Estimation of Hardwood Lignin Concentrations by Uv Spectroscopy and Chlorine Demethylation

Major projects are underway in our laboratory focusing on mildly acidic (pH>3) and alkaline (pH<10) pretreatments of hardwood chips prior to incineration for electric power or prior to pulping for paper manufacture. Production of lignocellulosic ethanol from the hemicelluloses in the hydrolyzates will be attempted. It is of great interest to quantify the concentrations of lignin in these hydrolyzates, since lignin fragments are suspected as fermentation inhibitors. UV spectroscopy is normally used to estimate the concentration of aqueous soluble lignin. However, the 203 nm absorbance gave unreliable results for these hydrolyzates, and on some occasions the 278 nm absorbance was unduly influenced by high absorbance in the 260-265 nm range. A credible method that uses chlorination to generate methanol from the methoxyl groups in lignin will be described. Model compound experiments showed that syringyl lignin units, with two methoxyl groups, gave a methanol yield of ~1.0 mmole/ mmole of aromatic rings.