Heat capacity and thermodynamic functions of crystalline and amorphous forms of the metal organic framework zinc 2-ethylimidazolate, Zn(EtIm)2

Abstract

Metal organic frameworks (MOFs) may be useful in a variety of applications, mostly related to their capacity to store gases and catalyze reactions. As several MOFs are mechanically milled, they transition through different structures, progressing toward denser and more energetically stable polymorphs. In this paper, we have measured the constant pressure heat capacities of four zeolitic imidazolate frameworks (ZIFs) based on the 2-ethylimadazolate (EtIm) linker exhibiting identical chemical compositions and different framework structures. Specifically, the crystalline Zn(EtIm)2 frameworks of zeolite rho (RHO), analcime (ANA), and β-quartz (qtz) topologies were compared to each other and to the amorphous form of the material prepared by milling. Molar heat capacities were measured from 1.8 K to 300 K using a Quantum Design Physical Property Measurement System (PPMS), and the data were fit to a sum of theoretical functions below 15 K, orthogonal polynomials from 10 K to 60 K, and a combination of Debye and Einstein functions above 50 K. These fits were then used to generate Cp,m°, Δ0TSm°, Δ0THm°, and Φm° values at smoothed temperatures from 0 K to 300 K. While these MOFs have somewhat different heat capacities reflecting their varying structures, they share an unusual feature in the heat capacity around 100 K that is likely due to some common vibrational behavior related to their common linker and metal node and/or their open frameworks. Though the enthalpies of transition scale with molar volume or density, the entropies of transition show more complex behavior and the Gibbs energies of the three energetically less stable polymorphs (RHO, am-RHO, and ANA) are very similar.