A Mechanistic Investigation of Acid-Catalyzed Cleavage of Aryl-Ether Linkages: Implications for Lignin Depolymerization in Acidic Environments
A Mechanistic Investigation of Acid-Catalyzed Cleavage of Aryl-Ether Linkages: Implications for Lignin Depolymerization in Acidic Environments
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DOI：
10.1021/sc400384w
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发表时间：
2014-03-01
影响因子：
8.4
通讯作者：
Beckham, Gregg T.
中科院分区：
文献类型：
--
作者：
Sturgeon, Matthew R.;Kim, Seonah;Beckham, Gregg T.
长期以来，酸催化一直被用于去除植物细胞壁多糖，并且酸影响碳水化合物的机制已被广泛研究。然而，木质素解聚并没有被很好地理解，主要是由于木质素的异质性和反应性。我们提出了一个实验和理论研究的酸催化裂解的两个非酚和两个酚二聚体，表现出β-O-4醚键，最常见的单体间键在木质素。这项工作表明，酸催化的β-O-4裂解的速率在二聚体表现出酚羟基比非酚二聚体快2个数量级。实验表明，所有模型化合物的主要产物分布是相似的，但在两个含γ-甲醇的模型化合物的酸解中观察到稳定的苯基-二氢苯并呋喃物种。通常在天然木质素中发现的甲氧基取代基的存在防止了该中间体的形成。用量子力学计算检查了反应途径，这有助于解释反应性的实质性差异。此外，我们使用自由基清除剂表明，通常提出的酚β-O-4键的均裂裂解途径是不可能在酸解条件下。总的来说，这项研究解释了在模型化合物实验和生物质的酸预处理中看到的β-O-4裂解速率之间的差异，并暗示在酸催化的预处理或分馏过程中木质素的解聚将通过hetcrolytic，解链机制进行，其中β-O-4键从支化的聚合物链的酚末端向内朝向聚合物的核心裂解。
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 beta-O-4 ether linkage, the most common intermonomer bond in lignin. This work demonstrates that the rate of acid-catalyzed beta-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 gamma-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 beta-O-4 linkages is unlikely in acidolysis conditions. Overall, this study explains the disparity between rates of beta-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 hetcrolytic, unzipping mechanism wherein beta-O-4 linkages are cleaved from the phenolic ends of branched, polymer chains inward toward the core of the polymer.