Hydrothermal stability of ordered mesoporous titanosilicate materials prepared at room temperature

摘要

A study of hydrothermal stability, performed in boiling water under static conditions, ofMCM-41 materials containing different titanium content, prepared by direct synthesis at ambienttemperature and pressure, using tetraethoxysilane, titanium ethoxide and octadecyltrimethylammoniumbromide is presented. The behaviour is compared with pure silica grades prepared by asimilar procedure. The samples were characterised by X-ray diffraction, adsorption of nitrogen at77K and diffuse reflectance UV–Vis spectroscopy. It was found that the stability improves as theamount of titanium increases and that Ti-MCM-41 samples prepared with Si/Ti≤50 are significantlystable. After 12h in boiling water the pore size uniformity was practically maintained and only asmall decrease in pore volume (5-9%), total surface area (2-7%) and mesopore width (3%) and aslight increase in pore wall thickness (3-6%) occurred. In contrast, some degradation of the MCM-41 structure for the pure silica and the lower Ti content (Si/Ti=100) samples was observed with theeffect being less pronounced for the latter. The higher hydrothermal stability of titanium substitutedsamples is probably correlated with a higher degree of polymerisation of the pore walls and with thepresence of extra framework titanium.