Exploring the Limits of Methane Storage and Delivery in Nanoporous Materials

The physical limits for methane storage and delivery in nanoporous materials were investigated, with a focus on whether it is possible to reach a methane deliverable capacity of 315 cm3(STP)/cm3 in line with the adsorption target established by the ARPA-E agency. Our efforts focused on how both geometric and chemical properties, such as void fraction (Vf), volumetric surface area (Sv), and heat of adsorption (Qst), impact methane deliverable capacity, i.e., the amount of methane adsorbed at some storage pressure minus the amount adsorbed at the delivery pressure. With the aid of grand canonical Monte Carlo (GCMC) simulations, we studied methane adsorption and delivery properties in a population of 122 835 hypothetical pcu metal–organic frameworks (MOFs) and 39 idealized carbon-based porous materials. From the simulation results, we developed an analytical equation that helped us delimit the necessary material properties to reach specific methane deliverable capacity targets. The maximum deliverable capacity between 65 and 5.8 bar among the hypothetical MOFs was 206 cm3(STP)/cm3 at 298 K. We found that artificially increasing the methane–MOF interaction strength by increasing the Lennard-Jones ε parameters of the MOF atoms by 2- and 4-fold only improved the maximum deliverable capacity up to 223 and 228 cm3(STP)/cm3, respectively. However, the effect on the amount stored at 65 bar was more significant, which suggested another strategy; raising the temperature of the system by 100 K can recover ∼70% of the methane stranded at the delivery pressure. By increasing the delivery temperature to 398 K, the ARPA-E target was reached by a few hypothetical MOFs with quadrupled ε values. This work shows the difficulty in reaching the ARPA-E target but also suggests that a strategy that combines a material with a large volumetric density of sites that interact strongly with methane and raising the delivery temperature can greatly improve the performance of nanoporous materials for methane storage and delivery. The optimal heat of adsorption in an isothermal storage and delivery scenario is approximately 10.5–14.5 kJ/mol, whereas in the nonisothermal storage and delivery scenario the optimal heats of adsorption fell within a range of 11.8–19.8 kJ/mol.