Development of novel solid acid catalyst based on polyanion nanosheets

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Two-dimensional metal oxide sheets in cation-exchangeable layered metal oxides such as HTiNbO_5 and HNb_3O_8 were examined as solid-acid catalysts. Exfoliation of HTiNbO_5 and HNb_3O_8 in aqueous solutions formed colloidal single-crystal TiNbO_5 and Nb_3O_8 nanosheets, precipitating under an acidic condition to form aggregates of the nanosheets. Although esterification of acetic acid, cracking of cumene and dehydration of 2-propanol were not catalyzed by original layered metal oxides because of the narrow interlayer distance, which prevents the insertion of organic molecules, the nanosheet aggregates functioned as strong solid acid catalysts for the reactions. NH_3 temperature-programmed desorption and ^1H magic-angle spinning nuclear magnetic resonance spectroscopy revealed that these nanosheet aggregates have strong Bronsted acid sites due to Nb^<5+> (or Ti^<4+>)-(OH)-Nb^<5+>.We also found that amorphous carbon composed of small graphen sheets with high density of sulfonic acid groups function as a replacement for sulfuric acid catalyst. The material prepared from inexpensive starting materials has high thermal and chemical stability.