Acid sites on SiO2-Al2O3 monolayer catalysts: P-31 NMR probes of strength and accessibility

We have examined the acidic properties of a SiO2-on-Al2O3 monolayer catalys t by P-31 NMR of adsorbed tricyclohexylphosphine and triphenylphosphine. Th ese large probe molecules permit the assessment of steric effects, by compa rison with trimethylphosphine, and reveal differences in acid strengths of surface sites. We find that there are significant constraints to accessing Lewis sites, but not Bronsted sites. The concentration of strong Bronsted s ites is slightly less than half of the total concentration of Bronsted site s.