Study of the surface acidity of TiO2/SiO2 catalysts by means of FTIR measurements of CO and NH3 adsorption

Samples were prepared by grafting different amounts of titanium isopropoxide (Ti(OPri)4) onto silica surface in N2 atmosphere, followed by steam hydrolysis and calcination. Either dioxane or toluene was used as a solvent. The surface properties of TiO2/SiO2 catalysts were studied by means of FTIR spectroscopy of adsorbed probe molecules, CO, and ammonia. Adsorption of CO at nominal 77 K exhibited different surface sites: (i) two families of Ti4+ cations, namely associated and isolated sites, with bands in the 2183–2194 and 2173–2177 cm−1 ranges respectively, shifting with coverage; (ii) surface SiOH groups; and (iii) at titanium loadings >8 wt% TiO2, corresponding to the alkoxide monolayer coverage of silica, TiOH species exhibiting an acidity higher than expected (observed shift Δν=200 cm−1Δν=200 cm−1), probably due to interaction with the support. With increasing the titanium loading, the band at 2176 cm−1 disappears, indicating that associated Ti4+ sites form at the expense of isolated ones. Adsorption of ammonia at room temperature confirms the data obtained with CO. The use of apolar solvents seems to cause anchoring of the Ti alkoxide on the dehydrated portions of the silica surface, with consequences for the final state of the catalyst. In agreement with this observation, activity was lower for catalysts prepared in toluene than in those prepared using dioxane. Prepared catalysts were rather effective in the transesterification of refined oils with methanol.