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

Several aspects of high field heat capacity observed in different types of singlet ground state system are discussed. The ground state energy gap and its magnetic field dependence are also studied in the Haldane gap compounds NENP, NINO and TMNIN. The origin of the observed minimum gap in applied magnetic field is discussed. In the spin-Peierls compound CuGeO 3 , the energy gap of the Cu 2+ spin system in the dimerized phase and the gapless state in the high-field magnetic phase are confirmed.

High field heat capacities of some singlet ground state systems at very low temperatures

Thermal properties of a ferromagnetic superconductor UGe2 were investigated by the heat capacity measurements. The bulk nature of the superconductivity was confimied by the observation of a peak in the heat capacity at the superconducting transition temperature TSC, From the pressure dependence of the linear heat capacity coefficient γ, it is suggested that the low energy magnetic excitation near the critical point Pc*, where T* becomes OK, might play an important role on the superconductivity. The pressure dependence of the superconducting peak and the temperature dependence of CIT above TSC suggest that the order parameter of the superconducting state might be different between the phases below and above PC*.

Thermal properties of a ferromagnetic superconductor UGe2

Heat capacity measurements are performed in Haldane-gap antiferromagnet NENP and TMNIN under magnetic field up to 13T. For NENP, the exponential temperature dependence of heat capacity gives strong evidence of existence of the energy gap. The field dependence of the gap energy shows characteristic increase above the critical field, which is considered to correspond to appearance of the staggered moment observed in 1 H-NMR measurement. We obtain similar results qualitatively for TMNIN. Moreover, the density of state in the lowest excited state is also discussed.

Heat capacities in Haldane-gap antiferromagnet NENP and TMNIN under magnetic field

High-quality single-crystal growth and a change of the electronic states under pressure in CeRh2Si2

We review an indenter-type pressure cell, which possesses a maximum pressure of 5 GPa and a sample space of q1.6×1.6 mm. This pressure cell is available not only for transport measurements but also for ac-susceptibility and NMR measurements. An advantage is the small size being able to change the direction of the pressure cell in the conventional superconducting magnet, which makes it possible to measure the anisotropy against magnetic field. [indenter type of pressure cell, hydrostaticity, electrical resistivity, NMR]

Tatsuo C. Kobayashi

Papers

High field heat capacities of some singlet ground state systems at very low temperatures

Thermal properties of a ferromagnetic superconductor UGe2

Heat capacities in Haldane-gap antiferromagnet NENP and TMNIN under magnetic field

High-quality single-crystal growth and a change of the electronic states under pressure in CeRh2Si2

Indenter Type of Pressure Cell and Its Applications