Well-defined Metal-Organic Framework (MOF) based catalysts for the direct utilisation of CO2

Abstract

Given current environmental concerns over greenhouse emissions, the need for technologies capable of the fixation and utilisation of carbon dioxide (CO2) remains a pivotal area of research. Despite this requirement, there remains a distinct shortage in research which adequately utilises CO2 as a direct C1 feedstock material. One widelyreported method, the catalytic ring-opening copolymerisation (ROCOP) of epoxides and CO2 to form poly(alkylene) carbonates, provides a promising and more sustainable route to polyurethanes, a material that represents a billion USD industry and finds ubiquitous usage across many commercial sectors. Providing an alternative to the traditional ringopening polymerisations (ROP) of cyclic carbonate monomers or toxic polycondensations with trans-diols and phosgene, the ROCOP pathway not only offers a safer, more sustainable route to polycarbonates, but directly consumes CO2 and reduces our dependence on petrochemical feedstocks.

To date, this process has been catalysed predominantly by homogeneous metalcomplexes, with scarce few examples of heterogeneous systems existing within the literature. Motivated by the distinct lack of heterogeneous counterparts, the development of suitable MOF-based catalysts has been established. Through the analysis of the current homo- and heterogeneous catalysts, structure-activity correlations pertaining to their Abstract ii activity towards specific stages of the copolymerisation mechanism have been realised, providing a catalyst design rationale on which this thesis is built. Using these correlations, a suitable MOF - MIL-100(Sc) - was synthesised, characterised and shown to provide unprecedented activity towards the copolymerisation under far milder conditions than those previously reported. To rationalise the source of the activity, in situ and operando characterisation techniques, in hand with computational calculations, have been utilised to reveal the nature of the two active sites within the MIL-100 architecture. Unambiguous evidence concerning the role of each site in the activation of CO2 and the epoxide is presented; evidence that has eluded the literature of copolymerisation catalysts.

Utilising these findings, further development of the catalyst was provided in the form of a surface-engineering protocol, successfully optimising active site populations at the surface of the MOFs through synthetic manipulation with monocarboxylic modulators, and through a mixed-metal strategy in which isomorphous substitution of the constituent metals was achieved. Through these strategies, catalysts capable of far superior catalytic activity compared to the current commercial catalyst was achieved. Through a range of complimentary characterisation techniques, this activity has been attributed to fundamental catalysis concepts, highlighting the importance of purposeful catalyst design and the synergy between the catalyst morphology and the active site.

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Identifiers

ORCID for Daniel J. Stewart: orcid.org/0000-0003-3409-6517

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

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