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[dbnd]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.