The synthesis of furan compounds by renewable carbohydrates has emerged as an important path toward biomass utilization. 5-Hydroxymethylfurfural (5-HMF) and furfural are two typical furan products from cellulosic and hemicellulosic materials, of which the primary components (hexoses and pentoses) undergo selective dehydration. It is worth noting that hemicellulose is rich in not only pentoses but also hexoses, indicating the feasibility of producing 5-HMF and furfural simultaneously. However, research on the production of 5-HMF from hemicellulose-derived hexoses remains lacking. Mannose, a major hexose component of hemicellulose, has been previously found to be a suitable feedstock for 5-HMF production in the presence of homogeneous catalysts, such as metal salts. In this work, Cr-, Al- and Sn/MCM-41 catalysts were prepared by a simple impregnation method and characterized. The conversion of mannose into 5-HMF was evaluated in dimethyl sulfoxide (DMSO) solvent. It was found that the as-synthesized Sn/MCM-41 catalyst showed a superior activity to Cr/MCM-41 and Al/MCM-41. Mannose could be effectively converted into 5-HMF with a yield of ∼45% and ∼88% conversion at 150 °C after 60 min, which were comparable to reported results over heterogeneous catalysts. The as-synthesized Sn/MCM-41 catalyst was also efficient for the conversion of glucose and fructose with reasonable 5-HMF yields. The Sn/MCM-41 catalyst could be reused for eight consecutive cycles without significant loss of catalytic activity.
As robust heterogeneous catalysts, stannosilicate zeolites (in particular Sn-Beta) exhibit excellent catalytic performance in aqueous Baeyer-Villiger oxidation due to their unique activation ability of carbonyl group. For a specific Sn-containing zeolite, pore-engineering and hydrophobicity are generally two coupled factors to impact catalytic behavior. How to distinguish the function of each factor in this selective oxidation is still a pending question. Bearing this in mind, five stannosilicate zeolites (Sn-MFI, Sn-MWW, Sn-Beta, Sn-MCM-41, and Sn-Y) were prepared to establish the relationship between the structure (Lewis acidity, pore-engineering, and hydrophobicity) and catalytic performance. Particularly, hierarchical Sn-Y zeolite was post-synthesized by a simple solid-state ion-exchange method, where Sn 4+ ions were incorporated into zeolite framework via reacting with silanol nests derived from dealumination process. In comparison with other stannosilicate zeolites, the obtained Sn-Y possessed excellent catalytic activity in Baeyer-Villiger oxidation irrespective of using aqueous hydrogen peroxide or bulky tert -butyl hydroperoxide as the oxidant. Structure-performance relationship revealed that Lewis acid site was the catalytically active center and the catalytic activity was depended on zeolite pore-engineering and hydrophobicity in aqueous B-V oxidation system. The relatively high catalytic activity of Sn-Y zeolite was ascribed to its excellent hydrophobicity and opened channel system. The former can be further enhanced by silanization treatment. Finally, the recyclability and deactivation mechanism were given insight. This work is aimed to provide some guidance for elaborately designing efficient stannosilicate zeolite catalyst for Baeyer-Villiger oxidation by precisely modulating pore-engineering and hydrophobicity.
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