Thermal Conductivity of Zeolitic Imidazolate Framework-8: A Molecular Simulation Study

While metal–organic frameworks have been extensively investigated for gas storage and separation, their thermal properties are scarcely studied and largely unknown. We report here a molecular simulation study to examine the thermal conductivity of zeolitic imidazolate framework-8 (ZIF-8). From equilibrium molecular dynamics simulation, the thermal conductivity of ZIF-8 is predicted to be about 0.165 W/mK. The low thermal conductivity is due to the short mean free path of the phonon in ZIF-8, which is estimated to be less than two unit cells. With increasing temperature from 300 to 1000 K, the thermal conductivity increases from 0.165 to 0.190 W/mK. The temperature effect is attributed to the enhanced overlap in the vibrational density of states between Zn and N atoms. The contributions of lattice vibrations in different directions to heat flux are examined from nonequilibrium molecular dynamics simulation. It is found that the longitudinal vibration contributes 60% to thermal transport in ZIF-8; in contrast, transverse vibration contributes 40%. Furthermore, the contributions from different forces to heat flux are analyzed. While the stretching and Lennard-Jones components have 31% contribution each, the bending and Coulombic components contribute 17% and 21%, respectively. However, the contribution of the torsional component is nearly zero. This simulation study provides quantitative understanding of the thermal conductivity of ZIF-8 and microscopic insight into the mechanism of heat transfer.