Liquid Metal-Organic Frameworks
Metal–organic frameworks are a novel family of chemically diverse materials, with applications in a wide field covering engineering, physics, chemistry, biology and medicine. Research so far has focused almost entirely on crystalline structures, yet a clear trend has emerged shifting the emphasis onto disordered states of MOFs, including “defective by design” crystals, as well as amorphous phases such as glasses and gels. Here we introduce a MOF liquid, a strongly associated liquid obtained by melting a zeolitic imidazolate framework (ZIF), with retention of chemical configuration, coordinative bonding modes, and porosity of the parent crystalline framework. We combine in-situ variable temperature X-ray, ex-situ neutron pair distribution function experiments, and first principles molecular dynamics simulations to study the melting phenomenon and the nature of the liquid obtained, focusing on structural characterization at the molecular scale, dynamics of the species, and thermodynamics of the solid–liquid transition.
R. Gaillac, P. Pullumbi, K. A. Beyer, K. W. Chapman, D. A. Keen, T. D. Bennett and F. X. Coudert, 2017, arxiv.
Shaping Metal-Organic Frameworks via Gel Formation
The ability of metal-organic frameworks (MOFs) to gelate under specific synthetic conditions opens up new opportunities in the preparation and shaping of hierarchically porous MOF monoliths, which could be directly implemented for catalytic and adsorptive applications. In this work, we present the first examples of xero- or aerogel monoliths consisting solely of nanoparticles of several prototypical Zr4+ -based MOFs: UiO-66-X (X = H, NH2, NO2, (OH)2), UiO-67, MOF-801, MOF-808 and NU-1000. High reactant and water concentrations during synthesis were observed to induce the formation of gels, which were converted to monolithic materials by drying in air or supercritical CO2. Electron microscopy, combined with N2 physisorption experiments, was used to show that an irregular nanoparticle packing leads to pure MOF monoliths with hierarchical pore systems, featuring both intraparticle micropores and interparticle mesopores. Finally, UiO-66 gels were shaped into monolithic spheres of 600 m diameter using an oil-drop method, creating promising candidates for packedbed catalytic or adsorptive applications, where hierarchical pore systems can greatly mitigate mass transfer limitations.
B. Bueken, N. Van Velthoven, T. Willhammar, T. Stassin, I. Stassen, D. A. Keen, G. V. Baron, J. F. M. Denayer, R. Ameloot, S. Bals, D. De Vos* and T. D. Bennett* Chem. Sci., 2017, 8, 3939-3948.
Structure-Property Relationships in MOF Glasses: Crystal Chemistry and Melting Temperature
Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of three-dimensional MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs.
T. D. Bennett*, Y. Yue, P. Lim A. Qiao, H. Tao, G. N. Greaves, T. Richards, G. I. Lampronti, Simon. A. T. Redfern, F. Blanc, O. K. Farha, J. T. Hupp, A. K. Cheetham and D. A. Keen, J. Am. Chem. Soc., 2016, 138, 3484-3492.
Metal-Organic Framework Glasses: Porous?
The porosity of a glass formed by melt-quenching a metal–organic framework, has been characterized by positron annihilation lifetime spectroscopy. The results reveal porosity intermediate between the related open and dense crystalline frameworks ZIF-4 and ZIF-zni. A structural model for the glass was constructed using an amorphous polymerization algorithm, providing additional insight into the gas-inaccessible nature of porosity and the possible applications of hybrid glasses.
A. W. Thornton, K. E. Jelfs, K. Konstas, C. M. Doherty, A. J. Hill, A. K. Cheetham and T. D. Bennett*, Chemical Communications, 2016, 52, 3750-2753
Connecting Defects and Amorphization in MOFs
Here we show, using 13C NMR, PDF, IR and DFT calculations, that the structural collapse of prototypical UiO-66 and MIL frameworks proceeds by successive breaking of metal ligand bonds. Curiously, the inorganic cluster of UiO-66 appears relatively undeformed upon amorphization, whilst linear ZrO backbones, like those in MIL-140, are, alongside the metal-ligand bonding, damaged.
Bennett, T. D,* Todorova, T. K, Baxter, E. F, Reid, D. G, Gervais, C, Van de Voorde, B, Bueken, B, De Vos, D, Keen, D, Mellot-Draznieks, C, Phys. Chem. Chem. Phys., 2016, 18, 2192-2201
Overturning the Crystalline Dominance in Hybrid Materials
Whilst over 50,000 MOF structures have been reported in the literature, attempts at characterizing their amorphous counterparts number under 50. This review seeks to piece together these attempts in a logical fashion, and shed led on the intersection between the hybrid and amorphous domains.
T. D. Bennett* and A. K. Cheetham*, Acc. Chem. Res., 47, 1555-1562