Phosphates of group III elements as models of acid–base catalysts

Abstract

The nature of active centres of phosphates of boron, aluminium and bismuth of composition varying in the range Me/P = 0.4–1.6 has been studied by infrared spectroscopy after adsorption of ammonia, pyridine, acetonitrile and HCl, and correlated with the activity of these catalysts in dehydration of ethyl alcohol, cracking of cumene and Beckmann rearrangement of the oxime of cyclohexanone to ε-caprolactam. In phosphate catalysts containing an excess of phosphorus, Brønsted-acid centres P—OH and P—OH—OH—B are responsible for the dehydration of ethyl alcohol to ethylene and for the cracking of cumene, but show no activity in the Beckmann rearrangement. In catalysts with an excess of boron two kinds of Lewis-acid centres become apparent: trigonal boron linked to normal tetrahedral boron (B_Tr—O—B) or to phosphorus (B_Tr—O—P). In this concentration range the cracking of cumene disappears and the dehydration of ethyl alcohol is diverted to the formation of diethyl ether instead of ethylene. The catalysts show very high activities in the Beckmann rearrangement of the oxime. Strong electron-acceptor centres (B_Tr—O—B) are involved in the intermolecular dehydration of ethyl alcohol to diethyl ether as well as in the Beckmann rearrangement, in which similar mechanisms may thus operate.