52 53 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Porous frameworkmaterials compress like a spring under high mechanical pressure. Chemistry - Materials Science - Metal-Organic Frameworks - Coordination Chemistry - Solid-State Chemistry Metal-organic frameworks (MOFs) are modular porous materials possessing a wide range of functions. Some MOFs reversibly switch between two different states, an open pore statewith voids accessible for guest molecules and a closed pore state, where the voids of the framework are inaccessible for guest molecules. Usually, this transition is discontinuous, i.e. thematerial behaves like a switch, and only two different states/structures are accessible. Researchers from Technische Universität Dortmund, the Technical University of Munich and the University of Edinburgh wanted to generate a MOF system featuring a continuum of available states/structures between the open and the closed states. Hydrostatic mechanical pressure is a suitable stimulus to probe the structural response of MOFs. They investigated the high-pressure structural behaviour of a series of MOFs of the ZIF-62 family, which feature the same framework structure but possess various fractions of small- and large-sized organic linker molecules. They studied the influence of the molar fraction of the larger organic linker on the relative stabilities of the open pore and the closed pore states. They used Diamond’s I15 beamline because it is ideally suited for High- Pressure Powder X-ray Diffraction experiments, the method of choice to study the behaviour of crystalline materials under hydrostatic pressure. At I15, they could use a dedicated hydraulic high-pressure cell for the experiments. The hydraulic cell is designed for studying materials under pressure in the range from ambient up to 4000 bar, which is ideal for studying soft MOFs. In addition, the high quality of the data collected during the experiment meant that atomist structural refinements were possible. The researchers found that all the studied ZIF-62 MOFs reversibly switch between an open pore and a closed pore state when a certain threshold pressure is reached. The required pressure for pore closure increases with increasing fraction of the large-sized linker included in ZIF-62. For very large fractions of the larger linker, ZIF-62 continuously transforms from the open pore to the closed pore form with increasing pressure. The structural change is similar to an inward folding of the network structure with increasing pressure. Structure refinement and detailed analyses revealed that the pore size of the continuously transforming ZIF-62 derivative also gets continuously narrower and narrower with increasing pressure. The pressure driven open-pore to closed-pore transition of ZIF-62 could lead to applications of these materials as shock absorbers or nano-dampers. Moreover, the continuously changing pore size of the ZIF-62 derivative with a large fraction of the bulkier linker could be used to design pressure-switchable gas separation membranes. The material can be fine-tuned for a specific molecular separation task by applying a pressure that sets an appropriate pore size cut-off. Related publication: Song, J. et al. Tuning the high-pressure phase behaviour of highly compressible zeolitic imidazolate frameworks: from discontinuous to continuous pore closure by linker substitution. Angewandte Chemie International Edition, (2022). DOI: 10.1002/anie.202117565 Funding acknowledgement: Deutsche Forschungsgemeinschaft (DFG): 447344931 Deutsche Forschungsgemeinschaft (DFG): SPP1928 COORNETs Max Buchner Foundation: 3699 China Scholarship Council Corresponding authors: Prof Sebastian Henke, Technische Universität Dortmund,
[email protected] 3 4 5 6 7 0 1000 2000 3000 4000 Pore diameter / Å Pressure / bar 0.01 0.03 0.05 0.07 0.09 0.11 Differential pore volume / cm 3 g -1 Å -1 Variation of the pore size distribution of ZIF-62 as a function of hydrostatic mechanical pressure visualised as colour map. Representative 8-ring fragments of the crystal structures of ZIF-62 at 1 bar (left) and 4000 bar (right) are shown as insets. Zinc atoms are dark blue, nitrogen atoms are green and carbon atoms are orange / light blue. Hydrogen atoms are not shown. Crystallography Group Beamline I15 Designing ‘smart’ crystallinemembranes for graded molecular sieving Surfaces - Physics - Chemistry - Materials Science - Interfaces and Thin Films - Organic Chemistry Membrane technology provides a promising means of separating molecules as it offers greater selectivity than energy-intensive methods like distillation and chromatography. However, achieving the ideal structure and porosity to separate molecules of similar size remains a significant challenge. A team of researchers from the University of Liverpool and Imperial College London aimed to fabricate a crystalline membrane using a porous organic cage molecule (POC). It has been reported that POCs are molecules with cavities that can create porosity in porous liquids, molecular crystals and amorphous solids, and host guest molecules. The researchers hoped the guest-accessible cavity would facilitate selective diffusion through the membrane structure. POCs can also undergo exciting structural transformations in crystalline solids in response to chemical stimuli. The first step in their investigation was proving the membrane was crystalline. They then needed to investigate the membrane’s dynamic behaviour to determine how it behaved in different experimental conditions, such as during filtration experiments. They used Diamond’s I11 beamline to perform Powder X-ray Diffraction studies and I07 beamline for Grazing Incidence X-ray Diffraction studies. The data collected at Diamond enabled them to determine the structure and study the dynamic behaviour of the crystalline membrane during in situ measurements. A vital aspect of the study is underpinned by the dynamic behaviour of the membrane and its ability to switch its pore aperture during filtration experiments in response to different chemical environments. At Diamond, the scientists could replicate the conditions used in larger-scale separation processes and study the structure and dynamic behaviour of the membrane. The study found a highly ordered crystalline membrane with a switchable phase transition between two crystalline forms with different pore apertures. Both forms showed excellent separation performances. By varying the water/ methanol ratio, the film can be switched between the two phases with different selectivities, giving a single, ‘smart’ crystalline membrane that can perform graded molecular sieving. The team used the dynamic behaviour of the membrane to perform graded molecular sieving experiments to separate a mixture of three organic molecules using a single, smart membrane. Smart membranes that perform graded molecular sieving experiments to separate complex mixtures of molecules would create a parallel technology to thewidespreadandhighlyeffectiveuseof solvent gradients inchromatography. At the same time, membranes with switchable pore apertures could also lead to new applications in triggered drug delivery, biosensors, or fermentation/ fractionation processes. Although the present method of synthesis poses a challenge to the scalability and implementation of POC membranes in commercial processes, there is potential for the development of a more scalable production method by utilising the solution processability of these molecular cages. In the future, computational techniques, such as crystal structure prediction, will be employed to design POC crystals with desired properties based on first principles. Related publication: He, A. et al. A smart and responsive crystalline porous organic cage membrane with switchable pore apertures for graded molecular sieving. Nature materials 21 , 463–470 (2022). DOI: 10.1038/s41563-021-01168-z Funding acknowledgement: EPSRC (EP/N004884/1, EP/R018847/1) Leverhulme Trust China Scholarship Council - studentship Royal Society of Chemistry (M19–2442) Corresponding authors: Marc A. Little, University of Liverpool,
[email protected] Andrew I. Cooper, University of Liverpool,
[email protected] a) Graphical representation of membrane structure in water (blue) and methanol (green). b) Crystal structure with its 3-D pore network shown in yellow, found in water. A second structure was formed by soaking the membrane in methanol, which has additional extrinsic solvent-filled channels, shown here in orange, that open up additional porosity in the membrane in response to the methanol solvent. c) Photograph of the crystalline membrane with a diameter of 7.4 cm deposited on polyacrylonitrile support. d) In situ diffraction patterns of the crystalline membrane showing the reversible phase transition between the water and methanol structures. Crystallography Group Beamline I11