Direct conversion of glucose and cellulose to 5-hydroxymethylfurfural in ionic liquid under microwave irradiation Changzhi Li a , Zehui Zhang a,c , Zongbao K. Zhao a,b, * a Dalian Institute of Chemical Physics, CAS, Dalian 116023, PR China b Dalian National Laboratory of Clean Energy, Dalian 116023, PR China c Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China article info Article history: Received 7 April 2009 Revised 28 May 2009 Accepted 10 July 2009 Available online 15 July 2009 abstract An efficient strategy for CrCl 3 -mediated production of 5-hydroxymethylfurfural (HMF) in ca. 60% and 90% isolated yields from cellulose and glucose, respectively, in ionic liquid under microwave irradiation is presented. Ó 2009 Elsevier Ltd. All rights reserved. Studies on conversion of cellulose into biofuels and other bio- materials have been currently one of the most intensive pursuits worldwide. 1 One major approach is to hydrolyze cellulose to glu- cose followed by fermentation for biofuels and related chemicals. However, the overall energy efficiency of these biochemical routes is under debate. 2 Thus, a workable non-fermentative process for the conversion of cellulose to bio-based products remains imperative. The five-membered ring compound, 5-hydroxy-methylfurfural (HMF), is one of the top bio-based platform compounds. HMF can be converted to a novel biofuel molecule 2,5-dimethylfuran 3 via selective hydrogenation. Thus, making HMF with renewable feedstock is highly demanding. Because glucose is liable to form a stable six-membered pyranoside structure, 4 it failed to form HMF with satisfactory yields under those known conditions. 5 Fur- thermore, a direct cellulose-to-HMF process (Scheme 1), which is economically more attractive, has yet to be developed. We present herein an efficient strategy for direct production of HMF in ca. 90% and 60% isolated yields from glucose and cellulose, respectively, in ionic liquids under microwave irradiation (MI) in the presence of CrCl 3 . This approach greatly advanced our utiliza- tion of lignocellulose for bioenergy through a chemical transforma- tion strategy. After a solution of glucose and CrCl 3 (3.6 wt%) in [C 4 mim]Cl was subjected to MI at 400 W for 1 min, the entire reaction mixture was purified by column chromatography on silica gel (ethyl ace- tate:petroleum ether = 1:10 to 1:1), affording HMF in 91% yield (Table 1, entry 1). In contrast, HMF was obtained in 17% yield if the reaction was done under oil-bath heating at 100 °C for 60 min (entry 2). If water was used as the solvent, glucose dehy- dration was essentially suffocated (entry 3). When sulfuric acid was employed in lieu of CrCl 3 , the dehydration reaction afforded HMF in only 49% yield (entry 4), and formation of insoluble humins was observed in this case. Careful comparison of these data indi- cated that (1) MI had a drastic effect on HMF formation; (2) [C 4 mim]Cl was a solvent superior to water, and (3) CrCl 3 was a powerful catalyst for selective dehydration of glucose. More de- tailed explanation will be discussed thereafter. Taken together, efficient dehydration of glucose to HMF was achieved in [C 4 mim]Cl with a catalytic amount of CrCl 3 under MI. We have demonstrated recently that cellulose could be effec- tively hydrolyzed in ionic liquids to produce glucose. 6 It was thus logic to pursue a direct transformation of cellulose into HMF. In- deed, we were able to obtain HMF in 61% isolated yield and total reducing sugars (TRS) in 16% yield when Avicel cellulose and CrCl 3 Á6H 2 O (10 wt%) were mixed in [C 4 mim]Cl and irradiated at 400 W for 2 min (Table 2, entry 1). These data are close to the best known result from glucose. 5b Other cellulose samples showed sim- ilar results in the range of 53–62% (entries 2–4). Apparently, this method worked almost equally well regardless of the cellulose type and the degree of polymerization. Although our method affor- ded TRS in around 20% yield based on analysis with the 3,5-dinitro- salicylic acid (DNS) reagent, 7 further analysis revealed that glucose had little contribution, indicating that other byproducts or sugar oligomers might be formed. In sharp contrast, when an identical reaction of Avicel cellulose was heated with oil-bath at 100 °C for 240 min, HMF and TRS were obtained in 17% and 45% yields, respectively (entry 5). It should be noted that the initial water con- tent in the system was estimated about 170 mM, a quite low con- centration compared to about 1000 mM in our early cellulose hydrolysis work. 6a Yet, as there are three water molecules released for the formation of one HMF molecule, cellulose hydrolysis is ex- pected auto-accelerated during the process. On the other hand, a lower water concentration would also slow down HMF rehydration and humins formation. 4 0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2009.07.053 * Corresponding author. Tel./fax: +86 411 8437 9211. E-mail address: zhaozb@dicp.ac.cn (Z.K. Zhao). Tetrahedron Letters 50 (2009) 5403–5405 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet