Probing surface acidity by 31P nuclear magnetic resonance spectroscopy of arylphosphines

The adsorption of triphenylphosphine, tri(p-tolyl)phosphine, tri(m-tolyl)phosphine, and tri(p-chlorophenyl)phosphine has been studied on SiO2, Al2O3, and a SiO2-Al2O3 cracking catalyst. P-31 solid-state NMR shows that all of these molecules are rotationally mobile in the physisorbed state. The interaction of the phosphines with Lewis and Bronsted acid sites on the surface can be observed. For these phosphines the chemical shifts of physisorbed and Lewis-bound species are too close to permit quantitative determination of the concentration of the latter. The concentration of Bronsted sites on SiO2-Al2O3 can, however, be measured. Using triphenylphosphine, we detect 0.18 mu mol/m(2) Bronsted sites, somewhat less than previously detected with stronger bases, indicating a spectrum of acid strengths on this catalyst. Steric effects appear to reduce the acid detectable with the tolylphosphines, and it is not clear that they are useful in studying the strength of acid sites on this catalyst. We find evidence for a small concentration of basic sites on the SiO2-Al2O3 catalyst.