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Metal Organic Frameworks as Heterogeneous Catalysts
By Francesc X. Llabrés i Xamena, Jorge Gascon
The Royal Society of ChemistryCopyright © 2013 The Royal Society of Chemistry
All rights reserved.
FRANCESC X. LLABRÉS I XAMENA AND JORGE GASCON
The last few decades have witnessed the unprecedented explosion of a new research field built around metal organic frameworks (MOFs). The first reports on metal organic frameworks (MOFs) or, more widely speaking, on co-ordination polymers date from the late 1950s and early 1960s, although it was not until the end of the last century when Robson and co-workers followed by Kitagawa et al., Yaghi and coworkers, and Ferey et al. rediscovered and boosted the field. Metal organic frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one- two-, or three-dimensional pore structures. The combination of organic and inorganic building blocks into highly ordered, crystalline structures offers an almost infinite number of combinations, enormous flexibility in pore size, shape and structure, and plenty opportunities for functionalization, grafting and encapsulation. These materials hold very high adsorption capacities, specific surface areas and pore volumes. Their porosity is much higher than that of their inorganic counterpart zeolites (up to 90%). In contrast to other nano-structured materials, many MOFs display a remarkable flexibility and respond to the presence of guests and external stimuli. Their thermostability is sometimes unexpectedly high, reaching temperatures above 400 °C and their chemo-stability is acceptable in many cases.
Indeed MOFs are fascinating porous solids. The assembly of organic and inorganic struts allows, in theory, the facile tuning of properties, either by the chemical functionalization of the organic building units or by selection of the inorganic constituents. Even within such a relatively short time span, the field has rapidly evolved from an early stage, in which the main scope was the discovery of new structures, to a more mature stage in which dozens of applications are currently being explored. High adsorption capacities and easy tunability have crystallized in perspective applications in gas storage, separation and molecular sensing. The possibility of synthesizing bio-compatible scaffolds infers a very promising future for medical applications. Magnetic, semi-conductor and proton conducting MOFs will certainly find their way towards advanced applications in several research fields. The easy compatibilization of MOFs with either organic or inorganic materials opens the door to advanced composites with applications varying from (opto)electronic devices to food packaging materials and membrane separation. Last but not least, their tunable adsorption properties and pore size and topology, along with their intrinsic hybrid nature, all point at MOFs as very promising heterogeneous catalysts, the topic of this book (see Figure 1.1 for a general picture).
According to the classical definition, a catalyst is a substance that increases the rate of a reaction towards equilibrium without being appreciably consumed. The word "catalysis" stems from Greek: [TEXT NOT REPRODUCIBLE IN ASCII] means "down" and [TEXT NOT REPRODUCIBLE IN ASCII.] means "loosening". The eastern approach to catalysis is different. The Chinese characters for catalyst refer to a marriage broker, emphasizing the fact that a catalyst brings together two different "species" resulting in a mechanism of production. By using a satisfactory catalyst the desired reactions proceed with a higher rate and selectivity at relatively mild conditions.
It is convenient to distinguish between heterogeneous and homogeneous catalysis. In the former case the catalyst and reactants are present in different phases, whereas in the latter case we are dealing with a single-phase system, usually a solution. Strictly speaking heterogeneous catalysis is not limited to solid catalysts. For instance, a system consisting of a liquid phase catalyst dispersed in a continuous liquid phase is heterogeneous. However, in practice only in the case of solid catalysts the term "heterogeneous catalysis" is used. How important is catalysis in practice? In the production of bulk chemicals catalysis is visibly present in nearly all plants. In the same lines, the role of catalysis is crucial in environmental protection, especially in emission control. In contrast, in the production of fine chemicals and pharmaceuticals, catalysis is developing at a slower pace, mostly due to the lack of efficient catalysts and to the high added value of the products.
With the discovery and explosion of MOFs, it was only a matter of time until the first catalytic applications were explored. First reports mostly consisted of demonstrating that a certain MOF contained the necessary catalytic centers to catalyze a given reaction. In many cases, the performance of the material was poor and many concerns existed regarding the stability of the materials under reaction conditions. The current challenge is to develop truly efficient and selective catalytic processes using MOFs, ideally exploiting the versatility of these materials. In this sense, catalysis by MOFs is at this moment a hot topic in research, with new catalytic applications being continuously described, including new materials and new reactions. Indeed, it would not be overly controversial to state that we have already passed from poor "proof-of-concept" solids to highly active catalysts, in some cases with performances comparable to (or even surpassing) state-of-the-art catalysts.
Because the field is reaching now a stage of maturity, we strongly believe that this is the perfect timing to publish, to the best of our knowledge, the first book fully devoted to MOF catalysis. We would like to stress that this book does not intend to be just a literature review of the main advances in MOF catalysis until 2012 but a lasting reference book with a didactic spirit, where results and synthetic strategies are thoroughly discussed rather than simply highlighted.
The book contains outstanding contributions from some of the main players in the field of MOF catalysis. In its second part, after Llabrés i Xamena et al. introduce the different strategies for the inclusion of catalytically active sites in MOFs (Chapter 7), Dirk de Vos and colleagues explain in detail the possibilities of MOF metal nodes as catalytic sites along with synthetic strategies to enhance activity and selectivity (Chapter 8). In Chapter 9 Joseph T. Hupp and co-workers challenge the reader with the almost infinite possibilities of catalysis at the organic linker. Gascon and co-workers explore in Chapter 10 the utilization of the MOF porosity to host slightly bigger catalytic species. Finally, Wenbin Lin et al., in Chapter 11, thoroughly investigate the limits of MOFs in asymmetric catalysis, probably one of the most promising catalytic applications together with photocatalysis, as rationally explained by Hermenegildo García and Belén Ferrer in Chapter 12. Since this is a rapidly developing research field, already outstanding catalytic reports on "brother" materials, the so-called Covalent Organic Frameworks (COFs) have been published during the last few years. In Chapter 13, Regina Palkovits, pioneer in the catalytic application of these materials, discusses the advantages and limitations of COFs.
As in the 21st Century catalysis is not a black-box anymore, characterization, rational design by synthesis, adsorptive properties and mechanistic insight are as important as the catalytic cycle itself. Indeed the development of structure–activity relationships in catalysis is the dream of every scientist involved in this field and the key towards rational design of new catalyst generations. For this reason, in the first part of the book, MOF synthesis and post-synthesis strategies are thoroughly discussed by Norbert Stock and Andrew D. Barrows, in Chapters 2 and 3 respectively. Carlo Lamberti, Silvia Bordiga and co-workers teach the reader on the most advanced spectroscopic and diffraction techniques for the characterization and structural determination of MOFs in Chapters 4 and 5. Last but not least, Evgeny Pidko and Emiel J. M. Hensen gather computation chemistry and MOF catalysis in Chapter 6.
The book finishes with a last Chapter where we not only speculate about future directions but also emphasize some of the main barriers that MOFs need to overcome to finally reach industrial catalytic applications.
We want to warmly acknowledge all the authors for their excellent contributions and the editorial team at RSC for their efforts on behalf of this book. We hope that this book will be valuable to the catalysis community both in industry and academia and especially to undergraduate students. We can only wish the reader as much joy as we had when editing this book.CHAPTER 2
Synthesis of MOFs
NORBERT STOCK, HELGE REINSCH AND LARS-HENDRIK SCHILLING
Christian-Albrechts-Universität, Max-Eyth-Straße 2, D-24118 Kiel, Germany
Metal organic frameworks are a highly diverse class of compounds, which are based on the assembly of defined organic and inorganic building units. One reason for the tremendous interest may lie in the structural beauty and variability of the framework compounds, which attracts chemists with a background in solid-state chemistry as well as in coordination chemistry. The potential applications of MOFs, related to their porous nature, also arouse interest among engineers and material scientists. This overlap in scientific background can be considered as an explanation for the manifold developments in this specific field of research. However, no matter which specific property of a MOF is of interest, the synthesis of the respective compound is always the beginning of the experiment. Various methods and approaches towards the understanding of MOF formation have been reported which cover a large variety of analytical methods and chemical parameters as well as reaction conditions. Nevertheless, up to now the designed synthesis of a new material must be considered as nearly impossible, especially since the variety of possible inorganic building units and topologies prohibits the prediction of the structure of a reaction product.
Understanding the principles of crystallization may give scientists a route towards reaction conditions and chemical parameters, which allow for the synthesis of new MOFs (section 2.2). The methods and common strategies for the discovery of new MOFs and their synthesis optimization are summarized in section 2.3. The concepts described can also give an insight into the complexity of the chemical systems and the diversity of possible structures that can be obtained. Attempts to achieve control over the morphology of the crystals and strategies for creating hierarchically porous materials are especially of interest for the application of MOFs in catalysis (section 2.4). Both aspects are important for the accessibility of catalytically active sites. Proper purification and complete activation is essential for the use of MOFs in catalysis (section 2.5), as the existence of impurities makes an understanding of the catalytic performance at least difficult, incomplete activation leads to a lower degree of porosity and thus to materials with inferior properties.
2.2 Mechanisms and Methods of Crystallisation
Generally, MOFs are crystallised from solution. Water and especially organic solvents have been shown to lead to highly porous materials in which the pores are filled with guest molecules such as solvent, structure directing agent or unreacted linker molecules. From an energetic point of view this is astounding, since dense structures are thermodynamically favoured, i.e. "Nature abhors open space in solid-state materials". Thus, the incorporation of guest molecules and especially the kinetics of the formation of the inorganic building units play a crucial role in the formation of MOF structures.
In general crystallisation can be regarded as an equilibrium reaction between the dissolved precursors and the solid compound (i.e. the MOF). The thermodynamics of this reaction at constant pressure is described with the Gibbs–Helmholtz equation (eqn (1)).
ΔG = ΔH - TΔS (1)
Due to the smaller number of microstates the entropy of a solid body is far lower than the entropy of a liquid or solution. It directly follows that higher temperature will cause the equilibrium to shift towards the dissolved compound, as depicted in Figure 2.1 (centre, left). Also, an increase in concentration will lead to precipitation, since the solubility of the reactants is finite. Thus, crystallisation can be induced by influencing the concentration and temperature of the solution. A simple example is recrystallisation for the purification of a substance, in which an increase in temperature and/or amount of solvent leads to dissolution of the substance and the recrystallisation can be caused by evaporating the solvent and/or decreasing the temperature.
The formation of a crystal can be seen as a two-step process in which nucleation is followed by crystal growth. Nucleation is the assembly of ions or molecules to form a cluster. Below a certain size the cluster is not stable and re-dissolves. Once the cluster attains a minimum size, the so called critical size rc, which is in the nm-range, it is thermodynamically stable and is called a nucleus. The Gibbs free energy of crystallisation (ΔGN) is composed of two terms, the surface term (ΔGS) and the volume term, which scale with r2 and r3, respectively (Figure 2.1, centre). For a spherical body the following equation applies (eqn (2)):
ΔGN = 4/3πr3 ΔGV + 4πr2γ (2)
Since ΔGV is a negative and the surface energy a positive term, the change in Gibbs free energy is positive up to the critical size rc. Once this point has been surpassed, ΔGN rapidly decreases and the growth of the crystal becomes an exergonic process (ΔGN< 0).
The crystallisation process depends not only on thermodynamic but also on kinetic factors. The time-dependent growth of a crystal from a solution can be described by the La Mer-diagram (Figure 2.1 centre, right). At t = 0 the reactants are combined. The reaction leads to the formation of precursors, i.e. inorganic building units and/or deprotonated organic linker molecules, and the concentration c increases. The concentration surpasses the thermodynamical solubility cs (described by the solubility product) and a supersaturated solution is formed. In this concentration regime heterogeneous nucleation, i.e. nucleation on surfaces (e.g. on a glass wall, an impurity, a seed crystal, bubbles etc.) can occur. Homogeneous nucleation takes place without preferential nucleation sites above the critical nucleation concentration c*min. After the period of nucleation, these seeds grow to form larger crystals until the concentration is lowered to cS. In addition, Ostwald ripening can occur by which larger crystals are formed at the expense of smaller ones.
Excerpted from Metal Organic Frameworks as Heterogeneous Catalysts by Francesc X. Llabrés i Xamena, Jorge Gascon. Copyright © 2013 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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