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

Silver is known to strongly affect the adsorptive properties of some zeolites. It is also known that thermal vacuum dehydration of some argentiferous zeolites leads to the formation of charged silver clusters within the zeolite. In this work we have synthesized silver zeolites of the types Y, X, and low-silica X. The zeolites were treated in such a way as to promote the formation of intracrystalline charged silver clusters. Equilibrium room-temperature isotherms were measured for adsorption of nitrogen for each of the zeolites after various heat treatments and dehydration. These materials were structurally characterized via Rietveld refinement using neutron powder diffraction data. Color changes upon heat treatment and subsequent X-ray photoemission spectroscopy confirmed some reduction of Ag+ → Ag0. The effects of various dehydration conditions, including the time, temperature, and atmosphere, on the room-temperature adsorption of nitrogen are discussed. Structural characterization, along with valence bo...

Silver ion-exchanged zeolites Y, X, and low-silica X: Observations of thermally induced cation/cluster migration and the resulting effects on the equilibrium adsorption of nitrogen

The catalytic activities of Fe/Beta (i.e., by incipient wetness impregnation) zeolite catalysts with various Fe loadings were investigated for the selective catalytic reduction (SCR) of nitric oxide by ammonia. Their stabilities and deactivation behavior were studied via hydrothermal aging and hydrocarbon coking. The results showed that NO conversion over 2.5 wt% Fe/Beta catalyst could reach 97.0% in the temperature range of 300–500 °C at a high space velocity (1.9 × 10 5  h −1 ). The Fe/Beta catalyst showed stability up to a hydrothermal aging temperature of 750 °C. XRD results showed that the structure of Fe/Beta was altered by hydrothermal aging over 750 °C, with significant de-alumination as well as a decrease of lattice constants at 900 °C. The catalysts after deactivation by the presence of propylene could be fully regenerated by burning off the coke in the reactants over 450 °C or in air at 550 °C. It was found that the deposition of carbon species on Fe/Beta decreased its activity for forming NO 2 as evidenced by FTIR. XPS results showed that the introduction of C 3 H 6 in the reactants resulted in partial reduction of Fe 2+ thereby decreased the activity of Fe/Beta for NO SCR.

Activity, stability and hydrocarbon deactivation of Fe/Beta catalyst for SCR of NO with ammonia

Fe 3+ -exchanged TiO 2 -pillared clay (Fe–TiO 2 –PILC) has been previously found to be more active than vanadia-based catalysts for selective catalytic reduction (SCR) of NO by NH 3 . The promoting role of rare earth oxides with variable valence (i.e. CeO 2 , PrO 1.83 and TbO 1.75 ) on the Fe–TiO 2 –PILC was studied in this work. The catalysts were also characterized by XRD, FT-IR and NO oxidation experiments. It was found that the addition of a small amount (≤2 wt.%) of CeO 2 and PrO 1.83 to Fe–TiO 2 –PILC increased the SCR activity (by 35%) both in the absence and presence of H 2 O+SO 2 , but TbO 1.75 did not improve the SCR activity significantly. XRD analysis showed that a cerium oxide phase was not observed on CeO 2 /Fe–TiO 2 –PILC when the amount of CeO 2 was below 2 wt.%. However, when more cerium oxide (5 wt.%) was doped on the Fe–TiO 2 –PILC, the SCR activity was decreased and a cubic CeO 2 phase was detected. This suggests that well dispersed cerium oxide is beneficial to the improvement of SCR activity. FT-IR spectra of NH 3 -adsorbed samples indicated that the addition of cerium oxide did not increase the surface acidity of Fe–TiO 2 –PILC. The improvement of SCR activity was attributed to an increase in the oxidation activity of NO to NO 2 by O 2 since NO 2 is an intermediate for the SCR reaction on the Fe–TiO 2 –PILC catalysts. By comparison, the SCR reaction on vanadia catalysts does not involve the adsorption and oxidation of NO for and as a result, the addition of CeO 2 to V 2 O 5 /TiO 2 does not increase their SCR activities.

The promoting role of rare earth oxides on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of nitric oxide by ammonia

Adsorption of gases on coals and heattreated coals at elevated temperature and pressure: 1. Adsorption from hydrogen and methane as single gases

Morphology Effects of CeO2 Nanomaterials on the Catalytic Combustion of Toluene: A Combined Kinetics and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study

Ralph T. Yang

Papers

Silver ion-exchanged zeolites Y, X, and low-silica X: Observations of thermally induced cation/cluster migration and the resulting effects on the equilibrium adsorption of nitrogen

Activity, stability and hydrocarbon deactivation of Fe/Beta catalyst for SCR of NO with ammonia

The promoting role of rare earth oxides on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of nitric oxide by ammonia

Adsorption of gases on coals and heattreated coals at elevated temperature and pressure: 1. Adsorption from hydrogen and methane as single gases

Morphology Effects of CeO2 Nanomaterials on the Catalytic Combustion of Toluene: A Combined Kinetics and Diffuse Reflectance Infrared Fourier Transform Spectroscopy Study