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

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

Luminescent Functional Metal–Organic Frameworks

Syntheses and functions of porous metallosupramolecular networks

Quick on the uptake: Palladium nanoparticles were fabricated simply by immersing {[Zn(3)(ntb)(2)(EtOH)(2)]4 EtOH}(n) (1) in an MeCN solution of Pd(NO(3))(2) at room temperature, without any extra reducing agent. 3 wt % PdNPs@[1](0.54+)(NO(3)(-))(0.54) significantly increase H(2) uptake capacities, both at 77 K and 1 bar and at 298 K and high pressures (see picture, red curve) compared to [Zn(3)(ntb)(2)](n) (black). ntb = 4,4',4''-nitrilotrisbenzoate.

Enhanced hydrogen storage by palladium nanoparticles fabricated in a redox-active metal-organic framework.

Various metal–organic frameworks (MOFs) have been prepared to obtain materials that show specific or multifunctional properties. Porous MOFs that contain free space where guest molecules can be accommodated are of particular interest because they can be applied in gas storage and separation, selective adsorption and separation of organic molecules, ion exchange, catalysis, sensor technology, and for the fabrication of metal nanoparticles. Secondary building units (SBUs) with a specific geometry have often been employed for the modular construction of porous MOFs as they make the design and prediction of molecular architectures simple and easy. In particular, {M2(CO2)4}-type paddlewheel clusters that can be formed from the solvothermal reaction of M ions and the appropriate carboxylic acid are widely used for the construction of porous frameworks. Three-dimensional porous frameworks with various topologies (Pt3O4, boracites, NbO, and PtS nets) can be built from paddlewheel-type metal cluster SBUs and trior tetracarboxylates, whereas pillared square-grid networks can be constructed from paddlewheel cluster SBUs and dicarboxylates in the presence of diamine ligands. Porous MOFs with accessible metal sites (AMSs) should have a higher hydrogen storage capacity than those without AMSs, although there are not yet enough experimental data to support this assumption. To determine the effect of AMSs in a MOF on H2 adsorption, the H2 uptakes should be compared for the same framework in the absence and presence of AMSs, or for two independent isostructural MOFs with and without AMSs. H2 uptake has previously been measured under several different outgassing conditions. Unfortunately, these experiments could not clearly demonstrate the effect of AMSs as the exact formula and structure at each stage were not known. Furthermore, even when coordinating solvent molecules are successfully removed with retention of the porous framework structure, the metal ion sometimes transforms its coordination geometry to the thermodynamically most stable form instead of keeping the AMSs. Herein we report two porous MOFs with the same NbOtype net topology, namely [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) and [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2 ; H4abtc= 1,1’-azobenzene-3,3’,5,5’-tetracarboxylic acid ), and compare the gas adsorption data for the MOFs with and without AMSs. Heating crystals of 1 and 2 under precisely controlled conditions allowed us to prepare [{Zn2(abtc)(dmf)2}3] (1a ; SNU-4) and [{Cu2(abtc)(dmf)2}3] (2a ; SNU-5’), which have no AMSs, as well as [{Cu2(abtc)}3] (2b ; SNU-5), which has AMSs. The framework structure of 1a is the same as that of 1 and those of 2a and 2b are the same as that of 2, as evidenced by the PXRD patterns. Solid 1a, 2a, and 2b exhibit higher adsorption capabilities for N2, CO2, CH4, and H2 than other previously reported MOFs. In particular, 2b adsorbs 2.87 wt% of H2 gas at 77 K and 1 atm, which is the highest value for H2 sorption under these conditions amongst a variety of other MOFs. The N2, CO2, and CH4 adsorption capacities per unit sample volume for 2b, which has AMSs, are 140–160% higher than those for 1a and 2a, which have no AMSs. The H2 adsorption capacity of 2b is also higher than those of 1a and 2a [at 77 K and 1 atm, 2.87 wt% for 2b vs. 2.07 wt% for 1a and 1.83 wt% for 2a ; excess adsorbed H2 at 77 K and 50 bar: 5.22 wt% (total 6.76 wt%) for 2b vs. 3.70 wt% (total 4.49 wt%) for 1a], although this is mainly due to the lower molecular weight effect of 2b. The H2 sorption capacity ratios 2b/1a and 2b/2a per unit sample volume at 77 K and 1 atm are 105% and 120%, respectively, and the ratio 2b/1a at 77 K and 50 bar is 106%. Our measurements of the isosteric heat of H2 adsorption (zero-coverage isosteric heats are 7.24, 6.53, and 11.60 kJmol for 1a, 2a, and 2b, respectively) suggest that the enhanced H2 adsorption in 2b can be attributed to the stronger interaction of H2 molecules with the AMSs of the MOF. Yellowish block-shaped crystals of [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) were prepared by heating a dmf solution of Zn(NO3)2·6H2O and H4abtc at 100 8C for 12 h. Greenish block-shaped crystals of [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2) were prepared by heating Cu(NO3)2·xH2O and H4abtc in a dmf/1,4-dioxane/H2O (4:3:1 v/v) mixture at 80 8C for 24 h. Solid 1 is insoluble in common organic solvents but is slightly soluble in water, where it dissociates into its building blocks. Solid 2 is insoluble in all common organic solvents and water. The temperaturedependent PXRD patterns show that the framework struc[*] Y.-G. Lee, H. R. Moon, Y. E. Cheon, Prof. M. P. Suh Department of Chemistry, Seoul National University Seoul 151-747 (Republic of Korea) Fax: (+82)2-886-8516 E-mail: mpsuh@snu.ac.kr

A comparison of the H2 sorption capacities of isostructural metal-organic frameworks with and without accessible metal sites: [{Zn2(abtc)(dmf)2}3] and [{Cu2(abtc)(dmf)2}3] versus [{Cu2(abtc)}3].

A fourfold interpenetrating diamondoid network, [{[Ni(cyclam)] 2 -(mtb)) n ]·8nH 2 O·4nDMF (1) (MTB= methanetetrabenzoate, DMF = dimethylformamide), has been assembled from [Ni(cyclam)][ClO 4 ] 2 (cyclam= 1,4,8,11-tetraazacyclotetradecane) and methanetetrabenzoic acid (H 4 MTB) in DMF/H 2 O (7:3, v/v) in the presence of triethylamine (TEA). Despite the high-fold interpenetration, 1 generates 1D channels that are occupied by water and DMF guest molecules. Solid 1, after removal of guest molecules, exhibits selective gas adsorption behavior for H 2 , CO 2 , and O 2 rather than N 2 and CH4, suggesting possible applications in gas separation technologies. In addition, solid 1 can be applied in the fabrication of small Pd (2.0±0.6nm) nanoparticles without any extra reducing or capping agent because a Ni II macrocyclic species incorporated in 1 reduces Pd II ions to Pd° on immersion of 1 in the solution of Pd(NO 3 ) 2 ·2H 2 O in MeCN at room temperature.

Multifunctional Fourfold Interpenetrating Diamondoid Network: Gas Separation and Fabrication of Palladium Nanoparticles

Selective gas adsorption in a microporous metal-organic framework constructed of CoII4 clusters

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