An attempt to achieve the direct hydrogenolysis of tetrahydrofurfuryl alcohol in supercritical carbon dioxide

The use of supercritical carbon dioxide (scCO2) in the hydrogenolysis of tetrahydrofurfuryl alcohol was investigated over a Rh supported MCM-41 catalyst (metal loading ≈ 1%). Without any additive, tetrahydrofurfuryl alcohol was converted to 1,5-pentanediol with high conversion (80.2%) and selectivity (91.2%) under mild reaction conditions (temperature = 80 °C and H2 pressure = 4 MPa). The effects of different variables like CO2 and H2 pressure, temperature and the reaction time were also optimized. A strong influence of CO2 pressure on the conversion was observed. The conversion was increased with pressure due to the enhanced solubility of tetrahydrofurfuryl alcohol in CO2 and reached the maximum at 14 MPa, as the single phase (CO2–H2–substrate) was formed, but the selectivity of 1,5-pentanediol remained unaltered. On the other hand, H2 pressure also changed the conversion as well as the selectivity. Temperature was found to play an important role in enhancing the catalytic efficiency; conversion of the substrate was increased along with the temperature, but the selectivity of 1,5-pentanediol dropped after reaching 120 °C. The Rh catalyst exhibited strikingly different product distribution under the solvent-less conditions compared to scCO2. In addition, instead of CO2, when the reaction was carried out in H2O, the conversion and the selectivity of 1,5-pentanediol decreased substantially. However, addition of only 7 MPa of CO2 modified the conversion and the product selectivity. Under similar reaction conditions, hydrogenolysis of THFA over a Pd catalyst provides a maximum of 50% conversion and the product distribution was different in comparison with Rh catalysts.