In this study, we methylated hydroxyl groups (phenolic hydroxyl: Phe-OH and aliphatic hydroxyl: Aliph-OH) in soda lignin (SL) prior to solvent liquefaction to improve storage stability of the resulting oil. We investigated two methylating reagents, dimethyl sulfate (DMS) and dimethyl carbonate (DMC), for selective Phe-OH and total hydroxyl group (Phe-OH and Aliph-OH) blocking. Samples of SL, DMS-SL, and DMC-SL were depolymerized into oils under supercritical ethanol (350 °C). Both methylated lignins produced higher amounts of oils and smaller amounts of char compared to untreated SL due to suppressed charring reactions. Oil produced from SL had relatively higher functional group contents than other oils from methylated lignins. Simulated one-year aging of oils was carried out and this revealed Mw increase of oil was the highest in SL (11.8 %), followed by DMC-SL (6.8 %) and DMC-SL (4.6 %). Interestingly, these increases showed linear correlation with decrease in Phe-OH after accelerated aging. Meanwhile, compositional difference in monomeric phenols in the oils were observed by GC/MS analysis. For example, methoxylated aromatics such as 4-ethylveratrole, 4-vinylveratrole, methylisoeugenol, and veratryl-2-propanone were detected only in oils from DMS-SL, and DMC-SL. Interestingly, there was no significant difference in monomeric compounds yields before and after accelerated aging among the oils, which suggests that repolymerization mainly occurred between oligomeric lignin fragments.
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