Journal
CATALYSIS LETTERS
Volume 71, Issue 1-2, Pages 95-103Publisher
SPRINGER
DOI: 10.1023/A:1016664610545
Keywords
acid strength; acidity measurements; bifunctional catalysts; carbocations; cationoidic intermediates; oxidation; superacid catalysts
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The probe bases used for measurement of strength of acid catalysts need to be structurally similar to the catalytic substrates. Traditional acidity measurements are inapplicable when the hydron transfer forms a tight ion pair, as on solid acids. Also, solid acids are much weaker than liquid acids of similar structure. Alkane activation by C-H and C-C cleavage, evidenced in strong superacids, does not occur in less strongly acidic media. The reactivity patterns and structure-reactivity relationships are the same for the cationoidic species (weakly coordinated carbocations) occurring in the less strong acids as for the full-hedged carbocations intervening in superacids. Even in trifluoromethanesulfonic acid, a weak superacid, the first step in alkane activation is an oxidation, forming unsaturated carbocations. On sulfated zirconia, the first step of alkane reactions is a one-electron oxidation. Mechanistic features incompatible with standard (or traditional) carbocationic processes, are observed in the reaction of 3-methylpentane on the zeolite HZSM-5.
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