Journal of Catalysis, Vol.369, 86-94, 2019
Mechanistic study of methylbenzene dealkylation in methanol-to-olefins catalysis on HSAPO-34
Methylbenzenes entrained within the cavities of silicoaluminophosphate zeotype HSAPO-34 react with methanol in H+-mediated dealkylation to give ethylene and propylene in methanol-to-olefins catalysis. Methylbenzenes dealkylation on solid acids is proposed to occur either via the side-chain mechanism, where an exocyclic C=C undergoes successive methylation prior to C-C cleavage for olefin elimination, or the paring mechanism, where ring contraction to a bicyclohexenyl cation precedes C-C cleavage for olefin elimination. Distinct dealkylation mechanisms prescribe distinct combinations of C atoms-from aromatic methyl, aromatic ring, and methanol/dimethyl ether-to comprise the olefin product. Site specific isotope tracing that distinguishes between isotope labels in aromatic methyl and aromatic ring positions for each methylbenzene shows that tetramethylbenzene gives ethylene via the side-chain mechanism and penta- and hexamethylbenzene give propylene via the paring mechanism. The ratio of propylene selectivity to ethylene selectivity increases in methanol reactions on HSAPO-34 entrained with a distribution of methylbenzenes deliberately manipulated towards increasing fractions of penta- and hexamethylbenzene, corroborating the conclusion that aromatic precursors and dealkylation mechanisms for ethylene diverge from those for propylene. (C) 2018 Elsevier Inc. All rights reserved.