Process for olive tree pruning lignin revalorisation

Second generation bioethanol plants are going to produce tones of lignin and its utilisation focuses the interest of the research community. Revalorisation of lignin was studied by proposing an entire process scheme. Agricultural residues are an interesting lignocellulosic raw material due to their high abundance and low price. However, olive tree pruning is undervalorised although it constitutes an attractive raw material. Starting from olive tree pruning, lignin was dissolved by treating the raw material in a water–ethanol mixture. The resulting black liquor was subjected to ultrafiltration in order to fractionate the dissolved lignin by molecular weight using different membrane cut-offs (300, 150, 50, 15 and 5 kDa). Lignin of each permeate was isolated and analysed to evaluate its composition and molecular weight. Afterwards, ultrafiltered lignin fractions were subjected to base catalysed depolymerisation in order to produce high value added products. Obtained products (oil, coke and residual lignin) were analysed in order to determine their composition and yield. Catechol and 4-methylcatechol were the main monomeric products in all ultrafiltrated lignin fractions but other interesting compounds were obtained such as phenol and cresols. Ultrafiltration was proved to produce specific molecular weight lignin fractions that presented different reactivity when applying base catalysed depolymerisation conditions. Furthermore, ultrafiltration process allowed lignin purification by removing hemicellulosic sugars.

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► Entire proposed process for lignin revalorisation into chemicals.
► Ultrafiltration allowed lignin purification and fractionation.
► Base catalyzed lignin depolymerisation mainly produced catechol and 4-methylcatechol.
► Lignin molecular weight was proved to be related with BCD efficiency.