Toluene alkylation with methanol

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

In the case of toluene alkylation with methanol an opportunity exists for para selectivity. Para-xylene ortho-xylene ratio was 3.1 over MFl and 0.6 over BEA framework types. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Acidity of the materials has been determined by ir and thermodesorption of ammonia and their catalytic properties for toluene alkylation with methanol have been studied. Relations between acidic strength, diffusion rate, catalytic shape selectivity and structural features are discussed. It is suggested that the microporous framework has to be considered as a "living material" under catalytic reaction conditions. 

Toluene alkylation with methanol and cumene cracking were both carried out using a conventional continuous-flow system under atmospheric pressure. The liquid products collected in an ice trap were analyzed by GLC using a Bentone 34 and DNP column (4 m). 

Figure 4.17. Zeolite transition-state selectivity. Toluene alkylation with methanol catalyzed by H-MOR showing the energies of the key reaction intermediates . Reaction energy diagram for ortho-, meta- and para-xylene are compared.

Toluene alkylation with methanol using Friedel-Crafts catalysts results in mixed products since isomerization reactions and fiirther methylation of the desired products readily occur under these reaction conditions [49,50]. Recent work has therefore been aimed at the development of processes with high selectivity, and zeolite catalysts appear to have the most promise in this regard. 

Cata t deactivation during toluene alkylation with methanol over B-HZSM-5 catatyst Terrperature 600°C, WHSV =3.8, pressure = 0.1 MPa, and toluene-methanol molar ratio = 2 l. Toluene conversion (circles) jgiene wt% in or nic product (open squares) and / -3giene wt% of total xylenes (filW squares). 

These effects have been used to explain the case of toluene alkylation by methanol in large zeolites, but other reactions are also being studied, and the results obtained so far are in agreement with the theoretical development. 

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. 

The only aromatic components that appeared at reaction temperatures below 300 °C were toluene and p-xylene. In the case of the small-crystalline H-ZSM-5(M), however, some m- and o-xylene were present in the product mixture even at 245 °C (WHSV = 6 h- ). This can be explained by xylene isomerization at the outer zeolite surface. At conditions where the para-selectivity was high (more than 90% para), the amount of p-ethyl-toluene (PET) in the product were one order of magnitude greater than that of any other Cg-component, but when it was low, the ratio 1,2,4-trimethyl-benzene (124TMB) PET was found to be about 10 1. These experimental facts indicate that 124TMB is mainly formed by secondary xylene alkylation with methanol. Toluene, p-xylene, PET and perhaps ethyl-benzene are more likely to be the primary aromatic products formed in the MTG-reaction. To confirm this suggestion the molar product ratios EB/PX,... 

Alkali-exchanged zeolites can be used for selective condensation reactions [32-37], double bond isomerization, alkylation of toluene by methanol, mono-methylation of phenylacetonitrile with methanol and dimethyl carbonate (DMC) [38], and selective N-alkylation with methanol, ethanol, and 2-propanol as alkylating agents [39-44]. However, the basicity of alkaline exchanged zeolites is rather low. 

Alkylaromatics-Alkylation with Methanol.- Basic catalysts lead to the side chain alkylation of toluene with methanol. In the presence of acid solid catalysts xylene isomers are formed. 

Various studies have reported the use of zeolite catalysts for the alkylation of alkylbenzenes other than toluene. Studies include the conversion of ethylbenzene and methanol to yield /7-ethyltoluene, which after dehydrogenation yields /7-methylstyrene. The latter compoimd can be polymerized to yield a polystyrene analog [63]. HZSM-5 zeolites prepared with Na, K, and Rb ions were examined by Kolboe et al [63] who reported selectivities of 1050% for ethylbenezene alkylation with methanol Hi para selectivity was observed, particularly for the Rb-containing zeolite that has a low Al content. In addition, the alkylation of phenol with methanol is a commercially viable process for the production of cresols and Tcylenols. The reaction occurs in the liquid phase at about 300°C and 5 MPa pressure. The catalyst is aluminum oxide, but recent work on HY and ZSM-5 zeolites has been reported that increases selectivity to /7-cresol as a result of shape selectivity effects [64]. 

The selective alkylation of toluene with methanol to produce -xylene as a predominant isomer can be achieved over shape-selective catalysts (99—101). With a modified ZSM-5 zeoHte catalyst, more than 99% -xylene in xylene isomers can be produced at 550°C. This -xylene concentration exceeds the equiHbrium concentration of 23% (99). The selective synthesis of -xylene using relatively low cost toluene is economically attractive however, this technology was not commercialized as of 1991. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Figure 4.28 Plot of CD intensities of poly -hexyl(j -/ -propoxyphenyl)silane (41) aggregates in toluene/series of (S)-primary chiral alkyl alcohols/methanol mixtures at 20°C. [For comparison, CD intensity with (A)-2-butanol is inserted.].

It has been found that the disproportionation of toluene over ZSM-5 catalyst can be directed such that p-xylene is the predominant xylene isomer (14-17). This reaction, designated STOP, is one of several in which disubstituted aromatics rich in the para isomer are produced. Others are the alkylation of toluene with methanol to produce p-xylene (15,18) and with ethylene to produce p-ethyltoluene (19,20), as well as the aromatization of olefins (20), paraffins (20) and of methanol... 

No commercial process is offered at this time for side chain alkylation of toluene with methanol for styrene and ethylbenzene production. In the literature the reaction is typically carried out at toluene to methanol molar ratios from 1.0 7.5 to 5 1 from 350 to 450 °C at atmospheric pressures. In some cases inert gas is introduced to assist vaporizing the liquid feed. In other cases H2 is co-fed to improve activity, selectivity and stability. Exelus recently claimed 80% yields in their ExSyM process at full methanol conversion using a 9 4 toluene methanol feed ratio at 400-425 °C and latm (101 kPa) in a bench-scale operation. This performance appears to be... 

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