The mechanism of methanol conversion to olefins has been the subject of debate since the discovery of this chemistry over 35 years ago. Previous work has shown evidence for the formation of light olefins from polymethylbenzene precursors. In this work, co-reactions of dimethyl ether with toluene, p-xylene, and 4-ethyltoluene with varying 12C/13C feed compositions were performed over H-ZSM-5 to discriminate between three proposed aromatic dealkylation mechanisms (paring, side-chain, and ring expansion). The effluent isotopologue distributions of multiple different polymethylbenzenes precursors were used to predict the total 13C-content of ethene and propene based on the three mechanisms. For the eight reactions performed using three different aromatic co-feeds, five different 12C/13C feed compositions, and a 200 K range in temperatures, the mean errors of the predicted 13C-contents compared to the experimentally observed 13C-contents for ethene and propene were consistently the lowest for ethene and propene formation from 1,2,4,5-tetramethylbenzene via the paring mechanism. These results show, for the first time, that on H-ZSM-5, aromatic dealkylation occurs through a paring mechanism and 1,2,4,5-tetramethylbenzene is more active for dealkylation reactions compared to either 1,2,4-trimethylbenzene or 4-ethyltoluene.

Mechanism of aromatic dealkylation in methanol-to-hydrocarbons conversion on H-ZSM-5: What are the aromatic precursors to light olefins?

251st National Meeting (2016)

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