Published in the RSC journal Energy & Environmental Science, the research demonstrates how lignin-derived compounds can first be converted to muconic acid via a biological process. Muconic acid can then be separated from the biological culture and catalytically converted into adipic acid. A patent application has been filed on this research, and the NREL Technology Transfer Office will be working with researchers to identify potential licensees of the technology.

Lignin is an alkyl-aromatic polymer present in plant cell walls for defense, structure, and water transport. Despite exhibiting a high-energy content, lignin is typically slated for combustion in modern biorefineries due to its inherent heterogeneity and recalcitrance, whereas cellulose and hemicellulose are converted to renewable fuels and chemicals. However, it is critical for the viability of third-generation biorefineries to valorize lignin alongside polysaccharides. To that end, we employ metabolic engineering, separations, and catalysis to convert lignin-derived species into cis,cis-muconic acid, for subsequent hydrogenation to adipic acid, the latter being the most widely produced dicarboxylic acid.

—Vardon et al.

The demonstration is an important step toward the goal of garnering more uses from lignin, which could be crucial for the economic success of the biofuels industry. It also shows that adipic acid can be produced from a renewable resource, which might have important ramifications for mitigating greenhouse gases.

Approximately 2.5 million tons of adipic acid are produced each year, mainly as a precursor for the production of nylon, and this global demand is expected to increase by up to 3.5% annually. Nylon is one of the most abundant materials in the world, used in everything from carpet and ropes to clothing and toothbrushes. With more than 4 million metric tons of nylon manufactured annually around the world, capitalizing on the production of adipic acid from lignin-derived aromatic molecules could one day have a significant impact on the bottom line of lignocellulosic biorefineries.

The current industrial pathway to produce adipic acid involves oxidation of cyclohexanol and cyclohexanone with nitric acid, which is a harsh process that produces nitrous oxide that must be cleaned up. The development of a new process toward converting renewable lignin feedstocks to adipic acid could potentially provide positive economic benefits and greenhouse gas offsets, but, certainly, major technology improvements must still be made.

While our research is still in the concept stage, this gets us one step closer to improving the technology to expand the slate of molecules that can be viably produced from lignin. As more lignocellulosic biorefineries come on line, large quantities of lignin will be generated, and valorization of these waste streams could eventually play a key role for the economic viability and environmental sustainability of biorefineries.

—Gregg Beckham, senior engineer at NREL and a co-author of the study

Lignin is one of the most abundant organic materials on Earth (second only to cellulose), and it potentially offers a vast, renewable source for the sustainable production of fuels, chemicals, and materials. Valorization of lignin has been shown by techno-economic analysis conducted at NREL to be essential to the development and success of the biofuels industry. However, because it is inherently difficult to remove from biomass, lignin is currently underutilized in fuel and chemical production.

The work reported in Energy & Environmental Science was funded by the Energy Department’s Bioenergy Technologies Office.

Resources

• Vardon, Derek R., Franden, Mary Ann; Joh;son, Christopher W.; Karp, Eric M.; Guarnieri, Michael T.; Linger, Jeffrey G.; Salm, Michael J.; Strathmann, Timothy J.; Beckham, Gregg T. (2015) “Adipic acid production from lignin” Energy Environ. Sci. doi: 10.1039/C4EE03230F