Structural factors determining thermal stability limits of ionic liquid/mof composites: imidazolium ionic liquids combined with cuBTC and ZIF-8

Abstract

Twenty-nine different imidazolium ionic liquids (ILs) were combined with two different metal-organic frameworks (MOFs), ZIF-8 and CuBTC, and the resulting IL/MOF composites were characterized in detail by combining X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), and Fourier transform infrared (FTIR) spectroscopy. Characterization data illustrated that MOFs remained structurally intact upon combining them with ILs. Thermogravimetric analysis performed on IL/MOF composites showed that most of the composites have lower thermal stabilities compared to the bulk ILs and pristine MOFs, whereas composites with ILs having a functional group in their anions showed thermal stability limits higher than those of bulk ILs. The derivative onset temperatures representing the maximum tolerable temperatures of the composites were analyzed based on the structural differences in MOFs and ILs, such as the changes in the alkyl chain length, methylation on the C2 site, and functionalization of the cation and the size/electronic changes on the anion. Data illustrated that the maximum tolerable temperatures of IL/MOF composites decrease with an increase in the alkyl chain length on the IL's imidazolium ring. Substitution of the alkyl group with functionalized groups in the IL's imidazolium ring also led to a decrease in the maximum tolerable temperatures of the composites. VVhereas, fluorination of the anion resulted in an increase in the thermal stability limits of the corresponding IL/MOF composites. Furthermore, ILs having a dicyanamide, acetate, and phosphate anion also showed an increase in their maximum tolerable temperatures when combined with CuBTC compared to their bulk counterparts. Moreover, simple structural descriptors for each cation and anion were defined by means of the density functional theory (DFT) calculations and used in the quantitative structure-property relationship (QSPR) analysis to correlate the maximum tolerable temperatures of IL/MOF composites to the IL's cation and anion structure. Results presented in this study will provide a guideline for the selection of proper IL-MOF pairs according to the application temperature of IL/MOF composites in various fields.