Intrinsically Microporous Polymers in Heterogeneous Redoxcatalysis

  • Lina Wang

Student thesis: Doctoral ThesisPhD

Abstract

Microporous composite materials have been studied for a considerable range of heterogeneous catalysis applications due to their large inner specific area and the potential of adsorbing small species into their microporous structure. Though prominent types of microporous crystalline hosts, zeolites and metal frameworks (MOFs), for example, are advantageous to many applications, they are not processable. Polymers of intrinsic microporosity (PIMs) have emerged as a new class of microporous materials which are molecularly rigid structures with poor packing and a high degree of open porosity at the nanometre scale. Characteristics of good solubility, processability (into a film by drop-casting or spin-coating), thermal and mechanical stability, and gas and ion species permeability enable PIMs to be applied in fields of electrochemistry. In this thesis, the opportunities for applications of PIMs in heterogeneous redoxcatalysis are investigated.

Redoxcatalytic reactions within microporous composites involve several interrelated processes, including diffusion and permeation of reactants and products, adsorption and desorption of active species, binding and decomposition of substances coupled with electron transfer processes, and on some occasions, involvement of non-reactive solvents. The whole reaction can be complex; however, simple redox systems based on PIM composites can be explored individually as a starting point. Examples are the binding behaviour and reactivity of ions (Fe(CN)63-/4-) and molecules (catechin and quercetin) inside the PIM host. More complicated systems (H2 and H2O2 generation from formic acid and O2) are studied next. Practical nano-catalysts (Pd, bipolar Pd/Au, and PdAu mixture) immobilised in PIMs for redoxcatalysis with gases, ions, and molecules are investigated.

In this thesis, the main aims are (i) to develop an understanding of the role of PIMs as host for heterogeneous catalysts for redoxcatalysis, (ii) to explore the mechanisms of reactions occurring within PIM composites, and (iii) to optimise and extend the applications of PIMs for useful product generation (H2 or H2O2 for example). The mechanism is investigated primarily with voltammetry to distinguish potential-dependent processes. Analysis methods are developed for quantification of products or species (catechin, quercetin, and H2O2) based on liquid chromatography coupled with mass spectrometry (LC-MS), for evaluation of O2 generation and H2 production based on a Clark-type sensor, and for colourimetric assay of H2O2 generation based on a sensor dye 3,5,3’,5’-tetramethylbenzidine (TMB). A computational DFT simulation is carried out for verification of PIMs as an active component during redoxcatalysis.

The electrochemical and binding behaviour of redox species is investigated by immobilisation of Fe(CN)63-/4- or ortho-quinol molecules, catechin and quercetin, into PIM-EA-TB. The interaction between Fe(CN)63-/4- and protonated amine sites in PIM-EA-TB is proposed to be linked to the retention and relatively slow leaching process. The PIM-EA-TB with immobilised Fe(CN)63-/4- is applied for the electrocatalytic oxidation of ascorbic acid. Hydrogen bonding is suggested to be responsible for ortho-quinol molecules immobilisation in the PIM-EA-TB host. A thin mono-layer film with one catechin or quercetin molecule binding to each monomeric PIM-EA-TB unit configuration is electrochemically active. Either spontaneous or electrochemically triggered leaching of guest ortho-quinol molecules is shown to depend on the state of the films.

Palladium nano-catalysts are incorporated in PIM-EA-TB. Competing processes of H2O2, water, and H2 production from formic acid and O2 are investigated for the Pd@PIM-EA-TB catalyst. Next, gold is electrolessly deposited and attached to the Pd in PIM-EA-TB. The Pd/Au@PIM-EA-TB product gives enhanced H2O2 production with suppressed H2 generation. It is suggested that Pd and Au work hand-in-hand as bipolar electrocatalysts. The Pd@PIM-EA-TB catalyst is also evaluated for formate oxidase reactivity as a nanozyme to produce H2O2. This, in turn, drives the TMB colour reaction in the presence of formic acid. A computational DFT simulation proves that, as an active component, PIM-EA-TB enhances both 2-electron formate oxidation and 2-electron oxygen reduction to H2O2 on palladium.

Furthermore, the reactivity of nano-Pd, -Au, and -PdAu mixture towards formic acid oxidation and oxygen reduction during H2O2 production is investigated and compared in two microporous polymer hosts (PIM-1 and PIM-EA-TB). PdAu@PIM-EA-TB shows the best reactivity towards H2O2 production. In contrast to PIM-1, PIM-EA-TB is proposed to directly contribute to the catalytic process with amine sites accepting protons during formate oxidation and ammonium sites donating protons for oxygen reduction.

Overall, this thesis highlights the reactivities and applications of PIMs as part of microporous composites in heterogeneous redoxcatalysis. The role of PIMs in the catalytic processes is investigated. New hypotheses are proposed for the mechanisms of the catalytic reactions studied in this thesis, and novel experimental approaches are developed to verify them.
Date of Award22 Feb 2023
Original languageEnglish
Awarding Institution
  • University of Bath
SponsorsChina Scholarship Council
SupervisorFrank Marken (Supervisor) & Adam Squires (Supervisor)

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