Abstract:

Three series of lignin-based chars were prepared from three raw materials, hardwood enzymatic hydrolysis lignin and their urea-modified and melamine-modified counterparts, through carbonization in the temperature range of 300-900 ℃ in order to elucidate the microstructure evolution of lignin chars. X-ray diffraction, nitrogen adsorption, electric resistivity analysis and scanning electron microscope were employed to characterize the graphite-like microcrystallite structure, electric conductivity and pore structure of their lignin-based chars. The results consistently showed that when the carbonization temperature was increased from 600 ℃ to 700 ℃, the electronic resistivity of the chars was sharply promoted leading to the rapid transition of lignin-based chars from an insulator to a semiconductor, and the development of the graphite-like microcrystallite component in the chars was remarkably enhanced, whether lignin as raw materials was modified by nitrogen-containing substances or not. However, urea or melamine modification lead to a reduction of interlayer spacing in the graphite-like microcrystallites, with a more reduction for melamine modification of more nitrogen content. Moreover, urea or melamine modification could obviously promote the electron conductivity of the chars prepared below 700 ℃ but lessen a little for the char prepared above 700 ℃. Outstandingly, urea and melamine modification remarkably suppressed the development of pore structure of lignin-based chars, especially totally the formation of microporosity. Specifically, the surface area of the chars prepared from the unmodified lignin, urea-modified one and melamine-modified one have a big difference with the value of 524, 102 and 69 m²/g, respectively.

Key words: lignin, carbon materials, electrocatalysis, urea, melamine, microstructure