Effects of transition-metal composition of protonated, layered nonstoichiometric oxides H_1-xNb_1-xMo_1+xO ₆ on heterogeneous acid catalysis

Protonated, layered transition-metal oxides H_1-xNb _1-xMo_1+xO6 with various Nb and Mo ratios were examined as solid acid catalysts. The effects of transition-metal composition on layered crystal structure, solid acid proprieties, and catalytic activities were investigated. X-ray diffraction (XRD) patterns revealed that a layered crystal structure was maintained in the range of x = -0.15 to 0.15. Beyond this range, the oxides did not form a layered structure, which resulted in negligible acid catalytic activity. The acid strength and acid amount determined by ammonia temperature-programmed desorption measurements indicated that increasing Mo concentration enhanced the acid strength. On the other hand, an increase in Nb concentration increased the acid amount. These layered oxides possess intercalation availability for benzyl alcohol, determined by XRD and thermogravimetric analysis, where the amount of intercalated benzyl alcohol tended to correspond to the acid amount of the layered metal oxides. Layered Nb-Mo oxides exhibited remarkable catalytic activity in Friedel-Crafts alkylation of toluene with benzyl alcohol, exceeding that of niobic acid, ion-exchange resins, and H-type zeolites, and this was attributed to the intercalation behavior. The alkylation activity over layered Nb-Mo oxides increased with increasing Nb concentration, which was related to the acid amount rather than the acid strength. For hydrolysis of cellobiose and esterification of lactic acid, on the other hand, the acid catalytic activity increased with increasing Mo concentration, H_0.9Nb_0.9Mo _1.1O₆ > HNbMoO₆ > H_1.1Nb _1.1Mo_0.9O₆, indicating that the acid strength was more important in these reactions.