Understanding Effects of Gas Molecules on Thermal Transport in Metal–Organic Frameworks

Metal–organic frameworks (MOFs)—porous crystals composed of metal nodes and organic linkers—are promising materials for a wide range of applications, notably in gas storage and separations. Despite ultrahigh porosities, large internal surface areas, and highly tunable structures, realizing the full potential of MOFs has been hindered in part by their inadequate thermal properties. MOFs typically possess low thermal conductivities (&lt; 2 W m^–1 K^–¹), which can dictate gas adsorption dynamics under practical loading rates. While it has been suggested that the presence of gas molecules within the pore can impact the propagation of phonons, it has been challenging to experimentally examine the effects of interactions at the pore wall. Recent studies have sought to measure the thermal conductivities of various MOFs, and molecular dynamic simulations have been employed to elucidate the factors governing their intrinsic transport properties, but the microscopic mechanisms underlying the effects of adsorbates remain underexplored.<br/> Here, we present experimental investigations of thermal transport in zeolitic imidazole framework-8 (ZIF-8) under systematically varied gas environments. We employ the frequency-domain thermoreflectance (FDTR) technique to probe thermal conductivity in single crystals of ZIF-8 with chemically diverse gas adsorbates. This approach enables us to investigate how the interaction energies between gas molecules and the pore walls influence transport pathways along the porous framework. Our FDTR measurements reveal that gases with higher adsorption capacity, such as CO₂, result in a notable decrease in the thermal conductivity of ZIF-8 compared to adsorbates with a lower adsorption capacity such as N₂. Additionally, variable-temperature FDTR measurements provide insight into the kinetics of gas molecules confined within the pores of ZIF-8. Our research illuminates the dynamic interplay between gas adsorption and the thermal properties of ZIF-8, offering new mechanisms to chemically modulate thermal transport in porous materials systems.