Cyclopentenyl Carbenium Ion Formation in Acidic Zeolites: An in situ NMR Study of Cyclic Precursors

Abstract

Previous in situ NMR studies of the reactions of propene and ethylene on zeolites have claimed the formation of small amounts of cyclopentenyl carbenium ions. The present investigation clearly demonstrates the formation of these ions in high yield from cyclic precursors on acidic zeolites. In situ {sup 13}C solid-state NMR experiments are reported for the reactions of cyclopentene, cyclopentanol, cyclohexanol, and cyclohexane on a variety of zeolites of differing acid strength. The reaction sequence for the alcohols was dehydration, double-bond migration, oligomerization, C{sub 6} to C{sub 5} ring equilibration, and accumulation of cyclopentenyl ions. In some of the experiments up to one-third of the C{sub 5} ring units of the oligomers were positively charged. A {sup 13}C chemical shift anisotropy measurement verified that the downfield signals are due to carbenium ions. Ex situ identification of the neutral extractable oligomers combined with comparison to model carbenium ion shifts permitted a plausible identification of the specific ions formed. The formation of products containing five-membered rings from cyclohexanol was modeled by the cyclohexane-methylcyclopentane skeletal rearrangements reaction. This investigation firmly establishes the formation of cyclopentenyl ions in zeolites and considers their significance in regard to reaction mechanisms. 61 refs., 7 figs.