Aluminium phosphate—zirconia catalysts: I. Structure, texture, acid—base properties and catalytic activity in cyclohexene isomerization of catalysts obtained with propylene oxide

Abstract

Nitrogen adsorption, X-ray diffraction measurements and infrared spectroscopy were used to study the effects of surface dehydration (773–1273 K) on the textural properties, crystallinity and structure of AlPO 4 ZrO 2 catalysts of different compositions and prepared by precipitation of aluminium orthophosphate (AlPO4) (Al/P= 1) with propylene oxide on commercial monoclinic zirconia (ZrO 2) (Aldrich Chemie, ceramic grade, 99.9 % ). The surface acid—base properties were studied through the irreversible adsorption of organic bases and acids from cyclohexane solutions at 298 K. In addition, the cyclohexene skeletal isomerization (CSI) acid-catalysed test reaction, giving 1- and 3-methylcyclopentenes, was used as a dynamic method for evaluating the number of strong acid sites, the only ones capable of giving rise to skeletal isomerization. Activation energies and selectivities with respect to 1-methylcyclopentene were calculated in terms of the Bassett—Habgood kinetic model for first-order processes in which the surface reaction is the controlling step and the partial pressure of the reactant is low. An increase in ZrO 2 content and/or calcination temperature decreases the surface area and pore volume. This is a consequence of AlPO 4 crystallization to the tridymite form with T d symmetry, in which the phosphorus atoms are surrounded by tetrahedra of oxygen atoms. The decrease in catalytic activity as calcination increases is consistent with not only the decrease in the amount of acid sites measured vs. weaker organic bases but also with the decrease in Brønsted acidity, as shown by the decrease in OH band intensity.