Influence of adsorbed gas molecules on thermal transport in metal-organic framework HKUST-1

Metal-organic frameworks (MOFs) have been regarded as promising candidates for gas fuel storage. It is known that a large amount of heat will be released (exothermic) or adsorbed (endothermic) during the gas storage process, which leads to a large temperature change in the system and, in turn, affects the usable gas uptake capacity. It is therefore necessary to investigate thermal transport in gas-adsorbed MOFs. By using atomistic simulations, we here systematically study the thermal transport in ${\mathrm{CH}}_{4}$-, ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$-, and ${\mathrm{H}}_{2}$-adsorbed MOF-199, which usually refers to HKUST-1. We find that the room temperature thermal conductivity of ${\mathrm{CH}}_{4}$- and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$-adsorbed systems first decreases greatly to approximately 40% of the original value and then converges with the amount of adsorbed gas molecules. However, for ${\mathrm{H}}_{2}$-adsorbed systems at room temperature, the corresponding thermal conductivity decreases from approximately 1.37 W/mK to approximately 0.69 W/mK and then increases to approximately 1.16 W/mK with increase of the amount of adsorbed ${\mathrm{H}}_{2}$ molecules. It is further shown that the thermal conductivity contributed by the HKUST-1 framework decreases monotonously with increase of the amount of adsorbed gas molecules owing to the increased scattering between the HKUST-1 framework and adsorbed gas molecules. The thermal energy transferred by the adsorbed ${\mathrm{CH}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ (${\mathrm{H}}_{2}$) molecules increases a little (greatly) with increase of the amount of adsorbed gas molecules. This is because the adsorbed ${\mathrm{H}}_{2}$ molecules can diffuse farther in HKUST-1 than the adsorbed ${\mathrm{CH}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ molecules, which is caused by the small size of ${\mathrm{H}}_{2}$ molecules and weak interactions between the HKUST-1 framework and ${\mathrm{H}}_{2}$ molecules. Therefore, the adsorbed ${\mathrm{H}}_{2}$ molecules can effectively transfer thermal energy through both conductive and diffusive pathways, and the thermal conductivity of the saturated-${\mathrm{H}}_{2}$ system is much higher than that of the saturated-${\mathrm{CH}}_{4}$ and saturated-${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ systems. Our results provide a physical picture of thermal transport in gas-adsorbed MOFs, which should facilitate the design of high-performance MOF-based gas storage systems.