(537b) Spectroscopic Investigation of Silica Nanopore Confinement Effects on a Halometallate Ionic Liquid

Halometallate ionic liquids (ILs) combine the tunable solvent characteristics of organic salts that melt near ambient conditions (ILs) with catalytic and electrochemical activity of the metal centers. When they contain earth-abundant metals (iron, zinc, and aluminum), they represent potential green catalysts for a wide variety of Lewis acid-catalyzed reactions. However, to minimize the volume of solvent needed, to promote reusability, and to overcome transport limitations, halometallate ILs may need to be used as supported systems confined in nanoporous materials such as silicon dioxide (silica).

Here, the effects of confinement of ionic liquid (IL) 1-butyl-3-methylimidazolium tetrachloroferrate [BMIM][FeCl₄] within 3.5 nm pores of mesoporous silica thin films are investigated using x-ray photoelectron spectroscopy (XPS). Mesoporous silica thin films with accessible pores oriented orthogonally with respect to the substrate were synthesized by evaporation induced self-assembly using cetyltrimethylammonium bromide (CTAB) as the template, and characterized by grazing-incidence small-angle x-ray scattering. The IL was physically confined within the pores and XPS used to probe the interface between the IL and the silica support. The IL was also mixed with pyridine and supported onto 8 nm mesoporous silica microparticles to evaluate the acidity using FTIR. FTIR showed that compared to [BMIM] chloride, the addition of iron gave the IL Lewis and Brønsted acidity properties that were maintained when confined within the pores. Comparing [BMIM][Cl] to [BMIM][FeCl₄] using XPS showed that the charge of anion was more delocalized with the addition of iron and was less able to accept hydrogen bonds. The confined [BMIM][FeCl₄] interacted with the pore wall in a different way than [BMIM][Cl]. Whereas the imidazolium group of [BMIM] preferentially interacts with the pore wall in [BMIM][Cl], the cation is displaced by FeCl₄^- perhaps due to the formation of a complex with the pore wall. The peak location of iron in XPS shifted downward when the IL was confined and suggests enhanced charge transfer with the pore wall. The results demonstrate the use of XPS to understand the solvent and catalytic environment of halometallate ILs of nanoconfined ILs.