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

Co-reactions of 7.5–9.3 kPa of DME with 4 kPa of toluene, p-xylene, and 4-ethyltoluene on H-ZSM-5 at 523–723 K at low conversions (<10 C%) with varying isotopic feed compositions of 13C/12C show that carbons originating from the aromatic ring are incorporated into ethene and propene. A comparison of the predicted 13C-contents of ethene and propene postulated on the basis of the paring, side-chain, and ring-expansion aromatic dealkylation mechanisms based on the experimentally observed isotopologue distribution of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, and 4-ethyltoluene reveal that the predicted 13C-content of ethene and propene from 1,2,4,5-tetramethylbenzene via the paring mechanism most closely match the experimentally observed 13C-contents of ethene and propene (<10% mean relative error), compared to the other mechanisms and aromatic precursors examined. This work quantitatively shows that aromatic dealkylation to form ethene and propene occurs through the paring mechanism and that 1,2,4,5-tetraMB is the predominant aromatic precursor for light olefin formation for MTO conversion on H-ZSM-5 for a 200 K range in temperature.