The Mannich reaction is utilized by lignin researchers both as an analytical technique and as a means for improving kraft lignin hydrophilicity and metal ion adsorptive capacity. Electrophilic substitution is conventionally agreed to take place at non-condensed C3 or C5 carbons of free phenolic lignin subunits. Recent studies have indicated the potential for amination yields greater than theoretical in the presence of acetic acid at elevated temperatures, yet the influence of pH on kraft lignin amination reaction mechanisms is not well understood. In this report, multiple analytical techniques (elemental analysis, 1H-13C HSQC, 13C, and 31P NMR spectroscopy, and model compound experiments) were used to elucidate the structures of different lignin amines derived from pine kraft lignin. The effect of lignin structure, amine species, and pH were investigated. Neither pH nor amine species were significant factors. The degree of amine substitution was two times higher for low molecular weight lignin compared to the high molecular weight fraction. New evidence was provided to confirm that amine substitution of pine kraft lignin takes place selectively ortho to non-etherified phenolic hydroxyl groups and at carboxylic acid groups. Other pathways were proposed and tested, but steric effects prevented substitution. Elemental analysis overestimated amine substitution likely due to the presence of nitrogen-containing side products and impurities. Total nitrogen content has been the most widely used indicator of Mannich condensation. The findings have implications for purification and characterization of modified technical lignins beyond the scope of this report as well as other complex, heterogeneous biopolymers.

“Practical and fundamental insights into technical lignin modification through the amination of pine kraft lignin”

ACS Spring Meeting 2021

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