Abstract:

This paper investigated the evolution of chemical structure of the chars prepared by carbonization of enzymatic hydrolysis lignin and the urea-modified and melamine-modified lignin. Fourier transform infrared spectra and X-ray photoelectron spectra were collected for these carbonized lignin in the temperature range of 300-900 ℃. Based on these spectra, a comprehensive analysis was conducted in order to elucidate the evolution in chemical functional groups and the oxygen and nitrogen-containing groups involved in these lignin-derived chars. The results showed that, 600-700 ℃ is an important carbonization temperature range where the remarkable evolution of the oxygen-containing and nitrogen-containing groups happens, independent of the lignin modification by urea or melamine. Below 600 ℃, carboxyl, carbonyl and hydroxyl predominate in the oxygen-containing groups of the carbonized lignin; above 600 ℃, the hydroxyls and carbonyls are dominant, of which hydroxyls are the most. For the urea-modified and melamine-modified lignin, the pyridine-like and pyrrole-like groups are the predominant nitrogen-containing groups in the carbonized lignin before 600 ℃. Above 700 ℃, with the increase in the carbonization temperature, pyrrole-like groups were gradually transformed into quaternary species and, as a result, the species of the pyridine-like, pyrrole-like, quaternary nitrogen become a predominant nitrogen groups in the carbonized lignin chars.

Key words: lignin, carbon materials, electrocatalysis, urea, melamine, chemical structure