Thermal transport and the impact of hydrogen adsorption in Linde Type A zeolitic imidazolate frameworks

Abstract: Thermal transport in metal-organic frameworks (MOFs) is of practical interest in diverse applications such as gas storage and separations, since insufficient heat dissipation can lead to detrimental effects. Despite investigations, influence of molecular infiltration on the heat transport remains unclear in many of MOFs due to poor understanding of mechanisms governing heat conductions. Here, we report molecular dynamics investigations of thermal transport properties in zeolitic imidazolate frameworks (ZIFs). We investigated Linde Type A topological ZIFs (ZIF-lta) exhibiting exceptionally low thermal conductivity with unusual trend of temperature dependence deviating from many crystalline materials, despite long-range crystalline order in them. We demonstrate that heat is predominantly carried by phonons with mean free paths comparable to their wavelengths, analogous to diffusons in amorphous solids owing to strong anharmonicity caused by complexity of unit cell consisting of a large number of metal centers. We further show that adsorbed hydrogen molecules increase thermal conductivity of ZIFs, mainly contributed by additional vibrational modes, as a result of gas-gas or gas-framework interactions. Our work advances fundamental understanding into the thermal transport in MOFs and suggests a means to engineer heat conduction via gas infiltrations.