Dynamics and Proton Transport in Imidazole-Doped Nanocrystalline Cellulose Revealed by High-Resolution Solid-State Nuclear Magnetic Resonance Spectroscopy

Abstract

Imidazole-doped nanocrystalline cellulose (CNC-Im) is a new proton conductor based on imidazole-functionalized nanocrystalline cellulose with a conductivity of approximately 10–1 S/m at 160 °C. Its conductivity is possible due to the transport of protons from imidazoles. The dynamics of local processes were studied by 15N and 13C nuclear magnetic resonance (NMR) spectroscopy under the conditions of 1H–15N and 1H–13C cross-polarization (CP) and magic angle spinning (MAS) and by heteronuclear correlation (HETCOR) spectroscopy. The 15N and 13C NMR spectra showed the coexistence of two fractions of imidazole molecules: slowly reorienting and exchanging protons and fast reorienting and fast exchanging protons. Analysis based on the two-phase model enabled the determination of the energy distribution of imidazole tautomerization, whose maximum value is 38 kJ/mol. The HETCOR experiment allowed determination of the binding of nitrogen protons from imidazoles to cellulose hydroxyl groups and possibly residual water. NMR studies conducted on the 13C isotope confirmed the reorientation of imidazoles. The proton transport in CNC-Im was shown to consist in the exchange of protons between imidazoles via the OH groups of cellulose and residual water conditioned by the reorientation of imidazole rings. The described proton transport leads to the observed conductivity in CNC-Im, assuming the dissociation of imidazole into anion and cation additionally.

Elucidations of Structure and Molecular Dynamics of Complex Polymers by State-of-the-Art Solid-State NMR Spectroscopy

“…290 They proposed that the proton transport mainly resulted from the proton exchange between neighboring imidazole, but through interacting with hydroxyl groups of cellulose or residual water, while the dynamic exchange process was activated by the reorientation of imidazole rings. 290 8.3.3. Organic Semiconductors.…”

“…Yin et al designed a semicrystalline and thermoplastic anhydrous proton exchange membrane for fuel cells via the cocrystallization of poly­(ethylene glycol) (PEG) and SiW, and the presence of PEG chains with two distinct dynamic modes, as revealed by 13 C ssNMR spectra, is closely related to its mechanical properties and proton transportation . Tritt-Goc et al studied the local dynamics and proton transport in imidazole-doped nanocrystalline cellulose (CNC-Im), and they found the imidazole simultaneously exhibit two different reorientation and exchange rate via variable temperature 1 H– 15 N and 1 H– 13 C CP experiments . HETCOR experiments further revealed the binding between nitrogen protons of imidazole and hydroxyl groups of cellulose or possibly residual water .…”

“…Tritt-Goc et al studied the local dynamics and proton transport in imidazole-doped nanocrystalline cellulose (CNC-Im), and they found the imidazole simultaneously exhibit two different reorientation and exchange rate via variable temperature 1 H– 15 N and 1 H– 13 C CP experiments . HETCOR experiments further revealed the binding between nitrogen protons of imidazole and hydroxyl groups of cellulose or possibly residual water . They proposed that the proton transport mainly resulted from the proton exchange between neighboring imidazole, but through interacting with hydroxyl groups of cellulose or residual water, while the dynamic exchange process was activated by the reorientation of imidazole rings …”

“…HETCOR experiments further revealed the binding between nitrogen protons of imidazole and hydroxyl groups of cellulose or possibly residual water . They proposed that the proton transport mainly resulted from the proton exchange between neighboring imidazole, but through interacting with hydroxyl groups of cellulose or residual water, while the dynamic exchange process was activated by the reorientation of imidazole rings …”

Solid electrolytes based on cellulose nanocrystals with protic ionic liquid for next-generation fuel cells

“…The spectra in a frequency range from 50 to 110 ppm exhibit well-de ned 13 C peaks attributed to the cellulose matrix (Bielejewski et al, 2020) and thus not seen in the ionic liquid spectrum. The observed 13 C peaks above 110 ppm are assigned to the imidazole carbons in CNC-Im and imidazolium cation in ionic liquid.…”