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14 декабря, 2021
Various solid-state 1 3C NMR techniques, such as CP/MAS and occasionally DP technique, have been employed to identify carbon functionality and aromaticity of biochars [1, 3, 7, 8, 10, 11, 13, 19, 23, 26]. In these studies, chemical structure of biochars to a greater extent depends upon the pyrolysis temperature, but is not much affected by heating rate and the nature of biomass [1, 13]. Biochars prepared at relatively low temperatures up to ~350°C retain spectral features of the original ligno — cellulosic composition of biomass [25]. For example, characteristic peaks of cellulose (0-alkyl carbons around 62, 72, 84 ppm, and di-O-alkyl carbons around 103 ppm), as well as those of lignin (methoxyl carbons ~57 ppm, aromatic carbons ~130 ppm, and aromatic C-O ~150 ppm) can still be observed in spectra of biochars within this temperature range (<350°C). For higher heat treatment temperature (HTT) biochars, a well-defined aromatic resonance evolves simultaneously with the decrease of the lignocellulosic resonances and signals of aliphatic, carboxyl, and carbonyl carbons. Lignin structures are more thermally stable than cellulose structures because characteristic peaks of lignin such as the phenolic shoulders around 150 ppm can survive even at temperatures about ~550°C [25]. For HTT about 600°C upwards, the general shapes of the 13C NMR spectra of biochars are very alike, with a strong broad resonance line near 128 ppm in the aromatic region [1, 8, 22, 25].
Chemical transformation of lignocellulosic materials into graphitic structures with the temperatures between 800 and 1,000°C is well documented [1, 13, 25]. Aromatic cluster typical of chars is also reported to grow with increasing HTT [21].