A spectroscopic investigation of the para-xylene oxidation catalyst system

Taylor, Rebekah 2025. A spectroscopic investigation of the para-xylene oxidation catalyst system. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Oxidation of para-xylene to terephthalic acid is one of the world’s largest industrial-scale processes, utilising a homogeneous catalytic mixture of Co/Mn/Br ions in an aqueous acetic acid solvent. Whilst this catalytic process has been extremely well-optimised over the past 70+ years of continuous operation, many unknowns remain on how the catalyst functions, both from a mechanistic understanding and a structural perspective. As the catalyst is primarily paramagnetic in nature, the aims of this Thesis are to apply Electron Paramagnetic Resonance (EPR) spectroscopy to study the catalytic system. The specific focus of the work was to investigate the previously unknown or poorly understood coordination spheres around the paramagnetic and catalytically active Co2+ and Mn2+ centres in the acidic solvent conditions. The initial investigations in this Thesis examined the influence of the highly sensitive H2O/AcOH solvent ratio on the Co2+ and Mn2+ coordination environments in the catalyst. A complex distribution of Co2+ and Mn2+ species was found across the composition range of 0 - 20 wt% H2O/AcOH. The differing speciation of Co2+/Mn2+ centres that were identified were notably separated into low- (≤ 8 wt% H2O) and high- (≥ 10 wt% H2O) water content regimes. This was primarily evidenced by continuous wave (CW) EPR studies through Mn2+ linewidth (lwpp) analysis. The origin of the lwpp changes was found to originate from the different extents of H2O coordination within these different regimes. UV-vis spectroscopy also revealed an increasing amount of Br coordination to the Co2+ centres (forming [CoBrxLn-x] y species) in the low-water regime. As the H2O content was decreased, the concentration of these [CoBrxLn-x] y species increased in turn. Crucially, it was shown that increasing Br ion coordination arises from the decreasing solution dielectric permittivity, as confirmed by variable temperature UV-vis and microwave dielectric spectroscopies. In the absence of Br coordination in the high-water regime, Electron Spin Echo Envelope Modulation (ESEEM) revealed that the Mn2+ centres were completely solvated. An ESEEM “water counting” method was subsequently employed to quantify the degree of hydration around the Mn2+ centres under different solvent conditions. Although this ESEEM approach has been well-established and utilised under physiological conditions, it has not IV been previously deployed to study catalytically relevant paramagnetic species under acidic conditions. A thorough consideration and adaptation of the experimental conditions was therefore required to ensure the robustness of this approach. The ESEEM results revealed that 0, 0.7, 1 and 4 H2O ligands coordinate to the Mn2+ ion at the solvent compositions of 3, 8, 13.7 and 20 wt% H2O/AcOH respectively. These key solvent compositions used in this study were based on the earlier CW EPR linewidth studies presented in Chapter 4. Finally, the effect of altering the Br/metal ion ratio on the catalyst coordination was also investigated (by UV-vis, EPR and ESEEM measurements). UV-vis illustrated the close relationship between H2O content and Br coordination, showing how varying their ratio can control the [CoBrxLn-x] y concentrations. EPR linewidth analysis showed results consistent with outer-sphere Mn-Br interaction in the high-water content regime, whilst the changing resonance intensities indicated a direct Mn-Br interaction in the low-water regime, consistent with the UV-vis observations of the Co2+ centres. Formation of [MnBrxL6-x] y species under extreme solvent conditions (0 wt% H2O and 10 eq. Br- ) did not contribute to this, instead microwave power saturation studies indicated a change in relaxation properties with increasing Mn-Br interaction (both outer and inner-sphere). Overall, the results presented in this Thesis have offered an original experimental insight into the coordination sphere of the Co2+ and Mn2+ centres in the para-xylene oxidation catalyst. Whilst it was known for many years that careful control of the H2O/AcOH composition or Br/metal ion ratio could in turn control reaction performance by altering the environment around the ions, the manner and nature of this coordination remained elusive. EPR spectroscopy and complimentary UV-vis studies have started to reveal some of these hidden secrets in the coordination chemistry of the Co/Mn/Br catalyst for the first time.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Chemistry
Funders:	EPSRC
Date of First Compliant Deposit:	16 May 2025
Last Modified:	16 May 2025 14:48
URI:	https://orca.cardiff.ac.uk/id/eprint/178330

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