![]() This suggests that, because the whole gas delivery system cannot be heated, some water remains bound on the cool surfaces of the gas system even when the crystal itself is heated and this water is readsorbed once the crystal is cooled. It was also found that if the activated samples were cooled, the amount of bound water increased even under (dynamic) vacuum. Some residual water may be trapped in blocked one-dimensional channels on surface contact of the crystal with the glue used to hold it in place. There are several possible reasons for this. However, complete activation, i.e., a residual occupancy of O w ~0% could not be achieved even after treatment under high vacuum and temperature a similar behaviour was also observed in a previous study 30. Samples were considered activated when the M-O water (O w) oxygen residual occupancy fell below 10%. An efficient activation protocol is determined and the MOF gas adsorption properties towards CO and NO are probed as previous work showed a reduced release of NO in the Co/Ni-4,6-dhip framework, while other works have disclosed a higher affinity of this framework toward H 2 10, 31, when compared to CPO-27.Įach selected crystal was first activated at high temperature (450–500 K) and vacuum (3.3 × 10 −6 mbar on the vacuum gauge). In this study we investigate the binding mechanism of CO and NO in Ni-CPO-27 and for the first time of its structural conformer MOF Co-4,6-dhip. We have previously published data on NO binding in Ni-CPO-27 30. By using the high intensity and high energy X-rays available at a synchrotron 29, it is possible to obtain, and model, data of high enough resolution to observe gas molecules within the MOF pore environment. ![]() Single crystal X-ray diffraction (scXRD) provides invaluable understanding of the binding motif in these materials. To fully understand the uptake and release thermodynamics it is important to know exactly how the gases bind within the frameworks. Gases such as CO also have medical uses, and MOFs such as BioMIL-3 16, Co-4,6-dhip and a variety of zeolites have also been studied 27, 28. However, NO is not the only gas capable of performing this function and CPO-27 not the only MOF. MOFs such as CPO-27 have shown great potential in storing NO and the subsequent controlled release in the presence of water 1. In high concentrations NO is toxic, but in low concentrations NO may be used in medical technologies 1, 25, 26. NO is an important signalling molecule in mammalian physiology, serving a vital function in various systems throughout the body 24. There has been significant research into using MOFs to store and release NO 1, 7, 16, 17, 18, 19, 20, 21, 22, 23. Another avenue is the storage and release of biologically active gases. However, the exact binding mechanisms of these gases often remain elusive. H 2 11 and CH 4 12, the storage of pollutants such as CO 2 13, SO x 14 and NO x 15 have received much attention in recent years. The adsorption of relevant gases for energy production, e.g. ![]() Gas storage and sequestration is amongst the most exciting and well researched areas of MOFs. The isomeric linker commands a slightly altered short range structure in the metal node, which results in minutely distorted hexagonal channels and a different space group (R3 m), but otherwise almost identical structural features (pore chemistry, and available open metal sites). We refer to this material as Co-4,6-dhip in the rest of this paper. ![]() A structural conformer of CPO-27, M 2( m-dobdc) where M is Co can be obtained by using the linker 4,6-dihydroxyisophthalic acid (4,6-dhip, an isomer of 2,5-dhtp) 10. These channels contain open metal sites upon removal of covalently bound water molecules, as such CPO-27 is an attractive option for the storage of gases, particularly Lewis acids such as CO and NO. It crystallises in the R-3 space group and consists of hexagonal channels running along the crystallographic c-axis. MOFs are well known for their high porosities and exceptional gas adsorption properties, due to their tuneable node chemistry and exceptionally high specific surface areas 3, 4.ĬPO-27 5, also referred to as MOF-74 6, is a well-studied and widely used MOF comprising M 2+ metal ions, where M can be a wide range of metals such as Mg, Co, Ni, Cu and Zn, linked by 2,5-dihydroxyterephthalate (2,5-dhtp) linkers 7, 8, 9. Their structures consist of metal nodes or oxoclusters bound by organic linkers to create large open lattice frameworks 2. Metal-organic frameworks (MOFs) are an extensively researched and exciting group of materials, with a wide range of applications including the storage and release of biologically active gases 1.
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