Evolution mechanism of surface hydroxyl groups of silica during heat treatment

The surface hydroxyl groups of nano-silica have a great influence on its application properties. In this paper, the changes of the number of various hydroxyl groups after dehydroxylation and rehydration progress of silica were quantitatively analyzed by thermogravimetric (TG) analysis and 29Si Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) measurements. According to the different stability of Si-O-Si bond formed after dehydroxylation, we propose two types of hydroxyl groups depending on the relative positions of them: chain adjacent hydroxyl groups (C-OHs) and spatially adjacent hydroxyl groups (S-OHs). DFT calculations indicate that two C-OHs dehydroxylation form a four-atoms ring structure, and the resulting OSiO bond angle deviation from optimal value is up to 19.56°, and the average bond length of the SiO bond increase more than 4.66%. Further more, its rehydration reaction activation energy (Ear) is very low, even only 0.21 kJ/mol, so it is easy to re-open and return to hydroxyl groups under environmental conditions. Conversely, the S-OHs dehydroxylation form a stable multi-atoms ring. The DFT calculations are a good illustration of the phenomenon in which some unstable siloxane bonds in a dehydroxylated sample are restored to hydroxyl groups in TG analysis and 29Si MAS NMR measurements.