Lignin monomer production integrated into the γ-valerolactone sugar platform

Abstract

We demonstrate an experimental approach for upgrading lignin that has been isolated from corn stover via biomass fractionation using γ-valerolactone (GVL) as a solvent. This GVL-based approach can be used in parallel with lignin upgrading to produce soluble carbohydrates at high yields (≥70%) from biomass without the use of enzymes, ionic liquids, or concentrated acids. The lignin was isolated after an initial hydrolysis step in which corn stover was treated in a high-solids batch reactor at 393 K for 30 min in a solvent mixture consisting of 80 wt% GVL and 20 wt% water. Lignin was isolated by precipitation in water and characterized by 2D HSQC NMR, showing that the extracted lignin was similar to native lignin, which can be attributed to the low acid level and the low extraction temperatures that are achievable using GVL as a solvent. This lignin was upgraded using a two-stage hydrogenolysis process over a Ru/C catalyst. The isolated lignin was first dissolved to form a mixture of 10% lignin, 80% THF, 8.5% H 3 PO 4 and 1.5% H 2 O, and treated at 423 K under hydrogen. The THF was removed by evaporation and replaced with heptane, forming a biphasic mixture. This mixture was then treated at 523 K in the presence of Ru/C and H 2 . The resulting heptane phase contained soluble lignin-derived monomers corresponding to 38% of the carbon in the original lignin. By adding 5% methanol during the second catalytic step, we produced additional monomers containing methyl esters and increased carbon yields to 48%. This increase in yield can be attributed to stabilization of carboxylic acid intermediates by esterification. The yield reported here is comparable to yields obtained with native lignin and is much higher than yields obtained with lignin isolated by other processes. These results suggest that GVLbased biomass fractionation could facilitate the integrated conversion of all three biomass fractions. Broader contextLignocellulosic biomass represents an attractive source of renewable carbon and could be used as a feedstock to substitute fossil-based fuels and chemicals. In addition, lignocellulosic biomass comprises non-edible plants that can grow on marginal lands and thus compete minimally with food production. In the context of increasing environmental issues related to climate change and because of increasing petroleum costs due to diminishing fossil reserves, it is urgent to find sustainable carbon-based substitutes to petroleum. However, growing, harvesting and processing biomass is often costly. Because of the expense of the raw material, the yield for conversion of biomass to useful products must be maximized. Lignocellulosic biomass typically contains about 50-70% polysaccharides (cellulose and hemicellulose) and 15-30% lignin by weight. Lignin is an irregular polymer composed of phenolic subunits. Given the relative uniformity of the carbohydrate polymers compared to lignin and the well-established chemical and biological upgrading pathways developed for sugar production and processing, much research and develop...

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…[201] Theisolation of pentose and hexose sugars from apacked bed of biomass (Corn stover,Maple wood, Loblolly Pine) has been recently demonstrated, by using the promoting effects of g-valerolactone (GVL) for mild aqueous acid hydrolysis. [243][244][245][246] High selectivity to different sugars obtained under varying conditions was attributed to the more facile hydrolysis of hemicelluloses relative to cellulose.B yi ntroducing at emperature gradient to the acid-catalysed flow reaction, hemicelluloses were hydrolysed and isolated in an early fraction (150-180 8 8C), and cellulose in al atter fraction (180-220 8 8C), enabling separation of xylose (the main monomeric sugar from hemicelluloses) and glucose (from cellulose). Removal of the GVL (e.g., by phase separation upon adding liquid CO 2 or NaCl), yielded as ugar stream concentrated at up to 127 gL À1 (i.e., 65 to 85 %ofthe highest concentrations obtained by enzymatic hydrolysis).…”

“…Up to 48 %ofthe carbon of the original lignin intake could be converted into monoaromatics that could then be extracted into ah eptane solution, with methanol acting as ac apping agent to form carboxylate esters. [246] Solvent-free,m echanocatalytic deep depolymerisation of polysaccharides has also been explored, beginning with either cellulose or crude lignocellulosic biomass. [248][249][250][251][252][253][254] Here,t he mechanocatalytic treatment affords aw ater-soluble,d epolymerised lignocellulose.T he saccharification of the watersoluble products renders high sugar yields (e.g., 88-92 % glucose,3.5-8 %cellobiose,93-98 %xylose relative to glucan and xylan fractions,respectively) and leads to precipitation of the lignin fraction as asulfur-free solid.…”

The influence of interunit carbon–carbon linkages during lignin upgrading

“…High yields of technical lignin were obtained using GVL as a solvent with low acid loadings (<2 wt% H 2 SO 4 ) at low temperatures (120 C) [26,27]. The lignin led to reasonable lignin monomer yields after hydrogenolysis (>30 mol% for corn stover and near 20 mol% for maple wood) [26,27]. NMR analyses showed that the native lignin structure was partially preserved in GVL-extracted corn stover lignin, including the dominant b-O-4 linkages.…”

“…Since both temperature and acid loading affect the rate of condensation, using mild conditions during pretreatment can contribute to limiting lignin condensation. High yields of technical lignin were obtained using GVL as a solvent with low acid loadings (<2 wt% H 2 SO 4 ) at low temperatures (120 C) [26,27]. The lignin led to reasonable lignin monomer yields after hydrogenolysis (>30 mol% for corn stover and near 20 mol% for maple wood) [26,27].…”

Process synthesis and analysis for catalytic conversion of lignocellulosic biomass to fuels: Separate conversion of cellulose and hemicellulose using 2-sec-butylphenol (SBP) solvent

“…Several studies have developed processes including a pretreatment step for the separate conversion of cellulose and hemicellulose to biofuels [2,34,43], and have focused on the technoeconomic analysis of thermochemical [23,24,44] and biochemical [34,[45][46][47] strategies. However, they are not many studies of technoeconomic analysis of large-scale production of biofuels from separate catalytic conversion of hemicellulose and cellulose.…”