In this paper, hydrogen adsorption in metal–organic framework MIL-101(Cr) is investigated through a combination of sorption experiments, modeling of experimental isotherms, differential scanning calorimetry (DSC), neutron scattering, in situ synchrotron powder X-ray diffraction, and molecular simulations. The molecular simulations at 77 K for H2 adsorption in the material show excellent correspondence with excess uptakes determined from experimental isotherms. The simulations also indicate that H2 adsorption at a low pressure is mainly located in the 0.7 nm supertetrahedron and that, with increasing pressure, H2 starts to accumulate in the small (2.9 nm) and large (3.4 nm) cages. The inelastic neutron scattering results show that, in contrast to reports for hydrogen adsorption under the same conditions for microporous carbons, there is no solid-like H2 or any higher density H2 phases adsorbed in the pores of MIL-101(Cr). This indicates that, with increasing pressures, the adsorbed density of the MIL-101(Cr) remains constant but the volume of adsorbate increases and that higher densities for adsorbed hydrogen require pore sizes smaller than 0.7 nm, which is the size of the smallest pore in MIL-101(Cr). The enthalpies of adsorption are also investigated for this material using simulations, the Clapeyron equation applied to the isosteres and DSC, with the direct calorimetric method showing good agreement at zero coverage with the other two methods. The simulations and the Clapeyron equation are also in good agreement up to 6 wt % coverage.
Bibliographical noteFunding Information:
The authors would like to thank the Engineering and Physical Sciences Research Council (EPSRC) for a Fellowship (EP/R01660X/1) and for Hydrogen and Fuel Cells SUPERGEN Hub projects (EP/E040071/1, EP/K021109/1, EP/L018365/1, and EP/J016454/1), STFC for the allocation of ISIS beamtime (proposal number RB1320122), ESRF for allocation of time for in situ X-ray diffraction on ID31 (MA-1761), and the University of Bath for International Mobility Funding to visit Stellenbosch University.
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- hydrogen storage
- metal-organic frameworks
- inelastic neutron scattering
- enthalpies of adsorption
- adsorbed density