THE EFFECT OF CATIONS ON THE THERMAL-DECOMPOSITION OF LIGNINS

The thermal decomposition of three lignosulphonates, three sodium salts of a spruce milled wood lignin (MWL) and a mixture of MWL and NaCl was studied by thermogravimetry/mass spectrometry (TG/MS). The TG curves and the evolution profiles of degradation products showed that the lignosulphonate cations (H+, NH4+, Na+) play a key role in the decomposition. The sulphonate groups degrade at low temperature and give rise to a large amount of SO2 in the case of lignosulphonic acid and NH4-lignosulphonate, whereas Na-lignosulphonate releases only 10-20% of the theoretical yield Of SO2. The catalytic effect of sodium was studied on spruce MWL in detail. Sodium enhances dehydration, demethoxylation, decarboxylation and char formation, whereas it decreases the yield of organic volatiles and CO. Tentative reaction routes are proposed for the rationalization of the influence of sodium on the thermal decomposition of lignins.