Influence of the nature of the rare earth element in rare earth orthophosphate used for light alcohol dehydration

Abstract

La, Nd, Sm and Gd rare earth orthophosphates with a Rhabdophane-type structure have been synthesized and tested as catalysts for the dehydration of ethanol and 1-butanol. These solids appear to be very active and selective. Contrary to a previously studied LaPO 4 series of catalysts, whose properties were shown to depend on their P/La surface ratio, this series of rare earth phosphates exhibits catalytic properties depending mainly on the nature of the rare earth elements themselves. The most efficient phosphates presented moderately strong Brønsted and Lewis acid sites and only small quantities of weak basic sites. These properties lead to an exceptionally high selectivity to alkenes for both studied dehydration reactions. Although a good correlation was observed between the Lewis acidity and the rate of ethanol conversion, a possible involvement of Brønsted acid sites, the strength of which increases with that of the Lewis sites, cannot be totally excluded. The results of the catalysts characterization are interpreted in terms of the possible nature of the different sites.