Selective glucose conversion to 5-hydroxymethylfurfural (5-HMF) instead of levulinic acid with MIL-101Cr MOF-derivatives

Abstract

The catalytic conversion of glucose to 5-hydroxymethylfurfural (5-HMF) is highly desirable, but the 5-HMF yield obtained using heterogeneous catalysts is still low compared to homogeneous catalysts, and the mechanism is not elucidated completely. In addition, the isolation and purification of 5-HMF still present a challenge as degradation reactions take place and side products form. The formation of 5-HMF from glucose has been reported using several solid acid catalysts; still metal–organic framework (MOF) catalysts could, so far, catalyze the cascade reaction of glucose to 5-HMF only in low yields of less than 16%. Glucose conversion using MOFs is little investigated and here sulfonated MIL-101Cr (MIL-SO₃H) was found to achieve 29% conversion of glucose to 5-HMF after 24 h in a THF : H₂O (v : v 39 : 1) mixture. The conversion of maltose resulted in 50% 5-HMF yield (saccharide solutions were 5 wt%). When the reaction was carried out in pure THF using MIL catalysts no product was formed, revealing the indispensability of water for the glucose-to-5-HMF conversion. Importantly, MIL-SO₃H preferentially leads to 5-HMF over levulinic acid (molar ratio 1 : 0.3), while the catalysts Amberlyst-15 and sulfuric acid form mostly levulinic acid in 5-HMF to levulinic acids ratios of 1 : 3 and 1 : 10, respectively. At the same time, MIL-NO₂ is the most selective, yielding only 5-HMF and showing no formation of levulinic acid. Using 5-HMF as a substrate did not result in any conversion to levulinic acid in the presence of MIL-SO₃H, thereby ruling out the catalytic formation of levulinic acid from 5-HMF. Catalyst recycle experiments showed that MIL-SO₃H stays porous and crystalline, but becomes deactivated through fouling by humin formation. With the use of ethanol as an alternative reaction medium, the formation of insoluble humins could be prevented, but the yield of 5-HMF and 5-ethyl-HMF decreased.