Preparation Of Acid-Base Dual-Functional Solid Catalysts For Conversion Of Glucose To 5-HMF

5-Hydroxymethylfurfural?5-HMF?is an important platform chemical,which can be further converted to a varity of valuable chemicals.Using cellulose as raw material is the most promising route to produce 5-HMF and other value-added chemicals.Mechanistically,the isomerization reaction of glucose to fructose is an extremely important step which has significant effect on the conversion efficiency of cellulose to 5-HMF as well as the furtherly derived compounds.In the process of conversion of glucose to 5-HMF,glucose firstly undergoes isomerization to fructose,and then subjects to dehydration reaction with eliminating three molecules of water.It is believed that the most effective catalysts for glucose isomerization to fructose are Lewis bases,while Bronsted acids are effective catalysts for the dehydration reaction of fructose to 5-HMF.Currently,fructose is mainly produced via isomerization of glucose,but the separation and purification of products are still cost-ineffective.In this sense,the isomerization and hydration reactions are expected to be conducted in one reactor so as to avoid the separation of fructose.Evidently,the single category of acidic active sites does not meet the reqirement to obtain high5-HMF selectivity.As such,it is very necessary to construct an efficient catalyst system with both acid/base active sites.In this paper,hydrotalcites and ion exchange resin were examined systematically for converting glucose to 5-HMF.Using Mg?NO₃?₂ and Al?NO₃?₃ as precursors,a series of mesoporous layered hydrotalcites?LDH?were prepared by co-precipitation method.The effects of the Mg/Al molar ratio and the calcination temperature on the microscopic structure and basicity over the hydrotalcites surface as well as on the catalytic performance of glucose-to-fructose reaction were investigated.Characterizations by SEM,TEM,XRD,and BET showed that the hydrotalcite materials with different Mg/Al molar ratios had similar mesoporous layered pore structures,but the calcination temperature had significant influence on the microstructure of hydrotalcites.High Mg/Al molar ratio and high calcination temperature can enhance the surface basicity of the hydrotalcites,but overly strong basicity is not conducive to the improvement of fructose selectivity.Mg₂Al₁-450 with Mg/Al=2?molar ratio?and calcined at450^oC exhibited the highest catalytic activity.Under the catalysis of Mg₂Al₁-450,as high as 30.00%of fructose yield with 92.10%of selectivity was obtained after reacting at 100o C for 1.0 h in aqueous solution.Study on the reaction mechanism found that the isomerization of glucose to fructose is an reversible reaction,both the forward and reverse reactions are quasi-first-order reactions with activation energies of 71.82 k J/mol and 48.15 k J/mol,respectively and humins as the byproducts can be formed either from glucose or fructose,which is a zero-order reaction with an activation energy of 94.22 k J/mol.The surface of the hydrotalcites was modified by means of ion exchange effect,and the catalytic activity for converting glucose to 5-HMF was further investigated.The structural characterizations showed that different types of hydrotalcites had different structures and basicity.Using the mixture of hydrotalcite with an acidic resin?D80A?,the influence of reaction media and reaction temperature were surveyed.The experimental results showed that the catalytic effect in DMSO is significantly better than that in water.The Ni-Al hydrotalcite catalyst?Ni₂Al₁-450?having relative weak basicity showed the best catalytic activity when combined with the ion exchange resin as a catailst.After reacting at 130°C for 2 h using DMSO as the solvent,the glucose conversion was 46.36%,and the selectivity of 5-HMF was up to 54.99%.It is manifested by four-run recycling that the catalyst is with the good catalytic activity and stability.