Ammonia Storage by Reversible Host–Guest Site Exchange in a Robust Metal–Organic Framework

Abstract MFM‐300(Al) shows reversible uptake of NH3 (15.7 mmol g−1 at 273 K and 1.0 bar) over 50 cycles with an exceptional packing density of 0.62 g cm−3 at 293 K. In situ neutron powder diffraction and synchrotron FTIR micro‐spectroscopy on ND3@MFM‐300(Al) confirms reversible H/D site exchange between the adsorbent and adsorbate, representing a new type of adsorption interaction.


Synchrotron Infrared Micro-spectroscopy
Infrared micro-spectroscopy experiments were carried out using the B22: Multimode Infra-Red Imaging and Microspectroscopy (MIRIAM) beam line at the Diamond Light Source, Rutherford Appleton Laboratories (UK). The instrument is comprised of a Bruker Hyperion 3000 microscope in transmission mode, with a 15x objective and liquid N2 cooled MCT detector, coupled to a Bruker Vertex 80 V Fourier Transform IR interferometer using radiation generated from a bending magnet source. Spectra were collected (512 scans) in the range 500-4000 cm -1 at 4 cm -1 resolution and an infrared spot size at the sample of approximately 20 × 20 µm. A microcrystalline powder of MFM-300(Al) was placed onto a S9 ZnSe disk and placed within a Linkam FTIR 600 gas-tight sample cell equipped with ZnSe windows, a heating stage and gas inlet and outlets. The N2, NH3 and ND3 were pre-dried using individual zeolite filters. The analysis gases were dosed volumetrically to the sample cell using mass flow controllers, the total flow rate being maintained at 100 cm 3 min -1 for all experiments. The gases were directly vented to an exhaust system and the total pressure in the cell was therefore 1 bar for all experiments. The sample was desolvated under a flow of dry N2 at 100 cm 3 min -1 and 393 K for 3 h. The sample was then cooled to 293 K under a continuous flow of N2. Dry NH3 was then dosed as a function of partial pressure, maintaining a total flow of 100 cm 3 min -1 . The sample was then regenerated with a flow of dry N2. To investigate the H→D exchange reaction, a flow of ND3 was introduced within the cell at a flow rate of 100 cm 3 min -1 at 293K for 1hr. N2 flushing was then repeated at 100 cm 3 min -1 , a scan taken and D→H exchange was implemented with a flow of NH3 at 100 cm 3 min -1 . The spectrum was then observed after a final N2 flushing at 100 cm 3 min -1 .
A graph of ln(p) versus 1/T at constant loading allows the differential enthalpy and entropy of adsorption and also the isosteric enthalpy of adsorption (Qst,n) to be determined. Four example fittings are displayed in Figure   S1. The calculated R 2 value for each fitting is > 0.99 indicating a reliable fit.

Dual-site Langmuir-Freundlich fittings and IAST selectivity of NH3 vs CO2, CH4 and N2.
Adsorption isotherms of NH3, CO2, N2 and CH4 in MFM-300(Al) at 293K were fitted with the dual-site Langmuir-Freundlich model (Equation 2), where n is the loading in mmol g -1 , P is the pressure in bar, qsat1 is the saturation capacity in mmol g -1 , b1 is the Langmuir parameter in bar −1 , and v1 is the Freundlich parameter for two sites 1 and 2. All R 2 values for the fits are >0.999 confirming they fit the model well. Ideal adsorbed solution theory (IAST) [4] was used to determine the selectivity factor, S, for binary mixtures    *-data not available. avalues estimated from published N2 isotherms at 77K.

Ammonia cycling stability determined by in situ high-res PXRD
Five separate cycles of dosing and removal of NH3 in MFM-300(Al) were studied at beamline I11, DLS. This was to determine the crystallographic stability of the material to repeated exposure to NH3 and whether it had any detrimental structural impact. Figure 1 shows the normalised diffraction patterns for this experiment and we observe no significant structural changes in MFM-300(Al) over 5 repeated cycles of NH3.
To further ratify this, a full width at half maximim (FWHM) analysis on the (110), (211) and (112) peaks was undertaken. These peaks were selected for their relative intensity within the diffraction pattern and the different direction each plane is oriented. No significant peak broadening is observed over 5 cycles indicating that NH3 is not causing structural changes ( Figure S4).     2 / º S13 therefore propose that the first site is filling almost exclusively, indicated by the cell contraction. After this point, the pores fill to saturation causing an expansion in cell volume.     Figure S11: View of the active binding site determined via NPD data of the MFM-300(Al)·1.5ND3  (4)    *-Information not available