EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Metal-organic frameworks (MOFs) represent a highly porous class of materials formed by metal nodes connected by organic linkers. Their modular nature enables an almost limitless pool of possible materials leading to a wide range of different applications like gas storage, gas separation or catalysis. Many of the processes occurring during these applications rely on the dissipation of heat. Therefore, it is crucial to understand the structure-dependent mechanism of heat transport to design MOFs tailored for specific applications.

Obtaining thermal conductivities with atomistic simulations is computationally highly demanding. Thus, accurate classical force fields are utilized to make the simulations feasible. To maximize the accuracy, we employ second-generation force fields parametrized system-specifically using periodic ab-initio reference data. Their functional form is based on MOF-FF, which has been specifically developed for MOFs and has provided accurate results in the past. A focus is laid on an excellent description of phonon properties, which are crucial for heat transport.

We employ non-equilibrium molecular dynamics simulations to determine the thermal conductivity for a selection of different MOFs and to spatially resolve barriers for heat transport. In this way, we identify the interface between node and linker, specifically the bond between the metal and oxygen atoms, as the major bottleneck for thermal energy flow. This bottleneck can be controlled by utilizing metals with different masses or by changing metal-linker bonding strengths, as shown by investigating a series of modified isoreticular MOFs. Complementarily, the impact of several different linkers on heat transport is examined. Additional insight is gained by identifying the most relevant phonons for thermal transport and by analyzing their harmonic and anharmonic properties.