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

Abstract

Layered niobic acids (H x K 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 3 O 8 - nanosheets. The thermal stability investigation of HNb 3O 8 also indicates a careful selection of characterization and application way for this layered niobic acid.