Effects of Molecular Organic Additives on Formation and Properties of Organosilicate and Silica Xerogels Correlated to Structural Properties of the Additive

This paper describes the effects of some polar organic additives on the sol−gel processing of organodisilicate and silica precursors. For a range of organic additives investigated, those for which molecular stacking is a feature of the crystal structure (type A) promote extensive fragmentation of organosilicate and silica xerogels into thin plates and small (∼1 mm diameter) granules during the final drying stages. Two organic additives that do not exhibit such stacking (type B) were investigated, these did not promote such fragmentation, and the organic additive-treated monoliths were formed. Sol−gel mixtures investigated with type A organic additives include: 1,4-bis(Cl3SiCH2)2C6H4 and organic additives trans-cinnamic acid (TCA), crotonic acid, or benzoic acid; 1,4-bis[(n-BuO)3SiCH2)]2C6H4 and m-nitroaniline; 1,4-bis[(EtO)3Si]C6H4 and [(EtO)4Si] with trans-cinnamic acid. Sol−gel mixtures containing type B organic additives investigated were 1,4-bis(Cl3SiCH2)2C6H4 and the organic additives phenol or pinacol; 1,4-bis[(EtO)3Si]C6H4 or (EtO)4Si with PhOH. The additives may be removed by extraction of the xerogel materials with THF. The effects of tetrahydrofuran solutions of types A and B additives on monolithic additive-free organosilicate xerogels and silica gels is also reported. The materials have been variously studied by SEM(EDAX), XRD, 29Si solid-state NMR, and 13C solid-state NMR. The THF-extracted products were studied by nitrogen sorption porosimetry. BET measurements on THF extracted materials showed that type A additives generally gave low surface area materials while type B additives gave high surface area materials. 29Si solid-state NMR and BET measurements revealed nonsystematic effects of TCA loading on local chemical structure and surface area, respectively.