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.

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Anion exchange membranes (AEMs) are of interest as hydroxide conducting polymer electrolytes in electrochemical devices like fuel cells and electrolyzers. AEMs require hydroxide stable covalently tetherable cations to ensure required conductivity. Benzyltrimethylammonium (BTMA) has been the covalently tetherable cation that has been most often employed in anion exchange membranes because it is reasonably basic, compact (limited number of atoms per charge), and easily/cheaply synthesized. Several reports exist that have investigated hydroxide stability of BTMA under specific conditions, but consistency within these reports and comparisons between them have not yet been made. While the hydroxide stability of BTMA has been believed to be a limitation for AEMs, this stability has not been thoroughly reported. We have found that several methods reported have inherent flaws in their findings due to the difficulty of performing degradation experiments at high temperature and high pH. In order to address these shortcomings, we have developed a reliable, standardized method of determining cation degradation under conditions similar/relevant to those expected in electrochemical devices. The experimental method has been employed to determine BTMA stabilities at varying cation concentrations and elevated temperatures, and has resulted in improved experimental accuracy and reproducibility. Alkaline membrane fuel cells (AMFCs) employing anion exchange membranes (AEMs) are of increasing interest in fuel cell research as they potentially enable the use of non-Pt fuel cell catalysts, a primary cost limitation of proton exchange membrane fuel cells.

Although lignin is one of the main components of biomass, its pyrolysis chemistry is not well understood due to complex heterogeneity.