Carbon capturing for sustainable polymer production: The first heterogeneous MOF catalyst for CO2 copolymerisation

Abstract

As environmental concerns over greenhouse emissions increase, the fixation and utilisationof carbon dioxide (CO2) remains a pivotal area of research. A widely-reported method forCO2 utilisation is the catalytic copolymerisation with epoxides to form poly(alkylenecarbonates). Formation of poly(propylene carbonate) (PPC) from propylene oxide (PO) isprevalent in catalysis as the final polymer is biodegradable and CO2 can contribute up to43% of the polymer weight.1 Currently, sustainable alternatives are being investigated withhomogeneous catalysts, with chromium(III)- or cobalt(III)- salens2 and Zn-b-diiminatecomplexes3 among the most active, however, scarce few examples of recyclable, easilyseparated heterogeneous systems, with high polycarbonate selectivity and catalyticturnover, exist within the literature. Due to their synthetic versatility, porosity and innate CO2sorption capabilities, MOFs have significant potential for use as CO2 utilisation catalysts, yetthey have only been shown to form the thermodynamically favoured cyclic carbonateproduct, with no examples to date of copolymerisation occurring, to the best of ourknowledge.Through the determination of structure-activity correlations drawn from literature sources andour own work, a suitable MOF was identified, synthesised and successfully shown to provideexceptional catalytic activity for the copolymerisation reaction. Herein, we report the firstexample of a highly selective, co-catalyst free, metal-organic framework (MOF), capable ofcatalysing the copolymerisation between CO2 and propylene oxide. The MOF catalystfacilitates >90% conversion in dramatically reduced timescales compared to those requiredfor widely used zinc glutarate catalysts, under milder reaction conditions, to produce PPCwith high CO2 insertion into the polymer backbone with a catalytic turn-over frequency inexcess of 1000 hr-1. In-situ MAS-NMR, XAS and FT-IR characterisation, supported by DFTmodelling, have been used to bring insight to the observed polymer selectivity and thebinding interaction between the framework and the CO2/PO substrates.The discovery of this MOF for this copolymerisation demonstrates the versatility of MOFcatalysts and provides a promising development in the ever-essential field of carbon captureutilisation technology.
The project leading to these results has received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under grant agreement N. 720783 - MULTI2HYCAT

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Identifiers

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

ORCID for Marina Carravetta: orcid.org/0000-0002-6296-2104

ORCID for Matthew Potter: orcid.org/0000-0001-9849-3306

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

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