Molecular Insights into the Correlation between Microstructure and Thermal Conductivity of Zeolitic Imidazolate Frameworks

Abstract

The thermal conductivity of metal−organic frameworks (MOFs) imposes significant impacts on the thermal transfer performance of related adsorption systems in engineering applications. However, how the structural properties of MOFs affect their thermal conductivities has yet to be unraveled. In this work, the thermal conductivities of 18 zeolitic imidazolate frameworks (ZIFs) were calculated by equilibrium molecular dynamics (MD) simulations. It was revealed that the thermal conductivities of ZIFs were not directly correlated with the commonly investigated structural properties. Thus, two parameters including alignment tensor (A i ) and pathway factor (P f ) were proposed to quantitatively evaluate the orientation and distribution of heat transfer pathways within frameworks, which was demonstrated to correlate better with the thermal conductivities of ZIFs. This study provides new insights into the thermal transfer mechanism within framework-based nanoporous materials, which may also facilitate fundamental understanding and guide the rational design of porous crystals with the thermal conductivity of interest.

Exploring the Impact of the Linker Length on Heat Transport in Metal–Organic Frameworks

“…A strong correlation between pore size and thermal conductivity was indeed found also for actual MOFs [19,46]. Different heat transport pathways due to different MOF topologies modify this simple picture [43,45]. The situation is further complicated by flexible MOFs, which undergo phase transitions that can change their pore sizes.…”

“…This suggests that increasing the length of the linker could lead to an improved thermal conductivity, as it decreases the density of interfaces in the heat-transport direction. However, longer linkers also mean a larger pore size, which, as mentioned above, is detrimental to thermal transport due to a reduced number of possible heat transport channels [19,[43][44][45]51]. In the present study, we analyzed the interplay between these effects employing non-equilibrium molecular dynamics (NEMD) simulations for a series of isoreticular MOFs (IRMOFs).…”

“…Changes in the phonon scattering behavior are also key to understanding variations in thermal conductivities due to different functional groups in the linkers of zeolitic imidazolate framework 8 (ZIF-8) systems [38]. In view of the above-mentioned applications of MOFs, several studies were also dedicated to understanding the impact of gas loading and the integration of guest molecules on heat transport, where, depending on the MOF and the loaded molecule, either a reduction [37,[39][40][41] or an increase [31,40,[42][43][44] in the thermal conductivity due to guest incorporation was observed.…”

Effect of Phase Transition on the Thermal Transport in Isoreticular DUT Materials

“…The SPC can be treated as a periodic framework, in which the heat transports mainly through the bonded secondary building units. 38 Thus, the main thermal transport pathway of the isoreticular DUT materials considered here is -Cu-O-C-N-Cu-that starts from the metal atom Cu, then passes through O, C, and N atoms in the ligand backbone, and finally returns to metal atom Cu, while the function group -H is absent from this main thermal transport pathway. It is noted here that to distinguish different carbons in DUTs, the carbon atom connecting to Cu and O atoms in carboxylate moieties is termed as C1, while that in the carbon ring of phenylene bridge (or the naphthalene bridge of DUT-47) is termed as C2.…”

“…To data, only the thermal conductivity of some conventional SPCs such as MOF-5, 23-26 ZIF-8, [27][28][29][30][31][32] and HKUST-1 21, 33,34 has been investigated in detail by experimental or computational approaches, while the thermal transport properties of the aforementioned DUT series still remain unknown. In addition, the existing studies mainly focus on the influence of gas and water adsorption, 29,34 framework architecture, 24,28,[35][36][37][38] defects, 33 and physical environment including pressure 18 and temperature 20 on the thermal conductivity of SPCs, little attention has been paid to the evolution of the thermal transport properties of SPCs during their dynamic process, i.e., the phase transition.…”