MOFs: New Useful Materials – A Special Issue in Honor of Prof. Susumu Kitagawa

Metal–organic framewrks (MOFs), also known as porous coordination polymers (PCPs), are an emerging class of porous materials. Due to their structural and functional tunability, the field of MOFs has become one of the fastest growing fields in not only chemistry but also material science. With the maturation of the synthesis methodology, structure characterization, and functionality modification, a new era of MOFs as a useful material has begun, with a huge number publications appearing in the last few years, dealing with their applications in a variety of directions, even devices. Here, we are greatly honored to assemble a compendium of the latest work on MOFs toward their applications from the materials angle, in this special issue in honor of Professor Susumu Kitagawa at Kyoto University, one of the pioneers of the MOF field. As early as in 1997, Kitagawa successfully demonstrated porosity in solid coordination networks with gas-sorption experiments. He synthesized a wide variety of MOFs and developed a comprehensive structural chemistry. In the early stage of MOF chemistry, he focused on sufficiently small and uniform pores, which possess comparable size to the oxygen molecule, and realized that gaseous oxygen can be made to form regular one-dimensional chains. This gave unprecedented insight into the properties of confined gases, with the resulting highly compressed packing of O2 comparable to that in solid oxygen at 2 GPa (2002). Acetylene is a gas with many industrial applications; however, it is hard to handle because of its explosive nature. A highly efficient method to separate it from its close relation, carbon dioxide (CO2), is a promising route for purifying and storing “strategic” gases in general. He succeeded in the highly selective capture and storage of acetylene (C2H2) gas against CO2 under ambient conditions (2005). The safe storage system involving the use of MOFs is a significant achievement for technologies for the storage and transport of C2H2 and other explosive gas. These two studies demonstrated early on the usefulness and versatility of MOFs. Beyond these robust porous frameworks, Kitagawa also developed the concept of the framework, possessing both porosity and structural flexibility. He predicted the existence and importance of this class of materials, and gave it the generic name of soft porous crystals. An important class of soft porous crystals is the flexible MOFs, which are regarded as cooperatively integrating “softness” and “crystallinity”. The precise combination of metal ions and organic ligands created a variety of flexible MOFs, and he demonstrated the supercritical gas recognition for separation (2003). A flexible framework can adsorb the target gas at a certain pressure via structural changes, and then release the adsorbed gas by the pressure-swing method. He optimized the flexibility in the structures, and achieved high CO2-separation ability over CH4 in low-regeneration-energy materials (2012). It has been shown that some of his flexible MOFs offer low-energy CO2 separation from biogas. He created MOF materials whose gas-adsorption properties are controllable by applying external stimuli such as light or electric fields. Switchable pore surfaces were synthesized that can be activated by the photoirradiation procedure (2010). On-demand trapping of a molecular decoding system used a structural entanglement to detect a single target molecule of benzene derivatives in a vapor form with a corresponding luminescent readout (2011). By the use of a redox-active MOF, the high selectivity for O2 and NO over various gases was demonstrated. MOFs contain an electron-donating organic linker and oxidation of the ligand occurs as specific gases are adsorbed (2010). Beyond this functionalization of the host framework, he also made a responsive MOF by using guest accommodation. He incorporated fluorescence signaling molecules in the porous structure, with the fluorescence of the hybrid material being switched ON/OFF via adsorption of CO2 gas (2011), allowing low-concentration gas sensing. He demonstrated reversible switching of magnetic states in MOFs by guest adsorption (2009). An Fe2+-based MOF with cyano-ligand bridging exhibits bistability between the high spin (HS) state and the low spin (LS) state at room temperature. The MOF can adopt both the HS and LS states depending on the adsorbed gases. These magnetic states are switched by gas adsorption/desorption at room temperature. The incorporation of such responsive groups in MOFs demonstrates their large potential to create advanced porous materials. Kitagawa noticed that the thermal transitions of confined organic polymers, important in molecule-/ion-transport technology, can be controlled within nanospaces. He showed that the pore size and surface functionality of MOFs can be tailored to study the transition behavior of confined polymers, providing properties characteristic of a few polymer chains (2010). The work demonstrated that the confined space of MOFs is useful to study and investigate the intrinsic nature of organic polymer chains, and can enable the synthesis of new types of organic polymers. He encapsulated the proton-carrier molecule imidazole in a metal–organic framework to create an anhydrous high-temperature proton conductor operating above 100 °C (2009). The open space of the MOF promotes the proton hopping pathway. For the application of MOFs, control of the crystal morphology and processing are essential. Kitagawa presented a new synthetic methodology, the “coordination modulation method”, for size- and shape-controlled MOF crystals (2009). He has also demonstrated the rapid preparation of porous coordination polymer nanocrystals by the use of an organic surfactant and MOF precursors (2010). The approach enables the synthesis of large quantities of uniform MOF nanocrystals, and would enhance the kinetics of molecule diffusion for gas adsorption and heterogeneous catalysis. To construct thin-film MOFs, he developed the direct growth approach of MOF microcrystals from the metal oxide substrate, the “coordination replication method” (2012), generating a hierarchical porous system possessing micro-, meso-, and macropores. He discovered a unique property of MOFs in the mesoscopic domain, and demonstrated that the crystal downsizing of twofold interpenetrated frameworks of [Cu2(dicarboxylate)2(amine)]n regulates the structural flexibility and induces a shape-memory effect in the coordination framework (2013). The flexible nature of MOFs for gas adsorption changed when the crystals were significantly downsized. He successfully observed functional control by tuning the crystal size of the MOF, and the work inspired the design of functions of MOFs by morphology control, especially in the mesoscale regime. Kitagawa's achievements have led to radical innovations in materials science with wide-ranging implications for both academia and industry. This special issue includes contributions from twelve groups whose research ranges from design and synthesis to applications of MOFs and MOF-related materials. Feng, Bu, and co-workers (article number 1705189) survey the recent progress of MOFs for separations of molecules in a broad range. Jiang, Xu, and co-workers (article number 1703663) present an overview of recent developments achieved in MOF catalysis, including heterogeneous catalysis, photocatalysis, and electrocatalysis over MOFs and MOF-based materials. Yaghi and co-workers (article number 1704304) highlight the chemistry and application of water in MOFs. Kaskel and co-workers (article number 1704679) give an overview of recent studies in adsorption and detection of hazardous trace gases by MOFs. Burtch, Allendorf, and co-workers (article number 1704124) discuss the emerging opportunities and challenges for MOF-enabled device functionality and technological applications that arise from their fascinating mechanical properties. Zhou and co-workers (article number 1704303) review the recent advances in the field of stable MOFs, covering the fundamental mechanisms of MOF stability, design, and synthesis of stable MOF architectures, and their latest applications. Kieslich, Fischer, and co-workers (article number 1704501) give and overview of the creation of defects in MOFs and their properties and applications, especially to catalysis. Zou, Xu, and co-workers (article number 1702891) summarize the recent progress of MOFs and MOF composites for energy storage and conversion applications, including photochemical and electrochemical fuel production, water oxidation, supercapacitors, and batteries. Nanosized MOFs are a current hot research topic, and three articles deal with nanosized MOFs in this special issue. Mirkin and co-workers (article number 1800202) give a progress report of the synthesis, properties and applications of MOF nanoparticles. Serre and co-workers (article number 1707365) review the advances in the application of MOF nanoparticles to in vivo efficacy in biomedicine. Lin and co-workers (article number 1707634) give an overview of nanoscale MOFs for therapeutic, imaging, and sensing applications. Day and Cooper (article number 1704944) demonstrate energy–structure–function maps as a new approach for the discovery of functional organic crystals. We hope that this special issue will provide the readers some representative and exciting views regarding the new development and utilization of MOFs and MOF-related materials. Due to the dynamic nature of this rapidly growing field, it is impossible to cover every main aspect of this field. There is no doubt that the research field of MOFs will continue to expand, with contributions from not only chemists, but also material scientists and engineers. We do hope that readers will enjoy the scope of topics presented here and perhaps find inspiration to push this field to the next stage. Qiang Xu received his Ph.D. in physical chemistry in 1994 from Osaka University. He is the Director of AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Adjunct Professor of Kobe University/Kyoto University, Distinguished Professor of Yangzhou University, and Distinguished Honorary Professor of The Hong Kong Polytechnic University. His research interests include the chemistry of nanostructured materials and their applications, especially for catalysis and energy. Hiroshi Kitagawa was born in December 1961 in Osaka; he finished his Ph.D. at Kyoto University in 1991 and, after working as Assistant Professor at IMS (Okazaki) and JAIST (Kanazawa), he was appointed as Associate Professor to the University of Tsukuba in 2000. He became Professor of Chemistry at Kyushu University in 2003 and moved to Kyoto University as Professor in 2009. He is now Vice Provost and Deputy Executive-Vice President for Research at Kyoto University. He is also engaged at the Japan Science & Technology Agency (JST) as a Research Director of ACCEL and is a Director of the network-type research institution “Science and Creation of Innovative Catalysts, PRESTO”. His research involves solid-state chemistry, coordination chemistry, nanoscience, low-dimensional electron systems, and molecule-based conductors.