Characterization of Wood Chars Produced at Different Temperatures Using Advanced Solid-State ¹³C NMR Spectroscopic Techniques

Abstract

Temperature is one of the controlling factors determining the chemical structure of char. We employed advanced solid-state 13C NMR techniques to characterize maple wood and its chars produced under N2 at temperatures from 300 to 700 °C. Our results indicated that 300 °C char was primarily composed of residues of biopolymers such as lignin and cellulose. Carbohydrates are completely lost for char prepared at 350 °C. At 400 °C, the char lost most of the ligno-cellulosic features and consisted predominantly of aromatic structures. By 500 °C, sp3-hybridized carbon had all but disappeared. Protonated aromatic carbons increased up to 400 °C chars but then decreased. Aromatic C–O groups decreased, whereas nonprotonated aromatic carbons, especially bridgehead carbons, increased as temperature increased. The minimum aromatic cluster sizes estimated from spectral analysis increased from 8 carbons in 300 °C char, to 20, 18, 40, 64, and 76 carbons, respectively, in 350 °C, 400 °C, 500 °C, 600 °C, and 700 °C chars. 1H–13C long-range dipolar dephasing displayed the same increasing trend of aromatic cluster sizes of wood chars with increasing temperature. We show for the first time quantitative changes of different aromatic C forms and aromatic cluster size as a function of heat treatment temperature.