Characterization of Maple Wood Chars Produced At Different Temperatures Using Advanced 13C Nuclear Magnetic Resonance: Quantitative Estimation of Aromatic Cluster Sizes.

Carbonaceous residues like char have great implications in global carbon cycling, transport and fate of organic contaminants in soil and sediments. Its chemical structural information is important for the understanding of its roles. The present work employed advanced ¹³C solid-state NMR techniques to characterize maple wood chars produced under N₂ at temperatures from 300-700 °C. These techniques included direct polarization/magic angle spinning, ¹H-¹³C long-range dipolar dephasing, and spectral-editing techniques such as dipolar dephasing and ¹³C chemical shift anisotropy filter. Especially we used the NMR-based protocol to quantitatively estimate aromatic cluster sizes. Our results indicated that the wood char produced at 300 °C were primarily composed of residues of biopolymers such as lignin and cellulose. With higher charring temperature, the spectra of wood chars became dominated by aromatics and aromatic bands became narrower and more symmetric due to the loss of aromatic C-O groups. The aromatic cluster sizes increased with increasing temperature.