Preparation Of Tungsten-niobium-zirconium Composite And Its Application To Catalytic Hydrogenation Of Cellulose

With the rapid development of world economy and technology,human society is facing a double crisis of resource shortage and environmental pollution.The excessive use of non-renewable fossil energy will also cause serious environmental pollution,so it is necessary to develop renewable green resources.Biomass energy is considered as an important substitute for fossil energy because of its renewability,wide source,abundant reserves and cleanliness.Cellulose is the most abundant biomass component in nature?40%⁶0%?,which can be converted into bio-chemicals and fuels by various methods such as enzymatic degradation,acid hydrolysis and catalytic hydrogenation,which has a large-scale application prospect.Rational use of biomass resources is conducive to alleviate the energy crisis and improve environmental degradation and many other issues.Therefore,this paper focused on the development of high-efficiency,low-cost catalysts and their application in the catalytic hydrogenation of cellulose to produce high value-added polyol products.Tungsten-based catalysts show excellent catalytic activity in the process of hydrogenation of cellulose to ethylene glycol.The solid acid catalyst WO₃-ZrO₂ has very strong acidity,which makes up for the deficiency of WO₃ in acid strength.Niobium oxide has excellent properties in the field of acid catalysis,and it belongs to non-precious metal with low cost.Therefore,WO₃-ZrO₂ composite oxide were synthesized by coprecipitation and hydrothermal method,Nb₂O₅ was added by impregnation method.Finally,non-precious metal was supported on the surface of catalyst by hydrogen reduction method to synthesize metal-supported bifunctional composite oxide catalyst.The physical and chemical properties such as crystal structure,surface acidity,morphology and valence state of the prepared catalyst were studied by XRD,BET,NH₃-TPD,SEM,TEM,XPS and other characterization methods,and then all catalysts were applied to the cellulose hydrogenation reaction.In the hydrogenation reaction,the effects of different reaction conditions and different kinds of catalysts on the catalytic performance and the reusability of the catalysts were investigated.The possible reaction mechanism of the catalytic hydrogenation of cellulose was also discussed.The WO₃-ZrO₂ composite oxide synthesized by hydrothermal method presents nano-flake structure.The WO₃ dispersed on the surface of ZrO₂ can promote the formation of tetragonal ZrO₂ and inhibit the formation of monoclinic ZrO₂,thus increasing the specific surface area,the non-precious metal Ni distributed uniformly on the surface of the catalyst with a small size.The introduction of Ni made the catalyst change from single acid catalytic center to acid center and active metal center dual-functional catalyst.The optimum reaction conditions for the hydrogenation of cellulose to polyol were:the reaction temperature was240 ^oC,the reaction time was 2.0 h,when the WO₃ content was 15.0 wt%,an active component of Ni and the non-precious metal loading of 15.0 wt%,and the optimum reaction conditions were:reaction temperature is 240 ^oC,reaction time is 2.0 h,initial hydrogen pressure is 4.0 MPa.Under the optimal reaction conditions,the conversion of cellulose reached 100.0%and the yield of ethylene glycol was up to 58.4%The Ni/WO₃-ZrO₂ catalyst was modified by adding promoter Nb₂O₅,and the Nb₂O₅distributed evenly on the surface of the catalyst.The addition of Nb₂O₅ made the acidity of the catalyst increased and the acidity distribution changed.The yield of ethylene glycol reached the highest value of 67.2%when the content of Nb₂O₅ was 3.0 wt%.After five cycles of Ni/WO₃-Nb₂O₅-ZrO₂ catalyst,the cellulose conversion rate is basically maintained at 100.0%,and the ethylene glycol yield is decreased by 3.0%to 64.2%,indicating that the catalyst has good repeatability and stability.In addition,the reaction process of cellulose hydrogenation catalyzed by Ni/WO₃-Nb₂O₅-ZrO₂ catalyst was explored.At first,cellulose was hydrolyzed to glucose under the co-action of acid center of catalyst and H⁺produced by high temperature water.And then glucose was formed into glycolaldehyde by reverse aldol condensation under the catalysis of W⁶⁺and Zr⁴⁺.Finally,ethylene glycol,1,2-propanediol and other polyol products were formed by hydrogenation reaction under the action of active metal Ni.