Characterization of lignin derived from water-only and dilute acid flowthrough pretreatment of poplar wood at elevated temperatures

Abstract

BackgroundFlowthrough pretreatment of biomass is a critical step in lignin valorization via conversion of lignin derivatives to high-value products, a function vital to the economic efficiency of biorefinery plants. Comprehensive understanding of lignin behaviors and solubilization chemistry in aqueous pretreatment such as water-only and dilute acid flowthrough pretreatment is of fundamental importance to achieve the goal of providing flexible platform for lignin utilization.ResultsIn this study, the effects of flowthrough pretreatment conditions on lignin separation from poplar wood were reported as well as the characteristics of three sub-sets of lignin produced from the pretreatment, including residual lignin in pretreated solid residues (ReL), recovered insoluble lignin in pretreated liquid (RISL), and recovered soluble lignin in pretreatment liquid (RSL). Both the water-only and 0.05 % (w/w) sulfuric acid pretreatments were performed at temperatures from 160 to 270 °C on poplar wood in a flowthrough reactor system for 2–10 min. Results showed that water-only flowthrough pretreatment primarily removed syringyl (S units). Increased temperature and/or the addition of sulfuric acid enhanced the removal of guaiacyl (G units) compared to water-only pretreatments at lower temperatures, resulting in nearly complete removal of lignin from the biomass. Results also suggested that more RISL was recovered than ReL and RSL in both dilute acid and water-only flowthrough pretreatments at elevated temperatures. NMR spectra of the RISL revealed significant β-O-4 cleavage, α-β deoxygenation to form cinnamyl-like end groups, and slight β-5 repolymerization in both water-only and dilute acid flowthrough pretreatments.ConclusionsElevated temperature and/or dilute acid greatly enhanced lignin removal to almost 100 % by improving G unit removal besides S unit removal in flowthrough system. Only mild lignin structural modification was caused by flowthrough pretreatment. A lignin transformation pathway was proposed to explain the complexity of the lignin structural changes during hot water and dilute acid flowthrough pretreatment.Graphical abstractLignin transformations in water-only and dilute acid flowthrough pretreatment at elevated temperaturesElectronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0377-x) contains supplementary material, which is available to authorized users.

Strengths, challenges, and opportunities for hydrothermal pretreatment in lignocellulosic biorefineries

“…However, removal of components soon aft er they are dissolved results in a very high fraction of oligomers, thereby requiring introduction of a downstream oligomer hydrolysis operation if fermentative organisms cannot metabolize such long chained polysaccharides. 32,63,78,81 Flowthrough pretreatments at about 200 o C with very dilute sulfuric acid (e.g. 0.05% H 2 SO 4 ) show similar results to those for fl owthrough with just hot water, with the exception that more cellulose is solubilized and monomeric sugar yields are higher.…”

“… Such hot water flowthrough systems quickly carry solubilized materials from the reaction zone, thereby limiting the opportunity for degradation. However, removal of components soon after they are dissolved results in a very high fraction of oligomers, thereby requiring introduction of a downstream oligomer hydrolysis operation if fermentative organisms cannot metabolize such long chained polysaccharides . Flowthrough pretreatments at about 200°C with very dilute sulfuric acid (e.g.…”

“…Formation of these propenyl end groups during pretreatment was plausibly due to cleavage of β‐O‐4´aryl ether by dehydration at the α‐position and oxidation at the γ‐OH position. In addition, linkages in insoluble lignin, such as resinol, β‐O‐4´ and the phenylcoumaran structures, were observed, and slight repolymerization formed new Cβ‐C5´ linkages in the insoluble lignin . For hardwood lignin, slight condensation of Cβ‐C5 and loss of γ–methyl groups occurred in the lignin fraction removed .…”

“…Schematic of water‐only and dilute acid flowthrough pretreatment of biomass along with picture of lignin solids left after hydrothermal pretreatment …”

“…18,60 Lignin is believed to depolymerize and form micelles via both homolytic and acidolytic cleavage into low molecular weight lignin globules. [61][62][63] Batch/co-current hydrothermal pretreatment As already mentioned, the CAFI project compared the costs for batch or co-current hydrothermal pretreatments to those for dilute acid in an integrated biorefi nery based on CAFI pretreatment data. 12,65 Assuming similar pressure ratings and identical reactor confi gurations, the pretreatment reactor capital cost would be about 10-15% lower if stainless steel is employed for liquid hot water pretreatment instead of corrosion resistant Incoloy cladding needed for dilute acid pretreatment.…”