The Effect of Organosolv Pretreatment Variables on Enzymatic Hydrolysis of Sugarcane Bagasse
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Chemical Engineering Journal 168 (2011) 1157–1162 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej The effect of organosolv pretreatment variables on enzymatic hydrolysis of sugarcane bagasse L. Mesa a, E. González a, C. Cara b, M. González a, E. Castro b, S.I. Mussatto c,∗ a Center of Analysis Process, Faculty of Chemistry and Pharmacy, Central University of Las Villas, Villa Clara, Cuba b Department of Chemical Environmental and Materials Engineering, Faculty of Experimental Sciences, University of Jaén, Jaén, Spain c Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal article info abstract Article history: Sugarcane bagasse pretreated with dilute-acid was submitted to an organosolv ethanol process with Received 10 November 2010 NaOH under different operational conditions (pretreatment time, temperature, and ethanol concentra- Received in revised form 27 January 2011 tion) aiming to maximize the glucose yield in the subsequent enzymatic hydrolysis stage. The different Accepted 3 February 2011 pretreatment conditions resulted in variations in the chemical composition of the solid residue as well as in the glucose recovered by enzymatic hydrolysis. All the studied variables presented significant (p < 0.05) Keywords: influence on the process. The optimum organosolv pretreatment conditions consisted in using 30% (v/v) Sugarcane bagasse ethanol at 195 ◦C, during 60 min. Enzymatic hydrolysis of the residue then obtained produced 18.1 g/l Organosolv Ethanol glucose, correspondent to a yield of 29.1 g glucose/100 g sugarcane bagasse. The scale-up of this process, Enzymatic hydrolysis by performing the acid pretreatment in a 10-l semi-pilot reactor fed with direct steam, was success- Glucose fully performed, being obtained a glucose yield similar to that found when the acid pretreatment was performed in autoclave. © 2011 Elsevier B.V. All rights reserved. 1. Introduction more accessible to enzymes. Pretreatment is non-trivial owing to the heterogeneity of the lignocellulosic materials and the tight The economic feasibility of second-generation bioethanol, i.e. three-dimensional structure of them due to the network of lignin, ethanol produced from lignocelluloses, depends among other fac- hemicellulose, and cellulose [3]. During the last decades, many pre- tors on the availability of cheap feedstocks [1]. In Cuba, sugarcane treatment processes, including the use of dilute-acid [4,5], steam bagasse (the solid residue obtained after extraction of the sugarcane explosion [1,6], wet oxidation [7,8], organosolvents [9–11], among juice) is a residue available in large quantities and its use to pro- others, have been developed for decreasing the biomass recalci- duce fuels and chemicals would contribute to decrease the nation’s trance, but only a few of them seem to be promising. dependence on oil importation. Currently, part of the sugarcane Among the pretreatment technologies, organosolv process has bagasse generated in the sugar-mills is used for producing steam been considered as one of the most promising for second genera- and electricity required for the cane processing plant. However, tion ethanol [12,13]. Treatment with organosolvents involves the large amounts still remain unused, and could be employed in many use of an organic liquid (methanol, ethanol, acetone, ethylene glycol practical applications, such as raw material for ethanol production. or triethylene glycol) and water, with or without addition of a cat- The conversion of lignocellulosic residues to ethanol is a topic alyst agent (acid or base). This mixture partially hydrolyzes lignin of great interest nowadays. This process, which consist in a pre- bonds and lignin–carbohydrate bonds, resulting in a solid residue treatment of the raw material for hemicellulose sugars extraction, composed mainly by cellulose and some hemicellulose [14,15]. followed by a treatment (usually enzymatic) for the cellulose Organosolv pretreatments efficiently remove lignin from ligno- conversion to glucose that will be converted to ethanol by fermen- cellulosic materials but most of the hemicellulose sugars are also tation, has been strongly studied but there are some challenges solubilized by this process. Therefore, a combined use of organosolv to be overcome to achieve an efficient production on commercial process with a previous stage of dilute-acid hydrolysis, to separate scale [2]. The main techno-economic challenge is the develop- hemicellulose and lignin in two consecutive fractionation steps, ment of cost-effective pretreatment methods to make cellulose would be useful to produce a pulp enriched in cellulose, avoiding losses of potential valuable sources from hemicellulose. Organo- solv process is also reported to be able to produce a large amount ∗ of a high-quality lignin that is relatively pure, primarily unaltered, Corresponding author. Tel.: +351 253 604 424; fax: +351 253 604 429. E-mail addresses: [email protected], [email protected] and less condensed than Kraft lignins. Such lignin is partially solu- (S.I. Mussatto). ble in many organic solvents and could be applied in the fields of 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.02.003 1158 L. Mesa et al. / Chemical Engineering Journal 168 (2011) 1157–1162 adhesives, films and biodegradable polymers [16]. The use of the Table 1 lignin and hemicellulose fractions obtained during the lignocellu- Experimental range and levels of the process independent variables evaluated for organosolv ethanol pretreatment of acid-pretreated sugarcane bagasse, according losic biomass fractionation is of large importance in a biorefinery toa23 full factorial design. concept. Low boiling point alcohols, mainly methanol and ethanol, seem Independent variable Symbol Range and levels to be the most suitable organic liquids for use in organosolv −10 +1 processes, due to their low cost and easy recovery. However, pre- Pretreatment time (min) x1 20 40 60 ◦ treatment with ethanol is safer because ethanol is less toxic than Temperature ( C) x2 175 185 195 methanol [17]. In addition, substrates pretreated by organosolv Ethanol concentration (% v/v) x3 10 20 30 ethanol process have been reported to have superior enzymatic digestibility over those pretreated by the other alternative pro- cesses [18]. The use of sodium hydroxide as catalyst agent during ratio of 1:7 w:w, at different pretreatment times, temperatures 3 organosolv ethanol pretreatment greatly improves the ethanol and ethanol concentrations, according to a 2 full factorial design selectivity with respect to lignin, i.e., improves the delignifying (Table 1). A 3% (w/w on dry fiber) NaOH concentration was used ability of ethanol [19]. Otherwise, ethanol also reduces the surface in the solutions of all the experiments. At the end of the reac- tension of the pulping liquor favoring the alkali penetration into tions, the reactors were immediately cooled in ice bath, and the the material structure, and the lignin removal, as a consequence obtained hydrolysate was separated from the residual solid by fil- tration. The pretreated solids were washed with water to remove [20]. ◦ The efficiency of the organosolv ethanol process with alkali residual ethanol and alkali, dried at 40 C, and a sample of each one may be significantly improved when the lignocellulosic material of them was analyzed to determine the remaining glucose, xylose has been previously submitted to an acid catalyzed pre-hydrolysis and lignin contents. All the reactions were carried out in duplicate. [21]. Considering this fact and all the above mentioned reasons, the present work had as objective to evaluate the sugarcane 2.3. Enzymatic hydrolysis bagasse fractionation by organosolv ethanol pretreatment with NaOH. More specifically, the effect of organosolv pretreatment vari- Enzymatic hydrolysis of the solid residues obtained after ables on enzymatic hydrolysis of sugarcane bagasse was evaluated. organosolv ethanol pretreatment was performed by using a Reactions were performed under different operational condi- commercial cellulase concentrate (Celluclast 1.5 L) supplemented tions (organosolv pretreatment time, temperature, and ethanol with -glucosidase (Novozym 188), both from Novozymes (A/S concentration), according to a 23 full-factorial design. The solid Bagsvaerd, Denmark). For the reactions, a cellulase loading of residue obtained in each experimental condition was enzymatically 15 filter paper units (FPU)/g substrate, and a -glucosidase load- hydrolyzed, being the released glucose concentration and the glu- ing of 15 international units (IU)/g substrate were added to 50 mM cose yield per gram of sugarcane bagasse determined. The design sodium citrate buffer (pH 4.8) and them mixed to the solid sub- allowed to define the pretreatment variables of great influence on strate to give a 5% (w/v) consistency. The enzymatic hydrolysis ◦ enzymatic hydrolysis of sugarcane bagasse and to define the con- experiments were performed in shaker at 50 C, 150 rpm for 24 h. ditions able to maximize the glucose recovery yield. Glucose content released in the reactions was quantified by HPLC. The glucose yield was expressed as the ratio between the amount of glucose released in the enzymatic hydrolysis and the amount of 2. Material and methods initial raw material. 2.1. Raw material and dilute-acid pretreatment 2.4. Experimental design