Controlled Formation of ZrO₂ in the Reaction of ZrCl₄ Vapor with Porous Silica and γ-Alumina Surfaces

The reaction of ZrCl 4 vapor at 300, 450, and 600 °C with silica and γ-alumina preheated at 300 and 600 °C was studied by XRD, FTIR, 1 H 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 ZrCl 4, leading to the formation of isolated M−O−ZrCl 3 and (M−O) 2 −ZrCl 2 species (M = Al or Si), respectively. Direct chlorination was associated with the formation of crystalline ZrO 2 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 ZrCl 4 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 ZrCl 4 -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 ZrO 2 agglomerates formed in the vicinity of siloxanes and (Al−O) pairs.