Pore Breathing of Metal–Organic Frameworks by Environmental Transmission Electron Microscopy

Abstract

Metal-organic frameworks (MOFs) have emerged as a versatile platform for the rational design of multifunctional materials, combining large specific surface areas with flexible, periodic frameworks that can undergo reversible structural transitions, or "breathing", upon temperature and pressure changes, and through gas adsorption/desorption processes. Although MOF breathing can be inferred from the analysis of adsorption isotherms, direct observation of the structural transitions has been lacking, and the underlying processes of framework reorganization in individual MOF nanocrystals is largely unknown. In this study, we describe the characterization and elucidation of these processes through the combination of in situ environmental transmission electron microscopy (ETEM) and computer simulations. This combined approach enables the direct monitoring of the breathing behavior of individual MIL-53(Cr) nanocrystals upon reversible water adsorption and temperature changes. The ability to characterize structural changes in single nanocrystals and extract lattice level information through in silico correlation provides fundamental insights into the relationship between pore size/shape and host-guest interactions.

Atomistic insight in the flexibility and heat transport properties of the stimuli-responsive metal–organic framework MIL-53(Al) for water-adsorption applications using molecular simulations

“…Although a substantial amount of MOFs possess a poor stability with respect to hydration, the MIL-53 family of soft porous crystals is known to have stable hydrated phases, which have been investigated both experimentally and computationally, focusing mainly on the chromium-and gallium-based variants. 51,[58][59][60][61][101][102][103][104][105][106][107][108] Here, the influence of the presence of water on the Helmholtz free energy profile of MIL-53(Al) is investigated, closely monitoring the changes in behaviour of the adsorbed species along the profile. Three different water loadings are considered: 2.5, 7.5, and 22.5 water molecules per conventional unit cell, varying from a ratio of less than one molecule per aluminium atom to almost six molecules per aluminium atom.…”

Enhancing the Separation Performance of Glassy PPO with the Addition of a Molecular Sieve (ZIF-8): Gas Transport at Various Temperatures

“…However, pure MOF membranes cannot reach the expected ideal selectivity towards molecules smaller than the pore diameter because the presence of imperfections such as pinholes and cracks is hard to completely avoid [30]. Such a drawback can be prevented by dispersing ZIFs in a polymer matrix, but in this case, the theoretical separation, obtained based on pore dimension considerations, is hard to reach experimentally due to the flexibility of the metal-organic cage, explained by the so-called "breathing" phenomenon [37,38]. Like all MOFs, ZIFs provide a wide range of designs that can be obtained by changing the imidazolate/imidazolate-like linkers and the coordination metal (e.g., zinc(II) or cobalt(II)) [34].…”

Natural and synthetic metal oxalates – a topology approach

“…Excellent examples are the "breathing" MOFs such as MIL-53. 40 Indeed we suggest that it should be possible to calculate a figure of merit for each network topology on the relative energetic cost of different distortions to further enhance their utility as blueprints for the design of molecule based materials.…”