The extraction of high-value chemicals from lignin in biomass is critical for biorefinery economics, so economical lignin depolymerization to produce aromatic monomers has been pursued for decades. While chemical treatment by alkaline oxidation is capable of producing high yields of monomers from isolated lignins, it does so only with very high base concentrations that must be neutralized post-reaction for extraction of phenolic monomers, which include valuable coproducts. When using fully soluble bases like NaOH or KOH, this means neutralizing the base and continually buying more or recycling all of the base with lime-cycle regeneration. The cost of the alkali and/or regeneration then outweighs the value of the monomeric products, even for products like the phenol vanillin valued at greater than $10/kg. Scientists at NREL have developed a method using reversibly soluble bases for alkaline oxidation of lignin and economical product recovery.
The treatment of lignin under alkaline oxidizing conditions generates a stream of aromatic monomers that can be upgraded to fuels or separated and sold as coproducts. Scientists at NREL have developed a method using Sr(OH)2 or Ba(OH)2 as reversibly soluble bases for simple and economical alkali recycling that is impossible with fully soluble bases or bases with low solubility at all temperatures. Due to the dramatic increase in solubility of Sr(OH)2 and Ba(OH)2 with increasing temperatures, at typical elevated reaction temperatures, these reversibly soluble bases provide the high hydroxide-ion concentrations needed for high monomer yields in the alkaline oxidation of lignin. However, these reversibly soluble bases also maintain the benefits of a mostly insoluble hydroxide source at room temperature; namely, cooling the post-reaction suspension to room temperature precipitates most of the Sr(OH)2 or Ba(OH)2, which can be removed by filtering. The remaining dissolved bases react with CO2 to convert soluble Sr and Ba to insoluble carbonate while neutralizing/acidifying the solution. The remaining solution is then extracted with an organic solvent and filtered again to recover the carbonate, which can be recycled by calcining to an oxide and contacting with water.
To learn more about Reversibly Soluble Bases for Lignin Oxidative Depolymerization, please contact Eric Payne at:
Applications and Industries
- Bio-based feedstocks for upconversion
NREL’s method for lignin oxidative depolymerization
- Provides an economical method for harvesting valuable aromatic monomers from biological lignin,
- Can produce increased concentrations of phenolic aldehydes, such as high-value vanillin, as coproducts, and
- Returns coproducts that can be separated as valuable chemicals or chemical precursors or upconverted to fuels.