Study On Hydrogenation Of Cellulose To Polyols Catalyzed By Zinc-Based Catalyst

With the rapid development of social economy,it is extremely urgent to seek a green and clean renewable resource instead of traditional fossil energy.Cellulose is a natural polymer compound formed by a large number of glucose monomers linked by?-1,4-glycosidic bonds.As the most widely distributed and abundant biomass resource in nature,cellulose has great potential for the production of high value-added chemicals and fuels.Therefore,it is of great significance to design synthetic high-efficiency catalysts to catalyze the hydrogenation of cellulose to prepare low-carbon polyol products such as 1,2-propylene glycol,ethylene glycol and glycerol.The surface of the zinc-based catalyst has more basic sites,which is beneficial to the key step of isomerization in the process of cellulose hydrogenation.In this thesis,ZnO with different morphologies were prepared by one-step calcination method and precipitation method.Then,five kinds of metals supported by Ru,Pt,Pd,Ni and Cu were obtained by sodium borohydride reduction method,and Characterization methods such as XRD,SEM,XPS analyse their physicochemical properties.The prepared catalyst was applied to the hydrogenation of cellulose.The effects of preparation methods,different metal loadings and different Ru loadings on the catalytic activity were investigated.The most cost-effective metal-supported ZnO catalyst was screened.The characterization and experimental results show that the one-step calcination method has higher catalytic activity than the ZnO prepared by the precipitation method.In the M/ZnO?M=Ru,Pt,Pd,Ni,Cu?catalyst,1.0 wt%Ru/ZnO has the highest target polyol yield of28.90%,of which the yields of 1,2-propanediol,ethylene glycol and glycerol are 19.27%,7.92%and 1.71%,respectively;1.0 wt%Ni/ZnO catalyst The polyol yield was second,reaching25.87%,but the cellulose conversion was the highest.Considering that the non-precious metal Ni has a price advantage over the noble metal Ru,Ni has a better application prospect and research value as a supporting metal for the hydrogenation reaction of cellulose.In order to solve the problem that cellulose can not be completely converted when using M/ZnO catalyst,solid acid ZrO₂ was added by coprecipitation method to modify it.Different proportions of ZnO-ZrO₂ composite oxide support were prepared,and Ni/ZnO-ZrO₂ catalysts with different Ni content were obtained by impregnation method.XRD,SEM,TEM,XPS,H₂TPR,NH₃ TPD and other characterization methods were used to analyze the crystal structure,micro-morphology,chemical composition and valence changes of the above catalysts.The catalysts with the best performance were screened through cellulose hydrogenation reaction.The effects of reaction temperature,initial hydrogen pressure and reaction time on the reaction were investigated,and the optimal reaction conditions were obtained.The results showed that the catalytic activity of the modified Ni/ZnO-ZrO₂ catalyst was significantly improved.Among them,20 wt%Ni/ZnO-ZrO₂ had the best catalytic effect.Under the optimum reaction conditions of 245°C,4 MPa initial hydrogen pressure and 3 h reaction time,cellulose was completely converted and the total yield of polyols reached 61.10%,1,2-propylene glycol,ethylene glycol and glycerol reached the highest value,and the rates were 37.57%,19.76%and3.77%respectively.After five cycles,the yield of target polyols remained almost unchanged,which proved that the Ni/ZnO-ZrO₂ catalyst had good recycling value and high temperature stability.In addition,the process of catalytic hydrogenation of cellulose was analyzed by 20 wt%Ni/ZnO-ZrO₂ catalyst.Firstly,cellulose is hydrolyzed to form glucose by the action of H⁺produced by solid acid ZrO₂ in water and obtained by adsorption and dissociation of H₂ from metal Ni;Then,most of the glucose is isomerized to fructose under the action of carrier ZnO-ZrO₂;Finally,the two low-carbon sugars further undergo subsequent reverse aldol condensation reaction and hydrogenation reaction to obtain a target polyol product such as ethylene glycol and 1,2-propylene glycol.