Continuous Synthesis of 5-Hydroxymethylfurfural from Glucose Using a Combination of AlCl3 and HCl as Catalyst in a Biphasic Slug Flow Capillary Microreactor

5-Hydroxymethylfurfural (HMF) was synthesized from glucose in a slug flow capillary microreactor, using a combination of AlCl3 and HCl as the homogeneous catalyst in the aqueous phase and methyl isobutyl ketone as the organic phase for in-situ HMF extraction. After optimization, an HMF yield of 53% was obtained at a pH of 1.5, 160 °C and a residence time of 16 min, and it could be further increased to 66.2% by adding 20 wt% NaCl in the aqueous phase. Slug flow operation in the microreactor greatly promoted mixing/reaction in the aqueous droplet and facilitated HMF extraction to the organic slug, enabling the reaction to run (largely) under kinetic control and an enhanced HMF yield by suppressing its further rehydration, degradation and/or polymerization. Confining reaction in the aqueous droplet prevented humin deposition on the microreactor wall. In line with the literature, [Al(OH)2]+ was confirmed by ESI-MS as the catalytically active species, and is responsible for the glucose isomerization to fructose under various pH values. The ratio between AlCl3 and HCl was optimized for the highest HMF yield and the best results were obtained with 40 mM AlCl3 and 40 mM HCl. Compared with batch results, a higher HMF yield was obtained in the microreactor at the same reaction time mainly due to a higher heating rate therein. The aqueous catalyst was recycled and reused three times without a noticeable performance loss. Thus, the present recyclable and stable homogenous catalyst system, combined with biphasic microreactor operation, is an attractive concept for the glucose conversion to HMF.