Furfural hydrogenation, hydrodeoxygenation and etherification over MoO2 and MoO3: A combined experimental and theoretical study.

Valorization of lignocellulosic biomass, particularly catalytic hydrotreatment of hemicellulose-based furfural (FUR), has been studied for the production of value-added chemicals. A three-phase batch reactor has been used for hydrotreatment in isopropanol over various commercially available unsupported MoO_x catalysts, at various temperatures (170–230 °C), pressures (0–80 bar H₂), catalyst loadings (0–2 wt%), and reactant concentrations (5–20 wt%). No significant difference in catalytic activity or selectivity has been observed among the three different MoO₃ and one MoO₂ catalysts, while NiMo/Al₂O₃, Mo₂C and WO₃ were much less active. Data-points collected have been used to propose a detailed reaction pathway network for a micro-kinetic model, which also took into consideration the thermodynamics, and adsorption, desorption, and surface reaction kinetics. The alcoholysis of FUR yielded valuable isopropyl levulinate (IPL) as the major product under all tested reaction conditions, while other value-added compounds (furfuryl alcohol, isopropyl furfuryl ether, furfuryl acetone, angelica lactone) were observed in smaller quantities. It was found that neither the presence nor the absence of the gaseous H₂ pressure contributes to the global reaction rate, or selectivity, since the solvent acts as a sufficient hydrogen donor. Additionally, density functional theory (DFT) calculations provided further insight into the active planes present by the implementation of the Wulff construction. Furthermore, the reaction mechanism was explained based on reaction energies, which were in silico determined and compared for several surfaces. The results were consistent with the characterization and activity-testing results. The furfural ring-opening reaction, yielding valuable IPL in the absence of gaseous H₂, over a cheap bulk MoO_x is reported for the first time.