A systematic kinetic study was conducted on the production of furfural from different sugar monomers, including xylose, arabinose and glucose, and xylan, a biopolymer rich in pentoses, in a subcritical water reaction medium intensified by the presence of CO2 as pressurization agent. The temperature range was from 150 to 200 °C, and different catalysts were employed, with CrCl3 as a Lewis acid catalyst. Glucose yielded very low amounts of furfural. Regarding pentoses, different reaction pathways were analyzed. The proposed mechanism involved the presence of the corresponding ketoses, xylulose and ribulose, from xylose and arabinose, respectively, as intermediates in furfural production. Formic acid was identified as the main degradation product in this reaction medium. It was concluded that formi acid was released into the medium from both pentoses and furfural. Pentose aldoses isomerization to ketoses, followed by furfural formation was the primary reaction pathway in this system. When using an acidic resin such as Nafion NR50 as catalyst, the isomerization of xylose was the rate-controlling step. When starting from xylan, similar furfural yields were obtained, but producitivity values evaluated over time were higher for xylose due to the two-step process involved in furfural production from xylan. The molecular weight distribution in the xylan system showed an initial increase for fractions in the range of 282–150 Da, while fractions of molecular weights higher than 1074 Da appeared only at the end of the treatment, likely due to the formation of different polymeric products.
