Thermochemical comparison of lignin separated by electrolysis and acid precipitation from soda black liquor of agricultural residues

Lignin samples were separated from soda black liquor of wheat straw and bagasse by electrolysis and also by acid precipitation. These were compared for their thermochemical characteristics by 1H NMR, TGA and DTA analyses. Lignins separated by electrolysis were found to have only 11–12 methoxyl groups per hundred phenylpropane units compared to 36–38 groups for acid precipitated lignins. They also had larger proportion of aromatic hydrogen and higher carbon to hydrogen ratio than acid precipitated lignins. All the lignin samples underwent a low temperature endothermic expulsion of adsorbed moisture. The distinctly exothermic nature of the thermal decomposition of acid precipitated lignins was missing for electrolytically separated lignins. Under comparable conditions, lignins separated by electrolysis could yield more amount of char compared to acid precipitated lignins.