Exfoliated Nanosheets as a New Strong Solid Acid Catalyst

Abstract

Two-dimensional metal oxide sheets in HTiNbO₅ and HSr₂Nb₃O₁₀, cation-exchangeable layered metal oxides, were examined as solid acid catalysts. Exfoliation of HTiNbO₅ and HSr₂Nb₃O₁₀ in aqueous solutions formed colloidal single-crystal TiNbO₅^- and Sr₂Nb₃O₁₀^- nanosheets, which precipitated under an acidic condition to form aggregates of HTiNbO₅ nanosheets and HSr₂Nb₃O₁₀ nanosheets. Although esterification of acetic acid, cracking of cumene, and dehydration of 2-propanol were not catalyzed by original HTiNbO₅ because of the narrow interlayer distance, which prevents the insertion of organic molecules, HTiNbO₅ nanosheets functioned as a strong solid acid catalyst for the reactions. Nanosheets of HSr₂Nb₃O₁₀ exhibited no or slight catalytic activity for these reactions. NH₃ temperature-programmed desorption and ¹H magic-angle spinning nuclear magnetic resonance spectroscopy revealed that HTiNbO₅ nanosheets have strong Brønsted acid sites, whereas HSr₂Nb₃O₁₀ nanosheets do not.