Mechanistic study of methylbenzene dealkylation in methanol-to-olefins catalysis on HSAPO-34

Andrew Hwang, Blake A. Johnson, Aditya Bhan

Research output: Contribution to journalArticle

7 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)86-94
Number of pages9
JournalJournal of Catalysis
Volume369
DOIs
StatePublished - Jan 2019

Keywords

  • HSAPO-34
  • Isotope tracing
  • Methanol-to-olefins
  • Methylbenzene flash chromatography
  • Methylbenzenes dealkylation
  • Quantitative C NMR
  • Site-specific

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