AlPO 4[sbnd]TiO 2 catalysts : I. Textural Properties, surface chemistry, and cyclohexene conversion of systems precipitated with propylene oxide

Abstract

A series of AlPO 4 TiO 2 (APTi) catalysts containing different weight compositions is prepared by precipitation of AlPO 4 on commercial TiO 2 (pure anatase, Aldrich Chemie) using propylene oxide and characterized with respect to crystallinity, textural properties, and surface chemistry (acidity, basicity, one-electron donor, and one-electron acceptor properties). The acidity (amount and strength distribution) is found by means of the irreversible adsorption of organic bases by using a spectrophotometric method and is correlated with the reactivity for cyclohexene skeletal isomerization (CSI) in a pulse microreactor. The acidity is found to be slightly dependent on the calcination temperature but a decrease is observed when the AlPO 4/TiO 2 ratio is lowered. Also, the presence of TiO 2 causes a homogenization in the acid strength distribution of AlPO 4 and in the inhibition of the AlPO 4 crystallization by treatment temperatures of 1073 K. Thus, AlPO 4 TiO 2 catalysts are found to have moderately strong acidic properties even at calcination temperatures of 1073 K. Apparent rate constants, activation energies, and selectivities to 1-MCP are calculated in terms of Bassett-Habgood's kinetic model for first-order processes and compared to those obtained using pure AlPO 4 catalysts. It is found that AlPO 4 TiO 2 catalysts are more active for the CSI process than AlPO 4 catalysts, especially at calcination temperatures over 923 K, indicating increased strength of acidity. Selectivity studies indicate that 1- and 3-methylcyclopentenes (1-MCP and 3-MCP) are competitive, stable, primary reaction products coming from cyclohexene (CH).