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Modelling and advanced characterization of framework materials
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Recent years have seen a rapid acceleration of research on framework materials, including, among others, metal–organic frameworks, covalent organic frameworks, supramolecular organic frameworks, porous organic polymers, and inorganic framework materials. These materials demonstrate properties beyond what was considered achievable for inorganic or organic porous materials in the past, and have potential applications in separation and storage, heterogeneous catalysis, sensing, drug delivery, and beyond.
While framework materials can display a range of desirable properties, the detailed study of their molecular and supramolecular structures, and characterization of the relationships between microscopic structure and macroscopic properties, is a very challenging area of research. It is rendered difficult by their inherent chemical and structural molecular complexity, as well as their propensity to display large-scale dynamic behaviours. Research into the structure and behaviour of framework materials thus requires the development of novel methodologies, as well as the combination of state-of-the-art techniques to provide a full picture of the different phenomena at play. Such advanced characterization techniques include in situ X-ray and neutron diffraction, total scattering methods, high-resolution transmission electron microscopy, magnetometry, calorimetry, in situ, operando, high spatial resolution and multi-dimensional spectroscopic methods, solid-state NMR and more. Such studies also require pushing the boundaries of computational chemistry methodologies for atomistic modelling, with methods such as first-principles molecular dynamics, free energy methods, development of next-generation force fields for flexible and reactive materials, coarse-graining methods, and many more.
This Collection brings together research focused on advanced characterization and computational modelling, providing new molecular insight on the structure and behaviour of framework materials. The Collection primarily welcomes original research papers, in the form of both full articles and communications. All submissions will be subject to the same review process and editorial standards as regular Communications Chemistry Articles.
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.
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.
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.
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.
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.
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.
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.
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.
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 2nd 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.