Using citric acid to suppress lignin repolymerization in the organosolv pretreatment of corn stalk

The esterification reactions that occur between citric acid (CA) and lignin on hydroxyl groups of side chains can inhibit the formation of reactive intermediates in acid-catalyzed pretreatments. The effects of CA on dilute hydrochloric acid (1.0%) pretreatments of corn stalk in hydrated 1,4–dioxane were investigated with sodium hypophosphite (0.5%) as the catalyst at 95 °C for 3.5 h. The resulting cellulose recovery (62.5–80.9%) and lignin removal (72.5–85.0%) after adding CA were significantly higher than those in the pretreatment without CA. Water content influenced pretreatment and delignification when CA was added into the treatment systems. The highest yield of total reducing sugar (97.1%) was achieved during the enzymatic hydrolysis of cellulose-rich materials obtained from the pretreatment with 11.4% water content. Meanwhile, β–O–4′ in the extracted lignin increased from 22.4/100Ar to 41.2/100Ar with increasing water content from 11.4% to 25.9%. These findings implied that CA as a stabilization agent could inhibit the repolymerization of degraded lignin fragments due to the esterification between CA and lignin and then improved the enzymatic saccharification of cellulose. [Display omitted] •Acid pretreatments with CA stabilization were investigated at 95 °C for 3.5 h.•CA stabilization suppressed cellulose acid-hydrolysis and lignin condensation.•Lignin with high β-O-4′ content (22.4–41.2/100Ar) was extracted.•Appropriate amount of water addition favors esterification reaction of CA.•TRS yield of 97.1% was achieved in pretreatment with 11.4% water content.