Sol-gel processing of tethered metal complexes: influence of the metal and the complexing alkoxysilane on the texture of the obtained silica gels

The metal complexes [M(AEAPTS)(2)](2+) or [M(TRIAMIN)(2)](2+) are formed by reaction of cobalt, nickel or copper acetate (or 1:1 mixtures thereof) with two molar equivalents of [N-(aminoethyl)aminopropyl]trimethoxysilane (AEAPTS) or [N((N-aminoethyl)aminoethyl)aminopropylltrimethoxysilane (TRIAMIN). Sol-gel processing of [M(AEAPTS)(2)](2+) /Si(OEt)(4) or [M(TRIAMIN)(2)](2+) /Si(OEt)(4) (1:6) mixtures results in xerogels in which the metal complexes are tethered to the silica gel network. The gelling behavior of the mixtures is influenced by the presence of metal ions and depends very strongly on the kind of complexing silane. For comparison, xerogels were also prepared from metal acetate/Si(OEt)(4) mixtures without a complexing silane. While the xerogels prepared from M(TRIAMIN)(2)(2+)/Si(OEt)(4) mixtures are essentially non-porous, the xerogels prepared from M(AEAPTS)(2)(2+)/Si(OEt)(4) mixtures or in the absence of a complexing silane are porous and have high surface areas. The latter samples have distinctly larger pores than the samples prepared in the presence of the complexing silanes. The xerogels were calcined in air at 550 degreesC to remove the organic groups and to obtain metal oxide/silica nanocomposites. The presence of the metal ions promotes the thermal degradation of the organic groups, i.e., both the onset and the end of the thermal degradation of the organic groups is shifted to lower temperatures. The decomposition temperature depends on the kind of metal. Calcination results in an increase of porosity, mainly by creation of micropores. Reduction at 500 degreesC results in metal/silica nanocomposites. The reduction step has only a minor influence on the texture; the proportion of micropores is only slightly reduced. Re-examination of the surface and pore structure of the xerogels and the metal oxide/silica nanocomposites after 2 years showed that the porosity had strongly decreased upon storage at ambient conditions, mainly due to the disappearance of micropores. (C) 2001 Elsevier Science B.V. All rights reserved.