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

Andrew Hwang, Blake A. Johnson, Aditya Bhan

Research output: Contribution to journalArticlepeer-review

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

Bibliographical note

Funding Information:
We acknowledge The Dow Chemical Company and the National Science Foundation ( CBET 1701534 ) for finanical support for this research, Mr. Linh Bui (University of Minnesota) for invaluable technical discussions, and Mr. Todd Rappe (Minnesota NMR Center) for assistance with NMR experiments. BAJ acknowledges an undergraduate research fellowship administered by the Office of Undergraduate Research at the University of Minnesota.

Funding Information:
We acknowledge The Dow Chemical Company and the National Science Foundation (CBET 1701534) for finanical support for this research, Mr. Linh Bui (University of Minnesota) for invaluable technical discussions, and Mr. Todd Rappe (Minnesota NMR Center) for assistance with NMR experiments. BAJ acknowledges an undergraduate research fellowship administered by the Office of Undergraduate Research at the University of Minnesota.

Publisher Copyright:
© 2018 Elsevier Inc.

Keywords

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

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