Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis

Production of ethanol by bioconversion of lignocellulosic biomass has attracted much interest in recent years. However, the pretreatment process for increasing the enzymatic digestibility of cellulose has become a key step in commercialized production of cellulosic ethanol. During the last decades, many pretreatment processes have been developed for decreasing the biomass recalcitrance, but only a few of them seem to be promising. From the point of view for integrated utilization of lignocellulosic biomass, organosolv pretreatment provides a pathway for biorefining of biomass. This review presents the progress of organosolv pretreatment of lignocellulosic biomass in recent decades, especially on alcohol, organic acid, organic peracid and acetone pretreatments, and corresponding action mechanisms. Evaluation and prospect of organosolv pretreatment were performed. Finally, some recommendations for future investigation of this pretreatment method were given.     

Steam pretreatment of lignocellulosic material for enhanced enzymatic hydrolysis

Pretreatment methods were compared with steam explosion, and differing views on the relative importance of mechanical and chemical effects were outlined. Hydrolysis was desirable; pyrolysis was undesirable. The effects of initial moisture content on steam consumption, mechanism and rate of heat transfer, pentosan solubilization, and subsequent glucose yield were summarized. The insignificant effect, after treatment at 240 degrees C, of 90% pressure bleed-down before explosion on subsequent simultaneous saccharification and fermentation (SSF) yields was described. Treatment at 190 degrees C with complete bleed-down (no explosion), when compared with that at 240 degrees C with explosion from full pressure, showed at least as good solubilizatoin of pentosan, enzymatic hydrolysis, and SSF but showed greater pentosan destruction for the same degree of pentosan removal. Water washing of unexploded steamed aspenwood chips was at least as efficient as that of similarly treated but exploded chips. Scanning electron micrographs of unexploded chips showed extensive rupturing of vessel pit membranes and other morphological features associated with steam-exploded wood. Neither the explosion nor the high temperatures (above 190 degrees C) are necessary.     

Comparison of different pretreatment methods for lignocellulosic materials. Part I: conversion of rye straw to valuable products

The conversion of lignocellulose to valuable products requires I: a fractionation of the major components hemicellulose, cellulose, and lignin, II: an efficient method to process these components to higher valued products. The present work compares liquid hot water (LHW) pretreatment to the soda pulping process and to the ethanol organosolv pretreatment using rye straw as a single lignocellulosic material. The organosolv pretreated rye straw was shown to require the lowest enzyme loading in order to achieve a complete saccharification of cellulose to glucose. At biomass loadings of up to 15% (w/w) cellulose conversion of LHW and organosolv pretreated lignocellulose was found to be almost equal. The soda pulping process shows lower carbohydrate and lignin recoveries compared to the other two processes. In combination with a detailed analysis of the different lignins obtained from the three pretreatment methods, this work gives an overview of the potential products from different pretreatment processes.     

Partial acid hydrolysis of cellulosic materials as a pretreatment for enzymatic hydrolysis

Partial acid hydrolysis was studied as a per treatment to enhance enzymatic hydrolysis, such a pretreatment was carried out in a continuous flow reactor on oak corn Stover, newsprint, and Solka Floc at temperatures ranging from 160 to 220°C, acid concentration ranging from 0 to 1.2%, and a fixed treatment time of 0.22 min. The resulting slurries and solids were than hydrolyzed with Trichoderma ressei QM 9414 cellulase at 50°C for 48 hr. For all substrates except Solka Floc, increased glucose yields were achieved during enzymatic hydrolysis of the pretreated materials as compared to hydrolysis of the original substrate. In several cases, after pretreatment, 100° of the potential glucose content of the substrate was converted to glucose after 24hr of enzymatic hydrolysis. It is felt that the increased glucose yields achieved after this pretreatment are due to acid's removal of hemicellulose, reduced degree of polymerization, and possibly due to a change in the crystal structure of the cellulose.     

Enzymic hydrolysis of lignocellulosic materials: I. Models for the hydrolysis process--a theoretical study

At the end of an enzymic hydrolysis process involving a solid lignocellulosic substrate, enzymes are found both in solution and absorbed to the substrate residue. Removal of residue from the system will result in loss of some of the enzymes, the extent of which will depend on the design of the process. To minimize enzyme loss, a study has been conducted in which six process models have been formulated and an enzyme loss function derived for each model based on the total amount of enzymes lost through residue removal. Model 1 is a reference model, characterized by an uninterrupted hydrolysis throughout the entire hydrolysis period. The residue is then washed in order to recover both sugar and adsorbed enzymes before the residue is discarded. Models 2-6 are all characterized by the removal of hydrolysate three times during the process, recirculation of dissolved and adsorbed enzymes to various points in the process and selection of a stage at which the residue is removed. The following conclusions could be drawn from the derived enzyme loss functions: Increased enzyme adsorption leads to increased enzyme loss.The enzyme loss decreases if the solid residue is removed late in the process.Both adsorbed and dissolved enzymes should be introduced at the starting point of the process. This is particularly important for dissolved enzymes. Three models were chosen for experimental studies, which are reported in a second, accompanying article. The experimental results obtained are compared with the theoretical study reported here.     

Cholinium carboxylate ionic liquids for pretreatment of lignocellulosic materials to enhance subsequent enzymatic saccharification

The present study is the first report demonstrating that ionic liquids consisting of cholinium cations and linear carboxylate anions ([Ch][CA] ILs) can be used for pretreatment of lignocellulosic materials to enhance subsequent enzymatic saccharification. Six variants of [Ch][CA] ILs were systematically prepared by combining cholinium cations with linear monocarboxylate anions ([C_nH_2n+1–COO]⁻, n = 0–2) or dicarboxylate anions ([HOOC–C_nH_2n+1–COO]⁻, n = 0–2). These [Ch][CA] ILs were analyzed for their toxicity to yeast cell growth and their ability to pretreat kenaf powder for subsequent enzymatic saccharification. When assayed against yeast growth, the EC₅₀ for choline acetate ([Ch][OAc]) was 510 mM, almost one order of magnitude higher than that for 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]). The cellulose saccharification ratio after pretreatment at 110 °C for 16 h with [Ch][OAc] (100.6%) was almost comparable with that after pretreatment with [Emim][OAc]. Therefore, [Ch][OAc] is a biocompatible alternative to [Emim][OAc] for lignocellulosic material pretreatment.     

Hydrolysis of lignocellulosic materials for ethanol production: a review

Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. There are mainly two processes involved in the conversion: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars, and fermentation of the sugars to ethanol. The cost of ethanol production from lignocellulosic materials is relatively high based on current technologies, and the main challenges are the low yield and high cost of the hydrolysis process. Considerable research efforts have been made to improve the hydrolysis of lignocellulosic materials. Pretreatment of lignocellulosic materials to remove lignin and hemicellulose can significantly enhance the hydrolysis of cellulose. Optimization of the cellulase enzymes and the enzyme loading can also improve the hydrolysis. Simultaneous saccharification and fermentation effectively removes glucose, which is an inhibitor to cellulase activity, thus increasing the yield and rate of cellulose hydrolysis.     

Comparison of different pretreatment methods for lignocellulosic materials. Part II: Influence of pretreatment on the properties of rye straw lignin

Several processes have been suggested to convert various types of lignocellulosic biomass into lignin products and saccharides. This paper evaluates the suitability of an organosolv process, a process using soda, a hydrothermal process and a process developed in this work, called the “Aquasolve process” for inclusion into a lignocellulosic biorefinery concept. Part II of this paper investigates the influence of the different pretreatment processes on the properties of rye straw lignin and evaluates their ability to produce high recoveries of high quality lignin.Specifications for high quality lignin products are defined and the isolated lignin fractions are analysed by Klason lignin, carbohydrate and ash content, elemental analysis, thermo-gravimetric analysis, ³¹P NMR, and size exclusion chromatography. The organosolv process shows the largest lignin recovery, followed by the soda and Aquasolve processes. Lignin products from the soda process, the Aquasolve process and with reservation the organosolv process show interesting properties for polymer applications.     

Differential speed two roll mill pretreatment of cellulosic materials for enzymatic hydrolysis

Differential speed two roll milling is an effective pretreatment for increasing the susceptibility of cellulose to enzymatic hydrolysis. Using mills with three, six, and ten in. diam rolls and processing times of 10 min or less results in the following percent increases in susceptibility over untreated controls: cotton, 1100; maple chips, 1600; white pine chips, 600; newspaper, 125. In comparison, ball milling of newspaper for 24 hr gives only a 62% increase. A further advantage of the roll mill is the increased wet density of the product permitting higher slurry concentrations during hydrolysis. Important parameters of mill effectiveness are roll clearance and processing time.     

Analysis of conversion and operation strategies for enzymatic hydrolysis of lignocellulosic biomass in a series of CSTRs with distributed feeding

Design considerations for enzymatic hydrolysis of lignocellulosic biomass in two and three continuous stirred tank reactors (CSTRs) in series with distributed feeding of substrate and enzyme, followed by a series of CSTRs, are discussed. A previously developed, fitted, and validated kinetic model is extended to accommodate distributed feeding and used along with the micromixing limiting situations of macrofluid and microfluid to describe the reaction system. The capabilities of the reaction system proposed are explored for a range of cumulative substrate concentration from 5 to 20% w/w (dry basis). Continuous distributed feeding does not show advantages in terms of cellulose conversion when compared with the operation where an equivalent mass of substrate is added at the first reactor of the series, but the potential to increase substrate concentration beyond the concentrations that can be handled in conventional CSTRs, and therefore, the volumetric productivity of reactors, is evident.     

Enzymatic hydrolysis of lignocellulosic materials: II. Experimental investigations of theoretical hydrolysis--process models for an increased enzyme recovery

Stream pretreatment of wheat straw solubilized most of the xylan present. Xylose and other sugars were recovered by washing the substrate with water but only a minor part (34%) was monomeric. Treatment of this solutions with celulases and hemicellulases improved the yield of monomeric sugars to 69%, the main product being xylose. Some xylose was also obtained during enzymatic hydrolysis of the solid substrate although the pretreatment step contributed 64% (mean value) of total xylose formed. A reference model, No. 1, and two other models, Nos. 2 and 4, described in the first part of this article series (this issue) have been studied experimentally and results confirm the theoretical conclusions. An uninterrupted hydrolysis over a given time period leads to a lower degree of saccharification than when hydrolysate is withdrawn several times. Saccharification is also favored if the residue is removed at a late stage, i.e., at the end of the 24 h hydrolysis cycle. Extended recirculation of the enzymes during a 4 x 24-h experimental period gave the following average yields of saccharification on a 24-h basis: 65% (Reference), 73% (Model 2), and 79% (Model 4). It is concluded that enzyme recovery with model 4 is 70% or more, while the Reference and Model 2 attain a lower level of recovery. The design of an improved hydrolysis model is also discussed.     

Pretreatment of lignocellulosic material with fungi capable of higher lignin degradation and lower carbohydrate degradation improves substrate acid hydrolysis and the eventual conversion to ethanol

Phanerochaete chrysosporium, Pycnoporus cinnabarinus,and fungal isolates RCK-1 and RCK-3 were tested for their lignin degradation abilities when grown on wheat straw (WS) and Prosopis juliflora (PJ) under solid-state cultivation conditions. Fungal isolate RCK-1 degraded more lignin in WS (12.26% and 22.64%) and PJ (19.30% and 21.97%) and less holocellulose in WS (6.27% and 9.39%) and PJ (3.01% and 4.58%) after 10 and 20 days, respectively, than other fungi tested. Phanerochaete chrysosporium caused higher substrate mass loss and degraded more of holocellulosic content (WS: 55.67%; PJ: 48.89%) than lignin (WS: 18.89%; PJ: 20.20%) after 20 days. The fungal pretreatment of WS and PJ with a high-lignin-degrading and low-holocellulose-degrading fungus (fungal isolate RCK-1) for 10 days resulted in (i) reduction in acid load for hydrolysis of structural polysaccharides (from 3.5% to 2.5% in WS and from 4.5% to 2.5% in PJ), (ii) an increase in the release of fermentable sugars (from 30.27 to 40.82 g L(-1) in WS and from 18.18 to 26.00 g L(-1) in PJ), and (iii) a reduction in fermentation inhibitors (total phenolics) in acid hydrolysate of WS (from 1.31 to 0.63 g L(-1)) and PJ (from 2.05 to 0.80 g L(-1)). Ethanol yield and volumetric productivity from RCK-1-treated WS (0.48 g g(-1) and 0.54 g L(-1) h(-1), respectively) and PJ (0.46 g g(-1) and 0.33 g L(-1) h(-1), respectively) were higher than untreated WS (0.36 g g(-1) and 0.30 g L(-1) h(-1), respectively) and untreated PJ (0.42 g g(-1) and 0.21 g L(-1) h(-1), respectively).     

Highly efficient solubilization of natural lignocellulosic materials by a commercial cellulase immobilized on various solid supports

Summary A commercial preparation of cellulase was immobilized on CNBr-sepharose, ConA-sepharose, and CNBr-glass beads. When filter paper was used as the substrate, the specific activity of the enzyme immobilized on ConA-sepharose was more than twice that of the soluble enzyme, while the activity of the enzymes immobilized on the other two substrates was either very slightly (CNBr-sepharose) or slightly (CNBr-glass beads) reduced. The immobilized enzymes showed alterations both in the Km and V max values: these were generally either slightly increased (Km) or reduced (V max). In addition, the immobilized enzymes were more resistant to inhibition both by glucose and cellobiose, they were all more stable than the soluble enzyme and solubilized three different natural lignocellulosic materials (alfa-alfa, wheat straw, and pine needles) to a much greater or significantly greater extext than the soluble enzyme: the ConA-sepharose cellulase was the most efficient. The possibility of reusing the immobilized enzyme was also tested. It was found that the ConA-sepharose cellulase could be reused five times with a final loss of activity that ranged between 30% and 50%.     

Thermodynamic analysis of lignocellulosic biofuel production via a biochemical process: guiding technology selection and research focus

The aim of this paper is to present an exergy analysis of bioethanol production process from lignocellulosic feedstock via a biochemical process to asses the overall thermodynamic efficiency and identify the main loss processes. The thermodynamic efficiency of the biochemical process was found to be 35% and the major inefficiencies of this process were identified as: the combustion of lignin for process heat and power production and the simultaneous scarification and co-fermentation process accounting for 67% and 27% of the lost exergy, respectively. These results were also compared with a previous analysis of a thermochemical process for producing biofuel. Despite fundamental differences, the biochemical and thermochemical processes considered here had similar levels of thermodynamic efficiency. Process heat and power production was the major contributor to exergy loss in both of the processes. Unlike the thermochemical process, the overall efficiency of the biochemical process largely depends on how the lignin is utilized.     

Effect of peracetic acid, sodium hydroxide and phosphoric acid on cellulosic materials as a pretreatment for enzymatic hydrolysis

Crystalline cellulose and cellulosic wastes have been treated with various concentrations of peracetic acid and other reagents at 100°C for various times, washed with water, ethanol and air dried. For each treated cellulose, the degree of enzymatic solubilization was measured with Trichoderma viride cellulase [1,4-(1,3;1,4)-β-d-glucan 4-glucanohydrolase, EC 3.2.1.4]. Cellulosic wastes such as sunflower stalks, wheat straw and sugar-cane bagasse were solubilized effectively by the enzyme. Delignification of wheat straw with 1% sodium hydroxide and treatment of this straw with peracetic acid enhanced the degree of enzymatic solubilization. Infrared spectra of the untreated and treated cellulosic wastes were recorded.     

Organosolv pretreatment for enzymatic hydrolysis of poplars: I. Enzyme hydrolysis of cellulosic residues

Aspen (Populus tremuloides) and black cottonwood (Populus trichocarpa) organosolv pulps produced in a wide range of solvent composition (between 30 and 70% by volume of methanol) and catalysts (H(2)SO(4) and H(3)PO(4)) such that the cooking liquor pH 相似文献   

Gamma-ray irradiation as a pretreatment for the enzymatic hydrolysis of cellulose

Summary The susceptibility of cellulose to enzymatic hydrolysis can be significantly affected through pretreatment by means of gamma-ray radiation. Experiments were carried out to investigate the effects of this radiation on enzymatic hydrolysis and on the two major structural features of cellulose that most influence hydrolysis, namely, specific surface area and crystallinity.D. H. Beardmore is currently with Phillips Petroleum Company, Bartlesville, OK 74004, U.S.A.     

Autohydrolysis extraction process as a pretreatment of lignocelluloses for their enzymatic hydrolysis

Autohydrolysis was studied as a pretreatment to enhance sugar yields from enzymatic hydrolysis of wheat and rape straw, beech, birch and poplar sawdust. Reaction temperatures were 185°C to 212°C and the reaction time 20 min. The pretreated slurries were hydrolyzed with “Novo” cellulase and Fusarium sp. 27 cellulase at 45°C and pH 4.8 for 24 h with addition of Fusarium sp. 27 cellbound cellobiase. From 85% to 90% sugar content of substrates were converted to reducing sugars after 24 h enzymatic hydrolysis, with exception of poplar wood. 10.8 g biomass was obtained after cultivation of Fusarium sp. 27 with water solution hemicellulose fraction from 100 g beech sawdust autohydrolyzed at 200°C during 20 min.     

The impacts of pretreatment on the fermentability of pretreated lignocellulosic biomass: a comparative evaluation between ammonia fiber expansion and dilute acid pretreatment

The sustainable production of biofuels will require the efficient utilization of lignocellulosic biomass. A key barrier involves the creation of growth-inhibitory compounds by chemical pretreatment steps, which ultimately reduce the efficiency of fermentative microbial biocatalysts. The primary toxins include organic acids, furan derivatives, and phenolic compounds. Weak acids enter the cell and dissociate, resulting in a drop in intracellular pH as well as various anion-specific effects on metabolism. Furan derivatives, dehydration products of hexose and pentose sugars, have been shown to hinder fermentative enzyme function. Phenolic compounds, formed from lignin, can disrupt membranes and are hypothesized to interfere with the function of intracellular hydrophobic targets. This review covers mechanisms of toxicity and tolerance for these compounds with a specific focus on the important industrial organism Escherichia coli. Recent efforts to engineer E. coli for improved tolerance to these toxins are also discussed.