A green organic, solvent-free, subcritical water-CO2 (subW-CO2) system was proposed for the production of furfural from the main hemicellulose's sugar, xylose. Despite subW itself showed potential for furfural generation due to its catalytic properties under subcritical conditions; when CO2 was added into the system as pressurization agent, higher yields were reached due to its effect as a Brønsted acid when dissolved in water. Furthermore, its combination with different Lewis acidic catalysts showed a synergistic effect, further improving furfural yield. In a batch configuration, five homogeneous and four heterogeneous catalysts were tested at 180 °C and 5.5 MPa and compared for the first time in a subW-CO2 reaction medium. Different furfural production paths were determined for the two catalysts types, with higher xylose isomerization rates to xylulose when homogeneous trivalent metal catalysts were used. CrCl3 and Nafion NR50 resin were selected as best catalysts from each of the groups due to higher furfural production rates (51.9 ± 0.9 % yield/h) and higher furfural selectivity (60.9 %), respectively. Furthermore, Nafion NR50 catalytic activity remained unchanged after 10 runs at 180 °C. The green subW-CO2 system furfural yields were comparable to water-organic solvent biphasic ones, proving to be a promising green alternative.
