Evaluating surface interactions in supported imidazolium ionic liquid phases by using ultrasensitive DNP NMR and DFT studies

Abstract

The intimate interaction between imidazolium-based ionic liquid (IL) salts, whether grafted or coated on silica, has been explored using low-temperature dynamic nuclear polarization (DNP) NMR and density functional theory (DFT) calculations. NMR experiments revealed interactions between resolved carbon sites on the IL and various silicon sites in the underlying surface. Such experiments are challenged by mere natural-abundance levels of ¹³C (1.1%) and ²⁹Si (4.7%) and the desire for specificity to only surface regions of the porous sample. DNP enhancements of ≥100× provided excellent signal-to-noise ratios (SNRs) such that 2D NMR heteronuclear correlation (HETCOR) spectra of surface-associated molecules via both ¹H–¹³C and ¹H–²⁹Si spectroscopy were obtained in <3 hours with SNRs of ∼10–140 among various sites, as well as more structurally definitive direct ¹³C–²⁹Si HETCOR with SNRs of ∼5–10 in <3 days. These results would be impossible to obtain without noted DNP gains. These results reveal the interaction between IL and silica, whereas the NMR and DFT combination further explores the orientation of the IL with respect to the surface, altogether showing that grafting is necessary for a well-defined relationship with the silica surface. The overall results can provide guidance on structure–function relationships for applications of this class of materials in heterogeneous catalysis and beyond.