Comparative study of lignin characteristics from wheat straw obtained by soda-AQ and kraft pretreatment and effect on the following enzymatic hydrolysis process

[Display omitted] •Characteristics of lignin after kraft or soda-AQ pretreatment was compared.•Quantitative information of different substructures in the lignin was obtained.•Lignin was more degraded during kraft pretreatment than soda-AQ pretreatment.•More cellulose was converted to glucose in the wheat straw after kraft pretreatment.•LCC played a role in the enzymatic hydrolysis of lignocellulosic material. To understand the structural changes of lignin after soda-AQ and kraft pretreatment, milled straw lignin, black liquor lignin and residual lignin extracted from wheat straw were characterized by FT-IR, UV, GPC and NMR. The results showed that the main lignin linkages were β-aryl ether substructures (β-O-4′), followed by phenylcoumaran (β-5′) and resinol (β-β′) substructures, while minor content of spirodienone (β-1′), dibenzodioxocin (5-5′) and α,β-diaryl ether linkages were detected as well. After pretreatment, most lignin inter-units and lignin-carbohydrate complex (LCC) linkages were degraded and dissolved in black liquor, with minor amount left in residual pretreated biomass. In addition, through quantitative 13C and 2D-HSQC NMR spectral analysis, lignin and LCC were found to be more degraded after kraft pretreatment than soda-AQ pretreatment. Furthermore, the subsequent enzymatic hydrolysis results showed that more cellulose in wheat straw was converted to glucose after kraft pretreatment, indicating that LCC linkages were important in the enzymatic hydrolysis process.