CONTROLLED FORMATION OF ZRO2 IN THE REACTION OF ZRCL4 VAPOR WITH POROUS SILICA AND GAMMA-ALUMINA SURFACES

The reaction of ZrCl4 vapor at 300, 450, and 600 degrees C with silica and gamma-alumina preheated at 300 and 600 degrees C was studied by X RD, FTIR, H-1 MAS NMR, and chemical etching with sulfuric acid. NMR an d FTIR revealed a consumption of OH groups in the reaction. However, s ubsequent water vapor treatment brought some of the OH groups of the s upport back, indicating that part of the surface had been directly chl orinated. The ratio between permanently consumed OH groups and directl y chlorinated OH groups depended on the reaction temperature. OH group s were permanently consumed in mono- and bimolecular reactions of ZrCl 4, leading to the formation of isolated M-O-ZrCl3 and (M-O)(2)-ZrCl2 s pecies (M = Al or Si), respectively. Direct chlorination was associate d with the formation of crystalline ZrO2 agglomerates, as detected by XRD in the samples prepared at 450 and 600 degrees C. These agglomerat es were insoluble in sulfuric acid. Although at the reaction temperatu re of 300 degrees C the samples were amorphous in XRD, the reappearanc e of some OH groups on the original surface after water treatment, tog ether with etching tests, suggested that agglomerates were also formed at that temperature. In addition to agglomeration and exchange reacti ons with OH groups, on 600 degrees C alumina ZrCl4 dissociated to (Al- O) pairs. This led to a higher Zr saturation density and Cl/Zr ratio o n 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 di minished the surface coverage. The surfaces of the ZrCl4-modified and water-treated silica and alumina surfaces consisted of Zr-OH groups fo rmed in the hydrolysis of isolated ZrClx species, original OH groups o f the supports, and ZrO2 agglomerates formed in the vicinity of siloxa nes and (Al-O) pairs.