Research Highlights

Identifying loading limits in metal-organic framework crystal-glass composites

Metal–organic framework crystal-glass composites (MOF-CGCs) are materials in which a crystalline MOF is dispersed within a MOF glass. In this work, we explore the room-temperature stabilization of the open-pore form of MIL-53(Al), usually observed at high temperature, which occurs upon encapsulation within a ZIF-62(Zn) MOF glass matrix. A series of MOF-CGCs containing different loadings of MIL-53(Al) were synthesized and characterized using X-ray diffraction and nuclear magnetic resonance spectroscopy. An upper limit of MIL-53(Al) that can be stabilized in the composite was determined for the first time. The nanostructure of the composites was probed using pair distribution function analysis and scanning transmission electron microscopy. Notably, the distribution and integrity of the crystalline component in a sample series were determined, and these findings were related to the MOF-CGC gas adsorption capacity in order to identify the optimal loading necessary for maximum CO2 sorption capacity.

Synthesis and Properties of a Compositional Series of MIL-51(Al) Metal-Organic Framework Crystal-Glass Composites

C. W. Ashling, D. N. Johnstone, R. N. Widmer, J. Hou, S. M. Collins, A. F. Sapnik, A. Bumstead, P. A. Chater, D. A. Keen and Thomas. D. Bennett*, J. Am. Chem. Soc., 2019DOI: 10.1021/jacs.9b07557.

 

Metal-organic Framework Crystal-Glass Composites

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The majority of research into metal-organic frameworks (MOFs) focuses on their crystalline nature. Recent research has revealed solid-liquid transitions within the family, which we use here to create a class of functional, stable and porous composite materials. Described herein is the design, synthesis, and characterisation of MOF crystal-glass composites, formed by dispersing crystalline MOFs within a MOF-glass matrix. The coordinative bonding and chemical structure of a MIL-53 crystalline phase are preserved within the ZIF-62 glass matrix. Whilst separated phases, the interfacial interactions between the closely contacted microdomains improve the mechanical properties of the composite glass. More significantly, the high temperature open pore phase of MIL-53, which spontaneously transforms to a narrow pore upon cooling in the presence of water, is stabilised at room temperature in the crystal-glass composite. This leads to a significant improvement of CO2 adsorption capacity.

Metal-organic framework crystal-glass composites

J. Hou, C. W. Ashling, S. M. Collins, A. Krajnc, C. Zhou, L. Longley, D. N. Johnstone, P. Chater, S. Li, M. V. Coulet, P. L. Llewellyn, F. X. Coudert, D. A. Keen, P. A. Midgley, G. Mali, V. Chen, T. D. Bennett,* Nat. Commun., 2019, 10, 2580.

 

High Pressure – High Temperature Phase Diagrams of MOFs

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Metal–organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions  of the pressure–temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy  reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially  achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure  makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid  possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low  temperatures, avoiding decomposition on heating at ambient pressure.

Pressure promoted low-temperature melting of metal-organic frameworks

R. N. Widmer, G. I. Lampronti, S. Anzellini, R. Gaillac, S. Farsang, C. Zhou, A. M. Belenguer, H. Palmer, A. K. Kleppe, M. T. Wharmby,  S. A. T. Redfern, F. X. Coudert, S. G. Macleod, T. D. Bennett,* Nat. Mater., 2019, DOI: 10.1038/s41563-019-0317-4.

 

 Liquid Metal-Organic Frameworks

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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.

Liquid Metal-Organic Frameworks

R. Gaillac, P. Pullumbi, K. A. Beyer, K. W. Chapman, D. A. Keen, T. D. Bennett and F. X. Coudert, Nat. Mater.2017, 16, 1149-1145

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Shaping Metal-Organic Frameworks via Gel Formation

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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.

Gel-based Morphological Design of Metal-Organic Frameworks

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.

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Structure-Property Relationships in MOF Glasses: Crystal Chemistry and Melting Temperature

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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.

Melt-Quenched Glasses of Metal-Organic Frameworks

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., 2016138, 3484-3492.

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Metal-Organic Framework Glasses: Porous?

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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.

Porosity in Metal-Organic Framework 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

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Connecting Defects and Amorphization in MOFs

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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.

Connecting Defects and Amorphization in UiO-66 and MIL-140 Metal-Organic Frameworks: A Combined Computational and Experimental Study

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

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Overturning the Crystalline Dominance in Hybrid Materials

mofgraphWhilst 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.

 

Amorphous metal-organic frameworks

T. D. Bennett* and A. K. Cheetham*, Acc. Chem. Res., 47, 1555-1562