Add time:07/25/2019         Source:sciencedirect.com

Metal organic frameworks (MOFs) are a novel class of materials that consist of a lattice of metal centers linked by organic molecules. Previously, heat capacities and thermodynamic functions have been reported for a series of crystalline polymorphs of Zn(EtIm)2, a zeolitic imidazolate framework family of MOFs that have potential applications in CO2 sequestration. In those studies, an anomalous trend in the heat capacity and thermodynamic functions was observed that was not predicted. To further investigate these materials and their thermodynamic data, the low-temperature heat capacities of imidazole, 2-methylimidazole, and 2-ethylimidazole, organic linkers that are present in Zn(EtIm)2 and similar materials, were measured by a Quantum Design Physical Property Measurement System (PPMS) from 1.8–300 K. The data collected was fit to theoretical functions below 10 K, orthogonal polynomials from 5–60 K, and a sum of Debye and Einstein functions above 50 K. These functions were used to generate Cp,m°, Δ0TSm°, Δ0THm°, and Φm° values at smoothed temperatures from 0–300 K. This investigation revealed the presence of a second-order phase transition in 2-ethylimidazole that may yield insight into the anomalous heat capacity behavior present in Zn(EtIm)2.

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