Synthesis and transformations of Ti-containing structures on the surface of silica gel

The influence of pretreatment temperature (T0) of the support (silica gel SchSK and KSS-4) and chemisorption temperature (Ts) on the composition of produced structures is examined by chemical analysis, gravimetric study, and IR-spectroscopy. During chemisorption at room temperature, TiCl3 groups are formed (molar ratio is Cl/Ti ≈ 3). The rise of the temperature (Ts) from 25 to 600°C with a step of 100° is accompanied by a decrease in the Cl/Ti ratio to 1.67. This indicates the formation of TiCl2 and TiCl surface groups. The content of Ti chemisorbed at 25°C is 0.67 mmol/g SiO2, and it increases to 0.75 mmol/g SiO2 at 200°C, and then it is reduced to 0.49 mmol/g SiO2 at Ts=400−600°C. Interactions between functional groups during the calcination process at temperatures above 100°C are supposed. This causes the release of TiCl4 molecules in the gas phase and the increase of the number of (SiO)nTiCl4−n (n = 2, 3) groups. We suppose that the structure of Ti-oxochloride groups is the most important factor for the stability of the SiOTi bonds at the stage of vapour hydrolysis. One of the most important reasons for bond destruction is HCl formed during the treatment of chlorine-containing structures by water vapour.