Cleavage of ethers and demethylation of lignin in acidic concentrated lithium bromide (ACLB) solution
Methoxyl group is the most abundant functional group of lignin, which affects the property, reactivity, and application of lignin. Efficient demethylation is always of interest to 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 the 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 the ACLB. The lignin demethylation resulted in more phenolic hydroxyl groups, which benefits some downstream applications of the lignin. This study also provided new insights into the cleavage of the ether bonds in lignin. In addition to the methyl-aryl ether bond, the ACLB can cleave other ether bonds of lignin in β-O-4, β-5, and β-β structures except for the 4-O-5 bond in diphenyl structure. The ether bonds are cleaved via the SN2 substitution except for the β-O-4 bond, which is primarily cleaved via the benzyl cation and enol ether intermediates, leading to the Hibbert’s ketones. A portion of the β-O-4 structures is transformed into benzodioxane (BD) structures, which are stable in the ACLB system.