Multiscale study of the structure and hydrogen storage capacity of an aluminum metal-organic framework

First-principles calculations based on density functional theory and Grand Canonical Monte Carlo (GCMC) simulations are carried out to study the structure of a new Aluminum Metal-Organic Framework, MOF-519, and the possibility of storing molecular hydrogen therein. The optimized structure of the inorganic secondary building unit (SBU) of MOF-519 formed by eight octahedrally coordinated aluminum atoms is presented. The different storage sites of H₂ inside the SBU and the BTB ligand are explored. Our results reveal that the SBU exhibits two different favorable physisorption sites with adsorption energies of ?12.2 kJ/mol and ?1.2 kJ/mol per hydrogen molecule. We have also shown that each phenyl group of BTB has three stable H₂ adsorption sites with adsorption energies between ?6.7 kJ/mol and ?11.37 kJ/mol. Using GCMC simulations; we calculated the molecular hydrogen (H₂) gravimetric and volumetric uptake for the SBU and MOF-519. At 77 K and 100 bar pressure, the hydrogen uptake capacity of SBU is considerably enhanced, reaching 16 wt.%. MOF-519 has a high gravimetric uptake, 10 wt.% at 77 K and 4.9 wt.% at 233 K. It has also a high volumetric capacity of 65 g/L at 77 K and 20.3 g/L at 233 K, indicating the potential of this MOF for hydrogen storage applications.     

A density functional theory and grand canonical Monte Carlo simulations study the hydrogen storage on the Li-decorated net-τ

To find ideal hydrogen storage media, hydrogen storage performance of Li decorated net-τ has been investigated by first-principles calculations. Maximum 6 Li atoms are adsorbed on net-τ, with the average binding energy of 2.15 eV for per Li atom. Based on 6Li-decorated net-τ, up to twenty H₂ molecules are adsorbed, with a high H₂ storage capacity of 12.52 wt% and an appropriate adsorption energy of 0.21 eV/H₂. Finally, H₂ uptake performance is measured by GCMC simulations. Our results suggest that Li-decorated net-τ may be a promising hydrogen storage medium under realistic conditions.     

Investigation of cryo-adsorption hydrogen storage capacity of rapidly synthesized MOF-5 by mechanochemical method

Comparisons were made between the samples mechanochemically (MOF-5(M)) and solvothermally (MOF-5(S)) prepared for the development of efficient hydrogen storage medium. Synthesized samples were undergone structural characterization as well as adsorption equilibrium measurements of hydrogen at temperature-pressure range 77 K–87 K and 0.1–10 MPa. Grand Canonical Monte Carlo (GCMC) simulations were further conducted to study the behaviors of hydrogen molecules adsorbed on MOF-5. It shows that, besides the advantage of large scale synthesis and a lower cost, mechanochemical method respectively brings about 207% and 90.5% increments in the specific surface area and the maximum excess adsorption capacity of hydrogen at 77 K within pressure range 0–10 MPa. Results also reveal that the crystal within MOF-5(M) is regular and distributing uniformly with a mean size only one tenth of that of the MOF-5(S); at 77 K within pressure range 0–10 MPa, Toth equation can predict the adsorption equilibrium data of hydrogen on two MOF-5 samples with a mean relative error less than 1.5%. It suggests that MOF-5(M) is more promising for hydrogen storage by adsorption for practical applications.     

Hydrogen storage in 1D nanotube-like channels metal–organic frameworks: Effects of free volume and heat of adsorption on hydrogen uptake

Metal–organic Frameworks generate significant interest for their potential application as Hydrogen storage materials. Grand Canonical Monte Carlo (GCMC) simulations were performed at two different temperatures 77 and 300 K over a wide range of pressures to describe H₂ adsorption in 7 metal–organic frameworks (MOFs), which all have the same framework topology but different surface chemistry and different pore sizes. DREIDING and UFF force fields were identified to be able to predict adsorption isotherms for H₂ in MOFs in a reasonable agreement with the experimental data from the literature. This work reveals that at 77 K the total amount of H₂ adsorbed correlates mainly with: the heat of adsorption at low pressure and the free volume at high pressure. While at 300 K the amount adsorbed mainly correlates with the available free volume at both low and high pressure. None of the MOFs studied fulfils DOE requirement, this is due to their low heat of adsorption. The required adsorption energy to meet the DOE targets is estimated to be 34 kJ/mol.     

Enhancement of hydrogen adsorption by alkali-metal cation doping of metal-organic framework-5

Over the past decade, metal-organic frameworks (MOFs) have been extensively studied as a novel approach to store hydrogen. The large surface area and volume of micropores that are intrinsic to MOFs make them ideal for gas adsorption. In addition, we chemically reduced MOF-5 by doping it with alkali metals (Li, Na, and K). We found that the H₂ uptake capacity of MOF-5 materials doped with Li, Na, and K exceeded that of a neutral framework by 24%, 68%, and 70%, respectively. Notably, at the same levels of doping, the Li⁺-doped framework exhibited the strongest H₂ binding, and the binding strength decreased sequentially in the order Li⁺ > Na⁺ > K⁺.     

Synthesis,characterization and hydrogen adsorption in mixed crystals of MOF-5 and MOF-177

Mixed MOF crystals with morphology similar to that of pure MOF-5 and pure MOF-177 were synthesized using two organic solvents: dimethylformamide (DMF) and diethylformamide (DEF). The mixed crystals were characterized with XRD, SEM and TGA for their physical properties and also evaluated for their hydrogen adsorption properties. The XRD and SEM results suggest that the mixed crystals are different from pure MOF-5 and pure MOF-177. The DMF-derived mixed MOF crystals have a slightly higher specific surface area, smaller pore diameter and greater pore volume than those of the DEF-derived crystals, and seem to be a better adsorbent than the DEF-derived crystals, which was confirmed by the higher hydrogen and nitrogen adsorption capacities on the DMF-derived crystals. The hydrogen adsorption capacities on the mixed MOF crystals are lower than those of pure MOF-5 and MOF-177. It was also observed that the hydrogen diffusion time constant increases with hydrogen pressure, and the heat of hydrogen adsorption decreases with adsorbed hydrogen amount on both mixed crystals.     

Synthesis and hydrogen-storage behavior of metal–organic framework MOF-5

Metal–organic framework MOF-5 (Zn₄O(BDC)₃), a microporous material with a high surface area and large pore volume, was synthesized by three approaches: direct mixing of triethylamine (TEA), slow diffusion of TEA, and solvothermal synthesis. The obtained materials were characterized by X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and nitrogen adsorption, and their hydrogen-storage capacities were measured. The different synthesis methods influenced the pore-structure parameters, morphologies and hydrogen-storage behavior of the obtained MOF-5. MOF-5 synthesized by the solvothermal approach showed a higher surface area and larger pore volume than the samples prepared by the other two approaches. Measurements of the hydrogen-storage behavior showed that the hydrogen-storage capacity was correlated with the specific surface area and pore volume of MOF-5.     

Theoretical calculation of hydrogen desorption energies of calcium hydride clusters

Recently calcium hydride has attracted attention as a possible component in ternary complex hydrides such as Ca(AlH₄)₂, Ca₂SiH_x and quaternary complex hydrides of the type Li–B–Ca–H. Although in bulk form CaH₂ decomposes reversibly above 600° centigrade we were motivated to see whether calcium hydride in cluster form has properties suitable for hydrogen storage. We report here the results of DFT calculations using VASP^® package for clusters Ca_nH_2n with n = 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20 to get the ground state geometries, energies, bond lengths, and desorption energies, after molecular dynamics optimization. The desorption energy vs. cluster size n curve showed that the desorption energy goes up sharply to ∼1.4 eV per H₂ for n up to 4, followed by a broad maximum of ∼1.8 eV per H₂ around n = 12–14, and then tapers off to a nearly constant value of 1.6 eV per H₂ approximating bulk behavior, which compares favorably with previously reported results. Comparison of these results with those of Mg_nH_2n shows that Ca_nH_2n has a lesser potential as a hydrogen storage medium.     

MOF-5 composites exhibiting improved thermal conductivity

The low thermal conductivity of the prototype hydrogen storage adsorbent, metal-organic framework 5 (MOF-5), can limit performance in applications requiring rapid gas uptake and release, such as in hydrogen storage for fuel cell vehicles. As a means to improve thermal conductivity, we have synthesized MOF-5-based composites containing 1–10 wt.% of expanded natural graphite (ENG) and evaluated their properties. Cylindrical pellets of neat MOF-5 and MOF-5/ENG composites with densities of 0.3, 0.5, and 0.7 g/cm³ are prepared and assessed with regard to thermal conductivity, specific heat capacity, surface area, and crystallinity. For pellets of density ∼0.5 g/cm³, we find that ENG additions of 10 wt.% result in a factor of five improvement in thermal conductivity relative to neat MOF-5, increasing from 0.10 to 0.56 W/mK at room temperature. Based on the relatively higher densities, surface areas, and enhanced crystallinity exhibited by the composites, ENG additions appear to partially protect MOF-5 crystallites from plastic deformation and/or amorphization during mechanical compaction; this suggests that thermal conductivity can be improved while maintaining the favorable hydrogen storage properties of this material.     

Hydrogen storage in zeolite imidazolate frameworks. A multiscale theoretical investigation

A multiscale approach is used to investigate the hydrogen adsorption in nanoporous Zeolite Imidazolate Frameworks (ZIFs) on varying geometries and organic linkers. Ab initio calculations are performed at the MP2 level to obtain correct interaction energies between hydrogen molecules and the ZIF structures. Subsequently, classical grand canonical Monte-Carlo (GCMC) simulations are carried out to obtain the hydrogen uptake of ZIFs at different thermodynamic conditions of pressure and temperature.     

Mechanochemical modification of LiAlH4 with Fe2O3 - A combined DFT and experimental study

LiAlH₄ is a promising material for hydrogen storage, having the theoretical gravimetric density of 10.6 wt% H₂. In order to decrease the temperature where hydrogen is released, we investigated the catalytic influence of Fe₂O₃ on LiAlH₄ dehydrogenation, as a model case for understanding the effects transition oxide additives have in the catalysis process. Quick mechanochemical synthesis of LiAlH₄ + 5 wt% Fe₂O₃ led to the significant decrease of the hydrogen desorption temperature, and desorption of over 7 wt%H₂ in the temperature range 143–154 °C. Density functional theory (DFT)-based calculations with Tran-Blaha modified Becke-Johnson functional (TBmBJ) address the electronic structure of LiAlH₄ and Li₃AlH₆. ⁵⁷Fe Mössbauer study shows the change in the oxidational state of iron during hydrogen desorption, while the ¹H NMR study reveals the presence of paramagnetic species that affect relaxation. The electron transfer from hydrides is discussed as the proposed mechanism of destabilization of LiAlH₄ + 5 wt% Fe₂O₃.     

On-board hydrogen storage in an adsorbent bed: Development of a multi-scale dynamic “1D-plus-1D” model

Reliable design, analysis and optimization of on-board adsorptive hydrogen storage systems require mathematical models that capture the key physics across multiple scales. Pellets of adsorbent may sometimes be preferred for adsorptive storage in fixed beds, compared to powders. Heat and mass transfer within individual pellets play a significant role in the overall dynamics of a fixed bed filled with adsorbent pellets. However, multiscale dynamical model that captures the effect of pellet behavior on the overall dynamics of adsorbent bed is so far missing. In this study, a combined bed–and–pellet “1D-plus-1D” model is developed and analyzed for hydrogen adsorption in a fixed bed of MOF-5 pellets at cryogenic temperatures. Specifically, a 1D axial bed model with mass, momentum and energy balance equations is coupled with a 1D radial pellet model comprising of mass and heat transfer with adsorption within the pellets present at different locations in the bed. The 1D bed and 1D pellet model equations are coupled through surface fluxes on the pellet. We show that internal diffusional resistances within pellets must be considered for pellets greater than 2 mm diameter, for parameters relevant to MOF-5 pellets. The role of various physical parameters of pellets is analyzed.