Zhang Libing, Yan Lishi, Wang Zheming, Laskar Dhrubojyoti D, Swita Marie S, Cort John R, Yang Bin
Bioproduct Sciences and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, WA 99354 USA.
Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99354 USA.
Biotechnol Biofuels. 2015 Dec 1;8:203. doi: 10.1186/s13068-015-0377-x. eCollection 2015.
Flowthrough 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.
In 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.
Elevated 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 temperatures.
生物质的流穿预处理是通过将木质素衍生物转化为高价值产品实现木质素增值的关键步骤，这一功能对生物精炼厂的经济效率至关重要。全面了解木质素在水预处理（如水洗和稀酸流穿预处理）中的行为和溶解化学，对于实现为木质素利用提供灵活平台的目标至关重要。
本研究报告了流穿预处理条件对从杨木中分离木质素的影响，以及预处理产生的三个木质素子集的特性，包括预处理固体残渣中的残留木质素（ReL）、预处理液体中回收的不溶性木质素（RISL）和预处理液体中回收的可溶性木质素（RSL）。在流穿反应器系统中，对杨木进行水洗和0.05%（w/w）硫酸预处理，温度为160至270°C，处理时间为2至10分钟。结果表明，水洗流穿预处理主要去除紫丁香基（S单元）。与低温水洗预处理相比，升高温度和/或添加硫酸可增强对愈创木基（G单元）的去除，从而使生物质中的木质素几乎完全去除。结果还表明，在高温下的稀酸和水洗流穿预处理中，回收的RISL比ReL和RSL更多。RISL的核磁共振光谱显示，在水洗和稀酸流穿预处理中均有显著的β-O-4裂解、α-β脱氧形成肉桂基样端基以及轻微的β-5再聚合。
在流穿系统中，升高温度和/或稀酸除了能去除S单元外，还能通过改善G单元的去除，将木质素去除率大大提高至近100%。流穿预处理仅引起轻微的木质素结构改性。提出了一种木质素转化途径来解释热水和稀酸流穿预处理过程中木质素结构变化的复杂性。
高温下水洗和稀酸流穿预处理中的木质素转化。
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