Methane storage in flexible metal–organic frameworks with intrinsic thermal management
Top: High-pressure CH4 adsorption isotherms (AIs). The usable capacity for a classical Langmuir-type AI (top left) and a ‘stepped’ AI (top middle), with the minimum desorption pressure and the maximum adsorption pressure indicated by the vertical lines. Right, CH4 AIs for Co(bdp) at 25 °C with Pdes = 5.8 bar and Pads = 35 bar indicated by dashed lines and filled/open circles representing adsorption/desorption. Middle: The crystal structures of the collapsed (0bar) and CH4- expanded (30bar CH4) phases of Co(bdp). Bottom: Left, Total Scattering XRPD data for collapsed Co(bdp) collected at the X04SA-MS beamline of the SLS at RT in vacuum (0 bar) at 16 keV. Empty capillary and air scattering contributions are shown in blue and green. Right, the calculated Pair Distribution Function for the collapsed phase of Co(bdp). At low r, the most prominent peaks are at: 1.40 Å (aromatic C-C, C-N, and N-N bonds of the bdp ligands); 2.11 Å (Co-N bonds); 3.7 Å and 7.1 Å (Co-Co distances). Other peaks, including that just below 3 Å, correspond to Co-C and Co-N distances for C, N non-bonded atoms.
As a cleaner, cheaper, and more globally evenly distributed fuel, natural gas has considerable environmental, economic, and political advantages over petroleum as a source of energy for the transportation sector. Despite these benefits, its low volumetric energy density at ambient temperature and moderate pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage. Adsorbed natural gas systems have the potential to store high densities of methane (CH4, the principal component of natural gas) within a porous material at ambient temperature and moderate pressures. Activated carbons, zeolites, and metal–organic frameworks have been investigated extensively for CH4 storage, showing practical challenges involved in designing systems with high capacities and in managing the thermal fluctuations associated with adsorbing and desorbing gas from the adsorbent. Here, we demonstrate that the flexible compounds Fe(bdp) and Co(bdp) (bdp = 1,4-benzene dipyrazolate) undergo a structural phase transition in response to specific CH4 pressures, resulting in adsorption and desorption isotherms that feature a sharp step. Such behaviour enables greater storage capacities than have been achieved for classical adsorbents, while also greatly reducing the amount of heat released or absorbed during adsorption and desorption, thanks to the mitigating effect of the phase transition enthalpy. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure. This work uses results from several experiments, including Total Scattering XRPD at the X04SA-Materials Science beamline of the SLS.
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