Structures and Brønsted acidity features for titanoniobates with different laminate composition

Abstract

Surface acidity is one of the important features for the layered titanoniobates. However, the effects of the laminate composition on the structure and acidity of titanoniobates are not clear. Herein, a series of layered solid acids including HTiNbO5, HTi2NbO7 and H3Ti5NbO14 are prepared through the high temperature calcination and proton exchange process. Their structures and physicochemical properties are investigated by the means of X-ray diffraction (XRD), thermal gravimetric–differential thermal gravimetric (TG-DTG), laser Raman spectroscopy (LRS) and 1H magic-angle spinning nuclear magnetic resonance (1H MAS NMR) spectra. The esterification of acetic acid and n-butanol is used to evaluate the acid catalytic properties of the as-prepared layered solid acids. The results show that the Brønsted acid sites on the as-prepared samples are derived from different hydroxyl species and the strength is obviously affected by its laminate composition, phase structure and hydroxyl species feature, especially the possible hydrogen bonding. More TiOH and TiOHTi bonds will be formed in those layered compounds with an increasing Ti/Nb atomic ratio, due to more exposure of the terminal TiO or TiOTi bonds on the laminate surface. The compound H3Ti5NbO14 possesses the strongest Brønsted intensity and acid catalytic activity among the investigated samples.