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−MMM-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 Brönsted acidity slightly higher to that of pure silicate. TiO2−SiO2 xerogels revealed the highest Brönsted acidity among the titanium−silicates studied. CO adsorption revealed two additional sites on the surface in comparison to pure silicate, characterized by ν(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 Ti4+ 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 Ti4+ 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 ∼ TiO2−SiO2 xerogel > Ti−MMM ∼ 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.