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

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 degrees C for 60 min in DMSO. The concentration of -SO3H in MOF-SO3H as well as the contribution of Bronsted 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.