Layered niobic acid with self-exfoliatable nanosheets and adjustable acidity for catalytic hydration of ethylene oxide

摘要

Layered niobic acids (H_xK_(1-x)Nb _3O_8, x = 0-1) are reported as new solid acid catalysts for the selective hydration of ethylene oxide (EO). They are prepared by simply calcinating Nb_2O_5-K_2CO_3 mixture followed by an ion-exchange process in HNO_3 solution of different concentrations. The highest selectivity for monoethylene glycol (MEG) is achieved over 95% with EO conversion of >99% at x of 0.7 under H _2O/EO ratio of 8. Combined with the results of first-principles density functional theory calculations and Hammett indicator method, it is revealed that the suitable acid strength is more crucial for MEG selectivity than acid amount. Furthermore, a self-exfoliation of layered HNb _3O_8 is also found during EO hydration, which proves to be another important factor for its good catalytic performance by exposing the abundant acid sites among the Nb_3O8- nanosheets. The thermal stability investigation of HNb_3O_8 also indicates a careful selection of characterization and application way for this layered niobic acid.