Supported niobium catalysts for methanol dehydration to dimethyl ether: FTIR studies of acid properties

A series of NbOx/TiO2 catalysts (0.9–9.0 at Nb nm−2) was prepared by incipient wetness impregnation of niobium (V) oxalate aqueous solutions on TiO2. Spectroscopic data reveal that NbOx phase is highly dispersed and relatively large Nb2O5 crystallites are not detected; highly distorted NbO6 octahedral structures are exclusively identified by Raman spectroscopy. Surface acidity measurements by NH3-temperature-programmed desorption experiments indicate that the overall number of Lewis acid sites in the solids decreases with Nb-loading, although their strength follows the opposite trend. A minor fraction of Brønsted acid sites is also detected for the sample with the highest Nb surface coverage used here (9.0 at Nb nm−2). The catalytic activity of NbOx/TiO2 in methanol dehydration to dimethyl ether is found to be determined mainly by the strength of the acid sites and the presence of Brønsted acid sites, being more active those catalysts with stronger acid sites and with a higher number of Brønsted acid sites.