Thermal decomposition of milled wood lignins studied by thermogravimetry/mass spectrometry

Abstract

Thermogravimetry/mass spectrometry has been applied to characterize 16 milled wood lignins isolated from grasses, softwoods and hardwoods. The samples were thermally decomposed in inert atmosphere using a heating rate of 20°C min −1. The effect of acetylation as well as ZnCl 2 and NaCl as catalysts on the thermal decomposition has been studied. The weight loss and evolution profiles of the most abundant degradation products were monitored as a function of temperature. Correlation has been found between the intensity of several products and the compositional parameters of lignins determined by wet chemical methods. The modification of OH groups by acetylation changed the decomposition patterns, but the acetic acid produced by thermolysis had no catalytic effect on the formation of monomers. The evolution profiles of H 2O and CO 2 from the original and acetylated lignins proved that free OH groups promote the scission of COOH groups. The additives ZnCl 2 and NaCl have different influence on the product distribution. NaCl promotes dehydration, demethoxylation and recombination of the primarily formed radicals, although the maximum of the evolution profiles is not shifted significantly. However, water and formaldehyde formation shifts to 60–80°C lower temperature in the presence of ZnCl 2 indicating the occurrence of ionic decomposition mechanisms.