Exploiting hydrophobicity and hydrophilicity in nanopores as a design principle for smart MOF microtanks for methane storage

Widespread use of methane-powered vehicles likely requires the development of efficient on-board methane storage systems. A novel concept for methane storage is the nanoporous microtank, which is based on a millimeter-sized nanoporous pellet (the core) surrounded by an ultrathin membrane (the shell). Mixture adsorption simulations in idealized pores indicate that by combining a pellet that features large, hydrophobic pores with a membrane featuring small, hydrophilic pores, it would be possible to trap a large amount of pressurized methane in the pellet while keeping the external pressure low. The methane would be trapped by sealing the surrounding membrane with the adsorption of a hydrophilic compound such as methanol. Additional simulations in over 2000 hypothesized metal-organic frameworks (MOFs) indicate that the above design concept could be exploited using real nanoporous materials. Structure-property relationships derived from these simulations indicate that MOFs suitable for the core (storing over 250 cc(STP)CH4 per cc) should have a pore size in the 12-14 angstrom range and linkers without appreciably hydrophilic moieties. On the other hand, MOFs suitable for the shell should have a pore size less than 9 angstrom and linkers with hydrophilic functional groups such as -CN, -NO2, -OH and -NH2. Simulation snapshots suggest that the hydrogen bonding between these groups and hydrophilic moieties of methanol would be critical for the sealing function.