²⁷A1 Cross Polarization of Aluminas. The NMR Spectroscopy of Surface Aluminum Atoms

The selective cross polarization of surface aluminum atoms is demonstrated and applied to γ-and α-aluminas. The relatively high hydration levels of the γ-alumina surface (out of the bottle) allow the observance of two surface species. Upon dehydration only one, somewhat broader resonance, with lower signal-to-noise ratio, can be resolved. Upon extensive dehydration this signal disappears. By comparison to Boehmite and a-alumina, these resonances are assigned to surface octahedral sites (3.0 ppm) and tetrahedral sites (~62.0 ppm). All chemical shifts are reported relative to Al(N0₃)₃.xH₂O. It is demonstrated that these resonances represent surface Br⊘nsted sites, and that they are not contaminated by Lewis acid sites or subsurface species. The Lewis acid sites can be selectively observed by replacing the surface hydroxyl protons with deuterons and adsoring a Lewis base probe, pyridine, on the surface. In this case the ¹H's on the pyridine provide the polarization for the ²⁷A1 resonance. The α-alumina used in this study appears to have a low number of surface hydroxyls concomitant with its low surface area (5.5 m²/g). This is reflected in the low (relative to γ-alumina)²⁷A1 signal-to-noise ratio obtained with this sample. Overall, this approach appears to be an excellent way to nondestructively characterize these surfaces. More importantly, the success of surface selective cross polarization methodology illustrates that other nuclides are equally amenable to such experiments, and that this approach could be very useful as a general surface chemistry method for characterization of surfaces.