Disassembly of catechyl lignin from castor shells by maleic acid aqueous and production of single catechols

Catechyl lignin (C-lignin) is recognized as a uniform and straight biopolymer that stands out as an exemplary archetype of what is often referred to as "ideal lignin." This difference is due to its unique ability to efficiently depolymerize into a single catechol product. This remarkable property makes C-lignin suitable for further applications and highlights its potential for the production of chemicals. In this study, a protocol utilizing maleic acid aqueous (MAA) was developed for the effective disassembly of coexisting C-lignin and G/S-lignin in castor shells under mild conditions. Thioacidolysis combined with nuclear magnetic resonance (NMR) spectroscopic analyses revealed a significant enrichment of benzodioxane units, without observation of β-O-4 structures from G/S-lignin. The mechanism underlying selective disassembly of coexisting C-lignin and G/S-lignin in castor shells was comprehensive elucidated by analyzing the chemical structure change of benzodioxane and β-O-4 lignin model compounds. Furthermore, the structural characteristics of the disassembled C-lignin were synthetically evaluated, highlighting its narrow molecular weight distribution (M_w = 2432–2763 Da) and high hydroxyl content (aliphatic OH = 7.7–8.4 mmol/g; catechol OH = 5.7–7.5 mmol/g). Compared with the feedstock containing C-lignin and G/S-lignin, the extracted C-lignin sample showed exceptional catalytic activity and selectivity. The results show that the Pd/C-catalyzed hydrogenation reaction mainly produces catechylpropanol compounds (selectivity as high as 97 %).