Medium-Range Order Structure Controls Thermal Stability of Pores in Zeolitic Imidazolate Frameworks

Metal-organic framework (MOF) glasses have multiplepotentialapplications, as they combine advantages of traditional glasses withthose of MOFs. The melt-quenching process used to form MOF glassestypically leads to a significant decrease in porosity, but the structuralorigin of this thermally induced pore collapse remains largely unknown.Here, we study the melting process of three zeolitic imidazolate frameworks(ZIFs), namely ZIF-4, ZIF-62, and ZIF-76, using ab initio molecular dynamics (MD) simulations. By analyzing the MD data usingtopological data analysis, we show that while the three ZIF systemsexhibit similar short-range order structural changes upon heating,they exhibit significant differences in their medium-range order structure.Specifically, ZIF-76 retains more of its medium-range order structuresin the liquid state compared to the other glass-forming ZIF systems,which allows it to remain more porous than ZIF-4 and ZIF-62. As such,our results may aid in understanding the structural features thatgovern the ability to maintain porosity in the melt-quenched glassystate.