(455f) Modulating the Structure of Aldehyde-Stabilized Lignin to Enhance Its Solubility and Hydrogenolysis

Despite its substantial carbon content, lignin has seen limited use as a renewable feedstock for chemical and material manufacture.¹ Existing lignocellulosic biomass fractionation processes produce lignin with random, interunit C–C bonds that inhibit its depolymerization and reduce its processability.^2–4 Recently, we discovered that aldehydes can stabilize lignin during its extraction, allowing for the full fractionation of lignocellulosic biomass into cellulose-rich solids, acetal-protected xylose, and aldehyde-stabilized lignin.^5,6 All aldehydes can facilitate this fractionation, but not all of them can produce fully upgradeable lignin.⁷ Hydrogenolysis of propionaldehyde stabilized lignin yields monomer quantities on par with that observed by reductive catalytic fractionation, while benzaldehyde-stabilized lignin yields far less.^8,9 By modulating the sterics and electronics of the aldehyde used during fractionation, we have determined the relationship between aldehyde structure and lignin monomer yield. Sterically bulky aldehydes can still produce highly upgradeable lignins, but electron-poor aldehydes, or those with competing nucleophilic functionality, lead to stabilized lignins with reduced upgradeability. The electronic effect is largely inductive as lignins stabilized by benzaldehydes with electron donating groups yield similar quantities of monomers upon hydrogenolysis as lignins stabilized by benzaldehydes with electron withdrawing groups.

Besides its influence on lignin stabilization, the aldehyde substituent also perturbs the physical properties of the extracted lignin. Using the lignin library, we have begun to create rules for tuning these properties with implications for its ultimate use. Specifically, we can control the solubilities of functionalized lignins, creating hydrophilic and lipophilic structures with solubilities exceeding 10 wt/wt% in aqueous and hydrocarbon mixtures, respectively. This control has allowed us to elucidate the effect of solvent on lignin hydrogenolysis. Briefly, depolymerization yields of select lignins in isooctane, tetrahydrofuran, and water correlated strongly with the lignins’ solubilities.¹⁰ Polar aprotic solvents were demonstrated to suppress hydrodeoxygenation of the lignin monomers. And notably, for the first time we demonstrated the selective formation (91%) of cyclohexanols from lignin solubilized in water.¹¹ Through the creation of this library of aldehyde-stabilized lignins, we have enhanced the predictability of the aldehyde stabilization of lignin, increased the solubility window of the aldehyde-stabilized lignin, and shown that the depolymerization products and yields can be tuned, furthering the use of lignin as a renewable feedstock for chemical synthesis.

References

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