FTIR spectroscopic study of titanium-containing mesoporous silicate materials

The surface acidity of different mesoporous titanium-silicates, such as well-organized hexagonally packed Ti-MMM, Ti-AIAM-2, Ti-SBA-15, and amorphous TiO2-SiO2 mixed oxides (aerogels and xerogels), was studied by means of FTIR spectroscopy of CO adsorbed at 80 K and CD3CN adsorbed at 293 K. The surface hydroxyl groups of mesoporous titanium-silicates with 2-7 wt % Ti revealed a Bronsted acidity slightly higher to that of pure silicate. TiO2-SiO2 xerogels revealed the highest Bronsted acidity among the titanium-silicates studied. CO adsorption revealed two additional sites on the surface in comparison to pure silicate, characterized by nu(CO) from 2185 (high pressure) to 2178 (low pressure) cm(-1) and from 2174 (high pressure) to 2170 (low pressure) cm(-1). These bands are due to CO adsorbed on isolated titanium cations in the silica surrounding or having one Till cation in their second coordination sphere and due to CO interactions with Ti-OH groups, respectively. CD3CN adsorption similarly revealed the existence of two additional sites, which were not detected for pure silicate: at 2289 cm(-1) due to CD3CN interaction with titanol groups and from 2306 (low pressure) to 2300 (high pressure) cm-1 due to acetonitrile interaction with isolated framework titanium cations with probably one Till cation in their second coordination shell. The spectroscopic results are compared with computational data obtained on cluster models of titanium-silicate with different titanium content. According to the IR data, the Ti accessibility on the surfaces for mesoporous titanium-silicates with similar Ti loading (2 wt %) was found to fall in the order TiO2-SiO2 aerogel similar to TO2-SiO2 xerogel > Ti-MMM similar to Ti-MMM-2 > Ti-SBA-15. This order (except TiO2-SiO2 xerogel) correlates with the catalytic activity found previously for titanium-silicates in 2,3,6-trimethylphenol oxidation with H2O2.