Studies of the characteristic and reaction mechanism of silica xerogel by sol–gel method catalyzed by alkylbiguanide as a strong organic base

Treatment of tetraethyl orthosilicate with 1,2-diisopropyl-4,4,5,5-tetra-methyl biguanide (A) as a highly strong base immediately gave silica gel by means of hydrolysis and condensation reaction at room temperature. The resulting wet gel was transparent and showed high density after dryness. From the results of gas adsorption and BET analysis, silica gel obtained by the treatment of strong base A had larger specific surface area and pore volume than silica gel that was prepared by a regular or less strong base such as tetramethylammonium hydroxide (TMAH). FTIR analysis revealed that the peak strength of Si-OH bond at 960 cm⁻¹ of silica gel prepared by highly strong base A was smaller than that of TMAH. To understand the mechanism behind such difference, a mixture of diphenylsilandiol and dimethoxydiphenylsilane were reacted with highly strong base A, and the resulting products comprised linear-chain siloxane oligomer and octaphenylcyclotetrasiloxane. Our results indicate that silanol generated by hydrolysis of TEOS is activated by A and the activated silanol undergoes subsequent direct reaction with unhydrolyzed alkoxy silane to give condensation products in ethanol. Such a direct polycondensation between silanol and alkoxy silane brought by highly strong base A led to three-dimensional crosslinking having a higher bulk density of silica gel.