Highly Efficient One-pot Two-step Synthesis Of 2,5-furandicarboxylic Acid Based On In-situ Oxidation Of 5-Hydroxymethylfufural Intermediate

2,5-furandicarboxylic acid(FDCA)is a bio-based monomer for the synthesis of bio-based high performance polyester,polyamide and epoxy resin.It has broad market prospects,but the synthesis of FDCA is still in the laboratory research stage.In order to avoid the problem of separation and purification of unstable intermediate HMF,we designs an "one-pot two-step method" in this paper to synthesize FDCA directly from fructose.In the first step,fructose is dehydrated to HMF using dimethyl sulfoxide(DMSO)as solvent.After separating the catalyst used in the first step,alkali,H2O,DMSO and oxidation catalyst were directly added to the reaction solution to construct H2O/DMSO system for HMF oxidation reaction.After catalyst separation,concentration,acidification and purification,FDCA product were obtained.The experiment on screening catalyst and solvent demonstrate that Amberlyst-15 and DMSO are high-efficiency catalyst and solvent respectively for the dehydration step.For the reaction system,too high temperature or fructose concentration results in serious side reaction or discoloration and decrease of HMF yield.When 5 wt%fructose dehydrated for 70 min under 120?,the HMF yield could reach 97%.When Pt/C and alkali were added into the in-situ formed HMF solution,the oxidation reaction could not occurred effectively.So it was difficult to prepare FDCA by one-pot method using DMSO as a single solvent.However,by directly adding H2O,DMSO,alkali and Pt/C catalysts to the in-situ formed HMF solution to construct an H2O/DMSO oxidation system,HMF could be efficiently oxidized to FDCA.Under the conditions of initial concentration of HMF 0.63 wt%,H2O/DMSO(3/1 w/w),K2CO3/HMF 2/1 mol/mol and temperature 75?,the highest FDCA liquid phase yield could reach 91.1%at 12 h.The total liquid phase yield of FDCA directly synthesized from fructose by "one-pot two-step method" was as high as 88.4%.In this reaction system,HMF is rapidly converted to HMFCA,which is rapidly converted to FFCA at the same time,and the rate at which FFCA is converted to FDCA is slow,which is a rate-limiting step.The presence of H2O facilitates the conversion of HMF to FDCA.DMSO inhibits the formation of by-products of HMF,and also indirectly promotes the formation of by-products of FDCA or intermediates,while the base facilitates the conversion of aldehyde groups of HMF to carboxyl groups.The oxidation product was separated by filtration,concentration,acidification and purification to obtain crude FDCA product,and the isolated yield is related to the oxidation reaction temperature and the initial concentration of HMF.Increasing the oxidation temperature and HMF concentration both led to more by-products,which was detrimental to FDCA separation,resulting in a decrease in separation yield.When the initial concentration of HMF was 0.63 wt%-2.52 wt%,the isolated yield of crude FDCA synthesized at 75? under the dosage of 20 g solvent was 55%-74%,while in the"amplification" experiment using 200 g solvent,the isolation yield of crude FDCA was up to 82%.After recrystallization,the pale yellow FDCA with purity of 98.2%was obtained."One-pot two-step method" has high yield(88.4%of FDCA total HPLC yield,82%of isolated yield),mild conditions(120 C for dehydration step,75?/0.1 MPa for oxidation step),and relatively short reaction time(total duration 13 h).The method could realize one-pot two-step continuous synthesis of FDCA by designing a simple solvent system,without the need to develop a new complex reactor or multifunctional catalyst,avoiding the separation and purification of unstable intermediate HMF.It is a simple and efficient FDCA synthesis technology,but the concentration of reactants and the endurance of the oxidation catalyst still needs to be improved.