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abstracts
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.
abstracts
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.
abstracts
Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally non-porous materials.1 Here we present a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC=1,4-benzenedicarboxylate)2 and its nitro-functionalized derivative (Sc2(NO2-BDC)3)3 and compare it to direct compression behaviour in a non-penetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically-identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against non-penetrative hydrostatic fluids at high pressure this study demonstrates a novel high pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favourable adsorption sites.
abstracts
In recent years the development of new methods of storing, trapping or separating light gases, such as CO2, CH4 and CO has become of utmost importance from an environmental and energetic point of view. Porous materials such as zeolites and porous organic polymers have long been considered good candidates for this purpose. More recently, the ample spectrum of existing metal organic frameworks (MOFs) together with their functional and mechanical properties have attracted even further interest. The porous channels found in these materials are ideal for the uptake of guests of different shapes and sizes, and with careful design they can show high selectivity. Adsorption properties of MOFs have been thoroughly studied, however obtaining in depth structural insight into the adsorption/desorption mechanism of these materials is challenging. For example, out of the hundreds of MOF structures published to date, there are less than 20 entries currently in the CSD in which the CO2 molecule can be located. Here we present our novel findings using the high-pressure gas cell at the Diamond Light Source on beamline I19, where we have studied the inclusion of CO2, CH4 and CO on the microporous scandium framework, Sc2BDC3 (BDC = benzene-1,4-dicarboxylate) and its amino-functionalised derivative, Sc2(BDC-NH2)3. Here, the different adsorption sites for CO2, CH4 and CO in both frameworks have been determined as a function of increasing gas pressure. These structures, coupled with Density Functional Theory calculations, have helped to elucidate the host-guest interactions governing the different levels of selectivity shown by both Sc2BDC3 and Sc2(BDC-NH2)3. Additionally, gas mixtures have also been studied; in particular CO2/CH4 mixtures of different compositions, explaining the selectivity of the frameworks for CO2 over other gases and showing the great potential of in situ structural experiments for investigation of the potential applications of MOFs.
