CHAPTER I

GENERAL INTRODUCTION CHAPTER I

GENERAL INTRODUCTION

1.1 Alkylation Reactions of Aromatic Hydrocarbons:

The importance of alkylation in organic preparations was realized as early as 1877 with the reaction of benzene and amyl chloride, to produce amyl benzene, by Charles Friedel and James Mason Crafts-^. This was apparently the first typical alkylation reaction and came to be known as the Friedel-Crafts alkylation reaction. Friedel-Crafts reactions now find a number of industrial applications such as m the manufacture of high octane gasoline, synthetic rubber, plastics, synthetic fibers, synthetic detergents etc. Besides its commercial importance, the area of organic chemistry dealing with this reaction encompasses classic examples of some of the most interesting aspects of modern organic chemistry: electrophilic aromatic substitution, carbocation formation and rearrangement. In the Friedel-Crafts reaction, the alkylating or acylating agent and the catalyst, such as aluminium chloride and hydrogen chloride, react to form either carbonium or acylium ion or complex. The ion or complex then attacks the aromatic ring^. Thus in the alkylation of olefins in the presence of the catalyst, the following reactions take place.

R-CH=CHo+ AICI^ + HCI R-CH-CH3AlC I4 CH-CH3 r -1- + R-CH-CH3AICI4 AlCl

^ +AICI4 + HCl

The aromatic ring to which the olefin gets attached may be that of benzene, substituted benzene or more complicated ring systems like naphthalene or anthracene. Friedel-Crafts reactions are complicated by the rearrangement of the attacking agent and in some cases of the aromatic starting materials . There is a tendency for a carbonium ion formed during the reaction, to rearrange to a stable secondary or tertiary carbonium ion. Although, the alkylating agent involved in the discovery of the Friedel-Crafts reaction was the alkyl halide, many other alkylating agents ha^f* been used. Today, the most commonly used alkylating agent is the alkene. Other alkylating agents which can be used for the reaction are alcohol, ethers and esters^'^.

1.2 Transalkylation Reactions of Aromatic Hydrocarbons:

Another feature of Friedel-Crafts alkylation reaction is polyalkylation. Since alkyl groups activate the aromatic ring towards further attack, there is a marked tendency for polysubstitution during the alkylation. This affects the yield of the monoalkyl product. It was shown that alkyl groups could be transferred from one aromatic ring to another by the catalytic effect of aluminium chloride and hydrogen chloride^. For example, when ethylbenzene was heated with the catalyst, a mixture of benzene, ethylbenzene, diethylbenzene (mainly m- and p- isomers), and higher boiling material was produced.

C2H5 C2H5

4- fOV-C2H5

Transfer of alkyl groups from one to another, similar or dissimilar aromatic rings, in the presence of an acid catalyst, is known as the transalkylation (or disproportionation) reaction. Industrially, this reaction is valuable as some of the low valued products, like polyalkylbenzenes, can be converted to their monosubstituted homologues having higher demands and values e.g. transalkylation of diethylbenzenes to ethylbenzene.

1.3 Catalysts for Alkylation and Transalkylation Reactions:

Friedel-Crafts alkylation or transalkylation is an acid catalyzed reaction. Both Bronsted and Lewis acids can catalyze these reactions^'-'. For the liquid phase alkylation reaction, anhydrous aluminium chloride is preferred as a catalyst, although a co-catalyst or promoter like hydrogen chloride is usually needed to obtain efficient alkylation. In the vapor-phase alkylation, the most commonly used catalyst is phosphoric acid supported on kieselguhr (also known as solid phosphoric acid (SPA) catalyst). Phosphoric acid in the presence of small amount of water, alkylate benzene effectively to products like ethylbenzene, isopropylbenzene etc. Besides these two catalysts, various other catalysts have been used for the alkylation reaction of aromatics^" . These include weak acidic metal halides (Lewis acids) such as AICI3.CH3NO2, BF3, FeCl3, TiCl^, SnCl^ or ZnCl2; protonic acids (Bronsted acids) such as BF3, HF, H2SO4, polyphosphoric acid (PPA); solid super acids such as perfluorinated sulphonic acid Nafion-H; and inorganic acidic oxides such as phosphorous pentoxide on alumina. Most of the catalysts mentioned earlier have to be used in the liquid phase. The use of such catalysts is often associated with problems of corrosion, toxicity and effluent pollution. This makes it desirable to replace homogeneous catalysts by heterogeneous catalysts. Furthermore, the gas phase also offers advantages in the field of process technology . Solid acid catalysts such as zeolites are free of environmental and corrosion problems. Further, the combination of acidity and shape selectivity in the case of zeolites makes them potential catalysts for different Friedel-Crafts alkylation and transalkylation reactions. Some of the important and interesting organic reactions which were not feasible because of low activity, selectivity and/or service life of the catalyst, can now be commercially exploited using zeolites.

1.4 Zeolites as Alkylation and Transalkylation Catalysts:

Zeolites are crystalline alumino-silicates with a framework based on extensive three dimensional network of SiO^ and AlO^ tetrahedral building blocks^. The tetrahedral co-ordination permits a variety of ring structures, which further link to form the cage or channel structures, that give rise to the molecular discriminatory nature of the zeolites. The isomorphous substitution of Si by Al gives rise to a net negative charge, compensated by cations, which are present in voids and channels of zeolites. The cations are generally from group I and II, although other metals, nonmetals, and organic cations may also be used. They are quite mobile and can be exchanged to varying degrees with other cations. All zeolites that are significant for catalytic and adsorbent applications can be classified by the number of T-atoms (T = Si or Al) that define the pore openings^' . There are only three types of pore openings known todate in the aluminosilicate zeolite system; they are descriptively referred to as the 8- membered ring (small pore), 10-membered ring (medium pore) and 12-membered ring (large pore) zeolites. The size and shape of the pore opening is determined by factors such as (1) configuration of the T and O atoms relative to each other, (2) silica to alumina ratio, (3) size and location of the cation, (4) temperature and (5) framework structure of the zeolites.

1.5 Active Sites in Zeolites:

The reactivity of molecular sieve zeolites as catalysts is determined by active sites provided by the imbalance in the charge between the silicon and aluminium ions in the framework. Thus, each aluminium atom present within the framework constitutes an active site^. Classical Bronsted and Lewis acid models have been used to classify active sites in zeolites. Bronsted acidity arises in the zeolite when the cation balancing the anionic framework charge is a proton (H"*") . Differences in acidic strength between different zeolites is often related to the T-O-T bond angles and lengths, and crystal resonance energy . ® H H 0.

Si Al ^ /' / \ 0/ \ 0 0/ ^

Bronsted Acidity

A trigonally co-ordinated aluminium atom, which acts as an electron acceptor, behaves as a Lewis acid. Higher temperature (>500°C) can result in conversion of the Bronsted acid sites to Lewis acid sites by dehydroxylation^. Dealumination by hydrothermal treatment of zeolites has also been found to produce a variety of cations and neutral species which function as Lewis acids • These cations also induce activity on nearby Bronsted sites. Strong electrical field in the small pore zeolites (including medium pore ZSM-5), arising from the presence of various charged species causes large energy gradients with in the molecular sieve, pores. They also may affect activity and selectivity of the catalyst^-*-.

1.6 Hydrothermal Synthesis of Zeolites:

Natural zeolites occur in the cavities of volcanic or metamorphic rocks, and on a much larger scale, sedimentary tufi deposits-^''. The formation conditions for these natural deposits are quite mild, with typical temperatures in the range of 70 to 350°C and they get accumulated over geological time scale. In typical modern procedure for synthesizing high silica zeolites, a solution of alumina and an organic base, such as tetra- aTcylammonium hydroxide, are mixed intimately with a sol or solution of the silica component^'^ '-"--^'-^'^. This highly alkaline mixture forms a thick gel that crystallizes in a few hours when maintained at or above 100°C, at hydrothermal conditions. Synthetic zeolites thus obtained by hydrothermal treatment are metastable phases. Changes in the nature and properties of these phases may be induced by many factors that affect the nucleation and crystal growth rates of the competing zeolite phases. This can determine the nature of the framework that finally crystallizes. Basic understanding of the crystallization mechanism is limited and largely empirical. Three variables have major influence on the zeolite crystallization; the gross composition of reaction mixture, temperature and time ' ' . Components of the reaction mixture that influence final crystalline material are silica to alumina ratio, hydroxide content of the gel, presence of inorganic cations, organic additives etc. Influence of all these components are interrelated.

1.7 Zeolite Structural Characterization:

Zeolites are crystalline and have complex structures with large unit cells. Typical diameter of zeolite crystals is 1 to 5 Jm. The crystals are often twinned, or intergrown with multiple structures at the unit cell or subunit-cell level"^^'-^^. Hence it is very rarely possible to establish the structure of a new synthetic zeolite by X-ray crystallographic techniques alone^^. Identification of a new zeolite phase begins with determining the X-ray powder diffraction pattern (PXD), as it is a fingerprint of individual zeolite structures'^^. Scanning electron microscopy (SEM) allows detection of impurities. SEM together with PXD are usually the first data sources indicating the possibility of a new zeolite. Thermogravimetric analysis (TGA) is a source of important information on the possible template trapping sites, and therefore possible structural subunits ". The temperature at which templates are released serve as an indication of the window size encompassing the template size. The next stage of characterization of a zeolite is to subject it to sorption tests to determine the total pore volume, as well as the diameter of the most prominent aperture of cages or channels^'"^'^. Probe molecules used for this purpose include oxygen, n-hexane, cyclohexane, H2O, CO2, Ar, N2, and n-butane. This technique also gives information about crystallinity of the sample. Water adsorption in the aluminosilicates has been shown to be directly related to the amount of aluminium present in the framework . Infra-red spectroscopy is another useful technique to investigate the structural features of zeolites^^"^^. In general, each zeolite structure has a characteristic infrared pattern; however some common vibrations are observed. These include (1) internal vibrations of the framework TO^ such as symmetric and asymmetric stretching, double ring vibrations, T-0 bending modes, and (2) vibrations related to the external linkages of the TO4 units in the structure. The latter are sensitive to the structural variations. Shift in bond positions in the symmetric and asymmetric stretching vibration modes are observed with change in Si/Al ratio in the material^^. Mid-infrared spectroscopy has been employed further to identify framework incorporation of phosphorous, gallium, iron, boron, and titanium into the silicate structure^^. Mossbauer spectroscopy in the case of Mossbauer active elements that form zeolites (like Fe) , can serve to indicate whether certain ions are present in the zeolite as a part of the framework or as exchangeable cations^^'^^. In a similar, but less widely applicable manner, ESR has been used to differentiate transition metals in different environments^^ '-^^ . The advent of magic-angle-spinning NMR has added significantly to the armory of techniques suitable for use in the structural chemistry of zeolites. By using magic angle spinning, if necessary with cross polarization, dipolar decoupling, or multiple pulse sequence, high resolution NMR spectra can be obtained from zeolite materials'^-'•"•^-^. Although, most of the work has been done on the ^^Si and ^"^Al nuclei, other probe nuclei including I'^o, ^Li, 205^;^, ^H, ^^B, ^^V, ^^C, 1%, ^^P, and ^^^Xe have also been studied-^^'-^^. For zeolites that have a single crystallographic silicon site, the ^^si NMR spectrum consists of upto five peaks, each corresponding to a different number of first neighbour aluminium [ SiO-(Alj^, Si^.j^) ; n=0 to 4]. The intensity of each peak is a direct measure of the number of corresponding units in the sample. Framework Si/Al ratio can be deduced from relative intensities of the five Si-nAl peaks, assuming the validity of Lowenstein's rule of Al-O-Al linkage avoidance-^ "^. An advantage in examining ^'Al NMR spectra is that 27 •^'Al has a 100 % natural abundance with I = 5/2 and a chemical shift range of around 450 ppm. But quadruple coupling must be separated from chemical shift effects to render useful chemical information. Two techniques frequently used to study the nature and role of template in zeolites are •'••^C NMR and -^^^xe NMR. The advantage of Xe is that it can be encapsulated in a large number of structures at high temperature and pressure-^^, whereas -^-^C method is only effective for the "as-synthesized zeolites". ^^^Xe shifts are also very sensitive to other factors apart from cavity dimensions. High resolution electron microscopy (HREM) can provide information on structural defects within the molecular sieve crystals. With enhanced computer imaging, it can provide structural information on the arrangement of channels and pores within the zeolite-^ . other techniques for characterization of zeolites are X-ray induced photoelectron emission (XPS, ESCA), X-ray absorption, extended X-ray absorption for fine structure (EXAFS), and to lesser degree, X-ray absorption near-edge structure (XANES)-^ Characterization of zeolites model reactions is a well established technique. Constraint index (C.I.)/ a comparative measure of the cracking rates of two hydrocarbons of different diameters (n-hexane and three methyl pentane) is widely used as a measure of the pore size of zeolites . More recently, a well eastblished but more complex hydrocracking test, the Spaciousness index (S.I.) has been developed . In the latter case, the complex product distribution obtained from the hydrocracking of decane is used to probe the pore size and shape.

1.8 Industrial Applications of Zeolites:

Major industrial applications of zeolites exploit different aspects of zeolite structure chemistry, like ion exchange and adsorption capacity, as well as catalytic activity^-^. The major use of zeolite ion-exchangers is in low phosphate detergents, in which zeolite A is used as the partial replacement for sodium thiophosphate builders and water softeners^"*. Zeolites are also used in agriculture and waste-water treatment plants^^. Zeolites with lower silica to alumina ratio reversibly desorb water and are widely used as desiccants^^. Siliceous zeolites, on the other hand are organophilic. Such zeolites, depending upon the dimensions of pore windows, can discriminate between molecules on the basis of their shape and size^'^^. The Parex process of UOP, for example, uses FAU framework zeolites to separate p-xylene from the mixture of other xylene isomers and ethylbenzene'*°''*^. Nitrogen molecule has a quadruple moment and is much more strongly adsorbed into zeolites with lower silica to

10 alumina ratio, than oxygen. Lindox and Unox processes exploit this difference in separating oxygen from air by means of 4 8 pressure swing adsorption . The third and most vital area of zeolite applications is in heterogeneous catalysis. The largest application of zeolite catalysis is in catalytic cracking processes. In the cracking process, e.g. fluidised catalytic cracking (FCC), a wide range of heavy petroleum fractions are converted to gasoline and light fuel oils with by-products like liquefied petroleum gas, fuel gas, heavy fuel oil, and petroleum coke. Today, the typical FCC catalyst comprises of 10 to 40 wt.% zeolite Y dispersed in an amorphous matrix such as clay or silica-alumina^. The use of high silica Y zeolite (USY), produced by high temperature steaming or by chemical treatment, reduces the rate of secondary cracking and hydrogen cracking activity. Hydrocracking, the second largest application of zeolite catalysis, involves cracking under high pressures of hydrogen in the presence of supported metal and acid functions. Large pore zeolites such as Y, X or mordenite, were an improvement over other acid catalysts in this process, because they proved to be more resistant to nitrogen and sulfur poisoning^-^. The reforming process converts low octane paraffins and naphthenes to substantially higher c octane isoparaffms and aromatics over a bifuntional catalyst such as Pt Re/Al203. Recently, Chevron Inc. has developed a process that uses Pt Ba/zeolite L catalyst to convert normal hexane and heptane to benzene and toluene in high yield^^. The catalyst is not readily poisoned by common poisonous intermediates as methylcyclopentane, but appears to be highly sensitive to sulfur. One of the largest petrochemical applications of zeolites is in the xylene isomerization process. In this process, the Cg aromatic stream from the reformer is converted to an equilibrium 11 mixtures of xylenes, from which the most valuable product, p- xylene can be separated. ZSM-5 based catalysts have displaced the conventional Pt/amorphous silica alumina based catalysts in most of the U.S. and 60% of the world's xylene isomerization units due to their high activity, stability, and low xylene loss^-^. Other petrochemical processes where ZSM-5 is used as a catalyst are selective toluene dispropo'tionation process (STDP) , Mobil-Badger ethylbenzene process (from benzene and ethylene), ALBENE one step ethylbenzene process (from benzene and ethanol), Mobil's methanol to gasoline process (MTG), selective alkylation of toluene with methanol, para-selective processes to produce para ethyl toluene and para methyl styrene etc.^ . In addition to these processes, application of zeolites for the production of numerous fine chemicals have been reported".

1.9 The Production of Phenol via Cumene:

Phenol is a commodity chemical, which has wide end-use pattern dominated by phenolic resins^^. Major applications of phenol are summarized in Table 1.1

Table 1.1

1) Phenolic Resins — Domestic application / 2) Caprolactum Nylon Phenol — 3) Bisphenol A Epoxy Resins, polycarbonates \\ 4) Alkyl phenols Surfactants, Antioxidants \ 5) Adipic acid Nylon 6) Other uses e.g. plastisizers

Until recently, there were six possible routes to phenol (Table 1.2). Currently the dominant route for phenol production

12 is based on cumene and accounts for 9 0% of phenol manufactured in western Europe. This is because of distinct advantages of the curaene process over the others.

Table 1.2

Feedstock Intersiediate Designation

1) Benzene Isopropylbenzene Cumene Process 2) Benzene Benzene sulphonic acid Sulphonation Route 3) Benzene Chlorobenzene Dow process 4) Benzene Chlorobenzene Hooker-Rasching Process 5) Toluene Benzoic acid Dow/ DSM process 6) Benzene Cyclohexane Scientific Design Process

The cumene route to phenol traces its origin to some academic work carried out during the second world war in Germany by Hock and Lang^^. BP Chemicals carried out pioneering work in this field and the BP-Hercules process is widely used through"out the world 56 The process is divided into three stages

1) Alkylation of benzene to cumene

,CH3 / O) + CH3-CH = CH2 CH €^ CH.

13 2) Oxidation of cumene to cumene hydroperoxide

CH3 CH3 0-(" + O2

3) Cleavage of cumene hydroperoxide to give phenol and acetone

CH7> \ C = 0 / CH3

The process gives a yield of phenol about 93% based on cumene, and about 84% based on benzene^^. About 0.6 ton of acetone is produced per ton of phenol made. Hence the economics of the process are heavily dependent on the price obtainable for the acetohe. At present, the demand for acetone appears to be growing faster than that for phenol. Hence this process is favoured for the manufacture of phenol. Oxidation of cumene and cleavage of cumene hydroperoxide are well established processes^^'^^. Cumene is oxidized with air under carefully controlled conditions in the liquid phase. Cleavage of cumene hydroperoxide to phenol and acetone is carried out isothermally at ambient temperature in the presence of small amounts of a suitable acid catalyst e.g. dilute sulfuric acid catalyst.

14 1.10 Alkylation of Benzene to Cumene:

Alkylation of benzene to cumene is a Friedel Crafts type of alkylation reaction and is catalyzed by both protons" ( H^PO^) and Lewis acids e.g. BF3, on different supports and amorphous and crystalline aluminosilicates^-'-'^^. The reaction proceeds through activation of the olefin by the catalyst and this activated olefin then reacts with benzene. When a zeolite is used as a catalyst, the mechanism can be presented as

ll§. Zeol// CH^ -CH = H® CH2 + H 0. Zeol CH2 iCH2 /40. Zeol / ® e ® •CH3 -CH -CH, ^"^^^../^s

n) + + H 0 Zeol

Riedel type of mechanism is proposed for the reaction with proposed interaction between free benzene molecule and activated olefin molecule attached to an active site. Isopropylation of benzene is a liquid phase catalytic reaction and is operated very close to critical conditions of the reactants, propylene and benzene. Harper et al.^-^ studied the kinetics of the reaction. The overall reaction scheme is given below;

15 CH3 CH3 CH K Q] + CH3 -CH=CH2 n

CH CH3 CH3 ^CH3 ^H^' K. + CH3 -CH = CH-

CH3 CH3 CH3 CH3 XH CH K 3 ^ K

The rate equations were incorporated into a differential mass-balance of a tubular, plug flow reactor. Nonlinear differential equations thus obtained were used to get a simulation model. Values of rate and equilibrium constant calculated at two different temperatures using the simulation model are presented in Table 1.3

Table 1.3

Temperature (°C) k^xlO'^ k2XlO>- 5 k3XlO -4 k4XlO -4 K

232°C 0.2485 0.8890 0.5805 0.1340 4.34

214°C 0.1578 0.5580 0.3250 0.0590 5.51

16 4 5 6 7 8 9 10 Benzene-to-Propylene Ratio

Fig. 1.1: Influence of Benzene to Propylene molar ratio on adiabatic temperature rise; Tin = Raection inlet temperature.

17 where rate constants k^ = kj_Q x e~^ /'^'^ (g Total Fluid) ^/mole.g cat.hr Equilibrium constants = K = KQ e~ "^ /

The equilibrium constant for the transalkylation reaction decreases with increasing temperatures. This means that the cumene content at equilibrium is favored by decreasing the temperature, although the effect is not particularly great. Both the alkylation reactions (reaction 1 and 2), in addition to transalkylation reaction are exothermic, and release considerable heat, resulting in increase in the local temperature. This will further influence the various reaction rates. The adiabatic rise at an inlet temperature of 170°C for various benzene to propylene mole ratios is presented in Fig. 1.1. This temperature rise varies between 3 0 to 60°C over the range of variables studied.

1.11 Catalyst and Process for the Manufacture of Cumene:

Two major processes most widely used are UOP's Cumox process and Monsanto-Lummus Cumene process. The Cumox process^^'^^ for the production of phenol is accomplished via UOP's Catalytic Condensation Process for cumene. The process utilizes a solid phosphoric acid (SPA) catalyst. The process is illustrated in a flow diagram presented in Fig. 1.2. As shown in the diagram, a mixture of propylene, propane, with excess benzene, is pumped upwards in the alkylation reactor (1) filled with the SPA catalyst. The reactor effluent is routed through a two stage flash system (2,3) and a clay treator (6), prior to the final fractionator (7) . Much of the benzene separated in the flash system (5) is recycled back to the alkylation reactor. Cumene (99% pure) is separated in the

18 Benzene feed Propane Benzene Clay Cumerve recycle treater product a

5 6 r 1.^

2 Propylene ' ,, Heovy feed ' ' aromotics 8

Fig. 12: Flow-diagram for the cumene process.:

19 fractionation column and used for the phenol production. The bottom from the fractionator is a highly aromatic material containing mainly diisopropylbenzene (DIPB) isomers. The DIPB is reacted back to cumene in a transalkylation reactor. During the process, 0.67 kg benzene and 0.38 kg propylene are consumed per kg of cumene formed. A very small regulated amount of water is added to the feed to keep the catalyst away from becoming friable and disintegrating. The process operates at 99.3 wt.% conversion of propylene with 92.5% selectivity to cumene. Today, the process is used for the production of 90% of the worlds open market cumene supply . Monsanto-Lummus Crest process was developed from the research on ethylbenzene/styrene rather than on cumene . During the process, dry benzene and propylene are mixed in the alkylation reactor with the aluminium chloride-hydrogen chloride catalyst, at controlled, temperature, catalyst concentration, and residence time. The reactor effluent is washed with water and caustic to separate the organic from strongly acidic catalyst. Major features of this process are, low benzene recycle ratio and transalkylation of polyisopropylbenznes to cumene. Inspite of many advantages, both the processes suffer substantially from the drawbacks arising from corrosion and environmental problems. Also, the use of an acid catalyst requires special type of reactor material increasing the capital cost of the process. Hence, there is a commercial interest in finding an alternative catalyst system, preferably zeolite based, for this process.

20 1.12 Scope of this Work:

The scope of the present work may be stated as follows:

(1) To synthesize and characterize medium pore zeolite H-EU-1 and large pore zeolites H-beta and H-ZSM-12. (2) To study the influence of acidity and structural differences on activity, selectivity and stability of the catalysts for alkylation and transalkylation reactions for the preparation of cumene. (3) To investigate the mechanism of cokeing in different zeolites and factors responsible for it.

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