NMR and theoretical study of acidity probes on sulfated zirconia catalysts

The measurement of the type and number of acid sites on sulfated zirconia c atalysts using the P-31 NMR spectrum of adsorbed P(CH3)(3) has been vexed b y spectral assignment controversies. Using a combination of NMR experiments and theoretical methods, including chemical shift calculations at the GIAO -MP2 level. we show that a previously observed P-31 resonance at +27 ppm is due to P(CH3)(4)(+), formed in a reaction that consumes a Bronsted site. T he coproduct of this reaction. PH(CH3)(2), is protonated on the surface to yield a P-31 resonance in the region expected for P(CH3)(3) On a Lewis site . Further complications result from a signal due to OP(CH3)(3), formed by o xidizing sites on the surface, complexed to unidentified acid sites. As an alternative, we show that carefully designed N-15 experiments using the les s reactive and less basic probe pyridine-N-15 provide more easily interpret ed measurements of Bronsted and Lewis sites on sulfated zirconias of divers e composition, preparation, and treatment. Quantitative studies revealed th at the number of Bronsted sites capable of protonating pyridine corresponde d to only similar to7% of the sulfur atoms on the catalyst we studied in th e greatest depth. Additional Bronsted sites were created on this catalyst w ith addition of water, a reaction not observed for sulfur-free zirconia.