New AlPO₄-sepiolite systems as acid catalysts, I. Preparation, texture, surface-chemical properties and cyclohexene skeletal isomerization conversion

Abstract

The effects of the addition of aluminium orthophosphate (Al/P=1) to sepiolite were investigated with respect to texture, porosity, surface chemistry and catalytic performance. The AlPO₄-sepiolite catalysts, containing different weight compositions, were prepared by precipitation of AlPO₄ (Al/P=1) with propylene oxide or aqueous ammonia on to commercial natural sepiolite. Sepiolite inhibits the AlPO₄ crystallization which is found when pure AlPO₄ is calcined at 873 K.

The surface acid-basic properties have been studied gas-chromatographically through the irreversible adsorption of organic acids and bases at temperatures in the range 473 to 673 K. In addition, the cyclohexene skeletal isomerization (CSI) acid catalysed test reaction to 1- and 3-methylcyclopentenes (1- and 3-MCP), was used to test catalyst performance. Catalytic activity, as an apparent rate constant, fits the Bassett-Habgood kinetic model for first-order processes. Marked changes were found in their catalytic properties as a function of the preparation method and the AlPO₄ to sepiolite ratio.

AlPO₄ addition induced acidity by increasing the strength of acid sites which resulted in a higher conversion which was accompanied by a greater selectivity for 1-MCP. The incorporation of AlPO₄ to sepiolite also inhibited the loss of activity with the prolonged use of the catalyst which was found for reference pure natural sepiolite.