High H2 Storage of Hexagonal Metal−Organic Frameworks from First-Principles-Based Grand Canonical Monte Carlo Simulations

Stimulated by the recent report by Yaghi and co-workers of hexagonal metal−organic frameworks (MOF) exhibiting reversible binding of up to 7.5 wt % at 77 K and 70 bar for MOF-177 (called here IRMOF-2-24), we have predicted additional trigonal organic linkers, including IRMOF-2-60, which we calculate to bind 9.7 wt % H2 storage at 77 K and 70 bar, the highest known value for 77 K. These calculations are based on grand canonical Monte Carlo (GCMC) simulations using force fields that match accurate quantum mechanical calculations on the binding of H2 to prototypical systems. These calculations were validated by comparison to the experimental loading curve for IRMOF-2-24 at 77K. We then used the theory to predict the effect of doping Li into the hexagonal MOFs, which leads to substantial H2 density even at ambient temperatures. For example, IRMOF-2-96-Li leads to 6.0 wt % H2 storage at 273 K and 100 bar, the first material to attain the 2010 DOE target

Improved H_2 Storage in Zeolitic Imidazolate Frameworks Using Li^+, Na^+, and K^+ Dopants, with an Emphasis on Delivery H_2 Uptake

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

Zeolitic Imidazolate Frameworks as H_2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation

We report the H_2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H_2 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 H_2 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 (Q_(st)), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H_2 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 H_2 at low pressure because of the high H_2 adsorption energy, and ZIFs have a variety of H_2 adsorption sites. For example, ZIF-11 has an initial Q_(st) value of ~15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H_2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs

Recent advances on simulation and theory of hydrogen storage in metal–organic frameworks and covalent organic frameworks

This critical review covers the application of computer simulations, including quantum calculations (ab initio and DFT), grand canonical Monte-Carlo simulations, and molecular dynamics simulations, to the burgeoning area of the hydrogen storage by metal–organic frameworks and covalent-organic frameworks. This review begins with an overview of the theoretical methods obtained from previous studies. Then strategies for the improvement of hydrogen storage in the porous materials are discussed in detail. The strategies include appropriate pore size, impregnation, catenation, open metal sites in metal oxide parts and within organic linker parts, doping of alkali elements onto organic linkers, substitution of metal oxide with lighter metals, functionalized organic linkers, and hydrogen spillover (186 references)

High H_2 Uptake in Li-, Na-, K-Metalated Covalent Organic Frameworks and Metal Organic Frameworks at 298 K

The Yaghi laboratory has developed porous covalent organic frameworks (COFs), COF102, COF103, and COF202, and metal–organic frameworks (MOFs), MOF177, MOF180, MOF200, MOF205, and MOF210, with ultrahigh porosity and outstanding H2 storage properties at 77 K. Using grand canonical Monte Carlo (GCMC) simulations with our recently developed first principles based force field (FF) from accurate quantum mechanics (QM), we calculated the molecular hydrogen (H2) uptake at 298 K for these systems, including the uptake for Li-, Na-, and K-metalated systems. We report the total, delivery and excess amount in gravimetric and volumetric units for all these compounds. For the gravimetric delivery amount from 1 to 100 bar, we find that eleven of these compounds reach the 2010 DOE target of 4.5 wt % at 298 K. The best of these compounds are MOF200-Li (6.34) and MOF200-Na (5.94), both reaching the 2015 DOE target of 5.5 wt % at 298 K. Among the undoped systems, we find that MOF200 gives a delivery amount as high as 3.24 wt % while MOF210 gives 2.90 wt % both from 1 to 100 bar and 298 K. However, none of these compounds reach the volumetric 2010 DOE target of 28 g H_2/L. The best volumetric performance is for COF102-Na (24.9), COF102-Li (23.8), COF103-Na (22.8), and COF103-Li (21.7), all using delivery g H_2/L units for 1–100 bar. These are the highest volumetric molecular hydrogen uptakes for a porous material under these thermodynamic conditions. Thus, one can obtain outstanding H_2 uptakes with Li, Na, and K doping of simple frameworks constructed from simple, cheap organic linkers. We present suggestions for strategies for synthesis of alkali metal-doped MOFs or COFs

Use of vanes in sharp bends to improve open-channel flow characteristics : laboratory and numerical experiments

Sharp open-channel bends are commonly encountered in hydraulic engineering design. The flow around these bends is highly three-dimensional (3D) due to combined effects of secondary flow cells, large variation of free surface in the bend, and flow separation along the inner bend wall. Disturbances such as secondary flows and flow separation caused by the bend may persist for considerable distances in the downstream channel. A simple way of reducing these disturbances is through the insertion of continuous vertical vanes in the bend section. First, a Laser Doppler Anemometry (LDA) unit was used to measure the three-dimensional mean and turbulent velocity components of flow in an experimental rectangular open-channel bend. Flow characteristics of the bend with no vane were compared with those of bends having 1 or 3 vertical vanes. The key bend flow characteristics such as the size of the flow separation zone at the inner wall of the bend, the intensity of secondary flow, the level of turbulence generated, the extent of energy loss and the fluctuation of water surface were determined. Several 3D numerical simulations were tested using various grid generation methods, different turbulence models and water surface treatment methods to determine the complex flow field in the sharp bend and in the channel downstream of the bend. The simulations revealed that the Reynolds stress turbulence model (RSM) with body-fitted coordinates, combined with the volume of fluid (VOF) method for water surface representation, gave the best validation when compared to experimental data. Following this, the simulations were extended for the bend with vanes. This allowed a more detailed quantification of the mean and turbulent characteristics of flow between vanes, without experiencing the limitations encountered in experiments. Results from both experimental and numerical investigations confirm that placing vertical vanes in the bend is a very effective way of reducing the intensity of the secondary flow and the extent of the flow separation zone along the inner wall. Energy loss for bends with vanes is also slightly reduced compared to the no-vane case. The 3-vane configuration is particularly efficient at creating a very uniform flow downstream of the bend. KEYWORDS: 90° open-channel sharp bend; Vertical vanes; Flow separation; Laser Doppler Anemometry; Secondary flows; 3D simulation; Turbulence models; Water surface slope; Reynolds Stress Model (RSM

Lithium-Doped Metal-Organic Frameworks for Reversible H_2 Storage at Ambient Temperature

To maximize reversible H_2 storage near room temperature and modest pressures, we propose Li doping of the metal-organic framework (MOFs) structures developed by the Yaghi group at UCLA (constructed using octahedral Zn−O−C clusters with aromatic carbon ring linkers). We tested this design using grand canonical Monte Carlo simulations with first-principles derived force fields and predict that at −30 °C and 100 bar the Li−MOF-C30 leads gravimetric H_2 uptake of 6.0 wt %, reaching the 2010 Department of Energy target (6.0 wt % in the temperature ranges of −30 to 80 °C and pressures ≤100 bar). Thus this promising material for practical hydrogen storage is worthy of developing experimental procedures for synthesis and characterization