Influence of the starting aluminum salt on the surface and acid properties of AlPO 4 catalysts precipitated with ammonium hydroxide

Abstract

In this paper, the influence of the starting aluminum salt (chloride, nitrate, or sulfate) and the pretreatment temperature (773–1273 K) on textural properties, crystal structure, and surface acidity of AlPO 4 ( Al P = 1 ) catalysts was studied in order to learn how preparation conditions affect catalyst activity in organocationic reactions. The catalysts were characterized using nitrogen adsorption, X-ray diffraction, infrared spectroscopy, and thermogravimetric analyses. The surface acid properties were determined using a dynamic method that consists of determining the AlPO 4's catalytic activity in cyclohexene skeletal isomerization (CSI), a reaction that requires the presence of strong surface acid sites. Catalytic activity (as apparent rate constants), activation energies, and selectivities to 1-methylcyclopentene (1-MCP) were calculated in terms of Bassett—Habgood's kinetic model for first-order processes in which the surface reaction is the controlling step and the partial pressure of the reactant is low. Significant differences in structure, texture, surface acidity, and catalytic activity in CSI were found, showing that the aluminum starting salt plays an important role in the final properties of AlPO 4 ( Al P molar ratio = 1 ) catalysts. Thus, aluminum nitrate yielded materials with higher surface area and low activity for CSI while aluminum sulfate resulted in higher surface acidity, and hence catalytic activity for CSI, although the sample is highly crystalline exhibiting low surface area. Aluminum chloride produces porous catalysts although they are less acidic.