Determination of chemical modification of eucalypt kraft lignin after thermal treatment by Py-GC–MS

Abstract

Kraft lignin is considered a potential valuable by-product in the pulp industry within the biorefinery concept. However, kraft lignin is characterized by a complex chemical structure, having a low reactivity and a high heterogeneity. Technical options to increase the reactivity of eucalypt kraft lignin are important for the manufacture of lignin based products. This study evaluates the effect of thermal treatment on the chemical composition of extracted kraft lignin from eucalypt kraft black liquor. The black liquor was heat-treated at temperatures of 175, 200 and 225 °C at reaction times of 30, 90 and 150 min. The lowest process yield (93 %) was found for the most drastic treatment conditions (225 °C and 150 min), with a minimum thermal degradation of the liquor, analyzed by TG/DTG curves. The lignins from the original liquor and the heat-treated liquors were extracted and characterized by their chemical composition, elemental analysis, heating value and Py-GC–MS to evaluate the changes caused by the thermal treatment. The thermal treatment promoted demethylation and demethoxylation on the eucalypt kraft lignin. A reduction in soluble lignin, oxygen and hydrogen content as well as an increase in insoluble lignin and carbon content were observed when higher temperature and retention time in the thermal treatment were applied. A reduction in the lignin S/G ratio of heat-treated lignins and an increase in the catechol and methoxycatechol content for the treatment of 225 °C and 150 min were also observed when compared to the reference. The eucalypt kraft lignin isolated from the heat-treated liquor may be more reactive than the original lignin due to changes in its chemical composition and molecular structure. This lignin should be considered in the production of high value-added lignin-based products.