Study on structure and thermal stability properties of lignin during thermostabilization and carbonization

• Evolution of soda lignin composition during thermostabilization was analyzed.
• Evolution of structures and mechanism during thermostabilization were expounded.
• How thermostabilization structures affect turbostratic structure was explained.
• The lignin thermostabilized at 260–290 °C obtained higher overall yields.

Soda lignin was first thermostabilized prior to carbonization. The composition, structures and thermal properties of treated lignin were investigated by EA, TGA–MS, 13C NMR, TGA, and Raman. EA and TGA–MS results showed that the hydrogen content decreased continuously and the oxygen content increased up to 260 °C then decreased and the carbon content increased passively during thermostabilization. 13C NMR results revealed that thermostabilization could be divided into three stages according to the evolution of the oxygenated structures content: <260 °C, 260–290 °C, >290 °C. Raman analysis showed that R values at each carbonization temperature were concave-down and non-symmetrical parabolic type and the 260 °C thermostabilized lignin after 1400 °C carbonized obtained the minimum R value 1.84. TGA results indicated that overall yields reflecting thermal stability of the thermostabilized samples, especially thermostabilized at 260–290 °C, were higher than that of untreated lignin.