Non-phenolic C₆-C₂- and C₆-C₃-type lignin model compounds with the β-O-4 bond were treated in tert-butyl alcohol (tBuOH) or dimethyl sulfoxide (DMSO) containing potassium tert-butoxide (KOtBu) or various other bases under mild conditions (at a base concentration of 0.5 mol/L and 30ºC) to examine how the reactions differ between these systems. The β-O-4 bond cleavage in KOtBu/tBuOH was slower than that in KOtBu/DMSO owing to the greater solvation of tBuO¯ in tBuOH than in DMSO. The β-O-4 bond cleavage of the erythro isomer of the C₆-C₃-type compound was slower than that of the threo isomer in all the reactions. This is explained by the preferred formation of a strong hydrogen bond between the α-hydroxy and the dissociated γ-alkoxy groups (or vice versa) of the former, which interferes with the cleavage. The rates of the β-O-4 bond cleavages in DMSO were in the order of the systems containing: NaOtBu ＞ KOtBu ≫ LiOtBu, which seems to relate to whether each base dissolves as an ion pair or free ions in DMSO. Those in DMSO were in the order of the systems containing KOtBu ≫ potassium hydride (KH) ≫ potassium iso-propoxide ＞ potassium ethoxide, which is consistent with their basicities except for KH.

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Aerobic oxidation of lignin in reaction media containing Bu₄NOH facilitates efficient production of vanillin (4-hydroxy-3-methoxybenzaldehyde) and vanillic acid (4-hydroxy-3-methoxybenzoic acid). This study presents production of these compounds from polyethylene glycol (PEG)-modified glycol lignin (PEG-lignin) from Japanese cedar (Cryptomeria japonica). A Bu₄NOH-based reaction medium was prepared by the addition of NaOH(s) to 1.25 mol/L Bu4NOH aq. Degradation of the PEG-lignin (14 mg) in 2.0 mL of the medium at 120 ℃ under O₂ in a sealed test tube produced vanillin and vanillic acid with 9.1 and 2.6 wt% yields, respectively, based on the Klason lignin amount of the PEG-lignin. The total yield of vanillin and vanillic acid reached 79.6 % of that in the corresponding alkaline nitrobenzene oxidation (14.7 wt%), indicating high performance of the medium. Bench-scale aerobic oxidation of the PEG-lignin (100 g /1.0 L) under O₂ pressurized at 0.7 MPa gave the products with their maximum yields similar to those in the above lab-scale experiment. Further increase in the O₂ pressure to 2.5 MPa significantly shortened the reaction time to achieve the maximum product yield. This pressure increase did not affect the vanillin yield, but suppressed the formation of O₂-sensitive vanillic acid, by which selective vanillin production was achieved.

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Nitrobenzene oxidation of lignin has long been one of the typical lignin chemical degradation methods. Previously, we reported a high-throughput and microscale protocol for the method. However, the reactor used in the protocol was available only in Japan and not supported outside Japan. In this study, we prepared an alternative protocol of alkaline nitrobenzene oxidation using an alternative reactor, which is available in many countries. Using the new protocol almost the same product yields as the previous protocol were obtained using a stable-isotope-dilution method. In addition, ethylvanillin, which is readily and commercially available, was also found to be usable as an alternative internal standard.

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