Metal-organic frameworks (MOFs) are a well-studied class of porous materials with the potential to be used in many applications such as gas storage and catalysis.[1] UiO-67 (UiO = University of Oslo), a MOF built from zirconium oxide units connected with 4,4-biphenyldicarboxylate (BDC) linkers, forms a face centred cubic structure. Zirconium has a high affinity towards oxygen ligands making these bridges very strong, resulting in UiO-based MOFs having high chemical and thermal stability compared to other MOF structures. Moreover, UiO-67 has become popular in engineering studies due to its high mechanical stability.[2] Using high pressure x-ray crystallography we can exert MOFs to GPa pressures, experimentally exploring the mechanical stability of MOFs to external pressure. By immersing the crystal in a hydrostatic medium, pressure is applied evenly to the crystal. On surrounding a porous MOF with a hydrostatic medium composed of small molecules (e.g. methanol), the medium can penetrate the MOF, resulting in medium-dependant compression. On compressing MOF-5 (Zn4O(BDC)3) using diethylformamide as a penetrating medium, the framework was shown to have an increased resistance to compression, becoming amorphous several orders of magnitude higher in pressure than observed on grinding the sample.[3] Here we present a high-pressure x-ray diffraction study on the UiO-based MOF UiO-67, and several new synthesised derivatives built from same metal node but with altered organic linkers, allowing us to study in a systematic way, the mechanical stability of the MOF, and its pressure dependence on both the linker, and pressure medium.

Materials with applications in gas storage and separation are of enormous interest across many disciplines of science. This has been driven in large part by advances in carbon sequestration technologies in line with global government targets for cleaner energy and the reduction of carbon dioxide (CO2) emissions. Potential applications in this field include the removal of CO2 from flue gas mixtures before it enters the atmosphere, for example. Porous metal-organic frameworks (MOFs) are currently seen as one of the most promising types of materials for this purpose since they combine the desired features of high porosity, thermal stability and chemical versatility. Fundamental to such research is the requirement to optimise the quantity of gas that can be stored in the MOF. Evaluation of gas storage capacities is usually performed using time-consuming gravimetric or calorimetric analyses over a range of temperatures and pressures, and often requiring gram-scale quantites of material. Here, we present a novel method for gas delivery and calculation of gas storage capacity using a perflourocarbon (PFC) liquid. PFCs have long been recognised as useful due to their predisoposition for dissolving large volumes of gases such as CO2 and oxygen. They are chemically inert hydrocarbons in which the hydrogen atoms have been replaced with fluorine. We have used a PFC containing dissolved CO2 as a pressure-transmitting liquid in an in situ high-pressure single-crystal X-ray diffraction experiment. Application of industrially-achievable pressure within a diamond anvil cell causes the CO2 to be squeezed out of the liquid into a host crystal of a porous MOF as the system seeks to fill empty space and redistribute density upon contraction. Diffraction data from a crystal of Sc2BDC3 (BDC = 1,4-benzenedicarboxylate)[1] has allowed us to determine its maximum CO2 storage capacity, which is in perfect agreement with other methods. This result is the first of its kind and the technique has several notable advantages over other methods: it requires just one experimental step, can be performed at room temperature, and requires only one single crystal of solid material.