Properties of Zeolitic Alkylation Catalysts

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

The technology and chemistry of isoalkane-alkene alkylation have been thoroughly reviewed for both liquid and solid acid catalysts (15) and for solid acid catalysts alone (16). The intention of this review is to provide an up-to-date overview of the alkylation reaction with both liquid and solid acids as catalysts. The focus is on the similarities and differences between the liquid acid catalysts on one hand and solid acid catalysts, especially zeolites, on the other. Thus, the reaction mechanism, the physical properties of the individual catalysts, and their consequences for successful operation are reviewed. The final section is an overview of existing processes and new process developments utilizing solid acids. 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

As indicated above (Table I), 2- and 3-valent cation forms of mordenite are considerably less active in cyclohexane isomerization than that of hydrogen. It is this property of mordenite-supported catalysts that distinguishes them sharply from zeolites of the faujasite type. Thus, in the case of faujasites, the activity of H-form (decationized form) and 2-valent modifications (such as Ca and Mg ) in acid-base reactions (cracking, isomerization, alkylation) is the same, while H-mordenite is many times as active as any cation form under study. 

The chemical selectivity obtained in the alkylation of aromatic molecules over zeolitic catalysts critically depends upon their acid base properties. While xylenes are the primary products in the methylation of toluene over acidic zeolites like HZSM5 [e.g. 1,2,3], ethylbenzene and styrene are formed over basic zeolites such as Rb-X and Cs-X [e g. 4,5,6]. Previous reports suggested the surface chemistry of chemisorbed methanol to be the most decisive parameter to determine the selectivity [7,8]. Recent experiments on toluene methylation to xylenes indicated, however, that various bimolecular precursors to the transition state in the rate determining step exist and may be important for the catalytic properties of zeolites [9,10],... 

The alkylation of aniline by methanol is carried out at 623-723K over LiY and NaY non-protonic and non-basic zeolites to study the effect of reaction temperature on the catalytic activity and the selectivity. The reaction temperature influences strongly the aniline conversion, the selectivity and the catalyst life. Two types of products are observed, C- or N- alkylates whether the alkylation takes place on the ring or on the N atom. A general trend is that as the reaction temperature increases the catalytic activity and selectivity of N-alkylation increase while the selectivity of C-alkylation decreases. The IR results of benzene adsorption have been used to try to explain the catalytic properties of these two catalysts. 

The purpose of this work was to increase the A3 selectivity at low conversion through a catalyst modification. Previous studies of phenol alkylation with methanol (the analogue reaction) over oxides and zeolites showed that the reaction is sensitive to acidic and basic properties of the catalysts [3-5]. It is the aim of this study to understand the dependence of catalyst structure and acidity on activity and selectivity in gas phase methylation of catechol. Different cations such as Li, K, Mg, Ca, B, incorporated into y-Al203 can markedly modify the polarisation of the lattice and consequently influence the acidic and basic properties of the surface [5-8] which control the mechanism of this reaction. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Recently, a combinatorial investigation of alkylation of toluene with methanol to produce styrene with basic zeolites and alkaline earth catalysts was performed (257). The results of tests involving preparation and testing of more than 200 catalysts were modest, and these results emphasize that fine-tuning of the acid and base properties necessary to achieve better alkylation catalysts is not an easy task. 

Alkylation of Benzene by Propylene to Cumene Table 6.7 Physical properties of a zeolite catalyst. 

Sodium forms of zeolites X and Y are known to be Inactive for alkylation. Calcium Introduction (catalyst 1) has resulted In a catalyst with some activity. Selectivity of the sample was not high about 57% of the alkylate were octanes with a ratio of TfV to DW of 2 1. The yield and quality of the alkylate were Improved, If Na" " cations were replaced with cations of rare-earth elements (catalysts 2 and 3). Product yield for catalysts 2 and 3 were 86.0% and 77.5% respectively with a TMP content In C0-fractionof about 85%. Unfortunately the stabilities of these two catalysts were rather low In both cases, and alkylates yields and quality declined after 3 or 4 runs. For example the percentage of unsaturated hydrocarbons In the hydrocarbon product for catalyst 3 Increased from 18 up to 30%, and TMP concentration decreased to 35% after several runs. Catalyst 4 has proved to be the most active and stable catalyst and the yields and quality of alkylates obtained over It have been the same even after many reaction-regeneration cycles. Further Increase of calcium content In the catalyst (catalyst 5) deteriorated Its catalytic properties. 

The information about synthetic mordenite properties was obtained in 1961 when Keough and Sand (7) found that H- and other forms of this crystalline aluminum silicate display high activity and selectivity in the reactions of hydrocarbon cracking and ethanol dehydration. Later this zeolite was shown (J, 2, 5, 7, 8, 10-13, 15, 16) an active catalyst in the reactions of isomerization, cracking, and alkylation of hydrocarbons and alcohol dehydration. However, the catalytic properties of mordenite have been studied insufEciently, compared with those of other zeolites. 

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