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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
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.
