Impacts of Functional Group Substitution and Pressure on the Thermal Conductivity of ZIF‑8

Metal–organic frameworks (MOFs) are promising candidates as natural gas adsorbents because of their porous feature and high structural tunability. In the gas adsorption/desorption process, MOFs are often under complicated physical environments, such as varied pressure and temperature; however, limited attention has been paid to the effect of pressure on their thermal properties. In this work, taking ZIF-8 with four different functional groups (−H, −CH₃, −Cl, and −Br) as an example, we investigate the influence of functional group substitution and pressure on the thermal conductivity of MOFs through equilibrium molecular dynamics simulations. A reduction in thermal conductivity induced by the functional group substitution is observed, which is caused by a damping effect of the acoustic mismatch. Regarding the impact of pressure, the thermal conductivity of ZIF-8 is found to decrease first with increasing hydrostatic pressure. When the pressure exceeds a critical value, a sudden rise is observed in the thermal conductivity of ZIF-8 because a phase transformation from the porous phase to the dense phase is found in this process. The complicated influence of pressure on thermal conductivity is explained by a competition between the aggravation of phonon scattering and the enhancement of volumetric heat capacity in ZIF-8 with increasing pressure. This work is expected to provide molecular insights into the functional group- and pressure-dependent thermal transport of MOFs and thus facilitate their applications in energy storage and gas absorption.