Advanced Model Compounds for Understanding Acid-Catalyzed Lignin Depolymerization: Identification of Renewable Aromatics and a Lignin-Derived Solvent

Abstract

ABSTRACT:The development of fundamentally new approaches for lignin depolymerization is challenged by the complexity of this aromatic biopolymer. While overly simplified model compounds often lack relevance to the chemistry of lignin, the use of lignin streams directly, poses significant analytical challenges to methodology development. Ideally, new methods should be tested on model compounds that are complex enough to mirror the structural diversity in lignin, but still of sufficiently low molecular weight to enable facile analysis. In this contribution we present a new class of advanced (β-O-4)-(β-5) dilinkage models that are highly realistic representations of a lignin fragment. Together with selected β-O-4, β-5 and β-β structures, these compounds provide a detailed understanding of the reactivity of various types of lignin linkages in acid catalysis in conjunction with stabilization of reactive intermediates using ethylene glycol. The use of these new models has allowed for identification of novel reaction pathways and intermediates and led to the characterization of new dimeric products in subsequent lignin depolymerization studies. The excellent correlation between model and lignin experiments highlights the relevance of this new class of model compounds for broader use in catalysis studies. Only by understanding the reactivity of the linkages in lignin at this level of detail can fully optimized lignin depolymerization strategies be developed.

Performances of Several Solvents on the Cleavage of Inter‐ and Intramolecular Linkages of Lignin in Corncob Residue

“…Clearly, solvent plays an important role in enhancing both the conversion of lignin and yield of lignin‐derived products, owing to increased mass and heat transfer, as well as the interactions between liquid lignin derivatives and the catalyst. Much attention has been paid to obtaining a high yield of monomers from lignin or lignin model compounds; this gives a detailed understanding of how the linkages are mechanistically cleaved in the model compounds. These results are very helpful for understanding the breakage of intramolecular bonds in lignin.…”

Catalytic Scissoring of Lignin into Aryl Monomers

“…Few experiments involved trimers or synthetic polymers containing the repeated linkage, which are closer to real lignin than dimers. Recently, advanced dilinkage models that contain two different linkages ((β‐O‐4)‐(β‐5)) were synthesized and tested for acid‐catalyzed approaches . The new models are helpful for analyzing more complicated processes.…”

“…1) The dehydration of C α HOH groups and the loss of formaldehyde generate an allyl ether structure, which is followed by hydrolysis to produce aldehydes. The aldehydes are unstable and easily recondensate under acidic conditions . 2) The benzylic C α + carbocation attacks the adjacent G‐ or H‐type of aromatic ring, generating a benzofuran structure and a new CC bond .…”

Breaking the Limit of Lignin Monomer Production via Cleavage of Interunit Carbon–Carbon Linkages

“…1,2 A great deal of effort has been devoted to developing strategies for the depolymerisation of lignin. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] However, the primary products from these processes are mixtures of oxygen-containing compounds with high boiling points, which can hardly be separated using conventional distillation techniques. Moreover, the yields of lignin monomers are limited due to the presence of stable interunit C-C bonds within native lignin or those formed during lignin extraction.…”