Designing Chemistry and Morphology in Metal-Organic Framework Gels, Liquids and Glasses

Metal-organic frameworks (MOFs) are three-dimensional structures composed of inorganic nodes, connected by organic linkers. The ease at which different elements or functionalities can be incorporated into a plethora of nanoscopic architectures has led to the population of an enormous class of compounds. Over 60,000 crystalline structures have been reported, which can be 'tuned' to exhibit exceptional selectivity for pre-determined target guest molecules. Two products, from companies span out of Northwestern and Queen's University Belfast, have been commercialised, in the areas of toxic gas storage and fruit packaging. Furthermore, huge societal and economic benefits from their use in highly selective CO2 capture, drug delivery, chemical sensing, toxic gas separations and harmful waste storage applications have been proposed, though not delivered. This is partly due to the gap between the significant strides made in chemical synthesis, and a dearth of work on their physical properties.

It is therefore highly surprising that even the most basic of all physical properties, the state of matter, represents one of the most under-researched areas in the MOF field. This proposal hence aims at addressing this bias towards the ordered, crystalline solid domain, which is vital given the research invested into the field and the potential benefits from the combination of the mechanical stability of the amorphous domain, with the chemical opportunities afforded by MOFs.

The recent discovery of the glass-forming ability of the MOF family offers the tantalising prospect of accurately designing functional glasses by first tuning the chemical properties of the parent crystalline framework, prior to subsequent melting and liquid quenching. The transfer of functionality from crystal to glass in this manner will enable the next generation of a plethora of functional glasses to be produced. Such materials will lie at the forefront of efforts to move MOFs away from the current focus on the crystalline porous state.

Two new classes of functional MOF-glasses will be produced in the course of the project; (i) porous MOF-glasses for separations and (ii) chiral MOF glasses for advanced optical applications. With respect to the former, the development of porous MOF-glasses or MOF-glass composite membranes for gas separations would prove a significant advancement in the field, given the current mixed matrix membranes suffer from chemical compatibility problems and a reduction in active component. In addition, reactions between MOFs in the liquid phase will be studied, and the structure and properties of the resultant multicomponent glasses produced investigated.

The 'soft' nature of MOFs also leads to the structural collapse of some frameworks during the post-processing processes (e.g. sintering, ball-milling, pelletisation) used to convert nano-crystalline powders to industrially useable morphologies. The development of a variety of morphologies (i.e. thin films, beads, and self-supporting, binder-free membranes from microcrystalline powders is therefore absolutely essential for the field to move forwards to application.

Concentrating on both fundamentally expanding our basic understanding of the field and at the same time bridging the gap between basic science and industrial relevance, this project is highly interdisciplinary in nature. The overarching aims of this proposal are thus to; (i) create new porous and chiral MOF-glasses (ii) create new composite materials for separation by combining crystalline MOFs and their glassy counterparts, (iii) produce a binary phase diagram for the reaction of two MOF liquids and analyse the products formed and (iv) cross-link unstable MOF spheres produced in previous work by the group, and extend the methodology to another MOF family.

The work performed on the formation of glasses and liquids from MOFs is internationally leading and will provide chemical industries e.g. Johnson Matthey with the ability to produce MOFs in an array of self-supporting, binder free, mechanically stable morphologies. Methods to synthesise coatings, thin films and glass fibres from MOFs are highly sought after by industries keen to bridge the gap between fundamental scientific promise and realised impact. Such morphologies would present advantages over microcrystalline powders or binder-containing membranes, in various catalytic, chemical sensing and gas separation contexts.
Aside from facilitating the industrial relevance of existing materials, the research here will also enable the manufacture of highly novel functional devices. Examples will include membranes in which highly selective crystalline MOF domains are chemically bound to a MOF-glass network - combining the separation abilities of crystalline MOFs with the stability of the glass domain. Other new materials include chiral hybrid glasses capable of acting as chiral waveguides, luminescent glasses formed from the doping of lanthanide species into crystalline MOFs pre-melting, and alloys formed by combining chemically intricate MOF-liquids together before vitrification. Our recent research into the mechanism of melting has demonstrated significant remnant porosity within the MOF liquids prior to quenching. Such a development in the hybrid field is completely novel, and, in a manner similar to the porous liquids reported by Cooper and James in Nature in 2015, is exciting in that liquid gas capture and transport solutions are more promising than their solid counterparts.
An immediate economic advantage provided by the project is through collaboration with Corning Inc, who are one of the world leading glass industries, and have over 40,000 employees worldwide. The glasses formed are a new category of melt-quenched glass, separate from the existing (i) inorganic, (ii) organic and (iii) metallic (e.g. silica, organic polymers and alloys respectively) currently known. Initial work on their mechanical and optical properties shows them to occupy a 'sweet-spot' in materials space, providing greater flexibility than inorganic glasses, yet greater strength than organics. Work to map out this area of materials space is planned using the world leading characterization facilities of Corning, based at the Research and Development headquarters in Elmira. Protective screens are a foremost priority, and I work closely with Dr. John Mauro, who is a senior research manager at Corning and develops novel glass compositions, including those used for Gorilla glass, i.e. that used on Iphone screens. Advanced optical solutions are also being actively pursued in this area, given that refractive indices of the few glasses studied to date are like those of silica and thus highly promising.
Inorganic porous frameworks called zeolites were used in both the Fukushima and Chernobyl nuclear incidents in post-release radioactive isotope (e.g. 131I, 137Cs) sorption, though a secondary encapsulation step in an inorganic glass was required to permanently immobilise the radioactive ions. Research into the use of MOFs in nuclear clean-up processes offers more selective ion sorption, though the prevention of guest desorption post-capture again necessitates encapsulation. Health benefits, arising from the introduction of the amorphous domain into projects which focus on using MOFs as drug delivery vehicles, will also be made possible in the long-term.
Researchers involved with this project will benefit from possessing an array of interdisciplinary skills, and they will prove particularly competitive for industry based jobs with knowledge of the transitioning of basic science to application.