Effect of Counter Cation on Alkaline Reaction of β-O-4-Type Substructure in Lignin

This study aimed to clarify the effects of counter cations on the alkaline-induced β-O-4 bond cleavage and further reactions of β-O-4-type substructures in lignin. For this purpose, a non-phenolic β-O-4-type lignin model compound, the erythro isomer of 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (veratrylglycerol-β-guaiacyl ether), was treated in a 100% water solution or an aqueous methanol, ethanol, or 1,4-dioxane solution containing LiOH, NaOH, or CsOH as an alkaline source at 150 °C. The rates of β-O-4 bond cleavage were in the order of CsOH > NaOH > LiOH in all solvents. This order can rationally be attributed to the strength of the interactions between HO– and the counter cations. Because Cs+ has the lowest positive charge density among the counter cations and hence interacts with HO– most weakly, HO– can exert its reactivity most actively in the reactions using CsOH. We also discuss how the counter cations affect the profile of reaction products.