Preparation of microporous silica gel by sol-gel process in the presence of ethylene glycol oligomers

A microporous silica gel was prepared by a sol-gel process in the presence of ethylene glycol (EG) and its oligomers (oligoEG), and the pore formation process was investigated by using NMR and TG-DTA. The micropore volume changed with the polymerization degree of oligoEG, and showed a minimum at the trimer of EG. An attractive interaction between silanol and organic additives plays a key role in micropore formation: the interaction inhibits the bond formation of Si-O-Si during drying and heating, and heterogeneous domains which convert into micropore are formed after the combustion of additives. There are two kinds of interaction: one is eater (Si-OR) bond formation of a silanol with an end hydroxy group of additives; the other is hydrogen bonding between a silanol and an ether oxygen of additives. The former interaction is significant for EG. With increasing length of oligoEG, however, the former interaction rapidly disappears, while the latter becomes stronger. Because of a minimum in the combination of these interactions, the minimal volume of the micropore is observed.