Modelling and advanced characterization of framework materials

Communications Chemistry is delighted to introduce a Collection of research works focused on the modelling and advanced characterization of framework materials. Here, the Guest Editors outline the themes within and look towards the future of the field.

Constructing crystalline materials with specific stimuli-responsive dynamics and controlled molecular motion affords opportunities for innovative functionality and applications. Here, the authors discuss recent developments in dynamic solid-state framework materials across a range of material classes, exploring key phenomena associated with such complex dynamics.

Metal–organic frameworks functionalized with photoresponsive molecules are of interest as materials with photoswitchable electronic properties, but designing such MOFs remains challenging. Here, the authors use in silico molecular design to explore photoswitchable MOF candidates that incorporate spiropyran photoswitches at controlled positions and with defined intermolecular distances and orientations.

Modelling the structural transitions of flexible metal-organic frameworks provides key insight into their breathing behaviours, but molecular dynamics simulations performed to date do not typically account for finite size and surface effects affecting the phase transition mechanism. Here, the authors present an approach that allows for the analysis and control of the volume of finite-size structures during molecular dynamics simulations, using a tetrahedral tessellation of nanocrystallite volume.

Understanding the mechanical properties of metal–organic frameworks (MOFs) is crucial for their industrial application, but the compressibility of structurally dynamic MOFs under pressure remains unclear. Here, in situ variable pressure powder X-ray diffraction experiments on two families of dynamic MOFs find that increased structural flexibility results in enhanced mechanical resilience.

Metal–organic frameworks containing rotatable dipolar linkers are of interest for sensor systems and in gas separation. Here, the authors investigate the molecular rotor behaviour of benzothiadiazole units in the carboxylate-based MOFs ZJNU-40 and JLU-Liu30 using dielectric spectroscopy and computational simulations.

Aluminium-based cationic metal–organic frameworks remain rare, yet offer opportunities for unusual framework structures and material properties. Here, a robust ultra-microporous cationic metal–organic framework based on aluminium building units and flexible tetracarboxylic acid linkers is reported, and three-dimensional electron diffraction combined with high-resolution powder X-ray diffraction show that the material comprises an ordered defective cationic framework with narrow quasi-1D channels.

Metal–organic frameworks have demonstrated great promise for gas separation membranes, but their structural flexibility hinders their stability and hence sieving performance. Here, the authors systematically tune the ratio of mixed linkers in CAU-10-based MOF membranes, finding the optimal ratio that suppresses structural flexibility and enhances CO₂/CH₄ separation performance.

Understanding the complex mechanical behaviour of metal–organic frameworks is of importance for their real-world application. Here, an in situ micropillar compression method is demonstrated to provide valuable insights into the anisotropic mechanical properties of the HKUST-1 single crystal.

Incorporating electron donor functionalities into porous coordination frameworks enables the strong binding of electron-withdrawing guests, but such binding typically occurs irreversibly. Here, a structurally dynamic 2D coordination network incorporating an electron donating group is found to selectively and reversibly bind oxygen and nitrous oxide, while also exhibiting large structural deformations after guest removal.

While stimuli-responsive metal-organic frameworks have been widely investigated, much less is understood about structural flexibility in their covalent counterparts. Here, 3D diamondoid covalent-organic frameworks are studied via dynamic free energy simulations, revealing key insight into how the nature of the building blocks and the degree of interpenetration contribute to framework flexibility.

Controlling the thermal expansion of a material is of importance for material longevity in applications with fluctuating temperatures. Here, the impact of nanoparticle loading on the thermal expansion behaviour of a pillared metal–organic framework is explored using variable temperature powder X-ray diffraction, and a significant reduction in the thermal stress is observed.

While the bulk structures of metal–organic frameworks can be solved by diffraction-based techniques, characterization of their local structures has been lacking. Here the authors review recent advances in (scanning) transmission electron microscopy that have made it possible to probe the local structures of MOFs at atomic resolution.

Covalent organic frameworks have demonstrated promise for a wide variety of applications, but their synthesis often results in materials with low crystallinity that are challenging to characterize. Here, coupling three dimensional electron diffraction with simulated annealing is shown to enable the structural determination of a low crystallinity pyrene-containing covalent organic framework.

Framework materials containing amorphous and nanocrystalline phases are challenging to characterize as they are poorly represented by average structural descriptors, and thus microscopic to nanoscale analysis is often required. Here, scanning electron diffraction combined with electron pair distribution function analysis and Bragg scattering analysis are used to probe spatial variations in the structure of metal–organic framework Fe-BTC, known to comprise crystalline nanoparticles and an amorphous matrix.

Metal-organic frameworks with desirable properties can be designed through careful choice of linker and node combinations, but achieving the synthesis of a desired MOF is complex and dependent on many experimental variables. Here, a genetic algorithm combined with experimental feedback and confirmation is used to obtain the optimal microwave-assisted synthesis conditions for a porphyrin-based aluminium MOF (Al-PMOF), achieving excellent crystallinity and a close to 80% yield in only the 2^nd generation.

Structural defects in metal–organic frameworks can be exploited to tune material properties. Here, a combined theoretical and experimental study demonstrates that the introduction of defects to UiO-66 alters its nature from hydrophobic to hydrophilic, affecting the adsorption mechanism of polar and non-polar molecules.

Computational methods enable the rapid evaluation of the gas uptake capabilities of metal–organic frameworks. Here, MOFs are screened for their CO₂ capture abilities using machine learning trained on molecular simulation data, with new effective point charge descriptors introduced to aid this process.

Molecular framework materials have shown great promise as heterogeneous catalysts, but understanding the structural changes that such catalysts suffer over time remains challenging. Here, the environment around the metal centers of a Cu₃[Co(CN)₆]₂-based double metal cyanide catalyst is studied before and after the synthesis of several phosphoramidates via an environmentally friendly route.

CHA zeolites display high selectivity toward CO₂ adsorption, but the influence of the nature and concentration of their extra-framework cations on the formation of silanol sites, and the respective impact on CO₂ sorption, is poorly understood. Here, the authors use high-resolution solid-state magic-angle spinning ¹H NMR and Fourier-transform infrared spectroscopy to study the effects of post-synthetic exchange of extra-framework alkali-metal cations on silanol sites in nanosized CHA zeolites.

Developing computational tools to study the phase behaviour of adsorbate molecules in complex pore architectures can greatly facilitate our understanding of phase transitions and phase equilibria in porous materials. Here, molecular simulations are used to study the hysteresis and phase equilibria of n-alkane adsorbate molecules in a metal–organic framework with both micropores and mesopores, and simulations in the canonical ensemble with Widom insertions are shown to be a powerful complementary approach to grand canonical Monte Carlo simulations.

The experimental realization of p-orbital systems with exotic quantum phases is desirable for the obtainment of strongly correlated materials. Here, two sublattices composed of molecules with p-orbital characteristics are combined to realize a p-orbital honeycomb-Kagome lattice in a two dimensional metal–organic framework on a Au(111) substrate.

Hydrophilic metal-organic framework NU-1500-Cr is a high performing water harvesting material, but the mechanism through which it adsorbs water remains unclear. Here, molecular dynamics simulations and infrared spectroscopy are used to follow the water adsorption process in NU-1500-Cr from the initial hydration stage to complete filling of the MOF pores.

Linker functionalization is a commonly used strategy to affect the electronic and catalytic properties of metal–organic frameworks, but its impact on the d-orbital energies of the metals to which the linkers are bound has been largely overlooked. Here, DFT calculations are used to study the energetics of d-orbital energy tuning as a function of linker chemistry in UiO-66, MIL-125, ZIF-8 and MOF-5.

Porous sorbents capable of high ammonia (NH₃) uptake capacities are of great interest for ammonia storage for industry, as well as for the environmental remediation of this toxic and corrosive gas. Here, NH₃ adsorption is investigated in four robust aluminium-based metal–organic frameworks, and in situ neutron powder diffraction, synchrotron IR micro-spectroscopy and ²⁷Al solid-state NMR studies show that the pore geometries, framework rigidity, and nature of the host–guest binding sites together dictate the high NH₃ capture and storage capacity.

Metal–organic frameworks have demonstrated promise for the storage and release of biologically active gases. Here, an in situ single crystal X-ray diffraction study using synchrotron radiation elucidates the binding mechanisms and geometries of nitric oxide and carbon monoxide gases in activated frameworks Ni-CPO-27 and Co-4,6-dihydroxyisophthalate.

Techniques capable of measuring adsorption processes in solution typically rely on indirect methods. Here, a magnetic sustentation technique is shown to rapidly and directly measure the mass of adsorbates in four paramagnetic metal–organic framework materials in solution.

Determining the spatial ordering and hydrogen bonding dynamics of confined water molecules within nanopores remains challenging. Here, Raman spectroscopy and crystallographic studies show ice-like spatial ordering of unbound water molecules within the pores of the metal-organic framework HKUST-1 at room temperature.

Metal–organic frameworks have been shown to adsorb and decompose chemical warfare agents, but their mechanism of action is not completely understood. Here the authors quantitatively track the binding and decomposition product structures of nerve-agent simulant dimethyl methylphosphonate in host UiO-67 through in situ X-ray total scattering measurements, pair distribution function analysis, and density functional theory calculations.

Flexible metal–organic frameworks that undergo structural transformations upon gas sorption show great promise for applications in gas storage and separation, but accurately describing their stepped isotherms and hysteresis remains a challenge. Here, the authors introduce an empirical model to describe hysteretic MOF sorption isotherms and determine their temperature dependence, which is asymptotic at low temperatures.