Molecular Simulation Study On Microscopic Heat Transfer Mechanism Of Zeolitic Imidazolate Frameworks

As a new type of porous materials,metal-organic frameworks(MOFs)have broad prospects in the field of gas adsorption,separation and storage due to their ultra-high specific surface area,high porosity,excellent thermal and chemical stability.In practical applications,the thermal conductivity of metal-organic frameworks(MOFs)imposes significant impacts on the thermal transfer performance of related adsorption systems in engineering applications.However,the correlation between the structural properties of MOFs and their thermal conductivity,and the effect of gas adsorption on their thermal conductivity have yet to be unraveled.Firstly,we investigated the thermal conductivity of 18 different topological Zeolitic Imidazolate Frameworks(ZIFs)with the same metal cluster and organic linker through the Molecular Dynamics(MD)simulation,and explored the correlation between their structural properties and thermal conductivity.The results show that there is no obvious correlation between the thermal conductivity of ZIFs and their pore size(Largest Cavity Diameter,LCD and Pore Limiting Diameter,PLD)density,and the overlap energy(E_overlap)between Zn and N atoms of ZIFs.Therefore,two new structure descriptors(i.e.,the spatial orientation of SBUs and the heat transfer pathway formed by SBUs)were developed to describe the heat transfer performance of ZIFs.The alignment tensor(A_i)was adopted to describe the components of SBUs spatial orientation in the i(i.e.,x,y,or z)direction(i.e.,the alignment degree of the SBUs with the x,y,and z directions),and the heat transfer pathway factor(P_f)was introduced to describe the number density of optimal heat transfer pathway in ZIFs.It was revealed that the spatial orientation of SBUs determines the relative thermal conductivity of one ZIF in different directions(i.e.,the larger the A_i value,the larger the thermal conductivity in the i direction),However,the overall thermal conductivity of different ZIFs can not be correlated with A_i,indicating that the heat transfer performance of ZIFs is not solely determined by the spatial orientation of SBUs,and the length and distribution of heat transfer pathways in ZIFs also affect the heat transfer performance.Therefore,the heat transfer pathway factor(P_f)was proposed and exhibited an obviously positive correlation with the thermal conductivity of ZIFs,that is,the greater the P_f value,the greater the thermal conductivity of a ZIF.To further explore the impacts of gas adsorption on the heat transfer performance of ZIFs,Grand Canonical Monte Carlo(GCMC)molecular simulations were performed to obtain the initial configurations of hydrogen(H₂),methane(CH₄)and ethanol(C₂H₅OH)-adsorbed ZIF-dft,ZIF-gis and ZIF-lta,respectively,followed by thermal conductivity calculation through MD simulations.The results shown that gas or guest molecules were able to improve the heat transfer performance of the ZIFs by forming additional heat transfer pathways between the Zn atoms of ZIFs regardless of gas type,and their thermal conductivities increased with the increased of adsorption amount.The thermal conductivities of different ZIFs after gas adsorption exhibited different trends,which is related to the variation of the overlap energy(E_overlap)between Zn atoms of ZIFs and the gases by affecting the additional heat transfer pathways.The larger the E_overlap,the stronger the coordination of the vibration frequency between the Zn atoms and the gases that is favorable for the formation of additional heat transfer pathways,leading to the higher thermal conductivity of ZIFs.Additionally,different gases exhibited different spatial distributions and diffusion coefficients within a ZIF.The more uniform gas distribution in a ZIF,the more additional heat transfer pathways and the faster the gas diffusion in a ZIF,the higher heat transfer efficiency of additional heat transfer pathways,both of which are beneficial for the heat transfer performance of ZIFs.