Influencing Metal-organic Framework Catalysis through Nanoscale Structuralisation

Abstract

Metal-organic Frameworks (MOFs) are a class of porous crystalline materials formed from the self-assembly of organic and inorganic components. Due to the modular nature of their synthesis and reversibility of metal-ligand bonding, these materials can be tailored at the nanoscale and functionalised to suit specific applications including heterogeneous catalysis. However, in order to apply MOFs on an industrial scale they must be incorporated within existing technologies and to do so they must be synthesised with defined particle sizes with precise control over their nanoscale structure (i.e. size, morphology, and surface chemistry). The work presented in this thesis investigates the influence of nanoscale structuralisation on MOF catalysis. The first chapter introduces MOFs as versatile materials for heterogeneous catalysis and discusses how their nanoscale structure can be altered to enable better integration within existing technologies. New materials phenomena have arisen from alterations to the particle size, morphology, and surface chemistry of MOFs, indicating the potential for the optimisation of MOF catalysts through nanoscale structuralisation. Chapter 2 presents a systematic investigation of the influence of particle size and morphology on surface catalysis of a zinc-based MOF, Zeolitic Imidazolate Framework 8 (ZIF- 8). Herein, ZIF-8 was synthesised at discrete particle sizes (50 nm to 100 μm) with three distinct crystal morphologies (rhombic dodecahedral, truncated rhombic dodecahedral and cubic) and the surface catalysed transesterification of hexanol with vinyl acetate was investigated. Work in Chapter 3 explored the impact of surface to volume ratios on the activity and reaction selectivity of a rhodium(I) homogeneous catalyst site isolated post synthetically at vacant chelating sites within the zirconium framework UiO-67-bpydc (UiO = University of Oslo, bpydc = 2,2’-bipyridyl-5,5’-dicarboxylic acid). The particle size and ligand loading of UiO-67-bpydc was controlled through crystal engineering techniques of coordination modulation and mixed ligand synthesis, resulting in discrete particle sizes of 100 nm, 1 μm and 10 μm, and 0-100% bpydc incorporation. Particle size and pore confinement effects were shown to influence the reaction selectivity (hydroformylation/hydrogenation) and activity of the samples post-synthetically metalated with [Rh(COD)(acetone)₂]BF₄. Chapter 4 extends the investigations initiated in Chapter 3 to gas phase catalysis of ethylene oligomerisation within a core-shell framework post-synthetically metalated with [PdMe(MeCN)]BF₄. Taking advantage of the reversibility of metal-ligand bonds within MOFs, diffuse core-shell framework composites were synthesised via slow diffusion solvent-assistedligand- exchange. The activity and selectivity of core-shell catalysts with active sites localised at the interior, exterior, and throughout framework were subsequently studied. Finally, in Chapter 5, the attempted isolation of reactive intermediates was investigated with an isostructural zirconium-based framework, UiO-67-Me₂bpydc (Me₂bpydc = 6,6’- dimethyl-(2,2’-bipyridine)-5,5’-dicarboxylic acid). Using protocols developed in preceding chapters, the crystal engineering techniques of coordination modulation, modulator exchange, and “crystal healing” large X-ray quality crystals of UiO-67-Me₂bpydc were synthesised and an X-ray crystal structure could be obtained. Preliminary experiments demonstrated that small tetrahedral transition metal complexes, such as those formed when CoCl₂ and ZnCl₂ coordinate at the 2,2’-bipyridine site, could be isolated within the framework and their structures determined via single crystal X-ray diffraction (SCXRD). In this manner UiO-67-Me₂bpydc was used as a crystalline matrix to isolate transition metal complexes.