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

Abstract

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 Å range and linkers without appreciably hydrophilic moieties. On the other hand, MOFs suitable for the shell should have a pore size less than 9 Å 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.