New Structures in Eucalyptus Kraft Lignin with Complex Mechanistic Implications
New Structures in Eucalyptus Kraft Lignin with Complex Mechanistic Implications
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DOI：
10.1021/acssuschemeng.0c03776
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发表时间：
2020-07-27
影响因子：
8.4
通讯作者：
Lawoko, Martin
中科院分区：
文献类型：
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作者：
Giummarella, Nicola;Pylypchuk, Ievgen, V;Lawoko, Martin
近年来，已经开发了技术上可行的方法来从黑液中以固体或液体形式回收硫酸盐木质素。这为将硫酸盐木质素定位为可再生芳香族聚合物前体提供了巨大的机会。然而，硫酸盐木质素的异质性是一个主要的障碍，并表现在其在很大程度上未知的分子结构，这在最近几年引起了进一步的关注。在这种情况下，我们在此研究了桉树硫酸盐木质素的详细结构，特别强调识别新的连接签名逆羟醛和随后的自由基偶联反应，我们最近发现这是一个关键的反应序列，有助于云杉硫酸盐木质素的结构。结合新的模型研究，我们通过桉树硫酸盐木质素的高级2D NMR表征明确地鉴定了新的结构，即，3，5-四甲氧基-对-联苯酚、3-二甲氧基-对-联苯酚和少量的3，5-二甲氧基-苯醌。这些结构是逆向羟醛随后进行自由基偶联反应的标志。通过1D C-13 NMR在桉树硫酸盐木质素中9%的单元间键处进一步定量两种二酚结构，这与我们先前在软木硫酸盐木质素中鉴定的量（10%）相当。因此，硫酸盐木质素自由基缩合形成碳-碳键并不区分异辛基木质素和愈创木基木质素单元。我们合理化这样的歧视，从自由基耦合在未取代的C-1在形成的枫香酚和愈创木酚木质素作为逆羟醛反应的结果的可能性。
Recent years have seen the development of technically feasible methods to retrieve kraft lignin from the black liquor as solids or liquids. This opens enormous opportunities to position kraft lignin as a renewable aromatic polymer precursor. However, the heterogeneity of kraft lignin is one major hurdle and manifests in its largely unknown molecular structure, which in recent years has drawn further attention. In this context, we herein studied the detailed structure of Eucalyptus kraft lignin with special emphasis on identifying new linkages signatory to retro-aldol and subsequent radical coupling reactions, which we recently showed to be a key reaction sequence contributing to the structure of spruce kraft lignin. In combination with novel model studies, we unequivocally identified new structures by advanced 2D NMR characterization of Eucalyptus kraft lignin, i.e., 3,5-tetramethoxy-para-diphenol, 3-dimethoxy-para-diphenol and small amounts of 3,5-dimethoxy-benzoquinone. These structures are signatory to retro-aldol followed by radical coupling reactions. The two diphenol structures were further quantified by 1D C-13 NMR at 9% of the interunit linkages in Eucalyptus kraft lignin, which was comparable to the amounts we previously identified in softwood kraft lignin (10%). Radical condensation of kraft lignin to form carbon-carbon bonds therefore does not discriminate between syringyl lignin and guaiacyl lignin units. We rationalize such indiscrimination to emanate from possibilities for radical couplings at unsubstituted C-1 in the formed syringol and guaiacol lignin as a result of the retro-aldol reaction.