Kapil, Nidhi; (2021) Supported gold catalyst materials with locally controlled chemical and geometric environments for enhanced propylene epoxidation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The conversion of propylene to propylene epoxide (PO) is a reaction of great industrial importance, as PO is used to synthesise polyurethane foams, which finds further applications in manufacturing cosmetics, adhesives, paints, etc. Currently, PO is commercially produced using the chlorohydrin process or the hydroperoxide process, which has many limitations and drawbacks. Ever since Haruta and Hutchings discovered the catalytic activity of gold nanoparticles, the direct gas phase epoxidation of propylene to form PO using H2 and O2 has gained attention as a simple, environmentally benign and less toxic route, using gold supported on titanium (Ti)-containing catalysts.1 Although direct gas phase epoxidation is the most favourable route, there is no economically viable catalyst available for industrial applications. Long term stability and high hydrogen efficiency, along with the high PO selectivity, are the critical parameters for the catalyst to be considered for commercialisation.2 Therefore, understanding and influencing the reaction mechanism, and designing a stable yet selective catalyst are topics of paramount importance both for academic research as well as industrial applications. The overarching goal of this thesis to design a stable, supported gold-based catalyst for the direct gas phase epoxidation of propylene to PO. In order to achieve that, this project takes a multiscale approach, integrating nano- (active site), meso- (porous catalyst architecture) and macroscale (reactor) efforts, which address the challenge of producing an improved propylene oxidation catalyst. On the nanoscale, small gold clusters in a range of sizes from 11-atom clusters to nanoparticles with a diameter of around 10 nm are successfully synthesised using a one pot methodology. Different organic bound ligands are used to influence the electronic and geometric properties of the gold nanoparticle’s surface. The successful synthesis of gold nanoparticles is also demonstrated in a microfluidic system, yielding similar results to the batch process. These nanoparticles are characterised using UV/Vis spectroscopy and high resolution transmission electron microscopy (HRTEM). In addition to the one pot methodology, an electro-spraying technique is also developed to synthesise gold and silver nanoparticles with tunable particle size. This technique is also extended to synthesise bimetallic gold-palladium nanoparticles. On the mesoscale, supports such as titanium silicalite (TS-1), mesoporous TS-1 (m-TS-1), ordered mesoporous silica with controlled surface curvature (SBA-15, MCM-41), and titanium grafted onto SBA-15 and MCM-41 have been prepared with desirable textural and physico-chemical properties for use in comparative studies. The pre-synthesised gold clusters and nanoparticles are successfully immobilised onto various metal oxide supports, such as titanium dioxide (TiO2), m-TS-1, TS-1, SBA-15 and MCM-41 without any change in the final size of the dispersed gold clusters. The strategy of nano-confinement and direct immobilisation is used to obtain these supported gold cluster catalysts. Different pre-treatment methods are employed to affect the interfacial sites between the metal nanoparticle and its support. Additionally, gold and silver nanoparticles synthesised using the electro-spraying technique are deposited onto carbonaceous support for electrochemical applications, further demonstrating the versatility and feasibility of the method. Multiple characterisation tools, such as DR-UV/Vis spectroscopy, HRTEM, SEM, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), 31P nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and N2 physisorption are employed for the characterisation of these catalysts. On the macroscale, an epoxidation reactor unit is designed and built in our lab. The catalyst performance, including long term catalyst stability, activity and selectivity, are evaluated and presented in detail. In summary, this study will aid in the design of a stable and selective catalyst for propylene epoxidation by modifying the environment at and around the active sites through the choice of support and through bulky organic ligands. These modifications aim to enhance the selectivity, stability, and activity of the catalyst in propylene epoxidation. These studies further suggest that catalyst performance is a function of the size of gold nanoparticles, choice of support, preparation methodology and pre-treatment routes.

Type:	Thesis (Doctoral)
Qualification:	Ph.D
Title:	Supported gold catalyst materials with locally controlled chemical and geometric environments for enhanced propylene epoxidation
Event:	UCL (University College London)
Language:	English
UCL classification:	UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Chemical Engineering
URI:	https://discovery.ucl.ac.uk/id/eprint/10136278

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