Very few commercial processes employ Au catalysts for the production of fine or commodity chemicals. The recent validation of Au/C catalysts to produce vinyl chloride monomer (VCM) via the acetylene hydrochlorination reaction, as a replacement of the traditionally used highly volatile and toxic mercuric chloride catalyst, represents a notable exception. However, the active form of the catalyst and the reaction mechanism are still not fully understood. The work presented in this thesis aims to provide detailed information about the nature of the catalyst's active species and the possible reaction pathway, combining commonly used characterisation techniques with more challenging in situ experiments. The first part of this thesis (Chapter 3) aims to understand the influence of the choice of solvent and metal precursor during the catalyst preparation in the activity of the final material in order to identify the active state of Au during catalysis and to propose a reaction mechanism (Chapter 3). This investigation was carried out via a in situ X-ray absorption fine structure (XAFS) spectroscopy experiment. This study led to the conclusion that under reaction conditions highly active catalysts comprise single-site cationic Au species whose activity correlates with the ratio of Au(I)/Au(III) present, providing a new insight to the structure-function relationship of this reaction, while the mechanism has been hypothesised to proceed through the oxidative addition of HCl to Au chloride, followed by the insertion of acetylene and reductive elimination of VCM. The deactivation of gold on carbon catalysts during acetylene hydrochlorination has been attributed to two possible deactivation mechanisms: the formation of oligomers on the catalyst surface blocking the active site and the reduction of active cationic gold to inactive metallic Au. These two deactivation pathways have been shown to be influenced by both reaction temperature used and the detrimental effect of acetylene rich feeds. The second part of this thesis (Chapter 4) investigates the role of each reactant on the catalyst composition to further elucidate both the reaction and deactivation mechanism via an in situ gas switching experiment while recording the XAS spectra at the Au L3-edge. This study led to the hypothesis that the oxidative addition of HCl across the Au(I) chloride species requires the concerted addition with C2H2, in partial modification of the reaction mechanism proposed in chapter 3. An inelastic neutron scattering (INS) study of the catalyst exposed to C2H2 showed the formation of oligomeric acetylene species on the catalyst surface, which, upon re-introduction of both reactants, led to significant catalyst deactivation associated with the formation of metallic Au nanoparticles. The formation of Au(0) has been directly correlated with a decrease in VCM productivity when under reaction conditions also using an higher Au loading catalys. The recently validated Au/C catalyst by Johnson Matthey, prepared by using a sulphur containing Au complex, under industrial conditions, is more active and stable than the traditional Au/C catalyst made using hard donor ligands such as Cl. Clearly the choice of the ligand plays a major role in the final activity and stability of those catalysts. Chapter 5 reports in situ ligand K-edge XAS characterisation of gold on carbon catalyst for the hydrochlorination of acetylene to understand chlorine and sulphur speciation in the catalysts under operating conditions. In both catalytic systems, Cl is bounded directly to the gold and is directly involved in the reaction mechanism, re-affirming that AuClx speciation are active site for the acetylene hydrochlorination reaction.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:768071 |
| Date | January 2018 |
| Creators | Malta, Grazia |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/119545/ |
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