High Ammonia Adsorption in MFM-300 Materials: Dynamics and Charge Transfer in Host-Guest Binding.

Ammonia (NH3) is a promising energy resource owing to its high hydrogen density. However, its widespread application is restricted by the lack of efficient and corrosion-resistant storage materials. Here, we report high NH3 adsorption in a series of robust metal-organic framework (MOF) materials, MFM-300(M) (M = Fe, V, Cr, In). MFM-300(M) (M = Fe, VIII, Cr) show fully reversible capacity for >20 cycles, reaching capacities of 16.1, 15.6, and 14.0 mmol g-1, respectively, at 273 K and 1 bar. Under the same conditions, MFM-300(VIV) exhibits the highest uptake among this series of MOFs of 17.3 mmol g-1. In situ neutron powder diffraction, single-crystal X-ray diffraction, and electron paramagnetic resonance spectroscopy confirm that the redox-active V center enables host-guest charge transfer, with VIV being reduced to VIII and NH3 being oxidized to hydrazine (N2H4). A combination of in situ inelastic neutron scattering and DFT modeling has revealed the binding dynamics of adsorbed NH3 within these MOFs to afford a comprehensive insight into the application of MOF materials to the adsorption and conversion of NH3.

Reversible coordinative binding and separation of sulfur dioxide in a robust metal-organic framework with open copper sites.

Emissions of SO2 from flue gas and marine transport have detrimental impacts on the environment and human health, but SO2 is also an important industrial feedstock if it can be recovered, stored and transported efficiently. Here we report the exceptional adsorption and separation of SO2 in a porous material, [Cu2(L)] (H4L = 4',4‴-(pyridine-3,5-diyl)bis([1,1'-biphenyl]-3,5-dicarboxylic acid)), MFM-170. MFM-170 exhibits fully reversible SO2 uptake of 17.5 mmol g-1 at 298 K and 1.0 bar, and the SO2 binding domains for trapped molecules within MFM-170 have been determined. We report the reversible coordination of SO2 to open Cu(II) sites, which contributes to excellent adsorption thermodynamics and selectivities for SO2 binding and facile regeneration of MFM-170 after desorption. MFM-170 is stable to water, acid and base and shows great promise for the dynamic separation of SO2 from simulated flue gas mixtures, as confirmed by breakthrough experiments.

Reversible adsorption of nitrogen dioxide within a robust porous metal-organic framework.

Nitrogen dioxide (NO2) is a major air pollutant causing significant environmental1,2 and health problems3,4. We report reversible adsorption of NO2 in a robust metal-organic framework. Under ambient conditions, MFM-300(Al) exhibits a reversible NO2 isotherm uptake of 14.1 mmol g-1, and, more importantly, exceptional selective removal of low-concentration NO2 (5,000 to <1 ppm) from gas mixtures. Complementary experiments reveal five types of supramolecular interaction that cooperatively bind both NO2 and N2O4 molecules within MFM-300(Al). We find that the in situ equilibrium 2NO2 ↔ N2O4 within the pores is pressure-independent, whereas ex situ this equilibrium is an exemplary pressure-dependent first-order process. The coexistence of helical monomer-dimer chains of NO2 in MFM-300(Al) could provide a foundation for the fundamental understanding of the chemical properties of guest molecules within porous hosts. This work may pave the way for the development of future capture and conversion technologies.

Optimal Binding of Acetylene to a Nitro-Decorated Metal-Organic Framework.

We report the first example of crystallographic observation of acetylene binding to -NO2 groups in a metal-organic framework (MOF). Functionalization of MFM-102 with -NO2 groups on phenyl groups leads to a 15% reduction in BET surface area in MFM-102-NO2. However, this is coupled to a 28% increase in acetylene adsorption to 192 cm3 g-1 at 298 K and 1 bar, comparable to other leading porous materials. Neutron diffraction and inelastic scattering experiments reveal the role of -NO2 groups, in cooperation with open metal sites, in the binding of acetylene in MFM-102-NO2.

Exceptional Adsorption and Binding of Sulfur Dioxide in a Robust Zirconium-Based Metal-Organic Framework.

We report a record-high SO2 adsorption capacity of 12.3 mmol g-1 in a robust porous material, MFM-601, at 298 K and 1.0 bar. SO2 adsorption in MFM-601 is fully reversible and highly selective over CO2 and N2. The binding domains for adsorbed SO2 and CO2 molecules in MFM-601 have been determined by in situ synchrotron X-ray diffraction experiments, giving insights at the molecular level to the basis of the observed high selectivity.

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

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.

Confinement of Iodine Molecules into Triple-Helical Chains within Robust Metal-Organic Frameworks.

During nuclear waste disposal process, radioactive iodine as a fission product can be released. The widespread implementation of sustainable nuclear energy thus requires the development of efficient iodine stores that have simultaneously high capacity, stability and more importantly, storage density (and hence minimized system volume). Here, we report high I2 adsorption in a series of robust porous metal-organic materials, MFM-300(M) (M = Al, Sc, Fe, In). MFM-300(Sc) exhibits fully reversible I2 uptake of 1.54 g g-1, and its structure remains completely unperturbed upon inclusion/removal of I2. Direct observation and quantification of the adsorption, binding domains and dynamics of guest I2 molecules within these hosts have been achieved using XPS, TGA-MS, high resolution synchrotron X-ray diffraction, pair distribution function analysis, Raman, terahertz and neutron spectroscopy, coupled with density functional theory modeling. These complementary techniques reveal a comprehensive understanding of the host-I2 and I2-I2 binding interactions at a molecular level. The initial binding site of I2 in MFM-300(Sc), I2I, is located near the bridging hydroxyl group of the [ScO4(OH)2] moiety [I2I···H-O = 2.263(9) Å] with an occupancy of 0.268. I2II is located interstitially between two phenyl rings of neighboring ligand molecules [I2II···phenyl ring = 3.378(9) and 4.228(5) Å]. I2II is 4.565(2) Å from the hydroxyl group with an occupancy of 0.208. Significantly, at high I2 loading an unprecedented self-aggregation of I2 molecules into triple-helical chains within the confined nanovoids has been observed at crystallographic resolution, leading to a highly efficient packing of I2 molecules with an exceptional I2 storage density of 3.08 g cm-3 in MFM-300(Sc).

Binding CO2 by a Cr8 Metallacrown.

The {Cr8 } metallacrown [CrF(O2 Ct Bu)2 ]8 , containing a F-lined internal cavity, shows high selectivity for CO2 over N2 . DFT calculations and absorption studies support the multiple binding of F-groups to the C-center of CO2 (C⋅⋅⋅F 3.190(9)-3.389(9) Å), as confirmed by single-crystal X-ray diffraction.

Amides Do Not Always Work: Observation of Guest Binding in an Amide-Functionalized Porous Metal-Organic Framework.

An amide-functionalized metal organic framework (MOF) material, MFM-136, shows a high CO2 uptake of 12.6 mmol g-1 at 20 bar and 298 K. MFM-136 is the first example of an acylamide pyrimidyl isophthalate MOF without open metal sites and, thus, provides a unique platform to study guest binding, particularly the role of free amides. Neutron diffraction reveals that, surprisingly, there is no direct binding between the adsorbed CO2/CH4 molecules and the pendant amide group in the pore. This observation has been confirmed unambiguously by inelastic neutron spectroscopy. This suggests that introduction of functional groups solely may not necessarily induce specific guest-host binding in porous materials, but it is a combination of pore size, geometry, and functional group that leads to enhanced gas adsorption properties.

Long-Term Stability of MFM-300(Al) toward Toxic Air Pollutants.

Temperature- or pressure-swing sorption in porous metal-organic framework (MOF) materials has been proposed for new gas separation technologies. The high tunability of MOFs toward particular adsorbates and the relatively low energy penalty for system regeneration indicate that reversible physisorption in MOFs has the potential to create economic and environmental benefits compared with state-of-the-art chemisorption systems. However, for MOF-based sorbents to be commercialized, they have to show long-term stability under the conditions imposed by the application. Here, we demonstrate the structural stability of MFM-300(Al) in the presence of a series of industrially relevant toxic and corrosive gases, including SO2, NO2, and NH3, over 4 years using long-duration synchrotron X-ray powder diffraction. Full structural analysis of gas-loaded MFM-300(Al) confirms the retention of these toxic gas molecules within the porous framework for up to 200 weeks, and cycling adsorption experiments verified the reusability of MFM-300(Al) for the capture of these toxic air pollutants.

Post-synthetic modulation of the charge distribution in a metal-organic framework for optimal binding of carbon dioxide and sulfur dioxide.

Modulation of pore environment is an effective strategy to optimize guest binding in porous materials. We report the post-synthetic modification of the charge distribution in a charged metal-organic framework, MFM-305-CH3, [Al(OH)(L)]Cl, [(H2L)Cl = 3,5-dicarboxy-1-methylpyridinium chloride] and its effect on guest binding. MFM-305-CH3 shows a distribution of cationic (methylpyridinium) and anionic (chloride) centers and can be modified to release free pyridyl N-centres by thermal demethylation of the 1-methylpyridinium moiety to give the neutral isostructural MFM-305. This leads simultaneously to enhanced adsorption capacities and selectivities (two parameters that often change in opposite directions) for CO2 and SO2 in MFM-305. The host-guest binding has been comprehensively investigated by in situ synchrotron X-ray and neutron powder diffraction, inelastic neutron scattering, synchrotron infrared and 2H NMR spectroscopy and theoretical modelling to reveal the binding domains of CO2 and SO2 in these materials. CO2 and SO2 binding in MFM-305-CH3 is shown to occur via hydrogen bonding to the methyl and aromatic-CH groups, with a long range interaction to chloride for CO2. In MFM-305 the hydroxyl, pyridyl and aromatic C-H groups bind CO2 and SO2 more effectively via hydrogen bonds and dipole interactions. Post-synthetic modification via dealkylation of the as-synthesised metal-organic framework is a powerful route to the synthesis of materials incorporating active polar groups that cannot be prepared directly.

Locating the binding domains in a highly selective mixed matrix membrane via synchrotron IR microspectroscopy.

The binding domains within a mixed matrix membrane (MMM) that is selective for CO2 comprising MFM-300(Al) and the polymer 6FDA-Durene-DABA have been established via in situ synchrotron IR microspectroscopy. The MOF crystals are fully accessible and play a critical role in the binding of CO2, creating a selective pathway to promote permeation of CO2 within and through the MMM. This study reveals directly the molecular mechanism for the overall enhanced performance of this MMM in terms of permeability, solubility and selectivity for CO2.

Direct observation of supramolecular binding of light hydrocarbons in vanadium(iii) and (iv) metal-organic framework materials.

Fine tuning of host-guest supramolecular interactions in porous systems enables direct control over the properties of functional materials. We report here a modification of hydrogen bonding and its effect on guest binding in a pair of redox-active metal-organic frameworks (MOFs). Oxidation of MFM-300(VIII) {[VIII2(OH)2(L)], LH4 = biphenyl-3,3',5,5'-tetracarboxylic acid} is accompanied by deprotonation of the bridging hydroxyl groups to afford isostructural MFM-300(VIV), [VIV2O2(L)]. The precise role of the hydroxyl groups, O-carboxylate centres and π-π interactions in the supramolecular binding of C2 hydrocarbons in these materials has been determined using neutron diffraction and inelastic neutron scattering, coupled with DFT modelling. The hydroxyl protons are observed to bind to adsorbed unsaturated hydrocarbons preferentially in MFM-300(VIII), particularly to C2H2, which is in a sharp contrast to MFM-300(VIV) where interactions with O-carboxylate centres and π-π interactions predominate. This variation in structure and redox leads to notably higher separation selectivity for C2H2/CH4 and C2H4/CH4 in MFM-300(VIII) than in MFM-300(VIV). Significantly, owing to the specific host-guest interactions, MFM-300(VIII) shows a record packing density for adsorbed C2H2 at 303 K and 1 bar, demonstrating its potential for use in portable acetylene stores.

Unravelling exceptional acetylene and carbon dioxide adsorption within a tetra-amide functionalized metal-organic framework.

Understanding the mechanism of gas-sorbent interactions is of fundamental importance for the design of improved gas storage materials. Here we report the binding domains of carbon dioxide and acetylene in a tetra-amide functionalized metal-organic framework, MFM-188, at crystallographic resolution. Although exhibiting moderate porosity, desolvated MFM-188a exhibits exceptionally high carbon dioxide and acetylene adsorption uptakes with the latter (232 cm3 g-1 at 295 K and 1 bar) being the highest value observed for porous solids under these conditions to the best of our knowledge. Neutron diffraction and inelastic neutron scattering studies enable the direct observation of the role of amide groups in substrate binding, representing an example of probing gas-amide binding interactions by such experiments. This study reveals that the combination of polyamide groups, open metal sites, appropriate pore geometry and cooperative binding between guest molecules is responsible for the high uptakes of acetylene and carbon dioxide in MFM-188a.

Stepwise observation and quantification and mixed matrix membrane separation of CO2 within a hydroxy-decorated porous host.

The identification of preferred binding domains within a host structure provides important insights into the function of materials. State-of-the-art reports mostly focus on crystallographic studies of empty and single component guest-loaded host structures to determine the location of guests. However, measurements of material properties (e.g., adsorption and breakthrough of substrates) are usually performed for a wide range of pressure (guest coverage) and/or using multi-component gas mixtures. Here we report the development of a multifunctional gas dosing system for use in X-ray powder diffraction studies on Beamline I11 at Diamond Light Source. This facility is fully automated and enables in situ crystallographic studies of host structures under (i) unlimited target gas loadings and (ii) loading of multi-component gas mixtures. A proof-of-concept study was conducted on a hydroxyl-decorated porous material MFM-300(VIII) under (i) five different CO2 pressures covering the isotherm range and (ii) the loading of equimolar mixtures of CO2/N2. The study has successfully captured the structural dynamics underpinning CO2 uptake as a function of surface coverage. Moreover, MFM-300(VIII) was incorporated in a mixed matrix membrane (MMM) with PIM-1 in order to evaluate the CO2/N2 separation potential of this material. Gas permeation measurements on the MMM show a great improvement over the bare PIM-1 polymer for CO2/N2 separation based on the ideal selectivity.

Modulating supramolecular binding of carbon dioxide in a redox-active porous metal-organic framework.

Hydrogen bonds dominate many chemical and biological processes, and chemical modification enables control and modulation of host-guest systems. Here we report a targeted modification of hydrogen bonding and its effect on guest binding in redox-active materials. MFM-300(VIII) {[VIII2(OH)2(L)], LH4=biphenyl-3,3',5,5'-tetracarboxylic acid} can be oxidized to isostructural MFM-300(VIV), [VIV2O2(L)], in which deprotonation of the bridging hydroxyl groups occurs. MFM-300(VIII) shows the second highest CO2 uptake capacity in metal-organic framework materials at 298 K and 1 bar (6.0 mmol g-1) and involves hydrogen bonding between the OH group of the host and the O-donor of CO2, which binds in an end-on manner, =1.863(1) Å. In contrast, CO2-loaded MFM-300(VIV) shows CO2 bound side-on to the oxy group and sandwiched between two phenyl groups involving a unique ···c.g.phenyl interaction [3.069(2), 3.146(3) Å]. The macroscopic packing of CO2 in the pores is directly influenced by these primary binding sites.