Study On The Catalytic Oxidation Of Biomass-derived 5-Hydroxymethylfurfural Into 2,5-Furandicarboxylic Acid And Furan-2,5-dimethylcarboxylate

As an important renewable carbon resource on earth,biomass can be used to produce many renewable chemicals,materials and fuels.These products can be considered as supplements or substitutes for fossil-derived ones to reduce the excessive use and dependence of human society on fossil fuels.5-Hydroxymethylfurfural(HMF),one of the most valuable biomass-based platform molecules,can be directly obtained from cellulose,a main component of lignocellulosic biomass,through acid-catalyzed reactions.The selective oxidation of HMF affords the formation of 2,5furandicarboxylic acid(FDCA)and 2,5-furan-dimethylcarboxylate(FDMC),both of which are ascertained as promising monomers for the production of high-performance renewable polymers.However,the previously reported catalytic systems for the preparation of FDCA or FDMC suffer from many problems,such as low substrate concentration and low catalytic efficiency.The problems of these catalytic systems seriously impede their large-scale application in industry.The key to solving the problem of poor oxidation selectivity in high concentrations of HMF is to improve the stability of HMF under alkaline conditions.Based on the physical and chemical properties of HMF,DMSO/H2O mixture was demonstrated as an effective medium for the production of FDCA from high concentration of HMF or fructose.For example,FDCA yield up to 93%was achieved at 10 wt%HMF concentration in DMSO/H2O over commercially available Ru/C(2 equiv.NaHCO3,130 ℃,4 MPa O2 and 12 h).The introduction of DMSO can shelter HMF from the harmful side reactions by solvation effect and reduce the solubility of base additive,which greatly contribute to improve the stability of HMF in DMSO/H2O mixture.Furthermore,an overall FDCA yield of 65%was achieved from concentrated fructose(19 wt%)through a facile two-step process.According to the mechanisms of HMF oxidation reaction,several highly effective catalysts were developed for the efficient production of FDCA or FDMC.Firstly,to improve the low catalytic efficiency of the liquid stoichiometric oxidant catalytic systems,an innovative NiOx catalyst was prepared by facile precipitation-oxidation method,which can afford high FDCA yields(94-97%)from HMF in only 30 min at 25℃ by using budget NaClO as an oxidant.Remarkably,an impressive FDCA formation rate of 404 μmol·g-1·min-1(9-81 folds higher than those of recently reported catalytic systems for FDCA production)could be achieved over the NiOx catalyst at 25℃.During the catalytic oxidation process,-Ni4+-O-sites over the NiOx catalyst are responsible for the efficient oxidation of HMF to FDCA via HMFCA and FFCA intermediates by using NaClO as oxidant.Moreover,when employing crude HMF,which was prepared from carbohydrates by acid hydrolysis,as raw material,excellent FDCA yields could also be achieved in NiOx/NaClO system,indicating the good tolerance of NiOx/NaClO system to the by-products in curde HMF,such as humins.Subsequently,in view of the low catalytic efficiency of non-precious Mn-based catalysts for the production of FDCA,we presented a Vitamin C(VC)-assisted solidstate grinding method for synthesizing mesoporous Mn-Co binary oxides with abundant oxygen vacancy(Ov),which could offer a satisfactory FDCA yield of 96%using air as the oxygen source(130℃,1.5 MPa air,3 h).Remarkably,Mn3Co2Ox-0.3VC offered an outstanding FDCA formation rate of 2611μmo1FDCA·gcat-1·h-1,which is 2.7-24.9 folds higher than those of recently reported Mn-based catalytic systems for FDCA production.Based on experimental studies,the catalytic performance of Mn-Co oxides for the oxidation of HMF corresponds well with their Mn-O bond intensities.The catalyst with higher Ov concentration exhibits weaker Mn-O bond intensity,which brings about a higher lattice oxygen(OL)reactivity.More importantly,Density functional theory(DFT)calculations also demonstrate that increasing the Ov amount not only boosts the OL reactivity of the catalyst by reducing the formation energy of Ov but also contributes to the adsorption and activation of O2 over the catalyst by significantly cutting down the O2 adsorption energy,thus leading to an enhanced catalytic activity for the oxidation of HMF.Besides,the catalyst with higher Ov concentration provides stronger substrate adsorption ability,which may also promote the HMF oxidation reactions.The recycling tests demonstrate the good reusability of the Mn3Co2Ox-0.3VC catalyst.Finally,in consideration of the low catalytic efficiency of the non-precious Cobased catalyst for the oxidative esterification of HMF,we reported an elaborately designed N-doped carbon-supported CoCu bimetallic catalyst(CoxCuy-NC),which could offer a desirable FDMC yield of 95%under mild and base-free conditions(Co7Cu3-NC,0.2 MPa O2,80℃,4 h).Notably,an FDMC formation rate of 6.1 molFDMC·molCo-1·h-1 was achieved over Co7Cu3-NC,which is 1.5-4.7 folds higher than those of recently reported Co-based catalytic systems for FDMC production.It has been demonstrated that Cu-doping in Co7Cu3-NC catalyst brings about more active sites(CoNx species)with stronger molecular oxygen activation ability.The increase of surface N content of Co7Cu3-NC also improves the basicity of the catalyst,which favors the hydrogen abstraction process during the HMF oxidative esterification reaction.