Effect of preparation method on the surface acidity and catalytic performance of iron orthophosphates in cyclohexene conversion

Abstract

A series of FePO₄(Fe/P = 1) catalysts have been prepared by precipitation of the corresponding chloride or nitrate iron(III) salt and orthophosphoric acid with aqueous ammonia or propylene oxide at pH 7.0. The resulting solids, after calcination at temperatures in the range 673–1273 K for 3 h, were characterized by surface area, X-ray diffraction (XRD) and diffuse reflectance IR (DRIFT) measurements.

All catalysts were amorphous for calcination temperatures below 923 K. Calcination at 923–1273 K develops FePO₄ with a quartz-like phase structure (ASTM 29–715). Amorphous samples exhibited relatively high surface areas which fell sharply as crystallization of FePO₄ occurred. Moreover, the surface acidity (Bronsted and Lewis), characterized by adsorption of pyridine using a pulse method and DRIFT measurements, strongly depended on the preparation method causing the amorphous FePO₄ catalysts, obtained from iron(III) chloride and aqueous ammonia, to be more acidic. This increased surface Brønsted acidity was responsible for their improved performance in cyclohexene skeletal isomerization (CSI). FePO₄ catalysts also showed catalytic activity for cyclohexene dehydrogenation to benzene.