Solid-state NMR spectroscopic investigation of supported fluorinated ionic liquids for SILP catalysts
Cindy Ly TAVERA MÉNDEZ1, Alexander BERGEN2, Karsten MEYER2, Martin HARTMANN1, Dorothea WISSER1
1Erlangen Center for Interface Research and Catalysis (ECRC) Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
2Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
Introduction
Ease in separation and reuse of metal complexes in homogeneous catalysis are relevant topics in current research [1]. In this regard, Supported Ionic Liquid Phase (SILP) catalysts stand out as promising materials with remarkable advantages. A thoughtful choice of support, with desired textural characteristics, and its right combination with a suitable ionic liquid, with optimum surface wetting and solubility of the substrates, allow us to tailor the catalytic properties of the active sites [2, 3]. Moreover, understanding the behavior of the IL film and its interactions with the support is essential to guide the design of new, tailor-made ILs with targeted interactions, aiming to locate the molecular catalyst either at the gas/liquid or at the liquid/solid interface, tuning its performance in the selected catalytic application.
Solid-state NMR spectroscopy under Magic Angle Spinning (MAS NMR) is a powerful technique to study the structure and dynamics of both solid and liquid phases inside a porous material on a molecular level. Here, we investigated the interaction of novel imidazolium-based task-specific fluorinated ionic liquids with a silica surface. The incorporation of a fluorinated moiety aims at the orientation and self-assembly of the ionic liquid molecules over the support. NMR spectroscopy techniques allowed us to gain insight into the molecule interaction, possible assembly on the surface, and potential as a ligand for a SILP catalyst application.
Results and discussion
Four task-specific alkyl aryl fluorinated ILs were deposited onto 29Si-enriched ordered mesoporous silica SBA-15. 19F NMR spectra of the obtained SILP materials exhibited chemical shift changes compared to the parent IL. New peaks, line broadening and, shorter longitudinal relaxation times (T1) suggest a different chemical environment due to the interaction of the IL with the silica surface. 19F {29Si} Cross Polarization (CP MAS) not only proved the proximity of the fluorinated moiety to the silica surface but also indicated the predominant magnetization transfer to silicon atoms involved in siloxane bridges (Q4) and single silanol (Q3) sites. Pointing out the interaction’s hydrophobic nature. The two-dimensional method 1H-19F HETCOR supported this observation.
To further complement this approach, REDOR experiments allowed us to determine the internuclear distance of the interacting spin pair, 29Si and 19F; according to the results, the two identified chemical environments present distances in the range of 2.0 to 2.9 Å and different mobility behaviour, depending on the type of impregnated IL. Finally, through 1H-29Si HETCOR and 1H-1H DQMAS, we evidenced the interaction of the alkyl or poly(ethylene glycol) moiety with the silica, their conformations, and its possible effect on the molecule configuration over the surface. Overall, the NMR data provided evidence that allowed us to propose a tentative surface configuration model with potential applications toward interface-enhanced supported ionic liquid phase catalysts.
References
[1] A. E. C. Collis and I. T. Horváth, Catal. Sci. Technol., 2011, 1, 912.
[2] A. Riisager, R. Fehrmann, M. Haumann and P. Wasserscheid, Eur. J. Inorg. Chem., 2006, 695.
[3] R. Fehrmann, A. Riisager and M. Haumann, Eds., Supported Ionic Liquids: Fundamentals and applications, Wiley-VCH Verlag GmbH, Weinheim, 2014