Tuning the Acid/Base and Structural Properties of Titanate-Loaded Mesoporous Silica by Grafting of Zinc Oxide

Mesoporous silica (SBA-15) loaded with TiOx species was synthesized by anhydrous grafting of titanium isopropoxide, and a novel procedure for the preparation of ZnOx/SBA-15 materials by grafting of Zn(acac)(2) was explored. The TiOx/SBA-15 and ZnOx/SBA-15 materials as well as subsequently prepared bifunctional ZnOx- and TiOx-containing SBA-15 materials were characterized in depth by combining N-2 physisorption measurements, UV-vis, X-ray photoelectron and X-ray absorption spectroscopy, and CO2 and NH3 temperature-programmed desorption experiments. The characterization results confirmed a close proximity of ZnOx and TiOx in the subsequently grafted materials. Because of strong interactions between the Zn precursor and the SiO2 surface, the order of the ZnOx and TiOx grafting steps affected the amount of Ti-O-Zn bonds formed in the materials. When ZnOx is present in SBA-IS, subsequently grafted TiOx is higher coordinated and more Ti-O-Zn bonds are formed compared to SBA-15 in which TiOx was introduced first, indicating strong interactions between the Ti precursor and ZnOx. While all TiOx and ZnOx-containing samples exhibit a large amount of acidic sites, ZnOx present as isolated species or small clusters in SBA-15 significantly improves the CO2 adsorption capacity by introducing basic sites. In the subsequently grafted samples the amount of acidic and basic sites is found to be unaffected by the order in which the two transition metals are Introduced.