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An alternative preparation of organofunctionalized silica gels and their characterization by solid-state NMR spectroscopy Zhang, Yugao


The theoretical consideration and experimental techniques required to obtain high -resolution solid-state NMR spectra of silica gels and their organofunctionalized derivatives are presented. A series of organofunctionalized silica gels has been prepared by the copolymerization of the two components, tetraethoxysilane (TEOS), (EtO)₄Si, and methyltriethoxysilane (MTES) (EtO)₃SiCH₃, which is an alternative to the conventional anchoring method. CP/MAS solid-state NMR techniques have been used to elucidate the structures of the organofunctionalized silica gels. The copolymerization method is superior to the anchoring method in that more functional groups can be incorporated into a silica gel matrix. CP/MAS solid-state NMR has been successfully applied in the investigation of the thermal stabilities of the functionalized silica gels as a complementary technique to the more traditional DSC technique and it has been concluded that in an inert gas (N₂) environment, the methyl groups on the methylsilica gel made by copolymerization method are more thermally stable to decomposition than that made by the anchoring method and both are much more stable than in a normal atmospheric environment. Two-dimensional heteronuclear ¹H-²⁹Si correlation NMR spectroscopy has been used to characterize three different types of modified silica gels; methylsilica gel prepared by the anchoring method; methylsilica gel prepared by the copolymerization method; and a mechanical mixture of silica gel and polymethylsiloxane. This technique successfully demonstrated the structural differences among the three materials which cannot be easily distinguished by other analytical techniques. It is shown that methylsilica gel prepared by the anchoring method consists of two distinct phases when the functional group loading is high (i.e., polymethylsiloxane built up "horizontally" on the surface of silica gel), and it is very like the mechanical mixture of silica gel and polymethylsiloxane. On the other hand, the methylsilica gel prepared by the copolymerization method consists of a distribution of the two components, i.e., there is no domain structure between silica gel and polymethylsiloxane. The methyl protons and hydroxyl protons in the functionalized silica gels were found to be sources of cross-polarization transfer to the Si(0-)₄ framework silicons, but the physically absorbed water does not contribute to this transfer.

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