Applications of Metal-Organic Frameworks in Heterogeneous Supramolecular Catalysis
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Chemical Society Reviews Applications of Metal -Organic Frameworks in Heterogeneous Supramolecular Catalysis Journal: Chemical Society Reviews Manuscript ID: CS-REV-02-2014-000094.R1 Article Type: Review Article Date Submitted by the Author: 07-May-2014 Complete List of Authors: Liu, Jiewei; Sun Yat-Sen University, Chen, Lianfen; Sun Yat-Sen University, Cui, Hao; Sun Yat-Sen University, Zhang, Jianyong; Sun Yat-Sen University, School of Chemistry and Chemical Engineering Zhang, Li; Sun Yat-sen University, School of chemistry and chemical engineering Su, Cheng-Yong; Sun Yat-Sen University, School of Chemistry and Chemical Engineering Page 1 of 52 Chemical Society Reviews Journal Name RSC Publishing ARTICLE Applications of Metal-Organic Frameworks in Heterogeneous Supramolecular Catalysis Cite this: DOI: 10.1039/x0xx00000x Jiewei Liu, Lianfen Chen, Hao Cui, Jianyong Zhang, Li Zhang* and Cheng-Yong Su* Received 00th January 2012, Accepted 00th January 2012 This review summarizes the use of metal-organic frameworks (MOFs) as a versatile supramolecular platform to develop heterogeneous catalysts for a variety of organic reactions, DOI: 10.1039/x0xx00000x especially in liquid-phase reactions. Following a background introduction about catalytic www.rsc.org/ relevance to various metal-organic materials, crystal engineering of MOFs, characterization and evaluation methods of MOF catalysis, we categorize catalytic MOFs based on the types of active sites, including coordinatively unsaturated metal sites (CUMs), metalloligands, functional organic sites (FOS), as well as metal nanoparticles (MNPs) embedded in the cavities. Throughout the review, we emphasize the incidental or deliberate formation of active sites, the stability, heterogeneity and shape/size selectivity for MOF catalysis. Finally, we briefly introduce their relevance to photo- and biomimetic catalysis, and compare MOFs with other typical porous solids such as zeolites and mesoporous silica with regard to their different attributes, and provide our view on future trends and developments of MOF-based catalysis. 1. Introduction (MOGs) which are jelly-like materials containing cross-linking coordination network.5 In point of catalysis, type-I MOMs may 1.1 Assembly of MOMs with Catalytic Relevance represent the simple but most popular structural models investigated in transition metal catalysis, in special cases the Supramolecular catalysis inherent in natural enzymatic systems supramolecular catalysis can be achieved.6 The type-II MOMs has been pursued for decades with the goals to develop are pertinent to supramolecular catalysis because they are able “artificial enzyme catalysts” for synthetic chemistry or to to provide specific substrates recognition and coordination understand fundamental mechanisms with regard to enzyme space for stereochemical confinement. 2 Both these two types of active sites. 1 Supramolecular coordination chemistry provides a coordination compounds could be studied in homogeneous promising platform for such supramolecular catalytic purposes, conditions. On the contrary, MOFs and MOGs are applicable in for example, to mimic pocket nature or host-guest interactions heterogeneous catalysis. 3,5 In contrast to MOGs and amorphous of enzymes by assembly of various size and shape predefined CPs, MOFs are typically crystalline porous solids resembling nanoreactors or molecular flasks.2 Recently, rapid progress in zeolites, able to use a minimum amount of metal ions and supramolecular catalysis was achieved for porous coordination organic ligands to build maximum surface areas with solids which can offer well-defined coordination nanospace and predictable, controllable, tailorable and post-modifiable pores functional groups/sites appropriate for catalysis.3 These and cavities. Such intrinsic structure attributes make MOFs persistent catalytic interests take the advantages that the emerge as the new interface between heterogeneous catalysis coordination assembly feature in good bond direction, moderate and supramolecular chemistry to behave as promising bond energy, self-correcting kinetic reversibility and plentiful candidates for heterogeneous supramolecular catalysis. metal geometries, thus driving the metal- or ligand-directed self-assembly to produce a variety of metal-organic materials (MOMs) 4 with varied relevance to catalysis. As exemplified in Fig. 1, four prototypes of MOMs with distinct structural identities and attributes are often pursued for multifarious catalytic purposes: (I) mon- or multinuclear metal complexes possessing specific functionality in a supramolecular sense; (II) discrete coordination rings, cages, tubes or capsules attributable to metal-organic polygons/polyhedra (MOPs) or metal-organic containers/cages (MOCs); (III) infinite metal- organic frameworks (MOFs) or coordination polymers (CPs) which often possess periodically repeating units in crystalline solid-state, including amorphous CPs which also subject to a Fig. 1 Schematic illustration of representative structural models wide range of catalytic studies,d,e and (IV) metal-organic gels driven by metal-ligand coordination assembly. This journal is © The Royal Society of Chemistry 2013 J. Name ., 2013, 00 , 1-3 | 1 Chemical Society Reviews Page 2 of 52 ARTICLE Journal Name Since MOFs possess uniform, continuous and permeable though, investigations on catalytic applications of MOFs are channels, they can provide active sites on the pore surface and relatively lagging behind other topics, which might be due to, in transport reactants/products to or from inner reactive vessels general, the relatively low chemical, hydrolytic and thermal (such as cagelike cavities inside the MOFs). In a stability (usually stable under 350–400 °C) of MOFs. supramolecular sense, the uniqueness of MOF structures lies in Nevertheless, the situation improved dramatically since 2009. the coordination-driven self-assembly, the tunable pore sizes, Fig. 2 shows the development of MOF fields on the basis of exceptionally high pore volume and surface area, and versatile articles appeared in the last twenty years. It is clear that MOF inner cavity (one- to three-dimensional pores capable of catalysis underwent a rapid development in recent five years. incorporating interior cages and windows), which endow the Paper of MOF catalysis catalytic microenvironment with a variety of tunable properties, 3000 Paper of MOF such as charge, polarity, chirality, redox potential, photoactivity, hydrophobicity/hydrophilicity, aromatic/lipophilic character, 2500 and stereochemistry. The porosity of MOFs spans from ultramicroporous to mesoporous materials, well bridging the 2000 gap between the small pore size of zeolites and large pore size of mesoporous silicate materials. This makes supramolecular 1500 catalysis easily taking place in an appropriate coordination space under nanoscale confinement, allowing multicomponent 1000 reactions as well as a wide range of substrates from small molecules to large polyaromatics and carbohydrates. 500 Meanwhile, using MOFs as heterogeneous supramolecular catalysts could take advantages of the following principal 0 characters: (1) enhanced catalyst reactivity and stability due to 2002 2004 2006 2008 2010 2012 Year the spatial separation of multiple catalytic sites in the framework, which also can contribute to cooperative catalysis character of enzymes; (2) permeable channels and coordination Fig. 2 Development of MOF fields in comparison to the MOF nanospace which bestow recognition effects and allow the catalysis in the last ten years (SciFinder until Jan. 15, 2014). facile access of substrates to the catalytically active sites The progress in MOF catalysis may thank to the gradual located within the cavities, endowing the catalytic reactions 8 with shape-, size-, chemo-, or enantio-selectivity; (3) maturation of crystal engineering. Engineering of crystalline framework flexibility and dynamics 7 in the solid-state, which MOF solid materials demands proper design and synthetic may bring new applications to heterogeneous catalysis by strategies to organize basic building blocks (BBs) strictly in a enforcing physical, chemical and environmental stimuli periodic way of specific network topology. This is implicit in responses, and utilize the guest-host responses to accomplish two interrelated steps: (a) choosing or creating BBs with transition-state recognition and regulation reminiscent of desired structure and property, and (b) connecting or arranging allosteric nature of enzymes; and (4) easy tunability and BBs through predefined interactions or crystal growth. modification of the polar-nonpolar and hydrophobic– Undoubtedly, advances in synthetic strategy and design hydrophilic properties of the porous solids, which is essential methodology have motivated in-depth study in functionality of for many bimolecular reactions, and helpful to practical crystalline solids, shifting research interests from original synthetic chemistry. Moreover, the ability to separate and reuse structural engineering of MOFs to the functional engineering of a heterogeneous catalyst would be highly attractive in large- MOFs targeting in collective or specific property like catalysis. scale reactions, where separation and waste disposal can be Crystal engineering of MOFs for catalytic purpose relies not costly. The last but important feature of MOFs for only on the permanent porosity, but also on other framework supramolecular catalysis is that, assembly process of MOFs attributes like reversible structural