Daeok Kim

Charged Covalent Triazine Frameworks for CO2 Capture and Conversion

The quest for the development of new porous materials addressing both CO2 capture from various sources and its conversion into useful products is a very active research area and also critical in order to develop a more sustainable and environmentally-friendly society. Here, we present the first charged covalent triazine framework (cCTF) prepared by simply heating nitrile functionalized dicationic viologen derivatives under ionothermal reaction conditions using ZnCl2 as both solvent and trimerization catalyst. It has been demonstrated that the surface area, pore volume/size of cCTFs can be simply controlled by varying the synthesis temperature and the ZnCl2 content. Specifically, increasing the reaction temperature led to controlled increase in the mesopore content and facilitated the formation of hierarchical porosity, which is critical to ensure efficient mass transport within porous materials. The resulting cCTFs showed high specific surface areas up to 1247 m2 g-1, and high physicochemical stability. The incorporation of ionic functional moieties to porous organic polymers improved substantially their CO2 affinity (up to 133 mg g-1, at 1 bar and 273 K) and transformed them into hierarchically porous organocatalysts for CO2 conversion. More importantly, the ionic nature of cCTFs, homogeneous charge distribution together with hierarchical porosity offered a perfect platform for the catalytic conversion of CO2 into cyclic carbonates in the presence of epoxides through an atom economy reaction in high yields and exclusive product selectivity. These results clearly demonstrate the promising aspect of incorporation of charged units into the porous organic polymers for the development of highly efficient porous organocatalysts for CO2 capture and fixation.

Graphene/ZIF-8 composites with tunable hierarchical porosity and electrical conductivity

The development of hierarchical metal–organic frameworks (MOFs) incorporating interconnected micro- and mesopores has been of considerable interest in gas separation, energy storage and catalysis due to the efficient mass transfer kinetics through mesopores. Here, we report the preparation of graphene/ZIF-8 nanocomposites with tunable hierarchical porosity and surface areas, wherein the distribution of micro- and mesopores along with the particle size of ZIF-8 crystals was controlled by simply varying the annealing temperature of graphene oxide sheets. These nanocomposites showed superior CO2 uptake capacities up to 17 mmol g−1 at 303 K, 35 bar to ZIF-8 due to the synergistic effect of the graphitic surface and ZIF-8 crystals, thus offering a new direction to further improve the gas uptake capacity of MOFs, while simultaneously achieving fast mass transfer of gas molecules into the adsorbent through mesopores. Furthermore, the presence of graphitic templates (20 wt%) introduced electrical conductivity up to 64 S m−1 into an insulating MOF such as ZIF-8. Importantly, we still observed an electrical conductivity of 2 S m−1 with graphene loadings as low as 2.5 wt%. This present approach not only provides a new direction for the effective and facile synthesis of hierarchical porous materials but also paves the way for the introduction of electrical/thermal conductivity into insulating MOF structures.

Unexpected Carbon Dioxide Inclusion in Water‐Saturated Pores of Metal–Organic Frameworks with Potential for Highly Selective Capture of CO2

Unusual CO2 storage in water-saturated MOFs was investigated by combining experiment and simulation. It was found that the micropores of HKUST-1 saturated with water provide an environment that is thermodynamically and kinetically favorable for CO2 capture, but not for N2 and H2 capture. We expect that this phenomenon have potential to be used for successful separation of CO2 from versatile flue streams and pre-combustion gas.

Enhanced water permeation based on nanoporous multilayer graphene membranes: the role of pore size and density

Bulk scale graphenes containing narrow and dense pores are realized via potassium hydroxide activation of pre-oxidized graphite (size: ca. 3 nm, density: ca. 1015 m−2). A film (20 nm thickness) comprised of this nanoporous graphene displays much enhanced water flux (ca. 37 L m−2 h−1 bar−1) compared to that of conventional graphene oxide membranes (∼6 times), while maintaining the seiving performances of GO laminates with ca. 20% rejection for NaCl and up to 99% rejection for various dyes around 1 nm diameter size. This advantageous property is a result of the fact that in addition to the effect of the interlayer stacking of graphene sheets, nanopores in the graphene generated by using the new method serve as additional channels through which water molecules can diffuse. We believe that the new approach will play a key role in preparing and designing graphene membranes with high flux.

Template-Directed Approach Towards the Realization of Ordered Heterogeneity in Bimetallic Metal–Organic Frameworks

Controlling the arrangement of different metal ions to achieve ordered heterogeneity in metal-organic frameworks (MOFs) has been a great challenge. Herein, we introduce a template-directed approach, in which a 1D metal-organic polymer incorporating well-defined binding pockets for the secondary metal ions used as a structural template and starting material for the preparation of well-ordered bimetallic MOF-74s under heterogeneous-phase hydrothermal reaction conditions in the presence of secondary metal ions such as Ni2+ and Mg2+ in 3 h. The resulting bimetallic MOF-74s were found to possess a nearly 1:1 metal ratio regardless of their initial stoichiometry in the reaction mixture, thus demonstrating the possibility of controlling the arrangement of metal ions within the secondary building blocks in MOFs to tune their intrinsic properties such as gas affinity.

Nanostructured ZnO as a structural template for the growth of ZIF-8 with tunable hierarchical porosity for CO2 conversion

We report a new approach to introduce hierarchical porosity into ZIF-8 by using three-dimensional nanostructured porous ZnO as a structural template. Importantly, the textural properties of these nanostructures can be tuned simply by varying the size of the polymeric templates used for the preparation of ZnO. Moreover, the growth of ZIF-8 crystals on ZnO and the resulting hierarchical porosity enabled enhanced catalytic activity for the conversion of CO2 into cyclic carbonates through an atom economy reaction in high yields with exceptional product selectivity.

Graphene oxide-templated preferential growth of continuous MOF thin films

We introduced a new strategy to fabricate defect-free continuous metal–organic framework (MOF) films using graphene oxide (GO) as an interfacial template on solid substrates. The unprecedented formation of a one-dimensional nanorod-shaped crystalline intermediate phase on the GO surface enabled the preferential growth of HKUST-1 films in the direction.

Phase behavior of gas hydrates in nanoporous materials: Review

A precise understanding of phase behavior for a variety of both artificial and natural processes is essential to achieving scientific and technological goals. There has been growing research interest in gas hydrates confined in nanoporous media aiming to simulate and analyze the unique behavior of natural gas hydrates in sediments. Moreover, the appearance of peculiar properties due to the confinement effect stimulates research on gas hydrate technology for gas separation, such as CO2 capture from versatile pre/post combustion emissions. In spite of their importance, reliable phase equilibrium data on gas hydrates confined at a nanoscale are scattered throughout the literature, while those in bulk state are abundant. Accordingly, we surveyed the previous studies on the phase behavior of gas hydrates in various nanoporous materials to include and provide valuable information and knowledge for start-up researchers in various gas hydrate fields.

Hydrophilic pore-blocked metal–organic frameworks: a simple route to a highly selective CH4/N2 separation

The separation of gas mixtures with similar thermodynamic and transport properties is a challenging issue. For that, various types of adsorbents and membranes have been introduced. To date, solving this difficult problem remains an urgent core task in the energy and environment fields. Herein, we introduce a simple method to achieve highly selective separation of CH4/N2. To demonstrate our concept, a CH4/N2 mixture was separated by Cu3BTC2 filled with water, with the water blocking the hydrophilic pore to reject the inclusion of gas molecules having weak affinity for water, and with the empty hydrophobic pores acting as gas storage sites. This led to high equilibrium selectivity of the CH4/N2 mixture, at 24.7, which is 6 times higher than the untreated Cu3BTC2 itself. Moreover, the formation of methane hydrate in the mesopore of MOF was observed.

Ultrathin graphene oxide membranes on freestanding carbon nanotube supports for enhanced selective permeation in organic solvents

Among the various factors required for membranes in organic solvent separations, the stability of membrane supports is critical in the preparation of membranes with universal chemical stability, mechanical flexibility, and high flux. In this study, nanoporous freestanding carbon nanotube (CNT) films were fabricated and utilized as supports for enhanced permeation in organic solvents. The excellent chemical stability of the CNT support allowed it to withstand various organic solvents such as toluene, acetone, and dimethylformamide. In addition, the structural stability and high pore density of CNT supports allowed the deposition of an ultrathin selective layer for an enhanced-flux membrane. Membrane performance was demonstrated by depositing a thin graphene oxide (GO) layer on the CNT support; GO was selected because of its high chemical stability. CNT-supported GO membranes effectively blocked molecules with molecular weight larger than ~800 g mol−1 while allowing the fast permeation of small molecules such as naphthalene (permeation was 50 times faster than that through thick GO membranes) and maintaining selective permeation in harsh solvents even after 72 hours of operation. We believe that the developed CNT support can provide fundamental insights in utilizing selective materials toward organic solvent membranes.