Lignin depolymerization for phenolic monomers production by sustainable processes

Biomass wastes (almond shell and olive tree pruning) were used in this work as raw materials for the extraction of high purity lignin by different delignification methods. A pretreatment stage was carried out to remove the major hemicelluloses content in the solid feedstocks. Afterward, two sulfur-free pulping processes (soda and organosolv) were applied to extract the largest fraction of lignin. The extracted lignin contained in the liquors was isolated using selective precipitation methods to design a tailor-made technique for obtaining high-purity lignin (in all cases more 90% of purity was reached). Soda process allowed the extraction of more lignin (around 40%-47%) than organosolv process (lower than 20%) regardless of the lignocellulosic source employed. Once the different lignin samples were isolated and characterized, they were depolymerized for the obtaining of small phenolic compounds. Three main streams were produced after the reaction: phenolic enriched oil, residual lignin and coke. After the purification of these fractions, their quantifications and characterization were conducted. The most abundant product of the reaction was residual lignin generated by the undesirable repolymerization of the initial lignin with yields around 30%-45%. The yield of the stream enriched in phenolic oil was higher than 20%. Coke, the lowest added-value product, presented a yield lower than 12% in all the cases. Lignin from organosolv presented higher phenolic oil yields, mainly due to their lower molecular size. This parameter was, thus, considered a key factor to obtain higher yields. (C) 2017 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.