Aqueous-phase tandem catalytic conversion of xylose to furfuryl alcohol over [Al]-SBA-15 molecular sieves

Abstract

Catalytically active sites were controllably assembled into an SBA-15 framework by direct hydrothermal synthesis. Incorporation of Al as a heteroatom at different Si/Al ratios was used to tune their surface properties and the catalysts were characterized by SAXRD, ICP-OES, FE-SEM, TEM, FTIR, ²⁹Si and ²⁷Al CP-NMR, pyridine adsorption and titration in water. Reactivity towards direct conversion of xylose to furfuryl alcohol was systematically investigated. Brønsted acid sites, generated by introducing Al into the siliceous SBA-15 framework, were found to be active for the pentose dehydration reaction, whilst the Lewis acid sites promoted the transfer hydrogenation of the adsorbed furfural intermediate to furfuryl alcohol. A remarkably high selectivity to furfuryl alcohol (90–95%) was observed using the mesoporous catalysts and it was dependent on the Al concentration in the siliceous framework. The vicinity of Brønsted and Lewis acid sites is suggested to be of major concern since the adsorbed furfural species is proposed to be reduced to furfuryl alcohol without being desorbed. The [Al]-SBA-15 molecular sieves were shown to be chemically and structurally stable demonstrating good recyclability in the aqueous-phase upgrading of xylose.