By-products characterization
Hemicelluloses were extracted from a lignin-free black liquor
sample by precipitation in 3 volumes of 95% ethanol at pH 6 [40],
with the objective of completing their chemical characterization
by Fourier transform infrared spectroscopy (FTIR). Lignin, obtained
in previous experiments, was also subjected to FTIR analysis. Measurements were performed in a PerkinElmer 16PC instrument by
direct transmittance using KBr pellet technique. Each spectrum was
recorded over 20 scans, in the range from 4000 to 500 cm−1
with a
resolution of 4 cm−1
. KBr was previously oven-dried to avoid interferences due to the presence of water, and background spectra were
collected before every sampling.
Gel permeation chromatography (GPC) was used to determine
lignin molecular weight (MW) and molecular weight distribution
(MWD). Lignin samples were subjected to acetylation in order to
enhance their solubility in THF, used in this technique as mobile
phase. This reaction, which causes the substitution of hydroxyl
functions by acetyl groups, was carried out by placing lignin in an
acetyl anhydride/acetyl acid mixture (1:1 w/w) containing sodium
acetate as a catalyst (0.5 equivalents per mole of acetyl anhydride). The final lignin concentration in the reactive mixture was
20 wt% [41], and the components were allowed to react for 48 h
at room temperature and then 1 h under reflux. Resulting product, obtained by precipitation in 1 wt% HCl ice-cold water, was
filtered, washed with distilled water and dried. Acetylated lignin