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

Layered niobic acids (HxK1−xNb3O8, x = 0–1) are reported as new solid acid catalysts for the selective hydration of ethylene oxide (EO). They are prepared by simply calcinating Nb2O5–K2CO3 mixture followed by an ion-exchange process in HNO3 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 H2O/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 HNb3O8 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 image nanosheets. The thermal stability investigation of HNb3O8 also indicates a careful selection of characterization and application way for this layered niobic acid.