(626g) Interfacial Interactions between Confined Ionic Liquids and Mesoporous Silica

Ionic liquids are tunable, low-volatility solvents for applications such as extraction, catalysis, and electrochemistry. However, they can be expensive and difficult to fully recover in some of these applications. To reduce costs and improve handling while still utilizing their advantages, ionic liquids are often confined within nanoscale pores, such as in mesoporous silica. Confinement leads to interactions at the interface between the ionic liquids and the mesostructured materials. In this work, the interactions between 1-butyl-3-methylimidazolium chloride or tetrachloroferrate ([BMIM][X]; X = Cl or FeCl₄) and mesoporous silica films with 2.5 or 9 nm pore diameters is investigated using x-ray photoelectron spectroscopy (XPS). In addition, effects of ionic liquid tethering to the pore walls are investigated since this is a common strategy to reduce leaching of valuable ionic liquids. Upon nanoconfinement, a new peak in the nitrogen XPS spectrum appears at lower binding energy consistent with the positively charged imidazolium head group of the ionic liquids preferentially interacting with the negatively charged silica pore wall. XPS shows that iron in [BMIM][FeCl₄] has multiple oxidation environments. Pyridine was used as a probe to provide evidence that the addition of metals to [BMIM][Cl] improves the Lewis and Brønsted acidity of the ionic liquids using Fourier transform infrared spectroscopy (FTIR) when the IL was confined within mesoporous silica particles. The enhanced acidity of confined ionic liquids makes them viable for use in catalysis. This work demonstrates that the addition of metal halides improves the acidity of confined ionic liquids, and this property is preserved upon confinement. The type of metal or cation of the ionic liquid can be changed to fine tune the properties of the confined system for the intended application.