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Faced with the depletion of fossil fuels, the future source of our chemical feedstocks, fuel and energy must be secured. As Society turns to biomass as a part of the solution to the problem, the abundance of aromatic compounds which can potentially be derived from lignin argues that this abundant biopolymer could be a source of renewable chemical feedstock. Further to this the highly functionalised nature of the aromatic compounds that can be derived from lignin, shows potential for their use in further chemical modification.

Lignin is an aromatic rich bio-polymer which is second only to cellulose as the most abundant renewable carbon source on the planet. For this reason, lignin could be a source of aromatic chemicals to the chemical industry. However, this is yet to become a reality due to the challenges inherent in lignin depolymerisation. This problem has reignited a great deal of interest in the chemistry of lignin and the research area of lignin depolymerisation has experienced an exponential growth over the last decade.[1][2]

Within the plant structure, lignin plays a major role – it provides structural rigidity to plants by cross-linking carbohydrate polymers.[1][2] Although, the exact lignin structure is still elusive, important characteristic sub-units in lignin are known. These key sub units are p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. The widely accepted notion is that, these three sub units, plus others yet to be identified sub-unit(s), polymerises to form lignin.[2]

The paper and pulp industry is the largest producer of lignin. In this case it is generated as a by-product during the Kraft process, used to obtain cellulose fibres from wood.[3] As the industry has grown so has the amount of lignin produced, thus providing a large scale source. However this is currently viewed as a waste product with the majority used as low value fuel.[2][4]

The lignosulfonate process is another method of isolating cellulose, and as a by-product, lignin. However the material obtained tends to be rich in sulfur, proving to be a problem in the downstream processing of lignin. Organosolv lignin is another well-known process for obtaining lignin and has advantages over the Kraft pulping process in that the cellulose, hemicellulose and lignin form separate product streams, allowing for valorisation of each component. In addition the absence of harsh conditions provides a more environmentally friendly route to obtain lignin. The higher purity of lignin obtained by the Organosolv process is advantageous for its valorisation into higher value chemicals.[2]

References

[1] http://www.ili-lignin.com/aboutlignin.php (accessed July 2015)

[2] J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius, and B. M. Weckhuysen,  Chem. Rev., 2010, 110, 3552–3599

[3] F. Tran, C. S. Lancefield, P. C. J. Kamer, T. Lebl and N. J. Westwood, Green Chem., 2015, 17, 244

[4] http://ietd. iipnetwork.org/content/pulp-and-paper#key-data (accessed July 2015)