Investigating the impact of linker length on the thermal conductivity in metal-organic frameworks

Description

Metal-organic frameworks (MOFs) are a versatile group of materials allowing for a vast range of possible applications like gas storage, gas separation, catalysis, or in
electronic devices. Many of the processes involved during these applications generate heat, which has to be dissipated to maintain effective operating conditions. Other applications exploit thermoelectric processes, where poor heat transport properties are desirable. Therefore, we aim to derive structure-to-property relationships for heat transport in MOFs to enable designing of materials for specific applications.
In our computational study, we start from isoreticular MOF-1 (IRMOF-1) as a reference structure and then investigate, how heat-transport related properties changes upon modifying the types and lengths of the linkers. In this context we expect two factors impacting the obtained trends: We have shown previously that the thermal interface between node and linker is the primary bottleneck for heat transport in IRMOF-1 variants [1], while especially the linkers act as excellent thermal conductors (see Figure 1). Therefore, one should expect that the reduction of the density of interfaces when employing longer linkers should have a distinct positive impact on the thermal conductivity. Conversely, it is known that the larger degree of porosity for longer linkers reduces the thermal conductivity [2] as it results in a reduction in the density of the heat transport paths. These considerations suggest that it should be possible to estimate the linker-length dependence of the thermal conductivity of IRMOFs with equivalent nodes on purely geometrical grounds. Interestingly, this expectation is neither met in the actually studied IRMOFs nor in a series of model systems developed to better understand the situation.
The computation of the thermal conductivity is based on non-equilibrium molecular-
dynamics simulations (NEMD). Due to the high computational cost of this methodology, classical force fields are employed to carry out the simulations. To
provide the best-possible description for the investigated systems, highly accurate
system-specific force-field (FF) potentials are carefully fitted to ab-initio reference data.
The structure of the force fields is analogous to the MOF-FF potential [3,4], which has been developed explicitly for MOFs. The resulting FFs are rigorously benchmarked based on various criteria including an excellent description of phonon modes, which are essential for an accurate description of heat transport [5].
To gain mechanistic insight into the origin of the heat transport properties, the total
thermal resistance is split into contributions from linkers, nodes and interfaces. This
allows us to show that an increase in linker length typically also leads to an increase
in thermal interface resistance, compensating the beneficial effect of the reduced linker density. Notably, this behavior is not only observed for the actual MOFs but also for the rather generic model systems. This suggests that the peculiar trends in the thermal conductivity are primarily a consequence of the framework architecture and topology rather than of the material specific properties of the nodes and the linkers.

[1] S. Wieser et al., Adv. Theory Simulations (2020) 2000211; [2] L. Han et al., Comput.
Mater. Sci. (2014), 94, 292–297 [3] S. Bureekaew et al., Phys. Status Solidi B, 250
(2013) 1128-1141; [4] 1. J. P. Dürholt et al., J. Chem. Theory Comput. 15 (2019) 2420–
2432; [5] T. Kamencek et al., J. Chem. Theory Comput. 16, 4 (2020) 2716–2735.

Period	7 Sept 2022
Event title	8th International Conference on Metal-Organic Frameworks and Open Framework Compounds: MOF 2022
Event type	Conference
Location	Dresden, Germany, SaxonyShow on map
Degree of Recognition	International