Biphasic catalytic dehydration of sugars to 5-hydroxymethylfurfural in a continuous centrifugal contactor separator

The worldwide depletion of fossil resources requires renewable carbon feedstocks (e.g., biomass) for producing chemicals. 5-hydroxymethylfurfural (HMF) is a promising biobased platform chemical that can be converted into polymer building blocks, resins, pharmaceuticals and many other value-added components. HMF is typically synthesized by the homogeneous or heterogeneous acid-catalyzed dehydration of sugars (e.g., fructose) in water. In this process two main side reactions occur: the rehydration of HMF to formic acid and levulinic acid (LA; another a value-added chemical)2 and the HMF condensation with sugar to humins. To maximize HMF yields, a non-reactive organic extraction phase may be added in which HMF has a high solubility, thus suppressing HMF-involved side reactions in water. To enhance the HMF extraction rate, a high liquid-liquid mass transfer is desired. In this context, dedicated reactor engineering studies should be performed to intensify the extraction and make it attractive for industrial-scale production. A promising intensification tool for liquid-liquid processes is the continuous centrifugal contractor separator (CCCS). In the CCCS, chemical reaction (in the annular zone) is combined with separation (in the inner centrifuge), so that HMF is easily retrieved from the organic phase (e.g., by distillation) and the aqueous phase can be recycled. Furthermore, the strong shear forces generated by centrifugal forces in the CCCS typically results in an enhanced HMF extraction rate. In this work, the biphasic synthesis of HMF (and LA) was performed in a CCCS (CINC V02) using a homogeneous H2SO4 catalyst in the aqueous phase. The influence of organic solvent type, addition of NaCl in the aqueous phase, acid catalyst concentration, rotatory speed and aqueous to organic flow ratio were tested. Reactions with 2-methyltetrahydrofuran (MTHF) as the organic extraction solvent produced the highest HMF yield/selectivity, due to the high HMF partition coefficient therein. Addition of NaCl in the aqueous phase also increased the partition coefficient but did not affect the HMF yield/selectivity. Under relatively mild conditions (1200 rpm, 140 °C and 10 bar) and 20 min residence time the fructose conversion was 70% with 40-50% HMF yield and 60-70% selectivity, respectively. The use of sucrose (a disaccharide of fructose and glucose from sugar beet thick juice) as the feedstock resulted in 70% conversion of the fructose fraction with 50% HMF yield (>70% selectivity), whereas the glucose fraction remained mostly intact. A basic techno-economic analysis of the process under the optimized conditions will be performed to estimate the potential for the industrial HMF synthesis using the CCCS.