NbP catalyst for furfural production: FT IR studies of surface properties

The surface acidic properties of niobium phosphate solid acidic catalyst (NbP) have been analyzed by FT IR spectroscopy before and after reactivity tests concerning the hydrolysis and dehydration to furfural of xylose, as model molecule, or of real corn stover hydrolysates. Nitriles adsorption over the fresh NbP catalyst evidences POH and NbOH groups acting as Brønsted acidic sites, whereas Nb5+ ions behave as Lewis exposed sites, showing medium strength acidity. The characterization of the used catalysts surface points out that, in spite of the strongly adsorbed surface species arising from the reactivity tests, both Lewis and Brønsted sites are still exposed, although slightly weaker in acidic strength and more heterogeneous than over the fresh catalyst. This effect can indeed explain the remarkably high reusability of this material, in terms of conversion and furfural yield. In situ IR studies of xylose conversion suggest that activation of xylose thorough coordination at Lewis sites is likely to occur in the reaction temperature range.