| 5-Hydroxymethylfurfural(HMF) is an important bio-based platform chemical. HMF has a primary hydroxy and aldehyde groups, thus offering the molecule highly active. Therefore, a series of valuable products such as 2,5-diformylfuran(DFF), 5-hydroxymethyl-2-furancarboxylic acid(HMFCA), 5-formyl-2-furancarboxylic acid(FFCA), 2,5-furandicarboxylic acid(FDCA), and HMF esters can be prepared from HMF. These bio-based chemicals can be used in pharmaceuticals, furan-based polyesters, spices, disesel additives, etc. However, these products are presently synthesized by chemical methods using stoichiometric oxidants and metal catalysts, which are environmentally unfriendly. Biocatalysis has attracted growing interest in the production of added value chemicals and pharmaceuticals, due to a lot of advantages such as mild reaction conditions, being efficient and highly selective. To date, there are limited reports on biocatalytic valorization of HMF. Therefore, the feasibility of production of DFF, HMFCA, FFCA and FDCA via oxidase-catalyzed selective oxidation of HMF was explored. In addition, the separation of the mixture of HMF and DFF using deep eutectic solvents was conducted. Then, the effects of various factors on GO-mediated selective oxidation of HMF to DFF were studied in detail. Finally, CAL-B-catalyzed esterification between HMF and levulinic acid(LA) in 2-methyl tetrahydrofuran(2-MeTHF) was studied.Alcohol oxidases(AOs) could catalyze oxidation of HMF to DFF. Of three AOs tested, AO from Candida boidinii was proved to be the optimal; DFF was obtained as the only product with a yield of 41% after 72 h at 25 ℃. Likewise, galactose oxidase(GO) was a good biocatalyst for the synthesis of DFF from HMF in the presence of catalase and horseradish peroxidase(HRP). In addition, GO was the best among the 4 oxidases that are able to transformate HMF to DFF. The optimal reaction medium, GO dosage, bubbing times, HMF concentration, HRP dosage, catalase dosage and temperature for GO-catalyzed HMF oxidation were deionized water, 4 U/m L, no bubbing, 30 mM, 0.13 mg/m L, 5 μg/m L and 25 ℃, respectively. At the optimal conditions, DFF yield of 81% was obtained with the selectivity of >99%. XO-mediated oxdiation of the aldehyde group on HMF proceeded quickly, affording HMFCA. HMFCA was obtained with 94% yield and >99% selectivity after 7 h at 37 ℃. However, XO could not almost accept DFF as the substrate. In HMF oxidation catalyzed by laccase-TEMPO system, FFCA was the main product. Of the three laccase-TEMPO systems tested, Panus conchatus laccase-TEMPO gave the best results; FFCA, FDCA and DFF yields were 82%, 10% and 4%, respectively after 96 h. In addition, FDCA was successfully synthesized with a total yield of 66% from HMF via GO- and CAL-B-catalzyed cascade oxidation.Among three deep eutectic solvents(DES), both choline chloride:glycerol(ChCl:Gly, 1:2, mol/mol) and choline chloride:urea(ChCl:Ur, 1:2, mol/mol) were found to be able to selectively extract HMF from the mixture of DFF and HMF, and DFF purity was improved from the original purity of 76% to 88-93% upon DES extraction. The DFF purity was increased to 97% after 3 times extraction using the equal volume of ChCl:Gly.Among six immobilized enzymes examined, CAL-B was found to be the best for the esterification between HMF and LA. The optimal reaction medium, CAL-B dosage, HMF concentration and reaction temperature for this enzymatic reaction were 2-MeTHF/LA(1:1, vol/vol), 10 mg/mL, 200 mM and 40 ℃, respectively. HMF conversion of 89% was obtained after the reaction of 12 h at the optimal conditions.The enzyme performances have been uncovered in the enzymatic oxidation and esterification of HMF in this work, whch will enrich the theoretical foundation for valorization of bio-based chemicals. In addition, new enzymatic approaches to synthesize valuable HMF oxidation derivatives and its levilunate have been established, thus laying technical foundation for the large-scale production of these compounds. |
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