Dou Xiaomeng, Li Wenzhi, Zhu Chaofeng, Jiang Xiao, Chang Hou-Min, Jameel Hasan
Laboratory of Basic Research in Biomass Conversion and Utilization, University of Science and Technology of China Hefei 230026 PR China
Hefei National Laboratory for Physics Science at Microscale, School of Chemistry and Materials Science, University of Science and Technology of China Hefei 230026 PR China.
RSC Adv. 2020 Dec 9;10(71):43599-43606. doi: 10.1039/d0ra08121c. eCollection 2020 Nov 27.
Efficient cleavage of aryl-ether linkages is a key strategy for generating aromatic chemicals and fuels from lignin. Currently, a popular method to depolymerize native/technical lignin employs a combination of Lewis acid and hydrogenation metal. However, a clear mechanistic understanding of the process is lacking. Thus, a more thorough understanding of the mechanism of lignin depolymerization in this system is essential. Herein, we propose a detailed mechanistic study conducted with lignin model compounds (LMC) a synergistic Co-Zn/Off-Al H-beta catalyst that mirrors the hydrogenolysis process of lignin. The results suggest that the main reaction paths for the phenolic dimers exhibiting α-O-4 and β-O-4 ether linkages are the cleavage of aryl-ether linkages. Particularly, the conversion was readily completed using a Co-Zn/Off-Al H-beta catalyst, but 40% of α-O-4 was converted and β-O-4 did not react in the absence of a catalyst under the same conditions. In addition, it was found that the presence of hydroxyl groups on the side chain, commonly found in native lignin, greatly promotes the cleavage of aryl-ether linkages activated by Zn Lewis acid, which was attributed to the adsorption between Zn and the hydroxyl group. Followed by the cobalt catalyzed hydrogenation reaction, the phenolic dimers are degraded into monomers that maintain aromaticity.
高效裂解芳基醚键是从木质素中生成芳烃化学品和燃料的关键策略。目前，一种使天然/工业木质素解聚的常用方法是结合使用路易斯酸和氢化金属。然而，目前仍缺乏对该过程清晰的机理认识。因此，更深入地了解该体系中木质素解聚的机理至关重要。在此，我们提出一项使用木质素模型化合物（LMC）和一种协同的Co-Zn/Off-Al H-β催化剂进行的详细机理研究，该催化剂反映了木质素的氢解过程。结果表明，具有α-O-4和β-O-4醚键的酚类二聚体的主要反应路径是芳基醚键的裂解。特别是，使用Co-Zn/Off-Al H-β催化剂时转化很容易完成，但在相同条件下，无催化剂时40%的α-O-4发生了转化，而β-O-4未发生反应。此外，还发现天然木质素中常见的侧链羟基的存在极大地促进了由Zn路易斯酸活化的芳基醚键的裂解作用，这归因于Zn与羟基之间的吸附作用。随后在钴催化的氢化反应作用下，酚类二聚体降解为保持芳香性的单体。
