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The reaction of ZrCl4 vapor at 300, 450, and 600 °C with silica and γ-alumina preheated at 300 and 600 °C was studied by XRD, FTIR, 1H MAS NMR, and chemical etching with sulfuric acid. NMR and FTIR revealed a consumption of OH groups in the reaction. However, subsequent water vapor treatment brought some of the OH groups of the support back, indicating that part of the surface had been directly chlorinated. The ratio between permanently consumed OH groups and directly chlorinated OH groups depended on the reaction temperature. OH groups were permanently consumed in mono- and bimolecular reactions of ZrCl4, leading to the formation of isolated M−O−ZrCl3 and (M−O)2−ZrCl2 species (M = Al or Si), respectively. Direct chlorination was associated with the formation of crystalline ZrO2 agglomerates, as detected by XRD in the samples prepared at 450 and 600 °C. These agglomerates were insoluble in sulfuric acid. Although at the reaction temperature of 300 °C the samples were amorphous in XRD, the reappearance of some OH groups on the original surface after water treatment, together with etching tests, suggested that agglomerates were also formed at that temperature. In addition to agglomeration and exchange reactions with OH groups, on 600 °C alumina ZrCl4 dissociated to (Al−O) pairs. This led to a higher Zr saturation density and Cl/Zr ratio on alumina than on silica. The reactions of the HCl evolved in the main reactions were considered minor. Increasing the preheat and reaction temperatures decreased the Zr concentration on the surface and thus diminished the surface coverage. The surfaces of the ZrCl4-modified and water-treated silica and alumina surfaces consisted of Zr−OH groups formed in the hydrolysis of isolated ZrCl x species, original OH groups of the supports, and ZrO2 agglomerates formed in the vicinity of siloxanes and (Al−O) pairs.

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Silica-, alumina-, and zirconia-supported tantalum oxide and supported vanadia-tantala mixed oxide catalysts were investigated by XRD, UV-vis-DR, FTIR, and FT-Raman spectroscopy to determine the nature of the surface tantala species. Two types of surface TaO x species were identified. On the SiO 2 support, at low coverages tetrahedral TaO 4 species exist, with a TadO bond and three bridging Ta-O-support bonds. At higher coverages, octahedral TaO 6 species occur. On Al 2 O 3 and ZrO 2 both surface species exist already at considerably low Ta loading. The acidic properties of the catalysts were investigated by an FTIR study of adsorbed pyridine. Supported tantala is clearly more Lewis acidic in comparison to vanadia. The oxidation of methanol was used to probe the catalytic performance of the supported oxide species and from the activity and product distribution it is concluded that supported tantala predominantly shows acidic over redox behavior.