Additive-Free Oxidative Cleavage of a Simple Lignin Model Compound in Air using Vanadium Complexes
Additive-Free Oxidative Cleavage of a Simple Lignin Model Compound in Air using Vanadium Complexes
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DOI：
10.1007/s11244-023-01902-8
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发表时间：
2024-02-06
影响因子：
3.6
通讯作者：
John，Alex
中科院分区：
文献类型：
--
作者：
Musharbash，Paul W.;Torres，Jerome B.;John，Alex
评价了三种具有不同空间、电子和金属中心氧化态(IVvsV)特征的Salophan和Salan配体支撑的钒配合物在一种简单的木质素模型化合物的氧化裂解中的催化性能。所有的络合物都是有效的，在优化的条件下(145℃，48小时，在空气中)以良好的产率(62-69%)和选择性(69-77%)生成了苯酚和苯甲酸的裂解产物，除了在较低的温度(125℃)外，没有观察到显著的反应活性差异。具有苯酚臂邻位/对位、环己基主干和V(V)中心的立体大分子叔丁基的络合物在135℃下72小时得到最高的产率和选择性(69-78%的产率；78-89%的选择性)。较长的反应时间和反应温度影响了两种裂解产物的产率和选择性。研究发现，氧化性气氛对观察到的反应性至关重要，因为在惰性气氛下尝试的反应不会导致显着的转化。初步的机理研究表明，木质素模型化合物在进行解理之前被氧化，与钒(IV)络合物相比，钒(V)络合物在裂解氧化产物方面更有效。值得注意的是，氧化裂解是在没有任何碱性或酸性添加剂的情况下实现的。
Three vanadium complexes supported by salophan and salan ligands featuring differences in steric, electronic, and oxidation state (IVvsV) of the metal center have been evaluated for their potential in catalyzing the oxidative cleavage of a simple lignin model compound. All complexes were found to be effective, and under optimized conditions (145 °C, 48 h, in air) produced the cleavage products, phenol and benzoic acid, in good yields (62–69%) and selectivity (69–77%); significant differences in reactivity were not observed except at a lower temperature (125 °C). Complex3cfeaturing sterically bulkytertbutyl groups atortho/parapositions of the phenol arms, a cyclohexyl backbone and a V(V) center resulted in the highest yields and selectivity at 135 °C over 72 h (69–78% yield; 78–89% selectivity). Longer reaction times as well as reaction temperatures were found to compromise yield and selectivity for both cleavage products. An oxidizing atmosphere was found to be crucial for the observed reactivity as reactions attempted under an inert atmosphere did not result in significant conversion. Preliminary mechanistic investigations suggest that the lignin model compound is oxidized prior to undergoing cleavage, and the vanadium(V) complex is more effective at cleaving the oxidized product in comparison to the vanadium(IV) counterparts. Quite significantly, the oxidative cleavage was achieved in the absence of any basic or acidic additives.Graphical Abstract