Kraft lignin

The following chemicals were obtained from Sigma–Aldrich Handels GmbH, Darmstadt, Germany: LS lignosulfonic acid sodium salt (average M_w≈54 000, M_n≈6000 g mol⁻¹), lithium bromide (≥99 %), and dimethyl sulfoxide (HPLC grade). PSS sodium salt and pullulan standards were obtained from Polymer Standard Service (PSS, Mainz, Germany). Technical lignin samples were kindly provided by associated companies: two softwood kraft lignins—Indulin AT (MeadWestvaco Corp., USA), Lignoboost (Innventia/RISE, Sweden), and soda lignin—Sarkanda (Granit S.A., Switzerland); two Organosolv lignins—OSL HW, hardwood (Fraunhofer CBP, Germany), and OSL Alcell, mixed hardwood (Repap, Canada); two lignosulfonates from different sulfite processes—Ammonium LS (Borregaard, Norway) and Magnesium spruce LS (Lenzing, Austria). Biolignin is based on wheat straw Organosolv processing (CIMV, France). Purified pine milled wood lignin (MWLp) was extracted according to the original procedure53 and purified according to the protocol described by Balakshin et al.54 DHP and trimer as side products of coniferyl alcohol polymerization were obtained according to the literature.55 A hexameric lignin model compound was obtained according to Kilpeläinen et al.56

First, 0.0875 g of
lignin was suspended in DI water with the Ni–Fe cocatalysts
on magnesium silicate supports in a batch reactor. To study the effect
of temperature on the depolymerization of kraft lignin, the system
was studied at temperatures of 250, 300, and 350 °C. Catalysts
were further loaded at 0.0044 g into the reactor. Subsequently, the
reactor was sealed and purged with N₂ to remove any reactive
air and achieve an inert atmosphere until reaching 10 bars of N₂. The reactor was operated for 1 h with vertical shaking at
40 rpm.
After completing the reaction, the products were separated
by a centrifuge consisting of solid and liquid phases. The solid phase
was defined as char, while the liquid phase contained lignin-derived
products and residual lignin. Next, the liquid phase was acidified
to a pH of 2.00 with 1 M hydrochloric acid. In this step, the residual
lignin was precipitated out as solids by a centrifuge at 15 °C.
Next, the lignin-derived products were separated using ethyl acetate.
Finally, the samples were characterized and quantified by a gas chromatography
mass spectrometer.
The liquid fraction qualification was analyzed
on a GC–MS
instrument (Shimadzu) equipped with a capillary column (30 m ×
0.32 mm × 0.25 mm). The GC heating ramp was as follows: the oven
temperature program increased from 40 °C (held for 3 min) to
300 °C at a rate of 5 °C/min under a helium atmosphere.
The main products from the depolymerization of kraft lignin are anisole,
phenol, p-cresol, 4-ethylphenol, creosol, catechol,
guaiacol, mequinol, 4-ethylguaiacol, syringol, 4-hydroxybenzaldehyde,
vanillin, 4′-hydroxyacetophenone, 3,4-dimethoxyenzaldehyde,
and vanillic acid. These products were analyzed using the database
from the National Institute of Standards and Technology (NIST) for
comparison of the molecular weight. The yields of each product and
the kraft lignin conversion were calculated using the following eqs 1 and 2.

Industrial bagasse soda
pulping black liquor was obtained from Environment Pulp and Paper
Company Limited, Nakhon Sawan, Thailand. Carbon dioxide (CO_2, 99.5%) was purchased from Praxair (Thailand) Co., Ltd. and used
in the acidification process. Kraft lignin (alkaline), potassium hydroxide,
and para-hydroxybenzoic acid were purchased from Sigma Aldrich. Sulfuric
acid (H₂SO₄, 98%) and hydrochloric acid (HCl,
37%) were purchased from Honeywell Fluka.

The starch (7 wt %) was gelatinized at 80 °C in a neck flask
for 3 h, while PVA was dissolved in deionized water at 90 °C
for 3 h. The resultant solutions were homogenized using a sonicator
(HD 2200, Bandelin electronic Co., Berlin, Germany) at 72% power for
10 min. Similarly, kraft lignin (7 wt %) and citric acid (7 wt %)
were dissolved in deionized water for 3 h and 15 min, respectively.
The lignin solution was homogenized using a sonicator at 72% power
for 15 min. The starch-soluble, PVA, and lignin solutions were centrifuged
at 11 000 rpm to remove undissolved particles. To prepare the
H-SCL slurry, starch, citric acid, and lignin were mixed in a ratio
of 51:9:40, while the H-PCL slurry was synthesized by blending PVA,
citric acid, and lignin in a ratio of 45.5:24.5:30. The solutions
were mixed using a magnetic stirrer for 1 h, then homogenized at 72%
power for 10 min, and finally degassed. The degassed slurry was cast
on a glass substrate using a doctor blade and partially dried to form
a flexible film that could be easily rolled into straws on a Teflon
rod. The partially dry films can easily be bonded at the edges without
needing binders and adhesives. The films and rolled straws were fully
dried in a dry oven and cured in a vacuum oven at 180 °C to form
esterified straws and films.