Chemical Transformations of Poplar Lignin during Cosolvent Enhanced Lignocellulosic Fractionation Process

Abstract

Converting lignocellulosic biomass to biofuels and bioproducts is significantly hindered by the innate recalcitrance of biomass to chemical and biological breakdown, and it usually requires a pretreatment stage in order to improve conversion yields. A promising novel pretreatment named Cosolvent Enhanced Lignocellulosic Fractionation (CELF) involving dilute acid treatment of biomass in a THF−water mixture was recently developed to overcome biomass recalcitrance. Detailed elucidation of physicochemical structures of the fractionated lignin that is precipitated from CELF pretreatment of hardwood poplar, also called CELF lignin, reveals transformations in its molecular weights, monolignol composition, and hydroxyl group content. Isolated CELF lignin revealed dramatic reductions in its molecular weight by up to ∼90% compared with untreated native lignin. Furthermore, CELF lignin's β-O-4 interunit linkages were extensively cleaved after CELF pretreatment as indicated by a semiquantitative HSQC NMR analysis. This is further evidenced by a 31 P NMR analysis showing a significant decrease in aliphatic OH groups due to the oxidation of lignin side chains, whereas the content of total phenolic OH groups in CELF lignin significantly increased due to cleavage of interunit linkages. In conclusion, the CELF process generated a uniquely tunable and highly pure lignin feedstock of low content aryl ether linkages, low molecular weight, and high amount of phenolic hydroxyl groups, suitable for its development into fuels, chemicals, and materials.

Determination of hydroxyl groups in biorefinery resources via quantitative 31P NMR spectroscopy

“…In the early 2000s, Ragauskas's group introduced NHND as a promising alternative IS in 31 P NMR analysis of lignin owing to its ability to be fully baseline resolved from lignin-derived resonances 98 . His group has applied 31 P NMR using this IS to diverse areas including the characterization of transgetic and pretreated biomass [99][100][101] . In addition, Argyropoulos's group carried out extensive studies to elucidate the spin-lattice relaxation times and solvent effects on the 31 P NMR chemical shifts and arrived at the now universally used relaxation additive (chromium acetylacetonate) and the mixture of pyridine and CDCl 3 at the ratio of 1.6:1 as the solvent 15,20 .…”

“…A large variety of depolymerization or condensation reactions typically occur within the lignin structure under such conditions. Quantitative 31 P NMR analyses of lignins isolated from various biomass pretreatment technologies have been highlighted in recent studies 101,[108][109][110][111][112][113][114][115][116][117] . As lignin has become a key genetic engineering target for the enhancement of wood quality and biofuel production 118,119 , quantitative 31 P NMR analyses also have been carried out on transgenic lignin 97,120 .…”

Lignin‐derived electrochemical energy materials and systems

“…Both of these methodologies can significantly alter the structure of lignin. In general, these delignification strategies are implemented primarily to degrade, solubilize, and reduce the overall recalcitrance of the lignin‐hemicellulose‐cellulose matrix, providing greater access to cellulose fibers for subsequent biological processes, and resulting in a lignin byproduct with unique properties and potential for biofuel and material applications . For example, ethanol organosolv pretreatment (EOP) is a process that is being examined for industrial biorefineries .…”

Performance of three delignifying pretreatments on hardwoods: hydrolysis yields, comprehensive mass balances, and lignin properties

“…For CELF pretreatment, THF-water is an excellent "theta" solvent for lignin that, when combined with dilute acid, achieves high lignin depolymerization and solubilization at lower severity conditions than water-only pretreatments [61]. To ensure that lignin fragmentation is dominant over condensation, CELF pretreatment is maintained at or below 160 °C to solubilize lignin, while avoiding the production of undesired lignin condensation products known to form at higher severities [29,62,63].…”

“…The first is that the CELF pretreatment is performed under acidic conditions such that the majority of the solubilized xylan is hydrolyzed to xylose [28,32], thereby resulting in minimal soluble xylan oligomers that are available to co-precipitate with the lignin. The second factor is that lignin precipitation by water dilution or by boiling off the THF also results in partitioning of the sugar monomers and low-MW oligomers into the aqueous phase rather than precipitation with the lignin [62]. At the other extreme, [Ch] [Lys] contains from 10.3% (poplar) to 15.3% (eucalyptus) polysaccharides in the recovered lignins.…”