Browsing by Author "Oliver, Clive"

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    A theoretical study of metal-organic frameworks
    (2018) Zwane, Reabetswe Robin; Venter, Gerhard; Oliver, Clive; Wilkinson, Karl
    Among the options for carbon sequestration, the development of CO2 capture materials has gained momentum over the past two decades. The design and construction of chemical and physical absorbents for the capture of CO2 and clean energy storage are a crucial technology for a sustainable low-carbon future. Metal-organic frameworks (MOFs) provide a new vision for the adsorption of molecules on solid surfaces. The interest in MOFs is owed to their ultrahigh porosity, high surface areas and tuneable pore sizes and shapes. The main objective of this thesis was to adopt a rational predictive capacity used in MOF design to control properties such as framework porosity and flexibility on a molecular scale. The in-silico studies were carried out by using ab initio quantum mechanical approaches such as density functional theory and perturbation theory. In addition, semi-classical methods like the Grand Canonical Monte Carlo (GCMC) approach was also used. A structural motif called vicinal fluorination was adopted to study MOF linkers in isolation and in a framework. An extensive conformational study, in various solvents, was carried out to investigate the effect of vicinal fluorination on the isolated MOF linkers and therefore elucidate their conformational stability. The effect of fluorination on adsorption isotherms was also investigated. Moreover, various fluorination patterns were explored. Adsorption isotherms of a non-fluorinated copperbased MOF based on experimental work, and its various fluorinated analogues were predicted using the GCMC method. It was found that vicinal fluorination is not dominant in controlling conformations of some MOF linkers. Rather, an interplay of interactions, including solute and steric interactions, influence the conformational stability on rotational profiles. However, vicinal fluorination was shown to control the flexibility of the linkers used in MOFs as it controls the force constants around the minima of rotational profiles of isolated MOF linkers. The study also highlighted the importance of the solvent on the relative energies of the linker conformations – this has a potential impact on the synthesis of MOFs. With the help of computational methods and validation from experimental data, the structural and sorption properties of the framework, upon fluorination, were shown to have consequences on the adsorption properties of the MOF. Vicinally fluorinated frameworks were shown to have higher uptakes at a low temperature and low pressures.
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    Crystal engineering of mixed-ligand metal-organic frameworks based on simple carboxylate and bipyridyl ligands
    (2018) Mutti, Marcello; Oliver, Clive; Pelagatti, Paolo
    Over the last few decades research in supramolecular chemistry and crystal engineering have seen an exponential growth. The synthesis of metal-organic frameworks (MOFs) has attracted much interest worldwide due to the possibility of obtaining a large variety of structures with a wide range of applications in fields pertaining to storage, separation and catalysis. This work focuses on the crystal engineering of MOFs based on mixed ligands which may ultimately be used in the gas storage of pollutants, greenhouse gases or fuel. Two novel 2D mixed-ligand MOFs, both based on manganese, 4,4’-bipyridine and 1,3,5- benzenetricarboxylic acid, have been prepared and fully characterized. The employment of dimethylformamide or dimethylacetamide, as the solvent, results in two isostructural MOFs. Another novel MOF, similar in structure to the previous two, with 5-nitroisophthalic acid instead of 1,3,5-benzenetricarboxylic acid has been also prepared and characterized. This MOF has the same metal and ligand combination as, and is largely isostructural to, a literature example, but differs in method of preparation and solvent content. These Mnbased MOFs have potential voids in the structure making them candidates for gas sorption experiments. A novel 2D mixed-ligand MOF based on cobalt, 4,4’-bipyridine and 5-nitroisophthalic acid has been synthesized and fully characterized. Its structure is the same of another MOF, based on manganese, present in this work and like its manganese analogue it exhibits potential void spaces in the framework that make it a candidate for gas sorption experiments. Finally, a novel 2D MOF based on 1,3,5-benzenetricarboxylic acid and cadmium bromide has been synthesized and fully characterized. Dehydration and rehydration studies performed by combining powder X-ray diffraction with thermogravimetric analysis show that it can lose coordinated water, that comes from the reaction solvent, upon heating, and reabsorb water from the atmosphere, ultimately regaining its original structure. All MOFs were synthesized via the solvothermal method and characterized with X-ray diffraction (single crystal and powder) and thermal analyses (hot stage microscopy, differential scanning calorimetry and thermogravimetric analysis).
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    Design, synthesis and analysis of Isoreticular, 2-Periodic, Co(II)/(Mn(II) mixed- and Zn(II) single-ligand metal-organic frameworks for sorption of energy- and environmentally-relevant molecules
    (2025) Muguru, Kedibone Gudani; Oliver, Clive; Mogodi, Wilson
    The study of porous materials, particularly Metal-Organic Frameworks (MOFs), has garnered significant attention due to their tunable structures and potential for diverse applications. MOFs, constructed from metal ions or clusters and organic ligands, offer structural versatility where the choice of ligand influences pore size, geometry, flexibility, and adsorption properties. Mixed-ligand MOFs, incorporating two or more distinct ligands, enhance structural complexity and enable tuning of pore features in multiple dimensions, making them highly suitable for gas sorption applications. This study reports the design, synthesis, and characterisation of novel 2-periodic, isoreticular Co(II)/Mn(II) mixed-ligand and Zn(II) single-ligand MOFs. The mixed-ligand systems, synthesised using isophthalic acid (H2ia) and 1,3-bis(4-pyridyl)propane-N,N'-dioxide (bppdo), yielded isoreticular frameworks with formulae [Co3(ia)3(bppdo)(MeOH)]n·n(DMF) (1) and [Mn3(ia)3(bppdo)(MeOH)]n·n(DMF) (2). The single-ligand systems, constructed using 4,4′-(hexafluoroisopropylidene)bis(benzoic acid) (H2hfipbb), produced two pairs of MOFs: doubly interpenetrated frameworks [Zn(hfipbb)(MeOH)]n·n(MeOH)0.5 (3) and [Zn(hfipbb)(EtOH)]n·n(EtOH)0.5 (4) and non-interpenetrated frameworks [Zn2(hfipbb)2(DMF)2]n·n(DMF)4 (5) and [Zn4(hfipbb)4(DMA)4]n·n(DMA)8·n(H2O) (6). The ligands' unique rigidity and functionality yielded MOFs with varying porosity and structural flexibilities. Desolvated studies of the mixed-ligand MOFs revealed drastic void space reductions (to 3.7 and 5.7% for 1′ and 2′, respectively, at 298 K) compared to the as-synthesised structures (potential void spaces of 21.6 and 22.3% for 1 and 2, respectively, at 298 K). Despite inaccessible void spaces at 195 K, significant CO2 sorption at this temperature indicated structural transformations enabling adsorption. Water vapour sorption at 298 K induced similar transformations, as confirmed by single-crystal X-ray diffraction, showcasing the dynamic adaptability of these frameworks. For the single-ligand MOFs, void space ranged from 12 to 25% of the unit cell volume, with notable differences between interpenetrated and non-interpenetrated structures. Comparison with Cu-based counterparts from the literature highlighted that the Zn-based MOFs exhibited superior N2 and H2 adsorption due to higher crystallinity and stability upon desolvation. However, Cu-based MOFs displayed stronger CO2 adsorption interactions, with two-step adsorption isotherms and higher Qst values (up to 31.8 kJ mol-1), reflecting greater framework flexibility. Water vapor sorption studies emphasised the stronger interaction of Cu-based MOFs with unsaturated Cu(II) metal sites. These findings underscore the potential of Zn-based MOFs for gas storage and the dynamic structural responses of the mixed-ligand frameworks.
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    Porosity studies of isoreticular mixed-ligand metal-organic frameworks
    (2019) Gcwensa, Nolwazi; Oliver, Clive
    The syntheses of four novel mixed-ligand metal-organic frameworks (MOFs) are reported. Isoreticular, Zn(II)-based mixed-ligand MOFs with formulae [Zn(μ2-ia)(μ2-bpe)]n·nDMF (1) and [Zn(μ2-mia)(μ2- bpe)]n·nDMF (2), where ia = isophthalate, mia = 5-methoxyisophthalate, bpe = 1,2-bis(4-pyridyl)ethane and DMF = N,N’-dimethylformamide were synthesised and characterised. Both compounds 1 and 2 exhibit sql, 2-periodic, 2D net coordination layers. Catenation of neighbouring frameworks form 2-fold interpenetrated bilayers which are interdigitated resulting in channel voids containing DMF. Experimental void calculations indicate 2′ has larger void space per unit cell than 1′; however, experimentally, 1′ showed higher water vapour and carbon dioxide 195 K sorption as well as significant hysteresis upon desorption of carbon dioxide 195 K. This hysteresis behaviour of 1′ is interchanged with 2′ for water vapour sorption at 298 K. Sorption isotherm inflection points indicate that structural changes occur, and empirical evidence point to weak bilayer···bilayer interactions in 1′ which allow the separation of the bilayers as well as the limiting effect on structural changes of the methoxy group present in 2′. Isoreticular mixed-ligand Cd(II)-based MOFs with formulae [Cd(μ2-mia)(μ2-bpe)1.5]n·n(DMF)0.5n(H2O)0.5 (3) and [Cd(μ2-nia)(μ2-bpee)1.5]n·nDMF (4), where nia = 5-nitroisophthalate and bpee = 1,2-bis(4-pyridyl)ethylene were also synthesised and characterised. Both compounds 3 and 4 exhibit sql, 2-periodic, 3D net coordination layers with disorder around a single bpe or bpee ligand. These structures are compared to published structure [Cd(bpee)1.5(nbdc)]n·nDMF (JECRAN) which is isoreticular to both MOFs. Activation of 4 and JECRAN occurs via single-crystal-to-single-crystal transformations. Potential and actual void space calculations indicate that 4′ has a larger void space than 3′ and JECROB. Liquid sorption experiments revealed that 3′ and 4′ showed affinities for different solvents. Although carbon dioxide 195 K sorption for 4′ is initially higher than for JECROB, structural changes, indicated by sorption isotherm inflection points, allow JECROB to adsorb more carbon dioxide than 4′