NMR and Theoretical Study of Acidity Probes on Sulfated Zirconia Catalysts

The measurement of the type and number of acid sites on sulfated zirconia catalysts using the ³¹P NMR spectrum of adsorbed P(CH₃)₃ has been vexed by 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 ³¹P resonance at +27 ppm is due to P(CH₃)₄⁺, formed in a reaction that consumes a Brønsted site. The coproduct of this reaction, PH(CH₃)₂, is protonated on the surface to yield a ³¹P 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 oxidizing sites on the surface, complexed to unidentified acid sites. As an alternative, we show that carefully designed ¹⁵N experiments using the less reactive and less basic probe pyridine-¹⁵N provide more easily interpreted measurements of Brønsted and Lewis sites on sulfated zirconias of diverse composition, preparation, and treatment. Quantitative studies revealed that the number of Brønsted sites capable of protonating pyridine corresponded to only ∼7% of the sulfur atoms on the catalyst we studied in the greatest depth. Additional Brønsted sites were created on this catalyst with addition of water, a reaction not observed for sulfur-free zirconia.