Comparison of the effects of three drying methods on lignin properties.

In the last few years, a serious effort has been initiated to develop standard methods for lignin characterization at the national and international levels. Thus, several Canadian and ISO standards were recently developed. The current results were generated in an effort to assist the ISO/TC6 Committee come up with a reliable standard method for the measurement of the dry solids content of lignins. In particular, this work investigated the drying of lignin using three different drying methods: conduction oven drying (105 °C), vacuum oven drying at (60 °C), and freeze drying. Ten different lignins were used in this study including wet and air-dried softwood and hardwood kraft lignins in the acid and base forms from the industrial LignoForce™ process and hydrolysis lignin from the TMP-Bio™ process. The results showed that 7 h, 48 h and 24 h were sufficient to reach a constant solids content in the case of all lignins when oven drying, vacuum oven drying under negative pressure (150 mbar), and freeze drying (25 mT) were used, respectively. Kraft lignins in the base form showed higher sensitivity to degradation compared to lignins in the acid form. The total hydroxyl group content of air-dried and wet hardwood lignins in the base form decreased by more than 50 % after vacuum oven-drying for 71.5 h or oven-drying for 16 h compared to freeze-drying for 68 h. The decrease in the total hydroxyl groups was more pronounced (70 %) when the wet softwood lignin in the base form was dried in the oven compared to freeze drying for 68 h.

New approach to recycle and valorize the first filtrate of the LignoForce system™: Lignin extraction and its use in rigid lignin-based polyurethane foams.

In this work, we report on the fractionation, recovery and characterization of softwood kraft lignin from the first filtrate of the LignoForce™ process. It is estimated that the lignin content in this stream could be >20-30 % of the lignin present initially in the black liquor. The use of a membrane filtration system was experimentally validated as an effective method for fractionating the first filtrate. Two membranes with different nominal molecular weight cut-offs (4000 and 250 Da) were tested. Higher lignin retention and recovery was obtained using the 250-Da membrane. This lignin (lignin_250) was found also to have a lower molecular weight and a tighter molecular weight distribution compared to the lignin obtained using the 4000-Da membrane (lignin_4000). The lignin_250 was characterized for it's hydroxyl group content and used to produce polyurethane (PU) foams. Up to 30 wt% petroleum-based polyol replacement by lignin_250, the resulting lignin-based PU (LBPU) foams presented the same thermal conductivity as the control (0.0303 W/m.K (control) vs 0.029 W/m.K (30 wt%)), as well as comparable mechanical (max stress: 145.8 kPa (control) vs 222.7 KPa (30 wt%), modulus 64.3 kPa (control) vs 75.1 (30 wt%)) and morphological properties to the petroleum polyol-based PU foams.

Sustainable Process for the Depolymerization/Oxidation of Softwood and Hardwood Kraft Lignins Using Hydrogen Peroxide under Ambient Conditions.

The present study demonstrated a sustainable and cost-effective approach to depolymerize/oxidize softwood (SW) and hardwood (HW) kraft lignins using concentrated hydrogen peroxide at temperatures ranging from 25 to 35 °C, in the absence of catalysts or organic solvents. The degree of lignin depolymerization could be simply controlled by reaction time, and no further separation process was needed at the completion of the treatment. The obtained depolymerized lignin products were comprehensively characterized by GPC-UV, FTIR, 31P-NMR, TGA, Py-GC/MS and elemental analysis. The weight-average molecular weights (Mw) of the depolymerized lignins obtained from SW or HW lignin at a lignin/H2O2 mass ratio of 1:1 after treatment for 120 h at room temperature (≈25 °C) were approximately 1420 Da. The contents of carboxylic acid groups in the obtained depolymerized lignins were found to significantly increase compared with those of the untreated raw lignins. Moreover, the depolymerized lignin products had lower thermal decomposition temperatures than those of the raw lignins, as expected, owing to the greatly reduced Mw. These findings represent a novel solution to lignin depolymerization for the production of chemicals that can be utilized as a bio-substitute for petroleum-based polyols in polyurethane production.

Effects of Process Parameters on Hydrolytic Treatment of Black Liquor for the Production of Low-Molecular-Weight Depolymerized Kraft Lignin.

The present research work aimed at hydrolytic treatment of kraft black liquor (KBL) at 200⁻300 °C for the production of low-molecular-weight depolymerized kraft lignin (DKL). Various process conditions such as reaction temperature, reaction time, initial kraft lignin (KL) substrate concentration, presence of a catalyst (NaOH), capping agent (phenol) or co-solvent (methanol) were evaluated. The research demonstrated effective depolymerization of KL in KBL at 250⁻300 °C with NaOH as a catalyst at a NaOH/lignin ratio of about 0.3 (w/w) using diluted KBL (with 9 wt. % KL). Treatment of the diluted KBL at 250 °C for 2 h with 5% addition of methanol co-solvent produced DKL with a weight-average molecular weight (Mw) of 2340 Da, at approx. 45 wt. % yield, and a solid residue at a yield of ≤1 wt. %. A longer reaction time favored the process by reducing the Mw of the DKL products. Adding a capping agent (phenol) helped reduce repolymerization/condensation reactions thereby reducing the Mw of the DKL products, enhancing DKL yield and increasing the hydroxyl group content of the lignin. For the treatment of diluted KBL (with 9 wt. % KL) at 250 °C for 2 h, with 5% addition of methanol co-solvent in the presence of NaOH/lignin ≈ 0.3 (w/w), followed by acidification to recover the DKL, the overall mass balances for C, Na and S were measured to be approx. 74%, 90% and 77%, respectively. These results represent an important step towards developing a cost-effective approach for valorization of KBL for chemicals.