Simplified process flow diagram illustrating paths to recover lignin. This can happen either after removal of most of the carbohydrates by hydrolysis and fermentation operations (top sequence) or by pretreatment before downstream carbohydrate conversion (bottom sequence). [Ragauskas et al., courtesy of Oak Ridge National Laboratory.] Click to enlarge.

The aromatic polymer lignin is found in most terrestrial plants in the approximate range of 15 to 40% dry weight and provides the plants with their structural integrity. Conventionally, most large-scale industrial processes that use plant polysaccharides burn lignin to generate the power needed to transform biomass. Cellulosic biorefineries seeking to convert biomass into liquid transportation fuels will generate substantially more lignin than necessary to power the operation, and therefore efforts are underway to transform it to value-added products.

At the same time, federal agencies and industry are funding research to simplify the process of taking biomass to fuels.

One of the very promising approaches to doing that is to genetically engineer plants so they have more reactive polysaccharides suitable for commercial applications, but also to change lignin’s structural features so that it’ll become more attractive for materials applications, chemicals and fuels.

—Arthur Ragauskas

Research highlighted in the review has shown it’s theoretically possible to genetically alter lignin pathways to reduce undesirable byproducts and more efficiently capture the desired polysaccharides and enhance lignin’s commercial value.

Through work on transgenic plants and wild plants that naturally have fewer undesirable constituents, biologists, engineers and chemists have recently improved the biorefinery field’s understanding of the chemistry and structure of lignin, which provides a better idea of the theoretical chemistry that lignin can do, Ragauskas said.

We should be able to alter the structure of lignin and isolate it in such a manner that we can use it for green-based materials or use it in a blend for a variety of synthetic polymers.

—Arthur Ragauskas

Doing so would create a stream of polysaccharides for use as ethanol fuels, with lignin waste that has structural features that would make it attractive for commercial applications such as polymers or carbon fibers.

The science could be applied to a variety of plants currently used for cellulosic biofuel production, such as switchgrass and poplar.

This review set out to describe a set of developments over the past few years that we suggest, when taken together, represent a tipping point in the prospects for lignin as a viable, commercially relevant sustainable feedstock for a new range of materials and uses.

First, the advent of new cellulosic biorefineries will introduce an excess supply of different, nonsulfonated, native and transgenically modified lignins into the process streams. Future research will continue to establish to what extent the lignin structure in plants can be altered to yield a product that can be readily recovered via pretreatment and has the appropriate tailored structures to be valorized for materials, chemicals, and fuels. Third, although lignin sequencing remains a vision, approaches based largely on NMR, high-performance liquid chromatography–mass spectrometry (LC-MS) or GC-MS, and specific binding antibodies have greatly improved our knowledge of the structures of lignin and its products.

These results need to be further integrated into improved force fields and high-performance computational modeling to provide a predictive tool of lignin’s chemical and physical properties and reactivities in multiple environments. Such insights may help redesign lignin within its cross-linked complex biological matrix to meet subsequent process and end product goals. Overall, the need to understand and manipulate lignin from its assembly within plant cell walls to its extraction and processing into value-added products aligns with our potential to obtain a deeper understanding of complex biological structures. This is especially true because the valorization of lignin cannot come at the expense of the effective utilization of other biopolymers, such as cellulose and hemicellulose.

—Ragauskas et al.

Co-authors on the review article included scientists from the National Renewable Energy, the University of British Columbia, the University of North Texas, Oak Ridge National Laboratory, and the University of California, Riverside.

Resources

• Arthur J. Ragauskas, et al., (2014) “Lignin Valorization: Improving Lignin Processing in the Biorefinery.” Science doi: 10.1126/science.1246843