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 sector1,2. Despite these benefits, its low volumetric energy density at ambient temperature and pressure presents substantial challenges, particularly for light-duty vehicles with little space available for on-board fuel storage3. 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 pressures4. Although activated carbons, zeolites, and metal–organic frameworks have been investigated extensively for CH4 storage5,6,7,8, there are 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 use a reversible phase transition in a metal–organic framework to maximize the deliverable capacity of CH4 while also providing internal heat management during adsorption and desorption. In particular, the flexible compounds Fe(bdp) and Co(bdp) (bdp2− = 1,4-benzenedipyrazolate) are shown to 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 adsorbents9, while also reducing the amount of heat released during adsorption and the impact of cooling during desorption. The pressure and energy associated with the phase transition can be tuned either chemically or by application of mechanical pressure.
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This research was supported by the Advanced Research Projects Agency – Energy (ARPA-E) of the US Department of Energy (DoE). Powder X-ray diffraction data were collected at beamline 17-BM-B at the Advanced Photon Source, a DoE Office of Science User Facility operated by Argonne National Laboratory under contract no. DE-AC02-06CH11357 and at beamline MS-X04SA of the Swiss Light Source (SLS) at the Paul Scherrer Institut. Single-crystal X-ray diffraction experiments were performed at beamline 11.3.1 at the Advanced Light Source, a DoE Office of Science User Facility operated by Lawrence Berkeley National Laboratory under contract no. DE-AC02-05CH11231. In addition, we thank M. Veenstra, D. A. Boysen, T. M. McDonald, D. J. Xiao, M. Nippe, Z. Hulvey, G. J. Halder, K. J. Gagnon, S. J. Teat, and the technical staff of the MS-X04SA beamline at SLS for experimental assistance and discussions. We also thank the National Science Foundation for providing graduate fellowship support for J.O. and J.E.B.
The authors and the University of California have filed for a patent on some of the results contained herein.
Metrical data for the solid-state structures of collapsed Co(bdp), expanded Co(bdp), collapsed Fe(bdp), 40-bar expanded Fe(bdp), 50-bar expanded Fe(bdp), DMF-solvated Fe(bdp) at 100 K, and DMF-solvated Fe(bdp) at 300 K are available free of charge from the Cambridge Crystallographic Data Centre under reference numbers CCDC 1058444–1058450.
Extended data figures and tables
a, Excess CH4 isotherms at 25 °C for Co(bdp) repeated four times on the same sample, which was regenerated under vacuum at 25 °C for 2 h between measurements. The adsorption step is at a slightly higher pressure during the first run because there is probably a slightly higher energy barrier to the first expansion of a freshly packed sample; however, the desorption steps occur at identical pressures for all four runs. b, The adsorption and desorption pressures are shown as green and red circles, respectively, for 100 CH4 adsorption–desorption cycles in Co(bdp) at 25 °C. c, Excess CH4 adsorption isotherms at 25 °C for Co(bdp) after 0, 25, 50, 75, and 100 cycles of 35-bar adsorption and 5-bar desorption. d, Excess CH4 isotherms at 25 °C for Co(bdp) before (green) and after (blue) the 100 adsorption–desorption cycles between 35 bar and 5 bar. Filled and open circles in a and d correspond to adsorption and desorption, respectively.
a, Total CH4 adsorption isotherms at 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Co(bdp), with adsorption after the step fitted independently at each temperature with an offset dual-site Langmuir–Freundlich equation. The small pre-step adsorption was fitted with a single-site Langmuir model. b, Total CH4 adsorption isotherms at 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Fe(bdp) with adsorption after the phase transition fitted independently at each temperature with an offset dual-site Langmuir–Freundlich equation. The pre-step adsorption was fitted with a single-site Langmuir model, and the isotherms were only fitted to a maximum loading of 10.6 mmol g−1, as indicated by the shading, to avoid complications from the second transition at higher CH4 loadings. As such, differential enthalpies are only calculated up to a maximum loading of 10.6 mmol g−1. c, Total CH4 adsorption isotherms at 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Co(bdp) with the corresponding single-site Langmuir fit for CH4 adsorption in the expanded phase. d, Total CH4 adsorption isotherms at 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Fe(bdp) with the corresponding single-site Langmuir fit for CH4 adsorption in the 40-bar expanded phase. The data were only fitted for the region of the isotherms that falls after the initial hysteresis loop closes and before the second isotherm step. All single- and dual-site Langmuir–Freundlich fits are shown as black lines.
a, Excess CH4 adsorption isotherms at −25 °C, 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Co(bdp). b, Excess CH4 adsorption isotherms at −25 °C, −12 °C, 0 °C, 12 °C, 25 °C, 38 °C, and 50 °C for Fe(bdp). Filled and open circles correspond to adsorption and desorption, respectively.
a, SEM image of DMF-solvated Co(bdp) microcrystalline powder. Scale bar, 10 μm. b, SEM image of Co(bdp) microcrystalline powder after more than 100 CH4 adsorption–desorption cycles. Scale bar, 1 μm. c, SEM image of desolvated Fe(bdp) microcrystalline powder. Scale bar, 1 μm.
a, Sample holder used for combined applied mechanical pressure and high-pressure CH4 adsorption experiments. The sample is located in the volume to the right of the fritted gasket and to the left of the blank gasket. A press is used to compact metal rods of different lengths against the sample, and the blank gasket is sealed behind the rod so that the uniaxial applied mechanical pressure (and constricted volume) is maintained throughout the high-pressure CH4 adsorption experiment. b, Excess CH4 isotherms at 25 °C for Co(bdp) before (green) and after (purple) the applied mechanical pressure studies. Filled and open circles correspond to adsorption and desorption, respectively.
a, b, The angles between the plane of the pyrazolate (light orange) and the Co–N–N–Co plane (light blue) are 38.1° and 17.3° in the collapsed and the CH4-expanded phases of Co(bdp), respectively. c, The angle between the plane of the pyrazolate (light orange) and the Fe–N–N–Fe plane (light blue) is 40.1° in the collapsed phase of Fe(bdp). d, Structure of the collapsed phase of Fe(bdp) under vacuum at 25 °C. e, Structure of the DMF-solvated phase of Fe(bdp) at 100 K. f, Idealized average structure of the 50-bar CH4-expanded phase of Fe(bdp) at 25 °C. In a–f, Grey, blue, red, purple, and orange spheres represent C, N, O, Co, and Fe atoms, respectively; H atoms are omitted for clarity.
a–d, Rietveld refinements for powder X-ray diffraction data (2θ is the diffraction angle) for Co(bdp) at 25 °C and under vacuum with λ = 0.77475 Å (a), for Co(bdp) at 30 bar of CH4 and 25 °C with λ = 0.75009 Å (b), for Fe(bdp) under vacuum at 25 °C with λ = 0.72768 Å (c), and for Fe(bdp) at 40 bar of CH4 and 25 °C with λ = 0.72768 Å (d). Red and blue lines represent the observed and calculated diffraction patterns, respectively. Grey lines represent the difference between observed and calculated patterns, and green and orange tick marks indicate calculated Bragg peak positions. The broad hump observed at 10° in the diffraction patterns is due to diffuse scattering from the sample holder (a thick-walled quartz glass capillary). The insets are magnified views of the main plots. e, Powder X-ray diffraction data for Fe(bdp) at 50 bar of CH4 and 25 °C (λ = 0.72768 Å). Green tick marks indicate Bragg angles for space-group-permitted reflections; the corresponding Miller indices are indicated for the most prominent peaks. Blue arrows indicate broad humps where multiple reflections overlap. f, The percentage of the expanded phase of Co(bdp) that is present in the variable-pressure experimental powder X-ray diffraction patterns as a function of CH4 pressure. The filled squares represent data collected during adsorption; the open squares represent data collected during desorption.
a, An illustration of the paracrystalline distortion in the crystallographic a–b plane of the collapsed phases of Co(bdp) and Fe(bdp) that leads to complex Bragg peak broadening. Black dashed lines represent the periodic crystal lattice; blue lines represent the paracrystal. Red circles represent the positions of metal–pyrazolate chains in the periodic lattice; blue circles represent their positions in a paracrystal. The magnitude of the paracrystalline distortion has been exaggerated for clarity. b, Simulated diffraction patterns are shown for a periodic collapsed Co(bdp) nanocrystal (75 nm × 60 nm × 43 nm; red trace) and for a paracrystal of equivalent size (blue trace). The upper trace (black) corresponds to the background-subtracted experimental diffraction pattern of the collapsed phase of Co(bdp) at 25 °C; the corresponding Miller indices are indicated for the most prominent peaks. For clarity, the three patterns have been given an arbitrary y offset; a.u., arbitrary units. Similar anisotropic peak broadening, which inflates hk0 peaks (but not h00 or 0k0 ones), is clearly visible in the experimental diffraction pattern and the paracrystalline simulation. The exact full-widths at half-maximum for the experimental and simulated Bragg peaks are given in Supplementary Table 16.
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Mason, J., Oktawiec, J., Taylor, M. et al. Methane storage in flexible metal–organic frameworks with intrinsic thermal management. Nature 527, 357–361 (2015). https://doi.org/10.1038/nature15732
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