Sodium dithionite is utilized as a reducing agent in the organosolv fractionation of lignocellulose to concommitantly produce cellulosic pulp and promote the reductive conversion of lignin into phenolic monomers. Reactions...
The reductive catalytic fractionation (RCF) is a promising method for the development of "lignin-first" biorefineries. Apart from the widely investigated virgin woody biomass, it is essential to explore the potential of waste biomass feedstocks. Herein, the RCF of wheat straw is examined to produce lignin mono-/oligomers along with a processable carbohydrate pulp. The use of different catalysts (Ru/C and Ru/Al 2 O 3 ) and catalyst loadings (0−20% w/w biomass ) revealed the superior performance of Ru/C, which resulted in the largest yield of phenolic monomers (up to ∼25 wt % of initial acid-insoluble lignin) and in the lowest formation of high-molecular-weight fragments in the extracted lignin oil. Furthermore, the operating temperature was shown to substantially affect both lignin extraction−depolymerization and polysaccharides preservation−processability. For a reaction time of 3 h, an increase of the temperature from 200 to 250 °C resulted in a >2-fold boost of the yields of lignin oil and monophenolics, while the recovery of polysaccharides decreased by about 30 wt % (with ∼20% lower enzymatic digestibility). An economic assessment highlighted that the high-temperature treatment becomes the most profitable configuration as the market price of lignin products increases. Overall, this work provides insight into the adoption of the RCF for the upgrading of lignocellulose from inexpensive and widely available wheat straw biomass.
The reductive catalytic fractionation (RCF) is an attractive method for the conversion of lignin toward valuable low-molecular weight aromatics during the pretreatment of lignocellulosic biomass. A prominent limitation to the upscaling of such technology is represented by the use of pressurized hydrogen gas. In this contribution, the role of hydrogen gas within the RCF of wheat straw biomass is investigated. The use of H2 was shown to enhance lignin depolymerization, by virtue of an improved hydrogenolysis and hydrogenation of lignin fragments, with a yield of phenolic monomers that increased from ~12 wt% of acid-insoluble lignin in the initial biomass under inert atmosphere, up to ~25 wt% under H2, for a reaction in methanol, at 250 °C, with Ru/C. The adoption of methanol, ethanol and isopropanol as hydrogen-donor solvents was also investigated in the absence of H2, and the use of ethanol was found to give the highest yield of monophenolics (up to ~20 wt%) owing to a better balance between solvolysis, hydrogenolysis, and hydrogenation of lignin. Nevertheless, a substantial loss of the carbohydrate fraction was observed for reactions performed at 250 °C, irrespective of hydrogen pressure and of the solvent employed. The use of a lower temperature of 200 °C in combination with H3PO4 resulted in an improved recovery of cellulose in the pulp and in the solubilization of hemicellulose and lignin, with the formation of monosaccharides (~14 wt% of polysaccharides in the initial biomass) and phenolic monomers (up to 18 wt%, in the absence of H2). Overall, the obtained results show that a tradeoff exists between the removal of H2 from the process and the production of low-MW phenolics during RCF, which can be improved by accurately tuning the process conditions.
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