Efficient hydrolysis of chitosan in ionic liquids Zehui Zhang a,b , Changzhi Li a , Qian Wang a , Zongbao K. Zhao a,c, * a Dalian Institute of Chemical Physics, CAS, Dalian 116023, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China c Dalian National Laboratory of Clean Energy, Dalian 116023, PR China article info Article history: Received 10 April 2009 Received in revised form 22 May 2009 Accepted 1 June 2009 Available online 6 June 2009 Keywords: Chitosan Hydrolysis Ionic liquids Glucosamine Chitin abstract Effective hydrolysis of chitosan, the N-deacetylated product of chitin, remains challenging. Here, we report acid-promoted hydrolysis of chitosan in imidazolium based ionic liquids with good total reducing sugars (TRS) yield under mild conditions. TRS yield reached over 60% in the presence of about 6.0 wt% concentrated hydrochloric acid at 100 °C within 7 h. Kinetic modeling of a typical experimental data set suggested that the hydrolysis most likely followed a consecutive first-order reaction sequence, where k 1 and k 2 , the rate constants for TRS formation and degradation, were determined to be 0.01372 and 0.00015 min 1 , respectively. Our method may be useful to explore new applications of natural chitin resources. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Chitin is a naturally occurring biopolymer composed of b-(1,4)- linked N-acetylglucosamine (GlcNAc) unit. It is the second most abundant natural polymer after cellulose and has a similar structure to cellulose. Chitin exists in several forms depending on its crystal- line structure. Two of these crystalline polymorphic forms are a-chi- tin and b-chitin (Muzzarelli, Ilari, Tarsi, Dubini, & Xia, 1994). The b-chitin crystallizes in a monoclinic cell. However, the packing structure of a-chitin is strongly stabilized by intrachain, intrasheet, and intersheet hydrogen bonds in three-dimensional configuration (Gardner & Blackwell, 1975). Therefore, a-chitin exhibits lower reactivity, swelling, and solubility than b-chitin (Kurita, Ishii, Tomi- ta, Nishimura, & Shimoda, 1994; Saito, Okano, Gaill, Chanzy, & Put- aux, 2000; Saito, Putaux, Okano, Gaill, & Chanzy, 1997). Yet, a- chitin extracted from crab shells or shrimp is much more abundant due to its marine product origin and is more widely utilized. Chito- san is the N-deacetylated product of chitin and its major resource is also from marine product. Recently, chitosan has attracted much attention owing to its broad rang of applications in the food, medi- cine, cosmetic, material etc. (Laudenslager, Schiffman, & Schauer, 2008; Li, Dunn, Grandmaison, & Goosen, 1992), but it remains undervalued partially due to its poor solubility. Hydrolysis of chito- san to gulcosamine and its oligomers perhaps is the most common way to improve its application. Several methods, including enzymatic hydrolysis, acidic hydroly- sis or oxidative depolymerization, have been applied to hydrolyze chitosan to produce glucosamine and its oligomers (Fig. 1). The enzymatic process takes place under mild conditions, yet the hydro- lysis rate is slow. Furthermore, the prices of the enzymes are high and the enzymes lose activity easily (Konieczna-Molenda, Fied- orowicz, Zhong, & Tomasik, 2008; Ming, Kuroiwa, Ichikawa, Sato, & Mukataka, 2006). Acid hydrolysis is routinely practiced to attain glucosamine and oligochitosans. Chitosan hydrolysis with concen- trated hydrochloric acid requires excess acid loading, complex reac- tors, and has major waste disposal problems (Einbu & Vårum, 2008; Horowitz, Roseman, & Blumenthal, 1957). Furthermore, an excess of acid treatment results in the breakdown of glucosamine, which sig- nificantly lowers the yield and interferes with downstream applica- tions. Oxidative depolymerization in concentrated nitrous acid provides chitosan oligomers with 9–18 monomeric units, and the fi- nal products contained 2,5-anhydromannose residues by deamina- tion (Allan & Peyron, 1995; Furusaki, Ueno, Sakairi, Nishi, & Tokura, 1996). Up to now, effective hydrolysis of chitin and chitosan remains challenging. Ionic liquids (ILs), combining good and tunable solubility proper- ties with a negligible vapor pressure and excellent thermal stability, have recently been used for dissolving biological macromolecules including cellulose, wool keratin and silk fibroin that are linked to- gether by intermolecular hydrogen bonds (Cuissinat, Navard, & Hei- nze, 2008; Swatloski, Spear, Holbrey, & Rogers, 2002; Zhang, Wu, Zhang, & He, 2005). Early data showed that chitosan had a good sol- ubility in 1-butyl-3-methylimidazolium chloride ([C 4 mim]Cl), and up to 10 wt% of chitosan can dissolve in this media to form a viscous solution (Xie, Zhang, & Li, 2006). 0144-8617/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2009.06.002 * Corresponding author. Address: Dalian Institute of Chemical Physics, CAS, 457 Zhongshan Road, Dalian 116023, PR China. Tel./fax: +86 411 84379211. E-mail address: zhaozb@dicp.ac.cn (Z.K. Zhao). Carbohydrate Polymers 78 (2009) 685–689 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol