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Conclusive Evidence for Two Unimolecular Pathways to Zeolite‐Catalyzed De‐alkylation of the Heptamethylbenzenium Cation
Author(s) -
Westgård Erichsen Marius,
Mortén Magnus,
Svelle Stian,
Sekiguchi Osamu,
Uggerud Einar,
Olsbye Unni
Publication year - 2015
Publication title -
chemcatchem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.497
H-Index - 106
eISSN - 1867-3899
pISSN - 1867-3880
DOI - 10.1002/cctc.201500793
Subject(s) - alkylation , zeolite , catalysis , chemistry , organic chemistry
The de‐alkylation of polymethylbenzenes is a key step in the zeolite‐catalysed methanol to hydrocarbons (MTH) reaction. As the accumulation of hydrocarbons in the zeolite pores commonly leads to numerous parallel reactions even at low conversions, the elucidation of the de‐alkylation mechanism is a challenging task. In this study, the gas‐phase dissociation of isotopically labelled heptamethylbenzenium ions was combined with reaction studies of its conjugate base over an H‐SSZ‐24 zeolite. The study provided conclusive evidence that unimolecular de‐alkylation pathways are favoured over side‐chain methylation in the zeolite‐catalysed reaction: the same alkenes, containing the same number of labelled carbon atoms, were formed in similar relative amounts in both the gas‐phase and catalytic studies. The observed labelling pattern (one ring‐carbon atom in C 2 , C 3 and i ‐C 4 , and two ring‐carbon atoms in n ‐C 4 and i ‐C 5 ) further revealed the existence of two different pathways for the unimolecular de‐alkylation of polymethylbenzenes.