There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

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

Metal–Organic Frameworks for Separations

Kenji Sumida, David L. Rogow, Jarad A. Mason, Thomas M. McDonald, Eric D. Bloch, Zoey R. Herm, Tae-Hyun Bae, Jeffrey R. Long

Carbon Dioxide Capture in Metal–Organic Frameworks

Erratum: Pressure-induced superconductivity in a ferromagnet UGe2 (J. Phys.: Condens. Matter (2001) 13 (L17-L23))

The magnetic properties of O2 adsorbed in nanochannels of porous metal-organic solids CPL-1 and CPL-p1 have been investigated by means of high-field magnetization process measurement. The contribution from adsorbed O2 shows the metamagnetic-like behavior in both compounds. We propose “the field-induced rearrangement mechanism” as the explanation for the metamagnetic-like behavior. It is known for the O2-O2 dimer that the magnetic interaction depends strongly on the intermolecular distance and the orientation of molecular axes. It is considered naturally that the stable arrangement with saturated magnetic moment in the sufficient high field is different from the H-structure which is stable at zero field.

/pdf/magnetic-properties-of-molecular-oxygen-adsorbed-in-micro-tvy9wqvc56.pdf

Magnetic properties of molecular oxygen adsorbed in micro-porous metal-organic solids

The magnetic properties of 4-methacryloyloxy- and 4-acryloyloxy-2,2,6,6-tetramethylpiperidine-l-oxyl (MOTMP and AOTMP, respectively) were studied. The magnetic susceptibility, magnetization, and molar heat capacity of MOTMP show that the crystal has one-dimensional ferromagnetic coupling, and that the coupling between the one-dimensional coupling chains was antiferromagnetic, resulting in a metamagnet below 0.16 K. On the other hand, AOTMP, where hydrogen atom is substituted for a methyl group at α-position, shows an antiferromagnetic coupling between the nitroxyl radicals.

Ferromagnetic and antiferromagnetic behavior of 4-Methacryloyloxy- and 4-Acryloyloxy-2,2,6,6-Tetramethylpiperidine-1-OXYL

The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of α-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin–orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu, Phys. Rev. Lett. 115, 207002 (2015); Y. Wang et al., Phys. Rev. B 93, 134512 (2016)]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningfu...

Pyrochlore oxide superconductor Cd2Re2O7 revisited

The magnetic behavior of the nickel cubane complex [Ni 4 (Ome) 4 -(sal) 4 (MeOH) 4 ](Hsal=salicylaldehyde) was studied by using a Hartshorn bridge equipped with a 3 He/ 4 He dilution refrigerator. This complex has long been known to have a well-isolated high-spin ground state with S =4 and it was suggested to have an easy-axis type anisotropy. AC magnetic susceptibility data clearly show a slow magnetization reversal below 0.3 K, characteristic of single-molecule magnets. The Arrhenius plot of the relaxation rate provides the activation energy of about 5 K.

Tatsuo C. Kobayashi

Papers

Erratum: Pressure-induced superconductivity in a ferromagnet UGe2 (J. Phys.: Condens. Matter (2001) 13 (L17-L23))

Magnetic properties of molecular oxygen adsorbed in micro-porous metal-organic solids

Ferromagnetic and antiferromagnetic behavior of 4-Methacryloyloxy- and 4-Acryloyloxy-2,2,6,6-Tetramethylpiperidine-1-OXYL

Pyrochlore oxide superconductor Cd2Re2O7 revisited

Slow Magnetization Reversal in [Ni 4 (OMe) 4 (sal) 4 (MeOH) 4 ]