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Catalysis at microstructured gas-liquid-solid interfaces

Wang, Kang 2024. Catalysis at microstructured gas-liquid-solid interfaces. PhD Thesis, Cardiff University.
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Abstract

Gas-liquid-solid (G-L-S) catalytic reactions are widespread in the chemical industries and in environmental chemistry. Such as hydrogenation, aerobic oxidation, hydro-refining, and ethynylation. Very often, the reaction rates are affected by the low gas solubility in liquids and mass/heat transfer resistances of reactants/products to/from the catalyst surface due to the physical separation of phases and the low G-L and L-S specific interface areas. In practice, high temperatures and pressures are often required to promote the G-L-S contact that impact negatively the energy efficiency, environmental footprint and safety of reactors. To overcome these drawbacks, this thesis will implement microstructured G-L-S reactors by designing smart particle-stabilised foams. Our starting hypothesis is that particle-stabilised foams can operate as a microreactor, increasing the G-L-S contact and allowing in principle potential enhancement of the catalytic activity at near-ambient pressures. In this thesis, we first designed a dual-particle system combining fluorinated organosilica particles and a novel type of fluorinated polyhedral oligomeric silsesquioxane (POSS) with isobutyl cage substituents allowing the genesis of stable foams in pure ethanol among other solvents with very low surface tension (γ <25 mN·m-1 at 20 °C). The interaction between organosilica and POSS particles was investigated in detail by combining contact angle, dynamic surface tension, and dynamic light scattering methods, and catalytic tests to ascertain the underlying mechanism promoting particle adsorption at the ethanol-air interface. Next, we synthesised amphiphilic silica Janus particles (JPs) with oleophilic and oleophobic (fluorinated, <8 wt% F) hemispheres and selective spatial location of Pd nanoparticles for generating oil foams based on the benzyl alcohol (BnOH)-xylene/O2 system at low particle concentration (1 wt%). The JPs exhibited an enhanced catalytic activity compared to randomly functionalised particles in the aerobic oxidation of BnOH due to the high gas-oil miscibility near the Pd sites in the presence of foam at 1 bar O2 pressure. The foamability of JPs exceeded systematically that of randomly functionalised particles for a mixture of aromatic alcohols and o-xylene. The JPs could be conveniently recycled after reaction and reused without loss of catalytic performance and foamability. II Finally, we designed a multiphase microreactor system for the photosynthesis of H2O2 from H2O and O2 in aqueous foam. We used commercial TiO2 (P25) as supporting material, onto which 1 wt% of Au nanoparticles were deposited. To adjust the surface properties, different densities of phosphonic acids were grafted on TiO2, leading to tuneable hydrophobicity and gas accessibility. The modified catalysts could generate foams in triethanolamine (TEA) aqueous solution and produce 3 times higher H2O2 concentration at room temperature and ambient O2 pressure when compared to control experiments on the parent TiO2 and Au@TiO2 without foam. Keywords: Particle-stabilised foam, gas-liquid-solid microreactor, interfacial catalysis, fluorinated material, H2O2 photosynthesis

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 13 November 2024
Last Modified: 15 Nov 2024 10:38
URI: https://orca.cardiff.ac.uk/id/eprint/173929

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