Formic Acid Stabilization on Supported Ionic Liquid Phases: Insights from Solid-State NMR Spectroscopy

Ionic liquids (ILs) are known to promote the catalytic hydrogenation of CO₂ to formic acid, in particular by shifting the reaction equilibrium through formic acid stabilization. While a broad range of ILs-formic acid interactions has been described, proposed, and discussed in pure IL systems, the molecular basis for such stabilizing effects is not yet fully understood. Fundamental insights are desired for practically relevant solid catalysts such as those made from supported ionic liquid phases (SILPs). Herein, we apply solid-state nuclear magnetic resonance (NMR) spectroscopy to probe interactions between formic acid and four selected catalysts comprising ruthenium nanoparticles (NPs) immobilized on SiO₂-based support materials including SILPs (Ru@SILPs). As evidenced by ¹H transverse relaxation times and the efficiency of ¹H-¹³C polarization transfers, a reduction in molecular motion of formic acid is observed upon its impregnation on Ru@SILPs with guanidinium- or imidazolium-based IL-type molecular modifiers compared to the surfaces with alkyl modifiers or pristine SiO₂. The NMR spectra reveal spatial proximities between formic acid and the cationic surface modifiers pointing to weak chemical interactions with the cationic modifiers. Our findings provide deeper mechanistic insights into the stabilizing role of SILPs for formic acid, with implications for the synthesis of formic acid from CO₂ hydrogenation.