Synthesis of multifunctional metal-organic frameworks and tuning the functionalities with pendant ligands.

A series of multifunctional metal-organic frameworks (MOFs), SNU-170-SNU-176, has been synthesized using ligands, in which various functional pendants such as -NH2, -SMe, -OMe, -OEt, -OPr, and -OBu are attached to the phenyl ring of 4-(2-carboxyvinyl)benzoic acid. The MOFs are isostructural but the interpenetration depends on the pendant group of the ligand. The MOFs exhibit high adsorption capacities for H2, CO2, and CH4 gases, ligand-based photoluminescence, and chemical sensing abilities, all being affected by the pendant group. All of the as-synthesized MOFs can sense nitroaromatics by luminescence quenching, and some of the activated MOFs can sense the type of solvents by the altered emission maxima with enhanced intensity. In particular, SNU-176 synthesized from a mixture of two different ligands with -SMe and -OMe pendants shows higher gas adsorption capacities than the MOFs synthesized from the individual ligands (SNU-171 and SNU-172). It also shows the ability to differentiate nitrobenzene (NB) and 2,4-dinitrotoluene (DNT) unlike the MOFs composed of single ligands.

50 Years as a Woman Chemist in South Korea.

In her guest editorial, Prof. Myunghyun Paik Suh reflects on her 50-year-long career devoted to chemistry, and the many challenges she had to overcome in order to become a world-leading scientist in South Korea. Furthermore, she gives some advice to young faculty members and students based on her experience.

Modeling adsorption properties of structurally deformed metal-organic frameworks using structure-property map.

Structural deformation and collapse in metal-organic frameworks (MOFs) can lead to loss of long-range order, making it a challenge to model these amorphous materials using conventional computational methods. In this work, we show that a structure-property map consisting of simulated data for crystalline MOFs can be used to indirectly obtain adsorption properties of structurally deformed MOFs. The structure-property map (with dimensions such as Henry coefficient, heat of adsorption, and pore volume) was constructed using a large data set of over 12000 crystalline MOFs from molecular simulations. By mapping the experimental data points of deformed SNU-200, MOF-5, and Ni-MOF-74 onto this structure-property map, we show that the experimentally deformed MOFs share similar adsorption properties with their nearest neighbor crystalline structures. Once the nearest neighbor crystalline MOFs for a deformed MOF are selected from a structure-property map at a specific condition, then the adsorption properties of these MOFs can be successfully transformed onto the degraded MOFs, leading to a new way to obtain properties of materials whose structural information is lost.

Copper-Organic Framework Fabricated with CuS Nanoparticles: Synthesis, Electrical Conductivity, and Electrocatalytic Activities for Oxygen Reduction Reaction.

To apply electrically nonconductive metal-organic frameworks (MOFs) in an electrocatalytic oxygen reduction reaction (ORR), we have developed a new method for fabricating various amounts of CuS nanoparticles (nano-CuS) in/on a 3D Cu-MOF, [Cu3 (BTC)2 ⋅(H2 O)3 ] (BTC=1,3,5-benzenetricarboxylate). As the amount of nano-CuS increases in the composite, the electrical conductivity increases exponentially by up to circa 109 -fold, while porosity decreases, compared with that of the pristine Cu-MOF. The composites, nano-CuS(x wt %)@Cu-BTC, exhibit significantly higher electrocatalytic ORR activities than Cu-BTC or nano-CuS in an alkaline solution. The onset potential, electron transfer number, and kinetic current density increase when the electrical conductivity of the material increases but decrease when the material has a poor porosity, which shows that the two factors should be finely tuned by the amount of nano-CuS for ORR application. Of these materials, CuS(28 wt %)@Cu-BTC exhibits the best activity, showing the onset potential of 0.91 V vs. RHE, quasi-four-electron transfer pathway, and a kinetic current density of 11.3 mA cm-2 at 0.55 V vs. RHE.

Metal-Organic Frameworks Incorporating Various Alkoxy Pendant Groups: Hollow Tubular Morphologies, X-ray Single-Crystal Structures, and Selective Carbon Dioxide Adsorption Properties.

Eight porous metal-organic frameworks (MOFs) incorporating various alkoxy pendant groups (-OC(n)H(2n+1); n=1-8) in the channels have been synthesized. All MOFs have macrosized, hollow, single-crystal morphologies independent of the type of alkoxy pendant groups. The X-ray single-crystal structures indicate that the MOFs have doubly interpenetrated 3D frameworks that generate clover-shaped 1D channels, the window sizes of which reduce as the length of the alkoxy pendant groups increases from -OCH3 to -OC8H17. The hollow axes of the tubular crystals are parallel to the 1D channels of the MOFs, which suggests that the hollow crystal is grown by the unidirectional addition of pillar ligands to the 2D network. Among the activated MOFs, only the MOFs with methoxy and ethoxy pendant groups show selective CO2 adsorption over N2 and CH4, whereas those with longer alkoxy pendant groups barely adsorb CO2 at room temperature, which implies that the pendant doors of the MOFs should have an appropriate length for selective CO2 capture.

Hydrogen storage in a potassium-ion-bound metal-organic framework incorporating crown ether struts as specific cation binding sites.

To develop a metal-organic framework (MOF) for hydrogen storage, SNU-200 incorporating a 18-crown-6 ether moiety as a specific binding site for selected cations has been synthesized. SNU-200 binds K(+), NH4(+), and methyl viologen (MV(2+)) through single-crystal to single-crystal transformations. It exhibits characteristic gas-sorption properties depending on the bound cation. SNU-200 activated with supercritical CO2 shows a higher isosteric heat (Qst) of H2 adsorption (7.70 kJ mol(-1)) than other zinc-based MOFs. Among the cation inclusions, K(+) is the best for enhancing the isosteric heat of the H2 adsorption (9.92 kJ mol(-1)) as a result of the accessible open metal sites on the K(+) ion.

Enhancing CO(2) separation ability of a metal-organic framework by post-synthetic ligand exchange with flexible aliphatic carboxylates.

A series of porous metal-organic frameworks having flexible carboxylic acid pendants in their pores (UiO-66-ADn: n=4, 6, 8, and 10, where n denotes the number of carbons in a pendant) has been synthesized by post-synthetic ligand exchange of terephthalate in UiO-66 with a series of alkanedioic acids (HO2 C(CH2 )n-2 CO2 H). NMR, IR, PXRD, TEM, and mass spectral data have suggested that a terephthalate linker in UiO-66 was substituted by two alkanedioate moieties, resulting in free carboxyl pendants in the pores. When post-synthetically modified UiO-66 was partially digested by adjusting the amount of added HF/sample, NMR spectra indicated that the ratio of alkanedioic acid/terephthalic acid was increased with smaller amounts of acid, implying that the ligand substitution proceeded from the outer layer of the particles. Gas sorption studies indicated that the surface areas and the pore volumes of all UiO-66-ADns were decreased compared to those of UiO-66, and that the CO2 adsorption capacities of UiO-66-ADn (n=4, 8) were similar to that of UiO-66. In the case of UiO-66-AD6, the CO2 uptake capacity was 34 % higher at 298 K and 58 % higher at 323 K compared to those of UiO-66. It was elucidated by thermodynamic calculations that the introduction of flexible carboxyl pendants of appropriate length has two effects: 1) it increases the interaction enthalpy between the host framework and CO2 molecules, and 2) it mitigates the entropy loss upon CO2 adsorption due to the formation of multiple configurations for the interactions between carboxyl groups and CO2 molecules. The ideal adsorption solution theory (IAST) selectivity for CO2 adsorption over that of CH4 was enhanced for all of the UiO-66-ADns compared to that of UiO-66 at 298 K. In particular, UiO-66-AD6 showed the most strongly enhanced CO2 uptake capacity and significantly increased selectivity for CO2 adsorption over that of CH4 at ambient temperature, suggesting that it is a promising material for sequestering CO2 from landfill gas.

Comparison of gas sorption properties of neutral and anionic metal-organic frameworks prepared from the same building blocks but in different solvent systems.

Two different 3D porous metal-organic frameworks, [Zn4O(NTN)2]·10DMA·7H2O (SNU-150) and [Zn5(NTN)4(DEF)2][NH2(C2H5)2]2·8DEF·6H2O (SNU-151), are synthesized from the same metal and organic building blocks but in different solvent systems, specifically, in the absence and the presence of a small amount of acid. SNU-150 is a doubly interpenetrated neutral framework, whereas SNU-151 is a non-interpenetrated anionic framework containing diethylammonium cations in the pores. Comparisons of the N2, H2, CO2, and CH4 gas adsorption capacities as well as the CO2 adsorption selectivity over N2 and CH4 in desolvated SNU-150' (BET: 1852 m(2) g(-1)) and SNU-151' (BET: 1563 m(2) g(-1)) samples demonstrate that the charged framework is superior to the neutral framework for gas storage and gas separation, despite its smaller surface area and different framework structure.

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.

Fabrication of metal nanoparticles in metal-organic frameworks.

In this review, we highlight various preparative strategies and characterization methods for metal nanoparticles fabricated in porous metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), and their applications in hydrogen storage and heterogeneous catalysis.

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.

Selective CO2 adsorption in a metal-organic framework constructed from an organic ligand with flexible joints.

A metal-organic framework (SNU-110) constructed from an organic ligand with flexible joints exhibits selective CO(2) adsorption over N(2), O(2), H(2) and CH(4) gases.

Control of interpenetration and gas-sorption properties of metal-organic frameworks by a simple change in ligand design.

In metal-organic framework (MOF) chemistry, interpenetration greatly affects the gas-sorption properties. However, there is a lack of a systematic study on how to control the interpenetration and whether the interpenetration enhances gas uptake capacities or not. Herein, we report an example of interpenetration that is simply controlled by the presence of a carbon-carbon double or single bond in identical organic building blocks, and provide a comparison of gas-sorption properties for these similar frameworks, which differ only in their degree of interpenetration. Noninterpenetrated (SNU-70) and doubly interpenetrated (SNU-71) cubic nets were prepared by a solvothermal reaction of [Zn(NO(3))(2)]⋅6 H(2)O in N,N-diethylformamide (DEF) with 4-(2-carboxyvinyl)benzoic acid and 4-(2-carboxyethyl)benzoic acid, respectively. They have almost-identical structures, but the noninterpenetrated framework has a much bigger pore size (ca. 9.0×9.0 Å) than the interpenetrated framework (ca. 2.5×2.5 Å). Activation of the MOFs by using supercritical CO(2) gave SNU-70' and SNU-71'. The simulation of the PXRD pattern of SNU-71' indicates the rearrangement of the interpenetrated networks on guest removal, which increases pore size. SNU-70' has a Brunauer-Emmett-Teller (BET) surface area of 5290 m(2) g(-1), which is the highest value reported to date for a MOF with a cubic-net structure, whereas SNU-71' has a BET surface area of 1770 m(2) g(-1). In general, noninterpenetrated SNU-70' exhibits much higher gas-adsorption capacities than interpenetrated SNU-71' at high pressures, regardless of the temperature. However, at P<1 atm, the gas-adsorption capacities for N(2) at 77 K and CO(2) at 195 K are higher for noninterpenetrated SNU-70' than for interpenetrated SNU-71', but the capacities for H(2) and CH(4) are the opposite; SNU-71' has higher uptake capacities than SNU-70' due to the higher isosteric heat of gas adsorption that results from the smaller pores. In particular, SNU-70' has exceptionally high H(2) and CO(2) uptake capacities. By using a post-synthetic method, the CC double bond in SNU-70 was quantitatively brominated at room temperature, and the MOF still showed very high porosity (BET surface area of 2285 m(2) g(-1)).

Enhanced isosteric heat of H2 adsorption by inclusion of crown ethers in a porous metal-organic framework.

Inclusion of 18-crown-6 or 15-crown-5 in a porous MOF increased the isosteric heats of H(2) adsorption significantly, which are comparable to MOFs containing open metal sites.

A highly porous metal-organic framework: structural transformations of a guest-free MOF depending on activation method and temperature.

A doubly interpenetrating porous metal-organic framework (SNU-77) has been synthesized from the solvothermal reaction of the extended carboxylic acid tris(4'-carboxybiphenyl)amine (H(3)TCBPA) and Zn(NO(3))(2)⋅6H(2)O in N,N-dimethylacetamide (DMA). SNU-77 undergoes single-crystal-to-single-crystal transformations during various activation processes, such as room-temperature evacuation, supercritical CO(2) drying, and high temperature evacuation, to afford SNU-77R, SNU-77S, and SNU-77H, respectively. These guest-free MOFs exhibited different fine structures with different window shapes and different effective window sizes at room temperature. Variable-temperature synchrotron single-crystal X-ray analyses reveal that the guest-free structure is also affected by changes in temperature. Despite the different fine structures, SNU-77R, SNU-77S, and SNU-77H show similar gas sorption properties due to the nonbreathing nature of the framework and an additional structural change upon cooling to cryogenic gas sorption temperature. SNU-77H exhibits a large surface area (BET, 3670 m(2) g(-1)), a large pore volume (1.52 cm(3) g(-1)), and exceptionally high uptake capacities for N(2), H(2), O(2), CO(2), and CH(4) gases.

Selective CO2 adsorption in a flexible non-interpenetrated metal-organic framework.

We have prepared a flexible metal-organic framework and demonstrated that when activated by supercritical CO(2) it has greater gas sorption capacities than that activated by the heat-evacuation method, and it selectively adsorbs CO(2) over N(2) at room temperature.

High gas sorption and metal-ion exchange of microporous metal-organic frameworks with incorporated imide groups.

Metal-organic frameworks (MOFs), {[Cu(2)(bdcppi)(dmf)(2)]·10DMF·2H(2)O}(n) (SNU-50) and {[Zn(2)(bdcppi)(dmf)(3)]·6DMF·4H(2)O}(n) (SNU-51), have been prepared by the solvothermal reactions of N,N'-bis(3,5-dicarboxyphenyl)pyromellitic diimide (H(4)BDCPPI) with Cu(NO(3))(2) and Zn(NO(3))(2), respectively. Framework SNU-50 has an NbO-type net structure, whereas SNU-51 has a PtS-type net structure. Desolvated solid [Cu(2)(bdcppi)](n) (SNU-50'), which was prepared by guest exchange of SNU-50 with acetone followed by evacuation at 170 °C, adsorbs high amounts of N(2), H(2), O(2), CO(2), and CH(4) gases due to the presence of a vacant coordination site at every metal ion, and to the presence of imide groups in the ligand. The Langmuir surface area is 2450 m(2) g(-1). It adsorbs H(2) gas up to 2.10 wt% at 1 atm and 77 K, with zero coverage isosteric heat of 7.1 kJ mol(-1), up to a total of 7.85 wt% at 77 K and 60 bar. Its CO(2) and CH(4) adsorption capacities at 298 K are 77 wt% at 55 bar and 17 wt% at 60 bar, respectively. Of particular note is the O(2) adsorption capacity of SNU-50' (118 wt% at 77 K and 0.2 atm), which is the highest reported so far for any MOF. By metal-ion exchange of SNU-51 with Cu(II), {[Cu(2)(bdcppi)(dmf)(3)]·7DMF·5H(2)O}(n) (SNU-51-Cu(DMF)) with a PtS-type net was prepared, which could not be synthesized by a direct solvothermal reaction.

Post-synthetic reversible incorporation of organic linkers into porous metal-organic frameworks through single-crystal-to-single-crystal transformations and modification of gas-sorption properties.

The porous metal-organic framework (MOF) {[Zn(2)(TCPBDA)(H(2)O)(2)]⋅30 DMF⋅6 H(2)O}(n) (SNU-30; DMF = N,N-dimethylformamide) has been prepared by the solvothermal reaction of N,N,N',N'-tetrakis(4-carboxyphenyl)biphenyl-4,4'-diamine (H(4)TCPBDA) and Zn(NO(3))(2)⋅6 H(2)O in DMF/tBuOH. The post-synthetic modification of SNU-30 by the insertion of 3,6-di(4-pyridyl)-1,2,4,5-tetrazine (bpta) affords single-crystalline {[Zn(2)(TCPBDA)(bpta)]⋅23 DMF⋅4 H(2)O}(n) (SNU-31 SC), in which channels are divided by the bpta linkers. Interestingly, unlike its pristine form, the bridging bpta ligand in the MOF is bent due to steric constraints. SNU-31 can be also prepared through a one-pot solvothermal synthesis from Zn(II), TCPBDA(4-), and bpta. The bpta linker can be liberated from this MOF by immersion in N,N-diethylformamide (DEF) to afford the single-crystalline SNU-30 SC, which is structurally similar to SNU-30. This phenomenon of reversible insertion and removal of the bridging ligand while preserving the single crystallinity is unprecedented in MOFs. Desolvated solid SNU-30' adsorbs N(2), O(2), H(2), CO(2), and CH(4) gases, whereas desolvated SNU-31' exhibits selective adsorption of CO(2) over N(2), O(2), H(2), and CH(4), thus demonstrating that the gas adsorption properties of MOF can be modified by post-synthetic insertion/removal of a bridging ligand.