Redox-Active Materials for CO Capture via Electroswing Sorption

Abstract

Carbon monoxide (CO) is a ubiquitous byproduct of the chemical, steel, and energy industries, but is currently mainly burned for its heat of combustion or for safety reasons. This not only leads to large CO₂ emissions, but is also wasteful because CO is a valuable starting chemical for C1 processes. Utilising CO from waste streams would therefore reduce anthropogenic CO₂ emissions and support a circular economy. The main obstacle is the difficulty of separating CO from N₂, which is almost always present in these waste gas streams.

Chapters 1 and 2 discuss the current state-of-the-art in CO separation from both academic and industrial perspectives. We review traditional swing sorption methods based on temperature or pressure, as well as the materials developed for these processes. While these approaches have achieved significant progress, they often suffer from limited selectivity and/or working capacity and are sensitive to process-relevant conditions such as humidity. We conclude that a novel separation method is required to achieve sufficient CO purity in an economically viable manner.

To address this challenge, we propose CO electroswing sorption. This method uses electrochemistry to manipulate the binding affinity between CO and metal ions in coordination complexes. Unlike traditional temperature- or pressure-based swing processes, electroswing sorption directly targets the electronic structure of the metal-carbonyl bond by changing the strength of the π-backbonding interaction through oxidation and reduction of the metal. However, this approach requires new materials containing redox-active unsaturated metal sites capable of forming metal-carbonyl bonds.

One potential material class is Prussian blue analogues, which are more commonly used in battery and electrocatalysis research. Chapter 3 discusses these materials for gas separation applications. We describe the synthesis and activation of Prussian blue analogues to utilise their open metal sites while preserving crystallinity and microporosity. The chapter highlights challenges associated with microporous adsorbents containing open metal sites in the presence of water, particularly competition between water and CO for these sites and the difficulty of removing water from them.

Chapter 4 presents a combined computational and experimental investigation of CO interactions with Fe- and Co-based tetraphenylporphyrin and phthalocyanine complexes at different oxidation states. The results show that the simple π-backbonding model cannot fully explain affinity switching in these systems. They also demonstrate that DFT screening can identify promising redox couples for CO electroswing sorption, although experimental validation remains necessary.

Chapter 5 addresses the challenge of accessing open metal sites in two-dimensional semiconductive metal-organic frameworks (MOFs). Attempts are made to exfoliate two hexahydroxytriphenylene-based MOFs using ultrasonication to improve access to these sites. The chapter highlights the practical difficulties of producing thin MOF sheets and discusses potential requirements for successful exfoliation.

Finally, Chapter 6 summarises the results of the thesis and provides an outlook for future research, including further material development and additional separations that may be achieved through electroswing sorption.