Superior Salting-Out of MgSO4 to Improve the Synthesis of 5-Hydroxymethylfurfural

The effect of salts on the 5-hydroxymethylfurfural (HMF) synthesis over the catalyst of AlCl3-HCl was investigated. An unprecedented R (distribution coefficient) value of 30.48 was achieved in the H2O/THF system when MgSO4 was used as a salting-out agent. Results indicated that the excellent salting-out effect of MgSO4 on HMF was attributed to the synergistic effect of Mg2+ and SO42-. MgSO4 showed more efficiency on the HMF synthesis than other salts in the ChCl-H2O/THF biphasic system. A 78.1% yield of HMF was achieved from glucose under optimal conditions, and higher than 95.0% of the produced HMF was partitioned into the organic phase due to the excellent salting-out effect of MgSO4. During the recyclability of the aqueous phase, a remarkably high yield of HMF (78.0%) and conversion of glucose (93.8%) were achieved even after 10 cycles. The MgSO4 also showed efficiency to improve the synthesis of HMF from other sugars. Finally, the salting-out effect of MgSO4 on sugar dehydration was investigated in other biphasic systems, and the yield of HMF showed an obvious increase compared with the literature reports.