Synthesis, characterisation and properties of benzenepolycarboxylate metal-orgnanic frameworks

Abstract

Metal-organic frameworks (MOFs) are coordination polymers that extend in one, two or three dimensions, forming networks. These materials need to be strongly bonding metal centres linked by organic ligands to form a geometrically well-defined structure. MOFs are of interest due to our ability to identify some of the principles that govern the design and assembly of the target framework. The careful selection of MOF building blocks such that their properties are retained by the framework can yield unique materials with a host of physical characteristics and applications. The nature of the organic linkers often leads to voids present in the structure; these are usually occupied by guest molecules such as solvent, which need to be removed or exchanged for the MOF to be activated. The design of frameworks that are robust and withstand the desolvation can lead to porous compounds with the ability to adsorb other guest molecules, giving a platform for numerous scientific and industrial applications. This thesis describes the synthesis, the structural characterisation and the study of thermostability and gas adsorption characteristics of MOFs. The structures of compounds of some transition metal ions [Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)] and some other metals with symmetrical benzenepolycarboxylates (terephthalic, trimesic, pyromellitic and mellitic acid) are described. A detailed adsorption study of one particular compound is presented. The compounds presented were prepared using a variety of methods: solvothermal, hydrothemal, ionothermal, vapour diffusion and solvent diffusion using specially adapted reaction vessels. Single-crystal X-ray crystallographic techniques were used for structure determination and characterisation, utilising in-house and synchrotron facilities. Further analysis was carried out using elemental analysis and powder X-ray diffraction to determine overall „bulk‟ sample purity. Thermogravimetric analysis and hot-stage microscopy were used to assess the thermal stability of selected compounds, producing the temperature profile needed for Intelligent Gravimetric Analysis which determined the pore characteristics and potential applications of the compounds. A total of forty-four metal–benzenepolycarboxylate complexes are presented, mostly with transition metals. Nine complexes containing the 1,4-benzenedicarboxylate ligand (terephthalate) (1-9) are reported. This ligand formed complexes with zinc (3,5,6,8), cobalt (2,4), nickel (1), cadmium (7) and copper (9). The structures of the compounds include one-dimensional chains (1,2), two-dimensional sheets (3-6) and three-dimensional coordinated frameworks (7-9). Twelve complexes containing the 1,3,5-benzenetricarboxylate ligand (trimesate) (10-21) are reported. This ligand formed complexes with zinc (11,12,17), cobalt (14,18), nickel (13), cadmium (15,16) and manganese (19,10,21). The structures of the compounds include two-dimensional sheets (10-12,16) and three-dimensional coordinated frameworks (17-21). Fifteen complexes containing the 1,2,4,5-benzenetetracarboxylate ligand (pyromellitate) (22-36) are reported. This ligand formed complexes with zinc (32,33,36), cobalt (22,25,34,35), nickel (26), cadmium (27,28,31), copper (23,24,29) and the alkali metal potassium (34). The structures of the compounds include two-dimensional sheets (22-27) and three-dimensional coordinated frameworks (28-36). Eight complexes containing the fully substituted 1,2,3,4,5,6-benzenehexacarboxylate ligand (mellitate) (37-44) are reported. This ligand formed complexes with cobalt (41), nickel (37,38), cadmium (42,43), copper (39,40,44). The structures of the compounds include one-dimensional chains (37), two-dimensional sheets (39,41,42) and three-dimensional coordinated frameworks (38,40,43,44). The adsorption characteristics of the porous MOF NEW105 (compound 17) were studied. N2 and CO2 adsorption at low temperatures gave a pore volume that was much lower than the value calculated from the crystallographic data. Virial analysis of the CO2 isotherm confirmed interactions that are equivalent to ultra-nanoporous materials, indicating that the structure has undergone a structural rearrangement. Further kinetic analysis of gas adsorption isotherms of CO2, N2 and O2 showed the new nanoporous material was capable of kinetic molecular sieving of gases, a process observed only once before in MOFs. The Adsorption studies were carried out by Northern Carbon Research Laboratories; M. Thomas, J. Bell and J. Armstrong.