Studies of acid catalyzed reactions: XII. Alcohol decomposition over hydroxyapatite catalysts

Abstract

Structure-reactivity relationships for alcohol elimination reactions were studied over two well-characterized hydroxyapatite catalysts. A “stoichiometric” preparation (HA), with the composition Ca 10(PO 4) 6(OH) 2, catalyzed both dehydration and dehydrogenation. A “non-stoichiometric” catalyst (NHA) with Ca P = 1.58 was active only for detrydration. The dehydration rate constants for fifteen acyclic alcohols over the NHA catalyst correlated with Taft σ α ∗ constants for substitution at the alpha carbon atom yielding ϱ α ∗ = − 5 at 230 °C. The dehydration rates were much slower over the less acidic HA catalyst, and a temperature of 400 °C was needed for a linear relation between σ α ∗ values and the logarithms of the rate constants. At that temperature ϱ α ∗ = − 2 for alpha substitution. The structure-reactivity relationships were remarkably similar to those for gas phase reactions such as HBr-catalyzed alcohol dehydration and the pyrolysis of esters. Like the latter, the eliminations over hydroxyapatite were mainly syn-eliminations. The necessary requirement is apparently the presence of a sufficiently acidic hydroxyl group at the surface. Beta elimination predominated in the alcohol dehydrations, but, over the NHA catalyst, both primary and secondary alcohols yielded products attributable to gamma elimination, and some of the primary alcohols gave cyclopropanes. Dehydrogenation of secondary alcohols over the HA catalyst could be correlated with σ ∗ values at 400 °C but in the opposite direction to dehydration; ϱ α ∗ = +1.5 for alpha substitution, suggesting that the transition state is negatively charged. An alkoxide ion is a likely precursor, with hydride transfer as the rate-limiting step.