Myunghyun Paik Suh

Highly Selective CO2 Capture in Flexible 3D Coordination Polymer Networks

Carbon dioxide capture has been in the center of interests in the scientific community in recent years because of the implications for global warming, and the development of efficient methods for capturing CO2 from industrial flue gas has become an important issue. It has been revealed that coordination polymer networks (CPNs) with channels or pores can be applied in gas storage, 2] gas separation, ion exchange, and selective adsorption of organic or inorganic molecules. 5–7] It has been reported that large amounts of CO2 can be adsorbed in some CPNs, for example 20 wt% at 195 K and 1 bar in [{Cu(pyrdc)(bpp)}2]n (pyrdc = pyridine2,3-dicarboxylate, bpp = 1,3-bis(4-pyridyl)propane), 16 wt % at 298 K and 50 atm in [Cu(dhbc)2(4,4’-bpy)] (dhbc = 2,5-dihydroxybenzoate, bpy = bipyridine), 114 wt% at 195 K and 1 bar in SNU-6, 150 wt % at 298 K and 42 bar in MOF-177, and 176 wt % at 303 K and 50 bar in MIL-101c. However, the selective capture of CO2, in particular at ambient temperature and pressure, from industrial emission streams that contain other gases such as N2, CH4, and H2O still remains challenging. [1c,10, 11] Our approach to selectively capturing CO2 with porous materials is based on the construction of highly flexible 3D coordination polymer networks whose channels or pores open and close depending on the gas type. Compared to N2 and H2, we expected that CO2 would interact with the host network more efficiently because of its quadrupole moment ( 1.4 10 39 C m) and open up channels that are closed for other gases. Our design strategy for flexible 3D networks is to use 2D grids formed from square-planar Ni macrocyclic complexes as linear linkers 6, 7a,b, 12] and 1,1’-biphenyl-3,3’,5,5’tetracarboxylate (bptc ) as a square organic building block, and then to connect the 2D grids with highly flexible alkyl pillars by utilizing alkyl-bridged Ni bismacrocyclic complexes such as [Ni2L ](ClO4)4 (A) and [Ni2L ](ClO4)4·8 H2O (B, Scheme 1). We previously prepared a 2D pillared bilayer network that behaves like a sponge from other Ni bismacrocyclic complexes and 1,3,5-benzenetricarboxylate. The formation of either 2D or 3D pillared network depends on how the bismacrocyclic complex connects 2D planes (Chart S1 in the Supporting Information), which is affected by the pore size of the 2D layer and the steric hindrance between the pillars. Herein, we report two flexible 3D coordination polymer networks, [(Ni2L )(bptc)]·6H2O·3DEF (1, DEF = N,N-diethylformamide) and [(Ni2L )(bptc)]·14H2O (2), which exhibit highly selective CO2 adsorption over N2, H2, and CH4 gases as well as thermal stability up to 300 8C and air and water stability. The CO2 adsorption isotherms of 1 and 2 show gate opening and closing phenomena as well as hysteretic desorption, which allow efficient CO2 capture, storage, and sensing. Compounds 1 and 2 are the first 3D pillared networks assembled from Ni bismacrocyclic complexes. The self-assembly of A and H4bptc in DEF/H2O/TEA (2:3:0.16, v/v; TEA = triethylamine) yielded violet crystals of 1. The self-assembly of B and Na4bptc in DEF/H2O (1:4, v/v) afforded 2. Compounds 1 and 2 are insoluble in water and common organic solvents such as MeOH, EtOH, MeCN, chloroform, acetone, toluene, dimethylformamide, and dimethylsulfoxide. In the X-ray crystal structure of 1 (Figure 1), each Ni macrocyclic unit of A is coordinated by two bptc ligand at the trans positions, and each bptc ligand binds four Ni ions belonging to four different bismacrocyclic complexes to construct 2D grids extending parallel to the ab plane. The 2D grid generates rhombic cavities with effective sizes of 3.32 8.14 , each of which involves four Ni macrocyclic units and four bptc units. The layer is not completely flat, as Scheme 1. a) Alkyl-bridged Ni bismacrocyclic complexes and H4bptc. b) Design strategies for construction of 3D networks. Bismacrocyclic complexes located upward and downward with respect to a 2D plane are indicated by the different colors (pink and orange).

Terminology of Metal-Organic Frameworks and Coordination Polymers (IUPAC recommendations 2013)

A set of terms, definitions, and recommendations is provided for use in the classification of coordination polymers, networks, and metal–organic frameworks (MOFs). A hierarchical terminology is recommended in which the most general term is coordination polymer. Coordination networks are a subset of coordination polymers and MOFs a further subset of coordination networks. One of the criteria an MOF needs to fulfill is that it contains potential voids, but no physical measurements of porosity or other properties are demanded per se. The use of topology and topology descriptors to enhance the description of crystal structures of MOFs and 3D-coordination polymers is furthermore strongly recommended.

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]

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

Coordination polymers, metal–organic frameworks and the need for terminology guidelines

Coordination polymers (CPs) and metal–organic frameworks (MOFs) are among the most prolific research areas of inorganic chemistry and crystal engineering in the last 15 years, and yet it still seems that consensus is lacking about what they really are, or are not.

Self-Assembly of a Molecular Floral Lace with One-Dimensional Channels and Inclusion of Glucose

A three-dimensional network with one-dimensional channels (see picture) has been self-assembled from the nickel(II) complex of cyclam and 1,3,5-benzenetricarboxylate in water through hydrogen-bond formation. The channels have an appropriate diameter (10.3 Å) to include D-glucose with a formation constant of Kf =(1.38±0.01)×104  M-1 . Under similar conditions maltose is not included.

Self-Assembly and Selective Guest Binding of Three-Dimensional Open- Framework Solids from a Macrocyclic Complex as a Trifunctional Metal Building Block

The nickel(II) hexaazamacrocyclic complex (1) containing pendant pyridine groups has been synthesized by the one-pot template condensation reaction of amine and formaldehyde. From the self-assembly of 1 with deprotonated cis,cis-1,3,5-cyclohexanetricarboxylic acid, H2CTC- and CTC3-, three-dimensional supramolecular open-frameworks of [Ni(C20H32N8)][C6H9(COOH)2(COO)]2 x 4H2O (2) and [Ni(C20H32N8)]3[C6H9(COO)3]2 x 16H2O (3), respectively, have been constructed. The solids 2 and 3 are insoluble in all solvents. X-ray crystal structure of 2 indicates that each nickel(II) macrocyclic complex binds two H2CTC- ions in trans position and two pendant pyridine groups of the macrocyclic complex are involved in hydrogen-bonding interactions with the hydroxy groups of H2CTC- belonging to the neighboring macrocyclic complexes, which provides the beltlike one-dimensional chain composed of rectangular synthons. The one-dimensional chains are linked together through lattice water molecules by the hydrogen-bonding interactions to generate two-dimensional networks, which are again connected to each other by the offset pi-pi stacking interactions between the pendant pyridine rings to give rise to a three-dimensional structure in which channels are present. The X-ray crystal structure of 3 indicates that each nickel(II) macrocyclic unit binds two CTC3- ions in trans position and each CTC3- ion coordinates three nickel(II) macrocyclic complexes to form a two-dimensional layer, in which pendant pyridine rings are involved in the hydrogen bonding and the herringbone pi-pi interaction. Between the layers, the pendant pyridine rings belonging to the neighboring layers participate in the offset pi-pi stacking interactions, which gives rise to a three-dimensional network structure. The network creates channels running parallel to the a, b, and c axes, which are filled with guest water molecules. The X-ray powder diffraction patterns indicate that the frameworks of 2 and 3 are deformed upon removal of water guests but restored upon rebinding of water. The host solids 2 and 3 bind [Cu(NH3)4](ClO4)2 in MeCN with a binding constant (Kf) of 210 M(-1) and 710 M(-1), respectively, while they do not bind [Cu(en)2](ClO4)2 (en = ethylenediamine). The dried solids of 2 and 3 do not interact with benzene and toluene, but they differentiate methanol, ethanol, and phenol in toluene solvent with the Kf values of 42, 14, and 12 M(-1), respectively, for 2, and 13, 8.2, and 8.9 M(-1), respectively, for 3. In terms of binding sites for guest molecules, the solid 3 has greater capacity than the solid 2.

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

A fourfold interpenetrating diamondoid network, [{[Ni(cyclam)]2-(mtb)}(n)].8n H2O.4n DMF (1) (MTB=methanetetrabenzoate, DMF=dimethylformamide), has been assembled from [Ni(cyclam)][ClO4]2 (cyclam=1,4,8,11-tetraazacyclotetradecane) and methanetetrabenzoic acid (H4MTB) in DMF/H2O (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 H2, CO2, and O2 rather than N2 and CH4, suggesting possible applications in gas separation technologies. In addition, solid 1 can be applied in the fabrication of small Pd (2.0+/-0.6 nm) nanoparticles without any extra reducing or capping agent because a Ni II macrocyclic species incorporated in 1 reduces Pd II ions to Pd 0 on immersion of 1 in the solution of Pd(NO3)2.2H2O in MeCN at room temperature.

High CO2‐Capture Ability of a Porous Organic Polymer Bifunctionalized with Carboxy and Triazole Groups

A new porous organic polymer, SNU-C1, incorporating two different CO2 -attracting groups, namely, carboxy and triazole groups, has been synthesized. By activating SNU-C1 with two different methods, vacuum drying and supercritical-CO2 treatment, the guest-free phases, SNU-C1-va and SNU-C1-sca, respectively, were obtained. Brunauer-Emmett-Teller (BET) surface areas of SNU-C1-va and SNU-C1-sca are 595 and 830 m(2) g(-1), respectively, as estimated by the N2-adsorption isotherms at 77 K. At 298 K and 1 atm, SNU-C1-va and SNU-C1-sca show high CO2 uptakes, 2.31 mmol  g(-1) and 3.14 mmol  g(-1), respectively, the high level being due to the presence of abundant polar groups (carboxy and triazole) exposed on the pore surfaces. Five separation parameters for flue gas and landfill gas in vacuum-swing adsorption were calculated from single-component gas-sorption isotherms by using the ideal adsorbed solution theory (IAST). The data reveal excellent CO2-separation abilities of SNU-C1-va and SNU-C1-sca, namely high CO2-uptake capacity, high selectivity, and high regenerability. The gas-cycling experiments for the materials and the water-treated samples, experiments that involved treating the samples with a CO2-N2 gas mixture (15:85, v/v) followed by a pure N2 purge, further verified the high regenerability and water stability. The results suggest that these materials have great potential applications in CO2 separation.

Enhanced isosteric heat, selectivity, and uptake capacity of CO2 adsorption in a metal-organic framework by impregnated metal ions

We demonstrate by experimental and theoretical studies that the impregnation of various metal ions such as Li+, Mg2+, Ca2+, Co2+, and Ni2+ in the pores of an anionic MOF, [Zn3(TCPT)2(HCOO)][NH2(CH3)2] (SNU-100′) significantly enhances isosteric heat, selectivity, and uptake capacity of the CO2 adsorption in the MOF. Due to the electrostatic interactions between CO2 and the extra-framework metal ions, the isosteric heats of CO2 adsorption are increased to 37.4–34.5 kJ mol−1 and the adsorption selectivities of CO2 over N2 at room temperature are increased to 40.4–31.0, compared with those (29.3 kJ mol−1 and 25.5, respectively) of the parent MOF (SNU-100′) containing [NH2(CH3)2]+ cations.

Magnesium Nanocrystals Embedded in a Metal–Organic Framework: Hybrid Hydrogen Storage with Synergistic Effect on Physi‐ and Chemisorption

Hexagonal-disk-shaped magnesium nanocrystals (MgNCs) are fabricated within a porous metal-organic framework (MOF, see picture). The MgNCs@MOF stores hydrogen by both physi- and chemisorptions, exhibiting synergistic effects to decrease the isosteric heat of H(2) physisorption compared with that of pristine MOF, and decrease the H(2) chemisorption/desorption temperatures by 200 K compared with those of bare Mg powder.