Kraft lignin behavior during reaction in an alkaline medium

The reaction of kraft lignin in an alkaline medium was studied using a microreactor set. Chemical changes in reacted kraft lignins that include ash content, Klason lignin, acid-soluble lignin and sugars were studied. Structural characteristics of these lignins were also determined in terms of phenolic-hydroxyl, aliphatic-hydroxyl, methoxyl, Mw, Mn and polydispersity. The techniques employed were HPLC, UV spectroscopy, FTIR spectroscopy, proton nuclear magnetic resonance spectroscopy and organic gel permeation chromatography (GPC).The effects of temperature and reaction time on lignin properties were studied using response surface methodology. The reaction temperature ranged from 116 to 180 °C and the reaction time ranged from 18 to 103 min. The obtained response surfaces show that both factors affected lignin properties within these ranges. The phenolic and aliphatic-hydroxyl content and the number of active sites increased when the treatment severity was increased. Weight-average molecular weight (Mw), number-average molecular weight (Mn) and solid-yield percentage decreased when the treatment severity was increased.

► We studied the chemical and structural changes of Kraft lignin during its reaction in alkaline medium.
► The phenolic and aliphatic hydroxyl content and the number of active sites increased when the treatment severity was increased.
► Weight-average MW, number-average MW and solid-yield percentage decreased when the treatment severity was increased.