Study On High Efficiency And Low Pollution Catalysts For Selective Hydrogenation Of Furfural

As an important product in the furan industry,furfural comes from renewable biomass resources such as corncob,cotton seed hulls and bagasse etc.Furfural is a very reactive molecule because of its unsaturated carbon-carbon double bonds in the furan ring and the carbonyl in the side chain,so a large number of derivatives can be obtained by catalytic hydrogenation.Furfuryl alcohol and 2-methyl furan are the products obtained by selective hydrogenation of the carbonyl over Cu-based catalysts which have appropriate hydrogenation capability.At present,the commercial catalysts for the furfuryl alcohol are Cu-Cr catalysts imported from abroad,which are very expensive.The catalysts for the 2-methyl furan are either copper chromite or Cu-Cr alloy.These catalysts not only bring harm to the operators, but also cause serious environmental pollution because they contain highly toxic carcinogen Cr.Therefore,to develop an environment friendly catalyst of low cost,high efficiency and pollution free for furfural hydrogenation has become an imminent task.This will play a significant role not only for the development of the fine chemical industry using the renewable resources such as agricultural products but also for the development of the green chemical process technology in China.This thesis aims at study on the low-Cr and the Cr free supported catalysts.The low-Cr Cu-Cr/γ-Al₂O₃ catalyst and the Cr free Cu-Ca/SiO₂ catalyst were prepared.Their structure was characterized by XRD,SEM,TEM,XPS,TPR,etc.techniques.Their catalytic performance for the selective hydrogenation of furfural was investigated.The supported Cu-Cr/γ-Al₂O₃ catalyst was prepared by impregnation method.The results of structural characterization and activity evaluation for the selective hydrogenation of furfural to 2-methyl furan showed that the interaction between Cu and Cr species was formed by adding Cr,which resulted in the transformation of some CuO into CuCrO₄ and improved the Cu dispersion state in the cartier.It was noticed that increasing the Cr content could lead to enhanced interaction between Cu and Cr species,and improved deep hydrogenation ability of the catalyst.An appropriate Cu / Cr molar ratio was 12/5 where the Cr content in the catalyst was only 6%(wt%of Cr₂O₃),which was much lower than the commercial copper chromite catalyst(Cr₂O₃ 45%).Based on this result,the interaction between Cu and Cr species was modified by adding Mg,Ca,Ba or Zn promoters to the catalysts.It was found that while increasing the alkaline-earth metal promoters the destructive effects of alkaline-earth metal Mg,Ca,Ba on CuCrO₄ structure was enhanced and the capacity of the hydrogenation catalyst was therefore reduced,resulting in decreased formation of 2-methyl furan.However,Zn is a more favorable promoter for the modification of the interaction between Cu and Cr species.The deep hydrogenation of 2-methyl furan was slowed down and the formation of 2-methyl furan was increased when using the catalyst modified by Zn promoter.The 100 hours stability test results for the optimum CuCr-Zn5 catalyst showed that the conversion of furfural remained at 100%and the selectivity of 2-methyl furan was stable between 91.3%and 92.2%.There were no visible structural changes of the catalysts after the reactions.Therefore,the CuCr-Zn5 catalyst possessed excellent stability of the structure and the stability of catalytic performance.According to the characteristics of sol-gel technique used for preparing the nano-particles catalyst,highly dispersed Cu-Ca/SiO₂ supported catalyst was prepared by the gelation of the flocculant for the commercial silica sol at a low price and its sedimentation for the metal ion.The research results indicated that the 20～40 nanometer ultrafine SiO₂ carrier can be prepared by using solution of 1.0 mol / 1 Na₂CO₃ and controlling pH at the neutrality. At the same time,the active component can be dispersed uniformly in the carrier in the form of 5 nanometer CuO particles,with no loss in the post-processing,to give the catalyst of good thermal stability.Compared with the impregnation method,the grain size of the active component was smaller and its dispersion degree was higher,the density of active site on surface was bigger,and the interaction between active component and the cartier or promoters was stronger for the catalyst prepared by the sol-gel method.The crystal grains of active component became larger,and the interaction between the active component and the carrier or promoters was enhanced,and the deep hydrogenation ability of the catalyst was increased with increasing the content of the active component.The stability of the catalyst was improved by adding Ca promoter in the catalysts.When furfuryl alcohol was the target product,the appropriate loading amount of CuO was 15%,and the yield of furfuryl alcohol could reach above 98%at the reaction temperature 120℃,the H₂/furfural molar ratio of 5-7 and liquid space velocity of furfural 0.36 h^-1.When 2-methyl furan was the target product,the appropriate loading amount of CuO was 20%,and the yield of 2-methyl furan was about 94% at the reaction temperature 220℃,the H₂/furfural molar ratio 5-6 and liquid space velocity of furfural 0.4 h^-1.The scale-up experiments for the optimum Cu-Cr/γ-Al₂O₃ catalyst and the Cu-Ca/SiO₂ catalyst were carried out in the single-tube reactor.The results showed that the Cu-Cr/γ-Al₂O₃ catalyst displayed good stability.The yield of 2-methyl furan was above 90%,which was superior to one of the commercial catalysts.The yield of 2-methyl furan was above 92%for the Cu-Ca/SiO₂ catalyst after the continuous operations of 1100 hours for the hydrogenation of superior to one of the commercial catalysts.The yield of 2-methyl furan was above 92%for the Cu-Ca/SiO₂ catalyst after the continuous operations of 1100 hours for the hydrogenation of furfural..The yield of furfuryl alcohol was above 98%for the Cu-Ca/SiO₂ catalyst after the continuous operations of 500 hours for the hydrogenation of furfural.The structural characterization of the used catalysts indicated that the deactivation of the catalyst is mainly due to the fact that furfural absorbed on the surface of the catalyst and the resin like polymer generated in the reaction have covered the surface of the catalyst or blocked the pores of the catalyst.It is believed that Cu-Ca/SiO₂ has superior catalytic performance and the ability of resisting the composition of the coke compared with the Cu-Cr/γ-Al₂O₃ catalyst,mainly due to its weaker surface acidity and appropriate pore structure.