A spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity

Abstract

Different titanium silicalite (TS-1) samples and TiO2/SiO2 aerogels samples have been prepared and characterized structurally by XRD, skeletal FT-IR and FT-Raman spectroscopies and diffuse reflectance UV spectroscopy. The surface acidity of all materials has been investigated by FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile. The TS-1 samples are found to be totally free from extra-framework Ti oxide species, while the characteristic features of framework tetrahedrally coordinated Ti cations are well evident. Framework Ti cations substituting for Si into the silicalite MFI structure act as quite strong Lewis acid sites towards acetonitrile. The presence of Ti neither changes significantly the Bronsted acidity of the silicalite silanols, nor results in the formation of new hydroxy groups. TS-1 samples present lower defectivity (smaller amount of clustered hydrolyzed Si–OH OH–Si bonds) than pure silicalite prepared in the same way. Extra-framework TiO2, instead, is well detectable into aerogel samples, although framework Ti cations are likely also present. The surface of Ti-containing aerogels is covered by large amounts of silanol groups, although these sites do not present enhanced Bronsted acidity. Additionally, Lewis sites are detected whose strength is similar to that observed on TS samples, and is superior to that of the predominant sites on TiO2. It is suggested that silica tends to cover the TiO2 core in the case of aerogels. The surface is consequently dominated by silica with Ti sites in substitutional position.