Catalyst-free synthesis of 5-hydroxymethylfurfural from fructose by extractive reaction in supercritical CO2 – subcritical H2O two-phase system

An extractive reaction configuration using supercritical carbon dioxide (scCO₂) as the extracting solvent was tested for the production of 5-hydroxymethyl furfural (HMF) from a 5 wt% fructose aqueous feed. In this configuration, extraction of HMF by scCO₂ prevents HMF degradation in the aqueous phase. Because of water co-extraction by scCO₂, the volume of the reactional mixture was maintained by continuous injection of water. Reaction was operated in a 90 mL high pressure reactor, where an HMF maximum yield of 62.4 % was achieved at 160 °C and 25 MPa, with a CO₂ flow rate of 20 g.min⁻¹ for 420 min. This is the first time that HMF is reportedly produced with such a yield by a catalyst-and organic solvent-free process. Besides, the separation efficiency reached 97.3 % and the relative purity of HMF in the extract was 95.8 wt%. Therefore, this configuration avoids post reactional purification which is needed in conventional batch processes or in extractive reaction processes using organic solvents. Based on kinetic and thermodynamic studies, modeling of the extractive reaction process was developed to perform a sensitivity analysis for CO₂ flow rate and extraction efficiency, upon the HMF yield. As an example, it was shown that for 800 min reaction duration, a CO₂ flow rate of 100 g.min⁻¹ or an extraction efficiency increase by a 10-fold factor could theoretically led to HMF yields of 73.0 % and 73.7 %, respectively.