Lignin is of significant interest due to its potential as a feedstock for the production of renewable fuels and bulk chemicals. To enable lignin valorization, novel catalysts must be developed that can depolymerize the biopolymer. Herein we present two approaches for the oxidative cleavage of the lignin β-ether units: 1) using a Li-Al layered double hydroxide (LDH) supported Au nanoparticle catalyst for conversion of the unit to an ester, which then allows bond cleavage by means of simple hydrolysis; and 2) a two-step depolymerization process involving selective oxidation of Ca-OH to Ca=O groups within the β-ether unit, followed by a second oxidative step to cleave the β-O-ether linkage. For the first strategy, preliminary data acquired using β-ether model dimers have shown the ability of Au/Li-Al LDH to transform the β-ether unit into an ester in the presence of 1 bar O2 at 120 °C. According to heteronuclear single-quantum coherence spectroscopy (HSQC), when identical reaction conditions are applied to kraft lignin, signals corresponding to Ca and Cβ within the β-ether units are no longer present, which is consistent with their conversion to esters given that the ester structure exhibits no C-H (i.e., Cα and Cβ) correlations observable by HSQC. For the second strategy, protocols from the recent literature for Ca-OH oxidation to Ca=O using DDQ or TEMPO as catalysts were applied, followed by cleavage of the oxidized β-ether using methods such as Baeyer-Villiger oxidation and Dakin oxidation. These literature methods were applied to several types of lignin in an effort to evaluate the effectiveness of this approach for the oxidative depolymerization of lignin feedstocks.

A comparative study on oxidative approaches for lignin depolymerization

255th National Meeting (2018)

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