Tetrabutylammonium Hydroxide 30-Hydrate as Novel Reaction Medium for Lignin Conversion

The efficient conversion of lignin, a major component of lignocellulosics, to monomeric aromatics is a hot topic in biorefining. In this study, we developed a method for selective degradation of several types of lignin [milled wood lignin, sodium lignosulfonate, soda lignin, and Japanese cedar (Cryptomeria japonica) wood flour] in Bu4NOH·30H2O (mp 27–30 °C). Degradation at 120 °C for 43–70 h gave low-molecular-weight (MW) compounds such as vanillin, vanillic acid, acetoguiacone, and p-hydroxybenzaldehyde in considerable yields. The total yields were 16.3, 6.5, 6.7, and 22.5 wt % from milled wood lignin, sodium lignosulfonate, soda lignin, and wood flour, respectively. Similar degradation in aqueous NaOH solution with the same OH– concentration (1.25 mol/L) as that in molten Bu4NOH·30H2O gave much lower yields of these products. This suggests that in addition to the effect of Bu4NOH·30H2O as a strong alkali, the Bu4N+ cation increased the selectivity of lignin degradation for low-MW products. Degradation under N2 gave significantly lower yields of the low-MW products, even in Bu4NOH·30H2O, suggesting that aerobic oxidation is involved in the formation of low-MW compounds and oxidation is affected by the presence of the Bu4N+ cation.