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

Abstract

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°C, 115°C, 125°C or 135°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 lnk₁=36.66+1.00lnC−108.0/(8.314T). The hydrolysis of acetyl groups followed a first-order kinetics. The corresponding kinetic coefficient (k_a, h⁻¹) was correlated with the operational conditions by the equation lnk_a=26.80+1.18lnC−73.37/(8.314T).

Introduction

The “biomass refinery” approach for biomass utilisation (Myerly et al., 1981) is based on the “fractionation” of lignocellulosics to obtain a variety of marketable chemicals from the polymeric fractions of the raw materials (cellulose, hemicelluloses and lignin). In this field, autohydrolysis can be conceived as a possible first processing step for an integral benefit of biomass. In this kind of treatment, hemicelluloses can be almost completely solubilised (Conner and Lorenz, 1986), whereas little alteration is caused in both lignin and cellulose, which are recovered in the solid phase.

In comparison with prehydrolysis (in which a mineral acid is added to the reaction media, see Springer, 1966; du Toit et al., 1984; Maloney et al., 1985; Ranganathan et al., 1985; Wayman and Yu, 1985; Parajó et al., 1993, Parajó et al., 1994, Parajó et al., 1995), in autohydrolysis reactions the catalytic species (hydronium ions) come from water autoionization and from the acetic acid generated from acetyl groups (Barnet et al., 1989; Overend and Chornet, 1989; Heitz et al., 1991; Aoyama et al., 1995; Saska and Ozer, 1996; Weil et al., 1997; Garrote et al., 1999a). Owing to this, autohydrolysis causes low environmental impact (no neutralisation sludges are generated) with limited equipment corrosion (Lamptey et al., 1985). Under mild operational conditions, the reaction is very selective towards cellulose decomposition and lignin repolymerization is avoided.

In most cases, autohydrolysis has been applied to lignocellulosics such as hardwoods or agricultural wastes, in which xylan was the main hemicellulose component (Garrote et al., 1999b). The liquors from typical autohydrolysis assays contain a mixture of sugar oligomers (mainly xylooligomers), monosaccharides (mainly xylose), sugar-decomposition products (such as furfural or hydroxymethylfurfural) and acetic acid (from acetyl groups). Under harsh operational conditions, condensation reactions between furfural, lignin and/or reaction intermediates may occur (Lora and Wayman, 1978; Dekker and Wallis, 1983; Muzzy et al., 1983).

The degradation of xylan during autohydrolysis has been intrepreted by means of kinetic models (Conner, 1984; Conner and Lorenz, 1986; Garrote et al., 1999a) closely related to the ones developed for prehydrolysis (Root et al., 1959; Ranganathan et al., 1985; Carrasco and Roy, 1992; Parajó et al., 1993). Under selected operational conditions, mild autohydrolysis causes the solubilisation of a substantial part of the xylan to xylose oligomers (Conner and Lorenz, 1986; Bouchard et al., 1991; Heitz et al., 1991; Garrote et al., 1999a), which cannot be directly metabolised by microorganisms. The hydrolysis of xylooligosaccharides in a sequential acid-catalysed step (posthydrolysis) provides a way to obtain xylose solutions to be used as fermentation media. For this purpose, a deep understanding of the posthydrolyis step is needed.

This work deals with the kinetic modelling of the posthydrolysis of xylooligomer-containing liquors obtained by autohydrolysis of Eucalyptus wood. The effects of temperature (in the range 100–135°C), catalyst concentration (in the range 0.5–2 wt% of sulphuric acid) and reaction time on the concentrations of xylooligomers and their degradation products as well as on the concentration of acetic acid were assessed by means of a kinetic model providing a sound basis for the studied process.