Catalytic conversion of alkylaromatics to aromatic nitriles
Citation for published version (APA): Stobbelaar, P. J. (2000). Catalytic conversion of alkylaromatics to aromatic nitriles. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR538872
DOI: 10.6100/IR538872
Document status and date: Published: 01/01/2000
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Catalytic Conversion of Alkylaromatics to Aromatic Nitriles
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr. M. Rem, voor een commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 28 november 2000 om 16.00 uur
door
Pieter Johannes Stobbelaar
geboren te Driebergen-Rijsenburg Dit proefschrift is goedgekeurd door de promotoren: prof.dr. R.A. van Santen en prof.dr. B.K. Hodnett
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Stobbelaar, Pieter J.
Catalytic conversion of alkylaromatics to aromatic nitriles / by Pieter J. Stobbelaar. - Eindhoven : Technische Universiteit Eindhoven, 2000. - Proefschrift. - ISBN 90-386-2612-6 NUGI 813 Trefwoorden: katalytische oxidatie ; ammoxidatie / heterogene katalyse ; zeolieten / overgangsmetaalverbindingen ; molybdeenverbindingen Subject headings: catalytic oxidation ; ammoxidation / heterogeneous catalysis ; zeolites / transition metal compounds ; molybdenum compounds
The work described in this thesis has been carried out at the Schuit Institute of Catalysis (part of NIOK: the Netherlands School for Catalysis Research), Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, The Netherlands. Financial support has been supplied by the European Community under the Industrial & Materials Technologies Programme (Brite-EuRam III).
Printed at Universiteitsdrukkerij, Eindhoven University of Technology.
Contents
Chapter 1: Nitrile formation and conversion reactions 1 Abstract 1 1. Aromatic nitriles: Production and applications 1 1.1 Ammoxidation reactions 1 1.2 (Potential) Applications of nitriles 3 1.3 Aromatic nitriles as intermediates in selective oxidation reactions 5 2. Scope of research 7 References 9
Chapter 2: Toluene ammoxidation mechanism 11 Abstract 11 1. Main reaction steps during toluene ammoxidation 11 2. Toluene activation 12 2.1 Hydrocarbon rupture 12 2.2 Effect of substituents on the aromatic ring 13 2.3 Effect of the catalyst basicity on the ammoxidation of 15 alkylaromatics 2.4 Nature of aromatic reaction intermediate 18 3. Ammonia activation 23 4. Catalyst reoxidation 27 5. Toluene ammoxidation reaction schemes 29 5.1 The propylene ammoxidation mechanism 29 5.2 The ammoxidation of toluene 32 6. Conclusions 35 References 36
Chapter 3: Screening of new toluene ammoxidation catalysts 41 Abstract 41 1. Introduction 41 2. Experimental methods 43 2.1 Catalyst preparation and characterization 43 2.2 Catalyst testing 45 3. Results and discussion 46 3.1 Catalyst screening 46 3.2 Catalyst deactivation 50 3.2.1 Performance of ion-exchanged catalysts 52 Contents
3.2.2 Performance of catalysts prepared by CVD of metal carbonyls 52 3.2.3 Performance of NaY based impregnated catalysts 53 3.2.4 Performance of γ-alumina supported catalysts 55 3.3 Benzonitrile selectivity 57 3.4 Temperature influence 60 3.5 Nitroxidation of toluene 61 4. Conclusions 64 References 64
Chapter 4: Faujasite encaged metal oxide toluene ammoxidation catalysts 67 prepared from metal carbonyl precursors Abstract 67 1. Introduction 67 2. Materials and methods 71 2.1 Catalyst preparation 71 2.2 Catalyst characterization 72 2.2.1 Determination of the catalyst composition 72 2.2.2 X-Ray Photoelectron Spectroscopy 73 2.2.3 Transmission Electron Microscopy 73 2.2.4 Temperature Programmed Oxidative Decarbonylation 73 2.3 Catalytic tests 74 2.3.1 2-Methyl-3-butyn-2-ol decomposition 74 2.3.2 Toluene ammoxidation 74 3. Results and discussion 75
3.1 Thermal activation of intra-zeolite Mo(CO)6 75
3.2 XPS analysis of Mo(CO)6/NaY and MoOx/NaY 80 3.3 Dispersion of molybdenum oxide clusters in NaY 84
3.4 Mo(CO)6 interaction with the faujasite lattice 87 3.5 Introduction of other transition metal carbonyls by CVD 92
3.5.1 Introduction of V(CO)6 into NaY 92
3.5.2 Introduction of Mn2(CO)10 into NaY 94
3.5.3 Introduction of Co(NO)(CO)3 into NaY 96 3.6 Catalytic activity in the ammoxidation of toluene 97 3.7 The effect of the Lewis acidity and basicity on the ammoxidation 99
of toluene over MoOx/Y 4. Conclusions 100 References 101
Contents
Chapter 5: The effect of molybdenum oxide reducibility on the ammoxidation of 105 toluene Abstract 105 1. Introduction 105 1.1 Preparation methods of supported Mo catalysts 106 1.2 Notation of different Mo species 107 1.3 Molybdate surface species 108 1.4 Characterization of Mo surface species 110 1.5 Molybdate and Mo oxide reduction 112 2. Materials and methods 114 2.1 Catalyst preparation 114 2.2 Catalyst characterization 114 2.2.1 Diffuse reflection UV-Vis spectroscopy 114 2.2.2 Temperature Programmed Reduction 115 2.2.3 Raman Spectroscopy 115 2.2.4 Transmission Electron Microscopy 115 2.2.5 X-Ray Diffraction 116 2.2.6 X-Ray Photoelectron Spectroscopy 116 2.2.7 Hydrogen–deuterium exchange reactions 116 2.3 Ammoxidation of toluene 117 3. Results and discussion 117 3.1 Addition of a second metal to Mo/Al 117 3.2 Variation of the molybdenum oxide loading 120 3.3 DR-UVVis Spectroscopy 121 3.4 Reduction of Mo/Al catalysts 124 3.5 Hydrogen-deuterium exchange over Mo/Al catalysts 126 3.6 Transmission Electron Microscopy on Mo/Al samples 131 3.7 In situ treatment of Mo/Al 131 4. Conclusions 137 References 138
Summary 143
Samenvatting 147
Dankwoord 151
Curriculum Vitae 153
Chapter 1
Nitrile formation and conversion reactions
Abstract The background of the research project is described. Ammoxidation of alkylaromatics is a simple gas-phase reaction that yields aromatic nitriles. These nitriles have versatile applications, mainly as raw material in the polymer industry. Additionally, alkylaromate ammoxidation can be applied in the production of selective oxidation products since nitriles can be converted by hydrolysis and hydrogenation reactions towards acids, aldehydes, amines and amides. This two-step approach cleanly yields the oxygenate without production of harmful side products. The project focuses on the ammoxidation of toluene. For this reaction the development of new faujasite-based catalysts was performed. Additionally, a comparison with more conventional γ-alumina supported molybdenum oxide catalysts has been made.
1. Aromatic nitriles: Production and applications
1.1 Ammoxidation reactions Aromatic nitriles can be formed by reacting an aromatic hydrocarbon with ammonia and oxygen. The simplest example is benzonitrile production from toluene, as shown in Equation 1.1.
CH3 + NH3 + 3/2 O2 CN + 3 H2O (1.1)
The reaction of a reducible hydrocarbon with ammonia and oxygen are referred to as ammoxidation reactions. Alkenes, alkanes and aromatics are used most often in ammoxidation reactions. The catalysts that are active in ammoxidation reactions consist mainly of mixed oxides containing variable-valence transition metals.
For the ammoxidation of propylene bismuth-molybdate based systems are applied industrially on a large scale [1]. The ammoxidation of propylene is well developed and is commercially applied by Sohio since the early sixties
1 Chapter 1
[2]. The annual world production amounts to 4600 ktons [3]. Until recently the production of acrylonitrile from propane could only be performed at very high temperatures (750 – 1000 °C) [4]. In the late eighties propylene ammoxidation to acrylonitrile has been patented frequently, for example by BP America (previously SOHIO) [5]. Recently, a large variety of new catalysts have been developed for the ammoxidation of propane. Mostly vanadium antimony oxide [6] systems are reported, though also molybdenum based multi-component catalysts are frequently patented [7]. Pilot-plant studies have been performed already [8] and commercial production of acrylonitrile from propane was announced [9]. The feedstock price of propane is significantly lower than that of propylene, but the acrylonitrile yields are markedly lower because of the poor acrylonitrile selectivity [10,11]. This lower acrylonitrile yield per mole of feedstock delays commercial production of acrylonitrile from propane. To date acrylonitrile production from propane has not started yet [12].
Aromatic ammoxidation reactions are performed mostly over vanadia based catalysts [13]. Several companies practice commercial ammoxidation of alkylated aromatics [14]. Showa Denko converts p-xylene and m-xylene to the corresponding di-nitriles, terephthalonitrile and isophtalonitrile. Also Mitsubishi Gas Chemical ammoxidizes m-xylene to isophtalonitrile on a commercial scale. They operate two plants in the USA and in Japan. BASF and Japan Catalytic Chem. Ind. produce phthalonitrile from o-xylene. Phthalonitrile is applied as an important precursor in the manufacturing of phthalocyanine dyes.
Typically, ammoxidation reactions are performed at temperatures between 400 and 500 °C. For propane ammoxidation the reaction temperature may be somewhat higher, because the dehydrogenation of propane needs higher temperatures to occur. During ammoxidation the catalysts is reduced by ammonia and the hydrocarbon. It is generally accepted that lattice oxygen reoxidizes the catalyst during ammoxidation. The oxygen insertion step and catalysts reoxidation can be performed in separate reactors [15]. Under these conditions the production of nitriles from hydrocarbons is referred to as oxidative ammonolysis. In this way explosion hazards can be eliminated, since the hydrocarbon mixture and oxygen are not mixed together in one reactor.
2 Introduction and background
1.2 (Potential) Applications of nitriles The most well-known ammoxidation reaction is the ammoxidation of propylene to form acrylonitrile. Acrylonitrile is basically used for the production of acrylic fibers, which can be used for manufacturing of clothing and carpets [16]. Worldwide, about 65 % of the acrylonitrile production is consumed for this purpose. Another important use of acrylonitrile can be found in the resin production, of which acrylonitrile- butadiene-styrene (ABS) and styrene-acrylonitrile (SAN) are the main applications. An extensive growth in the application of these resins has occurred during the past decade. The largest increase among the end uses of acrylonitrile, however, has come from adiponitrile, which is used as a precursor for hexamethylenediamine (HMD) by Monsanto [17]. HMD is used for the production of nylon-6,6. Recently, also the large-scale production of caprolactam from adiponitrile was reported [18]. Caprolactam is used as precursor for nylon-6, which can be produced in the same production site. Other applications of acrylonitrile are also found in the polymer industry.
Catalytic hydrogenation of nitriles may result in several products. Among these, amines, imines, aldehydes, amides and alcohols are the most important products. The main product depends on the catalyst, substrate and reaction conditions [19].
Aromatic nitriles find diverse applications, for example as dyes and in pesticide and fungicide production but also in various nylons and polyurethane foams. Benzonitrile is used as a precursor for resins and coatings. Benzonitrile is also used as an additive in fuels and fibers. Table 1.1 lists the main application of some relatively simple substituted aromatic nitriles. As already discussed nitriles derived from xylenes are primarily used as precursors for the corresponding di-acids, for ultimate use in esters and polyesters.
3 Chapter 1
Table 1.1: Applications of substituted benzonitriles. Compound Application Remarks intermediate in the production of 2- 2-chlorobenzonitrile azo dyes amino-5-nitrobenzonitrile red pigment 4-chlorobenzonitrile for plastics herbicide for intermediate in the production of 2,6- 2,6-dichlorobenzonitrile fruit and vine difluorobenzonitrile and 2,6- cultivation dichlorothiobenzamide 2,6-difluorobenzonitrile insecticides intermediate 2-chloro-4-nitro- azo dyes intermediate benzonitrile 4-chloro-2-nitro- intermediate in the production of 2- azo dyes benzonitrile amino-4-chlorobenzonitrile 2-amino-5-nitro- azo dyes intermediate benzonitrile intermediate in the production of 3,5- 4-hydroxybenzonitrile herbicides dibromo- and 3,5-diiodo-4-hydroxy- benzonitrile Data from [20].
The applications of more complex substituted aromatic and hetero aromatic nitriles were described by Grasselli et al. [21]. For example, high performance polymers are formed from atroponitrile. Related substituted aromatic aldehydes such as atropaldehyde and cinemaldehyde, which can be produced by direct gas-phase oxidation of the substrate, are used as flavors or perfumes in different products. In Vitamin B complex nicotinamide (niacinamide) and nicotinic acid (niacin) are used. These products are formed from nicotinonitrile, which can be obtained readily by ammoxidation of 3-methylpyridine over phosphorous molybdate-vanadate catalysts. Additionally, fungicides can be prepared from heteroaromatic nitriles such as 4-cyanothiazole. In Scheme 1.1 some examples are summarized of aromatic nitriles and their applications. Also the most commonly used catalysts are indicated for each example.
4 Introduction and background
Application as monomers for high performance polymers:
H2C CH3 H2C CN USb O + 4.6 x + NH3 + 3/2 O2 3 H2O
Application in resin and coating production:
V O + + 2 5 + CH3 NH3 3/2 O2 CN 3 H2O
Application in Vitamin B complex:
CN CH3 PVMoO + + x NH3 3/2 O2 + 3 H2O N N
Application as intermediate in fungicide production:
H3C NC N various catalysts N + + NH3 + 3/2 O2 3 H2O S S Scheme 1.1: Applications of several ammoxidation reactions.
1.3 Aromatic nitriles as intermediates in selective oxidation reactions During the eighties ammoxidation reactions were frequently investigated, especially the ammoxidation of alkylaromatics. The ammoxidation of toluene to form benzonitrile was often used as a model reaction for other alkylaromatics such as p-xylene. Showa Denko and Lummus ammoxidize xylenes to mono- and di-nitriles. In the Lummus process aromatic nitriles are prepared via an ammonolysis reaction. Xylene reacts with ammonia and lattice oxygen to form the aromatic nitrile. Gas-phase oxygen is used afterwards to regenerate the catalyst [15]. As described earlier by these authors terephthalic acid can be produced via hydrolysis of the nitriles [22]
5 Chapter 1 produced in this oxidative ammonolysis reaction. The conversion of alkylaromatics to oxidized products such as terephthalic acid is usually performed by direct oxidation reactions. Since the performance of alkylaromatic autoxidation reactions is relatively simple, because ring oxidation does not occur, these reactions are performed basically in the liquid phase [23]. The oxidizability of alkylaromatic hydrocarbons by liquid-phase autoxidation reactions decreases significantly in the order tertiary > secondary > primary benzylic C-H bonds [24]. Therefore, liquid- phase autoxidations have somewhat limited applications. Especially for primary alkylaromatics such as toluene, it is not possible to achieve high selectivity to hydroperoxide at reasonable high reaction rates. Since the oxidizability of toluene is about five orders of magnitude lower than the oxidizability of benzaldehyde [25] production of benzaldehyde by autoxidation is not possible. Nevertheless, terephthalic acid can be produced in high yields by liquid-phase direct oxidation using a Co/Mn/Br-acetic acid catalyst [26]. Though the yield of terephthalic acid by the conventional liquid-phase process is high, future regulations may restrict the application of this reaction, since the process conditions, which require the highly corrosive bromine–acetic acid environment, are reprehensible from environmental perspective. On the other hand due to the low solubility of terephthalic acid in the solvent, most of it precipitates as it forms. Separation of the product from the solvent is easy and the production process will only be changed if future legislation so obliges.
Based on the atom utilization concept described by Sheldon and Dakka [27], in general gas-phase oxidations are preferred over liquid-phase oxidation processes. Moreover, the use of gas-phase oxygen as oxidant is highly desirable since besides the oxidation product only water is produced. The Environmental Quotient (EQ), which is defined by the amount of waste per kilogram of product multiplied by an unfriendliness quotient (Q) is as low as possible for oxidation reactions. In this respect aromatic nitriles can be used as intermediates in selective oxidations. According to Equation 1.1 the aromatic nitrile is manufactured with high atom utilization; only water (having a low Q value) is formed as side product. Conversion of the aromatic nitrile in a second step cleanly yields the oxidation product.
Up to now the only industrially important manufacturing process for benzaldehyde is the hydrolysis of benzal chloride or the partial oxidation of
6 Introduction and background toluene [28]. The first route is highly productive and high benzonitrile selectivity is obtained (> 95%). However, for each molecule of benzaldehyde one hydrogen chloride molecule is produced as a side product. Direct selective oxidation of toluene is a clean route. To date, however, only moderate benzaldehyde yields are obtained. Very recently the group of Centi developed a bulk-type Fe-Mo-Ce-oxide catalyst that produces in high yield (50-55 mol%) 3-fluorobenzaldehyde from 3- fluorotoluene [29]. Via classical organic chemistry aldehydes can be formed from nitriles by performing a reduction with di-isobutylaluminumhydride [30].
Chatterjee et al. [31] produced benzaldehyde from benzonitrile over platinum and ruthenium loaded acidic zeolites with high selectivity by vapour-phase reductive hydrolysis. This reaction can also be performed in the liquid phase using Raney nickel [32] though a sulphuric acid or formic acid medium has to be applied in this case. Also nickel and iron precipitated on alumina catalysts have been described in literature for the liquid-phase reductive nitrile hydrolysis [33]. By hydrogenation aromatic nitriles can also be converted to aromatic amines [14]. The production of benzamide can be performed selectively over hydrotalcite-like catalysts as will be reported by Sychev et al. [34]. In our group theoretical work related to aromatic nitrile conversion, was carried out by Barbosa et al. [35], who studied the hydrolysis of acetonitrile over protonic zeolite catalysts.
2. Scope of research The research described in this thesis was aimed at the development of new, selective and clean processes for alkylaromatic side chain oxidation. A gas- phase process was chosen for the conversion of the alkylaromatic side- chain oxidation, based on the poor opportunities for liquid-phase processes. Catalyst leaching complicates severely the possibilities of liquid-phase heterogeneously catalyzed processes. Moreover, the higher cost of the oxidant does not favor the economics of the process. Therefore, a two-step vapour-phase process was studied, in which an alkylaromatic substrate is converted by ammoxidation to an aromatic nitrile. In a second step this aromatic nitrile is subjected to a hydrolysis reaction to form the oxygenate. This reaction pathway cleanly yields oxygenated aromatics, as sketched in Scheme 1.2.
7 Chapter 1
Amide formation:
+ + + CH3 NH3 3/2 O2 CN 3 H2O
O - + OH + + CN 2 H2O2 CNH2 O2 H2O
Overall: O + + 2 + O + CH3 NH3 H2O2 1/2 2 CNH2 4 H2O
Aldehyde formation:
+ + + CH3 NH3 3/2 O2 CN 3 H2O
O
CN ++H2 H2O + NH3 H
Overall: O CH + + 3 O2 H2O H
Scheme 1.2: Ammoxidation and sequential hydrolysis to cleanly produce oxygenated aromatic hydrocarbons.
For the ammoxidation reaction toluene was chosen as substrate, based on the relative simplicity of the molecule. An elementary screening study was performed in order to check the feasibility of faujasite-based catalysts for the ammoxidation reaction. The performance of zeolite Y encaged molybdenum oxide nanoclusters was compared to that of γ-alumina supported molybdenum oxide. The properties of the latter catalysts were studied in great detail by both in situ and ex situ characterization techniques.
This thesis focuses on the ammoxidation of toluene. Theoretical work on nitrile hydrolysis was performed by Barbosa [36]. The hydrolysis of
8 Introduction and background benzonitrile was studied in the National Technical University of Ukraine, Kiev by Prihod’ko [37].
References 1. J.F. Brazdil, Acrylonitrile, in: Kirk-Othmer Encyclopedia of Chemical Technology, Vol 1. Wiley New York, 4th edition, pp. 352-369. 2. J.D. Idol, US Patent 2904580, J.D. Idol, 1959. 3. http://www.chemweek.com/productfocus/1996/acrylonitrile.html 4. P.W. Langvardt, Acrylonitrile, in: Ullmann’s Encyclopedia of Industrial Chemistry, 6th (electronic) edition, 1999. 5. E.g. A.T. Gutmann, R.K. Grasselli, J.F. Brazdil, US Patent 4746641, (1988). 6. G. Centi, F. Marchi, S. Perathoner, J. Chem. Soc. Faraday Trans., 92, (1996), 5141-5149. 7. H. Midorikawa, N. Sugiyama, H. Hinago, US Patent 6973186, 1999. H. Kazuyuki, S. Komada, US Patent 5907052, 1999. K. Aoki, US Patent 5780664, 1998. H. Midorikawa, K. Someya, K. Aoki, O. Nagano, US Patent 5658842, 1997. H. Midorikawa, K. Someya, US Patent 5663113, 1997. R. Canavesi, F. Ligorati, R. Ghezzi, US Patent 4609635, 1986. 8. P. Fairley, Chem. Week, 160(36), (1998), 45. 9. P. Layman, Chem. Eng. News, 73, (1995), 13-15. 10. B.K. Hodnett, Heterogeneous Catalytic Oxidation, John Wiley & Sons, Chichester, 2000, pp. 240-263. 11. K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition, VCH, Weinheim, 1997, pp. 303-310. 12. R.K. Grasselli, Ammoxidation, in: Handbook of Heterogeneous Catalysis, Eds. G. Ertl, H. Knözinger, J. Weitkamp, Vol. 5, VCH, Weinheim, 1997, pp. 2302-2326. 13. R.G. Rizayev, E.A. Mamedov, V.P. Vislovskii, V.E. Sheinin, Appl. Catal. A, 83, (1992), 103-140. 14. K. Weissermel, H-J. Arpe, Industrial Organic Chemistry, 3rd edition, VCH, Weinheim, 1997, pp. 385-403. 15. M.C. Sze, A.P. Gelbein, Hydrocarbon. Proc., 1976, 103-106. 16. J.F. Brazdil, Acrylonitrile, In: Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, Vol. 1, pp. 352-369. 17. M.M. Baizer, C.R. Campbell, R.H. Fariss, R. Johnson, US Patent 3193480, 1965. 18. Chemical Market Reporter, 254, (1998), 5. 19. G.V. Smith, F. Notheisz, Heterogeneous Catalysis in Organic Chemistry, Academic Press, San Diego, 1999, p. 71-79. 20. P. Pollak, G. Romeder, F. Hagedorn, H-P. Gelbke, Nitriles, in: Ullmann’s Encyclopedia of Industrial Chemistry, 6th (electronic) edition, 1999. 21. R.K. Grasselli, J.D. Burrington, R. Di Cosimo, M.S. Friedrich, D.D. Suresh, in: Heterogeneous Catalysis and Fine Chemicals, Eds. M. Guisnet, J. Barrault, C. Bouchoulle, D. Duprez, C. Montassier, G.
9 Chapter 1
Pérot, Stud. Surf. Sci. Catal., Vol. 41, Elsevier Science Publishers B.V., Amsterdam, 1988, pp. 317-326. 22. A.P. Gelbein, M.C. Sze, R.T. Whitehead, Hydrocarbon Processing, (1973), 209-215. 23. R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, 1981, pp. 315-339. 24. J.A. Howard, Adv. Free Radical Chem., 4, (1972), 49-173. 25. R.A. Sheldon, Liquid Phase Autoxidations, in: Catalytic Oxidation, Principles and Applications, (Eds. R.A. Sheldon, R.A. van Santen), World Scientific, London, 1995, pp. 150-174. 26. W. Partenheimer, Catal. Today, 23, (1995), 69-158. 27. R.A. Sheldon, J. Dakka, Catal. Today, 19, (1994), 215-246. 28. F. Brühne, E. Wright, Benzaldehyde, in: Ullmann’s Encyclopedia of Industrial Chemistry, 6th (electronic) edition, 1999. 29. G. Centi, Private communications. 30. T.W.G. Solomons, Organic Chemistry, 5th edition, John Wiley & Sons Inc., New York, 1992, pp. 686-687. 31. A. Chatterjee, R.A. Skaikh, A. Raj, A.P. Singh, Catal. Lett., 31, (1995), 301-305. 32. P. Tinapp, Chem. Ber., 102, (1969), 2770-2776. B. Staskun, O.G. Backeberg, J. Chem. Soc., (1964), 5880-5881. T. Es, B. Staskun, J. Chem. Soc., (1965), 5775-5777. 33. Z. Bodnar, T. Mallat, J. Petro, J. Mol. Catal., 70, (1991), 53-64. 34. R. Prihod’ko, I. Kolomitsyn, M. Sychev, P.J. Stobbelaar, R.A. van Santen, Micr. Mesop. Mat., to be published. 35. L.A.M.M. Barbosa, R.A. van Santen, J. Catal., 191, (2000), 200-217. 36. L.A.M.M. Barbosa, Theoretical Study of Nitrile Hydrolysis by Solid Acid Catalysts, PhD Thesis, Eindhoven University of Technology, 2000. 37. R. Prihod’ko, Synthesis and Characterization of some Heterogeneous Catalysts for Fine Organic Chemistry, PhD Thesis, National Technical University of Ukraine, Kiev, in preparation.
10
Chapter 2
Toluene ammoxidation mechanism
Abstract The mechanism of the ammoxidation of toluene is reviewed. Ammoxidation of toluene is mainly studied over vanadia-based catalysts. Although the literature is not consistent with respect to the exact mechanism some general trends can be observed. The rate-determining step is the hydrocarbon activation. Most authors agree on the formation of an oxygenated adsorbed organic intermediate. Toluene is adsorbed on the catalyst surface as a benzyl fragment. This benzyl species is oxygenated to form an adsorbed benzaldehyde surface structure. This structure is sometimes also referred to as benzoate species. Additionally a reaction pathway via sequential dehydrogenation of adsorbed benzyl species to an adsorbed amine and imine is plausible. Oxygen is supplied as surface oxygen, according to a Mars-Van Krevelen like mechanism. The exact nature of the nitrogen insertion site is studied less extensively. The amount of ammonia plays a decisive role in the catalyst oxidation state. Strong ammonia adsorption leads to an inactive catalyst, whereas weak ammonia adsorption leads to combustion reactions.
1. Main reaction steps during toluene ammoxidation Several groups have studied toluene ammoxidation in the past years. Rizayev et al. [1], concentrating on Russian literature, have reviewed the ammoxidation of simple alkylaromatics over vanadium oxide based catalysts in 1992, but no literature overview exists that also discusses other catalyst systems. In addition, toluene ammoxidation over VPO-based catalysts as investigated intensively during the last five years by the group of Lücke et al. [2,3] was not included in this review. Recently Centi et al. [4] discussed in more detail ammonia activation with respect to the ammoxidation of alkylaromatic compounds. This section discusses in a more extensive manner the role of the several reaction steps in the ammoxidation of toluene.
11 Chapter 2
In toluene ammoxidation reactions three important processes occur: 1. Toluene activation, during which the methyl group has to be dehydrogenated. 2. Ammonia activation, leading to the formation of the nitrogen insertion species. 3. Reoxidation of the catalysts by consumption of gas-phase oxygen.
Scheme 2.1 summarises these steps in toluene ammoxidation.
+ (O) (I) (1) C6H5CH3
+ (I) + ( ) (2) NH3 C6H5CN
( ) (O) (3) 1/2O2 + Scheme 2.1: Fundamental steps during toluene ammoxidation
It is generally agreed that activation of the methyl group is the rate- determining step during toluene ammoxidation [E.g. 5,6]. The nature of this activated species, however, is still under debate. According to Golodets [7] partial oxidation reactions occur on oxide catalysts by a mechanism of alternating reduction and oxidation of the catalyst surface. Total oxidation reactions, on the other hand, proceed via both redox and associated mechanisms. This is also true for ammoxidation reactions.
In this chapter the literature on the role of each of these three steps in the ammoxidation of toluene is reviewed.
2. Toluene activation
2.1 Hydrocarbon rupture The pathway of hydrocarbon activation has been studied by several groups, mostly by applying kinetic studies or IR Spectroscopy. If the nitrile production occurs along the side chain three basic types of C-H activation must be considered: 1. Heterolytic rupture, producing a carbocation and an H--ion. This possibility is believed to occur over acidic catalysts. When this pathway of C-H rupture occurs, the H--ion binds to the acid centre to give hydrogen, which is oxidised to water in the presence of oxygen. This pathway, however, was never proven experimentally.
12 Toluene ammoxidation mechanism
2. Heterolytic rupture, producing a H+ ion and a carbanion. This mechanism is plausible over sufficiently strong basic sites. The hydrocarbon acts as an acid when this C-H rupture mechanism applies to the reaction. 3. Homolytic rupture. A hydrocarbon radical and a hydrogen radical are formed. Contrary to the two heterolytic C-H rupture mechanisms the presence of electron donating or electron withdrawing side groups on the benzene ring should have little influence on the activity or selectivity in the ammoxidation of toluene.
2.2 Effect of substituents on the aromatic ring To examine in more detail these types of C-H rupture several groups have studied the effect of electron donating and withdrawing side-groups on the aromatic ring. The addition of an electron-withdrawing group (especially in the para position) to the aromatic ring would increase the reactivity of the methyl group if C-H rupture occurs according to heterolytic rupture via the formation of a carbocation. In this case the formation of a carbanion would be favoured if an electron-donating group is attached to the aromatic ring. The effect of substituents on the aromatic ring can be divided into an inductive effect, in which charges are stabilized by the aromatic ring and a resonance effect, which applies to groups that contain a lone pair of electrons. Generally, the resonance effect is much stronger than the inductive effect. Moreover, the resonance effect is directed to the substituent position. Electron donating groups such as -NH2 stabilize cation formation only in the ortho- and para-position. Table 2.1 lists the most important substrates used in ammoxidation reactions.
Table 2.1: Substituent effect of the most important alkylaromatic ammoxidation substrates Inductive effect Electron withdrawing Electron donating
NO2 CH3
CN, CHO, COOH C2H5
OH C6H5
Resonance effect Halogens NH2
OCH3 OH Data from Morrison and Boyd [8].
13 Chapter 2
Over titania-supported vanadia catalysts the ammoxidation activity is increased when a substituted toluene is applied as substrate, both with electron withdrawing as with electron donating substituents as found by Busca et al. [9]. The relative alkylaromatic ammoxidation rates are listed in Table 2.2.
Table 2.2: Ammoxidation rates over substituted alkylaromatic substrates Substrate RelatiRelativeve ammoxidation rate Toluene 1.00 m-Xylene 1.12 p-Methoxytoluene 1.27 p-Chlorotoluene 1.42 p-Xylene 1.45 Activities measured at T = 300 ° C over a V-Ti-O catalyst [9].
These data support the occurrence of homolytic C-H rupture over catalysts that have well defined redox properties. Cavani et al. [6] reported the ammoxidation activity of a series of para-substituted alkylaromatics over V- Ti-O. Compared to toluene they found a higher activity towards the nitrile product for all substrates applied, irrespective the electron donating or withdrawing properties of the substituents. This does support a homolytic C-H rupture mechanism. The same group found similar ammoxidation activity with respect to toluene ammoxidation when a methyl group was situated in the meta-position [10]. The resonance effect predicts a strong difference between the ortho- and para-position on the one hand and the meta-position on the other hand. Differences that were found in selectivity towards the nitrile products could be explained well by steric effects. Earlier steric hindrance was found by Chmyr et al. [11] who found lower nitrile yields when the aromatic ring was substituted in the 2 and 6 positions. The methyl group was less accessible for reaction in this case as a result of the presence of these chloro substituents in the 2 and 6 position. 3,5 Chloro substitution protected ring oxidation without decreasing the ammoxidation activity.
Cavani et al. [6], however, found significantly lower selectivities towards alkylaromatic nitriles when a strong electron-donating group (methoxy) was attached to the aromatic ring in the para-position. This significantly lower nitrile production could not be explained by the homolytic rupture mechanism proposed. The higher electron density in this case led to a more
14 Toluene ammoxidation mechanism
- - pronounced attack by electrophilic centres such as O2 or O . This electrophilic attack in general leads to degradation of the aromatic ring for hydrocarbon oxidation reactions [12]. As a result the nitrile selectivity and thus nitrile yield is decreased. When weaker electron donating groups were applied the selectivity was similar to that in toluene ammoxidation.
Similar effects were
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