Phosphorus 31 NMR study of trialkylphosphine probes adsorbed on silica-alumina

Abstract

Three trialkylphosphines have been investigated as probe molecules for the study of acidic sites on amorphous silica-alumina. These molecules incorporate many favorable NMR features, such as high sensitivity of the /sup 31/P nuclide, high natural abundance, and short relaxation times, which allow convenient quantitative studies of acidic sites on surfaces. Phosphines bound at Bronsted and Lewis sites have been distinguished by chemical shift analogies to model systems. Unlike some other spectroscopic methods that typically measure both surface and bulk sites, the analysis of amorphous materials by probe molecules such as phosphines results in the detection of acidic sites that are strictly on the surface and thus catalytically accessible. Variation of the surface phosphine concentration yields a titration of acidic sites. At low phosphine concentrations only phosphines bound to Bronsted sites are observed. As the phosphine concentration is increased, phosphines bound to Lewis sites and physisorbed phosphines appear simultaneously. Because of the relatively large chemical shift separation of Bronsted bound phosphines from all other phosphine chemical species, it is possible to assay the absolute numbers of surface Bronsted sites directly. Different phosphines count different numbers of Bronsted sites, demonstrating the desirability of using more than one type of probe molecule tomore » characterize surfaces with specific binding sites. It has not been possible to quantitate the number of Lewis sites on silica-alumina because of small differences in chemical shifts and binding constants of Lewis-complexed and physisorbed phosphines.« less