Evidence of Si–O–C bonds in cellulosic materials modified by sol–gel-derived silica

The present study contributes to the controversial discussion in the literature whether Si–O–C bonds in wood–silica–gel composites exist. ¹³C NMR is a suitable method to proof such bonds. Because of the low concentration of ¹³C isotopes in natural wood, ¹³C cellulose was used as ¹³C-enriched substitute. A tailored sol for the impregnation of that cellulose was chosen by liquid ²⁹Si NMR pre-investigations of various sols whose reactivity and stability were time-dependently analysed. It is based on a sub-stoichiometric hydrolysis of tetraethoxysilane (TEOS) with 1 mol water per mol TEOS. Thermal analyses were performed to show a comparability of the thermal behaviour of wood–silica–gel and cellulose–silica–gel composites. There are two strong hints of the existence of stable Si–O–C bonds: (1) by thermal analysis, a shift in the fire behaviour of 100 K can be observed with a change in pyrolysis behaviour of the composite and (2) the proof by REDOR NMR that a dipolar coupling between ²⁹Si and ¹³C nuclei exists.