| As the monomeric sugar derived platform molecule,5-hydroxymethylfurfural(HMF)is regarded as an alternative precursor of liquid fuels and fine chemicals.Utilization of glucose or fructose as the starting substrate to synthesize HMF limits its cost competitiveness on an industrial scale,thus using abundant natural cellulose is more attractive for HMF sustainable production.However,the steric hindrance of lignin and the recalcitrant structure of cellulose in lignocellulose impose restrictions on its efficient conversion.Therefore,efforts for achieving value-added conversion of biomass should be focused on the development of green and efficient lignocellulose pretreatment technology for production of highly reactive cellulosic substrates.Aimed to joint with formic acid pretreatment of lignocellulose and catalytic conversion of cellulose for HMF,this thesis illustrated the evolution of three components in lignocellulose during the pretreatment process by formic acid,established the catalytic solvent system for HMF synthesis from formylated cellulose,revealed the relationship between physicochemical properties and catalytic activity of cellulose formate,and elaborated the mechanism of cellulose formylation and solvent media in the catalytic reaction.Specific research contents and results were as follows.1)Study on lignocellulose pretreatment and upgrading by formic acidWith corn stover pretreatment by formic acid,it was found that formic acid pretreatment at high temperature(>100℃)could promote the dissolution of lignin in formic acid solution and the fracture of β-O-4’ bonds,meanwhile degrading hemicellulose and leaving cellulose in solid residue.The concentration of recovered formic acid was close to the original formic acid.After formic acid pretreatment,the S and H units of lignin fraction were effectively removed or modified,leaving a large number of G units.The formylation of cellulose fraction could improve the hydrophobicity and thermal stability,but seriously limited the efficiency of enzymatic hydrolysis.It was found that maleic acid and aluminum chloride could catalyze formylated cellulose for HMF with a yield of 30.9%in acetonitrile-water media,which was better than the HMF yield of α-cellulose or microcrystalline cellulose as reactant under the same condition,demonstrating that cellulose with formylation had high activity and selectivity in polar aprotic organic solvent-water media.This study proposed a green and efficient pretreatment approach for lignocellulose and proved that the formylation of the components occurred during formic acid pretreatment,and provided a new method for the further utilization of formylated cellulose.2)Study on catalytic production of HMF from cellulose formateCellulose formate was prepared by one-step acid hydrolysis and formylation by recyclable formic acid as proton donor and formyl donor,and then HCl-AICl3 co-catalyst were used for direct catalytic conversion of cellulose formate to HMF in DMSO-H2O co-solvent media.The results showed that formic acid destroyed the hydrogen bonds of cellulose molecular chains to improve the structural accessibility of cellulose.The mutual attraction between the introduced formyl groups and the intrinsic alcohol hydroxyl groups in cellulose formate led to hydrogen bond rearrangement,which presented a unique spatial network,thus inhibiting the self-aggregation of cellulose chains.Cellulose formate with abundant reducing-end groups,low crystallinity and rich formyl groups showed excellent catalytic activity and selectivity in DMSO-H2O media with the HMF yield of 51.6%,which was 12 times higher than that generated from cellulose as substrate.This study developed a new process for HMF synthesis from cellulose formate in DMSO-H2O media,which provided a fresh idea for the synthesis of value-added chemicals from cellulose.3)Study on kinetics and mechanism of cellulose formate for HMFKinetic experiments were conducted with cellulose formate,glucose,fructose and HMF as substrates in DMSO-H2O co-solvent media and H2O media,respectively.The results showed that when the concentration of DMSO in co-solvent media was more than 60%,it could accelerate the isomerization of glucose to fructose and the dehydration of fructose to HMF,and the yield of HMF reached the maximum at 80%DMSO concentration.Additionally,DMSO could inhibit the rehydration of HMF to levulinic acid and the polymerization of HMF to humins.Based on the model of C6-formylated glucose molecule,the radial distribution functions of DMSO and H2O around C1,ring oxygen and formyl oxygen atoms in model were evaluated.It was found that the preferential coordination of DMSO and formyl group could promote the solvation of formylated substrate in DMSO-H2O media,and increase the binding strength of H2O molecule and model molecule,thus facilitating the synthesis of HMF.This study revealed the solvent effect of DMSO in catalytic reaction and the mechanism between cellulose formate and DMSO,which provided theoretical guidance for HMF synthesis from cellulose formate in DMSO-H2O media. |
