MOF-derived Magnetic Ni@C Catalysts With High Nickel Content And Well Dispersion For Efficient Hydrogenation Of Benzoic Acid: Effect Of MOF Structure On Ni@C Nanoparticle

The conversion of aromatics to high value-added chemicals is an important class of reaction.Among them,the chemoselective hydrogenation of benzoic acid?BA?to cyclohexane carboxylic acid?CCA?is a typical reaction with practical application significance,because CCA is an important organic intermediate for the synthesis of pharmaceuticals like praziquantel,hexanolactam and ansatrienin.However,the hydrogenation of benzoic acids needs to overcome the high resonance energy of aromatic ring.Therefore,the hydrogenation of benzoic acid is much more difficult than the reduction of olefins,alkynes,aldehydes,and carboxylic acid esters.On the other hand,the hydrogenation of aromatic ring with electron-withdrawing groups of the carboxylic group requires more severe conditions for avoiding the occurrence of some undesired side reactions than that with electron-donating groups.Therefore,high performance catalyst is desirable for the hydrogenation of benzoic acids.Currently the most effective catalyst for obtaining CCA is a supported noble metal catalysts.However,the high price and limited availability of noble metal catalysts have inhibited their further large-scale application in industry,and design and synthesis of more cost-effective and affordable noble metal-free catalysts are of particular interest.As well known,Ni-based catalyst is a kind of efficient non-noble metal catalyst for various hydrogenation reactions but it is seldom employed in BA hydrogenation to CCA due to the reaction needs high activation energy.Improving the Ni content of catalysts provides the possibility to overcome the high activation energy,but the high content might result in the agglomeration of Ni.Therefore,it is desirable to prepare Ni-based catalysts meanwhile possessing the high Ni content and high degree of dispersion for the BA hydrogenation to CCA.Metal–organic frameworks?MOFs?,which consist of organic ligands connecting metal ions into porous crystalline framework structures,have garnered significant interest because of the versatile nature of their structures.MOFs also represent the promising behavior as the precursors for preparing well dispersed metal-based catalysts due to the highly ordered metal ions being well separated by the organic linkers of MOFs.Moreover,MOFs have been demonstrated to be ideal precursors for the direct generation of porous carbon nanostructures by thermal treatment under controllable atmospheres.During the carbonization process,the well-organized framework in MOFs is chemically transformed into carbon that can effectively preserve the porous structure and high surface area of parent MOFs,synchronously achieving uniform heteroatom doping and metallic species decoration,which is quite effective for heterogeneous catalysis.In this paper,we chosed three kinds of Ni-MOFs to calcine in an inert atmosphere,so that we can obtain the high nickel content and well dispersion Ni@C nanomaterials.The main work is divided into the following two parts:1.Three kinds of MOFs were selected as precursors.Ni@C materials were obtained at different calcining temperatures.The process of the generation,growth,and agglomeration of Ni nanoparticles from the carbon substrate was observed by TEM.The Ni nanoparticles with the smallest particle size can be obtained at the decomposition temperature of the MOFs.The nanoparticles are limited by the graphite layer,which effectively prevents the agglomeration of high Ni content.And then discussing from the structure of MOFs,it was found that the speed that Ni particles are wrapped during carbonization is related to the number of carbon chains connected to the metal nodes.2.The Ni@C material prepared in the previous section was used for the hydrogenation of benzoic acid to cyclohexanoic acid.The high Ni content and ultra-small particle size can overcome the high activation energy needed for the reaction.The conversion rate of benzoic acid reached 98.9%at 150?,3 MPa H₂conditions,the reaction 2.5 h,which was much higher than that of Raney Ni,and even approached the catalytic activity of noble metals.In addition,the influence of different solvents in the reaction was also explored,and ethanol was selected as the most suitable solvent.The limited effect of the graphite layer on the Ni nanoparticles makes it show good stability in the filtration and cycling experiments,and proves the universality of the catalyst in the substrate expansion experiment.