Adsorptive separation is very important in industry. Generally, the process uses porous solid materials such as zeolites, activated carbons, or silica gels as adsorbents. With an ever increasing need for a more efficient, energy-saving, and environmentally benign procedure for gas separation, adsorbents with tailored structures and tunable surface properties must be found. Metal–organic frameworks (MOFs), constructed by metal-containing nodes connected by organic bridges, are such a new type of porous materials. They are promising candidates as adsorbents for gas separations due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability. This critical review starts with a brief introduction to gas separation and purification based on selective adsorption, followed by a review of gas selective adsorption in rigid and flexible MOFs. Based on possible mechanisms, selective adsorptions observed in MOFs are classified, and primary relationships between adsorption properties and framework features are analyzed. As a specific example of tailor-made MOFs, mesh-adjustable molecular sieves are emphasized and the underlying working mechanism elucidated. In addition to the experimental aspect, theoretical investigations from adsorption equilibrium to diffusion dynamics via molecular simulations are also briefly reviewed. Furthermore, gas separations in MOFs, including the molecular sieving effect, kinetic separation, the quantum sieving effect for H2/D2 separation, and MOF-based membranes are also summarized (227 references).

Selective gas adsorption and separation in metal–organic frameworks

A broad organic–inorganic series of hybrid metal iodide perovskites with the general formulation AMI3, where A is the methylammonium (CH3NH3+) or formamidinium (HC(NH2)2+) cation and M is Sn (1 and 2) or Pb (3 and 4) are reported. The compounds have been prepared through a variety of synthetic approaches, and the nature of the resulting materials is discussed in terms of their thermal stability and optical and electronic properties. We find that the chemical and physical properties of these materials strongly depend on the preparation method. Single crystal X-ray diffraction analysis of 1–4 classifies the compounds in the perovskite structural family. Structural phase transitions were observed and investigated by temperature-dependent single crystal X-ray diffraction in the 100–400 K range. The charge transport properties of the materials are discussed in conjunction with diffuse reflectance studies in the mid-IR region that display characteristic absorption features. Temperature-dependent studies show a ...

Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.

Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites.

Metal–organic frameworks (MOFs) are a unique class of crystalline solids comprised of metal cations (or metal clusters) and organic ligands that have shown promise for a wide variety of applications Over the past 15 years, research and development of these materials have become one of the most intensely and extensively pursued areas A very interesting and well-investigated topic is their optical emission properties and related applications Several reviews have provided a comprehensive overview covering many aspects of the subject up to 2011 This review intends to provide an update of work published since then and focuses on the photoluminescence (PL) properties of MOFs and their possible utility in chemical and biological sensing and detection The spectrum of this review includes the origin of luminescence in MOFs, the advantages of luminescent MOF (LMOF) based sensors, general strategies in designing sensory materials, and examples of various applications in sensing and detection

Luminescent metal–organic frameworks for chemical sensing and explosive detection

Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

Ferroelectric Metal–Organic Frameworks

Tetrazole compounds have been studied for more than one hundred years and applied in various areas. Several yeas ago Sharpless and his co-workers reported an environmentally friendly process for the preparation of 5-substituted 1H-tetrazoles in water with zinc salt as catalysts. To reveal the exact role of the zinc salt in this reaction, a series of hydrothermal reactions aimed at trapping and characterizing the solid intermediates were investigated. This study allowed us to obtain a myriad interesting metal–organic coordination polymers that not only partially showed the role of the metal species in the synthesis of tetrazole compounds but also provided a class of complexes displaying interesting chemical and physical properties such as second harmonic generation (SHG), fluorescence, ferroelectric and dielectric behaviors. In this tutorial review, we will mainly focus on tetrazole coordination compounds synthesized by in situhydrothermal methods. First, we will discuss the synthesis and crystal structures of these compounds. Their various properties will be mentioned and we will show the applications of tetrazole coordination compounds in organic synthesis. Finally, we will outline some expectations in this area of chemistry. The direct coordination chemistry of tetrazoles to metal ions and in situ synthesis of tetrazole through cycloaddition between organotin azide and organic cyano group will be not discussed in this review.

In situ hydrothermal synthesis of tetrazole coordination polymers with interesting physical properties

Hydrothermal reaction of (l)-N-(4'-cyanobenzy)-(S)-proline with CdCl2 as a Lewis acid catalyst and NaN3 gives colorless block compound 1, in which 1 displays a complicated 3D framework. Ferroelectric and dielectric property measurements reveal that 1 exhibits physical properties comparable to that of a typical ferroelectric compound with a dipole relaxation process and a dielectric constant of ca. 38.6 that makes it, by definition, a high dielectric material.

Ferroelectric metal-organic framework with a high dielectric constant.

Ferroelectric materials are of importance and interest in both fundamental scientific research and various technological applications. Metal–organic complexes (MOCs) represent a class of molecule-based ferroelectrics, which have shown various properties or functionalities due to their hybrid inorganic–organic nature. This tutorial review shows the recent development of the MOC ferroelectrics with particular emphases on the mechanism of ferroelectric-to-paraelectric phase transition, symmetry consideration, and multifunctionality.

Metal–organic complex ferroelectrics

Progress in metal–organic framework (MOF) research has recently opened up new possibilities to realize hybrid materials with unique solid-state electric properties, such as ferroelectricity, piezoelectricity, and dielectricity. Compared with conventional pure inorganic/organic compounds, MOFs take advantage of structural tunability and multifunctionality to develop polarizable molecular materials with rich dielectric properties. Among them, switchable molecular dielectrics, which undergo transitions between high and low dielectric states, are promising materials with potential applications especially in data communication, signal processing, and sensing. However, reports of such MOFs have remained scarce owing to a lack of knowledge regarding control of the motions of the dipole moments in the crystal lattice. From the microscopic point of view, the tunable dielectric permittivity closely relates to the positional freedom of molecular dipole moments. For instance, polar molecules in the liquid state show larger dielectric permittivities than in the solid state owing to the “melting” and “freezing” of the molecular reorientations. With regard to MOFs, the dipole moments are rigidly fixed in the crystal structures in most cases, usually resulting in small and almost temperatureindependent dielectric permittivities. Fortunately, there is still much room for the integration of flexible units into the frameworks; that is, the introduction of a polarization rotation unit in the form of a solid-state molecular rotator or host–guest systems, such as porous compounds. Cage compounds, which are assembled by the inclusion of guest species into the well-matched host cages, is a very promising class of switchable molecular dielectrics. The reorientations of the polar guests in the carefully designed cage compounds may give rise to large dielectric permittivities, which are characterized by a multidimensional liquidlike state, and their freezing will lead to low-dielectric systems. Herein, we present a novel organic–inorganic hybrid cage compound (HIm)2[KFe(CN)6] (1; HIm = imidazolium) with a perovskite-type structure, in which the order– disorder behavior of the HIm polar guests give rise to striking dielectric anomalies. The (HIm)2[KFe(CN)6] crystals were grown from an aqueous solution of K3[Fe(CN)6] and (HIm)Cl salts by slow evaporation at room temperature as large red hexagonal plate perpendicular to the c axis. The existence of HIm and CN groups in 1 is verified by IR spectra. The CN group in 1 exhibits several vibrations in the range 2102–2143 cm , distinct from a single peak of 2118 cm 1 in K3[Fe(CN)6]. Thermal analysis reveals that 1 undergoes two phase transitions, at 187 K (T1) and 158 K (T2). For convenience, we label the phase above T1 as the high-temperature phase (HTP), the phase between T1 and T2 as intermediate-temperature phase (ITP), and the phase below T2 as low-temperature phase (LTP). Variable-temperature X-ray diffraction analysis reveals that 1 crystallizes in the centrosymmetric space group R3̄m at 293 K and 173 K as the HTP and ITP, respectively, and in C2/c at 83 K as the LTP. The common structural feature of the compound is the anionic cage formed by Fe CN K units in which the HIm cation resides. The metal–cyanide bond is strong and covalent in the fragment {Fe(CN)6} (Fe C = 1.9 ) and much weaker and ionic in the fragment {K(NC)6} (K N = 2.9 ; Figure 1). In the HTP, the cation reorients around the threefold c axis perpendicular to the ring plane. The cation consists of three carbon and two nitrogen atoms, which were all refined as carbon atoms. The five atoms of the

Ren-Gen Xiong

Papers

Ferroelectric Metal–Organic Frameworks

In situ hydrothermal synthesis of tetrazole coordination polymers with interesting physical properties

Ferroelectric metal-organic framework with a high dielectric constant.

Metal–organic complex ferroelectrics

Exceptional dielectric phase transitions in a perovskite-type cage compound.