In this study, we report that phase separation of water/acetonitrile solution to a biphasic system can be induced by the presence of an acidic ionic liquid, i.e., 1-methyl-3-(butyl-4-chlorosulfonyl) imidazolium chlorosulfate ([IMBClm]SO3Cl), which allows to produce pseudo ternary phase of the/water/actonitrile/[MBClm]SO3Cl system at room temperature. Based on this finding, an efficient and recyclable aqueous acetonitrile biphasic system is established for catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) in the presence of [MBClm]SO3Cl at low temperature in a shorter period of time. The ionic liquids remaining in the organic phase can be efficiently removed by simple treatment with alumina. This leaves only HMF, with a yield of up to 88.7%, in the acetonitrile phase, which could be easily removable. (C) 2012 Elsevier B.V. All rights reserved.