Valorizing Kraft Lignin by a Catalytic Reductive Depolymerization in Ethanol/Water with Formic Acid as a Supplementary H2 Donor

Abstract

Lignin is a promising biopolymer to serve as a sustainable resource for a multitude of applications (e.g., thermoset materials, production of bulk chemicals) thereby substituting fossil-based carbon sources. In this work, the reductive depolymerization of Kraft lignin (KL) was studied in ethanol/water aided by formic acid (FA) as the way forward to valorize lignin. The effect of various process conditions was elucidated using 31 P-NMR, pH, GPC, CHNSO, GC-MS and 2D-LC analyses. It is found that the addition of a small amount of FA (3.6 vol%) is beneficial for obtaining smaller lignin fragments with more phenolic OH (PhOH) functionalities upon depolymerization at 250°C for 8h. Besides, a higher FA concentration causes acid catalyzed lignin repolymerization and a longer reaction time results in only a limited reduction in molecular weight of the obtained lignin fragments. The addition of a supported Pd catalyst leads to a more pronounced depolymerization (smaller lignin fragments with more PhOH functionalities) as well as a stronger decrease in oxygen and sulfur content. Furthermore, several experiments and multiple analysis techniques support the hypothesis that FA acts as H2 donor under the investigated conditions in this study. In conclusion, KL (MW ~16436 g mol -1 and 3.31 mmol g -1 PhOH) was successfully depolymerized into a low molecular weight lignin (MW ~3250 g mol -1 ) with more PhOH functionalities (5.29 mmol g -1 ).

Enhancing the performance of heterogeneous palladium based catalysts in the mild reductive depolymerization of soda lignin through addition of a non-noble metal and tuning of the preparation strategy

“…This aligns with the literature, where Pd catalyzed lignin depolymerizations consistently outperform solvolysis (0Cata and Al 2 O 3 ). 11,12,20,23,41…”

“…The derivatization with TMDP and analysis were based on the literature and the data processing was performed using TopSpin v4.2.0. 23 Phase and base line correction were performed automatically and the sharp peak of TMDP and H 2 O on the right side of the spectra was manually set to 132.2 ppm. The following integration ranges were applied: NHND (151.5–152.5 ppm), aliphatic OHs (145.2–150.0 ppm), condensed aromatic OHs (140.0–144.8 ppm, with subtraction of the syringyl aromatic OHs between 143.1 and 142.3 ppm), uncondensed aromatic OHs (137.0–140.4 ppm) and carboxylic acids (133.5–136.0 ppm).…”

“…11,12 Within the literature, it has been noted than Pd exhibits significantly less hydrodeoxygenation activity than Ru and Pt, making it a preferred choice for the production of lignin-derived functionalized aromatic compounds, despite its higher cost. 19,20 As a result, Pd based nanoparticle catalysts, supported on various supporting materials such as carbon, 21,22 alumina 23 or zeolites 24,25 have been implemented successfully to depolymerize lignin under mild reductive conditions. 11 Nonetheless, the high cost of Pd inhibits its application as a catalyst within actual industry.…”

Sustainable lignin modifications and processing methods: green chemistry as the way forward

“…Additionally, it was elucidated that the temperature (in combination with Pd/Al 2 O 3 ) played an important role in the decomposition of formic acid with only a limited decomposition at 200 °C compared to 250 °C. 287…”

“…Additionally, it was elucidated that the temperature (in combination with Pd/Al 2 O 3 ) played an important role in the decomposition of formic acid with only a limited decomposition at 200 °C compared to 250 °C. 287 A cheap alternative to the use of noble metals was investigated by Lou et al They used LaBO 3 (B = Co, Fe or Ni) to convert AL to aromatics in MeOH-assisted hydrothermal liquefaction (80 °C-260 °C) and concluded that all the perovskite catalysts promoted the lignin treatment, but LaCoO 3 showed the highest catalytic performance. 288 Recently, Xu et al explored an ultrasonic-assisted method (250 °C, 6 h) as a greener and more efficient heating process with LaNiO 3 supported by a mesoporous carrier in ethanol/water (1/1 v/v) of corn stover lignin, resulting in a bio-oil yield of 68.0 wt% and phenolic monomer yield of 11.46 wt%.…”