Control of Metal–Organic Framework Crystallization by Metastable Intermediate Pre‐equilibrium Species (original) (raw)
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CrystEngComm
Understanding how crystalline materials are assembled is important for the rational design of metal organic frameworks (MOFs), through streamlining their synthesis and controlling their properties for targeted applications. Herein, we report for the first time the construction of two 3-dimensional Tb(III) based MOFs; a metastable MOF acting as an intermediate phase, that partially dissolves and transforms into a chemically and thermodynamically stable MOF. This chemical transformation occurs solely in a N,N-dimethylformamide/water solvent mixture, and is triggered when additional energy is provided to the reaction. In situ studies reveal the partial dissolution of the metastable phase after which the MOF components are reassembled into the thermodynamically stable phase. The marked difference in thermal and chemical stability between the kinetically and thermodynamically controlled phases is contrasted by their identical chemical building unit composition.
In Situ, Time-Resolved, and Mechanistic Studies of Metal–Organic Framework Nucleation and Growth
Chemical Reviews, 2018
The vast chemical and structural diversity of metal−organic frameworks (MOFs) opens up the exciting possibility of "crystal engineering" MOFs tailored for particular catalytic or separation applications. Yet the process of reaction discovery, optimization, and scale-up of MOF synthesis remains extremely challenging, presenting significant obstacles to the synthetic realization of many otherwise promising MOF structures. Recently, significant new insights into the fundamental processes governing MOF nucleation and growth, as well as the relationship between reaction parameters and synthetic outcome, have been derived using powerful in situ, time-resolved and/or mechanistic studies of MOF crystallization. This Review provides a summary and associated critical analysis of the results of these and other related "direct" studies of MOF nucleation and growth, with a particular emphasis on the recent advances in instrument technologies that have enabled such studies and on the major hypotheses, theories, and models that have been used to explain MOF formation. We conclude with a summary of the major insights that have been gained from the work summarized in this Review, outlining our own perspective on potential fruitful new directions for investigation.
Angewandte Chemie International Edition, 2009
Metal-organic frameworks (MOFs) are an emerging, class of porous materials. MOFs are highly ordered, crystalline coordination polymers of persistent porosity with specific surface areas exceeding that of traditional adsorbents, such as zeolites and active carbons. Whereas initial research on MOFs was mainly driven by the interest to use them as gasstorage (e.g. CH 4 , H 2 , CO 2 ) materials, various other applications were proposed and demonstrated, including separation, sensing, catalysis, drug release, and the embedding of (metal and metal oxide) nanoparticles. Presently, the search for new types of MOFs is largely by trial-and-error, because very little is known about the details of the MOF crystal growth and nucleation process. In a 2006 review article on MOFs by Cheetham et al. it was noted that there was still no in situ characterization of an assembly process for MOFs on the molecular level. In this assembly process two subunits have to be combined, organic ligands and metal precursors. Whereas the organic ligands are
Angewandte Chemie International Edition, 2011
Metal-organic frameworks (MOFs) are among the most sophisticated nanostructured solids: they often possess high surface areas and pore volumes, with the possibility of finetuning their chemical environment by either selecting the appropriate building blocks or by postsynthetic functionalization. For many frameworks, flexibility of the lattice allows them to undergo a significant transformation in solid state. All these features make MOFs a special class of solids with the potential of transcending many common limitations in different technological disciplines, such as ferromagnetism, semiconductivity, gas separation, storage, sensing, catalysis, drug delivery, or proton conductivity. However, the crystallization mechanism of these complex structures is far from understood. Notwithstanding the plethora of publications that present new MOFs, and the effectiveness of the high-throughput approach, serendipity still governs the synthesis of new structures.
Journal of Chemical Education, 2011
Hydrothermal synthesis of MCM-22(P) was carried out with two different silica sources, colloidal silica (28%) and silicic acid with different gel composition. The synthesis was carried out in stirring and static conditions with different crystallization time. MCM-22(P) modified with swelling-sonication method resulted in swollen MCM-22, while alkali treatment yielded desilicated MCM-22. The materials were characterized by X-ray diffraction, low-angle XRD, FE-SEM-EDX, FT-IR, TGA, N 2 adsorption and NH 3-TPD analysis. The results revealed that MCM-22 has a layered sphere, doughnut like morphology and after modification, swollen and broken sphere was observed. Physicochemical analysis revealed that the materials' mesoporosity increased and acidity also changed. Energy dispersive X-ray analysis revealed the high amount of desilication in alkali-treated MCM-22(P).
Angewandte Chemie International Edition, 2010
The crystallization of metal-organic framework (MOF) materials is an extremely attractive way in which to produce functional solid materials with complex three-dimensional structures, since an element of "design" is considered possible in their synthesis: the idea being that by using chosen metal polyhedral units linked by polydentate organic ligands with a known coordination preference, a network structure of desired connectivity may be formed. This synthetic approach is presently the focus of some considerable attention and is yielding many novel hybrid inorganic-organic solids, often that possess some porosity on the nanoscale suitable for applications, such as gas separation, molecular sieving, and shape-selective catalysis. Although these uses are already well-established in silicate zeolite chemistry, there are distinct advantages offered by the MOFs for uses under mild conditions. For example, the choice of framework metal may offer desirable binding sites for gases (currently the focus of much attention in the topical areas of storage of hydrogen, methane, or carbon dioxide ), the functionalization of organic linkers (either pre-or post-synthesis) may allow tuning of the porosity, reactivity, and the selectivity towards binding of guest molecules, and the use of chiral ligands may result in chiral framework materials. In addition, MOF structures also often show great flexibility in the solid state, giving them properties distinct from the traditional inorganic zeotype materials. To explore the extent to which new MOF materials may be "designed" it is now important to elucidate the fundamental physico-chemical details of the crystallization of MOF materials: knowledge of how complex extended network structures are assembled from simple chemical precursors in solution could ultimately permit some fine tuning of synthesis conditions to test and realize the ideas of design in synthesis. Only a few such studies have been reported to date. These include extended X-ray absorption fine structure (EXAFS) spectroscopy studies of reactive solutions to examine the presence of structural building units in solution, through the amorphous intermediate to final crystalline product; light scattering from clear solutions to observe the formation of colloidal nanocrystals; and mass spectrometry to examine the interaction of Mg 2+ ions with (+)camphoric acid to identify possible building units for the construction of a MOF. Shoaee et al. recently used atomic force microscopy (AFM) to examine a growing face of a copper MOF after injection of a reactive solution and suggested that the growth unit from solution was actually smaller than the paddle-wheel-shaped building unit identified in its crystal structure. The study of MOF crystallization mechanism has so far been concerned with the local structure of solution species prior to the appearance of crystal order, but it is important to examine crystal growth over all length scales to build up a complete picture of crystallization. Herein, we describe observations of the emergence of the crystal order of MOFs from reactive solutions, above room temperature, by using the time-resolved energy-dispersive Xray diffraction (EDXRD) method for two established transition-metal carboxylate MOFs. The technique has been used successfully for the in situ study of the crystallization of a variety of inorganic materials, although to date it has not been applied to the study of hybrid MOF materials. Its advantage lies in the use of high intensity white beam X-rays, which allows the non-invasive penetration of laboratory-scale reaction vessels under elevated temperature and autogeneous pressure. Thus the evolution of Bragg peaks as a function of reaction conditions and time can be monitored with a time resolution of less than 1 min.
The Journal of Physical Chemistry C, 2019
Scheme 1. A representation of the volume variation occurring under the effect of the applied pressure during intrusion-extrusion experiments: System (a) before intrusion, (b) volume variation attributed to the intrusion of water in the porosity, (c) intrusion of water associated with a partial collapse (or phase transformation) of the porous structure, (d) complete collapse (or phase transformation) of the porous structure.
Liquid Metal-Organic Frameworks: Formation Mechanism, Structure and Properties
2017
Metal-organic frameworks (MOFs) are a family of chemically diverse materials, with applications in a wide range of fields 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. 2 Crystalline metal-organic frameworks have been proposed for application in a variety of situations which take advantage of their highly ordered and nanoporous structures, e.g. gas sorption and separation, 1-3 catalysis 4 and ion transport. 5 Inherent defects, 6 structural disorder 7 and nearubiquitous flexibility 8 present significant challenges in the development of highly robust, selective systems from perfect crystals. However, they also present opportunities, in creating functional "defective by design" materials. 9,10 Non-crystalline, or amorphous MOF systems are formed by avoidance of crystallization, or induced collapse of crystalline systems by pressure, temperature, or ball-milling. 11 In the case of the zeolitic imidazolate framework (ZIF) family, 12,13 which adopt similar structures to inorganic zeolites and are formed from M n+ (M n+ = e.g. Li + , B + , Zn 2+) inorganic nodes connected by Im based (Im = imidazolate, C 3 H 3 N − 2) ligands, such amorphous systems resemble the continuous random network of amorphous SiO 2. Recently, the formation of a molten ZIF state from a crystalline phase was observed in an inert argon atmosphere. No mass loss was observed during the formation of the liquid, which upon cooling yielded a melt-quenched glass, possessing an extended Zn-Im-Zn coordination network. 14 Unlike reversible solid-liquid transitions in 1D or 2D coordination polymers which occur below 500 K, 15 melting processes in ZIFs have only been observed at high temperatures, i.e. those exceeding 700 K. The novelty of the liquid and glass states means microscopic insight into the mechanism of melting, and the nature of the liquid produced are of great interest when considering the generality of the mechanism in the wider MOF family. However, thus far, the narrow temperature range and poorly understood kinetics-time stability of the fleeting liquid phase have precluded information on any liquid MOF state. Particularly salient and intriguing considerations pertaining to the liquid involve (i) "memory" of the crystalline framework conferred by remnant framework connectivity, (ii) coordinative framework dynamics and (iii) the proximity of structure to that of an ionic or a strongly associated liquid. 16 The ability to form hybrid "porous liquids", analogous to that of the organic systems of the Cooper and James groups, 17 would present a significant advance in the field, and help shift attention away from the solid state.