Metal–Organic Frameworks (MOFs)

Hong-Cai “Joe” Zhou a and Susumu Kitagawa b
aDepartment of Chemistry, Texas A&M University, College Station, TX 77842-3012, USA. E-mail:; Fax: +1 979 845 1595; Tel: +1 979 845 4034
bInstitute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. E-mail:

image file: c4cs90059f-p1.tif

Hong-Cai “Joe” Zhou

Hong-Cai “Joe” Zhou obtained his PhD in 2000 from Texas A&M University under the supervision of F. A. Cotton. After a postdoctoral stint at Harvard University with R. H. Holm, he joined the faculty of Miami University, Oxford in 2002. He moved to Texas A&M University in 2008 and was appointed a Davidson Professor in Science in 2014. His research focus is the study of metal–organic frameworks and porous polymer networks for biomimetic and clean-energy-related applications.

image file: c4cs90059f-p2.tif

Susumu Kitagawa

Susumu Kitagawa received his PhD at Kyoto University in 1979, and was a professor and lecturer at Kinki University before being promoted to Professor of Inorganic Chemistry at Tokyo Metropolitan University in 1992. He moved to Kyoto University as Professor of Functional Chemistry in 1998. He had been a visiting scientist in F. A. Cotton Laboratory, Texas A & M University from 1986–1987, and an exchange professor in City University of New York in 1996. He is now Director of Institute for Integrated Cell-Material Sciences (iCeMS) at Kyoto University. His main research fields are coordination chemistry, in particular, chemistry of coordination space, and his current research interests are centred on the synthesis and properties of porous coordination polymers/metal–organic frameworks.

Metal–organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are an emerging class of porous materials constructed from metal-containing nodes (also known as secondary building units, or SBUs) and organic linkers. Due to their structural and functional tunability, the area of MOFs has become one of the fastest growing fields in chemistry. This is demonstrated through the ever-escalating number of structures, publications, and citations emerging, as well as by the constant expansion of research scope and researcher engagements. This almost unprecedented surge in MOF research can be ascribed to the following five developments: (1) advances in cluster chemistry, (2) maturation of organic synthesis pertinent to ligand preparation and post-synthetic modification, (3) improvement in structure determination, particularly through X-ray crystallography, and development of hard- and software for evaluation of sorption properties, (4) interdisciplinary growth of MOF research with its neighboring fields, and (5) the ever-expanding potential in applications.

The MOF field has recently been reviewed extensively. In 2009, the first themed issue of Chemical Society Reviews dedicated to MOFs was published and it has been a tremendous success, as evidenced by the record number of citations it has received. Additional themed issues and monographs focusing on this research area have also been published elsewhere. Following on from these compilations, which have played a major role in catalyzing the evolution of the MOF field over the past five years, the advent of several new research directions, and the continuing progression of existing research themes, urge us to assemble another compendium of the latest MOF work. It is our ambition that this volume will not only offer highlights of new contributions for experienced researchers, but also provide an introduction to this fascinating research field for novices. Thus, both critical and tutorial reviews are incorporated in this themed issue. For the convenience of the reader, these are classified into five categories, which correspond to the five developments discussed above:

(1) Synthetic work focusing on metal-containing nodes or coordination bonds

An initial impetus for MOF research lies in the search for inorganic–organic hybrid porous materials analogous to zeolites. Some of the MOFs discovered initially were constructed from single-metal-ion nodes. The incorporation of metal clusters into MOFs has led to significant improvements in porosity and stability. However, the metal-containing building units and coordination bonds are normally formed in situ, almost exclusively from a one-pot procedure. The linkers, on the other hand, are pre-designed and generally keep their integrity during MOF formation. Although initial MOF researchers are predominantly inorganic chemists, ironically, MOF research in the past decade has been dominated by organic ligand design and post-synthetic modification of the linker. Only very recently, researchers have started to re-focus on the inorganic part of a MOF, the metal clusters and coordination bonds. This section starts with a highlight on the evolution of MOFs through building block replacement by Bury, Hupp, Farha, and coworkers (DOI: 10.1039/C4CS00067F). In this contribution, they highlight solvent-assisted linker exchange, non-bridging ligand replacement, and transmetalation for the functionalization of MOFs. Doonan and coworkers provide a review focusing specifically on the post-synthetic metalation of MOFs on the linkers (DOI: 10.1039/C4CS00076E); this includes addition, exchange, and encapsulation. Dincă and Brozek review cation exchange at the metal-containing nodes and attempt to establish a conceptual framework for the governing factors (DOI: 10.1039/C4CS00002A). Subjects such as which SBUs undergo exchange, why certain ions replace others, how the MOF influences the process, and what role the solvent plays, are addressed. These cation exchange reactions are based on substitution reactions, which include metal–ligand bond cleavage and formation. J.-R. Li, Guo, and coworkers review such reactions (DOI: 10.1039/C4CS00033A) in MOFs and metal–organic polyhedra including those for metal ions, bridging organic ligands, and guest molecules. Miras, Cronin and coworkers then describe a special class of MOFs, constructed based on polyoxometalates (POM) (DOI: 10.1039/C4CS00097H). An interesting comparison of POM-MOFs and other MOFs are given in this review. High valence 3p and transition metal-based MOFs are special due to their distinctive stability and application potential. Devic and Serre give an account of these MOFs, emphasizing the solution chemistry of these metals and the challenges in the preparation of MOFs based on phosphonates, carboxylates and other linkers, as well as their catalytic, redox- and photo-activities (DOI: 10.1039/C4CS00081A).

(2) Ligand design and post-synthetic modification on linkers

MOFs are generally made in a one-pot procedure, where metal-containing nodes are formed in situ. The consistent component throughout the reaction is the ligand, although occasionally the ligand is also generated during the solvothermal procedure. The geometry of the ligand determines the topology of the resulting MOF. The functional groups are generally attached to the linker through pre- or post-synthetic organic synthesis. The stability of the MOFs also relies on the type of metal-linker combination. In addition, if a flexible ligand is used, a stimuli-responsive or flexible MOF can be obtained. Typically, an individual pursuing MOF research spends more than eighty percent of his/her time performing organic synthesis. In this section of the themed issue, Zhou and coworkers highlight the progress made in tuning the structure and function of MOFs through linker design (DOI: 10.1039/C4CS00003J). MOFs constructed from ditopic, tritopic, tetratopic, hexatopic, octatopic, mixed, desymmetrized, metallo, and N-heterocyclic linkers are discussed. This overview is followed by a treatise from Kaskel, Fischer and coworkers specifically devoted to flexible MOFs, which respond to physical or chemical stimuli (DOI: 10.1039/C4CS00101J). Flexibility arising from metal nodes and linkers, experimental and theoretical mechanistic studies, as well as possible applications of these MOFs, are also explored. Cao and coworkers provide an analysis of the design, synthesis, and application of MOFs based on flexible linkers (DOI: 10.1039/C3CS60483G). More than fifty of such linkers and their corresponding MOFs have been covered. Ma and coworkers describe MOFs with metalloporphyrin in the linker (DOI: 10.1039/C4CS00001C). A snapshot of the historical perspective of these MOFs since 1991 is given and their possible applications are reviewed. At the end of this section, B. Chen and coworkers summarize the recent development of MOFs based on multicarboxylate linkers containing isophthalate moieties (DOI: 10.1039/C4CS00041B). The potential applications of these MOFs have also been highlighted.

(3) Symmetry-guided synthesis and structural characterization of MOFs from micro-, meso-, to macro-scale

A hallmark of MOFs is their crystallinity, which allows facile structural characterization through X-ray diffraction. Conventional solid-state materials discovery is essentially based on the method of trial-and-error in a black box. X-ray structure determination, together with symmetry-guided design methods such as the reticular approach, allows rational design of MOF topology, which plays an important role in MOF discovery. Rational design and synthesis of MOFs has now reached an even higher level of complexity. In this section, S. Furukawa, S. Kitagawa, and coworkers highlight recent achievements in pushing the envelope of MOFs to the mesoscopic scale and beyond, using MOF nanocrystals as building units (DOI: 10.1039/C4CS00106K). The superstructures of MOFs are classified into four dimensionalities (from zero to three) and the synthetic methodologies used to achieve these superstructures are conceptually categorized. Zaworotko and Z. Zhang provide an overview of template-directed synthesis of metal–organic materials (DOI: 10.1039/C4CS00075G). Templates such as solvent, organic compounds, coordination complexes, inorganic compounds, gas molecules, and surfactants are discussed and analyzed. Bradshaw and coworkers focus specifically on supramolecular templating of hierarchically porous MOFs (DOI: 10.1039/C4CS00127C). A variety of supramolecular templates and their resulting mesoporous MOFs containing micropores have been enumerated. Lah and Eddaoudi review the conceptual supermolecular building blocks and building layer approaches for the construction of MOFs (DOI: 10.1039/C4CS00135D). Ample examples corroborating these design principles are given and new hypothetical MOFs are proposed. At the end of this section, J.-P. Zhang, X.-M. Chen, and coworkers present an overview of single-crystal X-ray diffraction studies on structural transformations under various chemical and physical stimuli (DOI: 10.1039/C4CS00129J); these include the adsorption/desorption/exchange of solvent or other guest molecules, chemical reactions, and temperature changes.

(4) MOF interdisciplinary research

From the outset, MOF research was known for its interdisciplinary nature due to the exceptionally diverse research backgrounds of its early participants. This is evidently an advantage over other conventional disciplines. As the field expands at a record pace, more and more contributors with non-traditional backgrounds such as theorists, engineers, and entrepreneurs have been engaged in MOF research and development. The deep penetration of MOF research into neighboring fields has resulted in the new growth of MOF science and technology. In this section, Snurr and Colón review the increasingly popular high-throughput computational screening of MOFs, especially for gas adsorption and separation purposes (DOI: 10.1039/C4CS00070F). A detailed account on computational procedures in obtaining the structure, characterization, adsorption properties, high throughput screening, and data mining is given. Xu and Zhu provide an overview on MOF composites, which exhibit properties superior to those of individual components (DOI: 10.1039/C3CS60472A). Potential applications and synthetic strategies with respect to these composites are also presented. Qiu and coworkers highlight the most recent literature on MOF membrane (DOI: 10.1039/C4CS00159A). A brief introduction on fabrication methods and a review of potential applications of MOF membrane in separation are given. J. Li and coworkers offer an update on work published since 2011, focusing on the photoluminescence properties of MOFs and their potential utility in chemical and biological sensing and detection (DOI: 10.1039/C4CS00010B). The origin of the luminescence, the advantage of MOF-based sensors, the strategies of materials design, and potential applications in this regard are also discussed. Allendorf and coworkers focus on the basic requirements and structural elements needed for the fabrication of MOF devices in their review (DOI: 10.1039/C4CS00096J). They also analyze the most recent work on MOF-based electronic and opto-electronic devices, including the design of active MOFs, the creation of hybrid materials, and integration with device hardware. Finally, Falcaro and coworkers review the current technologies that enable the precise positioning of MOFs onto different platforms (DOI: 10.1039/C4CS00089G). Methods for permanent and dynamic localization as well as spatial control of functional materials within MOF crystals are also discussed.

(5) Potential application of MOFs

One of the main driving forces for this research field is the potential application of MOFs. Although challenges in commercialization still exist, strides have been made in the exploration of potential applications. Research hot spots shift as funding levels fluctuate. In addition, as researchers from other fields become engaged in MOF research, new ideas for applications emerge. The key is to take advantage of the intrinsic properties of MOFs to make new systems that are more efficient than existing ones. The first review in this section, written by Lin and T. Zhang, originated from the fundamental principles of energy transfer and photocatalysis (DOI: 10.1039/C4CS00103F). They contribute an overview of the latest progress in energy transfer, light harvesting, photocatalytic proton and CO2 reduction, as well as water oxidation using MOFs. Su, L. Zhang, and coworkers highlight the applications of MOFs in heterogeneous supramolecular catalysis, especially for liquid-phase reactions (DOI: 10.1039/C4CS00094C). An overview on the active sites such as coordinatively unsaturated metal centers, metalloligands, functional organic sites, and metal nanoparticles embedded inside MOFs is also given. In their review article, Dhakshinamoorthy and Garcia specifically discuss MOFs as solid-state catalysts for the synthesis of nitrogen-containing heterocycles such as pyrimidines, N-substituted piperidines, quinolones, indoles, N-substituted imidazoles, triazoles and heterocyclic amides (DOI: 10.1039/C3CS60442J). In their contribution, Shimizu and coworkers assess the challenges and opportunities in the application of MOFs as proton conductors (DOI: 10.1039/C4CS00093E). They focus on the design and synthesis of proton-conducting MOFs, as well as on their properties and the conditions under which they operate. Gas storage and separation has been a main theme in MOF research for more than a decade. In the next review, De Vos and coworkers give an account of liquid separation and purification using MOFs (DOI: 10.1039/C4CS00006D). In addition to an overview of the literature, separation mechanisms and structure–selectivity relationships are also analyzed. The MOF field has received much criticism in the past due to the water-sensitivity of some individual MOFs. In the next review, Farrusseng and coworkers present an overview of more than sixty MOF samples with reported water-adsorption properties (DOI: 10.1039/C4CS00078A). The applications of MOFs as materials for heat pumps and adsorbent-based chillers, as well as proton conductors, are also reviewed. Carbon capture using MOFs was extensively reviewed recently and is not included in this themed issue. However, in the next contribution, Navarro and coworkers highlight the recent literature on MOFs for the capture and degradation of toxic gases and vapors (DOI: 10.1039/C3CS60475F). Although hydrogen storage has not been reviewed in this volume, methane storage has seen significant recent advances. In the last review, B. Chen and coworkers provide an overview of the current studies on MOFs for methane storage (DOI: 10.1039/C4CS00032C). A comprehensive table of sorbents with methane adsorption data under a variety of conditions is also given in this article.

The MOF field is built upon cluster chemistry, organic chemistry, and X-ray crystallography. The pore space within MOFs provides a unique confined environment in which to develop new physical properties and chemical reactions. The time will come when MOF research will expand upon the three research fields, and change the foundation upon which it was built. For instance, we have already witnessed clusters that cannot be made in solution but can be stabilized in a MOF. We should be able to trap reactive intermediates and significantly alter the pathway, reactivity, and selectivity of an organic reaction. We could use a MOF as a matrix for crystal structure determination for macromolecules such as proteins. We have only scratched the surface in MOF research. In this volume, we have showcased the latest advances in MOF chemistry. We hope that you will find it beneficial to your future research and teaching endeavors. Finally, we would like to thank all the authors of this themed issue and the RSC editorial staff members for their invaluable contributions.

This journal is © The Royal Society of Chemistry 2014