Observation of Binding and Rotation of Methane and Hydrogen within a Functional Metal-Organic Framework

Binding of carbon dioxide and acetylene to free carboxylic acid sites in a metal-organic framework

The functionalisation of organic linkers in metal-organic frameworks (MOFs) to improve gas uptake is well-documented. Although the positive role of free carboxylic acid sites in MOFs for binding gas molecules has been proposed in computational studies, relatively little experimental evidence has been reported in support of this. Primarily this is because of the inherent synthetic difficulty to prepare MOF materials bearing free, accessible -COOH moieties which would normally bind to metal ions within the framework structure. Here, we describe the direct binding of CO2 and C2H2 molecules to the free -COOH sites within the pores of MFM-303(Al). MFM-303(Al) exhibits highly selective adsorption of CO2 and C2H2 with a high selectivity for C2H2 over C2H4. In situ synchrotron X-ray diffraction and inelastic neutron scattering, coupled with modelling, highlight the cooperative interactions of adsorbed CO2 and C2H2 molecules with free -COOH and -OH sites within MFM-303(Al), thus rationalising the observed high selectivity for gas separation.

Exceptional capture of methane at low pressure by an iron-based metal-organic framework

The selective capture of methane (CH4) at low concentrations and its separation from N2 are extremely challenging owing to the weak host-guest interactions between CH4 molecules and any sorbent material. Here, we report the exceptional adsorption of CH4 at low pressure and the efficient separation of CH4/N2 by MFM-300(Fe). MFM-300(Fe) shows a very high uptake for CH4 of 0.85 mmol g-1 at 1 mbar and 298 K and a record CH4/N2 selectivity of 45 for porous solids, representing a new benchmark for CH4 capture and CH4/N2 separation. The excellent separation of CH4/N2 by MFM-300(Fe) has been confirmed by dynamic breakthrough experiments. In situ neutron powder diffraction, and solid-state nuclear magnetic resonance and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modelling, reveal a unique and strong binding of CH4 molecules involving Fe-OH⋯CH4 and C⋯phenyl ring interactions within the pores of MFM-300(Fe), thus promoting the exceptional adsorption of CH4 at low pressure.

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal-Organic Framework

Conversion of methane (CH4) to ethylene (C2H4) and/or acetylene (C2H2) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH4 controllably, and separating C2+ products from unreacted CH4 can be challenging. Here, we report the direct conversion of CH4 to C2H4 and C2H2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal-organic framework material, MFM-300(Fe). The selectivity for the formation of C2H4 and C2H2 reaches 96% with a high time yield of 334 μmol gcat-1 h-1. At a conversion of 10%, the selectivity to C2+ hydrocarbons and time yield exceed 98% and 2056 μmol gcat-1 h-1, respectively, representing a new benchmark for conversion of CH4. In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe-O(H)-Fe sites in activating CH4 and stabilizing reaction intermediates via the formation of an Fe-O(CH3)-Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C2H4 and C2H2 from unreacted CH4 for direct use. Integrating the processes of CH4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Porous Metal-Organic Frameworks for Hydrogen Storage

The high gravimetric energy density and environmental benefit place hydrogen as a promising alternative to the widely used fossil fuel, which is however impeded by the lack of safe, energy-saving...

In Situ Aliovalent Nickle Substitution and Acidic Modification of Nanowalls Promoted Proton Conductivity in InOF with 1D Helical Channel

Proton-conductive materials have attracted increasing attention because of their broad explorations in chemical sensors, water electrolysis, fuel cells, and biological systems. Especially, metal-organic frameworks (MOFs) have been demonstrated to be extremely promising candidates as proton-exchange membrane (PEM) fuel cells. Compared with other configurations, MOFs with one-dimensional (1D) channels have the characteristics of enhancing the host-guest interaction and promoting the anisotropic motion of proton carriers in restricted volume, which are beneficial for acquiring rich proton sources and forming successive hydrogen bonds to improve proton conductivity. We are endeavored to screen and find a helical three-dimensional (3D) framework InOF-1, namely, [In₂(OH)₂(BPTC)]·6H₂O (BPTC^4- = 3,3',5,5'-biphenyl tetracarboxylate), as a typical 1D-channel MOF, which is pristinely grafted with spirally distributed -OH groups on the channel surface. Accompanied by an aliovalent substitution Ni(II) for In(III), isostructural NiOF-1 ([Ni₂(BPTC)(HCOOH)₂]·3H₂O) is successfully prepared and massive formic acids are anchored at interior walls, which are interacted with adsorbed water molecules via the formation of stronger O-H···O bonds. This interaction between host-guest molecules and dynamics of lattice water has already led to a remarkable conductivity of InOF-1 (σ = 7.86 × 10^-3 S/cm at 328 K under 95% RH). The synergistic effect of the acidic-modified nanowall, contracted volume, and enhanced adsorption of water molecules in the NiOF-1 channel contributes to a high conductivity value of 3.41 × 10^-2 S/cm (at 328 K under 95% RH). Moreover, the proton conduction mechanism is further visually presented by molecular dynamic (MD) simulation. In contrast to InOF-1, aliovalent-substituted and acidic-modified NiOF-1 has a stronger host-guest interaction and more abundant hydrogen-bond networks, resulting in shorter proton migration distances and more frequent proton hopping, in agreement with the experimental results.

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.

Zinc-tetracarboxylate framework material with nano-cages and one-dimensional channels for excellent selective and effective adsorption of methyl blue dye

An example of a zinc-tetracarboxylate framework material (FJI-11) was solvothermally synthesized and structurally characterized. FJI-11 presented 3D cage-stacking frameworks with octahedral cages, cuboctahedral cages and two kinds of 1D channel along the c-axis. In addition, FJI-11 exhibited the excellent selective and effective adsorption of methyl blue (MB) dye by guest molecule exchange, and its adsorption process was in accordance with the second-order kinetic model and the Freundlich model.

Salt- and gas-filled ices under planetary conditions

In recent years, evidence has emerged that solid water can contain substantial amounts of guest species, such as small gas molecules-in gas hydrate structures-or ions-in salty ice structures-and that these 'filled' ice structures can be stable under pressures of tens of Gigapascals and temperatures of hundreds of Kelvins. The inclusion of guest species can strongly modify the density, vibrational, diffusive and conductivity properties of ice under high pressure, and promote novel exotic properties. In this review, we discuss our experimental findings and molecular dynamics simulation results on the structures formed by salt- and gas-filled ices, their unusual properties, and the unexpected dynamical phenomena observed under pressure and temperature conditions relevant for planetary interiors modelling. This article is part of the theme issue 'The physics and chemistry of ice: scaffolding across scales, from the viability of life to the formation of planets'.

An In-Situ Neutron Diffraction and DFT Study of Hydrogen Adsorption in a Sodalite-Type Metal-Organic Framework, Cu-BTTri

Herein we present a detailed study of the hydrogen adsorption properties of Cu-BTTri, a robust crystalline metal-organic framework containing open metal-coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in-situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu-BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites.

Doubly interpenetrated indium-tricarboxylate frameworks mediated by small molecules with enhanced porosity

A synthetic strategy of indium-tricarboxylate frameworks by using small molecule regulators has been proposed to obtain four types of In-based coordination polymers with doubly interpenetrated structures.

Nanospace within metal-organic frameworks for gas storage and separation

Porous metal-organic frameworks (MOFs), also known as porous coordination polymers, represent a new class of porous materials, and one of their striking features lies in their tunable, designable, and functionalizable nanospace. This nanospace within MOFs provides virtually plenty of room for imagination, allowing designed incorporation of different size, shape, and functionalities for targeted gas storage and separation applications. Furthermore, the features of high porosities, tunable framework structures and pore sizes, and immobilized functional sites enable MOF materials to fully make use of their nanopore space for gas storage, to optimize their sieving effects, and to differentiate their interactions with gas molecules for gas separation. In this review article, we highlight some recent significant advances in developing microporous MOFs for some of the most important gas storage and separation applications.

Structural and dynamic studies of substrate binding in porous metal-organic frameworks

Porous metal-organic frameworks (MOFs) are the subject of considerable research interest because of their high porosity and capability of specific binding to small molecules, thus underpinning a wide range of materials functions such as gas adsorption, separation, drug delivery, catalysis, and sensing. MOFs, constructed by the designed assembly of metal ions and functional organic linkers, are an emerging class of porous materials with extended porous structures containing periodic binding sites. MOFs thus provide a new platform for the study of the chemistry and reactivity of small molecules in confined pores using advanced diffraction and spectroscopic techniques. In this review, we focus on recent progress in experimental investigations on the crystallographic, dynamic and kinetic aspects of substrate binding within porous MOFs. In particular, we focus on studies on host-guest interactions involving open metal sites or pendant functional groups in the pore as the primary binding sites for guest molecules.

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.

Three Cd(ii) coordination polymers constructed from a series of multidentate ligands derived from cyclotriphosphazene: synthesis, structures and luminescence properties

By utilizing three ligands derived from cyclotriphosphazene, three novel cadmium-based coordination polymers have been successfully synthesized and structurally characterized, which exhibit the corresponding photo-chemical properties.

Five transition metal coordination polymers driven by a semirigid trifunctional nicotinic acid ligand: selective adsorption and magnetic properties

Five coordination polymers have been synthesized by a new organic linker containing three distinct types of functional groups together with the mixed 2,2′-bipy or 4,4′-bipy co-ligand, revealing various framework structures and selective gas adsorption and magnetic properties.

Chiral metal–organic frameworks constructed from four-fold helical chain SBUs for enantioselective recognition of α-hydroxy/amino acids

Three chiral 3D metal–carboxylate frameworks have been successfully synthesized, featuring four-fold helical metal chains as SBUs. Co-MOFs could recognize enantio-selectively α-hydroxy/amino acids by the change of CD signals.

A Homochiral Multifunctional Metal-Organic Framework with Rod-Shaped Secondary Building Units

A new homochiral multifunctional metal-organic framework, [Zn₂(CTBA)₂·H₂O] (JUC-112), was synthesized under solvothermal conditions, through the design of chiral ligand 4-(3-carboxy-2,2,3-trimethylcyclopentanecarboxamido) benzoic acid (H₂CTBA) based on camphoric acid as building block. The crystal structure of the new material is a 2-dimensional (2D) chiral layer packed with infinite rod-shaped secondary building units (SBUs). The homochiral framework was identified by circular dichroism (CD) spectrum. Thermogravimetric measurement indicates its high thermal stability up to 450 °C. In addition, JUC-112 exhibits the capability of separating water from alcohols, second-order nonlinear optical effect, and photoluminescence.

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.

Selective adsorption behaviour of carbon dioxide in OH-functionalized metal–organic framework materials

The theoretically optimal adsorption locations in hydroxyl (OH)-decorated metal–organic frameworks show that the captured CO₂ molecules interact with the cis-μ₂-OH groups in an end-on mode, which shows a moderate to weak hydrogen bond.

Five 1D to 3D Zn(ii)/Mn(ii)-CPs based on dicarboxyphenyl-terpyridine ligand: stepwise adsorptivity and magnetic properties

Five coordination polymers with different dimensional structures have been solvothermally synthesized by utilizing H₂dtp ligand. Complexes1and2reveal strong solid-state luminescence, and complexes3–5display antiferromagnetic exchange.

Selective Adsorption of Sulfur Dioxide in a Robust Metal-Organic Framework Material

Selective adsorption of SO₂ is realized in a porous metal-organic framework material, and in-depth structural and spectroscopic investigations using X-rays, infrared, and neutrons define the underlying interactions that cause SO₂ to bind more strongly than CO₂ and N₂ .