Valorization of poplar, spruce and beech shredded wood : Lignin extraction and characterization

Abstract

This report is based on the extraction of lignin from by-products of the Walloon wood industry using green processes (meeting the standards of green chemistry). The woody materials studied correspond to shredded wood of beech, spruce, poplar and a mixture of species supplied by Groupe François, which wishes to develop these by-products. A percentage of lignin varying between 22 and 29 % (w/w) of the dry mass was found in the different batches studied. 4 types of pretreatments were tested: 2 alkaline extractions (NaOH 3 % (w/v) and Ca(OH)2 3 % (w/v)) and 2 acid extractions (H2SO4 3% (w/v) and H3PO4 3% (w/v)). The chemical composition of each of the phases produced during the pretreatments (a solid and a liquid phase after the acid pretreatments and a solid, a liquid and a precipitated phase after the alkaline pretreatments) was determined in order to envisage a utility for each of these phases. Specifically, the composition and mass of each of the phases involved in the pretreatments with NaOH and H2SO4 of spruce wood is presented in this document. In fact, these pretreatments are the most effective ones for each of the extraction pathways studied, according to the parameters chosen in this report, namely the recovery or extraction yield (the proportion of lignin found in the wood before pretreatment that was recovered after pretreatment, the recovery yield concerning the acid pretreatments and the extraction yield concerning the alkaline pretreatments) and the purity of this lignin. Thus, the water consumption of these 2 pretreatments was studied, as well as the possible uses of the related fluxes, according to their chemical composition. The lignins obtained by alkaline pretreatment were then characterized by elemental analysis, infrared, high performance steric exclusion chromatography by ultraviolet and electrophoresis. After determining the absence of sulfur and nitrogen in the precipitated lignins, lignin structural analyses showed little differences between the lignins obtained from the 3 treated wood species (beech, poplar and spruce). A special study has been made concerning the lignin molecular weight determination. This determination by HPSEC-UV analysis strongly depends on the hydrodynamic volume. However, this volume does not only depend on the lignin weight, but also on its chemical functions composition. Thus, to avoid this problem, a lignin molecular weight determination by SDS-PAG electrophoresis was carried out. The SDS (sodium dodecyl sulphate) was used to eliminate the involvement of non-covalent bonds on the hydrodynamic volume. This method seems to corroborate the results obtained by HPSEC-UV. However, the use of sodium polystyrene sulphonate as mass standards does not make it possible to establish a calibration curve. The determination of molecular weight by electrophoresis is rarely used in the study of lignin, it must be further improved to show a level of accuracy and precision, equivalent or superior to those of the HPSEC-UV analysis.