Chang Shenzhen, Chen Yanhong, An Haiyan, Zhu Qingshan, Luo Huiyun, Xu Tieqi
School of Chemical Engineering, Dalian University of Technology, Dalian 116023, People's Republic of China.
ACS Appl Mater Interfaces. 2021 May 12;13(18):21261-21271. doi: 10.1021/acsami.1c02558. Epub 2021 Apr 28.
Selective oxidation of alkyl-substituted phenols offers efficient access to -benzoquinones (BQs) that serve as key components for synthesizing biologically active compounds, but rational manufacture of efficient recyclable catalysts for such a reaction remains a severe challenge. Herein, two crystalline 2D polyoxometalate-based coordination polymers (POMCPs), formulated as H[Cu(L)][PMO]·HO (M = Mo, = 4 for ; M = W, = 6 for ; and HL = 4-(1-tetazol-5-yl)pyridine), are prepared by a mineralizer-assisted one-step synthesis strategy and explored as heterogeneous catalysts for -BQs synthesis. Both compounds have been characterized through elemental analysis, EDS analysis, infrared spectroscopy, UV-vis diffuse reflectance spectrum, EPR, XPS, BET, single-crystal, and powder X-ray diffraction. Single-crystal X-ray diffraction analysis indicates that both and exhibit an interesting 2D sheet structure composed of 2-connected Keggin type anions [PMO] and hexa-nuclear {Cu(HL)} cluster-based metal-organic chains via Cu···O interactions. When used as catalysts, POMCPs and have excellent catalytic activities in the selective oxidation of substituted phenols to -BQs with HO. Notedly, in the model reaction from 2,3,6-trimethylphenol (TMP) to the vitamin E key intermediate trimethyl--benzoquinone (TMBQ), the catalytic activities expressed by turnover frequency (TOF) of and can reach an unprecedented 2400 and 2000 h, respectively, at close to 100% TMBQ yield. The truly heterogeneous nature, stability, and structural integrity of both catalysts were ascertained by FTIR, PXRD techniques, and the following cycles. Mechanism studies reveal that both catalysts can involve a dual reaction pathway through a heterolytic oxygen atom transfer mechanism and homolytic radical mechanism. Moreover, the 2D POMCPs with highly accessible bilateral active sites and efficient mass transfer efficiency possess superior catalytic performance to their analogous 3D species.
烷基取代酚的选择性氧化为合成生物活性化合物的关键成分对苯醌（BQs）提供了有效途径，但为此类反应合理制备高效可回收催化剂仍然是一项严峻挑战。在此，通过矿化剂辅助的一步合成策略制备了两种晶体二维基于多金属氧酸盐的配位聚合物（POMCPs），其化学式为H[Cu(L)][PMO]·HO（M = Mo，对于 = 4；M = W，对于 = 6；HL = 4-(1-四唑-5-基)吡啶），并将其作为用于 -BQs 合成的多相催化剂进行探索。两种化合物均通过元素分析、EDS 分析、红外光谱、紫外可见漫反射光谱、EPR、XPS、BET、单晶和粉末 X 射线衍射进行了表征。单晶 X 射线衍射分析表明， 和 均呈现出由通过 Cu···O 相互作用连接的 2-连接 Keggin 型阴离子[PMO]和六核{Cu(HL)}簇基金属有机链组成的有趣二维片状结构。当用作催化剂时，POMCPs 和 在以 HO 将取代酚选择性氧化为 -BQs 方面具有优异的催化活性。值得注意的是，在从 2,3,6-三甲基苯酚（TMP）到维生素 E 关键中间体三甲基 - 对苯醌（TMBQ）的模型反应中， 和 的周转频率（TOF）所表示的催化活性在 TMBQ 产率接近 100%时分别可达到前所未有的 2400 和 2000 h⁻¹。通过 FTIR、PXRD 技术以及后续循环确定了两种催化剂真正的多相性质、稳定性和结构完整性。机理研究表明，两种催化剂均可通过异裂氧原子转移机理和均裂自由基机理涉及双重反应途径。此外，具有高度可及的双侧活性位点和高效传质效率的二维 POMCPs 比其类似的三维物种具有更优异的催化性能。
