Polymer immobilised ionic liquid phase :designing new tools for catalysis

Abstract

Ionic liquids (ILs) have been paid particular interest in the field of catalysis over the past 2 decades, offering not only enhancements in catalyst performance but also the ability to heterogenise molecular species and effectively combine the benefits homogenous and heterogeneous catalysis. Although advantageous, significant limitations remain, particularly with respects to leaching and mass transport. As a result the heterogenisation of IL moieties, in the form of the Supported Ionic Liquid Phase (SILP), has been widely investigated to avoid these issues. By extension of the SILP concept, polymer materials can offer additional benefits through the well-defined, controllable nature of polymer chemistry. The work presented herein focuses on the application of pre-fabricated ionic liquid tagged monomers to prepare well-defined polymer supports under mild, controllable conditions, termed the Polymer Immobilised Ionic Liquid Phase (PIILP). These well-defined materials were then used to support various homogeneous catalytic species as a means to give highly active, recyclable systems which effectively combine the positive aspects of homogeneous and heterogeneous catalysis with the enhancement effects associated with ionic liquids. Chapter 1 discusses the use of ionic liquids, SILP and PIILP-type systems in catalysis across the literature, before briefly assessing common polymerisation methods in order to determine the most suitable means to prepare PIILP materials. The initial evaluation of the PIILP methodology is discussed in chapter 2, wherein a linear pyrrolidinium-functionalised support was prepared by ring-opening metathesis polymerisation (ROMP). The resulting material was used to immobilise a peroxophosphotungstate species to give a heterogeneous catalyst which was highly active in epoxidation, alcohol oxidation and sulfoxidation reactions under relatively mild conditions with H2O2 as the oxidant. Chapter 3 discusses the implementation of the PIILP-catalysed sulfoxidation chemistry under continuous flow conditions, with the promising performance highlighting the potential application of PIILP in the preparation of pharmaceutical intermediates and in the purification of crude oil. In chapter 4 cross-linked polystyrene-based PIILP materials are shown to be effective supports in asymmetric Diels-Alder and Mukaiyama-Aldol reactions, with PIILP supports giving enhancements in ee compared to analogous IL and SILP systems. Similarly, in chapter 5 polystyrene-based supports are effectively used to immobilise Pd nanoparticles whilst also investigating the effect of heteroatom donating co-monomers. Although moderately promising catalytic performance was observed in a range of Suzuki Miyaura couplings, no clear trends with regards to nanoparticle size and catalyst performance were noted. The results obtained throughout this thesis highlight the vast potential of the PIILP methodology in heterogeneous catalysis, however the incredibly complex nature of the relationship between the catalyst, support and substrate makes it difficult to fully rationalise performance trends. As such, further, more extensive studies into the support properties will be required to realise PIILP to its fullest potential.