Preparation Of Nano-nickel-based Catalyst And The Application In The Selective Hydrogenation Of Cinnamaldehyde

The selective hydrogenation products ofα,βunsaturated aldehydes are often used as various chemical intermediates and play important roles in the field of fine chemicals.Among a range of hydrogenation catalysts,metallic nickel has always been widely investigated by researchers due to its low price,abundant reserves,and good hydrogen activation ability.However,Ni nanoparticles tend to agglomerate at high temperature;moreover,the as-prepared metallic Ni is easily oxidized to Ni O due to the low standard reduction potential of Ni²⁺（-0.25e V）,which limit its application.To deal with these problems,this dissertation focuses on the preparation of nano-nickel-based catalysts coated with carbon layer,which are expected to have good stability with uniformly dispersed metal nanoparticles.Metal-organic frameworks（MOFs）is composed of metal species and organic ligands via specific connection patterns and can be used as self-sacrificing templates to synthesize nano-metal catalysts wrapped by porous carbon layers by simple pyrolysis.Compared with traditional synthesis methods,the composition and morphology of the synthesized catalysts can be precisely controlled by treating MOFs at high temperatures.Based on this,in the first part of this dissertation,a MOF material Ni-BTC was used as the precursor,and the nano-nickel-based catalyst coated with porous carbon layer（Ni@C）was derived from the pyrolysis of Ni-BTC in inert atmosphere.The characterization results showed that the Ni species in the bulk phase of Ni@C catalyst were almost in the form of metallic Ni,and the Ni nanoparticles were wrapped with graphitic/amorphous carbon layer,which played good role in the dispersion and protection of Ni NPs.The catalyst Ni@C-600 showed high selectivity at high activity(Con.=96.2%,Sel._HCAL=92.4%)under the condition of 1.5 mmol cinnamaldehyde,120 ^oC and2 MPa H₂ for 6 h in isopropanol.It’s worth noting that the catalyst Ni@C-600 displays excellent storage stability-the dominant amount of Ni species are still in metallic state Ni after60 days storage;at the same time,Ni@C-600 shows excellent reusability,and its catalytic activity remained almost unchanged after 5 cycles when using a mixture of isopropanol and water（V:V=9:1）as the solvent.Based on the above results,the second part of this dissertation still concentrated on carbon-coated Ni nanoparticles,which were derived from carbon quantum dots（CQDs）as precursors.CQDs are highly dispersive,and there are abundant carboxyl,hydroxyl and other functional groups on the surface,which are easily complexed with metal ions to form a stable and highly dispersed metal-CQDs structure.Herein,citric acid,urea,and nickel nitrate were employed as raw materials to prepare Ni-CQDs precursors by a simple one-pot hydrothermal method,and the resultant Ni-CQDs precursors were carbonized in argon atmosphere at different temperatures to obtain a series of Ni-CN catalysts.The characterization results confirmed the formation of carbon-encapsulated structures.In addition,Ni-N_x bonds were formed on the surface of the catalyst,which can protect the metal nanoparticles and maintain their stability.The catalyst Ni-CN₂-600 showed high activity（The conversion was 100%）at 90^oC and 2 MPa H₂ for 3 h in the solvent of isopropanol-water,and the C=C bond selectivity was85%.At the same time,the catalyst Ni-CN₂-600 also displayed good storage stability and reusability,the conversion of cinnamaldehyde was still above 90%after 5 cycles.