Gil Garrote

Mild autohydrolysis: an environmentally friendly technology for xylooligosaccharide production from wood

Eucalyptus globulus wood samples were subjected to hydrothermal treatments under mild operational conditions (145–190 ° C, liquor to solid ratio 6–10 g g−1, reaction times up to 7.5 h). Residual xylan, xylooligosaccharides, other sugars, furfural, glucan and lignin contents were determined. Negligible effects were caused by hydrothermal treatments on both cellulose and lignin. Kinetic models were developed which describe the hydrolysis of hemicelluloses. Xylan degradation, xylooligosaccharide and xylose generation, and xylose dehydration to furfural were accurately described by models based on pseudohomogeneous, first-order kinetics with Arrhenius-type temperature dependence. These models are useful for a technical evaluation of this environmentally friendly technology. © 1999 Society of Chemical Industry

Autohydrolysis of corncob: study of non-isothermal operation for xylooligosaccharide production

Abstract Corncob samples were treated with water (autohydrolysis reaction) under non-isothermal conditions (reaction temperature up to 216°C) using a liquid to solid ratio of 8:1 kg/kg. The experimental variables considered were solid residue yield, solid residue composition (defined by the contents of cellulose, xylan, araban, acetyl groups and lignin in the solid residues) and composition of liquors (characterised by the concentrations of xylooligomers, xylose, glucose, arabinose, acetic acid and furfural). Most of the cellulose was retained in the solid phase, whereas partial delignification (up to 26% lignin removal) was achieved. Up to 94% of xylan was removed, producing xylooligomers (useful as food additives) and other reaction byproducts (including xylose and furfural). The decomposition of xylan into xylooligomers, with further generation of xylose, furfural and degradation products, was modelled by a series of parallel and consecutive reactions, which were assumed to be first order with coefficients showing an Arrhenius-type dependence on temperature.

Hydrolysis of sugar cane bagasse using nitric acid: a kinetic assessment

Abstract Sugar cane bagasse was hydrolysed using nitric acid at variable concentration (2–6%), reaction time (0–300 min) and temperature (100–128 °C). The concentration of sugars released (xylose, glucose and arabinose) and degradation products (acetic acid and furfural) were determined and the kinetic parameters of mathematical models for predicting them in the hydrolysates were obtained. The influence of temperature was also studied using the Arrhenius equation. Applying the kinetic models obtained, the optimal conditions selected were: 122 °C, 6% HNO3 and 9.3 min. Using these conditions, 18.6 g xylose/l; 2.04 g arabinose/l; 2.87 g glucose/l; 0.9 g acetic acid/l and 1.32 g furfural/l were obtained. Comparison of these results with those obtained using sulphuric and hydrochloric acids demonstrated that the nitric acid was the most efficient catalyst for hydrolysis.

Hydrothermally treated xylan rich by-products yield different classes of xylo-oligosaccharides

Four xylan rich by-products, namely wheat bran, brewerys spent grain, corn cobs and Eucalyptus wood, were characterised and subjected to a mild hydrothermal treatment in order to release and degrade the xylan from the starting materials. The chemical characterisation of the feedstock materials, with emphasis on the extracted xylan fractions and using enzymatic degradation of these xylans, resulted in rather detailed pictures of the xylans present. Depending on the feedstock material studied, the xylan present was substituted with arabinose, 4-O-methylglucuronic acid and acetyl groups. During the hydrothermal treatment, arabinose was rather easily removed from the xylan-backbone (wheat bran, brewerys spent grain and corn cobs). The acetyl groups were partly released from the feedstocks, becoming available to catalyse the depolymerisation of the xylan. Also, part of the uronic acids were released, mainly during the treatment of Eucalyptus wood. Due to the partial release of the substituents and cleavage of the xylan by the treatment performed, a wide variety of xylo-oligosaccharides with different structural features corresponding to the xylan-structure of the original feedstock were obtained. Xylo-oligosaccharides branched with arabinose were identified in the hydrolysate from brewerys spent grain, while in the hydrolysate of corn cobs and Eucalyptus wood xylo-oligosaccharides substituted with 4-O-methylglucuronic acid were present as well. Additionally, a series of partially acetylated (acidic) xylo-oligosaccharides was identified in the Eucalyptus wood hydrolysate.

Kinetic modelling of corncob autohydrolysis

Abstract Corncobs were reacted with water and treated at temperatures in the range 145–190°C during 0–12.3 h at a liquor to solid ratio of 8 or 10 kg/kg (autohydrolysis treatments), to hydrolyse the hemicellulose fraction to xylooligomers (useful as food ingredients) and xylose (a carbon source for further fermentation stages). The time-courses of xylan and xylan-degradation products (including xylooligomers, xylose, furfural and other degradation products) were established. The kinetics of xylan degradation was modelled by means of a mechanism involving sequential, first order, pseudohomogeneous kinetics. The values of the kinetic coefficients were calculated, and their dependence on temperature was established using Arrhenius-type equations. The proposed model provides a satisfactory interpretation of the experimental data, allowing the selection of optimized operational conditions.

Bioethanol production from hydrothermally pretreated Eucalyptus globulus wood

Eucalyptus globulus wood samples were pretreated in aqueous media under non-isothermal conditions to reach maximal temperatures (T(MAX)) in the range 195-250 degrees C, in order to assess the effects of the pre-treatment severity on the fractionation of wood and on the susceptibility of processed samples toward enzymatic hydrolysis. Both the fraction of cellulose susceptible to hydrolysis and the hydrolysis rate increased with the severity of the pre-treatments, but the overall glucose yield decreased for substrates pretreated at T(MAX) above 220 degrees C owing to cellulose losses. Using substrates pretreated at T(MAX)=220 degrees C, up to 94% of polysaccharides were recovered in the hydrolysis media as mono- or oligo-saccharides. High glucose to ethanol conversions were obtained operating at low enzyme charges in Simultaneous Saccharification and Fermentation mode.

Generation of xylose solutions from Eucalyptus globulus wood by autohydrolysis–posthydrolysis processes: posthydrolysis kinetics

Eucalyptus wood samples were treated with water under selected operational conditions (autohydrolysis reaction) to obtain a liquid phase containing hemicellulose-decomposition products (mainly acetylated xylooligosaccharides, xylose and acetic acid). In a further acid-catalysed step (posthydrolysis reaction), xylooligosaccharides were converted into xylose, a carbon source for further fermentation. The kinetic pattern governing the posthydrolysis step was established by reacting xylooligosaccharide-containing liquors at 100.5 degrees C, 115 degrees C, 125 degrees C or 135 degrees C in media containing 0.5, 1.0, 1.5 or 2 wt% of catalyst (sulphuric acid). The time course of the concentrations of xylooligosaccharides, xylose, furfural and acetic acid were determined, and the results were interpreted by means of a kinetic model which allowed a close reproduction of the experimental data. Almost quantitative conversion of xylooligosaccharides into xylose was achieved under a variety of experimental conditions. The first-order, kinetic coefficient for xylooligosaccharide hydrolysis (k1, h(-1)) varied with both temperature (T, K) and molar sulphuric acid concentration (C) according to the equation In k1 = 36.66 + 1.00lnC - 108.0/(8.314T). The hydrolysis of acetyl groups followed a first-order kinetics. The corresponding kinetic coefficient (ka, h(-1) was correlated with the operational conditions by the equation Inka = 26.80+ 1.18 InC - 73.37/(8.314T).

Eucalyptus globulus wood fractionation by autohydrolysis and organosolv delignification.

This work provides an assessment on the fractionation of Eucalyptus globulus wood by sequential stages of autohydrolysis (to cause the solubilization of hemicelluloses) and organosolv pulping (to dissolve lignin, leaving solids enriched in cellulose). With this approach, valuable products (hemicellulose-derived saccharides, sulphur-free lignin fragments and cellulosic substrates with low contents of residual hemicelluloses) are obtained in separate streams, according to the biomass refinery approach. Autohydrolysis was carried out under optimized operational conditions, and organosolv pulping was performed using uncatalyzed ethanol-water solutions. The effects of the most influential operational variables (autohydrolysis severity, delignification temperature and ethanol concentration in the organosolv stage) on solid yield, solid composition, cellulose susceptibility and recovery of the various fractions was assessed using statistical methods, which enabled the identification of the most favourable operational conditions.

Study on the deacetylation of hemicelluloses during the hydrothermal processing of Eucalyptus wood

Eucalyptus globulus wood samples were subjected to hydrothermal treatments at a liquor to wood ratio in the range 6–10 g/g and temperatures from 145 up to 190 °C. The effects caused by hydrothermolysis included extractive removal, hemicellulose degradation and deacetylation of both hemicelluloses and acetylated oligosaccharides. An analytical procedure based on the determination of the xylose and acetic acid contained in liquors before and after a quantitative posthydrolysis allowed the determination of acetyl groups bound to residual xylan and oligosaccharides. Since hydronium ions (the catalytic species involved in the degradation of the polymeric fractions of biomass) are mainly generated from acetic acid, special attention was paid to interpretate the time course of acetyl groups hydrolysis from both xylan and xylan-degradation products, and their interrelationship with the concentration of acetic acid is established.