Valorization of Grass Lignins: Swift and Selective Recovery of Pendant Aromatic Groups with Ozone
2018
Danby, Andrew M. | Lundin, Michael D. | Subramaniam, Bala
The lack of a simple process for valorizing the lignin produced during cellulosic ethanol manufacture is a major hurdle preventing such biorefineries from becoming economically sustainable. Here, we demonstrate a facile ozonolysis process in a continuous stirred tank reactor (CSTR) to generate a value-added product stream from lignin dissolved in either acetic acid (as in acetosolv lignin) or formic acid. At ambient pressure, relatively mild temperature (70 °C), and short residence times (few minutes), ozone is bubbled through the solution to selectively cleave the C═C bonds associated with specific pendant groups present in corn stover and wheat straw lignins. The resulting products, mainly vanillin and 4-hydroxybenzaldehyde, constitute approximately 7 wt % of the lignin and command high value with established applications in the flavoring, pharmaceutical, and electronics industries. These products are easily separated from the remaining lignin via membrane nanofiltration of the product mixture. The lignin resulting from ozone pretreatment and nanofiltration retains its polymeric structure and is available for further valorization. The lack of detectable secondary ozonide intermediates and the dominant formation of aromatic aldehyde monomers as the main products strongly suggest that the short chain carboxylic acid solvents may be acting as participating solvents, interrupting the classic Criegee mechanism.
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