Organosolv pretreatment for enzymatic hydrolysis of poplars: isolation and quantitative structural studies of lignins

Abstract

Lignins could become important sources of chemical feedstocks and fuels if fractionation of the polymeric constituents of lignocellulosic materials could be improved. To utilize these polymeric components, it is important to understand their composition, chemical characteristics, and properties as a function of how the separation of the polymers is conducted. This paper presents characterization of several lignins as a function of fractionation method and isolation. Organosolv lignins were isolated from pulping liquors obtained by pulping Populus tremuloides (aspen) at 165 °C for 1–2.5 h with methanol: water (70 : 30 by volume). Pulping catalysts were various concentrations of mineral acids (H2SO4 or H3PO4) or salts (NaHSO4). Lignins were isolated by: (1) precipitation through the addition of water of the major lignin fraction (water–insoluble lignin) and (2) solubilization in acetone to yield a small fraction of acetone-soluble lignin. The characterization of these materials was performed through extensive quantitative 13C-NMR and by use of the Distortionless Enhancement by Polarization Transfer (DEPT) NMR sequence to assess the major types of structures formed under these conditions. In addition infra-red spectral data obtained using the DRIFT technique were employed to investigate a few of the structural characteristics. Simple functional groups were also analyzed by chemical methods for comparison. Correlation of differences in the structures of the organosolv lignins was made with the severities of the organosolv pretreatments which were expressed in terms of combined severity factors that incorporated time, temperature, and catalyst effects (pH of the pulping liquor). At low severity, the alkyl aryl ether bonds, the predominant bonds in native lignin, were hydrolyzed leading to materials with low ether bond content, high phenolic hydroxyl content, some methanol incorporation at the side chain, and a small amount of carbon-carbon condensed structures. Quantitation of minor structures such as syringaresinol and p-hydroxybenzoate is given, and the key types of structures present. As the severity increases, a higher extent of carbon-carbon condensation and side-chain degradation was observed while the ether bonding content was decreased. These lignins have low apparent weight-average molecular weights and low polydispersities. The lignins isolated from high severity organosolv pretreatments were completely different from native lignins. The degree of side chain degradation was high, and the number of carbons conventionally employed to describe a lignin repeating unit was substantially decreased from nine to eight or less.