Synthesis Of MOF-Derived Nickel/Carbon Catalysts And Their Applications In The Selective Hydrogenation Of Nitroarenes

The catalytic hydrogenation of nitroarenes to aromatic amines is an important chemical transformation in industry,which is widely applied in the manufacture of pharmaceuticals,dyes,macromolecules,and so forth.However,it’s quite a challenge to reduce nitro groups selectively in the presence of halogens or other reducible functional substituents.As thus,the development of highly efficient and selective catalysts for the hydrogenation of nitroarenes has become a focal research,among which nickel-based catalysts have received great attention due to their excellent hydrogenation activities and abundant reserves.Metal Organic Framework(MOF)with nickel as the metal nodes is a kind of ideal precursor for preparing nickel/carbon catalysts.By means of pyrolysis,the highly ordered metal nodes within MOF can be transformed into high-density and high-dispersion catalytic sites while the organic bridging ligands can be converted into heteroatom doped carbon carriers.Besides,the formed mesoporous structure is beneficial to the diffusion and transfer of substrates and products.Hence,nickel-based MOF-derived materials deserve intensive research and are expected to be novel hydrogenation catalysts.Firstly,a needle-like Ni-MOF(MOF-Ni-1)with 4,4’-bipyridine and terephthalic acid as dual ligands and Ni(NO3)2·6H2O as nickel source was easily constructed by stirring at 100℃.MOF-Ni-1 was prolyzed at different temperatures,giving four kinds of composite catalysts Ni@NC-T.The effects of pyrolysis temperature on the composition and structure of Ni@NC-T were investigated by means of PXRD,SEM,TEM,XPS,etc.The results showed that Ni@NC-T not only possessed high nickel loading,but also had hierarchical porous structures(mesoporous and microporous).With the pyrolysis temperature increasing,Ni species was gradually reduced to Ni(0)and agglomeration occurred,and the ratio of graphite-nitrogen and graphite-carbon raised.Ni@NC-600,with highly dispersed Ni sites(Ni NPs average size was 7.5 nm)and well-developed pore structures,catalyzed hydrogenation of p-chloronitrobenzene with 100%conversion with more than 99%selectivity.Moreover,it also offered remarkable tolerance to ten kinds of nitroarenes substituted by sensitive groups such as halogen,nitrile,keto and carboxylic.However,the stability of the catalyst was poor in the air,which deactivated after three months.To improve the stability,in this chapter,urea was introduced to the MOF-Ni-1 and seven kinds of composite catalysts Ni@NC-xU were prepared by semi-closed pyrolysis.On the one hand,urea was employed as an additional nitrogen source to regulate electron distribution and defects.On the other hand,the produced NH3 during pyrolysis could etch the pore structures,forming 9～12 nm mesopores.Meanwhile,the Ni NPs in Ni@NC-xU were encapsulated by several layers of carbon shells,which significantly enhanced the stability of the catalysts.The best amount of urea was optimized and Ni@NC-0.5U performed the most outstanding catalytic activity,generating 100%conversion with more than 99%selectivity of p-chloronitrobenzene under mild conditions(room temperature,1 MPa H2).It was noteworthy that the catalyst demonstrated excellent stability whose catalytic performance could be retained after placed in the air for 6 months.Given that the low surface area and non-porous structure of MOF-Ni-1 may restrict the catalytic activity of the derived materials,a regular hexagonal MOF(MOF-Ni-2)with high specific surface area(1206.7 m2·g-1)was constructed with 4,4 ’-bipyridine and 1,3,5-benzenetricarboxylic acid as dual ligands and Ni(NO3)2·6H2O as nickel source.By stirring at 100℃,four types of surfactants-assisted MOFs were prepared.The catalytic test results showed that the Ni@NC(S)-P 123("S" stands for stirring method)derived from MOF-Ni-2-P123 performed the best activity in p-chloronitrobenzene hydrogenation.This is because obvious carbon shells were formed on the surface of Ni@NC(S)-P123,which retarded the collapse of the internal micropores and prevented the agglomeration of Ni NPs.To further explore the effect of the crystallization on the catalytic activity,three kinds of MOF-Ni-2 crystals with good crystallization and large size were prepared by solvothermal method.It was found that the well-developed MOF crystals were helpful to the formation of nickel active species and showed increased activities in the hydrogenation of p-chloronitrobenzene.To further explore the effect of the crystallization of MOFs precursor on the catalytic activity of derived materials,three kinds of MOF crystals with good crystallization and large size were prepared by solvothermal method.It was found that the large-size MOF crystals facilitated the formation of nickel active species,which remarkably improved the catalytic performance.