Production of lignin derived value-added compounds from kraft black liquors by electrochemi̇cal method

Every year, millions of tons of black liquor are generated as paper production waste and burned as low-value fuel. This study aims to transform black liquor, a waste material, into valuable chemicals such as vanillin with electrochemical analysis. With this purpose, the black liquor was obtained by Kraft cooking from the Pinus brutia Ten. (PB) and Populus tremula L.(PT) woods were used as material. The suitability of different solvents (chloroform, toluene, dichloromethane), acids (H3PO4, H2SO4, HCl), and pH degrees (2, 5.5, 7, 9) for extracting phenolic compounds from black liquor was tested. It was concluded that dichloromethane was the most suitable solvent for extraction of most of the phenolic compounds detected by HPLC (syringic acid, caffeic acid, epigallocatechin-3-gallate, t-ferulic acid, t-resveratrol, luteolin) from black liquor. Viscosity measurements of black liquor prepared under different acidic conditions showed that PB samples (1.8-24.4 cP) were more viscous than PT samples (1.57-3.63 cP). In HPLC analysis, it was determined that the black liquor contained p-hydroxybenzoic acid (819.5 mg/L), vanillin (467.7 mg/L), and syringaldehyde (438.1 mg/L), and vanillin was obtained with the highest efficiency in PB-H2SO4-pH9 sample. In the GC-MS analysis performed after liquid-liquid extraction with dichloromethane, it was determined that the PB black liquor sample was rich in resin and fatty acids, organic acids, vanillin, vanillic acid, acetovanillone, guaiacol, homovanillyl alcohol, and catechol, and PT samples were rich in syringaldehyde, syringic acid, phenol, p-hydroxybenzoic acid and benzoic acid. Vanillin, which was selected as the target substance, was obtained with the highest efficiency in PB-H2SO4-pH 5.5 (18.2%), and PT-H3PO4-pH 7 (5.36%) samples. In electrochemical analyses performed with Potentiostat/Galvanostat, the effects of the use of deep eutectic solvents (DESs) as electrolytes by using Choline chloride:Ethylene glycol (KK: EG), Choline chloride: Lactic acid (KK: LA), Lactic acid: 1,2-propanediol (LA: PR) in the electrochemical depolymerization of kraft lignin, and the number of alternating voltammetry (CV) scans on the production of valuable phenolics were investigated. In the HPLC analyses performed after CV scans in the potential range of -1.8 V to +1.8 V, syringaldehyde (234.03 mg/L) was obtained as the main product, and vanillin was obtained with the highest efficiency (6.83 mg/L) as a result of LA:PR 100 scanning. To determine the most suitable electrolyte, LA:PR solution was prepared at different water ratios (10%, 20%, 30%), and different pH degrees (2, 4, 6, 9), and CV scans were performed and it was seen that the most suitable environment for the electrochemical synthesis of lignin-derived compounds was LA:PR-pH 6 with 30% water content. CV scans of PT-Control, and PB-H2SO4-pH 9 lignin samples were performed in the range of 0 to +1 V in 30% aqueous LA:PR-pH 6 medium for 1-8 hours, and according to the results of HPLC analysis, an increase in the amounts of vanillin, fumaric acid, vanillic acid, caffeic acid, syringic acid, syringaldehyde, and guaiacol compounds was increased. Vanillin was obtained from PB lignin (20.16 mg/L) with higher efficiency than PT lignin (14.97 mg/L). The amount of vanillin increased by 23% in the PT sample after 8 hours and by 1.5% in the PB sample. As a result of the electrochemical analyses, an increase in the yield of vanillin and lignin-derived compounds was achieved, and the usability of DESs as electrolyte media in the electrochemical depolymerization of lignin was supported by this study.