Study On The Preparation、Characterization Of Ni-Al₂O₃ Catalysts And Catalytic Performance For Hydrogenation Of Levulinic Acid

Energy is closely related to the survival and development of human beings and directly affects the national economic development of the country.With the rapid development of the global economy,the human needs for energy is also increasing.Under the background of global energy insufficient supply,the development of renewable biomass energy and the transformation of energy supply methods are of great significance for ensuring sustainable energy development.Levulinic acid can be obtained from acid-catalyzed hydrolysis of widely available lignocellulose or bio-waste as raw materials.Compared with bio-based chemicals using food as raw material,levulinic acid has the unique advantage of not competing with food and not competing with others.And levulinic acid has achieved large-scale production,daily production up to tons and low prices.Therefore,the research on the catalysts and catalytic reaction mechanism for the synthesis of downstream high value-added chemicals from levulinic acid has attracted the attention of scientists from various countries.The catalyst used in the synthesis ofγ-valerolactone form levulinic acid hydrogenation includes both noble metal（Ru,Rh,Pt,Pd,Au）catalysts and non-noble metal（Ni,Cu）catalysts.Noble metal catalysts have higher hydrogenation activity and hydrogenation selectivity than non-noble metal catalysts,but their high price limits their application in large-scale industrialization.In recent years,the research on the synthesis ofγ-valerolactone by the hydrogenation of levulinic acid catalyzed by non-noble metal catalysts has caused great concern.Non-noble metal catalysts used in the hydrogenation of levulinic acid include Cu-based and Ni-based catalysts.In this paper,N₂ physisorption-desorption,XRD,H₂-TPR/TPD and NH₃-TPD were used to characterize the catalyst preparation methods,the molar ratio of Ni/Al and the introduction of ZnO promoter to the activity of Ni-Al₂O₃ catalyst.And their influence on the existence of the active ingredient,the surface properties,the effect of catalyzing the hydrogenation of levulinic acid,and the reaction mechanism.The specific research content was divided into the following aspects:1.Ni-Al₂O₃ catalysts were prepared by impregnation method,deposition precipitation method and co-precipitation method respectively.The effects of catalyst preparation methods on the dispersion,surface properties and catalytic hydrogenation of levulinic acid were investigated.The results showed that the catalyst precursors prepared by co-precipitation method have hydrotalcite-like structure compared with those prepared by impregnation and deposition-precipitation methods.The resulting catalysts had strong metal-support interactions,good active metal dispersion and rich surface acidity center.The rich acidity center and high metal dispersity make this catalyst show excellent performance in the synthesis ofγ-valerolactone,a hydrogenated synthesis of levulinic acid.Strong metal-support interaction prevents the loss of active metal Ni species during the reaction process.In turn,it showed high use stability.2.A series of hydrotalcite-like compounds with different Ni/Al molar ratios were prepared by co-precipitation method.Ni-Al₂O₃ catalysts were prepared by calcination and reduction.The dispersion,adsorption and activation of hydrogen,surface acidic sites,and the catalytic hydrogenation performance of active components in the catalysts were studied as the molar ratio of Ni/Al was changed.The results of the study indicated that the acid center of the surface of the catalyst plays a role in two aspects:（1）promoting esterification of levulinic acid with ethanol;（2）activating C=O to promote hydrogenation with hydrogen on adjacent nickel atoms.Ethyl 4-hydroxyvalerate is produced,which is cyclized to produceγ-valerolactone.With the increase of Ni/Al molar ratio,the acid center of the catalyst first increases and then decreases.At the same time,the hydrogen adsorption sites on the surface of the catalyst increase to a certain extent and remain constant.When the molar ratio of Ni/Al is 3:1,the catalyst had the most abundant acidic sites and exhibited excellent performance in the catalytic synthesis ofγ-valerolactone by the hydrogenation of levulinic acid.3.Based on the study of the first part and the second part,the effect of the introduction of ZnO promoter on the catalytic performance of Ni-Al₂O₃ catalyst for the hydrogenation of levulinic acid was studied.The results showed that Zn enters the lattice structure of active metal Ni species and there is a strong interaction between Ni-ZnO.The introduction of ZnO promotes the dispersion of the Ni species in the catalyst and,at the same time,generates a new acidic center on the catalyst surface.With the increase of the Zn/Al molar ratio in the catalyst,the dispersion of active Ni species in the catalyst first increased and then decreased.The surface acidity center first increased and then decreased,and the acid strength increased first and then decreased.When the Zn/Al molar ratio is0.5:1,the active metal dispersion in the catalyst is the highest,and the most abundant acidic sites are present,showing the best performance for the synthesis ofγ-valerolactone.Using this catalyst,the yield ofγ-valerolactone was highest at 86.9%under a reaction temperature of 160°C and a hydrogen pressure of 4 MPa.