Alkyl chain grafting on silica-zirconia mixed oxides: preparation and characterization

Silica-zirconia mixed oxide substrates were prepared and grafted with C18 alkyl chains using noctadecyltrichlorosilane. Si : Zr molar ratios in the mixed oxide samples before and after alkyl chain modification were determined by X-ray photoelectron spectroscopy (XPS). Nitrogen sorption measurements provided the porous features of these materials. By means of solid-state 29Si nuclear magnetic resonance (NMR) the amounts of the silanol groups present in the as-prepared, humidified and grafted mixed oxide systems as well as the degree of cross-linking of the silanes were obtained. 13C NMR spectroscopy was used to examine the conformational order of the n-octadecyl chains grafted on the silica-zirconia mixed oxide substrates. Similar information was available from variable temperature Fourier transform infrared (FTIR) investigations. Scanning electron microscopy (SEM) measurements and energy dispersive X-ray (EDX) analyses yielded the morphological features of the solid substrates and information about the distribution of the alkyl chains on the oxide surfaces. The annealing temperature for the preparation of the mixed oxides and the elemental composition of the samples, i.e. relative amount of the oxide components, were found to determine the surface properties and in turn the alkyl chain assembly on the mixed oxide surfaces. FTIR and solid-state 13C NMR data gave evidence for an unusual high conformational order of the grafted C18 alkyl chains in all samples, irrespective of the low surface coverage, which exceeds comparable C18 and C30 grafted silica and metal oxide systems. This finding was attributed to the formation of island structures with aggregated octadecyl chains of high conformational order due to strong intermolecular interactions. Differences in the conformational order of the various samples most likely arise from vertical polymerization which varies with the actual sample.