Toward a Hydrogen‐Free Reductive Catalytic Fractionation of Wheat Straw Biomass**

Abstract

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.

Selective Production of 4-Propylphenol from Lignin Oil without Exogenous Hydrogen over a RuNi/NiAl2O4 Catalyst

“…Hydrogen-free reductive catalytic fractionation (RCF) has been reported by other research groups. − However, alcohol solvents are inevitably used as hydrogen donors. Recently, we developed new self-hydrogen supplied catalytic fractionation (SCF) in which hemicellulose and water (solvent) were used as hydrogen sources.…”

A guide to lignin valorization in biorefineries: traditional, recent, and forthcoming approaches to convert raw lignocellulose into valuable materials and chemicals

“…447,477 The RCF of herbaceous feedstocks ( e.g. miscanthus, 465,478 corn stover, 472,479 wheat straw, 478,480,481 …) has been explored as well, and relatively high yields of delignification have been reported (up to ∼90%), 478 along with a yield of monophenolics that varies in the range of ∼10 –∼60% of the initial lignin content. 465,478,482…”

“…The adoption of semi-continuous flow-through reactors, in which the biomass and the heterogeneous catalyst are loaded in two separate compartments to facilitate the post-fractionation catalyst recovery and re-use, has been explored for RCF processes. 466,488,204,489–492 In addition, various works explored the development of H 2 -free RCF configurations in which the solvent or hemicellulose itself act as alternative reducing agents, reporting moderately high yields of lignin and carbohydrate products, 448,468,477,481,491,493–495 and a positive impact on the overall biorefinery economy. 485 Recently, the influence of solvent selection and on the RCF performance was inspected, and the choice of high-boiling solvents was reported to minimize capital costs.…”

Reductive Catalytic Fractionation of Straw Digestates for the Production of Biogenic Aromatic Monomers

“…A strong reductive atmosphere is necessary to effectively hydrogenate unsaturated monomers in RCF processing [54]. However, it has been shown in other studies using wood and herbaceous biomass that hydrogen does not necessarily have to be supplied externally via the gas atmosphere, since the solvent or components of the biomass can also act as hydrogen donors [10,14,16]. This could be confirmed also with washed industrial straw digestate, as under absence of external hydrogen with a nitrogen gas atmosphere (8 bar N 2 pressure at 200 • C), still around 36 mg/g of monomers per biomass (corresponding to 14.6 C-mol%) were released with a share of γ-OH monomers of around 27 mol% (Table S16).…”

“…With this stabilization, the repolymerization of the lignin fragments and released monomers can be avoided and lignin is obtained as an oil with a high monomer content [12,13]. It is believed that the addition of a hydrogenation catalyst furthermore supports ether bond cleavage by hydrogenolysis [10,11,14]. But essentially, the catalyst provides the crucial stabilization of monomers through hydrogenation.…”