Cleavage of ethers and demethylation of lignin in acidic concentrated lithium bromide (ACLB) solution

Abstract

The methoxyl group is the most abundant functional group of lignin and affects the properties, reactivity, and application of lignin. Efficient demethylation is always of interest in the area of lignin chemistry and application. This study demonstrated a new method for cleaving ether compounds and demethylating lignin in acidic concentrated lithium bromide (ACLB) solution under mild conditions. It was found that the ACLB system could universally cleave ether compounds except for diaryl ethers. The study on lignin model compounds (creosol, syringol, and 1,2,3-trimethoxybenzene) verified that ACLB could demethylate them to corresponding phenols. Four real lignin samples produced from various sources by different methods were also efficiently demethylated by 69–82% in ACLB. The lignin demethylation resulted in more phenolic hydroxyl groups, which benefits some downstream applications of lignin. This study also provided new insights into the cleavage of the ether bonds in lignin. In addition to the methyl–aryl ether bond, ACLB could cleave other ether bonds of lignin in β-O-4, β-5, and β-β structures except for the 4-O-5 bond in the diphenyl structure. The ether bonds were cleaved via the S_N2 substitution except for the β-O-4 bond, which was primarily cleaved via the benzyl cation and enol ether intermediates, leading to Hibbert's ketones. Some of the β-O-4 structures were transformed into benzodioxane (BD) structures, which were stable in the ACLB system.