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 authors have examined electropolymerized polypyrroles doped with p-toluenesulfonate (TOS), tetrafluoroborate (TFB), or trifluoroacetate (TFA), using scanning tunneling microscopy (STM). The initial stages of polymer growth involves microisland formation with concomitant polymer strands. In one case, the p-toluenesulfonate-doped polypyrrole, those strands have been shown to be helical.

Scanning tunneling microscopic imaging of electropolymerized, doped polypyrrole. Visual evidence of semicrystalline and helical nascent polymer growth

An ab initio molecular orbital study using the effective core potentials (ECP) is performed to determine the anion and cation effects on the adsorption of C2H4 and C3H6 on CuX and AgX (X = F, Cl, Br, I). Compared with all-electron calculations, the ab initio ECPs require only a fraction of the computational resources with accuracy that approaches that of the all-electron calculations. The following trends of anion and cation effects were obtained for the adsorption of C2H4 and C3H6 on the metal halides:  F- > Cl- > Br- > I- for anions, and Cu+ > Ag+ for cations. These trends are in excellent agreement with the experimental results. In addition, the theoretical metal−olefin bond energies are in fair agreement with the experimental data. A detailed analysis of the electronic distribution is also performed using natural bond orbital (NBO) theory. The NBO results show that the d−π* back-donation is about twice that of the σ donation for the Cu+−olefin bonds, whereas the two donations are of the same order for...

Anion and Cation Effects on Olefin Adsorption on Silver and Copper Halides: Ab Initio Effective Core Potential Study of π-Complexation

Adsorption of NO (with, and without O 2 ) on a commercial Degussa TiO 2 (designated TiO 2 - d ), and sol–gel TiO 2 (TiO 2 - g ) and their surface-sulfated forms were studied by TGA and in situ FTIR. Using TGA, these samples were studied as sorbents for selective, reversible adsorption of NO from hot combustion gases. Adsorption and desorption were measured, respectively, at 300 and 450°C in the same gas atmosphere (2000 ppm NO and 4% O 2 ). The sulfated TiO 2 - g sample showed the highest reversible amount of NO x adsorbed (≈14 mg/g) as compared to all known sorbents. The amount of NO adsorbed was higher on the unsulfated TiO 2 samples, however, only 20% of the adsorbed NO could be desorbed at 450°C. Thus, NO adsorption was weakened by the presence of surface sulfate. In situ FT-IR studies showed the following results: The unsulfated TiO 2 showed only (strong) Lewis acidity, with no Bronsted acidity. Bidendate SO 2− 4 ions (most likely a bridged form) are formed on TiO 2 upon sulfation (with 2000 ppm SO 2 and 4% O 2 ), which also generated Bronsted acidity. There was more Bronsted acidity than Lewis acidity (by NH 3 adsorption) at room temperature, which was reversed as the temperature increased. The adsorbed NO (in the presence of O 2 ) was predominantly in the form of bidendate NO − 3 ions bonded to Ti sites. On TiO 2 , the main IR band for NO − 3 was at 1542 cm −1 ; while the same band was shifted to 1582 cm −1 on the sulfated TiO 2 . Thus the TiO 2 surface was modified by SO 2− 4 ions, possibly by electron transfer through the S–O–Ti bridge to the neighboring Ti sites, and consequently weakened the adsorption of NO − 3 on the Ti sites. Furthermore, since TiO 2 is the support for V 2 O 5 in the selective catalytic reduction of NO (by NH 3 ), where the TiO 2 surface is also sulfated in combustion gases, the results of this study support the NO x spillover mechanism (from TiO 2 to V 2 O 5 ) as a possible role played by TiO 2 .

Reversible chemisorption of nitric oxide in the presence of oxygen on titania and titania modified with surface sulfate

Temperature-programmed desorption (TPD) of oxidized graphites shows significant amounts of CO desorption at temperatures about 1200 o C, in addition to that desorbed below 1200 o C (with a TPD peak at 980 o C). The low-temperature peak has been reported in the literature for a variety of carbons, both graphitic and nongraphitic

Strongly bonded oxygen in graphite : detection by high-temperature TPD and characterization

Cu 2+ ion-exchanged pillared clays are substantially more active than Cu 2+ -ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H 2 O (or SO 2 ) has only mild effects on their activities. First results on Cu 2+ -exchanged TiO 2 -pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu 2+ -ZSM-5. A delaminated pillared clay was subjected to Cu 2+ ion-exchange and studied for SCR by C 2 H 4 in this work. The Cu 2+ ion-exchanged delaminated Al 2 O 3 -pillared clay yielded substantially higher SCR rates than both Cu 2+ -exchanged TiO 2 -pillared clay and Cu 2+ -ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h −1 (with O 2 = 2%). The peak temperature decreased as the concentration of O 2 was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C 2 H 4 , the NO conversion peaked at 2% O 2 for all temperatures. H 2 O and SO 2 caused only mild deactivation, likely due to competitive adsorption (of SO 2 on Cu 2+ sites and H 2 O on acid sites). The high activity of Cu 2+ -exchanged Al 2 O 3 -pillared clay was due to a unique combination of the redox property of the Cu 2+ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O 2 ) on Cu 2+ O Al 3+ -on the pillars, and C 2 H 4 activation on the Lewis acid sites to form an oxygenated species.

Ralph T. Yang

Papers

Scanning tunneling microscopic imaging of electropolymerized, doped polypyrrole. Visual evidence of semicrystalline and helical nascent polymer growth

Anion and Cation Effects on Olefin Adsorption on Silver and Copper Halides: Ab Initio Effective Core Potential Study of π-Complexation

Reversible chemisorption of nitric oxide in the presence of oxygen on titania and titania modified with surface sulfate

Strongly bonded oxygen in graphite : detection by high-temperature TPD and characterization

Selective catalytic reduction of nitric oxide by ethylene in the presence of oxygen over Cu2+ ion-exchanged pillared clays