Hydrolysis of cellulose into reducing sugar via hot-compressed ethanol/water mixture

Abstract

Hydrolysis of cellulose by hot-compressed ethanol/water mixture is a promising way to obtain reducing sugar (RS) for biofuel production. The purpose of this research is to obtain high RS yield. In this work, the hydrolysis of microcrystalline cellulose by batch hot-compressed ethanol/water mixture was investigated. The influence of the ethanol/water mixture density corresponding to temperature and pressure on the RS yield was further examined. The hydrolysis conditions were optimized for high sugar yield and the crystallinity indexes (CI) of the residue and the original cellulose were analyzed. The RS yield reached as high as 98.22% under the conditions of ethanol mole fraction of 0.22, temperature of 260 °C, pressure of 5.75 MPa, reaction time of 35 s, and stirring speed of 10 Hz. The IR crystallinity index of cellulose was decreased to 0.211 from 0.618. It is shown that the RS can be produced effectively by cellulose hydrolysis in hot-compressed ethanol/water mixture.

Introduction

In recent years, renewable energy resources such as biofuels have attracted increasing attention due to the rapid increase of world energy demand and energy consumption [1], [2], [3], [4]. Globally, the means to utilize this abundantly available resource in a cost-effective way is a major research focus. Biomass contributes about 12% of the global primary energy supply and up to 40–50% in many developing countries [5]. Cellulose, which is widely present in straw, bagasse, wood and other agricultural and forestry wastes, has been applied in industries of papers, fiber, foods, and chemicals, etc. However, it has not yet been fully utilized. To further utilize cellulose materials, such methods as liquefaction and gasification are essential [6], [7]. For the conversion of cellulose into energy and chemical region, saccharification of cellulose and hemicellulose followed by fermentation is one of the methods to obtain ethanol which can be used not only as a useful chemical but also as a liquid fuel.

Thus, various methods such as enzymatic hydrolysis and acid hydrolysis as well as combination of both have been proposed and applied to hydrolyze lignocellulosic biomass as a way to recover saccharides [8], [9], [10], [11], [12], [13]. In order to further optimize the hydrolysis process, subcritical and supercritical water treatments have also been investigated to obtain saccharides for subsequent fermentation to ethanol [14], [15], [16], [17], [18], [19], [20], [21]. Firstly, Bobleter et al. [14] proposed the hydrothermal treatment of lignocellulose in subcritical water without catalyst. Sakaki et al. [15], [16] reported that cellulose was rapidly decomposed to water-soluble compounds in near-critical water by a batch reactor, and yield reached nearly 80%. Sasaki et al. [18] conducted supercritical treatment of cellulose, and found that cellulose can be converted to water-soluble saccharides more effectively in supercritical water (>374 °C, >22.1 MPa) than in subcritical water. Saka and Ueno [22] reported that large amounts of glucose and levoglucosan can be obtained from cellulose in supercritical water. Ando et al. [23] examined the decomposition behavior of plant biomass in hot-compressed water using a semi-batch reactor. The result was that more than 95% of the charged amount of biomass materials could be decomposed by hot-compressed water. However, these hydrolyzed products are further decomposed into various volatile and gaseous compounds in the severe conditions of supercritical water [21].

The above-mentioned researches have made significant breakthroughs in the field of preparing saccharides from cellulose. Nevertheless, the techniques proposed are difficult to be applied in industrial production due to the high reaction temperature and pressure, and low saccharide yield caused by the degradation. Yamazaki et al. [24] reported alcohols with longer alkyl chains could dissolve macromolecules and liquefy lignocellulose rapidly in supercritical region. Thus, we proposed to prepare saccharides from cellulose hydrolysis by hot-compressed alcohol/water mixture. Due to the low critical temperatures and pressures of this conventional alcohols (methanol, T_c: 239.5 °C, P_c: 8.1 MPa; ethanol, T_c: 240.8 °C, P_c: 6.1 MPa; isopropanol, T_c: 235.2 °C, P_c: 4.8 MPa), the critical point and the dielectric constant of the alcohol/water mixture would be lower than that of pure water, which led to milder conditions for the reaction and the increase of the solubility of relatively high molecular weight products from cellulose, hemicelluloses, and lignin. Earlier works [25], [26], [27], [28] have reported good conversion of lignocellulosic biomass into liquefied products by supercritical methanol treatment. However, up to now, few studies have been made on saccharides from cellulose hydrolysis by alcohol/water mixture treatment.

In this study, the chemical conversion of cellulose by hot-compressed alcohol/water mixture treatments was examined. The first goal of this work was to compare the hydrolysis of microcrystalline cellulose in different hot-compressed alcohol/water mixtures treatment, in which RS production was focused on. As a result, it was found that the ethanol/water mixture was the best one. Then an equation expressed the relationship between the RS yield and the density as well as the ethanol mole fraction in ethanol/water mixture system was proposed. Finally, the hydrolysis mechanism of cellulose in ethanol/water mixture was briefly discussed.