ROHdimethylphenylcarbinol)

cyclohexane in the liquid phase. A brief description of CH O12O HCOOH the different processes follows. 2 2 Direct synthesis of cyclohexanone and ROH C6H5C(CH3) CH2 H2O (a-methylstyrene) cyclohexanol. This process was developed by RORHC H C(CH )CH RH O Stamicarbon. It operates at 155°C. Conversion of 6 5 3 2 2 cyclohexane is 4%, with an overall selectivity olone of 77-80%, and a molar ratio ol/one1. At these of a Co catalyst. This process operates at 4-5% conditions, the consecutive reactions upon the desired conversion, with a selectivity in the range of 82 to products are minimized. The catalyst is cobalt 86% and a ratio ol/one0.4. In the presence of Cr, the naphthenate or octeate, both of which are soluble in ol/one ratio is 0.8. cyclohexane (used as the solvent for the reaction). The direct synthesis of adipic acid. In this process, The concentration of the catalyst is very low, ranging a large excess of cobalt catalyst is used (also with a from 3 ppm to 300 ppm. The mixture of alcohol and promoter) to lower the induction time (Tanaka, 1974). ketone is then oxidized with air in the presence of Cu The role of the promoter is to start the oxidation of and Mn based catalysts, with a selectivity to adipic Co(II) to Co(III). The solvent is acetic acid, in which acid of 70%. the ketone is soluble. High concentrations of catalyst Process in the presence of boric acid. In order to (feed ratio cyclohexane/catalyst 100) are necessary minimize the formation of by-products, boric acid is to obtain around 65% selectivity to adipic acid. Lower added to the reaction medium. The cyclohexanol catalyst concentrations cause a decrease in selectivity, formed reacts with boric acid, forming an ester and indicating that the mechanism is a classical preserving it from the consecutive reactions to autoxidation in the absence of the catalyst. cyclohexanone and to by-products. Since the water formed in the reaction of cyclohexanol formation is The oxidation of n-butane to acetic acid also a product in the reversible reaction of The synthesis of acetic acid by n-butane or naphtha cyclohexanol esterification, stripping of water from liquid phase oxidation, commercialized by Celanese in the reaction medium has been found to positively 1962, was the predominant process throughout the affect the selectivity to the desired product, being that 1960s. it favours the formation of the borate. The addition of n-C H 5/2O 2CH COOHH O boric acid (H BO ) allows operation to be carried out 4 10 2 3 2 3 3 DH°233 kcal/mol at relatively high conversion (12%), with an overall selectivity of 85-87% and a ol/one ratio as high as By 1973, approximately 40% of the acetic acid 10/1. The oxidation reaction occurs in the absence of a capacity was based on this process. Currently, only a catalyst. The ester is then hydrolized in a following small fraction of acetic acid is produced by liquid step to alcohol and the boric acid is recycled. This phase oxidation, due to the superior economics of the process has higher investment and operating costs, due Monsanto methanol carbonylation process. to the recovery and recycle of boric acid. The formation of acetic acid from an alkane is Process in two steps. In a first step, the explained by the fact that acetic acid is the most stable

hydroperoxide is synthesized in the absence of a product (aside from CO2) and thus tends to accumulate catalyst, and in a second step, the hydroperoxide is in the reaction medium, which is also the reason why it decomposed to the alcohol and ketone in the presence is used as the solvent for the reaction. The main

VOLUME II / REFINING AND PETROCHEMICALS 653 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

reaction path for the oxidation of n-butane, accounting for 70% of converted paraffin, occurs through the Table 9. Reactions occurring in n-butane oxidation abstraction of the methylenic hydrogen and the formation of the sec-butylperoxy radical; the n-C4H10 sec BuOO sec BuO remaining 30% is oxidized with the formation of the sec–BuO C HCH CHO primary peroxy radical. The consumption of these two 2 5 3 radicals is not as fast as the H abstraction from C2H5 C2H5OO C2H5O C2H5OH n-butane, and they accumulate in the absence of CH CHO12O CH COOH catalyst in the reaction medium. 3 2 3 The peroxy radicals react with the Co(II)-based sec–BuO CH3 C2H5CHO catalyst forming a more reactive secondary or primary C H CHO12O C H COOH alkoxy radical. The main reaction from the sec-butoxy 2 5 2 2 5 radical is the b scission, which leads to acetaldehyde sec–BuO CH3 C(O) C2H5 (precursor of acetic acid) and the ethyl radical and, to a n-C H n-BuOO n-C H COOH minor extent, the scission leading to propionic acid 4 10 3 7 and formic acid. Alternatively, sec-BuO is transformed to methylethylketone. The n-butoxy Witten-Dynamit process; Katzschmann, 1966). The radical is not subjected to b scission and is first step is the oxidation to p-toluic acid, occurring transformed to n-butyric acid. The main reactions with the classic free-radical autoxidation mechanism occurring are reported in Table 9 (usual passages are catalysed by Co/Mn naphthenates at 150°C and 6 atm omitted). The ethyl radical is the precursor of ethanol. pressure, without solvent: Once formed, it is quickly oxidized to acetaldehyde H CC H CH 1.5O and acetic acid (Hobbs et al., 1972b). A second route 3 6 4 3 2 H CC H COOHH O to acetic acid is from methylethylketone, through 3 6 4 2 oxidation at the b position (with respect to the CO The electron-attracting effect of the COOH group) and the formation of a ketoperoxyradical that group prevents the formation of the radical on the produces acetic acid by CC cleavage. second methyl group, and thus prevents oxidation to Two processes have been proposed: the first uses the diacid. Therefore, the carboxylic group is only cobalt as the catalyst, which operates at 160- esterified by reaction with methanol (second step): 220°C and 50-60 atm, and the second, together with H CC H COOHCH OH cobalt, uses a co-oxidant (methylethylketone) and 3 6 4 3 H CC H C(O)OCH H O operates at milder conditions (120-130°C, 30 atm). 3 6 4 3 2 Methylethylketone is recycled to the first reactor in Then, the second methyl group can be oxidized order to introduce a free-radical initiator able to (third step): abstract a hydrogen from the sec-CH bond. Performance varies considerably depending on the H3C C6H4 C(O) OCH3 1.5O2 reaction conditions and catalyst type – either Mn(III) HOOC C6H4 C(O) OCH3 H2O or Co(III); the reaction may also run in the absence of The final step is the esterification: catalyst. Selectivity to acetic acid can be in the range HOOCC H C(O)OCH CH OH between 50 to 65%; conversion per pass is kept below 6 4 3 3 H CO(O)CC H C(O)OCH 30%, to avoid over-oxidation. The air is diluted with 3 6 4 3 residual inert gases up to an oxygen concentration of The synthesis of p-toluic acid is accompanied by 8-11%, in order to keep its concentration at the exit the formation of by-products, such as p-toluicalcohol, below the explosion limits (Prengle and Barona, p-tolualdehyde (these two compounds are 1970a). intermediates in the pathway leading to p-toluic acid), toluene, terephthalic dialdehyde, The oxidation of p-xylene to terephthalic acid 4-carboxybenzaldehyde and polynuclear products or dimethylterephthalate (i.e. p-toluylester of p-toluic acid and p-toluic acid The oxidation/esterification of p-xylene to anhydride). 4-carboxybenzaldehyde is highly dimethylterephthalate: undesired, since the CHO group can not undergo condensation with ethylene glycol during PET 3O2 2CH3OH H3COOC COOCH3 polymerization. ° ° The selectivity to by-products (especially ∆H ∆H 340 340kcal/mol kcal/mol p-toluylester of p-toluic acid and p-toluic acid is industrially carried out in several steps (Hercules- anhydride) increases as the partial pressure of oxygen

654 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

decreases (Jacobi and Baerns, 1983). Therefore, the consumed via attack by RO2. An experimental value of complete depletion of oxygen in the bulk liquid is n/f intermediate between the two limit cases (1.5 and undesired for selectivity. 6) was found, indicating that the cobalt catalyst not The kinetics of liquid-phase oxidation of p-xylene only was involved in the ROOH decomposition, but to p-toluic acid has been studied by several authors also reacted with p-xylene through the Co(III) species (Cavalieri d’Oro et al., 1980; Hronec and Ilavsky, (Jacobi and Baerns, 1983). 1982; Jacobi and Baerns, 1983; Hronec et al., 1985; Other models have been developed that also take Raghavendrachar and Ramachandran, 1992; Cao et into account diffusional constraints and that assume a al., 1994). When the reaction is carried out in bubble first order dependency of rate upon p-xylene columns, regions where the process is controlled by concentration (Hronec and Ilavsky, 1982; Cao et al., different steps can occur. Typically, at high oxygen 1994). In all cases, the reaction is independent of partial pressure, the process is under chemical control oxygen partial pressure (in conditions of a (and the rate does not depend on oxygen partial reaction-controlled process). pressure), while at low oxygen partial pressure An alternative process is one developed by Amoco (for almost total oxygen consumption), the process (and by several other companies), which makes use of becomes mass-transfer controlled. In addition, a catalyst (Mid-Century MIC catalyst: Co/Mn/Br ions) temperature affects the nature of the rate-determining in acetic acid as the reaction solvent. Several step; at temperatures below 100°C, the reaction is variations of this process exist. The MIC catalyst is under chemical control and is zero order when either able to oxidize the second methyl group of p-toluic air or oxygen is used. Instead, at 130°C, the process is acid and, therefore, terephthalic acid is directly under mass-transfer control when air is used (the obtained as the final product. The acetic acid keeps the apparent activation energy falls from 15 to 4 intermediates and by-products in solution, but does not kcal/mol), while with pure oxygen, the reaction is still dissolve the products. The temperature is around under chemical control (Cao et al., 1994; Cincotti et 205°C and the pressure is 12-15 atm. The conversion al., 1997). is almost total. The concentration of oxygen in the Under chemical control, the rate equation for the waste gas is around 4%. The advantage of this process Co-catalysed first step of the process (i.e. oxidation of lies in the direct availability of terephthalic acid, which p-xylene to p-toluic acid) can be expressed as follows can be further employed for several different (Jacobi and Baerns, 1983): transformations. This process is now used to virtually d[p-C H ] cover the world production of fibre grade terephthalic 1442331218 10 2 acid. [21] k[ p-C8H10] dt The Teijin process is an improvement of the With respect to the hydrocarbon, a second order original Mid-Century process; with respect to the reaction arises by combining the rate propagation latter, the conditions are milder (100-130°C, 9 atm); a expression (Prengle and Barona, 1970b): high yield of terephthalic acid (higher than 97%) is achieved by using a cobalt catalyst and a large excess 144233121d[p-C8H10] [22] k [RO][ RH] of p-xylene. dt pr 2

where R is (CH3)C6H4(CH2) (with the assumption that Oxidations with oxygen-transfer agents the corresponding step is the rate-limiting reaction), The reactions of oxidation via catalysed oxygen with the termination reaction expression: transfer agents can be represented schematically as follows: 144233121d[p-C8H10] [23] k [RO] dt ter 2 SROY SORY (assuming that the termination reaction occurs mainly where S is the substrate and ROY the oxygen transfer by bimolecular reaction between RO2 fragments), and agent (hydroperoxides, H2O2 and peracetic acid). [ ] by deriving an expression for RO2 . It is also assumed The hydroperoxides of isobutane, ethylbenzene and that the concentration of hydroperoxide is quasi- cumene are the best oxygen transfer agents for stationary (Walling, 1969). The kinetic constant k is industrial applications for the following reasons: they

related to kpr and kter through the following expression: are safer to handle and more stable, soluble in 2 non-polar solvents, not acidic, present high selectivity 121kpr [24] k n in the reaction of oxygen transfer to the substrate, and 2fkter the by-products of the reaction are easy to separate. where n is the number of radicals produced for each Two main industrial applications can be evidenced: ROOH decomposed, and f is the fraction of RH epoxidations of olefins and hydroxylation of aromatic

VOLUME II / REFINING AND PETROCHEMICALS 655 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

compounds. The former process, which is by far the proposed for homogeneous epoxidation has been more important, is industrially carried out in either described above: Liquidi phase homogeneous homogeneous or heterogeneous catalysis. The metal-catalysed oxidations by molecular oxygen. hydroxylation of aromatic compounds, on the other The industrial process consists of three main parts: hand, is carried out in heterogeneous catalysis. synthesis of ethylbenzene hydroperoxide; epoxidation of propylene; dehydratation of phenylcarbinol to The epoxidation of propylene to propylene oxide styrene. All olefins can be epoxidized with a hydroperoxide The synthesis of the hydroperoxide is carried out in and a catalyst (see Section 11.1.3), but the most a bubble reactor with air as the oxidizing agent. After important process, which has been developed at an the removal of volatile compounds, the exit solution is industrial level, is the synthesis of propylene oxide. sent to a falling film evaporator for concentration. The Two alternative processes have been developed, one in hydroperoxide-containing solution is then fed together the homogeneous phase, developed by Halcon and with fresh propylene to the first reactor of three Atlantic Richfield (Oxirane process), with a in-series reactors. The final exit solution, after molybdenum complex as the catalyst (Landau, 1967; separation of unconverted propylene (recycled to the Landau et al., 1979), and the other in the first reactor), is sent to the purification section. heterogeneous phase, with silica-supported titanium oxide, developed by Shell (Wulff, 1975). New technologies of epoxidation under study Alkylhydroperoxides used as oxygen-transfer or development agents are either t-butylhydroperoxide or ethylbenzene The epoxidation of propylene can also be carried

hydroperoxide, due to the fact that the corresponding out catalysed by Ti-silicalite (TS-1), with H2O2 as the coproduct is valuable and contributes to the economy oxidizing agent (Neri et al., 1984). The process is not of the process. t-butylhydroperoxide is converted to yet commercially applied, though pilot plant operation t-butylalcohol, and then to isobutene. Ethylbenzene is operative. The catalyst was developed by Eni hydroperoxide is decomposed to phenylcarbinol and researchers and is made of a zeolite isostructural with the latter is converted to styrene. Cumylhydroperoxide, silicalite-1, where titanium substitutes for silicon in which could present advantages due its high rate of the framework of the zeolite (Taramasso et al., 1983). epoxidation and higher selectivity, is not used as an The solvent for the reaction is an alcohol oxygen-transfer agent due to the non-practical use of (methanol or t-butanol). At 50°C and atmospheric

the by-product. pressure, the yield of epoxide on an H2O2 basis is All processes, both homogeneous and around 90%, and the selectivity on a propylene basis is heterogeneous, operate at temperatures between 90 98% (Sheldon, 1973; Chong and Sharpless, 1977). and 120°C and at a pressure of 35 atm, in order to The main by-products observed for the different keep propylene in the liquid phase. The upper olefins are only those derived from the consecutive temperature limit is the temperature of hydroperoxide transformation of the oxirane ring by reaction with decomposition and the lower limit is dictated by the methanol (yielding glycol ethers), with water (yielding

activity of the catalyst. The catalysts for epoxidation glycols), or with H2O2 (yielding hydroperoxides). reactions are d0 metal complexes: Mo(VI), W(VI), Several Olin patents claim the synthesis of V(V) and Ti(IV) for homogeneous epoxidation propylene oxide from propylene using molecular

(Mimoun, 1980; Jørgensen, 1989), and TiO2 supported oxygen as the oxidizing agent in the presence of over SiO2 for heterogeneous epoxidation (Sheldon, molten salts (Meyer and Pennington, 1991; Fullington 1980). and Pennington, 1992). Although it has been claimed The reasons for the superior activity and selectivity that the salt acts as a catalyst, it is likely that the of these metal ions can be summarized as follows: they oxidation is not catalytic and that the salt probably have a very low oxidation power and, therefore, do not serves to remove the heat of reaction, allowing better decompose the hydroperoxide homolitically; they are control of the temperature, and thus allowing the very in their high oxidation state and have high Lewis-type exothermal reaction to run more smoothly and more acidity, necessary to activate the hydroperoxide selectively. Thus the mechanism of reaction is a towards nucleophilic attack, through withdrawal of classical radical chain reaction, and the molten salt electrons from the OO bond. Indeed, V(V) may possibly serve to facilitate the initiation step. complexes are worse than those of Mo(VI), because The feedstock contains propylene at a they are more active in hydroperoxide decomposition concentration of around 50%, with a deficiency of (and thus the selectivity is lower), while Cr(VI) oxygen of approximately 10%, and the remainder

complexes, which have high Lewis acidity, cannot be consists of inert gas (N2 and carbon oxides). The used due to their high oxidizing power. The mechanism reaction proceeds at 200-300°C, 5-20 atm pressure and

656 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

at relatively high residence time (higher than 10 s). reaction between catalyst and ethylene (at 110°C and Gas reactants may be either bubbled through a 9-10 atm), with the formation of acetaldehyde; the stirred–tank reactor filled with the molten salt, moved second step is the reoxidation of the reduced catalyst countercurrently in a tower through a downflow of through contact with air. molten salt or reinjected into a loop through which the In the single-step process, ethylene and molecular

salt circulates. The molten salt is a mixture of LiNO3, oxygen are fed into an air-lift tower, where the NaNO3 and KNO3. The composition of the mixture catalyst-containing aqueous solution is circulated via determines the temperature of the reaction. a separation vessel. After cooling and washing with Propylene conversion is around 10%, with a water of the gaseous stream, the liquid fraction, selectivity to epoxide in the 55 to 65% range. containing water and crude acetaldehyde, is sent to a By-products include carbon oxides (selectivity two–column separation, while the gaseous stream, 22-25%) and acetaldehyde (10-13%); minor amounts of containing unreacted ethylene and inerts, is recirculated formaldehyde, methanol, acrolein, acetone, allyl alcohol to the reactor. The ethylene per pass conversion is and propylene glycol are also formed. Unreacted between 20 and 50%; unconverted ethylene is recycled. propylene is recycled together with part of the In the two-step process, following the reaction, an by-product gases. The recycle of acetaldehyde improves acetaldehyde/water mixture is separated from the the yield to propylene oxide, since acetaldehyde is solution containing the reduced catalyst in a flash oxidized to peracetic acid, which epoxidizes propylene. tower. The latter is sent to the reoxidation reactor, Alternatively, acetaldehyde is oxidized to acetic acid in where it is put in contact with air. Ethylene and a second reactor in a two-step process. molecular oxygen almost completely react in the two Another technology under development is the steps, respectively. The crude acetaldehyde/water

in situ generation of H2O2, which finally acts as the mixture is first concentrated and then sent to a oxygen-transfer agent on propylene (Clerici and two-stage distillation section. In this process Ingallina, 1998). This process, proposed by configuration, the ethylene conversion is up to 90%, EniTechnologie (Clerici and Ingallina, 1993a), which eliminates the need for recycle of the epoxidizes olefins with molecular oxygen in the unconverted olefin. In both process configurations, the presence of Ti-silicalite and a redox couple generating yield of acetaldehyde is around 95%.

in situ H2O2 and is discussed in Section 11.1.3.

Redox processes Bibliography

The oxidation of ethylene to acetaldehyde Bailey C.L., Drago R.S. (1987) Utilization of dioxygen for Currently, acetaldehyde is mainly obtained through the specific oxidation of organic substrates with cobalt (II) the process developed by Wacker Chemie and catalysts, «Coordination Chemistry Reviews», 79, 321-332. Balci M. Hoechst: (1981) Bicyclic endoperoxides and synthetic applications, «Chemical Reviews», 81, 91-108. Benson S.W Nangia P.S. C H 1/2O CH CHO ., (1979) Liquid-phase oxidation of 2 4 2 3 isobutane. A reanalysis of the data, «International Journal DH°58 kcal/mol of Chemical Kinetics», 12, 169-181. The catalyst is made of an aqueous solution of Betts A.T., Uri N. (1968) Catalyst-inhibitor conversion in PdCl2 and CuCl2. Acetaldehyde is formed by reaction autoxidation reactions, «Advances in Chemistry Series», between ethylene and Pd chloride (i.e. the 76, 160-181. rate-determining step), which, at the same time, is Burstyn J.N. et al. (1988) Mechanisms of dioxygen activation reduced to metallic Pd. Pd is then reoxidized to PdCl2 in metal-containing monooxygenase. Enzimes and model systems, in: Martell A.E., Sawyer D.T. (editors) Oxygen by CuCl2, with the formation of CuCl, which is finally complexes and oxygen activation by transition metals, New reoxidized by molecular oxygen, yielding back CuCl2. (The reaction mechanism was described above: Liquid York, Plenum Press, 175-187. Choy V.J O’Cannor C.J phase homogeneous metal-catalysed oxidations by ., . (1972) Chelating dioxygen molecular oxygen). By-products of the reaction are compounds of the platinum metals, «Coordination Chemistry Reviews», 9, 145-170. acetic acid, oxalic acid, crotonaldehyde, chlorinated Collman P. et al. (1980) Oxigen binding to heme proteins hydrocarbons and chlorinated acetaldehyde. and their synthetic analogs, in: Spiro T.G. (editor) Metal The process can be carried out either in a single ions in biology, 2: Metal ion activation of dioxygen, New step, with pure oxygen as the oxidizing agent, York, John Wiley, 1-72. at 130°C and 4 atm, or in a two-step process, where Dadyburjor D.B. et al. (1979) Selective oxidation of the oxidizing agent of choice is air. hydrocarbons on composite oxides, «Catalysis Reviews. In the latter case, the first step consists in the Science and Engineering», 19, 293-350.

VOLUME II / REFINING AND PETROCHEMICALS 657 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Fox M.A., Chanon M. (editors) (1988) Photoinduced protic molecules and hydrogen peroxide, «Journal of electron transfer. Part A: Conceptual basis; Part B: Catalysis», 133, 220-230. Experimental techniques and medium effects, Amsterdam, Benson S.W. (1965) Effects of resonance and structure on the Elsevier. thermochemistry of organic peroxy radicals and the kinetics Gubelmann M.H., Williams A.F. (1983) The structures and of combustion reactions, «Journal of the American Chemical reactivity of dioxygen complexes of the transition metals, Society», 87, 972-979. in: Structure and bonding, 55: Transition metal complexes- Benson S.W., Shaw R. (1970) Thermochemistry of organic Structure spectra, Berlin, Springer, 1-65. peroxides, hydroperoxides, polyoxides, and their radicals, Kaufman S., Fisher D.B. (1974) Pterin-requiring aromatic in: Swern D. (edited by) Organic peroxides, New York, amino acid hydroxilases, in: Hayaishi O. (editor) Molecular John Wiley, 105-139. mechanisms of oxygen activation, New York, Academic Cao G. et al. (1994) Kinetics of p-xylene liquid-phase catalytic Press, 285-369. oxidation, «American Institute of Chemical Engineers Meyer T.J. (1988) Metal oxo complexes and oxygen Journal», 40, 1156-1166. activation, in: Martell A.E., Sawyer D.T. (edited by) Carrà S., Santacesaria E. (1980) Engineering aspects of Oxygen complexes and oxygen activation by transition gas-liquid catalytic reactions, «Catalysis Reviews-Science metals, New York, Plenum Press, 33-47. and Engineering», 22, 75-140. Murray R.W., Wasserman H.E. (edited by) (1979) Organic Cavalieri d’Oro P. et al. (1980) On the low temperature oxidation chemistry, 40: Singlet oxygen, New York, Academic Press. of p-xylene, «Oxidation Communications», 1, 153-162. Patai S. (edited by) (1983) The chemistry of peroxide, Chen Z. et al. (1985) Gas-liquid interphase mass transfer Chichester, John Wiley. coefficients in trickle beds, «Huagong Xuebao», 3, 339-346. Sheldon R.A. (1991) Fine chemicals by catalytic-oxidation, Chester A.W. et al. (1977) Zirconium cocatalysis of the cobalt- «CHEMTECH», 21, 566-576. catalyzed autoxidation of alkylaromatic hydrocarbons, «Journal of Catalysis», 46, 308-319. Sheldon R.A. (1991) Heterogeneous catalytic oxidation and fine chemicals, in: Guisnet M. et al. (editors) Studies in Chong A.O., Sharpless K.B. (1977) Mechanism of the surface science and catalysis, 59: Heterogeneous catalysis molybdenum and vanadium catalyzed epoxidation of olefins and fine chemicals II. Proceedings of the 2nd international by alkyl hydroperoxides, «Journal of Organic Chemistry», symposium, Poitiers, 2-5 October, Amsterdam, Elsevier, 42, 1587-1590. 33-54. Cincotti A. et al. (1997) Effect of catalyst concentration and Simandi L.I. (editor) (1991) Dioxygen activation and simulation of precipitation processes on liquid-phase homogeneous catalytic oxidation. Proceedings of the 4th catalytic oxidation of p-xylene to terephthalic acid, international symposium on dioxygen activation and «Chemical Engineering Science», 52, 4205-4213. homogeneous catalytic oxidation, Amsterdam, Elsevier. Clerici M.G. (1991) Oxidation of saturated hydrocarbons with Stevens B. (1973) Kinetics of photoperoxidation in solution, hydrogen peroxide, catalyzed by titanium silicalite, «Applied «Accounts of Chemical Research», 6, 90-96. Catalysis», 68, 249-261. Clerici M.G. Tsutsui M., Ugo R. (editors) (1977) Fundamental research et al. (1991) Synthesis of propylene oxide from in homogeneous catalysis, New York, Plenum Press. propylene and hydrogen peroxide catalyzed by titanium silicalite, «Journal of Catalysis», 129, 159-167. Vaska L. (1976) Dioxygen-metal complexes. Toward a unified Clerici M.G., Ingallina P. view, «Accounts of Chemical Research», 9, 175-183. (1993a) European Patent 0549013 to Eniricerche. Clerici M.G., Ingallina P. (1993b) Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite, References «Journal of Catalysis», 140, 71-83. Clerici M.G., Ingallina P. Andrigo P. et al. (1992) Phenol-acetone process. Cumene (1998) Oxidation reactions with oxidation kinetics and industrial plant simulation, in situ generated oxidants. New concepts in selective «Chemical Engineering Science», 47, 2511-2516. oxidation over heterogeneous catalysts, «Catalysis Today», Astarita G. et al. (1983) Gas treating with chemical solvents, 41, 351-364. New York, John Wiley. Dawson J.H. (1988) Probing structure-function relations in Bamford C.H., Tipper C.F.H. (edited by) (1980) heme-containing oxygenases and peroxidases, «Science», Comprehensive chemical kinetics, Amsterdam, Elsevier. 240, 433-439. Doraiswamy L.K., Sharma M.M Barton D.H.R., Doller D. (1991) Selective functionalization . (1984) Heterogeneous of saturated hydrocarbons. Gif and all that, in: Simandi reactions. Analysis, examples and reactor design, New L.I. (edited by) Dioxygen activation and homogeneous York, John Wiley, 2v. catalytic oxidation. Proceedings of the 4th international Drago R.S., Beer R.H. (1992) A classification scheme for symposium on dioxygen activation and homogeneous homogeneous metal-catalyzed oxidations by molecular catalytic oxidation, Amsterdam, Elsevier, 1-10. oxygen, «Inorganica Chimica Acta», 198, 359-367. Barton D.H.R. et al. (1989) On the mechanism of the Gif and Ellis P.E., Lyons J.E. (1989a) Effect of axial azide on the selective, Gif-Orsay systems for the selective substitution of saturated low temperature metalloporphyrin-catalyzed reactions of hydrocarbons, «New Journal of Chemistry», 13, 177-182. isobutane with molecular oxygen, «Journal of the Chemical Bellussi G. et al. (1992) Reactions of titanium silicalite with Society, Chemical Communications», 16, 1187-1189.

658 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Ellis P.E., Lyons J.E. (1989b) Effect of fluorination of the distribution of the p-xylene oxidation, «Erdoel Kohle Erdgas meso-phenyl groups on selective tetraphenylporphyrinato Petrochemie», 36, 322-326. metal(III)-catalyzed reactions of isobutane with molecular Jørgensen K.A. (1989) Transition-metal-catalyzed epoxidations, oxygen, «Journal of the Chemical Society, Chemical «Chemical Reviews», 89, 431-458. Communications», 16, 1189-1190. Katzschmann E. (1966) Oxidation of alkyl aromatic Emanuel N.M., Gal D. (1986) Modelling of oxidation compounds, «Chemie Ingenieur Technik», 38, 1-10. processes, Budapest, Akademiai Kiado. Kerr J.A. (1966) Bond dissociation energies by kinetic methods, Finn M.G., Sharpless K.B. (1985) On the mechanism of «Chemical Review», 66, 465-500. asymmetric epoxidation with titanium-tartrate catalysts, Kochi J.K. (1973) Oxidation-reduction reactions of free radicals «Asymmetric Synthesis», 5, 247-308. and metal complexes, in: Kochi J.K. (editor) Free radicals, Fleming J.B. et al. (1976) Safety in phenol-from-cumene New York, John Wiley, 2 v.; v. I. process, «Hydrocarbon Processing», 55, 185-190. Krzysztoforski A. et al. (1986) Industrial contribution to Franz G., Sheldon R.A. (1991) Oxidation, in: Ullmann’s the reaction engineering of cyclohexane oxidation, encyclopedia of industrial chemistry, Weinheim, VCH, «Industrial and Engineering Chemistry. Process Design 1985-1996, 37 v.; v. A18, 261-270. and Development», 25, 894-898. Fullington M.C., Pennington B.T. (1992) WO Patent WO Ladhaboy M.E., Sharma M.M. (1969) Absorption of oxygen 9209588 to Olin Co. by n-butyraldehyde, «Journal of Applied Chemistry», 19, Gates B.C. et al. (1979) Chemistry of catalytic processes, New 267-272. York, McGraw-Hill. Landau R. et al. (1967) Epoxidation of olefins, in: Proceedings Groves J.T. (1985) Key elements of the chemistry of the 7th World petroleum congress, Mexico City, April, of cytochrome-p-450. The oxygen rebound mechanism, 67-72. «Journal of Chemical Education», 62, 928-931. Landau R. et al. (1979) Propylene oxide by the co-product Hattori K. et al. (1970) Kinetics of liquid phase oxidation of processes3, «CHEMTECH», 9, 602-607. cumene in bubble column, «Journal of Chemical Lyons J.E. (1980) Up petrochemical value by liquid phase catalytic Engineering of Japan», 3, 72-78. oxidation, «Hydrocarbon Processing», 59, 107-119. Hayaishi O. (1974) General properties and biological functions Manor Y., Schmitz R.A. (1984) Gradientless reactor for gas- of oxygenases, in: Hayaishi O. (edited by) Molecular liquid reactions, «Industrial & Engineering Chemistry mechanisms of oxygen activation, New York, Academic Fundamentals», 23, 243-252. Press, 7. Mansuy D., Battioni P. (1989) Catalytically active Hendry D.G., Amer J. (1967) Rate constants for oxidation of metalloporphyrin models for cytochrome P-450, in: cumene, «Journal of the American Chemical Society», 89, Ruckpaul K., Rein H. (edited by) Basis and mechanisms 5433-5438. of regulation of cytochrome P–450, London, Taylor & Hiatt R. et al. (1968) Homolytic decompositions of Francis. hydroperoxides. IV: Metal-catalyzed decompositions, Melville H.W., Richards S. (1954) Photochemical «Journal of Organic Chemistry», 33, 1430-1435. autoxidation of isopropylbenzene, «Journal of the Chemical Hill C.L. (editor) (1989) Activation and functionalization of Society», 944-952. alkanes, New York, John Wiley, 243. Meyer J.L., Pennington B.T. (1991) US Patent 4992567 to Hobbs C.C. et al. (1972a) Mass-transfer rate-limitation effects in Olin Co. liquid-phase oxidation, «Industrial & Engineering Chemistry. Mimoun H. (1980) The role of peroxymetalation in selective Product Research and Development», 11, 220-225. oxidative processes, «Journal of Molecular Catalysis», 1, Hobbs C.C. et al. (1972b) Product sequences in liquid-phase 1-29. oxidation of paraffins, «Industrial & Engineering Chemistry. Mimoun H. (1982) Oxygen-transfer from inorganic and organic Process Design and Development», 11, 59-68. peroxides to organic substrates. A common mechanism, Howard J.A. (1972) Absolute rate constants for reactions of «Angewandte Chemie-International Edition in English», oxyl radicals, «Advances in Free-Radical Chemistry», 4, 21, 734-750. 49-173. Mimoun H. et al. (1970) Epoxidation of olefins with covalent Howard J.A. (1973) Homogeneous liquid-phase autoxidations, peroxomolybdenum(VI) complexes, «Tetrahedron», 26, in: Kochi J.K. (editor) Free radicals, New York, John Wiley, 37-50. 2 v.; v. II, 3-62. Moiseev I.I. et al. (1974) Kinetics of olefin oxidation by Hronec M., Ilavsky J. (1982) Oxidation of polyalkylaromatic tetrachloropalladate in aqueous solution, «Journal of the hydrocarbons. XII: Technological aspects of p-xylene American Chemical Society», 96, 1003-1007. oxidation to terephthalic acid in water, «Industrial & Morbidelli M. et al. (1986) Gas-liquid autoxidation reactors, Engineering Chemistry. Process Design and Development», «Chemical Engineering Science», 41, 2299-2307. 21, 455-460. Neri C. et al. (1984) European Patent 0100119 to ANIC. Hronec M. et al. (1985) Kinetics and mechanism of cobalt- Ortiz de Montellano P.R. (edited by) (1985) Cytochrome catalyzed oxidation of p-xylene in the presence of water, P–450. Structure, mechanism and biochemistry, New York, «Industrial & Engineering Chemistry. Process Design and Plenum Press. Development», 24, 787-794. Prengle H.W., Barona N. (1970a) Petrochemicals by liquid Jacobi R., Baerns M. (1983) The effect of oxygen transfer phase oxidation, «Hydrocarbon Processing», 49, 106- limitation at the gas-liquid interphase. Kinetics and product 118.

VOLUME II / REFINING AND PETROCHEMICALS 659 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Prengle H.W., Barona N. (1970b) Petrochemicals by liquid proteins, «Journal of the American Chemical Society», phase oxidation. 2: Kinetics, mass transfer, and reactor 112, 879-881. design, «Hydrocarbon Processing», 49, 159-175. Shilov A.E. (1989) Historical evolution of homogeneous Purcell K. (1985) Cycloreversion in metal-assisted olefin alkane activation systems, in: Hill C.L. (edited by) oxidation by peroxide. Molybdenum(VI) vs. rhodium(III), Activation and functionalization of alkanes, New York, «Organometallic», 4, 509-514. John Wiley. Raghavendrachar P., Ramachandran S. (1992) Liquid- Sifniades S. et al. (1982) US Patent 4358618 to Allied Corp. phase catalytic oxidation of p-xylene, «Industrial & Suresh A.K. et al. (1988) Mass transfer and solubility in Engineering Chemistry Research», 31, 453-462. autocatalytic oxidation of cyclohexane, «American Institute Santacesaria E., Pimpinelli N. (1986) Oxidation in the gas- of Chemical Engineers Journal», 34, 55-68. liquid phase in industry. I: Kinetics and mechanical aspects, Tanaka K. (1974) Adipic acid in one-step, «CHEMTECH», «La Chimica e L’Industria», 68, 69-79. 4, 555-559. Sheldon R.A. (1973) Molybdenum-catalyzed epoxidation of Taramasso M. et al. (1983) US Patent 4410501 to olefins with alkyl hydroperoxides. I: Kinetic and product Snamprogetti. studies, «Recueil des Travaux Chimiques des Pays-Bas», Trifirò F., Cavani F. (1994) Selective partial oxidation of 92, 253-266. hydrocarbons and related oxidations, Catalytica Studies Sheldon R.A. (1980) Synthetic and mechanistic aspects of Division, 4193 SO. metal-catalyzed epoxidations with hydroperoxides, «Journal Twigg G.H. (1962) Liquid-phase oxidation [of organic of Molecular Catalysis», 7, 107-126. compounds] by molecular oxygen, «Chemistry and Sheldon R.A., Doom J.A. van (1973) Metal-catalyzed Industry», 1, 4-11. epoxidation of olefins with organic hydroperoxides. I: Uri N. (1961) Physico-chemical aspects of autoxidation, in: Comparison of various metal catalysts, «Journal of Lundberg W.O. (editor) Autoxidation and antioxidants, New Catalysis», 31, 427. York, John Wiley, 55-106. Sheldon R.A., Kochi J.K. (1976) Metal-catalyzed oxidations Walling C. (1969) Limiting rates of hydrocarbon of organic compounds in the liquid phase. A mechanistic autoxidations, «Journal of the American Chemical Society», approach, «Advances in Catalysis», 25, 272-413. 91, 7590-7594. Sheldon R.A., Kochi J.K. (1981) Metal-catalyzed oxidations Wulff H.P. (1975) US Patent 3923843 to Shell Oil. of organic compounds, New York, Academic Press. Sheu C. et al. (1990) Activation of dioxygen by bis(2,6- Philippe Arpentinier dicarboxylatopyridine)iron(II) for the ketonization of methylenic carbons and the dioxygenation of acetylenes, Air Liquide - Centre de Recherche Claude-Delorme aryl olefins, and catechols. Reaction mimics for dioxygenase Jouy-en-Josas, France

660 ENCYCLOPAEDIA OF HYDROCARBONS 11.1.3 Oxidation processes The reaction shows, though, that together with with hydrogen peroxide the desired product SO a second product and hydroperoxides (coproduct) X is also formed, derived from the reduction of the oxidant. At the end of the reaction, Introduction the coproduct has to be recovered, recycled or upgraded, or – in the worst case – disposed of. Oxidants in industrial chemical processes Different oxidants form different coproducts Oxidation reactions are of particular importance in (Table 1), however, in every case, the formation of petrochemicals because, while the traditional raw a coproduct increases the costs of the process. In materials are hydrocarbons formed solely by carbon particular, its mere disposal can have an and hydrogen, the majority of the derivatives produced environmental impact, which, in the light of the from them consist of carbon, hydrogen and oxygen. growing sensitivity to problems of pollution, is From both an economic and environmental often unacceptable. For example, the production of standpoint, theoretically, the most preferable way by propylene oxide by the chlorohydrin process (still far to oxidize any substrate should be its reaction with in use) entails that, for every ton of epoxide, about molecular oxygen. In practice, however, this reaction 2.1 t of calcium chloride is coproduced and must presents certain negative aspects. In fact, although be disposed of; furthermore, it may be polluted by oxygen is a formidable oxidant from the chlorinated organic by-products. thermodynamic point of view, it is not very reactive. Therefore, from the environmental standpoint, only This scanty reactivity (enabling all living organisms to a few oxidizing agents (hydrogen peroxide, ozone, remain alive for a reasonable time, instead of burning nitrous oxide) are acceptable, as they lead to the

more or less slowly, forming water and CO2) is due to formation of harmless, easily disposable coproducts the fact that the ground state of the O2 molecule is a (water, oxygen, nitrogen). triplet state (with two unpaired electrons). On the

contrary, the ground states of the O2 reduction Hydrogen peroxide products (i.e. water or hydrogen peroxide), as in the Of the oxidizing agents presented in Table 1, the majority of the stable organic compounds obtained by most versatile and easiest to use by far is hydrogen oxidation, are all singlet states (with paired electrons). peroxide, whose molecule has a non-planar structure,

As in a chemical reaction the angular moment must be with a C2 symmetry (Fig. 1). It should be stressed that conserved, the reactions of molecules in a triplet state the bond between the two oxygen atoms is relatively with products in a singlet state are spin-prohibited and weak (51 kcal/mol, to be compared, for example, with can occur only with extremely slow kinetics. The 89.5 kcal/mol for the two OH bonds). obstacle can be overcome by supplying energy to the system, that is by conducting the reaction at high temperatures and pressures or by operating in Table 1. Different coproducts formed by reduction conditions that favour the onset of radical type of selected oxidants mechanisms (compatible with the triplet state of the molecular oxygen). Both solutions, however, are Oxidant Coproduct generally adverse to achieving selective oxidations and H O H O hence to a certain part of oxidations of industrial 2 2 2 interest. N2ON2 As an alternative to molecular oxygen, the O O oxidation of an organic substrate S can be obtained by 3 2 causing it to react with an appropriate oxidant XO, Hydroperoxides (ROOH) Alcohols (ROH) whose basic state is a singlet one: RCO3H RCO2H S XO → SO X NaClO NaCl

VOLUME II / REFINING AND PETROCHEMICALS 661 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

and oxidation cycle, this can undergo various degradation processes with the formation of various H by-products, bringing about complex regeneration and 96°52' purification processes of the working solution (apart ° from the partial loss of a relatively costly reagent). The 93 51' OO 1.49 Å solvent should be able to dissolve both the (apolar) H 0.97 Å anthraquinone and the (polar) anthrahydroquinone, and, in addition, must guarantee immiscibility with water so as to permit the extraction and recovery of the Fig. 1. The molecule of hydrogen peroxide. hydrogen peroxide. These problems are tackled, in part, by selecting the proper alkyl substituent of the anthraquinone Hydrogen peroxide is commonly produced by the (generally ethyl, amyl or tert-butyl) and by using a anthraquinone process, originally developed in mixture of solvents, chosen from two groups Germany by IG Farbenindustrie between 1935 and characterized by their different polarity. The first 1945, and used on an industrial scale as of the 1950s. group, suitable for dissolving alkylanthraquinone, This process is based on the capacity of the alkyl includes a number of aromatic compounds (toluene, derivatives of anthrahydroquinone to produce hydrogen methylnaphthalene). The second group, suitable for peroxide under typical autoxidation conditions: alkylanthrahydroquinone, includes the organic esters OH O of phosphoric acid, diisobutylcarbinol and R R alkylsubstituted ureas. Typically, the composition of O2 H2O2 the working solution varies from one producer to another; it is optimized to maximize the solubility of OH O alkylanthraquinone and hence the concentration of the O OH hydrogen peroxide produced, to guarantee high rates R R of hydrogenation and oxidation, as well as to minimize H2 the formation of degradation by-products. The other O OH characteristics of the plant are generally the result of specific choices and developments. Other organic compounds also possess this The many problems to be addressed are reflected property, such as secondary alcohols, hydroquinone in the process’ complex nature and high investment derivatives and azobenzenes, which has been exploited costs. In fact, the anthraquinone process is marketed for the development of alternative production throughout the world by a relatively small number of processes. Mention should be made, in this regard, to companies. The average capacity of the plants is the autoxidation of isopropyl alcohol, the Shell 40,000-50,000 t/y, with numerous examples of outputs process, and of the 1-phenylethanol recently developed of under 20,000 t/y. Plants with a productive capacity by ARCO up to the pilot plant stage. Today the only of between 70,000 and 110,000 t/y are less common. It process truly applied for the production of large is important to note that for certain applications (see volumes is the anthraquinone process. An below), significantly larger supplies of hydrogen electrochemical process also exists, which is used for peroxide are required. the production of small quantities at local scale. For all these reasons, considerable efforts have Typically, the anthraquinone process is based on been made in recent years to identify alternative the cyclic hydrogenation and oxidation of a working syntheses to hydrogen peroxide, resulting in two main solution consisting of an alkylanthraquinone and directions. The first one is the reaction of carbon various derivatives thereof, dissolved in a solvent mix. monoxide with oxygen and water to form hydrogen The recovery of the hydrogen peroxide includes an peroxide and carbon dioxide: extraction with water, a series of purification CO O H O H O CO treatments of the aqueous solution, removing the 2 2 2 2 2 organic impurities, and the vacuum distillation of the This reaction was reported for the first time in excess water. The commercial solution thus obtained 1979 by Yuri Yermakov (of the Catalysis Institute of generally has a concentration of 60-70% in weight. Novosibirsk, in Russia). The catalytic system used by The process is characterized by various critical Yermakov (based on soluble palladium salts and factors, linked to both the reactivity and the different triphenylphosphine) was rather inefficient, but solubilities of the oxidized and reduced forms of the recently the substitution of the phosphine with alkylanthraquinone. In fact, during the hydrogenation nitrogen ligands (phenanthrolines) has led to

662 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

considerable improvement in the reaction performance and (1-phenyl) ethyl hydroperoxide from and has enabled it to be conducted continuously ethylbenzene: (Bianchi et al., 1999). H3C H3C OOH Of even greater interest is direct synthesis from the CH2 CH elements: O2 H2 O2 H2O2 This reaction has been known for a long time (the For example, the oxidation of isobutane and of first patents date from the 1960s; Hooper, 1967), but it ethylbenzene is commonly conducted in the liquid has not yet been applied industrially due to the phase at 120-140°C and under moderate pressure problems linked in particular to safety in managing (30-40 and about 2 bar, respectively). Analogous hydrogen-oxygen mixtures. The two gases are made to conditions (90-130°C and 5-10 bar) are adopted also react in a solvent, typically water or methanol in the oxidation of cumene; however, in this case, the (Paparatto et al., 2003a), in the presence of a reaction must be conducted in the presence of an palladium base catalyst, possibly modified with the emulsified aqueous phase, weakly basic due to the addition of platinum or of other noble metals. At the addition of sodium hydroxide or carbonate. Inevitably, moment, this direct synthesis seems the most in fact, small quantities of acid by-products are promising candidate to replace the traditional formed, mainly formic acid. These acids favour the anthraquinone process in the near future. decomposition of cumyl hydroperoxide giving acetone and phenol; the latter is an excellent inhibitor of many Hydroperoxides radical reactions and, in particular, its presence is not Alkylhydroperoxides, or more briefly compatible with autoxidation. hydroperoxides, formally derive from the substitution of In general, the selectivity of all these reactions, a hydrogen atom of the hydrogen peroxide molecule between 60 and 95%, is satisfactory and depends with an alkyl R group, so that their general formula is mostly on the conversion of the hydrocarbon, which, ROOH. Hydroperoxides retain the weakness of however, is rather limited, approximately between 10 hydrogen peroxides oxygen-oxygen bond: in fact, the and 40%. energy of this bond is typically 40-44 kcal/mol, even Some uses of hydroperoxides, exploiting two very lower than that of hydrogen peroxide (51 kcal/mol). The different aspects of their reactivity, are described energy of the OH bond, instead, is 89-90 kcal/mol, below. On the one hand, their oxidizing properties are practically coinciding with that of hydrogen peroxide used, and in particular their capacity to transfer an (89.5 kcal/mol). Many hydroperoxides are known – oxygen atom to olefins, giving rise to the formation of primary, secondary or tertiary – but few have taken on an epoxide and an alcohol (which is the product of industrial importance; those produced and used at a hydroperoxide reduction). In this aspect, large scale are substantially three: two tertiary hydroperoxides (such as hydrogen peroxide) are not hydroperoxides, i.e. tert-butyl hydroperoxide and cumyl very reactive and are therefore used in the presence of hydroperoxide, and a secondary one, i.e. (1-phenyl) appropriate catalysts, generally based on transition ethyl hydroperoxide. metals (in particular, titanium and molybdenum). They are prepared by autoxidation of the The use of cumyl hydroperoxide in the traditional corresponding hydrocarbons, a typical radical cumene process for the production of phenol and reaction, and the hydroperoxide group is formed, acetone is completely different. In this case, the mostly, at the expense of the CH bonds having less mentioned property of cumyl hydroperoxide to energy (e.g. tertiary or benzylic ones). These two cases decompose in an acid environment, producing the two are exploited, respectively, in preparing tert-butyl final products, is exploited. hydroperoxide from isobutane and cumyl All hydroperoxides, and in particular those of hydroperoxide from cumene: lower molecular weights, tend to give rise to decomposition reactions, which can be of an explosive H C CH3 3 character. CH CH3 O2 H3CC OOH

H3C CH3 Oxidation with hydrogen peroxide Without any catalyst, hydrogen peroxide is not a OOH H3C CH3 particularly reactive oxidizing agent. However, it is CH H3C C CH3 employed in a series of non-catalytic uses, for example O2 to bleach paper and wood pulp, in detergency (in the preparation of perborate and sodium percarbonate) and

VOLUME II / REFINING AND PETROCHEMICALS 663 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

in the environmental field (e.g. for eliminating toxic 1936; Milas, 1937). The Milas reagents are formed by substances in waste water). reaction of metallic oxides with hydrogen peroxide in A completely different use of hydrogen peroxide is a tert-butanol solution and were often used for made in the Ugine Kuhlmann process for the oxidizing olefins to the corresponding vicinal diols, a

production of hydrazine, a process in which H2O2 is reaction that in most cases leads to the formation of an used in the presence of methyl ethyl ketone (MEK) intermediate epoxide that is successively hydrolyzed to and of a nitrile or an amide (Goor, 1992): diol by the acid medium. Although Milas used O NH vanadium(V), chromium(VI) and osmium(VIII), it NH3 H2O2, NH3, MEK very soon became clear that analogous reactions were C C possible, often with better results, using H2O 3H2O H3C C2H5 H3CC2H5 molybdenum(VI) or tungsten(VI). However, the doubt MEK remains that some of the reactions observed might have been caused not so much by the hydrogen H3CC2H5 H2O2, NH3, MEK peroxide as by the tert-butyl hydroperoxide, which, CNCN under the conditions adopted, can be formed by 3H2O reaction of the tert-butanol with the hydrogen peroxide. H5C2 CH3 2H2O Meanwhile, in the 1960s and 1970s, the reasons why it is difficult to use oxygen in selective oxidations N2H4 + 2MEK grew clearer. Thus, many industrial chemists began With respect to traditional processes (Raschig, assessing with interest the oxidizing properties of Bayer), which are based on the oxidation of ammonia hydrogen peroxide and other peroxides that are partly with chlorine or hypochlorite, the process using reduced forms of molecular oxygen. The already hydrogen peroxide avoids the coproduction of very mentioned derivatives of tungsten(VI) and, above all, large quantities of sodium chloride. of molybdenum(VI) were obvious candidates for the Regarding the far more numerous catalyzed role of catalyst and, in fact, they formed the basis of reactions, the first observation of a catalytic effect in the first industrial success in this sector: the oxidation reactions with peroxides dates from the epoxidation of propylene with hydroperoxides (see classic work by Henry John Horstman Fenton, who, in below). 1876, described the capacity of ferrous salts to However, the use of hydrogen peroxide remained catalyze the oxidation of tartaric acid by hydrogen an aim of great interest as it would have made it peroxide (Fenton, 1876, 1894). In its most classical possible to decouple the production of propylene oxide form, the Fenton reagent consists of a mixture of (or of other epoxides) from the coproduction of hydrogen peroxide and ferrous sulphate, but, more tert-butanol or of styrene. Yet the derivatives of generally, the hydrogen peroxide can be activated by vanadium(V) or of molybdenum(VI) and tungsten(VI), any other reducing term of a monoelectronic redox which catalyze the reactions of hydroperoxides couple such as, for example, copper (I). The question extremely well, produced very poor results if used regarding the nature of the active species that comes with hydrogen peroxide. To understand the reason for into play in oxidations with the Fenton reagent (or this failure, it must be considered that the catalytically with systems correlated with this) is still being active species is formed by reaction of the metal debated. The traditional view that it is the hydroxyl derivative with the oxidizing agent, whether this is a radical (OH) has been challenged in recent years and, hydroperoxide or hydrogen peroxide. In the presence instead, the intervention of high-valent iron species of water, however, the latter competes effectively with with oxidation state (IV) or (V) has been claimed (at the oxidizing agent, giving rise to inactive species least for reactions conducted in non-aqueous solvents). whose formation eventually inhibits the desired In any case, oxidations with the Fenton reagent are not reaction. However, water is always present in very selective and, with few exceptions that will be oxidations with hydrogen peroxide, both because the discussed below, they are of little use in organic latter is normally used in the form of an aqueous synthesis. They are applied, instead, in the solution, and because new water is formed as environmental field, for example to purify waters oxidation takes place and the hydrogen peroxide is polluted by phenolic compounds. consumed. Therefore, strategies were sought that Long after Fenton’s pioneering studies, in the would enable the problem of water to be eliminated or, 1930s, Nicholas Athanasius Milas described the at least, its consequences to be lessened. Basically catalytic effect of metals in electronic d0 configuration three approaches were followed: seeking anhydrous in oxidation reactions with hydrogen peroxide of reaction conditions, the application of the organic and inorganic substrates (Milas and Sussman, phase-transfer technique and the development of

664 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

microporous catalysts in which oxidation occurs in a lipophilic cation such as a tetraalkylammonium or relatively hydrophobic environment, such as inside the phosphonium (the phase-transfer catalyst, Q). In the cavities of a zeolite. organic phase, the peroxidic species reacts with the Maintaining an anhydrous reaction environment substrate forming the desired product (SO) and was pursued both by using stoichiometric quantities of regenerating the reduced form of the catalyst. dehydrating agents, and, in a more practical manner, Distribution of this between the organic and the by getting rid of the water through continuous aqueous phases closes the catalytic cycle (Fig. 2). azeotropic distillation. This latter technology Nevertheless, the first attempts by Charles Starks constitutes the basis of the Ugine Kuhlmann process (then with Continental Oil) to epoxidize simple olefins for the epoxidation of propylene, although this has in the transfer phase using tungstic acid and hydrogen never been done at the industrial scale. peroxide encountered little success due to the More unexpected and promising developments difficulty of limiting, under the reaction conditions, came, however, from the application of the phase- the unproductive decomposition of the hydrogen transfer technology and from the discovery of peroxide into oxygen and water. Only later was it microporous titanium silicates. discovered, unexpectedly, that a mixture of tungsten and phosphate (or arsenate) anions in an acid Homogeneous catalysts: phase-transfer processes environment is able to catalyze the epoxidation In the phase-transfer technology, the medium in reaction, provided that a tetraalkylammonium or which oxidation with hydrogen peroxide is conducted phosphonium salt is present in the system as a phase- is formed by two liquid phases that are mutually transfer catalyst (Venturello et al., 1983; Table 2). immiscible, one aqueous and the other organic, formed The reaction is applicable to a large number of by the substrate S possibly dissolved in a suitable olefins: linear, branched or cyclic, including hardly solvent. In the case of oxidations with hydrogen reactive ones such as terminal olefins or allyl chloride. peroxide, an anionic metal derivative reacts in the In all cases, it is carried out under mild conditions aqueous phase with the hydrogen peroxide, forming a (60-90°C and ambient pressure), even when very peroxidic complex that will be the catalytically active diluted (8-15%) aqueous solutions of hydrogen species in the process. This species, also of an anionic peroxide are used. nature, is extracted in the organic phase from a The reaction is stereospecific: trans-2-hexene gives only the trans epoxide and cis-2-hexene gives only the cis isomer. The catalytically active species that are formed from tungsten(VI) under phase-transfer O conditions and in the presence of phosphate or MO Na H2O2 M Na H2O [ ] [ O] arsenate anions belong to a new class of peroxidic water complexes, which are quaternary ammonium (or organic solvent phosphonium) salts of the PW O3 (Fig. 3), or 4 24 O AsW O3 anions (Venturello et al., 1985). These MO QSO M QS 4 24 [ ] [ O] complexes are the first peroxidic derivatives of a heteropolyacid whose structure has been solved. Epoxidations are conducted batchwise and, under Fig. 2. Oxidation under phase-transfer conditions. the reaction conditions, most of the catalyst is dissolved

2 3 Table 2. Epoxidation of olefins with diluted H2O2 catalyzed by WO4 /PO4 mixtures

Temperature Time H O conv. Epoxide yield Olefin pH 2 2 (°C) (min)(%)(%) 1-octene11.1.3 Cleri 1.6ci fig 02 70 45 98 82 1-dodecene 1.6 70 60 97 87 Allyl chloride 2 60 150 96 80 Styrene 3 40 180 93 77 a-methylstyrene 4.5 40 240 93 79 Cyclohexene 3 70 25 98 88

VOLUME II / REFINING AND PETROCHEMICALS 665 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

3 Fig. 3. The PW4O24 anion.

in the organic phase constituted by the olefin and and is a key intermediate in the synthesis of the possibly by a solvent (generally toluene or a chlorinated nootropic drug oxiracetam: hydrocarbon). At the end of the reaction (indicated by O O cat. the more or less complete disappearance of the O C H2O2 C hydrogen peroxide), the two phases easily separate and O H2O O the product is recovered with conventional techniques, generally by distillation. Typical yields, based on HO hydrogen peroxide, vary between 80 and 95%, and any O excess olefin is recovered unchanged: generally, in fact, N selectivity on the olefin is around 95%. Even CH2CONH2 sophisticated substrates, of pharmaceutical interest, can be epoxidized with excellent yields. A block diagram of Although epoxidation is the most interesting of the the epoxidation process of a typical olefin is shown in reactions promoted by these new catalysts, they are Fig. 4. able to use hydrogen peroxide efficiently for numerous At the end of the reaction, the catalyst is recovered, other transformations that are also of significant with small quantities of by-products, as distillation interest, including oxidations of primary alcohols and residues (or by ultrafiltration of the organic phase) and of aldehydes to form carboxylic acids, of secondary can be regenerated and partly recycled. alcohols to give ketones or of sulphides to give The epoxidation reaction has been developed on a sulphoxides (Ricci, 1996). Furthermore, an accurate commercial scale for the production of choice of reaction conditions makes it possible to 1,2-epoxydecane and of isobutyl 3,4-epoxybutyrate. obtain, in addition to epoxides, a whole series of other The latter, in particular, has been produced on the olefin derivatives, often difficult to prepare by other scale of 100 t/y by epoxidation of isobutyl vinylacetate methods. Thus it is possible to transform olefins into

olefin Fig. 4. Epoxidation of olefins with hydrogen olefin peroxide under phase-transfer catalystepoxidation phase distillation epoxide conditions. separation

H2O2 H2O residue

666 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

which followed the discovery of micro- and olefins mesoporous titanium-silicates. In particular, certain titanium-zeolites are to be included among the most promising oxidation catalysts. Their activity and selectivity, often very high, and their intrinsic stability epoxides 1,2-diols 1,2-hydroxyketones against oxidative degradation make them ideal catalysts for industrial applications. These titanium- zeolites are part of the more extensive family of mixed carboxylic 1,2-diketones oxides containing a transition metal at high dispersion, acids whose structure can vary from microporous crystalline (redox zeolites) to wholly amorphous, with an intermediate situation represented by amorphous metal Fig. 5. Products obtainable from oxidation of olefins silicates, with an orderly system of pores (MCM-41, in phase transfer. MCM-48, HMS). Among these titanium-silicates, the first, in order of time and importance, is titanium-silicalite-1 (TS-1), vicinal diols or into 1,2-hydroxyketones. Both of these discovered in the late 1970s (Taramasso et al., 1983). classes of compounds can be oxidized, in their turn, to This is a microporous crystalline titanium-silicate, give 1,2-diketones. Lastly, diols or even the original with an MFI structure (Baerlocher et al., 2001) which olefins can be oxidatively cleaved with the formation derives formally from silicalite-1 (S-1), wholly of carboxylic acids (Fig. 5). siliceous, by the substitution of part of the silicon with Recently, papers have been published which seem titanium, up to a maximum of around 2.5%. It is to suggest that, under appropriate conditions (e.g. in obtained by hydrothermal synthesis, from the

the complete absence of any chlorinated species), the progressive condensation of SiO4 and TiO4 tetrahedra performances of tungsten and phosphorus based around the template ion (i-C3H7)4N (Fig. 6). Its peroxide catalysts can be equalled by tungstic acid porosity is due to a three-dimensional system of alone or, perhaps, by polymeric species produced by interconnected channels, having an average diameter this in the reaction environment. However, peroxidic of about 0.55 nm. This is an important parameter, as it phosphotungstates retain the merit of having made sets an insuperable limit to the diffusion of reagents in possible the first commercial applications of the active titanium sites and thus to the feasibility of oxidations with hydrogen peroxide catalyzed by d0 the oxidation process. metals and, more generally, that of having rekindled Materials with an analogous composition, but of interest in their study, then further revitalized by the different structure, have been prepared by incorporating discovery of the new rhenium(VII) based catalysts, titanium in the S-2 (titanium-silicalite-2, TS-2), BEA

especially CH3ReO3 (Herrmann et al., 1991). (titanium-b, Ti-b), MOR (titanium-mordenite, Ti-MOR), and MWW (Ti-MCM-22) structures, and Heterogeneous catalysts: micro- and mesoporous others of less interest for catalysis. Mesoporous titanium-silicates titanium-silicates (Ti-MCM-41, Ti-MCM-48) are also As already mentioned, another successful approach known. Apart from titanium, other metals with redox in the field of oxidation with hydrogen peroxide is that characteristics have been incorporated into the

Fig. 6. Formation and growth of the crystal lattice of TS-1.

Si Si Ti Si Si Si Si Ti Si Si O O O O O O O O O H2O2 H H O SiO4 O O O O O O O O Si Si Si Si Si Si Si Si Si Si TiO4

VOLUME II / REFINING AND PETROCHEMICALS 667 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Table 3. Some zeolites structurally substituted with transition metals. Those most studied for catalytic applications are in bold type

Name Structure Metals Size of pores (Å) Dimensionality

TS-1 MFI Ti, Fe, V, Zr, Cr 5.6·5.3 3 TS-2, TS-3 MFI/MEL Ti, Fe, V 5.3·5.4 3 TS-12 MTW Ti, Fe 5.9·5.5 1 Ti-FER FER Ti, V 5.4·4.2 2 Ti-MWW MWW Ti 5.4·4.0 2

Ti-b BEA Ti, Fe, Sn, Cr 6.4·7.6 3 Ti-MOR MOR Ti 6.5·7.0 2 Ti-SSZ-33 CIT-1 Ti 6.4·7.0 3 Ti-UTD-1 DON Ti, V ca. 10 1 Ti-MCM-41 – Ti, V, Sn, Mn 20-100 1 Ti-MCM-48 – Ti, V, Cr 20-100 3

structures of many zeolites. Table 3 offers a selection of Reactivity tests, too, can help in the the materials described in scientific and patent characterization process. The hydroxylation of phenol, literature. The latter form a relatively small group for example, is very sensitive to the presence of compared with the number of materials proposed, the extraframework species and has been proposed as a majority of which, however, under reaction conditions, supplementary test in this regard (see below). are not sufficiently stable and in some cases are actually lacking in any catalytic activity. Physico-chemical properties and catalytic properties of titanium-zeolites Physico-chemical characterization of mixed oxides Catalytic properties depend on many factors, some A crucial factor for catalytic action is the location of which (position of the titanium, morphology of the of the metal in the zeolitic structure. It is thus essential crystals) have already been mentioned. In addition to to establish whether it is inserted in the crystal lattice these are the geometry and the surface properties of in atomic dispersion or is deposited on the surface in the pores, which impart to the titanium-zeolites the the form of discrete particles of oxide. It is equally properties of molecular sieves (i.e. the capacity to important to be able to exclude the presence of discriminate the compounds to be adsorbed on the occluded amorphous phases and to determine the basis of their dimensions and physico-chemical morphology of the crystals (dimension and shape). properties). The relations between surface properties Before any catalytic study, the physico-chemical and catalytic performance may be summed up as characterization of the potential catalyst must follows: therefore be accurately established. • Only the titanium present in the structure is Unfortunately no single technique exists that is catalytically useful. Titanium in an external both simple and at the same time able to provide clear position is actually harmful due to its capacity to answers (Boccuti et al., 1988; Millini and Perego, catalyze parasite reactions, such as the 1996). Generally, it is necessary to resort to the decomposition of the hydrogen peroxide and combined use of different techniques to obtain radical oxidation pathways. An analogous criterion sufficient certainty as to the quality of the catalyst. For applies for zeolites containing metals other than titanium-silicalite and the other titanium-zeolites, the titanium. most commonly used techniques are X-Ray • The titanium present in the structure confers to the Diffraction (XRD), UltraViolet absorption (UV), FTIR silicalite-1 very different catalytic properties from and Raman spectroscopy, X-ray absorption those conferred by vanadium, by iron and by any spectroscopy (EXAFS and XANES) and electron other transition metal (see below). microscopy (SEM, TEM). • The dimensions and geometry of the pores give

668 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Fig. 7. Chemical and steric properties of active sites of TS-1. Si Si Si Si Si O Si O O O OH2 H2O O H2O/H2O2 O OH2 Ti Ti Ti O OH O O O OOH OH2 O OH2 O Si Si Si Si Si Si H H

rise to what is termed shape selectivity (i.e. the R capacity of the material to discriminate the O H reagents and products on the basis of their shape O H and size; see below). Ti O H Ti • Selective adsorption phenomena, governed by the O O nature of the surface, concentrate specific AB components of the reaction mixture in the channels, in the vicinity of the catalytic sites, Fig. 8. Structure of catalytically active species decisively influencing the course of the reaction of TS-1. (activity, selectivity, deactivation). The capacity of TS-1 to adsorb selectively non-polar compounds, even in the presence of water and other polar and R R protic substances, explains the apparent difference O H O H of the catalytic properties of titanium in TS-1 and in soluble alkoxides. The latter are more or less Ti O H Ti O H inactive in an aqueous solution of hydrogen O O peroxide, that is under the conditions of maximum activity of TS-1. The phenomena that take place on the catalytic site are closely connected, on the one hand, with the Fig. 9. Mechanism of epoxidation tendency of framework titanium to expand its sphere catalyzed by TS-1. of coordination from tetrahedral to octahedral, chemiadsorbing polar molecules, and, on the other hand, with the poor resistance to hydrolysis of a oxidation processes with organic hydroperoxides. The TiOSi unit (Fig. 7; Bellussi et al., 1992; Clerici and other metal-zeolites are active for a more limited Ingallina, 1993). spectrum of reactions. Those with large pores and In accordance with this, the water and the alcohols mesoporous materials, however, permit the use also of are coordinated on the titanium and reversibly organic hydroperoxides as oxidants. hydrolyse a TiOSi bond, producing TiOH and TS-1, but also TS-2, Ti-b, Ti,Al-b and Ti-MWW SiOH species. The hydrogen peroxide behaves in catalyze the epoxidation of olefins, in a diluted like manner, producing the TiOOH species, which solution of hydrogen peroxide (10%) and at is at the origin of the oxidizing properties of the temperatures below 80°C. TS-1 already shows a good

TS-1/H2O2 system. The structures illustrated in Fig. 8 level of activity even below 0°C. In fact, epoxidation is have been proposed for the TiOOH species (for the the preferred reaction when other reactive groups are sake of clarity, the other ligands have been omitted; for present in the molecule. In this regard, these, see again Fig. 7). The structure represented in A chemoselectivity can be extremely high. is that which has the best chances of existing under the As opposed to what has been observed in the case conditions suitable for catalysis (i.e. in the presence of of soluble titanium catalysts, the solvents that favour water and/or of an alcohol). high reaction rates are protic and polar: methanol TS-1 catalyzes the oxidation of many organic (TS-1, TS-2, Ti-b, Ti,Al-b), acetone (TS-1) and functions with hydrogen peroxide, and in particular the acetonitrile. In a moderate quantity, water has a minor epoxidation of olefins and the hydroxylation of effect on yields and selectivity. On the other hand, its paraffins and aromatic compounds, as well as the presence in the reaction environment cannot be oxidation of alcohols, ethers and various sulphur and avoided, being added with the oxidant and produced nitrogen compounds. On the contrary, it is not active in by the same reaction. The decomposition of hydrogen

VOLUME II / REFINING AND PETROCHEMICALS 669 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

• The rate of reaction diminishes rapidly in the TS-1: order: TS-1Ti-bTi-MCM-41, becoming more 1.0 4.7 6.0 16.2 or less negligible on the mesoporous catalyst. Ti,Al-β: • Furthermore, Ti-b, Ti,Al-b and Ti-MCM-41 show, 1 1.1 1.6 1.6 to an increasing extent, a structural instability in aqueous hydrogen peroxide. The drawback is CH CO H: R1 3 3 AcO R R common to other metal-zeolites, in particular those 0.04 1 20-24 27 240 containing vanadium, and takes the form of a Fig. 10. Relative rate of epoxidation of some substituted progressive collapse of the crystal lattice, with the olefins with various oxidants. release of the metal in solution. • The mesoporous materials available have a substantially hydrophilic surface. This peroxide is generally slow with the catalysts indicated circumstance favours the adsorption of water to the or even negligible with TS-1. detriment of the olefin or other non-polar reagent, Yields and selectivity are generally high, and thereby lowering the reaction rate. Unfortunately, values close to 100% can be reached with TS-1, in water cannot be avoided in using hydrogen accordance with the heterolytic mechanism (Fig. 9; peroxide as the oxidant. Clerici and Ingallina, 1993; Neurock and Manzer, • A possible alternative is provided by the use 1996). of tert-butyl hydroperoxide (TBHP), for which the By-products of the reaction are generally products activity increases in inverse order: of the hydrolysis of epoxide. Their formation may be Ti-bTi-MCM-41 while TS-1 is practically reduced, neutralizing the residual acidity of the inactive for steric reasons. catalyst, with the addition of small amounts of bases in The above conclusions, therefore, do not imply any the reaction environment. decreasing activity of titanium sites with increasing All zeolitic catalysts, not only oxidation ones, pore size, but rather a decrease in the olefin adsorption possess shape selectivity: only olefins capable of capacity. It is the density of the surface SiOH being diffused inside the pores can be oxidized with groups and thus the hydrophilic nature of the pores high reaction rates. This explains the different that increases in the TS-1Ti-bTi-MCM-41 series, reactivity of cyclohexane on TS-1 (very poor) and on in parallel favouring the adsorption of water at the Ti,Al-b (good), as its molecular size is very close to expense of the olefin (or some other non-polar the pore diameter of the former and significantly reagent). The instability, too, is to some extent a smaller than that of the latter. More generally, shape consequence of the increase of the SiOH groups, selectivity completely reverses the order of reactivity which may be regarded as defective sites of the foreseen on the basis of the electronic properties siliceous matrix and the starting point of the hydrolytic of the double bond. Fig. 10 compares olefins with phenomena that cause the decay of the matrix.

a different steric size in epoxidation with TS-1/H2O2, TS-1 catalyzes the hydroxylation of paraffins, at Ti,Al-b/H2O2 and with peracetic acid, which is not temperatures below 100°C, in an aqueous or significantly affected by steric restrictions (the methanolic solution of hydrogen peroxide (Huybrechts numbers associated with olefins indicate their relative et al., 1990; Clerici, 1991): reactivity). The steric effects of the substituents on the OH OH double bond clearly prevail over the electron effects TS-1 with heterogeneous catalysts, whereas the opposite is the case for peracetic acid. H2O2 While the activity, the selectivity and the mild TS-1 H2O2 TS-1 H2O2 reaction conditions of the TS-1/H O system make it a 2 2 O O preferable alternative to conventional oxidants, the steric restrictions imposed by its microporous nature set evident limits to the range of its potential It may seem surprising that commonly inert applications. Ti-b and the other titanium-zeolites with compounds such as paraffins, even in a diluted large pores provide a partial solution to the problem. solution, can be oxidized preferentially with respect to The synthesis of mesoporous catalysts (Ti-MCM-41, the solvent methanol. The reason for this is probably Ti-MCM-48 and suchlike) could be an answer to the due to the hydrophobic nature of the catalyst, able to need. However, the results obtained with hydrogen selectively adsorb the paraffin in the vicinity of the peroxide are not very encouraging. The conclusions of active sites in accordance with the criteria already these studies may be summed up as follows: stated for the epoxidation of olefins. The same reason

670 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

likely explains the poor activity, or the lack of activity, the competition of the decomposition of hydrogen of the large-pore titanium-zeolites and of mesoporous peroxide has recently been proposed (Clerici, 2001). catalysts, for which the order of reactivity, already The titanium-zeolites resemble other catalytic seen for olefins, now appears far more accentuated. systems in the oxidation of primary and secondary In competition with hydroxylation, the decomposition alcohols, except for the effects of steric restrictions of the hydrogen peroxide also takes place, generally to a (Maspero and Romano, 1994). The products are notable extent. Only secondary carbons undergo aldehydes and the corresponding ketones, to which, in oxidation, the methyl groups being inert, which produces the case of the primary alcohols, the carboxylic acids a mix of secondary alcohols and ketones (formed by formed by consecutive oxidation can also be added consecutive oxidation). The vanadium-silicalites, instead, (Fig. 11). The primary alcohols are not as easy to are active also for the hydroxylation of the primary oxidize as the secondary ones. It is worth noting the carbons, forming a mix of primary and secondary scanty reactivity of methyl alcohol, which makes it alcohols and products of successive oxidation. suitable as a solvent for the other oxidations. As far as the aromatic compounds are concerned, Ketones are generally stable under the oxidation TS-1 and Ti-MOR catalyze their hydroxylation to the conditions of the secondary alcohols. They can, corresponding phenols (Romano et al., 1990): however, be oxidized to lactones and the corresponding esters, selecting the appropriate catalysts. Sn-b proves OH to be the most selective catalyst in this context, with TS-1 TS-1 OH OH values that can reach as high as 98% (Corma et al., H2O2 H2O2 2001). Contrary to the case of the titanium-zeolites, if double bonds are present, they do not undergo HO HOOH OH epoxidation, as exemplified by the oxidation of dihydrocarvone (Fig. 12). The reason for this must be The reaction follows the rules of electrophilic sought in the different mechanism, which for Sn-b is attack: the reactivity of phenol and toluene is high, based on the Lewis acid properties of the active site, while that of nitrobenzene, chlorobenzene and benzoic instead of redox properties as in the case of Ti-b. acid is negligible. In alkylbenzenes, the oxidation of In the oxidation of amines, various products can be the alkyl groups competes with that of the aromatic obtained, according to the nature of the amine and of nucleus, except for the methyl groups, which are the reaction conditions: almost completely inert. In this case as well, the behaviour of the vanadium-silicalites is different, as ArNHOH NH2 NHOH NO they preferentially catalyze the hydroxylation of the alkyl side chains, including methyl groups. ArNHOH NN The hydroxylation of paraffinic and aromatic compounds is regulated by severe steric restrictions O imposed by the size of the pores. The kinetics of the RCH2NH2 RCH NOH RCHO hydroxylation of toluene is already comparable with that of benzene, in spite of the fact that it is more From aniline, when hydrogen peroxide is scarce, nucleophilic. A second consequence of the steric the final product obtained is azoxybenzene, while with effects is that the decomposition of hydrogen peroxide an excess of oxidant, the main product is nitrobenzene can become the predominant reaction when the (not shown). Oximes, alkyl-hydroxylamines and other substrates are bulky molecules. products are formed in the oxidation of the primary The mechanisms of hydroxylation reactions have and secondary aliphatic amines. Also in the case of the not been studied in depth. However, there is strong oxidation of amines, there is a different selectivity on evidence of their homolytic nature, causing them to the various metal sites. In the oxidation of aniline, for differ from epoxidation, which is heterolytic. A example, the vanadium-silicalites form mainly mechanism able to explain both the hydroxylation and nitrobenzene even when hydrogen peroxide is scarce.

Fig. 11. Mechanism of alcohol RCH H oxidation catalyzed by TS-1. 2 H R O H O C OH H 2 Ti O H Ti O H Ti H RCHO O O O

VOLUME II / REFINING AND PETROCHEMICALS 671 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

O O the future, epoxidation with hydrogen peroxide, now in Sn the development stage, could be added to these processes. O O The manufacture of propylene oxide, which, O among the higher epoxides, is by far the most H2O2 O O important commercially, will be examined below. The production capacity for propylene oxide (PO) O in the year 2000 globally amounted to 5.7·106 t, with an annual demand of around 4.7·106 t. It is estimated R that about 7% of the propylene produced is used for O O O H the production of this derivative. Propylene oxide is a basic intermediate of the Ti O H O OH O chemical industry, used for producing a long series of OH commercial products. For this purpose, it must first be transformed into a number of its derivatives: polyether polyols (65%), propylene glycols (20%), and other intermediates such as glycolethers, alkanolamines, Fig. 12. Different oxidation mechanism of Sn-b and Ti-b 1.4-butanediol, allyl alcohol and propylene carbonate. catalysts. Polyether polyols, produced by the reaction of propylene oxide with polyhydric alcohols, are used in the production of polyurethane foams, of flexible type (applied, for The thioethers are easily oxidized to the example, in the automobile industry for seats, or for corresponding sulphones and sulphoxides: household or furnishing purposes for mattresses and carpets) or of rigid type (used, for example, for heat [12] RSR RSOR RSO R 2 insulation). Propylene glycol (1.2-dihydroxypropane) The reaction, initially hardly considered due to the is used in the production of polyester resins and, due to ease with which the thioethers oxidize, may take on its biocompatibility, in the formulation of cosmetic, importance as a means of removing sulphur from pharmaceutical and food products. For the same reason, motor fuels. It is clear that in this application the most dipropylene glycol and other short chain oligomers are suitable catalysts are those with a mesoporous used to produce antifreeze fluids, hydraulic fluids, structure, and consequently the preferred oxidant is cutting oils and lubricants. As opposed to analogous tert-butyl hydroperoxide. products derived form ethylene oxide, in fact, they are biodegradable and harmless to living organisms. Epoxidation of olefins Mention should also be made of certain uses of alkanolamines (detergents, anticorrosion products), of Industrial production of epoxides polyethers (antifoam agents, surface-active agents) and Among the products that can be obtained by of esters (solvents for inks and paints). oxidation of olefins, epoxides have always received special attention due to their high reactivity, which Chlorohydrin process makes them extremely useful (and used) intermediates Introduced about a century ago, the chlorohydrin in organic synthesis. process is still widely applied. Propylene is reacted As often happens, the simplest epoxide, ethylene with chlorine in an aqueous solution, producing a oxide, is produced with a method (i.e. direct oxidation of mixture of two chlorohydrins, from which propylene ethylene with oxygen in the gas phase) which differs oxide is obtained by treatment with lime or soda from that used for all the other ones. The higher epoxides, (global yield around 89%): instead, are prepared under milder conditions and with ula 9 more complex processes. In fact, at the relatively high Cl OH temperatures required by the use of molecular oxygen, in CH CHCH H O Cl n CH CH CH 3 2 2 2 3 2 addition to double-bond, also allylic C H are oxidized OHOH Cl Cl with the consequence of obtaining a broad spectrum of products. For this reason, the higher epoxides, when not (1(1 n)n CH) CH3 3 CHCH CHCH2 2 HClHCl obtained through oxidation of the corresponding olefins Cl OH OH Cl with percarboxylic acids (a troublesome reaction, also n CH CH CH (1n) CH CH CH NaOH from the standpoint of safety), are generally prepared by 3 2 3 2 oxidation of olefins with chlorine (through the CHCH3 3 CHCH CHCH2 2 NaClNaCl HH2O2O corresponding chlorohydrins) or with hydroperoxides. In OO

672 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

The chlorohydrin process is burdened by CH3 CH3 CH3 a series of problems, with environmental CH C H CH C OOH CH C OH implications, such as the formation of inorganic 3 3 3 and organic chlorinated coproducts and by- CH3 CH3 CH3 products. In fact, a large part of the chlorine used eventually forms a stoichiometric quantity of CH3 hydrogen chloride, which is neutralized CH3 C OOH CH3 CH CH2 – a process taking place simultaneously with the formation of epoxide – using lime or soda. CH3 CH3 A considerable quantity of organic chlorinated by-products is also formed, which CH3 CH CH2 CH3 C OH O uselessly consume propylene and complicate CH3 downstream operations. The effluent, consisting of a diluted solution of In the ARCO process, six stages are distinguished: sodium or calcium chloride and chlorinated oxidation of the isobutane, epoxidation of the organic impurities, must be properly treated and propylene, separation of the products, purification of then disposed of. A further problem arises from PO, purification of TBA, preparation and recovery of the risks of corrosion and hence from the need to the catalyst. In the first stage, isobutane is oxidized in use resistant materials. In spite of this, the the liquid phase with oxygen, at around 140°C under chlorohydrin process is economically viable, able pressure, producing a mixture of tert-butyl to satisfy approximately one-half of the world hydroperoxide, tert-butanol and various by-products, market of propylene oxide. such as acetone and carboxylic acids. The reaction is run in recycled tert-butanol and is a typical Epoxidation with hydroperoxides autoxidation requiring no catalyst, although the The processes in which an organic presence of initiators speeds up the initial phases. For hydroperoxide acts as the oxidizing agent were conversion values close to 35%, the tert-butyl originally developed by Halcon, ARCO hydroperoxide and tert-butanol selectivities are 53% and Shell and were applied in the industry as of and 40%, respectively. the 1970s. These processes produce approximately The stream from the autoxidation reactor is the other half of the propylene oxide marketed subjected to fractionation to remove the isobutane, in the world. They are liquid-phase oxidation diluted with recycled tert-butanol to bring the processes, in which the catalysts may be soluble concentration of TBHP to 40% and, after adding the (Halcon/ARCO) or supported (Shell). molybdenum catalyst solution, fed to the The oxidants are TBHP (tert-butyl epoxidation reactor. A large excess of propylene is hydroperoxide) or EBHP (ethylbenzene supplied to maximize yields and diminish reaction hydroperoxide or (1-phenyl)ethyl hydroperoxide), times. The reaction is conducted at about obtained by oxidation with air of isobutane or 120°C and under pressure, with up to 98% ethylbenzene, respectively. selectivity with respect to propylene. The alcohols coproduced, tert-butanol (TBA) Among the by-products are methyl formate or 1-phenylethanol, are used for the production of (particularly unwelcome since its boiling point is MTBE (methyl tert-butyl ether) and styrene, close to that of epoxide), carbonyl compounds, respectively. carboxylic acids and propylene glycol. The The commercial advantage of propylene oxide successive phases include the separation of the produced in this way depends largely on their products from the excess propylene (to be recycled), value. the recovery of the molybdenum and the purification A third process also exists, introduced recently by of the propylene oxide and of the tert-butanol. The Sumitomo, in which the oxidant is cumyl process is characterized by the production of PO and hydroperoxide and the alcohol coproduced is recycled tert-butanol in a ratio of 1:2.4. instead of being placed on the market. In a second version of the process, originally developed by Texaco, epoxidation is carried out in two Epoxidation processes with TBHP successive stages at 110°C and 135°C. This second Two versions of the process exist, as used by version differs from the preceding one also in the Lyondell (formerly by ARCO) and by Huntsman methods of preparing the catalyst and the separation (formerly by Texaco). The main reactions are the and purification of the products, as well as in the lower same: PO/tert-butanol ratio.

VOLUME II / REFINING AND PETROCHEMICALS 673 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Epoxidation processes with EBHP Oxidation process with cumyl hydroperoxide Two processes exist in which the oxidant is The process developed by Sumitomo, based on (1-phenyl)ethyl hydroperoxide, differing in type of catalyst: cumyl hydroperoxide as the oxidant, is characterized by the fact that the co-product, cumyl alcohol, is CH CH CH 3 3 3 dehydrated and hydrogenated to cumene and then CH C OOH COH recycled. The hydroperoxide route, in this particular case, avoids having to place another product on the H H H market. Furthermore, it is characterized by its CH3 temperature and pressure conditions, significantly milder than in other hydroperoxide processes. So far it C OOH CH3 CHCH2 has been applied only in a single plant in Japan, which H has been in operation since 2003 (200,000 t/y). CH3

CH3 CH CH2 COH Epoxidation with hydrogen peroxide catalyzed by TS-1 O H This process differs from the preceding ones in its use of hydrogen peroxide as the oxidant, made possible In the ARCO (now Lyondell) process, a soluble by adopting titanium-silicalite as the catalyst (Clerici et compound of molybdenum is used, whereas in the al., 1991). The coproduct in this process is merely water, process developed by Shell, a silica supported which makes it unnecessary to market or recycle it, and

titanium(IV), Ti/SiO2, is used. The ratio of the products offers indisputable environmental advantages due to the PO/styrene, instead, is similar and close to 1:2.2. absence of chlorinated by-products. In the first stage of both processes, the Epoxidation is conducted in aqueous methanol at a hydroperoxide solution is prepared by oxidation of temperature below 60°C and pressures slightly above 1 ethylbenzene with air at about 150°C and a pressure of atm. The reaction rate is high, with a TOF (Turn Over about 3.5 bar. For safety reasons, conversion is limited Frequency) of 1-2 s1 (measured in the laboratory at to values of less than 10%. Selectivity is 80-85% with 40°C). Its selectivity with respect to hydrogen respect to hydroperoxide, the remainder consisting peroxide is nearly quantitative. Hydrogen peroxide mostly of 1-phenylethanol and acetophenone. The consumption, due to methanol oxidation and to EBHP concentration required for the epoxidation decomposition into water and oxygen, is negligible. reaction (17-19%) is reached by removing the excess The by-products are propylene glycol and its two of ethylbenzene by distillation. monomethyl ethers, produced by the attack of Epoxidation is conducted at 100-115°C. In the methanol on the epoxide ring. The selectivity,

Shell process, the heterogeneous catalyst Ti/SiO2 generally high, can be made nearly quantitative makes it possible to adopt a packed fixed-bed reactor. (98%), by keeping the acidity of the environment A notable feature of this catalyst is its stability under control with the addition of small quantities of regarding the release of soluble species in solution. On bases (in the order of some ppm). the contrary, under analogous reaction conditions, The reaction takes place by contacting, in a slurry supported molybdenum, tungsten or vanadium reactor, a stream of propylene with a suspension of catalysts release soluble species. The regeneration of TS-1 in methanol/water, into which an aqueous

the deactivated catalyst Ti/SiO2 takes place by solution of hydrogen peroxide is fed (Romano, 2001). oxidizing the organic deposits with air. The high activity of the catalyst in a diluted solution What characterizes the processes with EBHP is the makes it possible to operate with oxidant stage dedicated to the production of monomeric concentrations of even less than 10%. The effluent is styrene. The mixture of 1-phenylethanol and subjected to distillation to recover the unreacted acetophenone, recovered from the effluent of the propylene, the propylene oxide and the methanol. The epoxidation reactor, is dissolved in triphenylmethane residual aqueous solution, after possible recovery of and subjected to dehydration at a high temperature in the propylene glycol and the monomethyl ethers, is the presence of an acid catalyst. After the separation of conveyed to an ordinary biological plant for final the styrene, the residue containing acetophenone is treatment. conveyed to a hydrogenation reactor, after which the Proper control of the operating conditions enables additional 1-phenylethanol rejoins the stream sent for the phenomena of catalyst deactivation, caused by the dehydration. Selectivity exceeds 98%. depositing of heavy organic by-products, to be From the standpoint of overall performance, in minimized. These by-products can be removed by both the ARCO and the Shell process the yields of washing with solvent at temperatures higher than epoxide are nearly 92% and those of styrene are 94%. 100°C or, simply, by combustion with air. No

674 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

CH3 O CH3 CH OH Ti (i-PrO)4 HCHOH H CCH H3CC OOH 2 H3COHC 2 2 D - (-) - DET H H CH3 (R)-glycidol CH3

H HO COOC H C 2 5 D - (-) - DET C HO H COOC2H5

Fig. 13. Enantioselective epoxidation of allyl alcohol.

phenomena of deactivation due to the leakage of The epoxidation of allyl alcohol was one of the titanium from the crystal lattice have been observed. first enantioselective epoxidation processes Compared with the chlorohydrin and developed at industrial scale (by ARCO; Fig. 13). hydroperoxide processes, the new technology is Using the tartaric esters of the one form or the characterized by its low environmental impact, a other, both the enantiomers of glycidol can be simpler process layout and reduced investment costs. obtained with enantiomeric excess (ee) between 91 For example, there is five times less aqueous waste and 95%. and it is suitable for normal biological treatment. A The isolation of the product is greatly facilitated by prototype plant of about 6 t/d capacity was set up by transforming in situ the glycidol (a labile, water EniChem in 2001. soluble product) into the corresponding m-nitrobenzenesulphonate, which, after crystallization, Enantioselective epoxidations is recovered with ee99%. Some rather peculiar reactions, enantioselective More or less at the same time as Sharpless’s epoxidations, will now be addressed, which have found enantioselective epoxidation, another efficient various industrial applications, although mostly in the procedure was published, using hydrogen peroxide as production of small quantities of compounds having the oxidizing agent. This was the Juliá-Colonna extremely high value added, typically used by the epoxidation, which uses polyamino-acids as chiral pharmaceutical industry. catalysts, typically polyalanine (Juliá et al., 1982). The The first procedure for enantioselective epoxidation enantiomeric excesses are very large (up to 96%), but with high enantiomeric excesses was published in 1980 limited to the epoxidation of a number of a, by Barry Sharpless (then at Stanford University), which b-unsaturated ketones. consists in the reaction of primary allyl alcohols with a New and important contributions followed. In hydroperoxide (e.g. tert-butyl hydroperoxide or cumyl 1988, it was again Sharpless who introduced hydroperoxide), promoted by stoichiometric quantities asymmetric dihydroxylation (which actually produces of an alkoxide of titanium(IV) and of enantiomerically diols and not epoxides), catalyzed by osmium pure diethyl tartrate (Katsuki and Sharpless, 1980). The tetroxide and chiral amines; then, in 1991, a protocol active species is formed in situ from the titanium that was defined for asymmetric epoxidation (catalyzed by simultaneously binds the enantiomerically pure tartrate, manganese complexes with chiral Schiff bases), which the hydroperoxide and the substrate (the latter through is applied to a large number of olefins and no longer the OH group). The synthetic versatility of the allyl only to allyl alcohols. In both cases, however, the alcohols caused the reaction to rapidly become a oxidants used are no longer peroxides, but are amine

synthetic instrument of primary importance. The road oxides, K3[Fe(CN)6] or sodium hypochlorite. towards industrial applications was further eased when, Enantiomerically pure epoxides, prepared by the in 1986, Sharpless discovered that, in the presence of one procedure or the other, are used in the synthesis of molecular sieves, it was possible to run the oxidations various pharmaceutical products (e.g. b-blocking or using catalytic quantities (5-10% in mols) of the antiviral drugs) and of products for agriculture titanium-tartrate complex. (pheromones).

VOLUME II / REFINING AND PETROCHEMICALS 675 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Oxidation of aromatics the oxidant and iron containing ZSM 5 zeolites as catalysts: Industrial production of phenol ormula 13 OH Phenol is one of the main chemical intermediates Fe-zeolite with increasing world production, amounting to about N2ON2 8·106 t in 2004. Above all, it is used in the production of intermediates for polycarbonates and epoxy resins, Oxidation, run at 350°C, enables high rates of phenolic resins (used, for example, in adhesives) and conversion of benzene (27%) to be achieved with 98% caprolactam, the raw material of Nylon 6. selectivity with respect to phenol (Panov, 2000).

At present, nearly all the global phenol production Despite this, the use of N2O can hardly be applied on a is based upon the cumene process, which is also the large scale; in fact, its supply is not adequate for main way of industrially producing acetone: for every phenol synthesis and its production for this purpose 10 t of phenol, in fact, about 6 t of acetone are (e.g. by pyrolysis of ammonium nitrate or by selective coproduced. The cumene process foresees three oxidation of ammonia) would be too costly. Thus, the stages: reaction of benzene with propylene only plausible scenario for the application of the (alkylation), which gives isopropylbenzene (cumene); Solutia process is that of foreseeing its implementation oxidation of the cumene to the corresponding side-by-side plants for adipic acid (an important hydroperoxide; acid decomposition of the intermediate in the nylon cycle; Bellussi and Perego,

hydroperoxide, which produces phenol and acetone: 2000). N2O co-produced in the latter, instead of being eliminated, could be used for the production of phenol, H3CCH3 CH practically at zero cost. A viable alternative to N2O is hydrogen peroxide: CH cat. O2 H3C CH2 OH cat. H3C H2O2 H2O H3C OOH C OH O The main problem encountered in the direct O H SO 2 2 4 C oxidation of benzene to phenol (not only with hydrogen peroxide) is the inadequate kinetic control of H3CCH3 the reaction: phenol, in fact, is oxidized more readily The first reaction, the alkylation of benzene, is than benzene and, therefore, rather than being catalyzed by acids: traditionally aluminium trichloride accumulated, is transformed into a whole series of or supported phosphoric acid. Recently, however, further oxidized products (catechol, hydroquinone, innovative processes have been developed that foresee benzoquinone, etc.), up to the complete degradation of the substitution of these acids with appropriate zeolites its cyclic structure or the formation of polymeric tars. which have a very reduced environmental impact Indeed, the ease with which phenol is oxidized with (Bellussi and Perego, 2000). hydrogen peroxide is exploited in the production of Also thanks to these improvements, the cumene mixtures of catechol and hydroquinone (see below). process is fully satisfactory in many aspects. Therefore, to obtain phenol selectively by the Nevertheless, any increase in the productive capacity oxidation of benzene, strategies must be found that of phenol implies the need to find commercial outlets enable the reactions of overoxidation to be slowed for a corresponding quantity of acetone. down, allowing the desired product to be accumulated. A first route to avoid such a need consists in The first step in this direction was taken by George recycling the acetone, transforming it back into Olah who, working with extremely concentrated propylene or, more briefly, into isopropyl alcohol, hydrogen peroxide (98%) in a superacid environment which is also able to alkylate the benzene (Girotti et (FSO3H-SbF5 1:1) at 78°C, obtained phenol with a al., 2003). yield of 54% based on both hydrogen peroxide and on The processes of direct oxidation of benzene into benzene (Olah and Ohnishi, 1978). Although the phenol provide a more effective and definitive prohibitive reaction conditions meant that this work solution, which would completely eliminate the can only be of historical value, the idea that in a coproduction of acetone. The first of these processes is superacid medium phenol could be protonated (and known as the Solutia process, from the name of the hence deactivated towards subsequent oxidations) is American company that recently finalized it in most definitely noteworthy. collaboration with the Boreskov Institute of Catalysis A second approach towards reducing the rate at

of Novosibirsk (Russia), using nitrous oxide, N2O, as which phenol is oxidized arises from the consideration

676 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

that many consecutive oxidation problems are The most recent developments in the oxidation of effectively handled and resolved by biological systems, benzene to phenol with hydrogen peroxide regard the which succeed in segregating the catalyst and the use of a particular solvent (sulpholane) in combination reaction product in different environments. In this way, with TS-1. Oxidizing benzene under these conditions the product substantially no longer has access to the in the most commonly used solvents, the selectivity catalytic site and consequently can no longer be for phenol decreases very rapidly with the increase in transformed, and is accumulated without any difficulty. conversion and, typically, falls below 50% already A typical example is constituted by several enzymes of with benzene conversions of around 3%. This drop is the oxygenase class, in which the active sites are buried strongly reduced when sulpholane is used as the into deep hydrophobic pockets easily accessible to solvent. In this case, the selectivity for phenol remains lipophilic substrates, whereas the more hydrophilic higher than 80% even with benzene conversions of products, once released, no longer return there. 8%. The by-products are catechol (7%), hydroquinone This important feature of biological systems can be (4%), 1,4-benzoquinone (1%) and tars (5%; Balducci reproduced, to some extent, by adopting a reaction et al., 2003). Probably sulpholane forms a complex medium consisting of two mutually immiscible liquid with phenol, by means of a hydrogen bond, once this phases, one aqueous and the other one organic; the has been desorbed by the catalyst: first one contains the catalyst and the second is very efficient in extracting the phenol produced, conveying O it away from the catalyst and thereby minimizing O H S further oxidations. O The best results have been obtained using acetonitrile. In fact, in the presence of benzene, a The complex is definitely larger in size than heterogeneous system is formed by water and phenol and so, although it is relatively labile, it is acetonitrile with two phases, one mainly aqueous and in any case able to delay, to some extent, a new the other mainly organic. The concentration of the entry of the phenol into the cavities of the zeolite. benzene in the mainly aqueous phase is more than In this way the sulpholane modifies, by reducing it, quadrupled. Simultaneously, a large part (85%) of the the coefficient of distribution of the phenol phenol formed is extracted in the organic phase and is between the volume of the intrazeolite cavities and thus protected against the overoxidation reactions. the external solvent, thereby delaying its The catalytic system consists of an acid (e.g. subsequent oxidation. trifluoroacetic or sulphuric), iron sulphate and a ligand A further improvement in the performance of the able to modulate its activity and its selectivity: catalytic system was obtained by subjecting the 2-methylpyrazine-4-carboxylic acid N-oxide: titanium-silicalite to a post-synthesis treatment with O aqueous solutions of NH4HF2 and hydrogen peroxide, at 80°C. The treatment removes part of the titanium CH N 3 and also substantially modifies the environment of part of the titanium not removed. The new catalyst HOOC N thus obtained (TS-1B) enables a 94% selectivity to With this system it has been possible to convert phenol to be reached, with benzene conversions of 8.4% of the benzene, obtaining phenol with a around 9% (Balducci et al., 2003). selectivity of 97% with respect to benzene and of 88% The selectivity of the whole process could be with respect to hydrogen peroxide (Bianchi et al., further improved by converting the by-products into 2000). phenol by treating them with hydrogen, in water, over

Fig. 14. Direct H O phenol NaOH H SO H oxidation of benzene 2 2 4 2 to phenol with H2O hydrogen peroxide benzene catalyzed by TS-1B. sulpholane diphenols HDO, oxidation distillationpurification recovery HDO hydrodeoxigenation. phenol

H2O2 sulpholane/benzene benzene H2O/ tars make-up Na2SO4

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commercial catalysts based on nickel and resorcinol continues to be an exception, as it is still molybdenum oxides (Bianchi et al., 2004). obtained by the alkaline fusion of m-benzendisulphonic A simplified block diagram of the process is acid or by the oxidation of m-diisopropylbenzene. shown in Fig. 14. The oxidation of benzene is carried out continuously, between 95 and 110°C and at 6 bar, Direct hydroxylation of phenol with hydrogen over a fixed bed containing TS-1B. At the end, the peroxide unconverted benzene, the water and the phenol are The direct hydroxylation of phenol with hydrogen removed through distillation, while the sulpholane peroxide was introduced during the 1970-1980s and used as a solvent is purified by extracting the was a major development from the standpoint of by-products (hydroquinone, catechol and tars) with environmental safeguard. aqueous soda and then recycled. The process permits The first commercial processes were developed by the complete conversion of the hydrogen peroxide and Brichima and Rhone-Poulenc. Two products are obtained, conversion of more than 15% of the benzene, with a catechol and hydroquinone, in a ratio that depends both selectivity to phenol higher than 97% based on on the process and on the working conditions. The main benzene and 71% on hydrogen peroxide. by-products are water and tars, the latter being possibly Conversion of the benzene obtained in direct reused for the plant’s energy requirements. oxidation with hydrogen peroxide, although low, is In the Brichima process, no longer in operation for fully comparable with that of the traditional process, in about two decades, the hydroxylation of the phenol which the overall conversion after the two stages does took place with a radical-type mechanism: not exceed 8.5% and, more often, is around 6%. While 2 . 3 it is still too early to know whether the results H2O2 Fe OH OH Fe described will lead to a new process for the industrial OH OH production of phenol without the coproduction of H OH acetone, undoubtedly it lays the foundations for it. .OH

Oxidation of phenol to hydroquinone and catechol OH OH H In 2002, the production capacity of hydroquinone OH OH . and of catechol in the developed countries amounted H2O2 OH H2O to about 50,000 and 32,000 t/y, respectively. The production of the former is mainly driven by the This was a typical chain reaction in which iron(II) demand of the photographic industry. That of catechol and cobalt(II) played the role of radical initiators, has undergone an appreciable development since the generating a hydroxyl radical. The propagation 1970s, with the introduction on the market of synthetic reaction, in which the radical species responsible for vanillin and other artificial aromas. In addition, both attacking the aromatic nucleus was also regenerated, are used for the production of various antioxidants and consisted mainly in the oxidation of the inhibitors of polymerization. cyclohexadienylic intermediate by the hydrogen In the past, diphenol synthesis processes foresaw the peroxide. In this phase, the intervention of the metal transformation of pre-existing functional groups, ions, present at the level of ppm, was overall regarded through a succession of stoichiometric reactions. The as negligible (Maggioni and Minisci, 1977). demand for hydroquinone for the budding photographic In the Brichima process, the chief product was industry at the beginning of the Twentieth century, for catechol, obtained in a ratio of about 2.0-2.3 to example, was first satisfied by reducing with sulphur hydroquinone. To minimize reactions of consecutive dioxide the benzoquinone produced by aniline oxidation oxidations, which produce tars, it was necessary to use a with chromic acid. Although the environmental large excess of phenol with respect to the oxidant, compatibility of the process was subsequently improved, limiting its conversion to less than 25%. This entailed by substituting chromic acid with manganese dioxide, having to separate considerable quantities of phenol by the quantity of by-products continued to be far greater distillation from the tars and to recycle them. Selectivity than that of hydroquinone. It is likely that the aniline with respect to hydrogen peroxide, the more costly of the oxidation process is still being applied in some two reagents, slightly exceeded 60%, no longer sufficient developing countries. Also an alternative method, the with the passing of time to guarantee an economic peroxidation of p-diisopropylbenzene, is based on a advantage to the process as compared with others. series of stoichiometric reactions. It was not until the In the Rhone-Poulenc process (Varagnat, 1976), the 1970-1980s that some catalytic processes using mechanism is heterolytic and the catalyst is a mixture of

hydrogen peroxide as the oxidant were developed and a strong mineral acid (H2SO4, HClO4) with phosphoric became part of industrial practice. The production of acid. The latter acts as a sequestering agent of metal

678 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Table 4. Yields of various hydroxylation processes of phenol with hydrogen peroxide

Conversion Catalyst Ortho/Para YIELD BASED YIELD BASED ON of phenol (%) ON H2O2 (%) PHENOL (%) TS-1 0.5-1.3 30 82 92 Co2+, Fe2+ (radical) 2.0-2.3 9 66 79 Acid (H+) 1.2-1.5 5 85-90 90

impurities that may be present, thereby preventing the deal of information was acquired on the hydroxylation establishment of undesired radical chains: of phenol, although this was not always reliable due to the uncertainties that sometimes existed regarding the H OH O H OH O 3 2 2 2 3 2 purity of the catalyst used. The most reliable OH OH information includes: HO H • Solvent: water, acetone-water, methanol. OH O H H2O • Temperature: 80-100°C.

H • H2O2/phenol (molar): 0.25-0.35. OHOH • Conversion of phenol (H2O2): 20-30% (100%). OHOH • Selectivity of phenol (H O ): 90-95% (80-90%). 2 2 HH33OO • Catechol/hydroquinone ratio: 0.5-1.3. The choice of solvent strongly influences the To favour selectivity on phenol and to diminish the performance of the process, first and foremost formation of tars, the process operates with a large the yields. Generally the acetone-water mixture is excess of phenol – greater than in the Brichima preferred, but high yields and selectivities have process –, so that conversion is limited to some 5%. been reported also with the use of methanol. The Selectivity with respect to hydrogen peroxide is catechol/hydroquinone ratio is the other instead higher, with values of over 85%. The parameter influenced by the choice of solvent. hydroquinone/catechol ratio can vary between 1.2 and This can vary between 0.5 and 1.3, reaching the 1.5, and is regulated by the choice of the acid and by highest values in acetone and the lowest ones in the possible addition of additives, giving the process a methanol. certain flexibility to meet market demands. High selectivity depends partly on the choice of optimal operating conditions, but even more so on Direct hydroxylation of phenol with hydrogen the purity of the TS-1 (i.e. the absence of titanium in peroxide catalyzed by TS-1 extraframework position). The presence of Hydroxylation of phenol was the first amorphous titanium-silicates or of titanium dioxide, commercial application of TS-1, accomplished in in the form of either amorphous nanoparticles or 1986 (Romano et al., 1990). This is a continuous crystalline anatase, increases the incidence of the process of slurry type; EniChem set up a production decomposition of hydrogen peroxide and of tar unit with a capacity of 10,000 t/y on this basis. The formation. strength of this process lies in its yields, appreciably Other zeolitic catalysts have been studied as higher than those of other direct hydroxylation possible substitutes of titanium-silicalite, but with processes (Table 4). TS-1, in fact, makes it possible disappointing results. Ti-b has less activity and its to operate at lower phenol/oxidant ratios and selectivity on hydrogen peroxide is comparable to and therefore with higher phenol conversion, without likewise insufficient as that of radical catalysts. selectivity suffering. The process is thus Ti-MOR seems to possess good activity and selectivity characterized by more efficient use of a relatively characteristics, but to date has not been used in costly reagent such as hydrogen peroxide, and by industrial production. lower costs of phenol separation and recycling. In addition to being industrially viable, the Ammoximation development of the process had the merit of attracting interest in the structural and catalytic properties of TS-1, Industrial production of nylon with two results: other zeolites were discovered able to Polyamide fibres, commonly called nylon, are one catalyze oxidations (see again Table 3), and a great of the earliest and most important typologies of

VOLUME II / REFINING AND PETROCHEMICALS 679 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

NH NH adipic acid 3 preparation of 3 O NH3 ammonium oxidation carbonate HO C O CO COH O H 2 2 C N N2O3 (NH4)2CO3 O [C N [ n preparation of CO2 NH2 O H ammonium H N 2 Nylon 6,6 nitrite hexamethylenediamine (NH4)NO2 H preparation of O SO2 N hydroxylamine NH sulphate NH [ [ n 3 O NH OH . H SO e-caprolactam Nylon 6 2 2 4

cyclohexanone preparation of (NH4)2SO4 cyclohexanone NH Fig. 15. Various types of nylon and their synthesis. 3 oxime 2.8 kg/kg oxime (NH ) SO synthetic fibres (Petrini et al., 1996; Bellussi and oleum preparation of 4 2 4 caprolactam Perego, 2000). Nylon 6,6, synthesized by William H. NH3 1.6 kg/kg Carothers at the DuPont company in the 1930s, was the first to be placed on the market. It is produced from adipic acid and hexamethylenediamine. A e-caprolactam different type of nylon, called Nylon 6, developed in Germany by I.G. Farben and marketed under the name Fig. 16. Raschig process for production of Perlon, is obtained by ring opening polymerization of e-caprolactam. of e-caprolactam (Fig. 15). e-Caprolactam, the world productive capacity of which amounts to about 4·106 t/y, is obtained 20 OH industrially mainly from benzene, through a complex O N series of transformations. The benzene can be NH OH.H SO 2NH hydrogenated to cyclohexane which can be oxidized to 2 2 4 3 cyclohexanone; alternatively, cyclohexanone can be obtained by the hydrogenation of phenol, which is also (NH4)2SO4 H2O produced from benzene. In its turn, cyclohexanone is transformed into its oxime and the latter into OH N e-caprolactam: O OH OH H2SO4 2NH3 NH (NH4)2SO4 O N O

NH In the Raschig process for the synthesis of e-caprolactam (Fig. 16), hydroxylamine sulphate is

produced from NH3, CO2 and SO2 through a complex The conventional technology used in these last two series of operations; these include the synthesis of

stages of production raises the problem of a NOx (obtained by combustion of ammonia in air) and considerable coproduction of a salt, ammonium of ammonium carbonate (obtained by the reaction of

sulphate, which is today of extremely low commercial ammonia and CO2), their combination to form value. ammonium nitrite and the subsequent reduction of this

Moreover, the process is penalized by the with SO2. complexity of the cycle connected with the inorganic In the synthesis of the oxime, about 2.8 kg of raw materials used and with the synthesis of ammonium sulphate is coproduced for every 1 kg of hydroxylamine, as well as by the problems linked with product. In the following conversion step of oxime to

the emission of nitrogen oxides (NOx) and sulphur caprolactam, by means of the Beckmann oxides (SOx). rearrangement conducted in the presence of oleum,

680 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

another 1.6 kg is coproduced, making a total of 4.4 kg Ammoximation of cyclohexanone of ammonium sulphate per 1 kg of caprolactam. In this panorama of unresolved problems, the This process has undergone constant improvement ammoximation process represents a radical to reduce the coproduction of ammonium sulphate, but innovation (Roffia et al., 1989; Petrini et al., with only partial results, or involving considerable 1996). The term ammoximation indicates the complexity in the operative cycle. Significant production of oxime directly from ammonia. In examples of this trend are the BASF and DSM fact, the process is based on a catalytic reaction processes, which exploit the reducing action of between cyclohexanone, ammonia and hydrogen hydrogen in the presence of palladium catalysts for the peroxide, and completely eliminates, on the one production of hydroxylamine. hand, the problems linked with the production and The rearrangement step of the oxime into caprolactam use of hydroxylamine and, on the other, the has also formed the object of intense studies aimed coproduction of sulphates: at cutting out the use of oleum and the coproduction OH of sulphate. Recently, Sumitomo has industrialized O N a vapour-phase catalytic process using a solid catalyst cat. TS-1 of a zeolitic nature instead of oleum, thus enabling the NH3 H2O2 2H2O coproduction of salt to be completely avoided. Other forms of synthesis have tackled the problem The catalyst in the process is TS-1. The of the production of oxime, or of caprolactam directly, cyclohexanone ammoximation reaction with ammonia in a completely different way. For example, in the and hydrogen peroxide (or oxygen) had already been Toray process, the photonitrosation of the cyclohexane described earlier, but scanty yields have been obtained. to oxime takes place; the SNIA process foresees the Only the use of TS-1 as the catalyst, patented for the oxidation of toluene to benzoic acid followed by first time in the 1980s by Paolo Roffia and hydrogenation to hexahydrobenzoic acid, and the co-workers, enabled a breakthrough to be obtained, conversion of the latter to caprolactam with nitrosyl allowing EniChem to develop the process on an sulphuric acid. These processes have had limited industrial basis. commercial success. Other alternative routes based on In this process (see schematic diagram in Fig. 17), butadiene, via oxidative carbonylation (DSM) or the reaction is carried out continuously in the liquid hydrocyanation (BASF/DuPont), are at present in the phase in a stirred reactor, in the presence of the final stage of development. catalyst dispersed in slurry in the reaction medium at a concentration of 2-3% in weight. The reaction is typically conducted between 80°C and 90°C, at a vent to treatment slight overpressure, supplying cyclohexanone, NH3 catalyst H O and aqueous H2O2 in molar ratio 1.0:2.0:1.1 in the make-up 2 solvent make-up reaction solvent consisting of water and tert-butanol, with a residence time of around 1.5 h. The reaction product is separated from the catalyst, through filters soaked in the reactor, and conveyed to a rectifying NH3 column for recovery of the unconverted excess ammonia and solvent (water/tert-butanol azeotrope), H2O2 which are recycled in the reaction. The aqueous cyclohexanone solution of crude oxime obtained from the bottom of the column is conveyed to the purification unit for recovery of the cyclohexanone oxime with the required purity, to be used in the subsequent rearrangement to caprolactam. During the process, the catalyst must undergo periodic purging and make-up operations since, in the presence of ammonia, the siliceous structure of TS-1 slowly dissolves, with loss of weight of the catalyst accompanied by migration of aqueous oxime to the titanium to the outside surface of the solid and loss spent catalyst purification of catalytic activity. Operating under these conditions, the conversion of Fig. 17. EniChem process for production the cyclohexanone is nearly complete (99.9%) and the of cyclohexanone oxime. selectivity to the oxime based on cyclohexanone is

VOLUME II / REFINING AND PETROCHEMICALS 681 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

Fig. 18. Mechanism of ammoximation reaction H2O(NH3) catalyzed by TS-1. NH2OH O

Ti H H2O O O O O H

H2O(NH3) O

OH2(NH3) OH2(NH3) Ti O OH O OH O O Ti Ti O O NH2OH O OH2(NH3) O O

OH2(NH3) O OOH Ti H2O2 O NH3(H2O) O

higher than 98%; the yield of oxime based on H2O2 able to catalyze the production of hydroxylamine from supplied is about 94%. The main inorganic by-products, NH3 and H2O2 with good yields, and bulky ketones derived from the oxidation of the ammonia or the unable to diffuse in the pores of the catalyst, such as

decomposition of the H2O2, are N2, N2O, O2, nitrites 4-tert-butylcyclohexanone, are transformed, with good and ammonium nitrates. The main organic by-products yields, into the corresponding oximes. stemming from the cyclohexanone include azine, More in detail, the mechanism proposed for the cyclohexenylcyclohexanone (the product of aldol formation of hydroxylamine on the basis of condensation of the cyclohexanone), nitrocyclohexane spectroscopic studies foresees that, in an aqueous and cyclohexenonoxime (the last two derived from ammoniacal medium, the tetrahedrally coordinated consecutive reactions on the oxime). These by-products titanium atoms present in the structure of TS-1 are

derive from competitive reactions in which the active able to coordinate up to two other ligands (H2O or sites of titanium present in TS-1 are not involved. NH3), assuming an octahedral coordination. Through Basically, two reaction mechanisms have been the action of hydrogen peroxide, a species would then proposed. The first one foresees the formation of be formed characterized by the simultaneous presence imine as an intermediate, by reaction of the of ammonia and hydroperoxide as ligands, which cyclohexanone with the ammonia: would then lead to the formation of hydroxylamine with the regeneration of the catalytic centre (Fig. 18). OH O NH N Salt-free production of caprolactam NH3 H2O2 The development of the ammoximation process cat. TS-1 has simplified the very complex part of the caprolactam production technology linked to the The second one foresees the formation of preparation of derivatives of hydroxylamine and to the hydroxylamine by the action of the hydrogen peroxide synthesis of cyclohexanone oxime, completely on the ammonia: avoiding, at this stage, the coproduction of ammonium sulphate and the emission of SO and NO . OH x x In turn, the catalytic vapour-phase process, O N developed in Japan by Sumitomo, makes it possible to H O 2 2 avoid the coproduction of ammonium sulphate in the NH3 NH2OH cat. TS-1 rearrangement stage of oxime to caprolactam (Ichihashi and Sato, 2001). The process is run with the The latter appears more likely (Zecchina et al., oxime dissolved in methanol, at 300-400°C and at 1992) since TS-1, in the absence of cyclohexanone, is approximately 1 atm, in a fluidized bed reactor that

682 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

permits frequent regeneration of the catalyst generation of hydrogen peroxide, made possible by the (necessary due to its rapid deactivation caused by the molecular sieve properties of TS-1. depositing of organic tars). The catalyst is a zeolite of On this basis, the direct use of the working solution MFI structure with a very low aluminium (and other of the anthraquinone process for the epoxidation of hetero-elements) content, basically a silicalite-1. Its propylene has been studied (Clerici and Ingallina, 1996): use, associated with that of alcohol as a component of OH O R R the reaction medium, has enabled complete TS-1 conversions of oxime and selectivities to caprolactam O2 of more than 95% to be achieved. These performances, OH O although slightly inferior to those of the conventional rearrangement conducted with oleum, in which the HHOO yield exceeds 99%, have been reckoned suitable for 2 2 OO industrial development due to the absence of the coproduction of any salts. In the process, formally, the oxidant is molecular The combined process bringing together the oxygen, with all the advantages thereof with respect to ammoximation and catalytic transposition the use of hydrogen peroxide. In effect, however, the technologies results in a totally salt-free production of latter is produced in the reaction environment by the caprolactam. This process was industrialized for the oxidation in situ of the alkylanthrahydroquinone. first time in Japan by Sumitomo, which, on the basis Successively, it diffuses to the active sites within the of an agreement with EniChem, set up a plant with a pores, where it is immediately consumed in the capacity of 60,000 t/y. The combined process, apart epoxidation reaction, with the mechanism already from allowing salt-free production, eliminates all illustrated in Fig. 9.

gaseous emissions of NOx and SOx and results in a The feasibility of the process is based on the significant reduction in investment and operating impossibility of the alkylanthraquinone and the other costs. This new technology will certainly play a components of the working solution to diffuse inside the primary role in caprolactam production processes pores, where they could undergo oxidative degradation based on cyclohexanone. processes or interfere with the redox mechanisms. Their molecular dimensions are, in fact, larger than the opening Conclusions and perspectives of the catalyst’s pores. However, it should be emphasized that the scheme envisaged does not enable the complexity The prospects of using hydrogen peroxide, even for of the anthraquinone process to be side-stepped, but just large-scale productions, seem encouraging. An to eliminate the stage relative to the separation, ammoximation plant has come into production in purification and concentration of the hydrogen peroxide. Japan, one or more pilot plants for the epoxidation of A variant foresees that the two processes, the propylene are operating in various chemical production of hydrogen peroxide and epoxidation, companies, and considerable research efforts are being remain substantially separate, but that the epoxidation made in many industrial laboratories. Nevertheless, it solvent (aqueous methanol) is used also for extracting cannot be denied that obstacles exist, not to be hydrogen peroxide from the working solution (Clerici underestimated, which are inherent in the current and Ingallina, 1996). method of producing hydrogen peroxide and which The principle foreseeing the use of aqueous hinder its diffusion in petrochemistry. The methanol as the solvent for the direct synthesis of anthraquinone process, in fact, is based on a complex hydrogen peroxide is quite similar; it is targeted on technology that requires considerable investments. obtaining a solution that can be supplied to the Moreover, it is anticipated that a propylene oxide epoxidation reactor without further treatments, except

plant, based on the TS-1/H2O2 technology, will require for the removal of any additives (Paparatto et al., 2003b). a hydrogen peroxide unit of a size never experimented. In general, this method (like the others mentioned above) Despite such considerations, however, some can also be used for other TS-1-catalysed oxidations, and chemical companies have recently announced the not only in the case of propylene epoxidation. setting up of large-scale propylene oxide plants that Then there is a second category of studies, to are based on this technology. which mention has already been made, on the possible The need to exceed the limits of the anthraquinone use of a mixture of hydrogen and oxygen directly in process gave fresh impetus to the studies on the direct the oxidation reactor. For this purpose, it is necessary synthesis of hydrogen peroxide, as previously to finalize a bifunctional catalyst that possesses both mentioned. At the same time, it led to exploring other the catalytic centres necessary for synthesis of the alternatives, such as process integration and the in situ hydrogen peroxide, and the active centres for the

VOLUME II / REFINING AND PETROCHEMICALS 683 SYNTHESIS OF INTERMEDIATES FOR THE PETROCHEMICAL INDUSTRY

oxidation reaction. TS-1, on which metallic palladium Clerici M.G., Ingallina P. (1996) Clean oxidation has been supported, is an adequate catalyst (Clerici technologies. New prospects in the epoxidation of the olefins, and Bellussi, 1993; Meiers et al., 1998). in: Anastas P.T., Williamson T.C. (editors) Green chemistry. Designing chemistry for the environment, Washington An interesting application of mixed oxides regards (D.C.), American Chemical Society, 59-68. the deep desulphuration of motor fuels. The large size Clerici M.G. et al. (1991) Synthesis of propylene oxide from of the sulphur molecules, in this case, entails the use propylene and hydrogen peroxide catalyzed by titanium of mesoporous mixed oxides, instead of zeolites, silicalite, «Journal of Catalysis», 129, 159-167. which are microporous. Consequently, for the reasons Corma A. et al. (2001) Sn-zeolite beta as a heterogeneous already discussed, the oxidant cannot be hydrogen chemoselective catalyst for Baeyer-Villiger oxidations, peroxide, but rather an organic hydroperoxide, a «Nature», 412, 423-425. Fenton H.J.H solution that, moreover, is best fitted for the actual . (1876) On a new reaction of tartaric acid, «Chemical News», 33, 190. set-up of the refinery. In principle, oxidants such as Fenton H.J.H. (1894) Oxidation of tartaric acid in presence tert-butyl hydroperoxide, tert-amyl hydroperoxide, of iron, «Journal of Chemical Society», 65, 899-910. (1-phenyl)ethyl hydroperoxide or other such Girotti G. et al. (2003) Alkylation of benzene with isopropanol hydroperoxides are obtainable by oxidation with air of on β-zeolite. Influence of physical state and water streams present in the refinery. Patents filed by various concentration on catalyst performances, «Journal of oil companies show the interest taken in this method, Molecular Catalysis A: Chemical», 204-205, 571-579. as an alternative to the more traditional process of Goor G. (1992) Hydrogen peroxide: manufacture and industrial deep hydrotreatment. use for production of organic chemicals, in: Strukul G. (edited by) Catalytic oxidations with hydrogen peroxide as oxidant, Dordrecht, Kluwer Academic Publishers, 13-43. Herrmann W.A. et al. (1991) Methyltrioxorhenium as catalyst References for olefin oxidation, «Angewandte Chemie. International Edition in English», 30, 1638-1641. Baerlocher C. et al. (2001) Atlas of zeolite framework types, Hooper G.W. (1967) US Patent 3336112 to ICI. Amsterdam, Elsevier. Huybrechts D.R.C. et al. (1990) Oxyfunctionalization of Balducci L. et al. (2003) Direct oxidation of benzene to phenol alkanes with hydrogen peroxide on titanium silicalite, with hydrogen peroxide over a modified titanium silicalite, «Nature», 345, 240-242. «Angewandte Chemie. International Edition», 42, 4937-4940. Ichihashi H., Sato H. (2001) The development of new Bellussi G., Perego C. (2000) Industrial catalytic aspects of heterogeneous catalytic processes for the production the synthesis of monomers for nylon production, «Cattech», of ε-caprolactam, «Applied Catalysis. A: General», 221, 4, 4-16. 359-366. Bellussi G. et al. (1992) Reactions of titanium silicate with Juliá S. et al. (1982) Synthetic enzymes. 2: Catalytic asymmetric protic molecules and hydrogen peroxide, «Journal of epoxidation by means of polyamino-acids in a triphase Catalysis», 133, 220-230. system, «Journal of Chemical Society. Perkin Transactions Bianchi D. et al. (1999) Biphasic synthesis of hydrogen peroxide I», 11, 1317-1324. from carbon monoxide, water, and oxygen catalyzed by Katsuki T., Sharpless K.B. (1980) The first practical method palladium complexes with bidentate nitrogen ligands, for asymmetric epoxidation, «Journal of the American «Angewandte Chemie. International Edition», 38, 706-708. Chemical Society», 102, 5974-5976. Bianchi D. et al. (2000) A novel iron-based catalyst for the Maggioni P., Minisci F. (1977) Catechol and hydroquinone biphasic oxidation of benzene to phenol with hydrogen from catalytic hydroxylation of phenol by hydrogen peroxide, peroxide, «Angewandte Chemie. International Edition», «La Chimica e l’Industria», 59, 239-242. 39, 4321-4323. Maspero F., Romano U. (1994) Oxidation of alcohols with Bianchi D. et al. (2004) European Patent 1424320 to Polimeri H2O2 catalyzed by titanium silicalite-1, «Journal of Europa. Catalysis», 146, 476-482. Boccuti M.R. et al. (1988) Spectroscopic characterization of Meiers R. et al. (1998) Synthesis of propylene oxide from silicalite and titanium silicalite, in: Structures and reactivity propylene, oxygen, and hydrogen catalyzed by palladium- of surfaces. Proceedings of a European conference, Trieste, platinum-containing titanium silicalite, «Journal of September 13-16, Amsterdam, Elsevier, 133-144. Catalysis», 176, 376-386. Clerici M.G. (1991) Oxidation of saturated hydrocarbons with Milas N.A. (1937) The hydroxylation of unsaturated substances. hydrogen peroxide, catalysed by titanium silicalite, «Applied III: The use of vanadium pentoxide and chromium trioxide Catalysis», 68, 249-261. as catalysts of hydroxylation, «Journal of the American Clerici M.G. (2001) The role of the solvent inTS-1 chemistry: Chemical Society», 59, 2342-2344. active or passive? An earlier study revisited, «Topics in Milas N.A., Sussman S. (1936) The hydroxylation of the double Catalysis», 15, 257-263. bond, «Journal of the American Chemical Society», 58, Clerici M.G., Bellussi G. (1993) US Patent 5235111 to 1302-1305. Eniricerche-Snamprogetti. Millini R., Perego G. (1996) Structural evidence for Clerici M.G., Ingallina P. (1993) Epoxidation of lower isomorphous substitution in microporous materials and olefins with hydrogen peroxide and titanium silicalite, methods of characterization. The case of titanium silicalite, «Journal of Catalysis», 140, 71-83. «Gazzetta Chimica Italiana», 126, 133-140.

684 ENCYCLOPAEDIA OF HYDROCARBONS OXIDATION PROCESSES

Neurock M., Manzer L.E. (1996) Theoretical insights on Romano U. et al. (1990) Selective oxidation with ti-silicalite, the mechanism of alkene epoxidation by H2O2 with titanium «La Chimica e l’Industria», 72, 610-616. silicalite, «Chemical Communications», 1133-1134. Taramasso M. et al. (1983) US Patent 4410501 to Olah G.A., Ohnishi R. (1978) Oxyfunctionalization of Snamprogetti. hydrocarbons. 8: Electrophilic hydroxylation of benzene, Varagnat J. (1976) Hydroquinone and pyrocatechol production alkylbenzenes, and halobenzenes with hydrogen peroxide, by direct oxidation of phenol, «Industrial & Engineering «Journal of Organic Chemistry», 43, 865-867. Chemistry. Product Research and Development», 15, 212-215. Panov G.I. (2000) Advances in oxidation catalysis. Oxidation Venturello C. et al. (1983) A new, effective catalytic system of benzene to phenol by nitrous oxide, «Cattech», 4, 18-32. for epoxidation of olefins by hydrogen peroxide under phase- Paparatto G. et al. (2003a) European Patent 1307399 to Eni- transfer conditions, «Journal of Organic Chemistry», 48, Enichem. 3831-3833. Paparatto G. et al. (2003b) US Patent 6541648 to Polimeri Venturello C. et al. (1985) A new peroxotungsten heteropoly Europa. anion with special oxidizing properties. Synthesis and structure Petrini G. et al. (1996) Caprolactam via ammoximation, in: of tetrahexylammonium tetra(diperoxotungsto)phosphate(3), Anastas P.T., Williamson T.C. (editors) Green chemistry. «Journal of Molecular Catalysis», 32, 107-110. Designing chemistry for the environment, Washington Zecchina G. et al. (1992) Ammoximation of cyclohexanone (D.C.), American Chemical Society, 33-48. on titanium silicalite. Investigation of the reaction Ricci M. (1996) Electrophilic activation of hydrogen peroxide. mechanism, in: Proceedings of the 10th International Some recent applications in organic synthesis, in: congress on catalysis, Budapest, 19-24 July, 719-728. Proceedings of the Seminars in organic synthesis. XXI Summer School A. Corbella 17-21 June, Milano, Società Mario G. Clerici Chimica Italiana, 247-266. Roffia P. EniTecnologie et al. (1989) Cyclohexanone ammoximation. A break San Donato Milanese, Milano, Italy through in the 6-caprolactam production process, in: New developments in selective oxidation. Proceedings of an Marco Ricci international symposium, Rimini (Italy), 18-22 September, 43-52. Franco Rivetti Romano U. (2001) Ossido di propilene. Nuova tecnologia Polimeri Europa produttiva, «La Chimica e l’Industria», 83, 30-31. Novara, Italy

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