Design of heterogeneous catalysts for the utilisation of carbon dioxide and tandem reactions

Abstract

The adoption of green technologies and principles by the chemical industry, and chemistry as a whole, is urgently required to help mitigate and ultimately reverse the alarming trends in global greenhouse gas emissions. Green processes such as carbon dioxide utilisation enable the incorporation of a waste material in the production of valuable synthetic chemicals. The catalysed reaction of CO2 with epoxides is a particularly attractive example as the cyclic carbonate product has wide-ranging commercial applications. Recyclable, heterogeneous catalysts are at the forefront of research efforts to produce cyclic carbonates in a sustainable manner.

Through the development of an imidazole based organocatalyst covalently bound via pendant silanols to mesoporous silica, the efficacy and principles of heterogenised imidazoliums as catalysts for cyclic carbonates production was established. A facile anchoring strategy allowed for manipulation of the active site, which, with detailed physico-chemical and spectroscopic characterisation, established the catalytic mechanism, allowed for progressive optimisation and improved catalytic application.

The robustness of the anchoring method and the excellent activity of the functionalised imidazole catalysts led to their incorporation into supports of greater complexity. By employing analogous immobilisation methods, an imidazolium species was anchored to hierarchically porous aluminophosphates and silicoaluminophosphates where careful tuning of the framework properties improved the reaction selectivity towards the cyclic carbonate product.

Further heterogenisation of functionalised imidazoles onto the coordinatively unsaturated sites within the metal organic framework (MOF), MIL-101(Cr), resulted in a hybrid MOF with exceptional activity for the reaction of CO2 and epoxides, under halide-free and solvent-free conditions. Comprehensive characterisation, in combination with molecular modelling simulations, allowed for structure-property relationships to be established, enabling optimisation to afford high cyclic carbonate yields whilst employing short reaction times, relatively benign conditions and low catalyst loadings.

To further illustrate the scope of hybrid MOFs for sustainable applications the robust MOF, UiO-66(Zr), was exploited in a tandem catalytic reaction. Colloidal deposition of gold nanoparticles onto UiO-66-NH2 framework enabled successive alcohol oxidation-Knoevenagel condensation reactions to be achieved at good yields, for a range of substrates. Furthermore the extension of the capabilities of the aforementioned MIL-101(Cr) framework to a different, epoxidation-CO2 utilisation cascade reaction demonstrated the vast scope of applications achievable with hybrid MOFs for green and sustainable catalytic processes.

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Identifiers

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

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