Hydrogenation Of Levulinic Acid Into γ-valerolactone Over CuAg/Al₂O₃ Bimetallic Catalyst And Mechanism Of Preventing Cu Leaching By Ag

Levulinic acid（LA）is a bio-platform chemical that is accessible from renewable lignocellulosic biomass.Hydrogenation of LA forms gamma-valerolactone（GVL）,a versatile bio-chemical that can be used as precursor of clean fuel,green solvent in fine chemical synthesis and food additives,or as intermediate in synthesis of value-added chemicals.Therefore,over the last two decades,researchers have developed a variety of methods for efficient synthesis of GVL from LA.Due to the strong acidity of LA,leaching of active metal Cu,Ni under mild reaction conditions is a serious issue.Metal leaching deactivates the catalysts,contaminates the product,and increases the cost for product purification.In this paper,a new strategy of preventing metal leaching was proposed for the first time through noble metal Ag modification of the Cu/Al₂O₃ catalyst.Firstly,the CuAg/Al₂O₃ catalyst and the other catalysts containing Cu and/or Ag were synthesized.Secondly,the geometric structure and chemical properties of the catalysts were investigated by using a series of characteristics,such as XPS,H₂-TPR,ICP-OES,TEM（HRTEM）,STEM-EDS and XRD.Thirdly,the properties of catalysts on the reaction of converting LA into GVL were investigated under mild reaction conditions（180℃,1.4MPa H₂）.Finally,the metal leaching characteristics and the mechanism of preventing Cu leaching by Ag in CuAg/Al₂O₃ catalyst were studied in detail.The main conclusions are as follows:The XPS spectra shows that fresh CuAg/Al₂O₃ catalyst mainly contains Cu²⁺;after reaction at 180℃and 1.4MPa H₂,Cu²⁺is converted into Cu⁰ metal state.It means that the catalyst has undergone an in-situ reduction process during reaction.The H₂-TPR patterns of Cu-and Ag-supported catalysts show that the Ag component can reduce the reduction temperature of CuO on the surface ofγ-Al₂O₃.After the in-situ reduction,Cu and Ag underwent self-assembly to form bimetallic Cu-Ag nanoparticles in cluster form onγ-Al₂O₃surface,which was confirmed by the TEM（HRTEM）and STEM-EDS images.The XRD patterns indicate that the close proximity and the strong interaction between Ag and Cu enhance the amorphization of crystallites.These findings show that Cu and Ag can form a strong interplay of geometric and electronic effects.Moreover,the metal sintering of the catalyst reduced in H₂/Ar is more severe than that of the reduced catalyst in the reaction solvent in situ.Therefore,in-situ reduction can alleviate the sintering effect.The CuAg/Al₂O₃ catalyst without reduction pretreatment led to the complete conversion of LA with approximately 100%selectivity to GVL without Cu leaching at 180°C under 1.4MPa H₂ in THF solvent.After nine consecutive reactions,the catalyst retained its high catalytic activity and good stability.The Cu/Al₂O₃ without reduction pretreatment almost has no catalytic activity;Ag/Al₂O₃ has certain catalytic activity.Both of which did not generate comparable result as that of CuAg/Al₂O₃.Therefore,the synergistic effect of Cu and Ag facilites the in-situ reduction of the catalyst,prevents Cu leaching,and leads to efficient conversion of LA into GVL under mild conditions.LA forms a soluble copper carboxylate complex with Cu species through the reaction of LA with Cu in the oxidation state rather than in the metal state.Thus,the key to preventing Cu leaching is to maintain Cu in the metal state.However,under mild reaction conditions of180°C and 1.4 MPa H₂,Cu/Al₂O₃ suffered severe metal leaching because of Cu reoxidation during the reaction even when it was thoroughly prereduced by H₂.While the CuAg/Al₂O₃catalyst extensively prevented Cu leaching.Based on the experiments and characterizations,the mechanism of Ag Preventing Cu leaching in CuAg/Al₂O₃ catalyst is that the synergistic effect between Cu and Ag maintains Cu at metal state through accumulation of electrons and then significantly suppresses Cu leaching from the bimetallic catalyst.