Chemical pretreatment is a widely used method. It effectively removes and recovers most of hemicellulose portion as soluble sugars, and disrupts lignin to be partially dissolved in aqueous acidic/basic solution (e. g., H2O2 and ammonia) [1]. Chemical pretreatment with acids has proven to be effective for breaking down hydrogen bonds leading to intra-crystalline cellulose swelling. Acid pretreatment is a process in which hydronium ions break down or attack inter- and intra-molecular bonds among cellu­lose, hemicellulose, and lignin. It increases the porosity of substrate and accessibility of cellulose to enzymes for subsequent hydrolysis. During acid pretreatment process, very little cellulose is hydrolyzed. However, the process is usually accompanied by further degradation of monomers, and it has drawbacks such as equipment corrosion and issues in the recovery and recycle of the acid.

Alkali (e. g., NaOH, KOH) solution is a swelling agent for both crystalline and amorphous celluloses that can destroy the linkages between lignin and carbohydrates by saponification of intermolecular ester bonds [20]. Kumar et al. [21] reported that NaOH pretreatment increased hardwood digestibility from 14 % to 55 % by reducing lignin content from 24-55 % to 20 %. Some chemical agents, such as peroxides and acidic alcohol solutions, have advantages for dissolving lignin and loosening hemicellulose from insoluble crystalline cellulose. Pan et al. [22] pretreated woody biomass with an organic solvent (1.25 % H2SO4 and 50 % ethanol) under conditions of 180 °C for 60 min, about 75 % lignin was removed from the substrate.

Ionic liquids (ILs) are efficient for the pretreatment and hydrolysis of lignocel- lulosic materials, and can dissolve biomass and overcome many of the physical and biochemical barriers for hydrolysis at ambient conditions. Many ILs have been shown to be effective solvents for cellulose. After IL pretreatment, glucose yield from subsequent cellulose hydrolysis is greatly increased with 97 % enzymatic glucose conversion rate being reported [23].