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

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

1. INTRODUCTION Among the classes of highly porous materials, metalÀorganic frameworks (MOFs) are unparalleled in their degree of tunability and structural diversity as well as their range of chemical and physical properties. MOFs are extended crystalline structures wherein metal cations or clusters of cations (\" nodes \") are connected by multitopic organic \" strut \" or \" linker \" ions or molecules. The variety of metal ions, organic linkers, and structural motifs affords an essentially infinite number of possible combinations. 1 Furthermore, the possibility for postsynthetic modification adds an additional dimension to the synthetic variability. 2 Coupled with the growing library of experimentally determined structures, the potential to computationally predict, with good accuracy, affinities of guests for host frameworks points to the prospect of routinely predesigning frameworks to deliver desired properties. 3,4 MOFs are often compared to zeolites for their large internal surface areas, extensive porosity, and high degree of crystallinity. Correspondingly, MOFs and zeolites have been utilized for many of the same applications

Metal–Organic Framework Materials as Chemical Sensors

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

Crystal chemistry and SAXS studies of an octahedral polyoxo-arsenotungstate nanocluster encapsulating four unprecedented thorium arsenate fragments ({Th3As2On} - n=25 or 26).

Synthesis and structural characterization of a new layered gallium phosphate using triethylenetetramine as structure-directing agent

Chemistry-Structure-Simulation or Chemistry-Simulation-Structure Sequences? The Case of MIL-34, a New Porous Aluminophosphate [J. Am. Chem. Soc. 2001, 123, 9642−9651].

Four hetero-polyoxometalate compounds were obtained in aqueous solutions, following the synthetic combination of Na9[B-α-AsIIIW9O33] with the 4f tetravalent cerium cation in the presence of the formate ligand at different pH values (ranging from 1.5 to 6). All four molecules [{Ce6(μ3-O)4(μ3-OH)4(HCOO)3,5}4(AsIIIW9O33)4]2− compound 1, [(Ce6(μ3-O)4(μ3-OH)4)(H2O)4(AsIIIW9O33)(HCOO)9]6− compound 2, [(Ce6(μ4-O)3(μ3-O)5 (μ2-H2O)3)(HCOO)3(AsVW9O34)3]22− compound 3, and [(W4O10){(Ce6(μ3-O)5(μ4-O)3(μ2-H2O))(AsVW9O34)3}2]44− compound 4 were constructed around one to four hexanuclear cerium clusters of [Ce6O8] type. For the compounds 1–3, the arsenato-polytungstate POM units and formate ligands were associated and participated in the stabilization of one cerium(IV)-bearing hexanuclear unit in a cyclic motif (1), a monomeric POM species for 2, and a trimeric POM species for 3. However, in the last molecular system (4), only inorganic entities were bonded to the hexameric cerium(IV) clusters. From the structural point of view, compound 4 appeared as the association of two compound 3 entities, in which the formate ligands were replaced by an inorganic tungsten-based sub-unit coming from the partial decomposition of the {AsIIIW9O33} precursor. The behavior and the association modes of the four molecular species were explained on the basis of the pH variation used in their syntheses. All four compounds were characterized by single-crystal X-ray diffraction, IR spectroscopy, TGA, SEM/EDX microscopy, and ICP analyses. The solution behavior was assessed on compound 3 by using small angle X-ray scattering (SAXS) measurements.

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Influence of pH on CeIV-[AsIIIW9O33]9− association for the formation of hexanuclear cerium(IV) oxo-hydroxo-clusters stabilized by trivacant polyanions

The reaction of Na10[SiW9O34] with the 4f tetravalent cerium cation associated with four different organic O/N-donor ligands at pH = 4–4.5 gives rise to the formation of five anionic polyoxometalates [Ce4(μ3-O)2(SiW9O34)2(C4H5O4)2]10− (1), [Ce4(μ3-O)2(SiW9O34)2(C5H7O4)2]10− (2), [Ce4(μ3-O)2(SiW9O34)2(C5H8O2–NH3)2]8− (3), [Ce4(μ3-O)2(SiW9O34)2(C6H10O2–NH3)2]8− (4) and [(Ce3(μ3-O)(μ2-OH)3)(Ce6(μ3-O)4(μ3-OH)4)(SiW9O34)4]21− (5). Compounds 1–4 consist of one tetranuclear Ce-based cluster fragment stabilized by two [SiW9O34]10− polyanions and two organic linkers. Compound 5 is composed of a hexanuclear core combined with a trinuclear cerium core, stabilized by four [SiW9O34]10− moieties. The molecular species were characterized by single-crystal and powder X-ray diffraction, IR spectroscopy, TGA, SEM/EDX, SAXS and ICP analysis. The solution stability of compounds 1–4 was also assessed by 29Si and 183W NMR.

Thierry Loiseau

Papers

Crystal chemistry and SAXS studies of an octahedral polyoxo-arsenotungstate nanocluster encapsulating four unprecedented thorium arsenate fragments ({Th3As2On} - n=25 or 26).

Synthesis and structural characterization of a new layered gallium phosphate using triethylenetetramine as structure-directing agent

Chemistry-Structure-Simulation or Chemistry-Simulation-Structure Sequences? The Case of MIL-34, a New Porous Aluminophosphate [J. Am. Chem. Soc. 2001, 123, 9642−9651].

Influence of pH on CeIV-[AsIIIW9O33]9− association for the formation of hexanuclear cerium(IV) oxo-hydroxo-clusters stabilized by trivacant polyanions

Bottom-up synthesis of functionalized {Ce4(SiW9O34)2(L)2} polyoxometalates