Preparation of Titania-Silica Mixed Oxides by a Sol-Gel Route in the Presence of Citric Acid

Titania-silica mixed oxides (denoted as TiO2-SiO2) with different Ti contents are prepared by a sol-gel route in the presence of citric acid (CA). Various characterization techniques, including XRD, FT-IR, N-2-adsorption, TEM, solid-state CP-MAS NMR spectroscopy, UV-vis, and NH3-TPD, have been carried out to investigate the physical-chemical properties, states of Ti species, and the formation process of TiO2-SiO2 materials. The characterization results show that the TiO2-SiO2 materials possess a mesoporous silicate framework with high specific surface area and thermal stability. Titanium species in the TiO2-SiO2 materials are mainly present in the form of two-dimensional low polymeric TiOx species, which are uniformly dispersed on the surface of mesoporous silica. Additionally, the titanium species have a strong resistence to high-temperature sintering, even for the sample with high Ti contents (25 wt %). Weak acid sites are observed over TiO2-SiO2 materials, and the amount of acidic sites increases with the increment of Ti contents. The acid catalytic performance of TiO2-SiO2 materials is investigated in the transesterification of dimethyl oxalate (DMO) with phenol. All the materials are active for the reaction, and the total selectivity to methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO) is very high (>99%). The formation mechanism of such materials is discussed concerning the interaction between citric acid and precursors of silicon and titanium. We propose that a portion of CA can act as a ligand to coordinate with titanium species to form the titanium-CA complexes. Both CA and the titanium-CA complexes may act as a template for the formation of a silica framework. After calcination, titanium species release from the titanium-CA complex and reach a position on the Surface of silica, forming the resultant TiO2-SiO2 materials.