Lignin

Given the potential of Appalachia for the production of biomass – either in the form of bioenergy crops grown on reclaimed mine land or timber from well-established forest areas – biomass utilization is a natural fit for the Appalachian economy. Produced biomass could be used to produce heat and electricity for distributed (off-grid) agricultural facilities such as greenhouses, or could be processed in biorefineries into fuels and higher value bioproducts such as chemicals and resins. However, the economics of cellulosic biorefineries are challenging. In this context, it is important to note that multiple studies have concluded that selling lignin as a co-product has the potential to contribute substantially to the economic viability of biofuels [1]. Lignin is the second most abundant terrestrial polymer after cellulose. Moreover, approximately 60% more lignin is generated in first generation cellulosic ethanol plants than is needed to meet internal energy demands by its combustion [2]. A 2013 report by the National Renewable Energy Laboratory (NREL) concluded that lignin valorization was essential if the target cost of $3.00/gal of gasoline equivalent fuel from lignocellulosic biomass was to be met [3]. In the same vein, Kautto et al. modeled a process for ethanol production from hardwood with lignin, furfural, and acetic acid as co-products. Similar to the NREL findings, the value of the lignin co-product(s) was a strong determinant of the minimum ethanol selling price [4].

To date, no process has been validated at commercial scale to depolymerize and economically convert lignin into high-value products. However, recent work by UK researchers has resulted in the development of a heterogeneous catalyst, 1 wt% Au/Li-Al layered double hydroxide, that can effectively depolymerize lignin under mild, oxidative conditions [5-7], O₂ (1 atm.) being used as the oxidant. The products are aromatic monomers, typically comprising aromatic aldehydes and acids, viz., vanillin, vanillin acid, syringaldehyde, syringic acid and ferulic acid. Depending on the lignin feed, monomer yields can reach 40 wt%. Notably, unlike the phenolic compounds formed in reductive lignin depolymerization processes, which are generally of low value (<$1400/ton), the monomers obtained from our oxidative process command high selling prices ($400-$1400/kg for vanillin, vanillin acid, and syringaldehyde). On-going work is focused on converting the current batch process to a continuous one, while simultaneously reducing the cost of the catalyst.

In the NRT, this work will be supported by TEA and LCA to develop a pathway for the application of this approach to biomass utilization. Data for the TEA (output quanties, quality, extraction costs, input costs and waste streams) will be collected experimentally. In order to assess the probability of $3.00/gal of gasoline equivalent, a simulation approach will be implemented to allow for key cost factors of the depolymerization process to be investigated. This information will be relayed back to the team to guide their experiments towards economic feasibility. In conjuction there will be an LCA utilizing a cradle to grave approach to examine the sustainability of the depolymerization process. In summary, the research activities in this strand of the NRT will span chemistry, horticulture, and agricultural economics. 

References

1. Davis, K. M.; Rover, M.; Brown, R. C.; Bai, X.; Wen, Z.; Jarboe, L. R., Recovery and utilization of lignin monomers as part of the biorefinery approach. Energies Vol. 9 (10), pp. 808, 2016.
2. Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M., Lignin valorization: improving lignin processing in the biorefinery. Science Vol. 344 (6185), pp. 1246843, 2014.
3. Davis, R.; Tao, L.; Tan, E.; Biddy, M.; Beckham, G.; Scarlata, C.; Jacobson, J.; Cafferty, K.; Ross, J.; Lukas, J. Process design and economics for the conversion of lignocellulosic biomass to hydrocarbons: dilute-acid and enzymatic deconstruction of biomass to sugars and biological conversion of sugars to hydrocarbons. National Renewable Energy Laboratory (NREL), Golden, CO., 2013.
4. Kautto, J.; Realff, M. J.; Ragauskas, A. J.; Kässi, T., Economic analysis of an organosolv process for bioethanol production. BioResources Vol. 9 (4), pp. 6041-6072, 2014.
5. Song, Y.; Mobley, J. K.; Motagamwala, A. H.; Isaacs, M.; Dumesic, J. A.; Ralph, J.; Lee, A. F.; Wilson, K.; Crocker, M., Gold-catalyzed conversion of lignin to low molecular weight aromatics. Chemical science Vol. 9 (42), pp. 8127-8133, 2018.
6. Morrey, K., Gold rush for lignin conversion. Chemistry World, 09/19/2018. https://www.chemistryworld.com/news/gold-rush-for-lignin-conversion/3009516.article (February 2019).
7. Crocker, M.; Song, Y.; Mobley, J. Oxidation catalyst. U.S. Pat. Application Serial No. 16/234,950 (2018).