Organosolv Fractionation of Walnut Shell Biomass to Isolate Lignocellulosic Components for Chemical Upgrading of Lignin to Aromatics

Abstract

Renewable carbon sources are a rapidly growing field of research because of the finite supply of fossil carbon. The lignocellulosic biomass walnut shell (WS) is an attractive renewable feedstock because it has a high lignin content (38–44 wt %) and is an agricultural waste stream. Lignin, a major component of lignocellulosic biomass that is currently a waste stream in pulping processes, has unique potential for chemical upgrading because its subunits are aromatic. In the interest of improving the sustainability and reducing the environmental impact of biomass processing, valorization of agricultural waste streams is important. Herein, three lab-scale, batch organosolv procedures are explored in the interest of optimal isolation of protected WS lignin (WSL). One system uses acetic acid, one MeOH, and the final EtOH as the primary solvent. The optimal condition for protected WSL isolation, which resulted in a 64% yield, was methanol and dilute sulfuric acid with formaldehyde to act as a protecting group at 170 °C. Select samples were upgraded by hydrogenolysis over a nickel catalyst. Protected lignin recovered from the optimal condition showed 77% by weight conversion to monomeric phenols, demonstrating that the protected WSL can selectively afford high value products. One key finding from this study was that MeOH is a superior solvent for isolating WSL versus EtOH because the latter exhibited lignin recondensation. The second was that the Ni/C-catalyzed reductive catalytic fractionation (RCF) directly of WS biomass was not selective relative to RCF of isolated WSL; conversion of raw WS to monomers produced significantly more side products.

Catalytic conversion of high S-lignin to a sustainable tri-epoxide polymer precursor

“…We also compared the above approach to one where several organosolv methods were used to separate lignins from the high-S poplar biomass prior to the hydrogenolysis to monomers. 30,[35][36][37][38][39] However, the drawbacks of these methods are obvious given that they require extra usage of solvents, time, and energy. 31,[38][39][40][41] More importantly, the properties of organosolv lignins are often altered during these fractionation treatments, including re-condensation reactions forming unnatural and recalcitrant C-C bonds.…”

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

“…within the organosolv treatment of biomass under acidic conditions, to suppress the repolymerization of the solubilized lignin and hemicellulose intermediates by formation of relatively stable acetal structures. [508][509][510][511][512][513][514][515] AAF is carried out in acidied aqueous mixtures of aprotic organic solvents (e.g. dioxane), at temperatures in the range of 80-120 °C.…”

Beyond Yield Optimization: The Impact of Organosolv Process Parameters on Lignin Structure

“…In their work, Nishide et al investigated various parameters affecting the organosolv process, such as different solvents, catalysts, and process temperatures. [79] They found that in the environment catalyzed by alcoholic acids, an increase in temperature leads to a decrease in polydispersity and average molecular weight. A temperature-dependent trend is also observed by Yao et al [80] They isolated organosolv lignins from Broussonetia papyrifera at 150, 160, 170 and 180 °C, respectively, and examined their structural differences.…”