Catalytic Conversion Of Xylose To Polyols

Due to rapid decreasing fossil resources and the ever-increasing demand for energy,it is necessary to explore new energy,especially renewable energy,to replace the current fossil resources.As a renewable resource,lignocellulose has great potential because of its abundant,available and inedibility.Xylose is the main component of hemicellulose in lignocellulose.The structure of its polyhydroxyl structure enables it to undergo hydrogenation,hydrogenolysis,oxidation,dehydration and other reactions.However,on the one hand,its active chemical properties make it easy to transform into a variety of chemicals,on the other hand,it poses a challenge to high-selectively convert it into the desired products.Therefore,how to design an efficient and robust catalyst and selectively crack C-C and C-O bonds for the production of high-value xylitol/diols.In this paper,three supported metal catalysts were designed with xylose as the substrate to convert xylose into valuable products.The structure-activity relationship of the catalyst was by correlation of characterization results with catalytic performances.The main content is described as follow:（1）Nitrogen-doped activated carbon（NC）was prepared with sawdust,NH₃ and K₂CO₃ as carbon,nitrogen source and activator,respectively.Ru/NC catalyst was prepared by wet impregnation method.The effects of pore structure and surface structure of support carbon on the hydrogenation performance of xylose were investigated.The results showed that the specific surface area,the type and distribution of nitrogen-containing species have an important influence on the valence state and dispersion state of Ru nanoparticles,thus affecting its xylose hydrogenation performance.Ru/NC-800-1.5 catalyst had the highest xylose hydrogenation activity.The conversion of xylose was 71.4%and the yield of xylitol is 71.2%at 120℃for 0.5 h.Extending the reaction time to 2 h can achieve 100%xylose conversion and 98.5%xylitol yield.（2）Cu-Re/AC catalysts were prepared by wet impregnation method.The effects of different Cu/Re ratio,catalyst loading and reduction temperature on the surface structure of the catalyst and the catalytic performance of xylose hydrogenolysis were investaged.The results showed that the addition of Re promoted the catalysts’ability to selectively cleavage the C-C and C-O bond of xylose,so as to improve the yield of diols.The 20%Cu-Re（5:1）/AC-450 catalyst showed the best xylose hydrogenolysis activity,and the total yield of ethylene glycol and 1,2-propanediol was the17.7%.The effects of different promoters on xylose hydrogenolysis were also investigated.The results showed that CeO₂ could promote xylose hydrogenolysis,and the total yields of ethylene glycol and 1,2-propanediol were increased to 25.8%.（3）Ni-W/CeO₂ catalysts were prepared by incipient wetness impregnation method.The effects of Ni/W ratio,support type and catalyst loading on the xylose hydrogenolysis activity were investigated.The results showed that W species promoted the retro-aldol condensation of xylose,improved the ability to selectively cleavage C-C and C-O bonds,and improved the yield of diols.The 5%Ni-W（10:6）/CeO₂-A catalyst showed the highest activity with the yields of ethylene glycol and 1,2-propylene glycol of 14.6%and 32.6%,respectively,at 180℃and 4 MPa H₂ for 2 h.When the substrate is xylan,the yields of ethylene glycol and 1,2-propanediol can reach 17.4%and 41.3%,respectively.