Chemical pretreatment was originally developed and have been extensively used in the paper industry for delignification of cellulosic materials to produce paper prod­ucts. Chemical pretreatment to date has primary goal to improve the biodegradabil­ity of cellulose by removing lignin and hemicellulose and to lesser degree of crystallinity of the cellulose component. Chemical pretreatment is the most studied pretreatment technique among pretreatment categories. The various commonly used chemical pretreatment are acid, alkali, organic acids, pH-controlled liquid hot water, and ionic liquids.

Acid pretreatment uses dilute sulfuric acid (H2SO4), the most common chemical pretreatment which has been commercially used for a wide variety of biomass types. Other acids also have been studied such as hydrochloric acid (HCl) (Wang et al. 2010), phosphoric acid (H3PO4) (Zhang et al. 2007; Marzialetti et al. 2008), and nitric acid (HNO3) (Himmel et al. 1997). Concentrated acids such as H2SO4 and HCl have been widely used for lignocellulosic material pretreatment because they are powerful agents for cellulose hydrolysis (Sun and Cheng 2002). Due to its ability
to remove hemicellulose, acid pretreatment has been used in fractionating the components of lignocellulosic biomass. In this sense, sulfuric acid can resolve hemicelluloses and activate the enzymatic activity to cellulose.

Alkaline pretreatment uses base compounds like sodium hydroxide, potassium, calcium, and ammonium hydroxides. The major effect of alkaline pretreatment is the removal of lignin from the biomass, thus improving the reactivity of the remaining polysaccharides. NaOH causes swelling, increasing the internal surface of cellulose and decreasing the degree of polymerization and crystallinity, which provokes lig­nin structure disruption (Taherzadeh and Karimi 2008). NaOH has been reported to increase hardwood digestibility from 14 to 55 % by reducing lignin content from 24 to 55 % to 20 % (Kumar and Wyman 2009). In the case of pretreatment of corn stover by aqueous ammonia, 70-85 % lignin was removed, and 40-60 % of hemi­celluloses were solubilized (Kim et al. 2003). Whereas pretreatment of Oil palm EFB using 6 % sodium hydroxide can remove 60-70 % lignin (Sudiyani et al. 2013). The characteristic of alkaline pretreatment is that it can remove the lignin without hav­ing big effects on other components (McMillan 1996). Some other alkaline pro­cesses reported were alkaline peroxide which combined sodium hydroxide and hydrogen peroxide or alkali and microwave. Some reports mentioned that pretreat­ment using alkali is one of the processes close to commercialization. Alkaline-based methods are generally more effective at solubilizing a greater fraction of lignin while leaving behind much of the hemicellulose in an insoluble polymeric form. The alkaline process involves soaking the biomass in alkaline solutions and mixing it at a target temperature for certain amount of time. A neutralizing step to remove lignin and inhibitors (salts, phenolic acid, furfural, aldehydes, others) is required before enzymatic hydrolysis. Alkaline pretreatment are more effective on agriculture residue and herbaceous crops than on wood materials (Hsu 1996).