Venezia, Baldassarre; (2022) Reactor Designs for Safe and Intensified Hydrogenations and Oxidations: From Micro- to Membrane Reactors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The current and pressing environmental challenges are leading towards an important paradigm shift within the chemical industry. Green chemistry can be performed by using selective catalysts, and renewable and environment-friendly feedstock. For this reason, it is essential to provide scientists with platform tools that can allow safe and reliable catalyst testing, screening and studies. At the same time, as the use of green feedstock, such as oxygen and hydrogen, can pose new hazards, the design of intensified reactors can represent an unmissable opportunity to drive this green shift within the safe and scalable production of valuable molecules. This thesis reports reactor design solutions of different scale that have been devised to guarantee safe and intensified catalytic hydrogenations and oxidations for catalyst testing and continuous production purposes. Starting with the aim of studying a catalyst under realistic operating conditions, a silicon microfabricated reactor was designed and tested for the gas phase combustion of methane and carbon monoxide over palladium and platinum catalysts. Owing to its small volume and to its isothermal temperature profile, this microreactor proved to be a safe and effective tool for performing information-rich experiments, while exhibiting a plug-flow behaviour with negligible external and internal mass transfer resistances. Reactions were performed in combination with X-ray absorption and IR spectroscopy, allowed by the detailed microfabrication reactor design, to investigate the catalyst structure-activity relationships in steady-state and transient experiments. Boosting the catalyst activity can be achieved using catalytic nanoparticles, which offer an increased surface area compared to their bulk equivalents and hence an improved reaction rate. However, accessibility of the reactants to supported nanoparticles can be limited by the diffusion phenomena occurring around and inside a catalyst support. A recent trend of supporting nanoparticles onto surfaces modified using polyelectrolyte assemblies has attracted attention owing to the low temperature, ease and environmentally friendly preparation process. Finely tuned ex situ synthesised palladium nanoparticles were adsorbed on the inner surface of a tubular Teflon AF-2400 membrane, which was modified with polyelectrolytes in a layer-by-layer configuration. The membrane was used as a tubular reactor inside an outer tube with pressurised hydrogen, and nanoparticles of different size and shape were tested in the continuous hydrogenation of nitrobenzene to aniline. The observed reactivity depended on the different nanoparticle size and on the palladium oxidation state. The use of a tube-in-tube membrane reactor ensured process safety owing to the small volume of gas stored in the tube annular section and to the continuous processing. Alcohol oxidations using molecular oxygen can be dangerous due to the risk of creating explosive mixtures with the organic substrate. Two slurry loop reactors were developed using the same Teflon AF-2400 membrane in different configurations: a tube-in-tube and a flat membrane configuration for scalable reactions. These were designed and tested to carry out safe aerobic oxidation of alcohols. The membrane separated the oxygen from the organic phase and allowed a controlled dosing of the gaseous reactant. In order to boost the turnover frequency, the catalyst was used in the form of a slurry which was recirculated in a loop where it contacted the membrane saturator and a crossflow filter. This allowed the withdrawal of the liquid products from the loop. The reactors could be operated continuously, and provided improved process safety and comparable catalyst turnover frequency to conventional batch processes. When scaling up reactors, inadequate mixing can occur impacting on process safety and product quality. A Taylor-vortex membrane reactor is presented for the first time, combining the benefits of a flexible baffle structure inside a Taylor-vortex system that can hinder axial dispersion, and a supported tubular membrane for safe gas-liquid reactions. Stable conversion and product selectivity were achieved in the homogeneously catalysed continuous aerobic oxidation of benzyl alcohol. No pervaporation of organics through the membrane was detected during reaction, making this reactor a safe and a scalable tool for continuous gas-liquid reactions.

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