Degradation Of Cellulose Catalyzed By Rhenium(Molybdenum) Containing Ionic Liquids

Cellulose,being as one of the most renewable energy,is a promising feedstock for the production of biofuels and value-added chemicals.Traditionally,acid hydrolysis and enzymatic hydrolysis are used for the degradation of cellulose.However,there are problems such as equipment corrosion,environmental pollution,catalytic instability of enzyme and so on in the process of cellulose degradation.Therefore,it is necessary to develope a green and efficient process for the hydrolysis of cellulose.Rhenium?molybdenum?containing ionic liquids have the advantages of easy preparation,good thermal and chemical stability,and environmental friendliness,which makes them be widely used in epoxidation of olefins and dihydroxylations.Thus,it should be of great significance to degrade the cellulose effectively using rhenium?molybdenum?containing ionic liquid as catalyst.Perrhenate and molybdenum polyoxometalates ionic liquids were designed synthesized and used as catalyst to degrade microcrystalline cellulose?MCC?in this paper.10 kinds of perrhenate and 8 kinds of polyoxomolybdate based ionic liquids were synthesized.The structure of these compounds were characterized by mass spectrometry?ESI-MS?,infrared?FT-IR?and nuclear magnetic resonance?NMR?.The physical and chemical properties such as melting point,solubility and thermal stability were tested,and the results showed that the catalysts have good thermal stability.MCC was dissolved in ionic liquids,and the structure and properties of original and regenerated cellulose were analyzed by FT-IR,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and thermal gravimetric analysis?TGA?.The results indicated that the cellulose I?original cellulose?was changed to cellulose II?regenerated cellulose?and crystallinity of cellulose decreased.In the meantime,the decomposition temperature of cellulose droped after the treatment with ionic liquid,and it is in favor of degradation of cellulose.MCC was hydrolyzed to produce the total reducing sugar?TRS?in ionic liquids using rhenium?molybdenum?containing ionic liquids as catalyst under microwave irradiation conditions.The results indicated that the perrhenates and polyoxomolybdates synthesized in this paper display good catalytic performance for cellulose hydrolysis,and the catalytic activity of perrhenates is better than that of polyoxomolybdates in general.While[?CH₃?₄N]ReO₄ is the best catalyst among the 10 kinds of perrhenates,where 97.1%of TRS and 42.0%of glucose yields can be obtained with adding 5 mol%of catalyst and 70?L water when 0.1 g MCC was hydrolyzed at 150?for 30 min in 1-allyl-3-methylimidazolium chloride?[Amim]Cl?ionic liquid.Among the 8 kinds of polyoxomolybdates,[?CH₃?₄N]₄Mo₈O₂₆6 is the best catalyst where 75.3%of TRS and 38.4%of glucose yields can be obtained when 0.1 g MCC was hydrolyzed at 160?for 30 min with 1 mol%of catalyst[?CH₃?₄N]₄Mo₈O₂₆6 as catalyst.Different Lewis acid catalysts,CrCl₃·6H₂O,CrCl₂·6H₂O,FeCl₃·6H₂O,CuCl₂,CuCl and composite metal chlorides were used as catalyst to degrade the cellulose to give HMF.The results indicated the composite metal chlorides CuCl₂/CrCl₃ display good catalytic performance.Perrhenate or polyoxomolybdate was combined with CuCl₂/CrCl₃ respectively and used as catalysts in process of cellulose degradation.The results showed that the composite catalyst composed with rhenium?molybdenum?containing ionic liquid and CuCl₂/CrCl₃ can be more effective than CuCl₂/CrCl₃ for conversion of cellulose to HMF,and the yield of HMF was improved in a large extent.0.1 g MCC was degradated using 5 mol%of[?CH₃?₄N]ReO₄ and 4 mol%of CrCl₃ and CuCl₂ as catalyst,The maxium yields of TRS and HMF?79.6%and 58.8%?can be obtained under microwave irradiation for 30min at150?with the addition of 50?L water.When 1 mol%of[?CH₃?₄N]₄Mo₈O₂₆6 was combined with 4 mol%of CrCl₃ and CuCl₂ and used to catalyze the degradation of cellulose under microwave irradiation for 20 min at 160?with adding 50?L of water,the highest yields of TRS and HMF obtained were 78.5%and 55.1%respectively.Products were extracted with ethyl acetate,and the extracted mixture was separated by silica gel column chromatography.The final product,HMF was characterized by ¹H NMR and ¹³C NMR.The reaction mechanism of cellulose hydrolysis catalyzed with perrhenates was discussed.It is assumed that the interaction of Re=O with hydroxyl of cellulose molecule is the key of cellulose hydrolysis.The reaction mechanism of glucose conversion to HMF was also discussed.With the catalysis of process of CuCl₂/CrCl₃,?-glucose converted to?-glucose firstly,and then converted to HMF after ring cleavage,H proton transfer and dehydration.Based on the cellulose degradation data,the kinetics of the degradation of cellulose to HMF was analyzed,and the kinetic equation was obtained by calculating the reaction rate constant,activation energy and pre-exponential factor with MATLAB.The results indicated that the reaction rate of cellulose degradation to HMF at all levels increased with the increase of reaction temperature.The reaction of cellulose degradation to glucose is slow reaction,and determes the overall reaction rate.