Lignin may emerge as versatile feedstock for carbon fiber, engineering plastics

The U.S. Department of Energy (DOE) is funding two projects to develop alternative routes to carbon fiber as detailed in reports here and here. The second beneficiary of the program, the National Renewable Energy Laboratory (NREL; Golden, CO) will be funded to the tune of $5.3 million to investigate and optimize multiple pathways to bio-acrylonitrile, a precursor for carbon fiber. The research may also lead to precursors for engineering plastics and other polymeric materials.

NREL's work traces back to an old saying in the biofuels industry: "You can make anything from lignin except money." However, NREL has demonstrated a concept that provides opportunities for the successful conversion of lignin into a variety of materials including precursors for carbon fiber and engineering plastics. A paper entitled "Lignin Valorization Through Integrated Biological Funneling and Chemical Catalysis" was recently published in the Proceedings of the National Academy of Sciences detailing the NREL's effort.

NREL bioplant is used to convert lignin into useful precursors for biopolymers, engineering plastics and carbon fiber, for example.

NREL is not the only organization looking to utilize lignin as a raw material. Two Swedish research insitutes, for example, are also investigating lignin's use as a carbon fiber precursor, while British company Biome Bioplastics is developing processes for lignin's uses a bioplastics precursor.  Oak Ridge National Laboratory (Oak Ridge, TN) is also investigating the use of lignin as a carbon fiber precursor

The process for converting glucose from biomass into fuels such as ethanol has been well established. However, plants also contain a significant amount of lignin - up to 30 percent of the cell walls. Lignin is a heterogeneous aromatic polymer that plants use to strengthen cell walls, but it is typically considered a hindrance to cost-effectively obtaining carbohydrates, and residual lignin is often burned for process heat because it is difficult to depolymerize and upgrade into useful fuels or chemicals.

"Biorefineries that convert cellulosic biomass into liquid transportation fuels typically generate more lignin than necessary to power the operation," NREL Senior Engineer and a co-author of the study Gregg Beckham said.  "Strategies that incorporate new approaches to transform the leftover lignin to more diverse and valuable products are desperately needed."

Although lignin depolymerization has been studied for nearly a century, the development of cost-effective upgrading processes for lignin valorization (raising value through artificial means) has been limited. On the other hand, in nature, some microorganisms have figured out how to overcome the heterogeneity of lignin. "Rot" fungi and some bacteria are able to secrete powerful enzymes or chemical oxidants to break down lignin in plant cell walls, which produces a heterogeneous mixture of aromatic molecules.

This new study shows that developing biological conversion processes for one such lignin-utilizing organism, Pseudomonas putida KT2440, may enable new routes to overcome the heterogeneity of lignin. And, that may enable a broader slate of molecules derived from lignocellulosic biomass. In the lat

"The conceptual approach we demonstrate can be applied to many different types of biomass feedstocks and combined with many different strategies for breaking down lignin, engineering the biological pathways to produce different intermediates, and catalytically upgrading the biologically-derived product to develop a larger range of valuable molecules derived from lignin," Beckham said. "It holds promise for a wide variety of industrial applications. While this is very exciting, certainly there remains a significant amount of technology development to make this process economically viable."

In recent research, NREL has synthesized medium chain-length polyhydroxyalkanoates (mcl-PHAs) from lignin-enriched streams derived from pilot-scale biomass preteatment. These mcl-PHAs are reported to be similar in physicochemical properties to conventional carbohydrate-derived mcl-PHAs, which have applications as bioplastics. In a further demonstration of their utility, mcl-PHAs were catalytically converted to chemical precursors that could potentially be further converted into low-cost carbon fiber, engineering plastics, thermoplastic elastomers, polymeric foams and membranes.

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