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.