Development of novel solid acid catalyst based on polyanion nanosheets

研究了阳离子交换层状金属氧化物HTiNbO_5和HNb_3O_8中的二维金属氧化物片作为固体-酸催化剂。HTiNbO_5和HNb_3O_8在水溶液中剥离形成胶体单晶TiNbO_5和Nb_3O_8纳米片，在酸性条件下沉淀形成纳米片的聚集体。虽然由于层间距离较窄，阻碍了有机分子的插入，原始层状金属氧化物无法催化乙酸的酯化、异丙烯的裂解和2-丙醇的脱水，但纳米片聚落对这些反应起到了强固体酸催化剂的作用。NH_3程序升温解吸和^1H角自旋核磁共振谱分析表明，由于Nb^<5+>（或Ti^<4+>）-(OH)-Nb^<5+>，这些纳米片聚集体具有很强的Bronsted酸位。我们还发现，由具有高密度磺酸基团的小石墨烯片组成的无定形碳可以替代硫酸催化剂。由廉价原料制备的材料具有较高的热稳定性和化学稳定性。

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.