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

Catalytic performance and characterization of VO2+-exchanged titania-pillared clays for selective catalytic reduction of nitric oxide with ammonia

New sorbents for benzene/cyclohexane separation based on π-complexation were prepared by dispersion of transition metal salts on a high-surface-area substrate. PdCl2 or AgNO3 dispersed on SiO2 gel exhibited high equilibrium adsorption ratios of benzene over cyclohexane. PdCl2 loading of 0.88 g/g of SiO2 showed the best benzene/cyclohexane ratio of 3.2. The heats of adsorption of benzene on these sorbents were in the range 7−12 kcal/mol and followed the rank order CuCl > PdCl2 > AgNO3 > AuCl3 > PtCl4, compared to 5−7 kcal/mol for cyclohexane. Molecular orbital calculations for the bonding of benzene and chlorides of these metals were performed at the Hartree−Fock (HF) and density functional theory (DFT) levels using effective core potentials. The theoretical bond energies were (in kcal/mol) 12.5 (CuCl), 10.8 (PdCl2), 8.6 (AgCl), 6.5 (AuCl3), and 5.2 (PtCl4), in fair agreement with the experimental results. The M−C interactions followed classical π-complexation in most cases, and differences in the bonds wi...

Aromatics/Aliphatics Separation by Adsorption: New Sorbents for Selective Aromatics Adsorption by π-Complexation

Pressure swing adsorption (PSA) air purification/vapor recovery was studied by experiments and model simulations using dimethyl methylphosphonate (DMMP) vapor and activated carbon

Pressure swing adsorption: experimental and theoretical study on air purification and vapor recovery

A direct comparison of adsorption of CO and ethylene on AgCl and CuCl is made. An ab initio molecular orbital study using the effective core potential is performed to determine the bond energies and the nature of the bonds between the adsorbates and adsorbents. Experimental results show that both CO and C2H4 adsorb more strongly on CuCl than on AgCl. However, CO adsorbs much more strongly on CuCl than on AgCl, while the difference in heats of adsorption is far less for C2H4 on these two sorbents. Ab initio molecular orbital calculations correctly predict the trends. The natural bond orbital (NBO) theory is used to explain the results. The NBO results show that in the two-way π-complexation bonding, the d−π* backdonation plays a major role in determining the bonding for these systems.

Comparison of π-complexations of ethylene and carbon monoxide with Cu+ and Ag+

Clinoptilolites were tailored through controlled ion exchange with both indigenous cations, such as Na+ and Mg+, and other cations, such as Li+, Sr2+, and Ce3+. These partially exchanged clinoptilolites are characterized through neutron activation analysis, and the adsorption isotherms and diffusion rates of nitrogen and methane were measured. Partially exchanged Ce clinoptilolite showed reversal of equilibrium selectivity from nitrogen to methane as the extent of Ce3+ exchange increased, while mixed Na/Li (also referred to as partially exchanged Li+) clinoptilolites showed reversal of equilibrium selectivity as the extent of Li+ exchange decreased. High-pressure isotherms on mixed Mg/Na clinoptilolites and partially exchanged Ce clinoptilolites were measured through the differential adsorption bed technique. Pressure swing adsorption (PSA) simulations were performed on a two-stage PSA process operating on a five-step PSA cycle for the tailored clinoptilolite sorbents. The simulation results of tailored c...

Ralph T. Yang

Papers

Catalytic performance and characterization of VO2+-exchanged titania-pillared clays for selective catalytic reduction of nitric oxide with ammonia

Aromatics/Aliphatics Separation by Adsorption: New Sorbents for Selective Aromatics Adsorption by π-Complexation

Pressure swing adsorption: experimental and theoretical study on air purification and vapor recovery

Comparison of π-complexations of ethylene and carbon monoxide with Cu+ and Ag+

Tailored Clinoptilolites for Nitrogen/Methane Separation