Furfural production from biomass and its derivate sugars has been studied over the last 100 years, but there is still room for improvement in its production process. Furfural degrades into other compounds when dissolved in water, and the use of organic solvents in biphasic systems has been extensively used to solve this problem. To avoid the use of organic solvents, a biphasic system formed by only subcritical water and carbon dioxide as reaction medium was proposed as a more sustainable approach to produce furfural from xylose. In this semi-continuous system, CO2 acts as a Brønsted catalyst by dissolving in the water phase and as a selective extracting agent for furfural, by a continuous feeding of the gas and a continuous furfural enriched gas release. CrCl3 was used as a Lewis catalyst to increase furfural yield. Best results were 46.3 % furfural yield at 50 bar after 1.5 h at 180 °C, with 13.5 % of the furfural collected through the CO2 phase at 35 g/L. Adding an organic solvent such as CPME further improved furfural production due to its lower degradation, although its combination with CO2 did not produce any further improvement. Furfural in the CO2 phase presented high purity, since CO2 can be easily removed, contrary to typical water or organic phases extractions. The semi-continuous operation of the subW-CO2 system at 50 bar was selected as the best option due to its higher furfural yield, lower degradation products production, increased furfural recovery and lower energy consumption and cost of furfural production.
