Catalytic conversion of carbon dioxide to formate using novel metal organic frameworks

Abstract

The combustion of fossil fuels has significantly increased the concentration of carbon dioxide (CO2) in the atmosphere. CO2 is a greenhouse gas and a major contributor to global warming. To rectify the CO2 problem, its capture and conversion have been proposed. Metal-organic framework (MOF) based materials, a relatively new class of porous materials, with unique structural features, high surface area, chemical tenability and stability, have been extensively studied for various applications. Advances in the design of these materials, including functionalisation of the linker with catalytically active species, add to the variation in the structures, providing a further means of tailoring MOF properties. In this thesis, a series of MOFs, prepared from 2,2’-bipyridine-4,4’-dicarboxylate linker with LaIII, MnII, MgII, CdII and ZnII are presented. The choice of the linker was to allow MOF formation through the carboxylate moiety while the bipyridyl units act as anchoring sites for catalytically active metals such as RuII and PdII. Structural elucidation of the compounds was performed by single crystal X-ray diffraction. The topological analysis was performed on the networks to get a better understanding of network connectivity. Bulk material was characterised using thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), scanning electron microscopy energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy, high resolution transmission electron microscopy (HR-TEM) and nuclear magnetic resonance spectroscopy (NMR). The topological analysis revealed that the MOF constructed using LaIII, JMS-1 (Johannesburg and Midlands State) had a new network topology, zaz (South Africa and Zimbabwe). The porosity of the activated phases was tested using CO2 (298, 273, 195 K), nitrogen (77 K) and hydrogen (77 K) sorption experiments. CO2 sorption experiments on JMS-1a, Ru(II)@JMS-1a, JMS-3a and JMS-4a showed a Type I isotherm, which is typical of microporous materials. The hydrogenation of CO2 to formate was performed under various conditions (solvent, base, temperature, pressure, time and catalyst loading) to establish the optimum conditions. The formate product was detected and quantified by 1H NMR spectroscopy, with acetone as an internal standard. The functionalised MOFs, Ru(II)@JMS-1a, Pd@Mn: JMS-2a and Pd@Mg: JMS-2a displayed excellent catalytic hydrogenation of CO2 under mild conditions. On the contrary, the native MOF, JMS-1a required harsh conditions to produce significant yields of formate. This work demonstrates that the catalytic activity of homogeneous systems can be enhanced under heterogeneous conditions by incorporating them in the MOFs.