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

The stabilities of three moisture-stable MOFs containing different metal clusters, i.e., HKUST-1 (Cu), MIL-53(Al), and ZIF-8 (Zn), were investigated in dihydrogen and dissociated hydrogen (caused by doped Pt nanoparticles) environments. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray-excited Auger electron spectroscopy (XAES) results showed that all three MOFs were stable in dihydrogen environment. However, the structure of Pt-doped HKUST-1 collapsed in the presence of dissociated hydrogen due to the higher reduction potential of Cu compared with H, and the degree of reduction that occurred to the divalent copper in HKUST-1 increased with temperature. Unlike HKUST-1, MIL-53 and ZIF-8 maintained their structures in both dihydrogen and dissociated hydrogen environments at temperatures up to 150 °C. Moreover, comparison of Pt-doped HKUST-1 samples synthesized by chemical vapor deposition (CVD) and incipient wetness impregnation showed that the contact between the doped Pt particles...

Investigation on Hydrogenation of Metal–Organic Frameworks HKUST-1, MIL-53, and ZIF-8 by Hydrogen Spillover

Crack-free carbon molecular sieve membrane supported on a macroporous substrate was formed by coating a layer of poly(furfuryl alcohol) followed by controlled pyrolysis. Molecular sieving membranes of desired thicknesses can be formed by repeating this procedure. Steady-state diffusion fluxes of single-component and binary mixtures of CH 4 /C 2 H 6 through the membrane were measured. Although large overall concentration gradients were used (to achieve accuracy), concentration-dependent diffusivities were obtained through integral analyses of the flux data. From the concentration dependence of the single-component diffusivities, it is possible to predict binary diffusivities using a simple theory developed by the authors. Good agreement was obtained between theoretical predictions and experimental data for binary diffusion

Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane

Desulfurization of a commercial diesel fuel by different layered adsorbents and regeneration of the latter were studied in a fixed-bed unit operated at ambient temperature and pressure. In general, a layered bed consisting of 12 wt % activated carbon, 22 wt % of activated alumina (Selexsorb CDX), followed by Cu(I)−Y, activated carbon/Selexsorb CDX/Cu(I)−Y, is capable of producing 41 cm3 of desulfurized diesel fuel/g of adsorbent. The matrix is capable of processing 27 cm3 of deep-desulfurized diesel with a weighted average content of 76 ppbw S. These low-sulfur fuels are suitable for fuel cell applications. For layered-bed regeneration, it was determined that calcination of the adsorbed sulfur moieties with air at 350 °C followed by autoreduction of the copper species recovered all of the original desulfurization capacity when activated aluminas are used as a guard layer. Solvent elution experiments indicate that carbon tetrachloride and N,N-dimethylformamide are suitable solvents to recover all of the ad...

New sorbents for desulfurization of diesel fuels via π complexation : Layered beds and regeneration

Fe-exchanged ZSM-5 has been found previously to be much more active than commercial vanadia-based catalysts for selective catalytic reduction (SCR) of NO with NH3. The kinetics of the SCR reaction is studied in this work. The results show that the rate of NO conversion on Fe-ZSM-5 is first-order with respect to NO, zeroth-order w.r.t. NH3 and nearly half-order w.r.t. O2, at 260–300 °C. This is in good agreement with our previous FTIR result that the catalyst surface is nearly completely covered by ammonia adsorbed species (i.e. NH4+ ions) under reaction conditions. The present results further support that the SCR mechanism on Fe-ZSM-5 involves NO2 as an intermediate and the formation of NO2 from NO oxidation (NO+(1/2)O2→NO2) is probably the rate-determining step. The apparent activation energy for the reaction is found to be 54 kJ/mol.

Ralph T. Yang

Papers

Investigation on Hydrogenation of Metal–Organic Frameworks HKUST-1, MIL-53, and ZIF-8 by Hydrogen Spillover

Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane

New sorbents for desulfurization of diesel fuels via π complexation : Layered beds and regeneration

Kinetics of selective catalytic reduction of NO with NH3 on Fe-ZSM-5 catalyst

FTIR and Kinetic Studies of the Mechanism of Fe3+-Exchanged TiO2-Pillared Clay Catalyst for Selective Catalytic Reduction of NO with Ammonia