Synthesis Of Macroporous Metal-Organic Frameworks With Immobilized Phosphotungstic Acid And Catalytic Hydrolysis Of Cellulose

Cellulose is a renewable raw material that can be converted into glucose by catalytic hydrolysis,and then glucose is further transformed into fructose,5-hydroxymethyl furfural and other important energy materials.The conversion of the cellulose into glucose is the key to achieve cellulose utilization.Phosphotungstic acid（PTA）is a new type of cellulose catalyst in recent years as a solid acid with the highest acid strength and electronegativity of outer oxygen atoms.PTA is completely soluble in water which needs to be extracted and recovered with ether after reaction.However,PTA loss is severe.Therefore,in this study,metal-organic frameworks（MOFs）take convenience of functioning which graft organic ligands containing basic groups and encapsule PTA into the pore structure because of the electrostatic interaction between the basic group and PTA.However,the pore size of MOFs is difficult to make cellulose enter the interior pore,hindering the catalytic activity of PTA,so choose three reaming methods to regulate the MOFs pore structure,surfactant template method,extended organic ligand method and regulator induced defect method,so that the MOFs can make full contact with PTA,exerting the acid catalytic effect.Functionalizing organic ligands with-NH₂ receives the loading of PTA which obtained by electrostatic between-NH₂ and PTA.Improving the amount of organic ligand2-aminoterephthalic acid（TA-NH₂）can increases the electrostatic interaction between-NH₂and PTA,and sample still maintains a complete Keggin structure.Grafting-NH₂ will occupy part of the pores,so the specific surface area and pore volume of MOFs will gradually decrease.When the TA-NH₂ increase to 5 mmol,the electrostatic interaction between-NH₂and PTA increases to be strong enough,although the pore size of the sample is reduced,it can still attract more PTA.The PTA loading of the Cr-MOF and Fe-MOF samples were up to 12.8wt%and 19.5 wt%.Surfactant template method forms micelles in water by adding CTAB,the volume of micelles increases with the increase of CTAB addition amount.The template reagents can maximize the pore diameter of Cr-MOF to 46.1 nm,and the Fe-MOF to 78.2 nm.However,CTAB can destroy the crystal surface growth of the MOFs as well as the crystal structure of Cr-MOF.Under the combined effect of static electricity between CTAB and PTA and electrostatic interaction between PTA and-NH₂,the loading capacity of PTA increases first,then decreases with the addition of CTAB.As the amount increases,the PTA loading of Cr-MOF and Fe-MOF can reach 14.8 wt%and 23.9 wt%,respectively.The helper template TMB can enter the interior of the CTAB micelles to further increase the micelle diameter.The synergistic effect of CTAB and TMB can significantly increase the largest pore diameter of Fe-MOF to 72.2 nm.Also the TMB addition influences the crystal surface growth of the MOFs carrier.But the solid loading of PTA increases with the amount of TMB addition,the loading PTA of Fe-MOF obtained maximum capacity reaches 26.2 wt%.The length of the organic ligand directly determines the size of the polymer pore size,and the pore size of the synthetic MOFs carrier enlarges with the increase of the long chain4,4’-biphthalic acid.The pore sizes of Cr-MOF and Fe-MOF reached 101.8 nm and 115.3 nm,respectively.The MOFs carriers all have the crystal structure of MIL-101.4,4’-diphenic acid will promote the growth of the MOFs 911 crystal plane.However,when the amount of4,4’-biphthalic acid is 4 mmol,the Cr-MOF pores occur throughout and collapse and the crystal structure was destroyed.The PTA loading of Cr-MOF and Fe-MOF increased with the increase of 4,4’-biphthalic acid addition,reaching a maximum of 21.2 wt%and 24.5 wt%respectively.Inducer-induced defect method is to add the organic ligand incompletely replace the previously added dodecanoic acid and the metal ion coordination.MOFs were activated by concentrated hydrochloric acid and DMF to remove the dodecanoic acid,forming defects and achieving the purpose of reaming.The largest pore diameter of Fe-MOF reaches 123.2 nm.The method of Cr-MOF hole expansion will cause the disappearance of crystals.Increasing the amount of TA-Cl promotes the growth of 911 crystal surface of Fe-MOF.The PTA loading capacity decreased with the increase of TA-Cl addition,which influenced by the pore size of MOFs and the electrostatic interaction between-NH₂ and PTA.And the maximum PTA loading is 14.8 wt%.Catalytic hydrolysis of microcrystalline cellulose by Cr-MOF and Fe-MOF catalysts which were reamed by surfactant template method,extended organic ligand method and modifier-induced defect method with equal amout.The larger the pore size of the MOFs carrier,the more microcrystalline cellulose that can enter the interior of the carrier and in full contact with the PTA,the stronger the acidity,the more acid catalytically active centers,and the better the catalytic effect of hydrolyzing the microcrystalline cellulose.After the pore-enlarging of Cr-MOF by surfactant template method,extended organic ligand method and regulator-induced defect method,the maximum glucose yield by Cr-MOF sample hydrolysising microcrystalline cellulose were 26.8%and 34.8%,respectively.The maximum glucose yields obtained by Fe-MOF sample were 40.4%,34.4%and 22.7%,respectively.Although the largest pore size of the samples obtained by the modifier-induced defect method is the largest among the three methods,the maximum pore size is only 4%of the total pore size,and the glucose yields had not significant increase,witch indicates that uniform channels are more favorable for catalytic reaction.Though microcrystalline cellulose hydrolyzed by three methods in three times,the drop of glucose yield was less than 6%,indicating that the PTA obtained favorable immobization stability.