A Mechanistic Investigation of Acid-Catalyzed Cleavage
of Aryl-Ether Linkages: Implications for Lignin Depolymerization in
Acidic Environments
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Abstract
Acid catalysis has
long been used to depolymerize plant cell wall
polysaccharides, and the mechanisms by which acid affects carbohydrates
have been extensively studied. Lignin depolymerization, however, is
not as well understood, primarily due to the heterogeneity and reactivity
of lignin. We present an experimental and theoretical study of acid-catalyzed
cleavage of two non-phenolic and two phenolic dimers that exhibit
the β-O-4 ether linkage, the most common intermonomer bond in
lignin. This work demonstrates that the rate of acid-catalyzed β-O-4
cleavage in dimers exhibiting a phenolic hydroxyl group is 2 orders
of magnitude faster than in non-phenolic dimers. The experiments suggest
that the major product distribution is similar for all model compounds,
but a stable phenyl-dihydrobenzofuran species is observed in the acidolysis
of two of the γ-carbinol containing model compounds. The presence
of a methoxy substituent, commonly found in native lignin, prevents
the formation of this intermediate. Reaction pathways were examined
with quantum mechanical calculations, which aid in explaining the
substantial differences in reactivity. Moreover, we use a radical
scavenger to show that the commonly proposed homolytic cleavage pathway
of phenolic β-O-4 linkages is unlikely in acidolysis conditions.
Overall, this study explains the disparity between rates of β-O-4
cleavage seen in model compound experiments and acid pretreatment
of biomass, and implies that depolymerization of lignin during acid-catalyzed
pretreatment or fractionation will proceed via a heterolytic, unzipping
mechanism wherein β-O-4 linkages are cleaved from the phenolic
ends of branched, polymer chains inward toward the core of the polymer