| Catalytic hydrogenation,as an important chemical process,has been widely used in the production of numerous chemicals.Therefore,efficient catalyst developing has been a long-standing hotspot in this field.Recently,the excellent catalytic performance of M-N-C catalysts has attracted widespread attention of scientists,which is possibly attributed to the presence of metal-nitrogen(M-Nx)sites in M-N-C catalysts.Thus,the key factor in promoting their catalytic hydrogenation performance is to enhance the formation of M-Nx sites efficiently.Based on this,this paper focuses on the effects of pyrolysis temperature,cobalt source and heteroatom doping species on the formation of M-Nx sites,and the construction of catalysts with rich M-Nx sites.Firstly,N-acetyl-alanine modified chitosan was used as an organic nitrogencontaining ligand,and a series of Co-N-C catalysts(Co/CNAA-T,T = 700,800,900,1000)were prepared by pyrolyzing the precursors from modified chitosan and cobalt acetate at different temperatures.The effects of pyrolysis temperature on their structure and catalytic performance were evaluated.The XPS results show that as the pyrolysis temperature increases,the content of Co-Nx sites in the catalyst increases first and then decreases.The above catalysts were used in the catalytic hydrogenation of methylphenylsulfoxide,in which Co/CNAA-900 exhibits the best activity.Subsequently,the active sites of the catalysts were explored by hydrochloric acid etching and potassium thiocyanate poisoning experiments,the highest activity of the Co/CNAA-900 catalyst can be ascribed to the abundant Co-Nx sites.In addition,the catalyst presents nice stability and resistance to toxicity in the recycle test.In order to further study the effect of precursor composition and heteroatom doping species on the content of M-Nx sites in M-N-C catalysts,a series of Co-N,S-C catalysts(Co-NSPC-X,X = N,C,S,Cl)were successfully prepared by pyrolysis of sulfurcontaining porous organic polymers impregnated with different cobalt salts,respectively.The characterization results indicate that the anions of cobalt salts can significantly influence the formation of Co-Nx sites.Moreover,the doping of S element into the carbon layer can promote the generation of Co-Nx sites.The catalytic hydrogenation reaction of nitrobenzene was used as a template to evaluate the performance of these catalysts.Among them,Co-NSPC-N exhibits optimal activity,which is mainly attributed to the largest content of Co-Nx sites.In addition,the catalyst also displays good recycle stability and wide applicability of substrates. |
