Quantifying the Dynamic Response in Metal-Organic Frameworks (MOFs): A Platform for Tuning Chemical Space in Porous Materials

Metal-organic frameworks (MOFs) are periodic crystalline materials with molecular-scale pores that are among the most widely studied classes of materials across a range of scientific and engineering disciplines. Their modular construction from metal-ion-containing nodes linked by organic ligands enables both spatial and chemical tuning to selectively trap molecules in the pore space. These features allow the performance of MOFs to be optimised for numerous applications including storage and separation of gases, detection of molecules, environmental remediation, catalysis and drug delivery. Their potential impact therefore spans the energy, transport, environmental and health care sectors.

The periodic crystalline nature of MOFs makes them amenable to atomic-level characterisation by diffraction methods and extensive characterisation by a variety of spectroscopic techniques, which collectively provide far greater detail pertinent to materials design and optimisation than for non-crystalline competitor materials such as activated carbons. MOFs also present advantages over established crystalline porous materials such as zeolites and similar oxide materials as the modular construction of MOFs from metal ions and organic ligands and the opportunity for post-synthesis chemical modification enables almost limitless versatility in pore size, pore shape and spatial arrangement of chemical functionality. Some 10s of thousands of MOFs have been reported in the past 20 years. Most MOFs have fixed pore sizes and shape, but less than 1% are known to be flexible i.e. they change their pore space in response to an external stimulus. This allows the design of materials that can respond to a variety of such stimuli, including temperature, pressure, light and molecular guests, allowing finer control of molecular capture properties at the heart of applications of MOFs.

This project builds on our recent discovery of a new flexible 'breathing' MOF Me2NH2[In(NH2BDC)2] (SHF-61) (NH2BDC = aminobenzenedicarboxylate), which exhibits a substantial guest-responsive pore opening and closing behaviour. The MOF exhibits excellent CO2/N2 and CO2/CH4 adsorption selectivity, indicating potential for industrially relevant gas separation, and has markedly different flexible responses to different small molecule guests, which suggests an underlying host-guest behaviour that can be exploited for many applications in separations, detection or catalysis. What further sets this MOF apart, even from most other flexible MOFs, is that it retains its integrity as single crystals during dynamic behaviour, providing an almost unprecedented opportunity for accurate and detailed structural characterisation by single-crystal X-ray diffraction.

This project will exploit this extraordinary opportunity for insight into guest-responsive flexible behaviour as a platform for development of responsive materials. We will develop a new family of materials by chemical modification and reticular synthesis (pore-space expansion). These materials will be studied systematically to provide a broad range a fundamental knowledge applicable to the MOF field, and exploited in the short-term for selective molecular recognition including gas separation, but also to build a foundation for longer-term applications in catalysis and other areas. The research will be conducted by a multi-disciplinary team of chemists and chemical engineers. The Brammer-Düren-Fletcher-Oswald team provide extensive experience and the necessary expertise in synthesis, characterisation and computational modelling/simulation of MOFs and an established record of collaboration. Specialised expertise supported by excellent laboratory facilities and complemented by extensive engagement with national facilities will enable a systematic and quantitative investigation leading to development of a versatile family of MOF materials and a source of fundamental information for research worldwide on flexible MOFs.

Although this project is fundamental research, it has the potential for impact on many current research problems and for longer-term impact across the science and technology area. The outcomes of the studies will extend well beyond the family of materials studied, and inform approaches to development of new materials to further strengthen pathways to future materials applications. The project will develop, quantify and further our understanding of dynamic behaviour in flexible "breathing" metal-organic framework materials (MOFs). Such dynamic materials are sought after as responsive materials platforms for a diverse range of applications, including selective adsorption and separation of gases and other small molecules, molecular sensing, environmental remediation, energy storage, catalysis and drug delivery. The project will capitalise on a rare opportunity for detailed structural characterisation and insight arising from our recent discovery of the dynamic behaviour in the parent MOF material. The structural characterisation will be integrated with adsorption measurements, including thermodynamic and kinetic data, and studies of response to pressure as a stimulus. Experimental studies will both inform and be informed by extensive computational modelling of framework dynamics and adsorption of gases and vapours by the MOFs.

The initial impact of the project will stem from fundamental, quantitative experimental data and explanatory theoretical models that will underpin future development of MOFs, and in particular flexible MOFs, as responsive materials, thereby providing a vital contribution to the continued development of MOFs along the pathway from scientific enquiry into engineering products. The most immediate beneficiaries of this impact will be academic researchers worldwide who are studying and developing MOFs. Academic research more broadly across materials science, and areas such as host-guest supramolecular chemistry and crystallography, will also benefit from the anticipated advances. The project aligns particularly well with the EPSRC Grand Challenge in Directed Assembly, which through its large community of academic and industrial members has developed a roadmap for the design of functional materials with both short-term (years) and long-term (decades) horizons. All team members are engaged in this Grand Challenge. Beyond academic research, companies already engaged in development of MOFs may realise more immediate impact, but the longer-term impact of the project will span industries in the energy, transport, environmental and health care sectors where MOFs will be deployed in future. Positive societal impact will indirectly stem from these developments, such as more efficient synthesis (catalysis) with less waste, reduced environmental impact and improvements in health care delivery.

Research on MOFs is a multi-disciplinary enterprise. The new project establishes a multi-disciplinary team with a strong foundation to address a range of future problems for which MOFs may provide a solution. The project will bring together four UK research teams and collaborators in the UK and Australia. The team members provide complementary scientific and engineering experience, and have previously collaborated in bilateral projects. The project will produce three early-career scientists (PDRAs) with training and expertise that reflect this multi-disciplinary approach. Their expertise will collectively span synthetic chemistry, diffraction and spectroscopy, adsorption phenomena, high-pressure science and computational modelling. This project will enable them to work in a multi-disciplinary environment and understand the demands and capabilities brought about by a much wider range of experimental and computational approaches. These individuals will be well-trained scientists and engineers for 21st century needs in academic and industrial research.