Layer-by-layer assembly of metal-organic framework films

Background: The creation of thin, porous films comprised of metal organic frameworks (MOFs) is a challenging task yet one with significant potential reward. Porous MOF films have great potential as nanofiltration membranes with high permeability and selectivity for applications such as water purification. However, strategies to create thin MOF films in the absence of a solid support or substrate are limited, which in turn limits these potential applications. Controlling the thickness of the films is also extremely challenging, as MOF syntheses normally follow uncontrolled growth patterns.

This proposal builds upon the recently realised rationale of Bloch et al., whereby a "MOF", a porous salt, is constructed through aggregation of discrete, oppositely charged metal organic cages (MOCs),1 and the recent methodology reported by Nitschke et al. to deposit charged MOCs onto an alumina surface and remove them using a pH stimulus.2 We will combine these principles to realise the synthesis of MOF films of controllable thickness; the deposition of alternate layers of positively and negatively charged MOCs onto alumina (Figure 1) will yield layered materials, and covalent cross-linking will be used to confer stability before template removal through pH control. We will use a combined experimental and computational approach to elucidate and predict key experimental parameters and match MOCs of suitable dimensions to create films with tailored porosity.

Objectives: (i) Use computational screening and high-throughput approaches to identify potential co-mixtures of MOFs that lead to stable, well-ordered, layered structures; (ii) synthesise target MOCs and establish their layer-by-layer deposition onto and removal from alumina surfaces; (iii) develop new organic ligands which will enable us to chemically cross-link the distinct layers in the films to improve the robustness of the materials; (iv) characterise our MOC-films, with a focus on investigating the porosity, dimensions (thickness) and robustness of the films.

Novel methodology: this will be the first study of the layer-by-layer deposition of MOCs onto a surface, and we will utilise both computational and experimental tools in establishing this new concept. For the computational work, we expect to be able to use established forcefields such as MOF-UFF or QuickFF to undertake these screening calculations, supplemented with finer level sifting using either tightbinding schemes such as GFN-xtb or density functional theory, both of which are implemented in the CP2K code. Experimental work will be exploratory and will utilise adsorption isotherms to identify conditions under which MOCs can be adsorbed from solution onto our surfaces. Our combined computational and experimental methodology will serve to streamline the physical experiments undertaken and can feed in to automated materials discovery processes.

Alignment to EPSRC's research themes: (i) the core aims of this project align with EPSRC's Manufacturing the future and Physical Sciences themes, specifically within the portfolios of Synthetic Supramolecular Chemistry, Computational & Theoretical Chemistry, Synthetic Coordination Chemistry, Materials Engineering - composites; (ii) the potential for this work to feed into an automated discovery pipeline aligns with the EPSRC Artificial intelligence and robotics theme; (iii) the potential application of thin MOC-films in water purification, chemical separations and nanofiltrations align with EPSRC's Manufacturing the Future theme, and fits within the Manufacturing Technologies portfolio.