The design and use of heterogeneous and homogeneous catalysts for the activation of small molecules

This thesis presents research conducted on the activation of small molecules. The traditional gases are considered by reacting them with sterically hindered metal complexes. Additionally, the complete oxidation of siloxanes, and partial oxidation of methane, is investigated using photoactive titania. The work approaches the field from both a heterogeneous and homogeneous viewpoint, focusing on nanophotocatalysts, whilst also investigating the design and synthesis of possible model molecular systems. Siloxanes are present in biogas, posing a risk to equipment that uses biogas as an energy source. The study on their removal using photocatalytic titania nanofibres is presented, both in the laboratory and in an industrial setting. Removal is achieved by complete oxidation of the siloxanes to silica, reducing their vapour pressure and thereby effectively removing them from the gas stream. The synthesis of titania nanofibres was designed and developed, using an adapted novel electrospinning method. The catalytic performance of the nanocatalysts was evaluated for the removal of siloxane. Doping of the nanofibres with silver, tungsten and cobalt is also reported, with 1~WO3 doping showing a three-fold increase in activity, compared to pristine titania nanofibres. Titania, as an efficient photocatalyst, is then investigated to determine its ability to partially oxidise methane, to form a more valuable feedstock chemical. Tungsten trioxide, iron(III) oxide and uranium dioxide are also investigated as alternative photocatalysts. Conversion of methane was monitored by GC-MS and FTIR. Titania showed the most activity, generating carbon dioxide. However, no partial oxidation products were detected. The use of perfluorocarbons as a phase is also investigated, in an attempt to improve solubility and selectivity. These experiments demonstrated the photocatalytic oxidation of ethanol to acetaldehyde. In order to tailor the electronic property and catalytic activity, molecular catalysts were also synthesised. Consequently, the use of triptycene as a ligand for transition metals was studied for the first time. The synthesis, isolation and full characterisation of a novel triptycene precursor is reported, including the single crystal X-ray structure. This precursor is ligated to a number of metal centres, including iron, cobalt and chromium for the first time. Many of the synthesised species are new examples of the rare low-spin state in a tetrahedral geometry. Of these novel complexes, the cobalt triptycene species was shown to successfully activate dioxygen. The catalytic performance of molecular catalysts was also investigated in the activation of oxygen and water. New molecular mimics of a previously reported heterogeneous cobalt oxide catalyst have been synthesised, using a number of phosphate ligands with a cobalt metal centre. The resulting complexes are fully characterised, with a single crystal X-ray structure reported. In-depth analysis of the complexes is reported, including electronic absorption, variable temperature magnetic studies and cyclic voltammetry.