Deconstructing poplar lignin from ionic liquid pretreatment for biological conversion through sulfonation and Fenton chemistry

Abstract

Generating value from lignin through deconstruction and biological conversion is promising but limited by several factors including lack of economically viable deconstruction methods and low bioconversion of the breakdown products. Due to the complex chemical structure of natural lignins, high yield deconstruction requires cleaving both carbon–carbon and ether bonds. The high strength of C–C bonds poses a great challenge for economically viable high conversion of lignin to valuable products or intermediates. Prior work has shown that a Fenton reaction can efficiently cleave C–C bonds in sulfonated polymers at or near room temperature. In the present work, poplar lignin isolated from a cholinium lysinate ionic liquid pretreatment was sulfonated and then treated with a Fenton reaction using conditions that minimized H₂O₂ and avoided unwanted repolymerization. The deconstruction process was performed at room temperature and ambient pressure. We explored the tradeoff between the extent of deconstruction and the amount of carbon lost as CO₂, with total carbon recovered as soluble products ranging up to 40% depending upon conditions. The reaction products were analyzed by size exclusion chromatography, infrared spectroscopy, total dissolved organic carbon and elemental analysis. The results indicated that the products are rich in acid, aldehyde, alcohol, and sulfonate functionalities. A panel of microorganisms were tested for growth using the lignin breakdown products as the sole carbon source and five showed robust growth. A bisabolene-producing strain of Rhodosporidium toruloides was used to demonstrate conversion to product. Several ideas are discussed to improve yields for each step in the process.