The effect of chemical and physical characteristics of spruce SEW pulps on enzymatic hydrolysis

Conifers, which are the most abundant biomass species in Nordic countries, USA, Canada and Russia, exhibit strong resistance towards depolymerization by cellulolytic enzymes. At present, it is still not possible to isolate a single structural feature which would govern the rate and degree of enzymatic hydrolysis. On the other hand, the forest residues alone represent an important potential for biochemical production of biofuels. In this study, the effect of substrate properties on the enzymatic hydrolysis of softwood was studied. Stem wood spruce chips were fractionated by SO₂–ethanol–water (SEW) treatment to produce pulps of varying composition by applying different operating conditions. The SEW technology efficiently fractionates different types of lignocellulosic biomass by rapidly dissolving hemicelluloses and lignin. Cellulose remains fully in the solid residue which is then treated by enzymes to release glucose. The differences in enzymatic digestibility of the spruce SEW pulp fibers were interpreted in terms of their chemical and physical characteristics. A strong correlation between the residual lignin content of SEW pulp and enzymatic digestibility was observed whereas cellulose degree of polymerization and hemicellulose content of pulp were not as important. For the pulps containing about 1.5 % (w/w) lignin, 90 % enzymatic digestibility was achieved at 10 FPU enzyme charge and 24 h of hydrolysis time.     

Pretreatment of sugarcane bagasse by acidified aqueous polyol solutions

Pretreatments of sugarcane bagasse by three high boiling-point polyol solutions were compared in acid-catalysed processes. Pretreatments by ethylene glycol (EG) and propylene glycol solutions containing 1.2 % H₂SO₄ and 10 % water at 130 °C for 30 min removed 89 % lignin from bagasse resulting in a glucan digestibility of 95 % with a cellulase loading of ~20 FPU/g glucan. Pretreatment by glycerol solution under the same conditions removed 57 % lignin with a glucan digestibility of 77 %. Further investigations with EG solutions showed that increases in acid content, pretreatment temperature and time, and decrease in water content improved pretreatment effectiveness. A good linear correlation of glucan digestibility with delignification was observed with R² = 0.984. Bagasse samples pretreated with EG solutions were characterised by scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction, which confirmed that improved glucan enzymatic digestibility is mainly due to delignification and defibrillation of bagasse. Pretreatment by acidified EG solutions likely led to the formation of EG-glycosides. Up to 36 % of the total lignin was recovered from pretreatment hydrolysate, which may improve the pretreatment efficiency of recycled EG solution.     

Fractionation of Sugarcane Bagasse Using a Combined Process of Dilute Acid and Ionic Liquid Treatments

Biorefineries processing lignocellulose will produce chemicals and fuels from chemical constituents, cellulose, hemicelluloses, and lignin to replace fossil-derived products. Fractionation of sugarcane bagasse into three pure streams of chemical constituents was addressed through dissolution of constituents with the ionic liquids, 1-ethyl-3-methylimidazolium acetate ([EMiM]CH₃COO) or 1-butyl-3-methylimidazolium methyl sulfate ([BMiM]MeSO₄). Constituents were isolated from the reaction mixture with the anti-solvents acetone (ā), acetone–water (AW), and sodium hydroxide (NaOH). Delignification was enhanced by NaOH, although resulting in impure product streams. Xylose pre-extraction (75 % w/w) by dilute acid pretreatment, prior to ionic liquid treatment, improved lignin purity after anti-solvent separation. Fractionation efficiency of the combined process was maximized (84 %) by ionic liquid treatment at 125 °C for 120 min, resulting in 80.2 % (w/w) lignin removal and 76.5 % (w/w) lignin recovery. Ionic liquids achieved similar degrees of delignification, although fully digestible cellulose-rich solids were produced only by [EMiM]CH₃COO treatment.     

Effect of lignin content on thermal degradation of wood pulp

A series of pulps containing between 3.6 and 23% of lignin was prepared by a careful delignification of a high-yield bisulfite pulp. The pulps were subjected to isothermal pyrolysis in a Perkin-Elmer TGS-1 thermobalance. The measurements were carried out at 8 different temperatures from 325 to 360°C under nitrogen atmosphere. The results obtained indicate that the effect of lignin on degradation depends strongly on temperature. Below 330°C, the rate of degradation varied only little with lignin. This variation becomes more important at temperatures above 330°C in that the rate of degradation increases with decreasing lignin content. The apparent activation energy of degradation ranges from 41.4 kcal mol^?1 at 23% of lignin to 67.0 kcal mol^?1 at 3.7% of lignin.     

Precipitation of lignin and extractives on kraft pulp: effect on surface chemistry, surface morphology and paper strength

The effect of pH on the formation of precipitates (lignin, extractives and metals) on kraft pulp surfaces was examined by electron spectroscopy for chemical analysis, time-of-flight secondary ion mass spectrometry and atomic force microscopy (AFM). A softwood kraft pulp slurry from an oxygen delignification stage was diluted to 3% consistency with water or an acidic Z filtrate. After heating to 70 °C the pH was lowered from 11 to 2–5, using sulphuric acid. Lignin and extractives precipitated at pH values below 6, and their amounts increased with decreasing pH. Most of the precipitated lignin was found on the pulp surface after sheet forming, whereas the main part of the precipitated extractives could be easily washed away with water. The layer of precipitated lignin was apparently thicker than the layer of extractives. AFM showed the precipitated material as a granular phase. Neither surface morphology nor surface coverage depended on the addition of Z filtrate. The amount of metals ID the pulp and on the pulp surface decreased when pH was lowered to 2. More metals, such as Ca and Mg, were detected ID the pulps as well as on the sheet surfaces when the pulp was diluted with Z filtrate. Strength and bonding properties of the pulp sheets were slightly impaired by the precipitated material. Acidification appears to be the main reason for the precipitation of both lignin and extractives on the pulp surfaces. This should be taken into account when filtrates are recycled ID the bleaching or washing of pulps.     

Mesoporous structures in never-dried softwood cellulose fibers investigated by nitrogen adsorption

Nitrogen adsorption was used to characterize mesoporous structures in never-dried softwood cellulose fibers. Distinct inflections in desorption isotherms were observed over the relative vapor pressure (P/P₀) range of 0.5–0.42 for never-dried cellulose fibers and partially delignified softwood powders. The reduction in N₂ adsorption volume was attributed to cavitation of condensed N₂ present in mesopores formed via lignin removal from wood cell walls during delignification. The specific surface areas of significantly delignified softwood powders were ~150 m² g^?1, indicating that in wood cell walls 16 individual cellulose microfibrils, each 3–4 nm in width, form one cellulose fibril bundle surrounded with a thin layer of lignin and hemicelluloses. Analysis of N₂ adsorption isotherms indicates that mesopores in the softwood cellulose fibers and partially delignified softwood powders had peaks ranging from 4 to 20 nm in diameter.     

Effects of nitren extraction on a dissolving pulp and influence on cellulose dissolution in NaOH–water

A commercial dissolving pulp was treated with aqueous solutions containing 3, 5 and 7 % of an organometalic complex (nitren) with the aim to selectively extract xylan and study its impact on the conventional physical–chemical properties of the pulp. The influence of these treatments on the pulp dissolution in a moderate solvent (8 % NaOH aqueous solution) was assessed by measuring the dissolution yields and the dissolution mechanisms. The results of this study show that nitren treatment has the effect of removing a large part of the xylan present in a dissolving pulp. It is also removing mannans and most important, it is influencing cellulose in two ways, (1) extracting it with more intensity when the nitren concentration increases, and (2) decreasing its mean molecular mass, also more evident with nitren concentration increase. The nitren extractions are favourable for the dissolution in cold NaOH–water, being more effective with higher concentrations. This chemical modification of the fiber surface leads to the disassembly of the primary wall. This allows an easier access of the NaOH reagent to regions not accessible on the initial fibres, which with the decrease of the cellulose molecular weight allows an easier dissolution and gives different dissolution mechanisms.     

Dissolution of chitin in aqueous KOH

Our NMR experiments show that chitin can dissolve well in aqueous KOH through a freeze-thawing process, and the dissolution power of the alkali solvent systems is in the order of KOH > NaOH > LiOH aqueous solution, which is totally contrary to that of cellulose in the alkali aqueous solution (i.e., LiOH > NaOH ? KOH). In this work, we systematically study the dissolution process in KOH and KOH/urea aqueous solutions. Chitin has good solubility (solubility ~80 %) in 8.4–25 wt% KOH aqueous solution at ?30 °C. The role of urea also has been investigated: unlike aqueous chitin-NaOH solutions, urea indeed enhances the solubility of chitin in KOH aqueous solutions, but the increased degree becomes unobtrusive with decreasing temperature and increasing dissolution time; the DA decline curves of chitin-KOH and chitin-KOH/urea aqueous solutions are nearly overlapping, indicating that the effect of the urea on the degree of acetylation of chitin in KOH aqueous solutions is small, similar to the NaOH/urea solvent.     

Enhanced deconstruction and dissolution of lignocellulosic biomass in ionic liquid at high water content by lithium chloride

Fabricating an aqueous ionic liquid (IL) for deconstruction and dissolution of lignocellulose is attractive because addition of water could reduce the cost and viscosity of the solvent and improve the biomass processing, but the solvating power of the IL is usually depressed in the presence of water. In the present study, an aqueous IL consisting of 1-butyl-3-methylimidazolium chloride (BmimCl), water, and lithium chloride was fabricated for efficient deconstruction and dissolution of lignocellulose (bamboo). The dissolution of cell wall components (cellulose, lignin, and hemicelluloses) in the aqueous IL was investigated. The results indicated that the presence of water significantly reduced the solvating power of BmimCl; For example, 11.5 % water decreased the dissolution of bamboo in BmimCl from ~97 to ~53 %. Dissolution of cellulose and lignin was specifically depressed. However, addition of lithium chloride was able to improve the tolerance of BmimCl to water and enhance the deconstruction and dissolution of biomass in BmimCl with high water content. It was found that approximately 80 % bamboo could be dissolved in solvent consisting of 45 wt% BmimCl and 55 wt% LiCl·2H₂O (25 wt% overall water content in the solvent). In particular, lignin and hemicelluloses were selectively dissolved by 96 and 92 %, respectively. The undissolved residue was predominantly composed of cellulose (~86 %) with a small amount of lignin (<5 %). BmimCl-LiCl-H₂O is a promising and effective solvent system with low cost and viscosity for biomass processing.     

Enhancement of the Enzymatic Digestibility and Ethanol Production from Sugarcane Bagasse by Moderate Temperature-Dilute Ammonia Treatment

In this study, sugarcane bagasse was pretreated with ammonium hydroxide, and the effectiveness of the pretreatment on enzyme hydrolysis and ethanol production was examined. Bagasse was soaked in ammonium hydroxide and water at a ratio of 1:0.5:8 for 0–4 days at 70 °C. Approximately, 14–45 % lignin, 2–6 % cellulose, and 13–22 % hemicellulose were removed during a 0.5- to 4-day ammonia soaking period. The highest glucan conversion of sugarcane bagasse soaked in dilute ammonia at moderate temperature by cellulase was accomplished at 78 % with 75 % of the theoretical ethanol yield. Under the same conditions, untreated bagasse resulted in a cellulose digestibility of 29 and 27 % of the theoretical ethanol yield. The increased enzymatic digestibility and ethanol yields after dilute ammonia pretreatment was related to a combined effect of the removal of lignin and increase in the surface area of fibers.     

The thermodynamics study on the dissolution mechanism of cellobiose in NaOH/urea aqueous solution

NaOH/urea aqueous solution is a novel, green solvent for cellulose. To explain why cellulose just be dissolved in this solvent under ?13 °C, we studied and discussed the dissolving process of cellobiose in water, urea solution, NaOH solution and NaOH/urea aqueous solution. Dissolving cellobiose in water and the urea solution absorb heat, which is an entropy-driven process. Dissolving cellobiose in NaOH solution and mixed NaOH/urea solution is exothermic, which is an enthalpy-driven process. OH^? plays an important role in the dissolving process by forming a hydrogen-bonding complex. From the thermodynamic point of view, negative entropy can well interpret why cellulose must be dissolved in cold NaOH/urea aqueous solution.     

Role of urea in alkaline dissolution of cellulose

To elucidate the role of urea in dissolution of cellulose in aqueous alkali-urea solvent, the dissolution process was monitored by differential scanning calorimetry and X-ray diffractometry. Urea had no direct interaction with cellulose in dissolution process, but promoted the decrease of crystallinity. Moreover, the addition of urea increased the dissolved fraction of cellulose in the solvent by 1.5–2.5 times and improved the thermal stability of the solution. Urea might help alkali hydrate to penetrate into crystalline region of cellulose by stabilizing the alkali-swollen cellulose molecules, leading to an increase in dissolved fraction of cellulose. This stabilization may be due to the local accumulation of urea on the hydrophobic surface, preventing the hydrophobic association of dissolved cellulose molecules.     

Mechanical deconstruction of lignocellulose cell walls and their enzymatic saccharification

Laboratory mechanical softwood pulps (MSP) and commercial bleached softwood kraft pulps (BSKP) were mechanically fibrillated by stone grinding with a SuperMassColloider^®. The extent of fibrillation was evaluated by SEM imaging, water retention value (WRV) and cellulase adsorption. Both lignin content and mechanical treatment significantly affected deconstruction and enzymatic saccharification of fibrillated MSP and BSKP. Fibrillation of MSP and BSKP cell walls occurs rapidly and then levels off; further fibrillation has only limited effect on cell wall breakdown as measured by water retention value and cellulase adsorption. Complete (100 %) saccharification can be achieved at cellulase loading of 5 FPU/g glucan for BSKP after only 15 min fibrillation with energy input of 0.69 MJ/kg. However, the presence of lignin in MSP affects the extent of fibrillation producing fibrils mainly above 1 μm. Lignin binds nonproductively to cellulases and blocks cellulose thereby reducing its accessibility. As a result, the cellulose saccharification efficiency of MSP fibrils (6 h of fibrillation, energy input of 13.33 MJ/kg) was only 55 % at same cellulase loading of 5 FPU/g glucan.     

Enzymatic hydrolysis of mercerized and unmercerized sisal pulp

Enzymatic saccharification of sisal cellulosic pulp has been investigated. Brazil leads global production of lignocellulosic sisal fiber, which has high cellulose content, an important property for producing glucose via saccharification. Hence, sisal pulp can be a good alternative for use in biorefineries. Prior to enzymatic hydrolysis, the starting pulp [85 ± 2% α-cellulose, 15 ± 2% hemicelluloses, 1.2 ± 2% insoluble lignin, viscometric average molar mass (MMvis) 19,357 ± 590 g mol^?1, crystallinity index (CI) 74%] was pretreated with alkaline aqueous solution (mercerization, 20 g of pulp L^?1, 20% NaOH, 50 °C). The changes in the properties of the cellulosic pulp during this pretreatment were analyzed [α-cellulose content, MMvis, CI, pulp fiber dimensions, and scanning electron microscopy (SEM)]. The unmercerized and mercerized (97.4 ± 2% α-cellulose, 2.6 ± 2% hemicelluloses, 0.3 ± 0.1% insoluble lignin, MMvis 94,618 ± 300 g mol^?1, CI 68%) pulps were subjected to enzymatic hydrolysis (48 h, commercial cellulase enzymes, 0.5 mL g^?1 pulp); during the reactions, aliquots consisting of unreacted pulp and liquor were withdrawn from the medium at certain times and characterized (unreacted pulp: MMvis, CI, fiber dimensions, SEM; liquor: high-performance liquid chromatography). The changes in pulp properties observed during mercerization facilitated access of enzymes to cellulose chains, and the yield of the hydrolysis reaction increased from 50.2 (unmercerized pulp) to 89.0% (mercerized pulp). These initial results for enzymatic hydrolysis of sisal pulp indicate that it represents a good alternative biomass for bioethanol production.     

Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions

Rapid dissolution of cellulose in LiOH/urea and NaOH/urea aqueous solutions was studied systematically. The dissolution behavior and solubility of cellulose were evaluated by using (13)C NMR, optical microscopy, wide-angle X-ray diffraction (WAXD), FT-IR spectroscopy, DSC, and viscometry. The experiment results revealed that cellulose having viscosity-average molecular weight ((overline) M eta) of 11.4 x 104 and 37.2 x 104 could be dissolved, respectively, in 7% NaOH/12% urea and 4.2% LiOH/12% urea aqueous solutions pre-cooled to -10 degrees C within 2 min, whereas all of them could not be dissolved in KOH/urea aqueous solution. The dissolution power of the solvent systems was in the order of LiOH/urea > NaOH/urea > KOH/urea aqueous solution. The results from DSC and (13)C NMR indicated that LiOH/urea and NaOH/urea aqueous solutions as non-derivatizing solvents broke the intra- and inter-molecular hydrogen bonding of cellulose and prevented the approach toward each other of the cellulose molecules, leading to the good dispersion of cellulose to form an actual solution.     

The influence of hemicellulose on fibril aggregation of kraft pulp fibres as revealed by FE-SEM and CP/MAS 13C-NMR

Three Norway spruce pulps were produced using different kraft pulping methods, in order to obtain large differences in cellulose and hemicellulose proportions at a similar lignin content. The hemicellulose content in the three pulps varied between 10% and 22%. The aim of the study was to evaluate the influence of cellulose and hemicellulose on fibre ultrastructure and correlate this with the differences observed in the mechanical properties between the pulps. The ultrastructure of the pulp fibres were studied using Field Emission Scanning Electron Microscopy (FE-SEM) and Solid-State Cross Polarisation Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS ¹³C-NMR) in combination with spectral fitting. CP/MAS ¹³C-NMR measured the average bulk properties of the pulp fibres, while FE-SEM allowed for observations on the ultrastructure of fibre surfaces. The ultrastructure of the fibres varied with varying hemicellulose content. The pulp with a high hemicellulose content had a porous surface structure. In fibres with a low hemicellulose content, the fibril aggregates (macrofibrils) formed a much more compact surface structure. With CP/MAS ¹³C-NMR this change was reflected by an increase in average fibril aggregate width with decreasing hemicellulose content. Results from FE-SEM and CP/MAS ¹³C-NMR correlated well. The changes recorded in ultrastructure may explain the very different mechanical properties reported previously for pulps with different hemicellulose content.     

Comparison of hot-water extraction and steam treatment for production of high purity-grade dissolving pulp from green bamboo

The performance of hot-water extraction (HWE) and steam treatment (ST), followed by kraft pulping were compared for production of high purity-grade dissolving pulp from green bamboo. With the same prehydrolysis intensity (represented by the P-factor), the fractionation efficiency of HWE is far lower than that of ST. Because of lower removal of non-cellulosic components, the solid residue from HWE (even at approximately double the prehydrolysis intensity, P-factor = 1,379) required more active alkali (AA) during kraft pulping to obtain a cellulose purity equivalent to that achieved by the ST (P-factor = 756)-kraft process. To reach equivalent hemicellulose removal, HWE required more severe intensity than ST. However, FTIR and SEM characterizations of solid residue confirmed that intensified HWE resulted in significant lignin condensation. Antagonistic effects of hemicellulose removal and lignin condensation extent on subsequent kraft pulping were therefore more apparent in HWE than that in ST. Under the same kraft pulping conditions, lignin condensation from a severely intensified HWE process (P-factor = 2,020) caused greater cellulose yield and viscosity loss than that found for ST. Finally, at a given residual pentosan or lignin content, the cellulose yields from all HWE-kraft pulps were about 3 % lower than those from ST-kraft pulps. Consequently, based on an optimally setup chlorine dioxide bleaching stage, a cellulosic pulp with alpha-cellulose content of 97.6 % and viscosity of 927 mL/g was successfully produced from a ST-kraft pulp (P-factor = 756, AA = 19 %).     

Development and Characterization of an Environmentally Friendly Process Sequence (Autohydrolysis and Organosolv) for Wheat Straw Delignification

The present work describes the delignification of wheat straw through an environmentally friendly process resulting from sequential application of autohydrolysis and organosolv processes. Wheat straw autohydrolysis was performed at 180°C during 30 min with a liquid–solid ratio of 10 (v/w); under these conditions, a solubilization of 44% of the original xylan, with 78% of sugars as xylooligosaccharides of the sum of sugars solubilized in the autohydrolysis liquors generated by the hemicellulose fraction hydrolysis. The corresponding solid fraction enrichment with 63.7% of glucan and 7.55% of residual xylan was treated with a 40% ethanol and 0.1% NaOH aqueous solution at a liquid–solid ratio of 10 (v/w), with the best results obtained at 180°C during 20 min. The highest lignin recovery, measured by acid precipitation of the extracted lignin, was 3.25 g/100 ml. The lignin obtained by precipitation was characterized by FTIR, and the crystallinity indexes from the native cellulose, the cellulose recovered after autohydrolysis, and the cellulose obtained after applying the organosolv process were obtained by X-ray diffraction, returning values of 21.32%, 55.17%, and 53.59%, respectively. Visualization of the fibers was done for all the processing steps using scanning electron microscopy.     

Nanofibrillation of dried pulp in NaOH solutions using bead milling

The drying process in typical pulp production generates strong hydrogen bonding between cellulose microfibrils in refined cell walls and increases the difficulty in obtaining uniform cellulose nanofibers. To investigate the efficacy of alkaline treatment for cellulose nanofibrillation, this study applied a bead-milling method in NaOH solutions for the nanofibrillation of dried pulps. NaOH treatments loosened the hydrogen bonding between cellulose microfibrils in dried pulps and allowed preparation of cellulose nanofibers in 8 % NaOH with a width of approximately 12–20 nm and a cellulose I crystal form. Both the nanofiber suspensions prepared in 8 and 16 % (w/w) NaOH were formed into hydrogels by neutralization because of surface entanglement and/or interdigitation between the nanofibers. When the dried pulp was fibrillated in 16 % (w/w) NaOH, the sample after neutralization had a uniquely integrated continuous network. These results can be applied to the preparation of high-strength films and fibers with cellulose I crystal forms without prior dissolution of pulps.     

Enhancing the Enzymatic Hydrolysis of Corn Stover by an Integrated Wet-milling and Alkali Pretreatment

An integrated wet-milling and alkali pretreatment was applied to corn stover prior to enzymatic hydrolysis. The effects of NaOH concentration in the pretreatment on crystalline structure, chemical composition, and reducing-sugar yield of corn stover were investigated, and the mechanism of increasing reducing-sugar yield by the pretreatment was discussed. The experimental results showed that the crystalline structure of corn stover was disrupted, and lignin was removed, while cellulose and hemicellulose were retained in corn stover by the pretreatment with 1% NaOH in 1 h. The reducing-sugar yield from the pretreated corn stovers increased from 20.2% to 46.7% when the NaOH concentration increased from 0% to 1%. The 1% NaOH pretreated corn stover had a holocellulose conversion of 55.1%. The increase in reducing-sugar yield was related to the crystalline structure disruption and delignification of corn stover. It was clarified that the pretreatment significantly enhanced the conversion of cellulose and hemicellulose in the corn stover to sugars.