Kraft lignin (KL) used in this study was provided by FPInnovations, produced using the
proprietary LignoForce process (Kouisni, 2012) in its pilot plant in Thunder Bay, Ontario
and was completely soluble in aqueous alkali (pH >10). It is a yellow-brown powder with weak odor and specific gravity of 0.80. The relative weight-average molecular weight
(Mw) of KL is ≈10,000 g/mol (PDI ≈2.0) based on GPC-UV analysis. The proximate and
and 5.2 wt.% sulfur (on dry ash free basis). The ash content of lignin was determined
gravimetrically in a muffle furnace at 700 oC for 4 hours. The ultimate analysis was done
on a CHNSO Elemental Analyzer and reported on a dry and ash free basis. Other
chemicals used include NaOH (96%), sulfuric acid (99%), acetone (99.5%), d6-DMSO
and d-chloroform, tetrahydrofuran (THF, HPLC grade), pyridine, acetic anhydride and
dibromomethane, all CAS reagent grade, purchased from Sigma-Aldrich and used without further purification.
Table 3-1 Proximate & ultimate analysis of original Kraft lignin (KL) Proximate analysis, wt.% (d.b)a Ultimate analysis, wt.% (d.a.f.)f
VMb FCc Ashd TS/MCe C H N S Og
56.3 43.1 0.57 98.5/1.5 63.8 5.4 0.02 5.2 25.6
a On dry basis; b VM: volatile matter; c FC: Fixed carbon (VM and FC was determined by
thermogravimetric analysis (TGA) in N2 at 10 oC/min to 900 oC); d Ash content
determined gravimetrically in a muffle furnace at 700 oC for 4 hours; e TS/MC: Total
solids /moisture contents in the sample was determined by placing 1-2 g of sample in an
oven at 105 oC for 24 hours; f On dry and ash free basis; g By difference.
3.2.2 Kraft lignin hydrolysis experiments
The hydrolysis experiments were carried out in a 100 mL Parr Model 4848 reactor, equipped with a pressure gauge, thermocouple, stirrer, gas line and sampling line. In a typical run, 12 g KL, 33 g NaOH (10 wt.% solution in distilled water) and 15 g of distilled water were loaded into the reactor. The reactor was sealed, evacuated and purged
thrice with N2 to ensure complete removal of residual air. The reactor was then finally
pressurized with N2 to a cold pressure of 2 MPa and tested for leaks. The reactor was
heated under a fixed stirring rate (390 rpm) and allowed to run over a pre-specified length of reaction time after reaching the required temperature. During the reaction the pressure of the reactor system will increase depending on the temperature mainly due to the water
vapor pressure (e.g., 5 MPa at 250 oC, 8 MPa at 300 oC up to 16 MPa at 350 oC). After
the pre-set reaction time elapsed, the reactor was immediately quenched with water to stop further reaction. After the system reached a low temperature (near room
temperature), the gas was collected in a gas cylinder of known volume (2800 mL) and the pressure of the gas cylinder was adjusted to 1.0 atm (abs.) using high purity nitrogen as a makeup gas. The gaseous products were analyzed using a Micro-GC-TCD analyzer and
the overall gas yield was determined. The gaseous products are mainly composed of H2,
CO, CH4 and C2-C3. Each experiment was conducted 2-3 times to ensure that the relative
experimental errors in DL yield be within ±10%.
The reactor contents were then completely rinsed into a beaker using distilled water. The pH value of the washed reactor contents (varying from 11.0 to 9.5 depending on the
reaction conditions) was adjusted to approximately 2.0 using 1.0 M H2SO4 solution to
precipitate the DKL products. The acidified reaction mixture was then filtered through a Buchner funnel. The aqueous (Aq) phase was analyzed by TOC-analyzer. As the gas
yield was found to be very low (≤ 1 wt %) in all tests, a lumped yield of (Gas+Aq) phase
was reported in this study for simplicity. The solid cake containing depolymerized KL was dissolved in acetone (20-25 mL) under sonication and then filtered under vacuum with Buchner funnel to get acetone soluble depolymerized lignin (DKL) or polyols and
solid residues (SR). The SRs were dried at 105 oC for 24 h in an oven and weighed to
obtain SR yield as wt.% of the original KL on a dry basis. The acetone soluble filtrate was transferred to a pre-weighed Erlenmeyer flask to remove acetone with rotary
evaporator at 60 oC followed by 24 hr drying in a vacuum oven to obtain the DL
products. The yield of DKL was calculated based on the mass of original KL on dry basis. As mentioned previously, the data presented in this work are the average of triplicate runs.
3.2.3 Product characterization
The DKL were analyzed by Nicolet 6700 Fourier Transform Infrared Spectroscopy (FT-
IR) with smart itr/ATR accessory to verify the presence of hydroxyl groups in the polyols
structure and other functional groups present in them the range of 500-4000 cm-1 with
attenuated total reflectance (ATR). Proton nuclear magnetic resonance (1H-NMR) spectra
for DLs were acquired at 25 oC using a Varian Inova 600 NMR spectrometer equipped
were accumulated using a 2s recycle delay, 3.6s acquisition time, a 45-degree tip angle
(pw =4.8 us), and a spectral width from -2 ppm to 14 ppm (sw =9000.9 Hz). d6-DMSO
and d-chloroform were used as the 1H-NMR solvents for qualitative study. Quantitative
1H-NMR analysis was realized using acetylated samples of the KL and DLs. Briefly, 1 g
of dried KL or DKL was dissolved in a 1:1 (v/v) mixture of pyridine (5 mL) and acetic anhydride (5 mL) in a vial followed by stirring for 24 to 48 hr. The well-stirred mixture was then transferred into a beaker containing 100 mL of ice-cooled 1 wt.% HCl solution. The resulting precipitates of acetylated samples were washed with distilled water to pH
≈7. The samples were then dried at 105 oC for 24 hr to remove residual water before
further utilization. Dibromomethane (CH2Br2) was selected as an internal standard as its
characteristic peak at 4.9 ppm does not overlap with any other peaks in the KL/DKLs.
Also the solvent selected for the quantitative analysis was d-chloroform instead of d6-
DMSO as peak of the latter overlaps with that of aliphatic acetate. For determining
hydroxyl number through 1H-NMR, the samples were prepared by first weighing 15 mg
of the acetylated KL or DKL and 10 mg of internal standard in a vial and then the sample
was transferred into a 5 mm NMR tube via a transfer pipette using d-chloroform (≈1000-
1500 mg) for the subsequent NMR analysis.
The relative molecular weight distributions (Mw and Mn) of DKLs were measured with a
Waters Breeze GPC-HPLC instrument (1525 binary pump, UV detector set at 270 nm,
Waters Styragel HR1 column at 40 oC) using THF as the eluent at a flow rate of 1 ml/min
with linear polystyrene standards for the molecular weight calibration curve). The range of linear polystyrene standard was 100 to 1 million and the reported molecular weights are based on polystyrene equivalent weight. Although there is a limitation on utilization of linear polystyrene standard for KL or DKL because of KL/DKL multi-branched structure however, relative molecular weights can provide useful information for their further utilization. Elemental analysis of the DKLs was obtained using a CHNS-O Flash Elemental Analyzer 1112 series (Thermo) for determining the contents of CHNS (carbon, hydrogen, nitrogen and sulfur) in the samples. The ash contents of KL and DKLs were obtained by combustion of 1-2 g of the pre-dried sample of KL or DKLs in a crucible in a
muffle furnace at 700 oC for 4 hours. The total organic carbon contents in the aqueous phase were obtained with a TOC-analyzer.