Probing the hydrogen equilibrium and kinetics in zeolite imidazolate frameworks via molecular dynamics and quasi-elastic neutron scattering experiments

ResponZIF Structures: Zeolitic Imidazolate Frameworks as Stimuli-Responsive Materials

Zeolitic imidazolate frameworks (ZIFs) have long been recognized as a prominent subset of the metal-organic framework (MOF) family, in part because of their ease of synthesis and good thermal and chemical stability, alongside attractive properties for diverse potential applications. Prototypical ZIFs like ZIF-8 have become embodiments of the significant promise held by porous coordination polymers as next-generation designer materials. At the same time, their intriguing property of experiencing significant structural changes upon the application of external stimuli such as temperature, mechanical pressure, guest adsorption, or electromagnetic fields, among others, has placed this family of MOFs squarely under the umbrella of stimuli-responsive materials. In this review, we provide an overview of the current understanding of the triggered structural and electronic responses observed in ZIFs (linker and bond dynamics, crystalline and amorphous phase changes, luminescence, etc.). We then describe the state-of-the-art experimental and computational methodology capable of shedding light on these complex phenomena, followed by a comprehensive summary of the stimuli-responsive nature of four prototypical ZIFs: ZIF-8, ZIF-7, ZIF-4, and ZIF-zni. We further expose the relevant challenges for the characterization and fundamental understanding of responsive ZIFs, including how to take advantage of their flexible properties for new application avenues.

Dynamics of propene and propane in ZIF-8 probed by solid-state 2H NMR

We present a detailed ²H NMR characterization of molecular mobility of propene and propane propagating though the microporous ZIF-8, a zeolitic imidazolate framework renowned for its outstandingly high separation selectivity for industrially relevant propene/propane mixtures. Experimental characterization of both propene and propane diffusivity in ZIF-8 has been provided. Using ²H NMR spin relaxation analysis, the motional mechanisms for propene and propane guests trapped within the ZIF-8 framework have been elucidated. Kinetic parameters for each type of motion were derived. The characteristic times for microscopic translational diffusion and activation barriers (E_C3H8 = 38 kJ mol^-1, E_C3H6 = 13.5 kJ mol^-1) for propane and propene diffusivities have been estimated. A notable difference in the observed activation barriers emphasizes that the ZIF-8 window crossing is associated with the "gate-opening" and represents an extremely shape selective process. Finally, we show that the ²H NMR technique is capable of providing reliable information on microscopic diffusivity in the ZIF-8 MOF even for molecules with slow diffusivity (<10^-14 m² s^-1).

Thermodynamic Evidence of Structural Transformations in CO2-Loaded Metal-Organic Framework Zn(MeIm)2 from Heat Capacity Measurements

Metal-organic frameworks are a class of porous compounds with potential applications in molecular sieving, gas sequestration, and catalysis. One family of MOFs, zeolitic imidizolate frameworks (ZIFs), is of particular interest for carbon dioxide sequestration. We have previously reported the heat capacity of the sodalite topology of the zinc 2-methylimidazolate framework (ZIF-8), and in this Article we present the first low-temperature heat capacity measurements of ZIF-8 with various amounts of sorbed CO₂. Molar heat capacities from 1.8 to 300 K are presented for samples containing up to 0.99 mol of CO₂ per mol of ZIF-8. Samples with at least 0.56 mol of CO₂ per mol of ZIF-8 display a large, broad anomaly from 70 to 220 K with a shoulder on the low-temperature side, suggesting sorption-induced structural transitions. We attribute the broad anomaly partially to a gate-opening transition, with the remainder resulting from CO₂ rearrangement and/or lattice expansion. The measurements also reveal a subtle anomaly from 0 to 70 K in all samples that does not exist in the sorbate-free material, which likely reflects new vibrational modes resulting from sorbate/ZIF-8 interactions. These results provide the first thermodynamic evidence of structural transitions induced by CO₂ sorption in the ZIF-8 framework.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

A comprehensive overview of characterization tools for the analysis of well-known metal–organic frameworks and physico-chemical phenomena associated to their applications.

Molecular Sieves for the Separation of Hydrogen Isotopes

Stable molecular hydrogen isotopes, D₂ and T₂, are both scarce and essential in several energy, industrial, and large-scale fundamental research applications. Due to the chemical similarity of these isotopes, their extraction and purification from hydrogen has relied for decades on expensive and energy-demanding processes. However, factoring in the phenomenon of quantum sieving could provide a new route for these separations. In this work, we have explored how to separate hydrogen isotopes by adsorption taking these quantum effects into account. To this end, we have conducted adsorption measurements to test our deuterium model and performed a widespread computational screening over 210 pure-silica zeolites for D₂/H₂ and T₂/H₂ separations. Based on low-coverage adsorption properties, a reduced set of zeolites have been singled out and their performance in terms of adsorption capacity, selectivity, and dynamic behavior have been assessed. Overall, the BCT-type zeolite clearly stands out for highly selective separations of both D₂ and T₂ over H₂, achieving the highest reported selectivities at cryogenic temperatures. We also identified other interesting zeolites for the separation of hydrogen isotopes that offer an alternative way to tackle similar isotopic separations by an aimed selection or design of porous materials.

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.

Characterization of Znq+–imidazole (q = 0, 1, 2) organometallic complexes: DFT methods vs. standard and explicitly correlated post-Hartree–Fock methods

Benchmarking DFts for the characterization of the Zn^q+–imidazole (q= 0, 1, 2) complexes.

Constant Pressure Path Integral Gibbs Ensemble Monte Carlo Method

We present the implementation of a real-space constant pressure path integral Gibbs ensemble Monte Carlo (CP-PIGEMC) method for the simulation of one-component fluid consists of distinguishable quantum particles (henceforth referred to as Boltzmannons) in an external potential field at finite temperatures. We apply this simulation method to study the para-H2 adsorption in NaX zeolite at 77 K and pressures up to 100 bar. We present a new set of effective solid-fluid parameters optimized for path integral simulations of hydrogen isotope adsorption and separation in synthetic zeolites. The agreement among CP-PIGEMC, experiment, and the path integral grand canonical Monte Carlo method (PIGCMC) is very good, even at high pressures. CP-PIGEMC is a particularly useful method for simulation of one-component quantum fluid composed of Boltzmannons at finite temperatures, when the chemical potential is difficult to measure or calculate explicitly.