Syntheses,structures And Properties Of Zn/Co-MOFs Based On Different N-containing Heterocyclic Ligands And Their Derivatives

Metal organic frameworks with an infinite network structure self-assembled by metal ions or metal clusters and organic ligands through coordination bonds.Owing to its various structural types,high porosity,high specific surface area,designability and tailorability,it has become a multi-functional material with wide application such as gas gas storage and separation,optical materials,magnetic materials,catalysis and biomedicine.The ligands in MOFs are rich in carbon and heteroatoms such as nitrogen,sulfur and phosphorus.The porous carbon materials with high specific surface area,uniform pore size and highly dispersed heteroatoms can be obtained by calcining MOFs at high temperature without adding porogenand templates.The derived carbon materials not only have good conductivity,but also possess catalytic active sites for electro catalytic reaction.Therefore,MOFs derived carbon materials have attracted much attention in recent years due to their promising applications in the field of electrocatalysis.In this paper,neutral polyimidazolyl and polypyridyl nitrogen-containing heterocyclic ligands were chosen as the primary ligand,and aromatic polycarboxylic acid ligands were utilized as ancillary ligands to react with Co(?)/Zn(?)metal ions under the urothermal environment,such that twenty-two coordination polymers with mixed-ligand frameworks have been successfully obtained and characterized.The catalytic performance of porous carbon materials derived from three of these different cobalt-based porous coordination polymers for oxygen reduction reaction and the adsorption and adsorption and separation properties of one of these compoundswere studied.The main research results are as follows:(1)Three 3D polymeric complexes containing polynuclear Co(?)units were obtained froml,4-bis(1-imidazolyl)benzene and biphenyl polycarboxylic acid ligands..Complex 1 exhibits an unusual 3D networkbased on binuclear Co(?)cluster and Co₂+centers bridged by bpda2-and bib ligands.Complex 2,containing a novel pentanuclear Co(?)cluster,features a porous 3D MOF with regular nanosized tunnel.Complex 3 contains an interesting tetranuclear Co(?)cluster and displays an amazing 3D porous MOF built from double-layered 2D networks pillared by two rows of bib spacers.Compound 3,with high surface area,high yield,high purity,high chemical stability,was used as a precursor to prepare Co,N-codoped porous carbon materials by simple carbonization under Ar atmosphere.The obtained derived carbon material(denoted as CoNC-3)has better catalytic performance for ORR,better stability and stronger methanol resistance than Pt/C.It can be concluded that the MOFs based on polyimidazolyl ligands can be used as precursors for the preparation of carbon materials as catalystsfor oxygen reduction reduction.(2)Different aromatic polycarboxylic acids are employed as the auxiliary ligands to give rise to structure diversities in the Co(?)-tpt(tpt=2,4,6-tris(4-pyridyl)-1,3,5-triazine)frameworks.Five 3D polymeric complexes were obtained.Complexes 4 and 5 reveal amazing 3D networks in which the polycarboxylate ligands and Co(?)ions connect with each other to form regular 3D porous frameworks with 1D cylindrical channels partitionedby virtue of tpt ligands.Complexes 6-8 exhibit unusual 3D frameworks constructed from the Co-polycarboxylate layers pillared by tpt ligands.In addition,compound 4 was chosen as a precursor to prepare Co,N-codoped porous carbon materials(denoted as CoNC-4)as an eletrocatalyst for oxygen reduction reaction.Compared with CoNC-3,the catalytic activity of CoNC-4 for oxygen reduction is improved.Especially.when the dicyandiamide used as a additional nitrogen source was introduced into synthesis process,the content of the pyridine-N and Co-Nx active sites in the obtained porous carbon material(denoted as CoNC-(4+DCD))obviously increases,so that the electrocatalytic performance of CoNC-(4+DCD)is significantly improved and shows almost identical onset potential but remarkably more positive half-wave potential as well as higher limiting current density as compared with commercial Pt/C catalyst.Moreover,the whole synthesis process of CoNC-(4+DCD)became more efficient and facile which makes it more suitable for practical application prospects.(3)Five Co-MOFs were synthesized by 5-(2-methyl-l-imidazol-1-yl)-isophthalic acid(H₂miipa)and different bis-(1-imidazolyl)ligands.Three Co-MOFs(10-12)based on H₂miipa and three linear bis-(1-imidazolyl)ligands with different lengths exhibit similar three-fold interpenetrated three-dimensional(3D)"pillared-layer" structures.While the"V"-shaped 4-(1-imidazolyl)phenyl ether was used in place of linear bis-(1-imidazolyl)ligands as reactants,a 2D double-layered network(compound 13)was obtained.Interestingly,the porous double-layered networks are interlocked to each other to form an infinite 3D network.CoNC-(13+DCD)with high specific surface area was also obtained by using low porosity compound 13 as a precursor and adding DCD as a additional nitrogen source,and the catalytic activity of the derived carbon material was even better than that of Pt/C.It can be seen that the derived carbon materials with high specific surface area and good catalytic performance can also be obtained by using compounds with low porosity as precursors in the case of addingadditional nitrogen source,which greatly expands the selection range of precursors and provides great possibility for reducing cost of catalyst.(4)When the tris(1-imidazolyl)benzene and 4-(1-imidazolyl)phenyl ether were used as primary ligands in the presence of aromatic polycarboxylic acid as ancillary ligands to react with Co(?)/Zn(?)metal ions,nine Zn(?)/Co(?)coordination polymers(compounds 14-22)were successfully isolated.In compound 14,the(1-imidazolyl)benzene coordinatesto Zn(?)cations to form a 1D ribbon-like chain,and 1,4-naphthoic acid link one-dimensional chain to form a three-dimensional structure.In compounds 15-20,aromatic polycarboxylic acid ligands and metal ions form two-dimensional layers,and the layers are connected by polyimidazole ligands to form a three-dimensional structure.Complexes 15 and 16 feature porous 3D MOFs with regular nanosized tunnels.The gas adsorption properties of compound 15 were studied by the adsorption experiments for a number of small gases including CH₄,C₂H₆,CO2,C₂H₄,C₂H₂ at 273 K/298K and 1 bar.The adsorption capacities of C₂H₂ and C₂H₄ are greatly over that of C₂H₆ and CH₄,which makes it suitable for the separation of saturated hydrocarbons(C₂H₆ and CH₄)and unsaturated hydrocarbons(C₂H₂ and C₂H₄).