Post-synthetic modification of metal-oxo clusters of metal-organic frameworks (MOFs) for aqueous applications

Abstract

The unreliable supply of critical materials, including rare earth elements (REEs), and greenhouse emissions associated with typical H2 production processes, force us to find alternative strategies for smoothing the transition towards green energy. Therefore, the development of new methods and novel advanced materials, as well as investigation of the reaction mechanisms are required.

Metal-organic frameworks (MOFs) are promising three-dimensional crystalline structures with a tuneable nature formed by self-assembled organic and inorganic building units. This dissertation focuses on two specific MOFs, MIL-101(Cr) and NH2-MIL-125(Ti), due to their water-stable and highly porous nature. It explores the postsynthetic modification of their metal-oxo clusters to adjust their physicochemical properties for desired applications.

Firstly, a Cr-based MOF, MIL-101(Cr), was selected to meet the challenge of the selective adsorption of REEs from leachate solutions of potential urban mining sources. The coordinatively unsaturated Cr3+ sites of MIL-101(Cr) were decorated with the organophosphorus compounds, namely Cyanex-272, HDEHP and TBP. This resulted in promising materials with a strong affinity towards heavy REEs, high adsorption capacity (up to 57.5 mg g-1 for Er3+), and exceptional separation ability of Er3+ – over 90% selectivity against M2+ ions and up to 95% in a mixture with Nd3+ and Gd3+. In addition, with the help of quantum mechanical and electrostatic principles calculations, the simulated structures and modelled adsorption processes provided insights into the grafting mechanisms of the metal-oxo cluster of the MOF, elucidated the selectivity principles and revealed two unique coordination modes for Mn+–MOF interactions.

Secondly, a Ti-based MOF, NH2-MIL-125(Ti), was studied to enhance its photocatalytic performance in hydrogen evolution reaction (HER) under visible-light exposure. A postmodification NH2-MIL-125(Ti) using plain Co(NO3)2 led to the incorporation of single CoII active sites into the metal clusters of the MOF, enhancing its visible light photocatalytic activity. The performed pump-flow-probe XANES spectroscopy revealed the formation within microseconds of transient CoI sites responsible for the increased H2 evolution. Furthermore, the reuse of the catalysts led to a sharp rise in HER performance.

After thorough characterisation of the MOFs, two concurrent mechanisms for the formation of highly active proton reduction sites – a non-destructive linker elimination resulting in coordinatively unsaturated Ti sites and the restructuring of single CoII sites, were revealed.

Overall, the demonstrated simple post-synthetic modifications of metal-oxo clusters of the MOFs and subsequent characterisations coupled with computational studies provided a fundamental understanding of the key factors that govern their performance. Besides, this dissertation offers a pathway for designing stable functionalised materials that can contribute to the transition towards climate-neutral energy production.