Promotive or suppressive effect of co-existing nucleophiles on lignin condensation in alkaline pulping processes

This study aimed to quantitatively examine how the addition of a strong or weak nucleophile (1.0 mol/L) to a soda cooking treatment (1.0 mol/L NaOH) at 150 °C affects the disappearance and condensation reactions of a simple phenolic lignin model compound, vanillyl alcohol (Va). All the observed phenomena could basically be attributed to the adduct formation of each reagent with the quinone methide intermediate derived from Va (QMVa) and the stabilization of the adduct. The adduct is irreversibly stabilized to be its conjugate base when the reagent was MeNH2 or Me2NH, resulting in the accumulation of the adduct and consequent consumption of QMVa without the reformation of Va from QMVa and condensation reactions of Va with QMVa. The adduct is not stabilized to be its conjugate base and reproduce QMVa when the reagent was Me3N, CO32¯, or SO42¯ (the sodium salt), resulting in the increased concentration of QMVa to promote the condensation reactions of Va with QMVa. These suppressive and promotive effects were more visible in the same treatment of Va with each nucleophile without the addition of NaOH, in which the concentration of hydroxide anion was low.