| Biomass is the only renewable resource with fixed carbon on the Earth.Therefore,efficient use of biomass is of great significance in alleviating the energy and resource crisis and ensuring the implementation of the sustainable development strategies.In general,biomass contains three main components as cellulose,hemicellulose and lignin.However,current techniques for biomass utilization intensively focus on the conversion of cellulose to the versatile chemicals through three consecutive processes of cellulose hydrolysis,glucose isomerization and fructose dehydration.Amongst which,glucose isomerization is confirmed to be the rate-determining step.Enzymatically catalysis is a potential approach for the efficient isomerization of glucose to fructose,but this strategy is limited by the high cost of enzyme and the rigorously controlled reaction condition.Chemical isomerization using traditional Lewis acidic and base is another choice.However,the harsh condition,poisonous catalysts and serious side-reactions are inevitable.Therefore,novel strategies using eco-friendly ionic liquids would be fascinating.In this study,we proposed a new strategy for the aqueous isomerization of glucose into fructose in the presence of guanidine ionic liquids.The effects of ionic liquid catalysts and reaction conditions were extensively investigated and the plausible catalytic mechanism was also proposed.Details are listed as follows.(1)A series of novel ionic liquid catalysts,which are comprised of tetramethylguanidine cation coordinated by carboxylate,lactate,formate,and amino acid anions(proline,lysine arginine,etc)were synthesized and characterized by using TG,~1H-NMR,FT-IR and ESI-MS.The base strengths of these ionic liquids were also investigated by the measurement of p H value and the method of potentiometric titration.Subsequently,the catalytic activities of ionic liquids were studied in the aqueous isomerization of glucose to fructose.The results demonstrated that guanidine ionic liquids involved amino acidic anions were highly efficient for the isomerization of glucose to fructose.The fructose yield reached 34.1%when the reaction was carried out under the optimized conditions(the concentration of[TMG]Pro was 0.02 mol/L;the dosage of glucose was 1 mmol/10 m L,the reaction time was 30 min,and the reaction temperature was70℃.The kinetic analysis showed that this process confirms the first-order reaction kinetics,and the apparent activation energy is 57.05±1.66 k J/mol,which is lower than that of many current base-catalyzed glucose isomerization systems.This low active energy explains well the high activity of this ionic liquid[TMG]Pro on the isomerization of glucose.The catalytic mechanism was also proposed based the e isotope labeling study.It was found that,in the guanidino ionic liquid[TMG]Pro catalyzed glucose isomerization,the C-2 position of glucose undergoes a process of deprotonation to form 1,2-enediol intermediate,and then,fructose as the final product after capturing a proton from the solvent of water.(2)Homogeneous ionic liquids show better activity in catalyzing glucose isomerization in the aqueous phase than many current techniques using traditional catalysts,but the separation and recovery of the ionic liquid is still a challenge.In this respect,heterogeneous catalysts would be more attractive.Therefore,we further investigated the catalytic isomerization of glucose in the presence of ionic liquids supported on the metal-organic framework of Ui O-66.~1H-NMR,FT-IR,TG,XRD and BET were used to characterize the structure and properties of the supported ionic liquids.The influences of reaction conditions on the isomerization performance of glucose were also investigated.The results demonstrated that 18.5%of the fructose yield can be obtained at the optimized condition.More importantly,this supported ionic liquid showed excellent recyclability,where insignificant loss of catalytic activity was shown even after 30 consecutive runs,indicating its great potential in industrial application. |
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