Article pubs.acs.org/EF
Near-Complete Recovery of Sugar Monomers from Cellulose and Lignocellulosic Biomass via a Two-Step Process Combining Mechanochemical Hydrolysis and Dilute Acid Hydrolysis Yun Yu,* Yu Long, and Hongwei Wu* Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia S Supporting Information *
ABSTRACT: This study reports a two-step hydrolysis process for achieving near-complete recovery of sugar monomers from crystalline cellulose or lignocellulosic biomass. The first step is mechanochemical hydrolysis of the acid-impregnated sample in the solid state via ball milling at room temperature. It was found that mechanochemical hydrolysis not only effectively breaks the hydrogen-bonding network within the crystalline cellulose but also drives the acid-catalyzed hydrolysis reactions to form watersoluble products, mainly consisting of glucose and its oligomers, with a degree of polymerization up to 15. However, mechanochemical hydrolysis appears to be incapable of further hydrolyzing these oligomers into monomers and, hence, is not suitable for producing sugar monomers directly. Therefore, the second step is dilute acid hydrolysis of the mechanochemically hydrolyzed sample in the aqueous phase under low-severity conditions, i.e., at a low acid concentration of 0.25 wt % and a low temperature of 150 °C. The second dilute acid hydrolysis step can be completed rapidly (within 30 min) and achieves remarkable glucose recovery, up to ∼91% from cellulose. A key innovation of the two-step hydrolysis process is that deep depolymerization in the first step (mechanochemical hydrolysis) is not required for completely converting crystalline cellulose into water-soluble products because all sugar oligomers can be effectively hydrolyzed into monomers in the second step (dilute acid hydrolysis). Our results also show that near-complete recovery of sugar monomers (∼94%) can be achieved from wood biomass via the twostep hydrolysis process, suggesting that this technology has the potential to replace the conventional enzymatic hydrolysis to recover sugar monomers from lignocellulosic biomass.
1. INTRODUCTION Because of the diminishing fossil fuel reserves, biofuels and biochemicals produced from various biomass resources via the concept of biorefinery become increasingly important.1,2 Especially, biochemical conversion of biomass into bioethanol is a promising near-term route for producing drop-in biofuel.3 Bioethanol is by far the most widely used biofuel worldwide, with an annual production of 1.1 × 1011 L in 2011.4 The commercial bioethanol (so-called first-generation bioethanol) is mainly produced from corn in the U.S. or sugar cane in Brazil5 but only provides marginal energy and environmental benefits over gasoline, triggering the debate on “food versus fuel” competition.6 Therefore, bioethanol from non-food lignocellulosic biomass (so-called cellulosic ethanol as a secondgeneration biofuel) is more promising.7 However, cellulosic ethanol production at a commercial scale is a great challenge as a result of its significantly higher production cost. Lignocellulosic biomass is a complex mixture of three nature polymers (cellulose, hemicellulose, and lignin) that are strongly intermeshed and also chemically bonded by covalent crosslinkages and non-covalent forces.8 Its complex structure makes the material recalcitrant toward efficient chemical and biological treatments. The hydrolysis of cellulose is more difficult than that of hemicellulose9 as a result of the presence of inter- and intramolecular hydrogen-bonding networks within cellulose.10,11 Acid hydrolysis and enzymatic hydrolysis are the two main technologies to recover glucose from cellulose,11 each having its major disadvantages that limit further commercialization. Acid hydrolysis suffers from a low sugar yield, a high acid © XXXX American Chemical Society
consumption, and the formation of large amounts of gypsum as waste.11 The use of concentrated acid increases the reactor costs because of the expensive corrosion-resistant materials. Enzymatic hydrolysis is costly as a result of the high enzyme cost and expensive reactor cost because of slow enzymatic reaction. Especially, a pretreatment process is essential to enhance the efficiency of enzymatic hydrolysis of lignocellulosic biomass, further increasing the process cost.12−15 Therefore, a cost-effective and energy-efficient hydrolysis process is required to recover the sugar monomers from lignocellulosic biomass. Several novel pretreatment methods have been developed to disrupt the crystalline structure of cellulose, with the key objectives of making it more accessible and improving the reactivity.16,17 Ionic liquid was found to dissolve cellulose, even wood,18,19 significantly improving the reactivity of cellulose during the following hydrolysis process.20 However, the expensive costs associated with ionic liquid manufacturing and recycling limit its practical application. Hydrothermal processes at high temperatures (i.e., >230 °C) can also effectively break down the hydrogen bonds in cellulose and depolymerize cellulose into sugar oligomers that can be easily converted into glucose via acid hydrolysis or enzymatic Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 23, 2015 Revised: October 29, 2015
A
DOI: 10.1021/acs.energyfuels.5b02196 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels hydrolysis.10,21−23 However, dependent upon the conditions, hydrothermal processes can lead to significant degradation of sugar products and the reactor may also be costly. Mechanical treatment (e.g., ball milling) was also reported to effectively break the hydrogen bonding in cellulose, increasing the reactivity during the following hydrolysis.24−27 However, the required treatment time is typically in the time scale of hours, which requires significant energy input. Recently, mechanochemical treatment has emerged as a solvent-free technique to completely depolymerize cellulose and biomass into water-soluble sugar oligomers and monomers via driving acid-catalyzed hydrolysis reactions by mechanical force.28−30 However, there are at least two disadvantages that greatly limit its future application for the production of sugar monomers.31 One is that the energy input during mechanochemical treatment is extremely high because the treatment is reported to take several hours to complete.32 The other is that the sugar products from mechanochemical treatment mainly consist of sugar oligomers, and the yields of monomers are low (1%) and temperatures (>150 °C).43 Second, enzymes are expensive (i.e., ∼16% of the ethanol production cost43), and the enzymatic reaction is very slow, generally taking more than 3 days to complete. Because of the long reaction time, the enzymatic hydrolysis process generally requires sizable reactors of large footprints. In contrast, the two-step hydrolysis process only requires a lowseverity dilute acid condition (i.e., a low acid condition of 0.25 wt % and a low temperature of
