Abstract
The SUNY-ESF biorefinery protocol begins with the pre-treatment of hardwoods with hot-water (160 °C, 2 hours) to extract fermentable hemicellulosic sugars. However, this process also releases some lignin/lignin degradation products that are fermentation inhibitors. Lignin is a complex, amorphous polymer formed primarily by oxidative combinatorial coupling of phenylpropanoids. Among the predominant side-chain linkages in lignin, the most reactive are the alkyl-aryl ether linkages. They are susceptible to cleavage under acid conditions; and the ESF biorefinery hot-water extraction (HWE) protocol is inherently a `mildly' acidic process. To determine if the alkyl-aryl ether linkages are cleaved in this pre-treatment process, model compounds and milled-wood lignin were subjected to HWE. Sugar maple (Acer saccharum) and two paulownia (Paulownia elongata and tomentosa) wood species were used in this study. Studies on the effect of irradiation on wood prior to HWE were also initiated. From the model compounds utilized in this study it was shown that the β-O-4 type alkyl-aryl ether linkage is cleaved whereas the β-5/α-O-4 type linkage, also known as phenylcoumaran, remains largely unaffected. These studies also showed that the lignin fraction separated from the hot-water extracts of sugar maple was composed mainly of syringyl units and had relatively high free phenolic hydroxyl group content. Two-dimensional nuclear magnetic resonance spectrometry (2D NMR) was employed to further understand the effect of HWE on lignin samples, which included milled-wood lignin isolated from sugar maple before and after HWE. Dibenzodioxocin structure containing the alkyl-aryl ether linkages was shown to have cleaved during HWE based on 2D NMR. Gas chromatography-mass spectrometry (GC-MS) was used to identify the low-molecular weight phenolic compounds generated during this process. Antioxidizing activity of different fractions isolated in this study was shown to be comparable to that of vitamin C. 3-hydroxytyrosol, a potent antioxidant, was identified in the hot-water extract of P. tomentosa.