Catechyl Lignin Extracted from Castor Seed Coats Using Deep Eutectic Solvents: Characterization and Depolymerization

Abstract

Catechyl lignin (C-lignin), a homobiopolymer usually existing in the seed coats of Vanilla and Cactaceae plants, was recognized as an “ideal lignin” archetype leading to well-defined aromatic compounds. Herein, we report that deep eutectic solvents (DESs) can serve as green and recyclable solvents to extract C-lignin from cheap and abundant castor seed coats for the first time, which led to C-lignin biopolymer in good yields with high purity. High abundance of benzodioxane structures in DES-extracted C-lignin samples was corroborated by HSQC NMR experiments. Catalytic depolymerization of ChCl/LA DES C-lignin with Pd/C resulted in monomeric catechol derivatives in 29.6 wt % (or 81 mol %) yields with 86% selectivity toward catechylpropanol. This study provided a green and cost-effective C-lignin extraction procedure from bulk sustainable feedstock, thus paving the way for C-lignin valorization into unique catechol compounds.