A naturally occurring catechyl lignin (C-lignin) was recently discovered in the seed coats of vanilla orchid and cactus species. C-lignin is biosynthesized via oxidative polymerization from a sole monolignol, caffeyl alcohol, with benzodioxane as the exclusive linkages. Compared to traditional G/S lignin derived from wood and grass plants, C-lignin showed acid-resistant properties, which, together with its homogeneous structure, render it an ideal lignin archetype for valorization. For example, C-lignin can be depolymerized into a simplex catechol product in high yield, being superior to G/S lignin in terms of both activity and selectivity.<br/>Although C-lignin was identified as the only lignin specie in the seeds of vanilla and some cactus species, the low output and high cost (such as vanilla, $350–500 kg<sup>−1</sup>, 8000 tons per year) will hardly meet the requirements of future industrialization. Recently, C-lignin was also discovered in some seed coats of Euphorbiaceae plants, the cheap and plentiful feedstocks. However, coexistence of C-lignin and G/S lignin was detected in these seed coats. Herein, we reported our recent results on C-lignin chemistry, which involved the purification, characterization and depolymerization. We have developed different disassembled methods to separate C-lignin and G/S coexisting in Euphorbiaceae seed coats, and characterizations revealed that the isolated lignin samples were entirly composed of benzodioxane units. The disassembled C-lignin samples can produce a single catechylpropanol product (selectivity up to 97%) through Pd/C-catalyzed hydrogenolysis. We also developed an atomically dispersed Ru catalyst, which was capable to cleave all C−O bonds in benzodioxane structures efficiently and circumvent the reduction of C=C bonds selectively, thus depolymerizing C-lignin biopolymer into propenylcatechol preferentially. The derivatization of C-lignin-derived catechols with a unique end-chain into valuable products (such as intermediates of pharmaceutical) would also be discussed.

Purification, characterization and depolymerization of catechyl lignin in Euphorbiaceae seed coats

ACS Spring 2022

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