Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.805322
Title: Functional supports for heterogeneous reduction reactions subtitle : understanding the relationship between structure and performance
Author: Stones, Rebecca
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2019
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Abstract:
The work presented in this thesis describes the preparation and application of novel heterogeneous catalyst systems. Two avenues for the support/stabilisation of catalytically active transition metal nanoparticles (NPs) have been investigated: carbon nanoreactors and polymer immobilised ionic liquids. The effects of simple changes to the fabrication method on the resultant structure of these materials was investigated using a number of characterisation techniques and the efficiency of the support systems was investigated using exploratory chemical reactions to gain insight into the structure activity relationship. The first systems consist of hollow carbon nanostructures as supports for catalytically active metal NPs. A series of mono- and bimetallic ruthenium and nickel based nanoparticles were encapsulated in graphitic nanofibers (GNFs) using a versatile sublimation deposition approach. The effect of varying the metal loading and fabrication conditions, as well as the Ru:Ni ratio and addition sequence for bimetallic systems, was explored both in terms of structure and catalytic performance of the GNF supported catalysts. By examining the structural changes in the resultant materials this study aims to provide some understanding of how the composition of ruthenium and nickel based bimetallic nanoparticles supported within GNFs can be altered simply through changes in the fabrication method to tailor the activity of the catalyst materials for a given reaction. It was found that lower metal loadings result in a more well-defined structure with a more controlled distribution of MNPs. Additionally, the addition sequence employed during the fabrication of bimetallic systems can drastically alter the resultant structure of the material and therefore their catalytic performance. Exploratory hydrogenation reactions were employed to probe the catalytic performance, in terms of activity, selectivity and recyclability, of the mono- and bimetallic materials fabricated. It was found that the concerted addition fabrication method for bimetallic RuNiNPs afforded the only active material towards the reduction of nitrobenzene to aniline with the optimum ratio of Ru:Ni being 1:1 (Ru0.5Ni0.5NPs@GNFs (concerted addition) had the highest TOF of 24.1 ± 1.7 molAnmolM-1/h). The effects of nanoscale confinement in carbon nanoreactors has previously been shown to dramatically affect the activity, selectivity and stability of catalytic chemical transformations. In this study, the confinement effects imposed by the GNF support structure were explored using competitive reduction reactions, where the starting materials contain the same nitro functional group, but differing sizes, shapes and degrees of aromaticity, to gain further understanding of the interactions between reactant molecules and support structure by looking at which molecule preferentially reacts. These results demonstrate a general methodology for the controlled fabrication of active and robust, mixed MNP catalysts supported in carbon nanoreactors. The second systems consist of polymer supports, with differing functionalities, for catalytically relevant Pd and AuNPs. Modification of the polymer support structure in various ways (selective inclusion of various heteroatom donors, inclusion of an ionic liquid (IL) and inclusion of a polyethylene glycol unit (PEG)) were successfully achieved to enable the MNP stabilisation. The effect of varying the multifunctional support structure on the size of MNPs formed was investigated using transmission electron microscopy. Changing the components of the supported structure e.g. selective removal of the IL, had little effect on average MNP size. Numerous exploratory reactions were employed to probe the catalytic performance, in terms of activity, selectivity and recyclability of the polymer supported materials. These materials were found to achieve high activities/selectivities for a range of reactions (e.g. hydrogenation and Suzuki Miyaura cross-couplings) under mild reaction conditions with each component of the system plays a vital role in their efficiency; ionic liquid provides stabilisation of MNPs, polymer support prevents leaching of the ionic liquid during catalysis and affords easier recyclability, the heteroatom donor provides further stabilisation of the MNPs, and the polyethylene glycol unit improves the dispersibility of material in aqueous media. These results demonstrate the ability to tune the activity of a material based on the support system employed.
Supervisor: Chamberlain, Thomas Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.805322  DOI: Not available
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