Study On The Direct Hydrolysis Of Cellulose To Glucose By Metal-organic Frameworks Supported Phosphotungstic Acid

Cellulose is an inexhaustible natural renewable resource.The treatment of cellulose waste mainly produces heat through direct combustion or uses ash as fertilizer,which causes great waste of resources and serious pollution to the ecological environment.The utilization of cellulose resources has attracted extensive attention of researchers.The hydrolysis of cellulose to glucose is the necessary process of its conversion and utilization.Therefore,the key to efficient utilization of cellulose is to realize the directional hydrolysis of cellulose to glucose.At present,the hydrolysis products of cellulose are widely distributed in the research of cellulose hydrolysis,and the directional hydrolysis of cellulose to glucose can not be realized.Designing a catalyst which can catalyze the directional hydrolysis of cellulose and keep acid catalytic activity is of great significance for the realization of cellulose resource utilization.The key to realize the directional hydrolysis of cellulose lies in the selective adsorption of cellulose by catalysts and the reduction of the contact between the catalytic center and the hydrolysate glucose.In this paper,MIL-101 was used to immobilize PTA,which effectively solved the problem of difficult recycling of catalysts.However,the pore structure of MOFs was difficult to make the cellulose with macromolecular structure enter the catalyst and contact with phosphotungstic acid.Which result in poor catalytic performance.Therefore,in this study,the MIL-101 structure was functionalized by grafting different electronegative groups to achieve directional adsorption of cellulose and increase the contact frequency between cellulose and PTA.By analyzing the effects of different electronegative groups on the structure,acid activity,pore size distribution and thermal stability of the catalyst,it was preferred that the electronegative group-NH₂,-Cl simultaneously functionalized the MIL-101structure to synthesize the catalyst PTA@MIL-101-NH₂-Cl.The bonding form of the electrostatic interaction between-NH₂ and PTA,and the mechanism of the strong electro-negative group-Cl in catalyzing the directed hydrolysis of cellulose were investigated.A new method for improving the water stability of MIL-101?Cr?by incorporating Ni???ions into the framework by pre-synthesis metal ion doping was proposed for the poor water stability of MOFs.The crystal structure,surface morphology and pore structure of MIL-101structure before and after nickel ion doping were analyzed.and the water stability of the material in different pH aqueous solutions was explored.Based on this,the catalyst PTA@Ni/MIL-101-NH₂-Cl was synthesized and applied to the targeted hydrolysis of cellulose.The research results were as follows:Grafting of different electronegative groups did not have a significant effect on the crystal structure of MIL-101.As the electronegativity of the grafting group increased,the specific surface area and pore volume of the material tend to decrease.The larger pore size distribution of all samples ranged from 1.65 to 2.31 nm,which with the risk of PTA being lost through the MIL-101 channel.The grafted electronegative group can form a hydrogen bond with the abundant-OH in cellulose to achieve specific adsorption of cellulose.In the simulated adsorption test,as the electronegativity of the grafting group increased,the cellobiose and The adsorption performance of glucose increased gradually,and it showed selective adsorption of cellobiose.The grafting of-NO₂ achieved the highest adsorption effect on glucose and cellobiose,respectively,reaching 52%and 63%,respectively,and got the highest glucose yield reached 32.4%,but-NO₂ was easily reduced to-NH2 during the reaction.Ni???-doped Ni/MIL-101?Cr?material retains the original structure of MIL-101.The doped Ni element exists in the sample in both ionic and reduced states,respectively of 80.1%and 19.9%.Ni???and Cr??I?together with organic ligands form a highly stable Ni^?Cr^?ICUS secondary building unit.At the same time,the introduction of Ni???causes the organic ligand to incompletely coordinate,resulting in a hydrophobic aromatic wall caused by the uncoordinated complete organic ligand in the pores of the skeleton.The hydrophobic aromatic wall repeled water molecules in the pores,preventing attack and skeleton dissolution of the Cr^?ICUS unit by water molecules.The relative crystallinity of Ni???-doped samples after seven days of reaction under different pH solutions can be maintained above 0.77.Under acidic and central conditions,the relative crystallinity of samples doped with Ni ions can reach above 0.8,much higher than the undoped Ni???sample.The catalyst PTA@Ni/MIL-101-NH₂-Cl was synthesized by Ni-doped synchronously supported PTA.The yield of glucose obtained by Ni???-doped catalyst is 41%-56%of that obtained by PTA with the same loading.