Comparison of surface properties of silica xero- and hydrogels hydrothermally modified using mechanochemical, microwave and classical methods

Abstract Several series of silicas of different porous structure and surface chemistry were prepared hydrothermally by applying the classical (HTT), microwave (MWT) or mechanical (MChT) energy. The starting material was silica in the form of wet gel (hydrogel) or xerogel (dry gel). The influence of such parameters as the form of starting silica, concentration of water, temperature, duration time and value of rotation speed on the structural and chemical parameters of prepared silica samples was studied. The prepared samples were characterized using the low-temperature adsorption-desorption of nitrogen, TG/DTG analysis, Scanning Electron Microscopy (SEM) and FTIR spectroscopy methods. The concentration of surface hydroxyls was determined using the TG/DTG data. Depending on the form of initial silica subjected to modification and a kind of heat energy supplied to the system the obtained silica samples are characterized by different porous structure, various concentration of physically adsorbed water, and surface hydroxyl groups (δ OH ). New patterns, namely an increase in specific surface area and total pore volume due to formation of more open porosity during wet milling gel and xerogel were established. As a result, mesoporous silicas possessing high specific surface area and uniform pore size distribution were prepared. For the samples modified in the form of hydrogel the δ OH value is about 3 OH/nm 2 , which is lower than that for completely hydroxylated silica (4.6 OH/nm 2 ). So, it is assumed that only in the case of samples modified in the form of xerogel, the intraglobular water is released during heating. New, not previously described IR absorbance bands at 3260–3280 and 3170–3185 cm −1 appearing in the spectra can be hypothetically attributed to vibration of water molecules disposed in micropores and places at globule contacts, correspondingly. Similar dependence was also observed for 1645–1655 cm −1 that is assigned to deformation vibration of water molecules. The images obtained using SEM method reflect well the morphology of studied silicas.