Meta-Xylene Disproportionation

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

Fig. 1.5 Schematic representation of shape selective effects a) Reactant selectivity Cracking of an n-iso C6 mixture, b) Product selectivity Disproportionation of toluene into para-xylene over a modified HFMI zeolite, c) Spatioselectivity Disproportionation of meta-xylene over HMOR. The diphenylmethane intermediate A in formation of 1,3,5 trimethylbenzene is too bulky to be accommodated in the pores, which is not the case for B...

The disproportionation of toluene was the dominant reaction on Mo exchanged ZSM-5 and mordenite (0.4 wt % Mo). Compared with H-ZSM-5 and H-mordenite, the Mo exchanged samples had a much better resistance to deactivation, and this efftect was more pronounced on Mo mordenite, especially when was used as carrier gas. The variation in the distribution of xylene isomers with the conversion is plotted in Fig. 2. The Mo zeolites enhanced the relative yield of para + meta xylenes. 

If it can be shown that the photooxidation of hydrocarbons in zeolites is a general method, then the shape and size-selective properties of zeolites may potentially be used to control the selectivity of specific oxidation reactions (2,3). For example, ZSM-5 is an important shape-selective catalyst in many reactions, such as the disproportionation of toluene (4). Para-xylene is the dominant product because the transport of the other isomers, ortho- and meta-xylene, is restricted due to the pore size of ZSM-5. Thus, stereochemical aspects of selective photooxidation reactions may also be influenced by the zeolite and may be used to design environmentally benign processes for the synthesis of industrially useful molecules. 

Despite the results illustrated in Example 2.1. benzene has been produced for the last 50 years and is a viable starting material for a host of petrochemical products. Therefore, how is this possible We must conclude that benzene can be produced via at least one other route, which is less sensitive to changes in the price of toluene, benzene, and natural gas. One such commercial process is the disproportionation or transalkylation of toluene to produce benzene and a mixture of para-, ortho-, and meta-xylene by the following reaction. 

Young et al. suggested the following kinetic situation under the toluene disproportionation conditions. The transalkylation reaction to form benzene and xylenes within the pores is relatively slow. Benzene diffuses out of the pores rapidly. The xylenes isomerize rapidly within the pores. (Xylene isomerization is about 1000 times faster than toluene disproportionation.) para-Xylene diffuses out moderately fast while the ortho and meta isomers move within the pores relatively slowly and further convert to para isomer before escaping from the channel system. 

Mass transport selectivity is Ulustrated by a process for disproportionation of toluene catalyzed by HZSM-5 (86). The desired product is -xylene the other isomers are less valuable. The ortho and meta isomers are bulkier than the para isomer and diffuse less readily in the zeoHte pores. This transport restriction favors their conversion to the desired product in the catalyst pores the desired para isomer is formed in excess of the equUibrium concentration. Xylene isomerization is another reaction catalyzed by HZSM-5, and the catalyst is preferred because of restricted transition state selectivity (86). An undesired side reaction, the xylene disproportionation to give toluene and trimethylbenzenes, is suppressed because it is bimolecular and the bulky transition state caimot readily form. 

With the percentages of the three xylenes from the various sources differing so much, it s not likely that a company, or the industry for that matter, will produce just the amount of the xylene isomer it wants. Para-xylene has the biggest demand and meta- the smallest, but none of the processes, cat reforming, olefins plants, or disproportionation, have commensurate yields. 

Xylenes. Mixed xylenes are generally obtained by the catalytic reforming of petroleum fractions (see Section 6.2.1.7, above) or by the recovery of the Cg fraction from an aromatic concentrate (pyrolysis gasoline) stream. An increasingly important source of mixed xylenes is from the disproportionation of toluene. The separation of mixed xylenes into para, meta, and ortho isomers can be accomplished by several methods (e.g., fractional crystallization). 

Figure 11 is a highly simplified diagram of this process. Toluene enters the ZSM-5 crystal and disproportionates at the acid catalyst sites to benzene and the three xylene isomers para-, meta-, and ortho-. Because of their larger size, meta- and ortho-xylenes diffuse slower than para-xylene. As you expect, the longer they stay inside the ZSM-5 crystal, the richer the product will be in para-xylene. 

The significant finding was that the xylene fraction was 99% para. The other fractions are not lost. Toluene can be disproportionated to p-xylene and benzene with H-ZSM-5 treated with a little hexamethyldisiloxane to give 99% p-xylene, so that the usual separation of the ortho- and meta isomers with another zeolite would not be required.177 Benzene can be transalkylated with the higher aromatics to give toluene. Ethylbenzene can be isomerized to p-xylene. Ethylbenzene can be alkylated with ethanol in the presence of a modified ZSM-5 catalyst to produce p diethylbenzene with 97% selectivity.178... 

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