BACKGROUNDIonic liquids have shown excellent performance in biomass delignification. However, there is still a strong demand to improve delignification by developing new biomass pretreatment methods to take full advantage of ionic liquids. RESULTSA two-step ionic liquids method for cornstalk pretreatment was developed by combining choline hydroxide (ChOH) and 1-ethyl-3-methylimidazolium dimethyl phosphate ([Emim][DMP]) ionic liquids. First, the cornstalk sample was soaked in ChOH aqueous solution for 3h, the residue was then dissolved completely by [Emim][DMP] in 10min. The cellulose rich material (CRM) was regenerated from the [Emim][DMP] solution by adding anti-solvent (acetone/water (v:v=1:1)). The results showed that 77.28% lignin was removed and cellulose content was increased to 52.14% in the CRM. Simultaneously, the crystalline structure of cellulose was altered and amorphous cellulose was obtained in the two-step method. The amorphous cellulose was converted into 5-Hydroxymethylfurfural (HMF) and a high HMF yield of 87.21% was achieved. CONCLUSIONSThe two-step method enhanced the cornstalk pretreatment through improving delignification and the disruption of cellulose crystalline structure under mild conditions, which made the two-step method more competitive compared with other conventional pretreatment methods. (c) 2014 Society of Chemical Industry