The interest of using lignins as raw material for different chemical applications has importantly increased in the last years. The emergence of new processes allowing producing lignin from pulp mills at industrial scale such as Lignoboost and the green chemistry approaches explain this gain of interest. However, depending on vegetal sources (softwoods, hardwoods, annual plants or residues) and the extraction processes (kraft, soda, bisulfite, organosolv), lignins structure can be very different. Therefore, the initial step for considering the use of different lignins as source of chemicals, either as a macromolecule for green adhesives or as oligomers/monomers the chemical industry, is a detailed characterization of their chemical structures. To address such important topic, nine kraft lignins issued from different vegetal materials (softwoods, hardwoods, annual plants) isolated from alkaline black liquors by two methods (addition of CO2 or H2SO4) were characterized by different chemical and spectroscopic/spectrometric methods. Lignins purity from 87 to 96 %, C content from 50.2 to 65.2 %, O content from 27.2 to 34.4 %, H content from 4.4 to 5.7 % and S content from 2.1 to 4.2 %. Phenolic OH content was calculated using two methods: UV-vis difference spectra (15-31 OH per 100 C9 units) and 31P NMR after phosphitilation (16-45 OH groups per 100 C9 units). 31P NMR spectra also allowed quantifying different phenolic units (S, G or H units), condensed phenolic units, aliphatic OH groups, and carboxylic acids. Generally, aliphatic OH groups content is lower for hardwoods lignins (0.6-0.7 mmol/g) that the others (1-1.7 mmol/g). Important amounts of phenolic S units are logically observed for hardwoods, especially for eucalyptus (1.9 mmol/g). Softwoods lignins contain considerable amount of phenolic G (1.2-1.4 mmol/g) and condensed units (0.8-1.1 mmol/g). However, very surprisingly, annual plants lignins contain few amounts of phenolic H units (0.2 mmol/g). SEC analysis revealed the average molecular weight (Mw) ranges between 980-2546 g/mol. Finally, a fine analysis of the lignins units using a quantitative the QQ-HSQC 2D NMR sequence was performed revealing an important variety of structures. A synthesis of the results based on the analysis of the products issues from the wet chemistry methods and 2D NMR analysis will be presented as a guide for the users of lignin in adhesives or chemicals production applications.

Features of different lignin sources for adhesives and aromatic chemicals production

249th National Meeting (2015)

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