Design Requirements for Metal-Organic Frameworks as Hydrogen Storage Materials

Storing an acceptable density of hydrogen in porous materials by physisorption at room temperature and reasonable pressures is a challenging problem. Metal-organic frameworks (MOFs) are a new class of nanoporous materials that have shown early promise for meeting this goal. They have extremely large specific surface areas, but the heats of adsorption to date are too low to provide significant storage at room temperature. In this work, molecular simulations are used to provide guidelines for the design of MOFs for hydrogen storage. To learn how much the heat of adsorption must be increased to meet current targets, we artificially increase the hydrogen/MOF Lennard-Jones attraction. The correlation of the amount of hydrogen adsorbed with the heat of adsorption, the surface area, and the free volume is revisited. We also review the distinction between excess and absolute adsorption and show that comparing the density of hydrogen within the free volume of materials provides useful insight. The simulation results yield a graph showing the required heats of adsorption as a function of the free volume to meet gravimetric and volumetric storage targets at room temperature and 120 bar

Effects of Surface Area, Free Volume, and Heat of Adsorption on Hydrogen Uptake in Metal−Organic Frameworks

Grand canonical Monte Carlo simulations were performed to predict adsorption isotherms for hydrogen in a series of 10 isoreticular metal−organic frameworks (IRMOFs). The results show acceptable agreement with the limited experimental results from the literature. The effects of surface area, free volume, and heat of adsorption on hydrogen uptake were investigated by performing simulations over a wide range of pressures on this set of materials, which all have the same framework topology and surface chemistry but varying pore sizes. The results reveal the existence of three adsorption regimes:  at low pressure (loading), hydrogen uptake correlates with the heat of adsorption; at intermediate pressure, uptake correlates with the surface area; and at the highest pressures, uptake correlates with the free volume. The accessible surface area and free volume, calculated from the crystal structures, were also used to estimate the potential of these materials to meet gravimetric and volumetric targets for hydrogen storage in IRMOFs

Heats of Adsorption for Seven Gases in Three Metal−Organic Frameworks: Systematic Comparison of Experiment and Simulation

The heat of adsorption is an important parameter for gas separation and storage applications in porous materials such as metal−organic frameworks (MOFs). There are, however, few systematic studies available in the MOF literature. Many papers report results for only one MOF and often only for a single gas. In this work, systematic experimental measurements by TAP-2 are reported for the heats of adsorption of seven gases in three MOFs. The gases are Kr, Xe, N2, CO2, CH4, n-C4H10, and i-C4H10. The MOFs studied are IRMOF-1, IRMOF-3, and HKUST-1. The data set provides a valuable test for molecular simulation. The simulation results suggest that structural differences in HKUST-1 experimental samples may lead to differing heats of adsorption

Understanding Inflections and Steps in Carbon Dioxide Adsorption Isotherms in Metal-Organic Frameworks

Adsorption isotherms for CO2 in IRMOF-1 exhibit inflections that grow into pronounced steps at lower temperatures. The isotherm shapes can be predicted by molecular simulations using a rigid crystal structure, indicating that changes in the MOF crystal structure are not responsible for the steps in this system

Separation of CO₂ from CH₄ Using Mixed-Ligand Metal−Organic Frameworks

The adsorption of CO2 and CH4 in a mixed-ligand metal−organic framework (MOF) Zn2(NDC)2(DPNI) [NDC = 2,6-naphthalenedicarboxylate, DPNI = N,N′-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide] was investigated using volumetric adsorption measurements and grand canonical Monte Carlo (GCMC) simulations. The MOF was synthesized by two routes: first at 80 °C for two days with conventional heating, and second at 120 °C for 1 h using microwave heating. The two as-synthesized samples exhibit very similar powder X-ray diffraction patterns, but the evacuated samples show differences in nitrogen uptake. From the single-component CO2 and CH4 isotherms, mixture adsorption was predicted using the ideal adsorbed solution theory (IAST). The microwave sample shows a selectivity of ∼30 for CO2 over CH4, which is among the highest selectivities reported for this separation. The applicability of IAST to this system was demonstrated by performing GCMC simulations for both single-component and mixture adsorption