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

New materials hold the key to fundamental advances in energy conversion and storage, both of which are vital in order to meet the challenge of global warming and the finite nature of fossil fuels. Nanomaterials in particular offer unique properties or combinations of properties as electrodes and electrolytes in a range of energy devices. This review describes some recent developments in the discovery of nanoelectrolytes and nanoelectrodes for lithium batteries, fuel cells and supercapacitors. The advantages and disadvantages of the nanoscale in materials design for such devices are highlighted.

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Nanostructured materials for advanced energy conversion and storage devices

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

Starting from the relationship between the mean field (Φ) and the characteristic adsorption energy (E), relationships are derived for the dependence of the characteristic energy on pore size. Specifically, relationships between the characteristic adsorption energyEin the Dubinin–Radushkevich (D–R) and Dubinin–Astakhov (D–A) equations and the pore dimension of the slit-shaped pores are derived and discussed for three adsorption regions: the Henry's law region, the moderate coverage region, and the high coverage region. The results show that the characteristic energyEis a complicated function of many factors including pore size and coverage (θ), as well as temperature (T). There is no simple correlation between characteristic energyEand pore sizeLfor the whole range of θ. Nevertheless, in the moderate coverage region, θ ≈ 0.2–0.8, the simple inverse proportionality may apply although it is only a crude approximation. The characteristic energy depends on the pore size more strongly in the high coverage region and less strongly in the Henry's law region. Moreover, the results reveal that, when the pore size is in the range of 1–2 times the diameter of the adsorbate molecule, different potential average schemes yield very close values for the mean field, indicating that the average scheme is not important in this case.

Theoretical Investigation of Relationships between Characteristic Energy and Pore Size for Adsorption in Micropores

Fluidized-Bed Combustion of Coal with Lime Additives. Kinetics and Mechanism of Regeneration of the Lime Sorbent

CuO/TiO2 has been shown to be a superior sorbent for selective, reversible adsorption of NO from hot combustion gases. High NOx adsorption capacities at 200 and 300 °C were obtained with a 5% CuO/TiO2 sorbent, and the NOx desorbed rapidly at 450 °C. Doping with 2% Ce on the CuO/TiO2 sorbent further increased both uptake rates (50% increase in initial rate) and NOx capacity (by 30%). In a direct comparison with the most promising known sorbent, MnO/ZrO2 (1:1 molar ratio), reported by Eguchi et al. (J. Catal. 1996, 158, 420), the Ce-CuO/TiO2 sorbent showed both higher uptake rates (by 100% in initial rate) and higher NOx capacity (by 15%). The effects of CO2, H2O, and SO2 on NOx sorption on the Ce-CuO/TiO2 sorbent were studied at 200 °C. CO2 slightly decreased the initial uptake rate but increased the NOx capacity. H2O coadsorbed with NOx on different sites, both reversibly (i.e. desorbed at 450 °C). SO2 irreversibly adsorbed (likely to sulfate the surface of TiO2) and decreased the NOx capacity by approxim...

Ralph T. Yang

Papers

Theoretical Investigation of Relationships between Characteristic Energy and Pore Size for Adsorption in Micropores

Fluidized-Bed Combustion of Coal with Lime Additives. Kinetics and Mechanism of Regeneration of the Lime Sorbent

Selective Adsorption of NOx from Hot Combustion Gases by Ce-Doped CuO/TiO2

Adsorption of gases on coals and heattreated coals at elevated temperature and pressure: 2. Adsorption from hydrogen-methane mixtures

Zeolites and Molecular Sieves