Structure analysis of biomass after alkali-freeze pretreatment for one-pot production of C5 and C6 sugars with magnetic acid catalyst

[Display omitted] •KOH-freeze pretreatment is effective for producing C5 and C6 sugars.•One-pot hydrolysis of hollocellulose in pretreated biomass was achieved.•CrII reached themaximum (61.9%, 60.8% for wheat, corn straw) at 10 wt% KOH.•Hemicellulose sugars increased by 2.3 times vs original samples.•In catalyst cycle 5, glucose & C5 sugars reached 52.2% & 70.3 %, respectively. In this work, freeze pretreatment of biomass with 1–13 wt% KOH was applied for one-pot production of C5 and C6 sugars with C-SO3H@Fe3O4/C catalyst. Crystalline structures indicated cellulose I to cellulose II appeared at 4 wt% KOH, and cellulose II reached the maximum (40.1 % to 61.9 % for wheat straw vs 34.8 % to 60.8 % for corn straw) at 10 wt% KOH. High yields of cellulose (60.4 % for wheat straw vs 68.3 % for corn straw) and hemicellulose sugar (84.2 % for wheat straw vs 83.8 % for corn straw) were achieved in 10 wt% KOH-freeze pretreated biomass with C-SO3H@Fe3O4/C at 180 °C. Moreover, the catalyst was magnetically separated with high recovery rate (90.1 %). Yields of cellulose (52.2 %, 50.2 %) and hemicellulose sugar (70.3 %, 60.1 %) for wheat and corn straw were obtained after 5 cycles, individually. This study demonstrates freeze-KOH pretreatment can effectively promoteone-pot production of C5 and C6 sugars by crystallinity amelioration.