NMR and Theoretical Study of Acidity Probes on Sulfated Zirconia Catalysts James F. Haw,* ,² Jinhua Zhang, § Kiyoyuki Shimizu, ² T. N. Venkatraman, ² Donat-Pierre Luigi, ² Weiguo Song, ² Dewey H. Barich, § and John B. Nicholas* ,‡ Contribution from the Loker Hydrocarbon Research Institute and Department of Chemistry, UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1661, the EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, and the Department of Chemistry, Texas A&M UniVersity, P.O. Box 300012, College Station, Texas 77842-3012 ReceiVed July 27, 2000. ReVised Manuscript ReceiVed October 23, 2000 Abstract: The measurement of the type and number of acid sites on sulfated zirconia catalysts using the 31 P NMR spectrum of adsorbed P(CH 3 ) 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 31 P resonance at +27 ppm is due to P(CH 3 ) 4 + , formed in a reaction that consumes a Brønsted site. The coproduct of this reaction, PH(CH 3 ) 2 , is protonated on the surface to yield a 31 P resonance in the region expected for P(CH 3 ) 3 on a Lewis site. Further complications result from a signal due to OP(CH 3 ) 3 , formed by oxidizing sites on the surface, complexed to unidentified acid sites. As an alternative, we show that carefully designed 15 N experiments using the less reactive and less basic probe pyridine- 15 N 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. Sulfated zirconia has attracted intense interest as a catalyst for alkane isomerization and related reactions at low tempera- tures. It is very active for the alkylation of butenes with butanes to form trimethylpentanes, for which thermodynamics mandates low temperatures to avoid dimethylhexanessproducts with much lower octane numbers. Sulfated zirconia materials have motivated several recent reviews 1-5 and ongoing studies of their application and function. 6-13 Because low-temperature reactions of alkanes are also observed in superacid solutions, sulfated zirconia was early and frequently classified as a solid superacid, and this view persisted after similar views were overturned for zeolite solid acids. 14 The growing recognition that sulfated zirconia is also not a solid superacid 15 has removed the most convenient explanation for catalysis by this material and increased rather than diminished the need for fundamental study of this material. Sulfated zirconia has been studied by theoretical methods 16,17 and spectroscopy, 18,19 including NMR. 20-26 It is undisputed that the surface of sulfated zirconia contains both Brønsted and Lewis acid sites, and much work has been carried out in attempts to ² University of Southern California. ‡ Pacific Northwest National Laboratory. § Texas A&M University. (1) Song, X. 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