Alkylation zeolite active sites

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

A comparative study with various types of zeolite showed that Cs-exchanged X and Y zeolites were active for toluene alkylation but primarily catalyzed the decomposition of methanol to CO.431 L and Beta zeolites, in turn, were less active and required higher reaction temperature but were much more selective, providing only very little CO. Adding boron to Cs-exchanged zeolites promotes the alkylation reaction 432 It appears that boron reduces the decomposition of methanol to CO without inhibiting active sites for side-chain alkylation. 

A further possibility for side-chain alkylation of toluene is oxidative methylation with methane. Catalysts with occluded alkali metal oxides, prepared by impregnating zeolites with alkali metal hydroxides followed by calcination, usually exhibit better performance.441 Further enhancement was achieved by impregnating ion-exchanged zeolites 442 Significant improvements in stability and the yields of Cg hydrocarbons were also observed when NaX was impregnated with 13% MgO which was found to increase the amounts of active sites.443... 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

The rate decay time constant is independent of cation form of the zeolite in the ethene system (Figure 4) although the alkylation activity of the three forms is considerably different (Figure 2). This indicates that the active site within the zeolite (at least for deactivation) is the same for all three cation forms as expected from our current picture of active sites for acid-catalyzed reactions in these zeolites (8, 18, 19). The three catalysts should have different numbers of active sites because of their individual response to activation at 823°K, but these sites should be similar thus M2 should be independent of cation form, Mi should depend on it. 

Some zeolitic and non-zeolitic molecular sieve catalysts are claimed to be capable for ortho- and para-selective alkylation using olefin as alkylating agent (refs. 1,2). Zeolite catalysts are less active and selective in the methylation of aniline by methanol (refs. 3,4). Reaction is usually carried out with a large excess of methanol since a large fraction of the alcohol decomposes without participating in the alkylation. Numerous N- and C-alkylated aniline derivatives appear in the reaction product. It was found that N-alkylation requires basic sites while C-alkylation occurs mainly on acidic sites (refs. 5-7). 

As with HZSM5-la, we attribute the initial deactivation to blocking of catalytically active sites by adsorbed xylene molecules preventing toluene methylation to occur at these sites. The longer residence time of the bulkier xylene isomers in the larger crystals of HZSM5-2 (see Table 1) seems to favour further alkylation of m- and o-xylene to trimethylbenzenes over isomerization to p-xylene Once trimethylbenzene is formed, dealkylation is rather difficult at 573 K and its rate of transport is too low to be able to diffuse out of the zeolite pores. It forms, thus, a dead end product that decreases the availability of active sites and reaction intermediates (leading to slow deactivation). 

Armengol et al. [227] used protonated Al-MCM-41 molecular sieve for alkylation of bulky aromatic compounds such as 2,4-di-rerr-butylphenol with a bulky alcohol (cinnamyl alcohol). This reaction did not occur in the presence of large pore HY zeolite indicating the importance of the mesoporous structure of the H-MCM-41 catalyst and the accessibility of active sites. Kloetstra et aL [228] obtained excellent results during the tetrahydropyranylation of alcohols and phenols over Al-MCM-41 (Scheme 3). Bulky alcohols including cholesterol, adamantan-l-ol and 2-naphthol were converted into the corresponding tetrahydropyranyl ethers in relatively short periods of time. 

An important item related to the hardness and softness of the active sites of the zeolites is the change in selectivity in the alkylation reactions of aromatic compounds. In the methylation of toluene, orbital control tends to give p-xylene [19]. In this context, it has been shovra that H-ZSM-5 and H-ZSM-11 with a Si/Al ratio of 17 is a soft electrophile [20]. Soft electrophile have a low-energy LUMO. A soft-soft reaction is fast because of a large interaction between the HOMO of the nucleophile and the LUMO of the electrophile... 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

It can be seen that, under similar conditions, the reactivity of the dialkyl sulfides is directly linked to their molecular size Et2 S > Pr2 S > Bu2 S, and saturated sulfides are more reactive than allyl or aryl sulfides Pr2 S > Me S Allyl > Allyb S > Me S Ph > Ph2 S. These results can be explained, first, if we take into account the relative easiness of thioethers accessibility to the Ti active sites of the catalytic species located in the zeolite framework. The diffusion of the bulkier molecules, such as Ph2S is very difficult even inside the large pores of Ti-beta zeolite. Secondly, the reactivity of thioethers is in agreement with the nucleophilicity of the sulfur atom, so that alkyl sulfides are more easily oxidized than allyl or aryl sulfides by H2O2 (an electrophilic oxidant) in agreement with reported results [1-9]. It must be pointed out, that in the case of allyl methyl sulfide and di-allylsulfide, the epoxidation of the allyl system is not observed under our experimental conditions. 

Recently basic sites in ALPOS and SAPOs have been detected by IR spectra [13] of chemisorbed pyrolle and it has been reported that small amounts of basic sites in zeolite exhibit more activity in the methylation of aniline. However an excessive amount covers the active sites and deactivates the catalyst. ALPO and its derivatives contain both acidic and basic sites. The basic sites are due to high aluminium content but small amount of protons, resulting in a highly negative charge on frame work oxygen. Introduction of Mg increases total basicity and decreases total acidity of the material. Due to this the successive alkylation of NMA to NNDMA is suppressed. AEL type materials have steady activity in this reaction. 

Adsorption of toluene on zeolites Li-X, Na-X, K-X, Rb-X, and Cs-X has been investigated with quantum chemical methods. Calculations of geometries, Mulliken partial charges, and C chemical shift parameters of clusters representing the catalytically active site are presented. The polarisation of the toluene carbons is the first step in alkylation reactions catalysed by zeolites and, at an early stage, will influence the outcome of the reaction. We show the simultaneous influence of the Lewis acidic cation and the basicity of the zeolite is responsible for altering the electron distribution within the toluene and thus affecting the outcome of an alkylation reaction. 

Reaction conditions have to take into account that disproportionation is a bimolecular reaction, while isomerization is unimolecular, and furthermore the activation energy of the former is higher. From the catalyst point of view it has to be considered that disproportionation needs stronger acidity to stabilize the slightly less stable alkyl carbocations and moreover a higher active site density will favour disproportionation versus isomerization. This intum means lower zeolite Si/Al ratios. 

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