AlPO4/TiO2 catalysts. Part 2.—Structure, texture and catalytic activity of systems precipitated with ammonia or ethene oxide

Abstract

Two series of AlPO₄/TiO₂(APTi) catalysts containing different weight compositions have been prepared by the precipitation of AlPO₄(Al/P = 1) on commercial TiO₂( > 99.9 % Anatase, Aldrich) using ammonia (APTi-A series) or ethene oxide (APTi-E series) as the precipitation agents. Physicochemical characterization of these catalysts was carried out by nitrogen adsorption, TG, X.r.d., i.r. spectroscopy and surface-acid measurements. The surface area, pore volume, crystal structure and surface-acid character were found to be dependent on the precipitation agent, chemical composition and calcination temperature. Changes in surface-acid character were determined by application of a dynamic method that consists in determining both the AlPO₄/TiO₂'s cataytic activity and selectivity in cyclohexene skeletal isomerization using the Bassett–Habgood kinetic model for first-order processes. X-ray diffraction studies show that the incorporated AlPO₄ inhibits the polymorphic anatase–rutile transformation in the calcination process of the APTi-A system. Also, APTi-A systems are less affected by thermal treatment than APTi-E ones.

It was found that both the apparent rate constant and the selectivity are dependent on the calcination temperature although the most important influence is that of catalyst composition i.e. the AlPO₄/TiO₂ weight ratio. Thus, a decrease in catalytic activity is observed when TiO₂ content increases. Selectivity studies carried out by the OPE curves and the Wheeler criterion show that isomerization products, 1- and 3-methylcyclopentene, are competitive stable primary reaction products coming from cyclohexene across a parallel reaction network.