Conversion of fructose into 5-hydroxymethylfurfural catalyzed by recyclable sulfonic acid-functionalized metal–organic frameworks

Abstract

A series of sulfonic acid-functionalized metal–organic frameworks (MOF-SO₃H) were prepared by postsynthetic modification (PSM) of the organic linkers within the MOF with chlorosulfonic acid. The obtained MOF-SO₃H, including sulfonic acid-functionalized MIL-101(Cr) [MIL-101(Cr)-SO₃H], UIO-66(Zr) [UIO-66(Zr)-SO₃H], and MIL-53(Al) [MIL-53(Al)-SO₃H], have been systematically studied as solid acids in fructose transformation to 5-hydroxymethylfurfural (HMF). With MIL-101(Cr)-SO₃H as catalyst, a HMF yield of 90% with a full fructose conversion was obtained at 120 °C for 60 min in DMSO. The concentration of –SO₃H in MOF-SO₃H as well as the contribution of Brønsted acidity of MOF-SO₃H parallels its –SO₃H grafting rate. Under a lower –SO₃H grafting level, a good linear correlation between catalytic activity, in terms of turnover frequency, and sulfonic acid-site density of MOF-SO₃H was found. Moreover, the sulfonic acid groups, which function as the catalytic sites, are equivalent in all MOF-SO₃H for fructose-to-HMF transformation, regardless of precursor MOFs. Both conversions of fructose and selectivities towards HMF increase with the sulfonic acid-site density of MOF-SO₃H at an initial stage of fructose-to-HMF transformation. Kinetics studies reveal that the MIL-101(Cr)-SO₃H promoted fructose-to-HMF transformation may follow pseudo-first-order kinetics with observed activation energy of 55 kJ mol⁻¹ under the investigated conditions. Moreover, MIL-101(Cr)-SO₃H behaves as a heterogeneous catalyst and can be easily recovered and reused. The research highlights a good prospect for catalytic application of MOF-derived solid acid catalysts for biomass carbohydrate valorization.