2.8 Molecular weight of recovered lignin
The weight-average molecular weight (MW), number-average molecular weight (Mn), and dispersity (Đ) of the isolated lignin were measured by an Agilent gel permeation chromatography (GPC) SECurity 1200 system equipped with Waters Styragel columns (Waters Corporation, Milford, MA) and a UV detector at 270 nm. The isolated lignin was acetylated in a pyridine/acetic anhydride 1:1 (v/v) mixture for 48 h, rotary evaporated, dissolved in THF, and filtered through 45 μm PTFE filters before injection.
2.9 Cellulose degree of polymerization
Dried sorghum bagasse samples (~0.3 g) were reacted in 6 ml of 25 wt% peracetic acid aqueous solution at 25 °C for 24 hours to isolate holocellulose. The obtained holocellulose samples were washed with distilled water via centrifugation until neutral pH was achieved. The holocellulose samples were further treated in two steps with 17.5% and 8.75% sodium hydroxide aqueous solution, respectively, to obtain α-cellulose. Prior to molecular weight measurement by gel permeation chromatography (GPC), α-cellulose was converted to cellulose tricarbanilate as described in a previous study.21 In brief, 15 mg of oven-dried α-cellulose was mixed with 4 ml anhydrous pyridine in a glass bottle, followed by the injection of 0.5 ml phenyl isocyanate. The mixture was reacted at 70 ℃ for 48 h. Another 0.5 ml of phenyl isocyanate was injected after 24 hours. The reaction was quenched by adding methanol. Cellulose tricarbanilate was precipitated in a methanol/water 7:3 solution, washed via centrifugation, and air-dried. An Agilent GPC SECurity 1200 system equipped with Waters Styragel columns (Waters Corporation, Milford, MA), a refractive index (RI) detector, and a UV detector was used to measure the weight average molecular weight (MW ) and number-average molecular weight (Mn ). Cellulose tricarbanilate was dissolved in tetrahydrofuran (THF) and filtered through 0.45 μm PTFE filters before injection.