Hydrogenation of 5-hydroxymethylfurfural in supercritical carbon dioxide–water: a tunable approach to dimethylfuran selectivity

Abstract

The use of supercritical carbon dioxide–water on the hydrogenation of 5-hydroxymethylfurfural (HMF) was investigated over a Pd/C catalyst. It was possible to achieve a very high yield (100%) of DMF within the reaction time of 2 hours at 80 °C. A significant effect of CO₂ pressure was observed on the product distribution. Simply by tuning the CO₂ pressure it was possible to achieve various key compound, such as tetrahydro-5-methyl-2-furanmethanol (MTHFM) (<10 MPa), 2,5-dimethylfuran (DMF) (10 MPa) and 2,5-dimethyltetrahydrofuran (DMTHF) (>10 MPa) with very high selectivity. Optimization of other reaction parameters revealed that H₂ pressure, temperature, as well as the CO₂–water mole ratio, played an important role in the selectivity to the targeted DMF. It is interesting to note that a very high yield of DMF was achieved when a combination of CO₂ and water was used. For instance, in the absence of water or CO₂, the selectivity of DMF was low; similarly, an excess of water against the fixed pressure of CO₂ reduced the selectivity to DMF. Hence, an optimized amount of water was mandatory in the presence of CO₂ for the formation of DMF with high selectivity. This method was successfully extended to the hydrogenation of furfural, which could afford 100% selectivity to 2-methylfuran with complete conversion within a very short reaction time of 10 min. The studied catalyst could be recycled successfully without significant loss of catalytic activity.