Ionic liquid‐mediated selective extraction of lignin from wood leading to enhanced enzymatic cellulose hydrolysis

Lignocellulose represents a key sustainable source of biomass for transformation into biofuels and bio‐based products. Unfortunately, lignocellulosic biomass is highly recalcitrant to biotransformation, both microbial and enzymatic, which limits its use and prevents economically viable conversion into value‐added products. As a result, effective pretreatment strategies are necessary, which invariably involves high energy processing or results in the degradation of key components of lignocellulose. In this work, the ionic liquid, 1‐ethyl‐3‐methylimidazolium acetate ([Emim][CH₃COO]), was used as a pretreatment solvent to extract lignin from wood flour. The cellulose in the pretreated wood flour becomes far less crystalline without undergoing solubilization. When 40% of the lignin was removed, the cellulose crystallinity index dropped below 45, resulting in >90% of the cellulose in wood flour to be hydrolyzed by Trichoderma viride cellulase. [Emim] [CH₃COO] was easily reused, thereby resulting in a highly concentrated solution of chemically unmodified lignin, which may serve as a valuable source of a polyaromatic material as a value‐added product. Biotechnol. Bioeng. 2009;102: 1368–1376. © 2008 Wiley Periodicals, Inc.     

Ultrasound mediated enzymatic hydrolysis of cellulose and carboxymethyl cellulose

A recombinant Trichoderma reesei cellulase was used for the ultrasound‐mediated hydrolysis of soluble carboxymethyl cellulose (CMC) and insoluble cellulose of various particle sizes. The hydrolysis was carried out at low intensity sonication (2.4–11.8 W cm^?2 sonication power at the tip of the sonotrode) using 10, 20, and 40% duty cycles. [A duty cycle of 10%, for example, was obtained by sonicating for 1 s followed by a rest period (no sonication) of 9 s.] The reaction pH and temperature were always 4.8 and 50°C, respectively. In all cases, sonication enhanced the rate of hydrolysis relative to nonsonicated controls. The hydrolysis of CMC was characterized by Michaelis‐Menten kinetics. The Michaelis‐Menten parameter of the maximum reaction rate V_max was enhanced by sonication relative to controls, but the value of the saturation constant K_m was reduced. The optimal sonication conditions were found to be a 10% duty cycle and a power intensity of 11.8 W cm^?2. Under these conditions, the maximum rate of hydrolysis of soluble CMC was nearly double relative to control. In the hydrolysis of cellulose, an increasing particle size reduced the rate of hydrolysis. At any fixed particle size, sonication at a 10% duty cycle and 11.8 W cm^?2 power intensity improved the rate of hydrolysis relative to control. Under the above mentioned optimal sonication conditions, the enzyme lost about 20% of its initial activity in 20 min. Sonication was useful in accelerating the enzyme catalyzed saccharification of cellulose. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1448–1457, 2013     

BSA treatment to enhance enzymatic hydrolysis of cellulose in lignin containing substrates

Cellulase and bovine serum albumin (BSA) were added to Avicel cellulose and solids containing 56% cellulose and 28% lignin from dilute sulfuric acid pretreatment of corn stover. Little BSA was adsorbed on Avicel cellulose, while pretreated corn stover solids adsorbed considerable amounts of this protein. On the other hand, cellulase was highly adsorbed on both substrates. Adding a 1% concentration of BSA to dilute acid pretreated corn stover prior to enzyme addition at 15 FPU/g cellulose enhanced filter paper activity in solution by about a factor of 2 and beta-glucosidase activity in solution by about a factor of 14. Overall, these results suggested that BSA treatment reduced adsorption of cellulase and particularly beta-glucosidase on lignin. Of particular note, BSA treatment of pretreated corn stover solids prior to enzymatic hydrolysis increased 72 h glucose yields from about 82% to about 92% at a cellulase loading of 15 FPU/g cellulose or achieved about the same yield at a loading of 7.5 FPU/g cellulose. Similar improvements were also observed for enzymatic hydrolysis of ammonia fiber explosion (AFEX) pretreated corn stover and Douglas fir treated by SO(2) steam explosion and for simultaneous saccharification and fermentation (SSF) of BSA pretreated corn stover. In addition, BSA treatment prior to hydrolysis reduced the need for beta-glucosidase supplementation of SSF. The results are consistent with non-specific competitive, irreversible adsorption of BSA on lignin and identify promising strategies to reduce enzyme requirements for cellulose hydrolysis.     

Effect of xylan and lignin removal by batch and flowthrough pretreatment on the enzymatic digestibility of corn stover cellulose

Compared with batch systems, flowthrough and countercurrent reactors have important potential advantages for pretreating cellulosic biomass, including higher hemicellulose sugar yields, enhanced cellulose digestibility, and reduced chemical additions. Unfortunately, they suffer from high water and energy use. To better understand these trade-offs, comparative data are reported on xylan and lignin removal and enzymatic digestibility of cellulose for corn stover pretreated in batch and flowthrough reactors over a range of flow rates between 160 degrees and 220 degrees C, with water only and also with 0.1 wt% sulfuric acid. Increasing flow with just water enhanced the xylan dissolution rate, more than doubled total lignin removal, and increased cellulose digestibility. Furthermore, adding dilute sulfuric acid increased the rate of xylan removal for both batch and flowthrough systems. Interestingly, adding acid also increased the lignin removal rate with flow, but less lignin was left in solution when acid was added in batch. Although the enzymatic hydrolysis of pretreated cellulose was related to xylan removal, as others have shown, the digestibility was much better for flowthrough compared with batch systems, for the same degree of xylan removal. Cellulose digestibility for flowthrough reactors was related to lignin removal as well. These results suggest that altering lignin also affects the enzymatic digestibility of corn stover.     

六种白腐菌预处理对毛白杨木材酶水解效果的影响

通过化学分析和酶水解试验,研究了不同的白腐菌对毛白杨的预处理效果及不同组分的降解对酶水解的影响。毛白杨木片经6种白腐菌预处理30d后,各组分都发生了降解,其中半纤维素的损失最为显著,Trametes ochracea C6888引起半纤维素降解率高达47.19%,其次是纤维素和酸不溶木素的降解。在后续酶水解过程中,6种白腐菌处理后的样品显示出不同的水解模式,菌株Trametes ochracea C6888、T. pubescens C7571和T. versicolor C6915预处理效果最为显著,还原糖得率在整个酶水解过程中一直高于对照,其中T. ochracea C6888在水解96h后还原糖得率达到15.93%,比未处理样品提高了25%。分析酸不溶木素降解率及半纤维素降解率与还原糖得率的关系发现,不同菌株在作用同一种基质时,预处理效果差异显著,木质素和半纤维素的脱除都会影响木质纤维素的酶水解。     

The influence of lignin migration and relocation during steam pretreatment on the enzymatic hydrolysis of softwood and corn stover biomass substrates

To be effective, steam pretreatment is typically carried out at temperatures/pressures above the glass transition point (Tg) of biomass lignin so that it can partly fluidize and relocate. The relocation of Douglas-fir and corn stover derived lignin was compared with the expectation that, with the corn stover lignin's lower hydrophobicity and molecular weight, it would be more readily fluidized. It was apparent that the Tg of lignin decreased as the moisture increased, with the easier access of steam to the corn stover lignin promoting its plasticization. Although the softwood lignin was more recalcitrant, when it was incorporated onto filter paper, it too could be plasticized, with its relocation enhancing enzymatic hydrolysis. When lignin recondensation was minimized, the increased hydrophobicity suppressed lignin relocation. It was apparent that differences in the accessibility of the lignin present in Douglas-fir and corn stover to steam significantly impacted lignin fluidization, relocation, and subsequent cellulose hydrolysis.     

Changes in the enzymatic hydrolysis rate of Avicel cellulose with conversion

The slow down in enzymatic hydrolysis of cellulose with conversion has often been attributed to declining reactivity of the substrate as the more easily reacted material is thought to be consumed preferentially. To better understand the cause of this phenomenon, the enzymatic reaction of the nearly pure cellulose in Avicel was interrupted over the course of nearly complete hydrolysis. Then, the solids were treated with proteinase to degrade the cellulase enzymes remaining on the solid surface, followed by proteinase inhibitors to inactive the proteinase and successive washing with water, 1.0 M NaCl solution, and water. Next, fresh cellulase and buffer were added to the solids to restart hydrolysis. The rate of cellulose hydrolysis, expressed as a percent of substrate remaining at that time, was approximately constant over a wide range of conversions for restart experiments but declined continually with conversion for uninterrupted hydrolysis. Furthermore, the cellulose hydrolysis rate per adsorbed enzyme was approximately constant for the restart procedure but declined with conversion when enzymes were left to react. Thus, the drop off in reaction rate for uninterrupted cellulose digestion by enzymes could not be attributed to changes in substrate reactivity, suggesting that other effects such as enzymes getting "stuck" or otherwise slowing down may be responsible.     

Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment step

Hydrolysis of cellulose to glucose in aqueous media catalyzed by the cellulase enzyme system suffers from slow reaction rates due in large part to the highly crystalline structure of cellulose and inaccessibility of enzyme adsorption sites. In this study, an attempt was made to disrupt the cellulose structure using the ionic liquid (IL), 1-n-butyl-3-methylimidazolium chloride, in a cellulose regeneration strategy which accelerated the subsequent hydrolysis reaction. ILs are a new class of non-volatile solvents that exhibit unique solvating properties. They can be tuned to dissolve a wide variety of compounds including cellulose. Because of their extremely low volatility, ILs are expected to have minimal environmental impact on air quality compared to most other volatile solvent systems. The initial enzymatic hydrolysis rates were approximately 50-fold higher for regenerated cellulose as compared to untreated cellulose (Avicel PH-101) as measured by a soluble reducing sugar assay.     

Structural comparison and enhanced enzymatic hydrolysis of eucalyptus cellulose via pretreatment with different ionic liquids and catalysts

Eucalyptus was fractionated with mild alkaline process, and the obtained cellulose fraction was pretreated with various ionic liquids (ILs) to enhance the enzymatic saccharification. The results showed that the ILs used was efficient for the hydrolysis of cellulose, with the maximum total reducing sugars (TRS) yield over 80% at 50 °C. The regenerated cellulose substrate exhibited a significant improvement about 4.4–6.4 folds enhancement on saccharification rate during the first 4 h reaction. The crystallinity index (CrI) of cellulose via 1-ally-3-methylimidazolium ([AMIM]Cl) pretreatment was significantly decreased from 70.2% to 31.2%, resulting in structural change from cellulose I to cellulose II, which enabled the cellulase enzymes easier access to hydrolyze cellulose. However, 1-butyl-3methylimidazolium acesulfamate ([BMIM]Ace) pretreatment had no large effect on the CrI although a high conversion yield in glucose was obtained. The surface morphologies of the regenerated substrate which was pretreated via 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) showed more porous and incompact network of cellulose when compared with the untreated substrate. This result indicated a better accessibility by cellulases to the cellulose surface. Besides, a certain amount of catalysts such as MgCl₂ and H₂SO₄ could improve the rate of enzymatic saccharification.     

The study of factors affecting the enzymatic hydrolysis of cellulose after ionic liquid pretreatment

Cellulose resource has got much attention as a promising replacement of fossil fuel. The hydrolysis of cellulose is the key step to chemical product and liquid transportation fuel. In this paper a serials of chloride, acetate, and formate based ionic liquids were used as solvents to dissolve cellulose. The cellulose regenerated from ILs was characterized by FTIR and X-ray powder diffraction. From the characterization and analysis, it was found that the original close and compact structure has changed a lot. After enzymatic hydrolysis, different kinds of ionic liquids (ILs) have different yields of the reducing sugar (TRS). They are 100%, 90.72%, and 88.92% from 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium formate ([BMIM][HCOO]) respectively after enzymatic hydrolysis at 50 °C for 5 h. The results indicated that the yields and the hydrolysis rates were improved apparently after ILs pretreatment comparing with the untreated substrates.     

热碱预处理对菊芋茎秆组成和酶水解影响

为了深入了解菊芋茎秆用于生物能源转化的潜力,在对菊芋茎秆的全秆、韧皮以及髓芯的组成分析基础上,采用不同浓度的Na OH在121℃对菊芋茎秆进行预处理,并对预处理后的茎秆进行酶水解。结果表明:菊芋茎秆具有较高木质素含量(32.0%),且韧皮中木质素含量最高;茎秆中碳水化合物总含量与传统农作物秸秆相当,但纤维素含量相对较高(40.5%),半纤维含量相对较低(19.6%)。经不同浓度Na OH预处理后,相对于未处理茎秆,全秆、韧皮以及髓芯中木质素含量分别降低13.1%–13.4%、8.3%–13.5%和19.9%–27.2%,半纤维素含量分别降低了87.8%–96.9%、87.6%–95.0%和74.0%–90.2%。纤维素含量在全秆、韧皮和髓芯中相应增加了56.5%–60.2%、52.2%–55.4%和62.7%–73.2%。酶水解的结果显示,增加预处理过程中Na OH的浓度,全秆和韧皮的水解率可被提高2.3–2.6倍和10.3–18.5倍。虽然热Na OH预处理可以有效地改善髓芯水解性能,但经过高浓度的Na OH(2.0 mol/L)预处理,髓芯的水解性能下降明显。由此可见,菊芋用于生物能源转化技术中,热碱法可较好地适用于菊芋秸秆预处理。提高碱浓度,有利于半纤维素和木质素的去除,并实现酶水解糖化产率的提高。但鉴于碱浓度过高会造成髓芯糖产率降低,热碱预处理菊芋秸秆工艺条件需进一步优化。     

Novel renewable ionic liquids as highly effective solvents for pretreatment of rice straw biomass by selective removal of lignin

Cholinium amino acids ionic liquids ([Ch][AA] ILs), a novel type of bio‐ILs that can easily be prepared from renewable biomaterials, were investigated for pretreatment of rice straw by selective extraction of lignin from this abundant lignocellulosic biomass material. Of the eight ILs examined, most were demonstrated to be excellent pretreatment solvents. Upon pretreatment using these ILs, the initial saccharification rates of rice straw residues were substantially improved as well as the extent to which polysaccharides could be digested (>90% for cellulose and >60% for xylan). Enzymatic hydrolysis of pretreated rice straw by Trichoderma reesei cellulase/xylanase furnished glucose and xylose with the yields in excess of 80% and 30%, respectively. Detailed spectroscopic characterization showed that the enhancement of polysaccharides degestibility derived mainly from delignification rather than changes in cellulose crystallinity. The yields of fermentable reducing sugars were significantly improved after individual optimization of pretreatment temperature and duration. With [Ch][Lys] as the solvent, the sugar yields of 84.0% for glucose and 42.1% for xylose were achieved after pretreatment at 90°C for 5 h. The IL [Ch][Lys] showed excellent reusability across five successive batches in pretreatment of rice straw. These bio‐ILs performed as well as or better than previously investigated non‐renewable ILs, and thus present a new and environmentally friendly way to pretreat lignocellulose for production of fermentable sugars and total utilization of the biomass. Biotechnol. Bioeng. 2012; 109: 2484–2493. © 2012 Wiley Periodicals, Inc.     

Influence of steam pretreatment severity on post‐treatments used to enhance the enzymatic hydrolysis of pretreated softwoods at low enzyme loadings

It is recognized that some form of post‐treatment will usually be required if reasonable hydrolysis yields (>60%) of steam pretreated softwood are to be achieved when using low enzyme loadings (5 FPU/g cellulose). In the work reported here we modified/removed lignin from steam pretreated softwood while investigating the influence that the severity of pretreatment might have on the effectiveness of subsequent post‐treatments. Although treatment at a lower severity could provide better overall hemicellulose recovery, post‐treatment was not as effective on the cellulosic component. Pretreatment at medium severity resulted in the best compromise, providing reasonable recovery of the water soluble hemicellulose sugars and the use of post‐treatment conditions that significantly increased the enzymatic hydrolysis of the water insoluble cellulosic component. Post‐treatment with alkaline hydrogen peroxide or neutral sulfonation resulted in 62% cellulose hydrolysis at an enzyme loading of 5 FPU/g cellulose, which was four times greater than was obtained when the cellulosic fraction was not post‐treated. When the enzyme loading was increased to 15 FPU/g cellulose, the post‐treated cellulosic fraction was almost completely hydrolyzed to glucose. Despite the higher lignin content (44%) of the sulfonated substrate, similar hydrolysis yields to those achieved after alkaline peroxide post‐treatment (14% lignin content) indicated that, in addition to lignin removal, lignin modification also plays an important role in influencing the effectiveness of hydrolysis when low enzyme loadings are used. Biotechnol. Bioeng. 2011;108: 2300–2311. © 2011 Wiley Periodicals, Inc.     

Enhancement of ethanol and biogas production from high‐crystalline cellulose by different modes of NMO pretreatment

Pretreatment of high‐crystalline cellulose with N‐methyl‐morpholine‐N‐oxide (NMO or NMMO) to improve bioethanol and biogas production was investigated. The pretreatments were performed at 90 and 120°C for 0.5–15 h in three different modes, including dissolution (85% NMO), ballooning (79% NMO), and swelling (73% NMO). The pretreated materials were then enzymatically hydrolyzed and fermented to ethanol or anaerobically digested to biogas (methane). The pretreatment at 85% NMO, 120°C and 2.5 h resulted in 100% yield in the subsequent enzymatic hydrolysis and around 150% improvement in the yield of ethanol compared to the untreated and water‐treated material. However, the best results of biogas production were obtained when the cellulose was treated with swelling and ballooning mode, which gave almost complete digestion in 15 days. Thus, the pretreatment resulted in 460 g ethanol or 415 L methane from each kg of cellulose. Analysis of the structure of treated and untreated celluloses showed that the dissolution mode can efficiently convert the crystalline cellulose I to cellulose II. However, it decreases the water swelling capacity of the cellulose. On the other hand, swelling and ballooning modes in NMO treatment were less efficient in both water swelling capacity and cellulose crystallinity. No cellulose loss, ambient pressure, relatively moderate conditions, and high efficiency make the NMO a good alternative for pretreatment of high‐crystalline cellulosic materials. Biotechnol. Bioeng. 2010; 105: 469–476. © 2009 Wiley Periodicals, Inc.     

Compatible ionic liquid-cellulases system for hydrolysis of lignocellulosic biomass

Ionic liquids (ILs) have been increasingly recognized as novel solvents for dissolution and pretreatment of cellulose. However, cellulases are inactivated in the presence of ILs, even when present at low concentrations. To more fully exploit the benefits of ILs it is critical to develop a compatible IL‐cellulases system in which the IL is able to effectively solubilize and activate the lignocellulosic biomass, and the cellulases possess high stability and activity. In this study, we investigated the stability and activity of a commercially available cellulases mixture in the presence of different concentrations of 1‐ethyl‐3‐methylimidazolium acetate ([Emim][OAc]). A mixture of cellulases and β‐glucosidase (Celluclast1.5L, from Trichoderma reesei, and Novozyme188, from Aspergillus niger, respectively) retained 77% and 65% of its original activity after being pre‐incubated in 15% and 20% (w/v) IL solutions, respectively, at 50°C for 3 h. The cellulases mixture also retained high activity in 15% [Emim][OAc] to hydrolyze Avicel, a model substrate for cellulose analysis, with conversion efficiency of approximately 91%. Notably, the presence of different amounts of yellow poplar lignin did not interfere significantly with the enzymatic hydrolysis of Avicel. Using this IL‐cellulase system (15% [Emim][OAc]), the saccharification of yellow poplar biomass was also significantly improved (33%) compared to the untreated control (3%) during the first hour of enzymatic hydrolysis. Together, these findings provide compelling evidence that [Emim][OAc] was compatible with the cellulase mixture, and this compatible IL‐cellulases system is promising for efficient activation and hydrolysis of native biomass to produce biofuels and co‐products from the individual biomass components. Bioeng. 2011; 108:1042–1048. © 2010 Wiley Periodicals, Inc.