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

A theoretical model using the method of characteristics is presented for a Skarstrom-type pressure swing adsorption (PSA) cycle using carbon molecular sieve for the kinetic separation of nitrogen/methane mixtures. Good agreement with experimental results was obtained. The solution of the reduced nonlinear ordinary differential equations along characteristics, a variable computational grid from the changing bed velocity, and a constant product flow boundary condition are complications that prohibited using the method of characteristics in favor of finite difference or orthogonal collocation methods. These complications have been overcome in the present study to develop a theoretical model, incorporating nonlinear isotherms and a linear driving force rate for diffusion, which requires less computer time and is particularly amenable for studying transient bed dynamics. The effects of feed pressure and concentration, half-cycle time, and product flow rate on kinetic PSA performance are also illustrated.

Kinetic separation by pressure swing adsorption : method of characteristics model

Surface diffusion on energetically heterogeneous surfaces

Control of microporosity of Al2O3-pillared clays: effect of pH, calcination temperature and clay cation exchange capacity

Pt-Rh/TiO 2 /Al 2 O 3 was investigated as a NO x storage-reduction catalyst using lean-rich cycles. It was found to be highly effective for NO x reduction and highly resistant to SO 2 and H 2 O. In the presence of 100 ppm SO 2 and 2.3% H 2 O, approximately 90% of NO x conversion was obtained at 250°C and a GHSV of 30,000 h −1 after 5 h lean-rich cyclic runs. A model NO x trap catalyst, Pt-Rh/BaO/Al 2 O 3 , was used for comparison. In the absence of SO 2 and H 2 O, nearly complete removal of NO x was achieved. NO x conversion dropped to only 30% after SO 2 /H 2 O were introduced into the reactant gases at 300°C. With Pt-Rh/TiO 2 /Al 2 O 3 , over 70% NO conversion was still obtained at 400°C. Although, the BaO containing catalyst has much higher capacity for NO x than that containing TiO 2 , it is much more susceptible to SO x poisoning. In situ FT-IR spectra of the titania catalyst showed that nitrate species was the final adspecies (i.e. at the end of the lean phase) on the storage catalyst and its intensity was only slightly weakened in the presence of SO 2 and H 2 O.

A highly sulfur resistant Pt-Rh/TiO2/Al2O3 storage catalyst for NOx reduction under lean-rich cycles

Hydrogen storage capacity by hydrogen spillover on metal-doped carbons can be significantly enhanced by introducing surface oxygen functional groups to the carbon. It is shown that the enhancement is greater by oxygen plasma treatment compared to air oxidation because different surface groups were formed. Oxygen plasma treatment generated mainly semiquinone (C═O) groups while air oxidation formed mainly hydroxyl (C−OH) groups. Experimental heats of adsorption, X-ray photoelectron spectroscopy (XPS) analyses, and ab initio molecular orbital calculations showed that the binding energies between the spiltover hydrogen and the different groups followed the following order: lactone > semiquinone > carboxyl > basal plane. Thus, the H2 storage capacity at 298 K and 10 MPa was increased from 1.17 wt % (without O2 treatment) to 1.74 wt % on Pt-doped on a templated carbon that was pretreated with O2 plasma. Similar enhancements were seen on Pt doped on a superactivated carbon, AX-21. However, there was a decrease i...

Ralph T. Yang

Papers

Kinetic separation by pressure swing adsorption : method of characteristics model

Surface diffusion on energetically heterogeneous surfaces

Control of microporosity of Al2O3-pillared clays: effect of pH, calcination temperature and clay cation exchange capacity

A highly sulfur resistant Pt-Rh/TiO2/Al2O3 storage catalyst for NOx reduction under lean-rich cycles

Enhanced Hydrogen Spillover on Carbon Surfaces Modified by Oxygen Plasma