Exceptional Packing Density of Ammonia in a Dual-Functionalized Metal–Organic Framework

We report the reversible adsorption of ammonia (NH3) up to 9.9 mmol g–1 in a robust Al-based metal–organic framework, MFM-303(Al), which is functionalized with free carboxylic acid and hydroxyl groups. The unique pore environment decorated with these acidic sites results in an exceptional packing density of NH3 at 293 K (0.801 g cm–3) comparable to that of solid NH3 at 193 K (0.817 g cm–3). In situ synchrotron X-ray diffraction and inelastic neutron scattering reveal the critical role of free −COOH and −OH groups in immobilizing NH3 molecules. Breakthrough experiments confirm the excellent performance of MFM-303(Al) for the capture of NH3 at low concentrations under both dry and wet conditions.

Details of refinement of the single crystal structure of MFM-303(Al).
The X-ray diffraction of the crystal had a resolution limit of around 0.95 Å, and data beyond this resolution were omitted from the refinement using a SHEL command. As a consequence the structure has a low data to parameter ratio, and so where possible restraints were applied to the geometries and atomic displacement parameters of the atoms to aid refinement. The possibility of further unmodelled disorder in the ligand position is suggested by the shape of the displacement ellipsoids and large U3/U1 ratio for the average U(i,j) tensor (4.8). However, more complex disorder models did not yield a plausible structure or better fit with the data.
Rigid bond restraints have been applied to the anisotropic displacement parameters of all atoms in the structure (RIGU).
The carboxylic acid group C61-O63 is disordered over two orientations whose occupancies have been refined and constrained to sum to unity, resulting in values of 0.61(5) and 0.39(5). The position and anisotropic displacement parameters of the carbon atom of each disorder component have been constrained to be identical S5 (EAXY, EADP). The 1,2 and 1,3 bond distances of the two components have been restrained to be similar (SAME) and each component has been constrained to have planar geometry coincident with phenyl carbon atom C25 (FLAT). The anisotropic displacement parameters of the disordered oxygen atoms have been restrained to be similar and isotropic (SIMU, ISOR).
The isotropic displacement parameters of water residues O1W, O2W and O3W were fixed to have values of 0.08 whilst their occupancies were allowed to refine resulting in values of 0.94(1), 0.48(1) and 0.39(1) respectively. The water residues have been included in the unit cell contents at full occupancy.
Hydrogen atoms on the phenyl rings were geometrically placed and refined with a riding model. Electron density peaks in suitable positions for hydrogen atoms were observed close to the hydroxyl-oxygen atom O1H, carboxylic acid hydroxyl-oxygen atom O42 and the water residue O2W. The positions of these hydrogen atoms were allowed to refine under the influence of geometric restraints. Further electron density peaks were observed close to water residues O1W and O3W; however, they could not be developed into sensible models for hydrogen atoms. These potential hydrogen atoms were not included in the model, but were included in the unit cell contents at full occupancy.
The hydroxyl hydrogen position at O1H is disordered over two positions with each refined to have half occupancy. The O-H bond lengths were restrained to suitable target values (DFIX) whilst all the H...Al 1,3 distances were restrained to be the same (SADI). Their isotropic displacement parameters were fixed at a value of 0.1. The positions of both atoms were weakly restrained to donate hydrogen bonds to water residue O2W. The 1,2 and 1,3 distances of the hydrogen atoms of water residue O2W were restrained to have suitable values (DFIX). Further weaker restraints were applied to their positions to make them donate hydrogen bonds to adjacent water residue O3W and carboxylic acid oxygen O63A. Their isotropic displacement parameters were fixed at a value of 0.12. An electron density peak was observed almost equidistant from, and directly between carboxylic acid oxygen atoms O42 and O62A. The electron density was modelled as a full occupancy hydrogen atom and its position allowed to refine freely. The isotropic displacement parameter of the hydrogen atom was fixed at 1.5 x Ueq of oxygen atom O42. Further disordered solvent molecules could not be sensibly modelled, so the structure was treated with PLATON SQUEEZE. A total of 32 electrons were accounted from the P1 cell in this, equating to a quarter of a water molecule per asymmetric unit, which have been included in the unit cell contents and calculation of derived parameters.

Density function theoretical (DFT) calculations
Modelling by Density Functional Theory (DFT) of the bare and NH 3 /CO 2 -loaded MOFs was performed using the Vienna Ab initio Simulation Package (VASP). 5 The calculation used the Projector Augmented Wave (PAW) method 6,7 to describe the effects of core electrons, and Perdew-Burke-Ernzerhof (PBE) 8 implementation of the Generalized Gradient Approximation (GGA) for the exchange-correlation functional.
Energy cutoff was 800 eV for the plane-wave basis of the valence electrons. The lattice parameters and atomic coordinates determined by X-ray single crystal diffraction in this work were used as the initial structure. Due to the large unit cell (~300 atoms), all electronic structure and phonon calculations were performed on the Γ point only. The total energy tolerance for electronic energy minimization was 10 -8 eV, and for structure optimization it was 10 -7 eV. The maximum interatomic force after relaxation was below 0.001 eV/Å. The optB86b-vdW functional 9 for dispersion corrections was applied. The vibrational eigen-frequencies and modes were then calculated by solving the force constants and dynamical matrix using Phonopy. 10 The OClimax software 11,12 was used to convert the DFT-calculated phonon results to the simulated INS spectra. Simulation of the INS spectra for NH 3 -loaded MFM-303(Al) was conducted with NH 3 presenting at site II due to its strong interaction with the -OH group of the framework. The simulation of NH 3 at site I breaks the symmetry of the model and unfortunately makes the calculation impractical.
Supplementary Figure S13. View of NH 3 site II derived from the model used to simulate INS spectra and its proximity to carbon atoms on the aromatic ring and to the µ-OH moiety.  (1), 112-122.