2H and 19F solid-state NMR studies of the ionic liquid [C2Py][BTA]-d10 confined in mesoporous silica materials

Abstract

Ionic liquids confined in porous materials are important solvents which allow a simple heterogenization of homogenous liquids. The perdeuterated ionic liquid N-ethylpyridinium-bis(trifluoromethanesulfonyl)amide ([C₂Py][BTA]-d₁₀) was prepared and its bulk phase behavior was studied by differential scanning calorimetry (DSC) and temperature-resolved ²H and ¹⁹F solid-state NMR spectroscopy. Its bulk properties were compared to [C₂Py][BTA]-d₁₀ confined in a mesoporous silica support material as model material usable in SILP catalysts. The line shape analysis of the temperature-dependent NMR spectra of the bulk material reveals two phase transitions, one at 287–289 K (solid II/solid I) and one extending over a temperature range of 298–306 K (solid I/liquid). While the first phase transition is caused by the onset of an intramolecular rotation of the ethyl group of the cation, the second is due to the melting of the ionic liquid. In the bulk material, a hysteresis between the transition temperatures in heating and cooling scans occurs. In confinement, the dynamics of the ionic liquid changes considerably: no hysteresis is observed for [C₂Py][BTA]-d₁₀ confined in the mesopores. Instead, only a broad transition from solid II to the liquid state, which spans the temperature range of 215–245 K, is observed. This transition is identified as the result of a broad distribution of molecular environments of the confined ionic liquid, which thus forms an amorphous phase inside the pores. Hence, the behavior of the ionic liquid in confinement is similar to the behavior of non-ionic guest molecules in the mesoporous silica. Finally, it was found that the anion and cation of the ionic liquid exhibit the same dynamic behavior in confinement.