Structural evolution upon thermal heating of nanostructured inorganic-organic hybrid materials to binary oxides MO2-SiO2 (M = Hf, Zr) as evaluated by solid-state NMR and FTIR spectroscopy

The structural and compositional evolution of inorganic-organic silica-based hybrid materials and their conversion to mixed oxides, consisting of host silica with variable amounts of zirconia or hafnia (1:1 Si:M, 40:1 Si:M, M = Zr, Hf), was studied by multinuclear solid-state NMR spectroscopy, FTIR spectroscopy, and thermogravimetry. It is shown that the as-prepared materials, composed of M4O2-(OMc)(12) oxoclusters (M = Zr, Hf; OMc = methacrylate) and methacryloxymethyltriethoxysilane (MAPTMS), are not completely polymerized. Rather, a finite number of double bonds remain independent of the actual sample composition. Thermolysis between 300 and 500 degrees C is accompanied by drastic changes in the structural composition, as reflected by the complete loss of all organic components. The incorporation of oxoclusters is found to catalyze the cleavage of Si-C bonds. At the same time, continuous alterations in the inorganic/oxide part are registered. The degree of condensation, as derived from the Si-29 NMR data, increases steadily with calcination temperature and is found to depend on the oxocluster amount. Thus, for the samples with the highest oxocluster content, a pronounced temperature dependence for the degree of condensation is observed that is absent for the samples with a small oxocluster content or for pure MAPTMS. Zr-91 NMR measurements on the sample with the highest zirconium content indicate that upon calcination at 1000 degrees C, only a small amount of crystalline zirconia with tetragonal coordination exists. The major zirconia fraction distributed in the silica matrix is therefore in an amorphous state.