Research On The Synthesis Of Ni-W Bifunctional Catalysts Based On Metal-Organic Frameworks And Their Catalytic Cellulose Hydrogenolysis

The rapid development of the economy and society highly rely on traditional fossil resources such as coal,oil,and natural gas.However,fossil energy is difficult to regenerate,and its excessive use has caused serious environmental pollution problems.Therefore,green and environmentally friendly renewable resources play a crucial role in sustainable development.As an abundant renewable biomass resource,the conversion of cellulose into lower diols is regarded as an attractive way of utilization.Low-valent tungsten species could promote retro-aldol condensation,and Ni has low cost and efficient hydrogenolysis,Ni-W bimetallic catalysts have unique advantages for cellulose hydrogenolysis.In this paper,the high specific surface area and high porosity of metal-organic frameworks（MOFs,such as UiO-66（Hf）,Ni-MOFs）are exploited,Ni-W bimetallic catalysts supported by MOF derivatives were prepared by impregnation and pyrolysis.Firstly,by using UiO-66（Hf）as a precursor,the Ni-WO_x/Hf O₂ catalyst was prepared by the impregnation,and then by the calcination and reduction.Under mild conditions（4 MPa,245℃,2 h）,the highest yield of ethylene glycol（51.48%）and 1,2-propylene glycol（19.62%）was obtained,and the cellulose was completely converted.The characterization results confirmed the Ni-WO_x/Hf O₂ catalyst has defective tungsten species,strong acidity,a high specific surface area（366.45 m²/g）,and a suitable average pore size（4.02 nm）.However,after 5 cycles of the Ni-WO_x/Hf O₂catalyst,the yield of lower diols decreased significantly due to coke deposition.In order to solve the problems of poor reuse performance of Ni-WO_x/Hf O₂ catalysts,the W-Ni@C catalyst was synthesized by in-situ pyrolysis of Ni-MOFs impregnated with ammonium metatungstate.For comparing the effects of other Ni-W bifunctional catalysts with different carbon supports on the cellulose hydrogenolysis reaction system,the W-Ni/AC catalysts prepared by the impregnation method and the W-Ni@C（CA）catalysts prepared by the sol-gel method.The W-Ni@C catalysts for cellulose hydrogenolysis can achieve higher ethylene glycol yield（67.14%）and cellulose conversion（100%）.The characterization results indicate that the W-Ni@C catalyst maintains the hollow structure of the Ni-MOFs precursor,with abundant mesoporous structure and uniformly dispersed active centers,with a specific surface area of 299.98 m²/g.Compared with the W-Ni/AC catalyst and the W-Ni@C（CA）catalyst,the W-Ni@C catalyst has the highest content of defective WO_x,which can effectively promote the C-C bond cleavage of glucose.Cycling test experiments show that the presence of hierarchical porous carbon in the W-Ni@C catalyst effectively inhibits the leaching and sintering of active centers,so the catalyst still has good reactivity after 5 cycles.In addition,according to the experimental results,the main reaction pathway for the W-Ni@C catalyst to promote the conversion of cellulose to lower diols is elucidated,the conversion of cellulose to ethylene glycol mainly includes the following three key reactions:cellulose hydrolysis,retro-aldol condensation reaction,and hydrogenation.The conversion of cellulose to 1,2-propanediol mainly involves hydrolysis,isomerization,retro-aldol condensation,and hydrogenation reaction.