Hydrocarbon pool side-chain methylation

The first brick to the hydrocarbon pool concept was brought in the early 1980s. First, Mole and coworkers [94,95] reported that deliberately introduced toluene acted as a co-catalyst for methanol conversion over H-ZSM-5. They proposed a mechanism by which methyl substituents on benzene rings undoxvent side-chain alkylation followed by olefin eliminafion (Fig. 15). 

In the hydrocarbon pool mechanism (ie, aromatics carbon pool), two parallel routes, namely the side-chain route or exocyclic methylation route and the "paring route, have been suggested for the formation of olefins [97]. 

Some very recent first-principle calculations together with kinetic Monte Carlo simulations have shown that the MBs with five or six methyl groups are not more active than those with fewer methyl groups [103], Propylene is intrinsically more favorable than ethylene when the reaction is not diffusion limited based on a side-chain hydrocarbon pool mechanism. The theoretical results are consistent with some experimental observations and can be rafionafized based on the shape selectivity of key reaction intermediates and transition states in the pore of catalyst [61,103],... 

Recently, Wang et al. [103] suggested that alkene methylation, firstly proposed by Dessau, should receive more attention in the MTO conversion even on SAPO-34 [109]. The overall energy barriers for the production of ethylene and propylene are much lower than those in side chain and paring hydrocarbon pool mechanisms. That is to say, hydrocarbon pool mechanism, where alkenes themselves are the organic active centers, may be operative in the MTO conversion [61,103]. 

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