Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance studies of octadecyl modified metal oxides obtained from different silane precursors

Octadecyl (C18) modified metal oxide substrates, including titania, zirconia, hafnia, and alumina, are prepared using two types of silylating reagents, n-octadecyltrihydridosilane and n-octadecyltrichlorosilane. Fourier transform infrared (FTIR) and solid-state 29Si nuclear magnetic resonance (NMR) measurements are performed to examine the cross-linking of the silanes. Solid-state 13C NMR spectroscopy provides information about the conformation and mobility of surface-immobilized alkyl chains. Variable temperature FTIR investigations are carried out to study the influence of the organosilane precursors and metal oxides on the conformational order of the alkyl modified systems. It is found that grafting by means of n-octadecyltrichlorosilane yields higher grafting densities than surface modification with n-octadecyltrihydridosilane. Combined pyridine adsorption and diffuse reflectance infrared Fourier transform (DRIFT) measurements are performed on the titania and hafnia substrates to evaluate potential surface heterogeneities, i.e. Lewis and Brّnsted sites. Differences in the alkyl chain conformational order within the series of C18 modified metal oxides are explained by the presence of island structures. The reduced C18 conformational order for the samples grafted with n-octadecyltrihydridosilane is traced back to the lower grafting density which in turn points to a lower reactivity of this silylating reagent. The most striking result is the higher conformational order of the C18 chains grafted in the present surface modified metal oxides when compared with silica-based systems. This finding is attributed to the lower porosity of the metal oxide supports along with more closely packed chains on the surface.