The catalytic performance of acidic ionic liquids (AIL) containing HSO4- anion and acidic-functionalized imidazolium cations for biomass liquefaction was firstly investigated in polyethylene glycol 400-glycerol mixture at atmospheric pressure in order to explore green and efficient ways to convert woody biomass into sustainable energy and chemicals. The results showed that fir sawdust (10 g) was up to 99% liquefied using 60 g of solvent at a concentration of 1-(3-sulfopropyl)-3-methylimidazolium hydrogensulfate ionic liquid of 0.3 mol.L-1, at a temperature of 423 K, in 60 min. The introduction of the sulfoalkyl substituent into the imidazolium cation of AILS was found to significantly improve their catalytic activity. The catalytic performances of 1-(3-sulfopropy1)-3-methylimidazolium hydrogensulfate and 1-(4-sulfobutyI)-3-methylmidazolium hydrogensulfate are thus similar to that of H2SO4 due to their double acidic sites. More than 90 wt% of the bio-oil was the heavy oil that derives from lignin. (c) 2015 Elsevier Ltd. All rights reserved.