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Premethylation of Lignin Hydroxyl Functionality for Improving Storage Stability of Oil from Solvent Liquefaction

Kim, Jae-Young, Hafezi-Sefat, Parinaz, Cady, Sarah, Smith, Ryan G., Brown, Robert C.
Energy & fuels 2019 v.33 no.2 pp. 1248-1255
accelerated aging, carbonates, dimethyl sulfate, ethanol, fuels, gas chromatography-mass spectrometry, lignin, liquefaction, methylation, moieties, oils, phenols, solvents, storage quality
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 because of suppressed charring reactions. Oil produced from SL had relatively higher Phe-OH, Aliph-OH, and carboxyl content 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 was observed by gas chromatography–mass spectrometry 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 compound yields before and after accelerated aging among the oils, which suggests that repolymerization mainly occurred between oligomeric lignin fragments.