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Seongnam, South Korea

Han S.S.,Korea Research Institute of Standards and Science | Choi S.-H.,Insilicotech Company Ltd | Goddard W.A.,California Institute of Technology
Journal of Physical Chemistry C | Year: 2010

We report the H2 uptake behavior of 10 zeolitic-imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H 2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H 2 uptake amount, isosteric heat of adsorption (Qst), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H2 adsorption behaviors in contrast to the metal-organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H2 at low pressure because of the high H2 adsorption energy, and ZIFs have a variety of H2 adsorption sites. For example, ZIF-11 has an initial Qst value of ∼15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs. © 2010 American Chemical Society. Source


Han S.S.,Korea Research Institute of Standards and Science | Choi S.-H.,Insilicotech Company Ltd | Goddard W.A.,California Institute of Technology | Goddard W.A.,Korea Advanced Institute of Science and Technology
Journal of Physical Chemistry C | Year: 2011

We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li +, Na +, and K +)-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H 2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H 2, Na-ZIF-70 to 2.19 wt % H 2, and K-ZIF-70 to 1.62 wt % H 2, much higher than 0.74 wt % H 2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H 2 binding energies to the dopants. Moreover, the total H 2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H 2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H 2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H 2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H 2 binding energy needs not always lead to higher delivery H 2 uptake. © 2011 American Chemical Society. Source


Han S.S.,Korea Institute of Science and Technology | Han S.S.,Korea Research Institute of Standards and Science | Jung D.H.,Insilicotech Company Ltd | Choi S.-H.,Insilicotech Company Ltd | Heo J.,Sangmyung University
ChemPhysChem | Year: 2013

We have used grand canonical Monte Carlo simulations with a first-principles-based force field to show that metal-organic frameworks (MOFs) with Li functional groups (i.e. C-Li bonds) allow for exceptional H2 uptake at ambient temperature. For example, at 298 K and 100 bar, IRMOF-1-4Li shows a total H2 uptake of 5.54 wt % and MOF-200-27Li exhibits a total H2 uptake of 10.30 wt %, which are much higher than the corresponding values with pristine MOFs. Li-functionalized MOF-200 (MOF-200-27Li) shows 11.84 wt % H2 binding at 243 K and 100 bar. These hydrogen-storage capacities exceed the 2015 DOE target of 5.5 wt % H 2. Moreover, the incorporation of Li functional groups into MOFs provides more benefits, such as higher delivery amount, for H2 uptake than previously reported Li-doped MOFs. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source


Han S.S.,Korea Research Institute of Standards and Science | Jung D.-H.,Insilicotech Company Ltd | Heo J.,Sangmyung University
Journal of Physical Chemistry C | Year: 2013

Using grand canonical Monte Carlo (GCMC) simulations with our recently developed first-principles-based force fields, we report the effects of porosity and interpenetration on the CO2 uptake in 14 prototypical MOFs (metal organic frameworks). The maximum CO2 capacity for both total and excess uptakes at high pressures (e.g., 50 bar) correlates well with the pore volume of MOFs and zeolitic imidazolate frameworks, rather than the surface area, which agrees well with the experimental results. The interpenetration between MOFs leads to smaller pore volume (higher density) lowering the maximum CO2 uptake at high pressures. However, the interpenetrating MOFs produce new CO2 adsorption sites with high binding affinity (approximately twice that of noninterpenetrating MOFs), such as shared spaces created by two organic linkers of adjacent MOFs, enhancing CO2 uptake at low pressures (e.g., 2 bar). For H2 uptake at 298 K, on the other hand, the interpenetration does not provide positive effects. For these reasons, the interpenetration of MOFs remarkably enhances the selectivity of CO2 over H2, by more than 3 times that of noninterpenetrating MOFs. These results also show that smaller pores in MOFs are, indeed, advantageous for the CO2/H2 separation. © 2012 American Chemical Society. Source


Han S.S.,Korea Research Institute of Standards and Science | Kim D.,Insilicotech Company Ltd | Jung D.H.,Insilicotech Company Ltd | Cho S.,Korea Advanced Institute of Science and Technology | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

For a reliable prediction of CO 2 loading in metal-organic (MOFs) and zeolitic-imidazolate frameworks (ZIFs) by molecular simulation, accurate description of the van der Waals (vdW) and Coulomb interactions is undoubtedly the most critical component. However, there have been some strong recent indications that the use of generic force fields (FFs) widely used in most current CO 2/MOF simulations that were not particularly parametrized for CO 2/MOF and ZIF systems could lead to serious discrepancies compared to experimental results. Here, we develop accurate vdW FFs for CO 2 uptake simulations in MOFs and ZIFs using high-level ab initio calculations, and validate the method by comparing the simulated and experimental CO 2 uptakes for various known MOFs and ZIFs. The agreements between simulations and experiments are shown to be excellent for several known MOFs and ZIFs, although the FF parameters are not specifically fitted to those particular systems, showing the potential transferability of the FF. Atomic charges of the adsorbents are computed via the rapid charge equilibration method. Using these highly accurate FFs, we reveal the origin of the enhanced CO 2 uptake in recently reported MOFs having multivariate functional groups (MTV-MOFs). We find that the capacity enhancement in MTV-MOFs arises from an increase in both vdW (due to tighter geometry) and electrostatic (due to increased dipole moment) interactions between CO 2 and MTV-MOF. We further predict that MOF-5-NO 2 can have an even higher CO 2 uptake than these MTV-MOFs due to a larger local dipole moment of the NO 2-functionalized linker than those found in the reported MTV-MOFs. As a further designing of the high capacity material, we then introduced both electron-donating and -withdrawing functional groups simultaneously in the same organic linker to increase the local dipole moment of the linker significantly. The resulting CO 2 uptake indeed increases substantially due to the favorable electrostatic interactions that can be tested experimentally. © 2012 American Chemical Society. Source

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