(513f) Lignin-First Strategy for Upgrading Lignin into Value-Added Products: Maximizing Feedstock Utilization

Lignocellulosic biomass - a bountiful source of renewable carbon for the sustainable production of fuels and chemicals - is composed of 70-85 wt.% polysaccharides (cellulose and hemicellulose) and 15-30 wt.% lignin, a heteropolymer of aromatic compounds. For a successful bio-refinery, it is imperative to maximize the value obtained from all lignocellulosic biomass fractions. Polysaccharides have been extensively studied and various processes have been developed to produce valuable liquid fuels and commodity chemicals from the polysaccharide fraction of lignocellulosic biomass. Lignin, being difficult to fractionate and process, is generally burnt for its calorific value. However, lignin is the largest source of renewable aromatics and, as such, strategies to valorize lignin are required for a bio-refinery to economically compete with fossil-fuel refinery.

“Lignin-first” strategies have been employed to valorize lignin before upgrading polysaccharide fractions [1]. This strategy liberates lignin in “native-like” form from the plant cell wall and further prevents degradation through either catalytic processing [2] or by protection-group chemistry [3].

In our previous work, we developed a lignin-to-bioproduct processing chain for the production of 2-pyrone-4,6-dicarboxylic acid (PDC) through microbial funneling of the phenolic monomers obtained from catalytic depolymerization of lignin [4]. We used chemical and biological upgrading in tandem to extract greater value from the lignin fraction by converting a fraction of lignin to high value PDC (Figure 1A). In this previous work, we first isolated lignin from lignocellulosic biomass under mild reaction conditions using g-valerolactone (GVL) and water as the solvent system and dilute sulfuric acid as a catalyst. In this scheme, lignin degradation was minimized during biomass fractionation step due to the protection by the solvent system and low process temperatures (<120 °C). However, the techno-economic analysis of the processing chain showed the cost of lignin isolation to have the largest impact on the minimal-selling-price (MSP) of PDC.

In this work, we modified the lignin-to-bioproduct processing chain by eliminating the costly biomass fractionation step (Figure 1B). In the new scheme, we combine biomass fractionation and catalytic depolymerization of the lignin into a single step, while simultaneously preserving the structure of the polysaccharide fraction of the lignocellulosic biomass. We demonstrate that the lignin fraction of the whole-cell-wall poplar can be successfully depolymerized by hydrogenolysis over a Pd/C catalyst into a mixture of monomeric and oligomeric phenolic compounds. We show that, using this strategy we were able to increase the monomer/oligomer product yield on a per kg of biomass basis. We further show that using an engineered strain of Novosphingobium aromaticivorans DSM12444, this complex mixture of aromatic compounds containing syringyl, guaiacyl, and p-hydroxyphenyl can be upgraded to PDC. Moreover, we show that PDC can be extracted from the culture broth with a simple separation and purification step (e.g., precipitation with sodium chloride). Furthermore, the sugar stream was subjected to enzymatic and microbial digestion to liquid fuels to produce value added products from both the lignin (phenolics) and polysaccharide fractions.

Combining these improvements over the previous process we demonstrate that the economics of PDC production from lignocellulosic biomass can be further improved. Most importantly, we show that tandem processes utilizing both chemical and biological upgrading can significantly improve the upgrading of a complex feedstock like lignocellulosic biomass.

References:

[1] Abu-Omar M.M., Barta K., Beckham G.T., Luterbacher J.S., Ralph J., Rinaldi R., Roman-Leshkov Y., Samec J.S.M., Sels B.F., Wang F., Energy Environ. Sci., 2021, 14, 161-292.

[2] Luterbacher J.S., Azarpira A., Motagamwala A.H., Lu F., Ralph J., Dumesic J.A., Energy Environ. Sci., 2015, 8, 2657-2663.

[3] Shuai L., Amiri M.T., Questell-Santiago Y.M., Heroguel F., Li Y., Kim H., Meilan R., Chapple C., Ralph J., Luterbacher J.S., Science, 2016, 354(6310), 329-333.

[4] Perez J.M., Umana G.E., Sener C., Misra S., Coplien J., Haak D., Li Y., Maravelias C., Karlen S.D., Ralph J., Donohue T.J., Noguera D.R., “Integrating lignin depolymerization and microbial funneling processes using agronomically relevant feedstocks”, in preparation.

This material is based upon work supported by the Great Lakes Bioenergy Research Center, U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Award Number DE-SC0018409.