Structure, Texture, Surface Acidity, and Catalytic Activity of AlPO4–ZrO2(5–50 wt% ZrO2) Catalysts Prepared by a Sol–Gel Procedure

A series of AlPO4–ZrO2(APZr) systems with various zirconia loadings (5–50 wt%) were prepared by sol–gel deposition of zirconia onto AlPO4and characterized by TGA/DTA, XRD, DRIFT, laser-Raman spectroscopy, SEM–EDX, XPS,27Al and31P MAS NMR, and nitrogen adsorption. Surface acid characterization was carried out through pyridine chemisorption and cyclohexene skeletal isomerization reaction. Addition of ZrO2onto AlPO4prevented the formation of monoclinic zirconia and increased the temperature of crystallization into the tetragonal form, so that APZr systems were still amorphous after calcination at 1073 K. This higher thermal stability was responsible for its large surface area as well as pore volume in relation to pure components. SEM showed nonhomogeneous distribution in morphology, texture, and particle size. Also, EDX and XPS indicated that the surface concentration ratios were practically the same as the expected values of stoichiometric composition. Moreover,27Al and31P MAS NMR showed that the local structure of the AlPO4support did not change with zirconia. Good correlations were obtained between catalytic activities in cyclohexene skeletal isomerization and surface acid densities of AlPO4–ZrO2systems (as determined by pyridine chemisorption at 373 and 573 K).