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.

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

Binding of C₂H₂ in MFM-300(V^III) showing interactions to O–H, carboxylate O-centres and intermolecular packing.

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(V^III) {[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(V^IV), [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(V^III), particularly to C₂H₂, which is in a sharp contrast to MFM-300(V^IV) 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(V^III) than in MFM-300(V^IV). Significantly, owing to the specific host–guest interactions, MFM-300(V^III) shows a record packing density for adsorbed C₂H₂ at 303 K and 1 bar, demonstrating its potential for use in portable acetylene stores.

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

The location of binding domains in a new CO₂ selective mixed matrix membrane has been established via in situ synchrotron IR microspectroscopy.

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(V^III) {[V^III₂(OH)₂(L)], LH₄=biphenyl-3,3′,5,5′-tetracarboxylic acid} can be oxidized to isostructural MFM-300(V^IV), [V^IV₂O₂(L)], in which deprotonation of the bridging hydroxyl groups occurs. MFM-300(V^III) 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(V^IV) 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.

Gaining molecular-level insight into host–guest binding interactions is fundamentally important, but experimentally challenging. Here, Schröder and co-workers study CO₂–host hydrogen bonding interactions in a pair of isostructural redox-active V^III/V^IV MOFs using neutron scattering and diffraction techniques.

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.