Physicochemical transformation of rice straw after pretreatment with a deep eutectic solvent of choline chloride/urea

•Rice straw and its fractions were pretreated with ChCl/urea.•The fractions perform different swell ability and dissolution to ChCl/urea.•ChCl/urea was an efficient delignifination solvent for lignin.•Rice straw residue had an increased crystallinity index.•Rice straw residue was inefficient to enzymatic hydrolysis. This study focused on the pretreatment with deep eutectic solvent of choline chloride (ChCl)/urea mixtures on rice straw and its chemical fractions of holocellulose, α-cellulose, and acid-insoluble-lignin (AIL). The pretreatment of ChCl/urea was significantly affected by the treated temperature prior to the treated time, and 130°C and 4h was an optimum condition for ChCl/urea pretreatment. The separation capacity of ChCl/urea on the chemical fractions was in an order of AIL (22.87%)> hemicellulose and amorphous cellulose (16.71%)>α-cellulose (9.60%). ChCl/urea had a higher selective solubility on lignin. The solubility of the whole fractionation of rice straw affected by ChCl/urea was a combination of solubilization on cellulose, hemicellulose and lignin. ChCl/urea pretreatment increased crystallinity index (CrI) of rice straw residue and α-cellulose, while had no obvious influence on CrI of holocellulose. The effect of structural properties of rice straw residue on enzymatic hydrolysis was also explored.