AbstractBiorefining is a potential pathway to cover the shortage of fuels, power, and chemicals from lignocellulosic biomass in the future. However, pretreatment of the biomass is recognized as a technological bottleneck for the cost-effective development of biorefineries, especially for the production of bio-fuels and chemicals. Active oxygen pretreatment is both an eco-friendly and efficient pretreatment process. To elucidate the effect of different chemicals on corn stalk and its hemicellulosic structure, five pretreatment processes were formed with MgO, H2O2, and O2. Additionally, the MgO was also replaced by NaOH and Mg(HO)2. Results show that MgO, which can be completely replaced by Mg(OH)2, is an alkali source and a protective agent in preventing raw material from carbonizing and cellulose from degrading during pretreatment. High pressure oxygen is the main chemical for depolymerizing corn stalk. The removal degrees of lignin and hemicelluloses in the pretreatment processes with oxygen were 81.1 to 87.7% and 73.3 to 83.0%, respectively. Without oxygen, much lower removal degree were achieved (19.3 to 49.0% and 55.5 to 67.6%, respectively). Corn stalk hemicelluloses were composed of (1→4)-β-D-xylopyranose substituted with α-L-arabinofuranosyl residues and 4-O-methyl-α-D-glucuronic acid units. The molecular weight of hemicelluloses decreased from 22,000 g/mol to the range 3100 to 6400 g/mol.