Evtushok Vasiliy Yu, Ivanchikova Irina D, Podyacheva Olga Yu, Stonkus Olga A, Suboch Arina N, Chesalov Yuri A, Zalomaeva Olga V, Kholdeeva Oxana A
Department of Fine Organic Synthesis and Renewable Energy Sources, Boreskov Institute of Catalysis, Novosibirsk, Russia.
Department of Natural Sciences, Novosibirsk State University, Novosibirsk, Russia.
Front Chem. 2019 Dec 13;7:858. doi: 10.3389/fchem.2019.00858. eCollection 2019.
In this work, we elaborated heterogeneous catalysts on the basis of the Venturello complex [PO{WO(O)}] (PW) and nitrogen-free or nitrogen-doped carbon nanotubes (CNTs or N-CNTs) for epoxidation of alkenes and sulfoxidation of thioethers with aqueous hydrogen peroxide. Catalysts PW/CNTs and PW/N-CNTs (1.8 at. % N) containing 5-15 wt. % of PW and differing in acidity have been prepared and characterized by elemental analysis, N adsorption, IR spectroscopy, HR-TEM, and HAADF-STEM. Studies by STEM in HAADF mode revealed a quasi-molecular dispersion of PW on the surface of CNTs. The addition of acid during the immobilization is not obligatory to ensure site isolation and strong binding of PW on the surface of CNTs, but it allows one to increase the PW loading and affects both catalytic activity and product selectivity. Catalytic performance of the supported PW catalysts was evaluated in HO-based oxidation of two model substrates, cyclooctene and methyl phenyl sulfide, under mild conditions (25-50°C). The best results in terms of activity and selectivity were obtained using PW immobilized on N-free CNTs in acetonitrile or dimethyl carbonate as solvents. Catalysts PW/CNTs can be applied for selective oxidation of a wide range of alkenes and thioethers provided a balance between activity and selectivity of the catalyst is tuned by a careful control of the amount of acid added during the immobilization of PW. Selectivity, conversion, and turnover frequencies achieved in epoxidations over PW/CNTs catalysts are close to those reported in the literature for homogeneous systems based on PW. IR spectroscopy confirmed the retention of the Venturello structure after use in the catalytic reactions. The elaborated catalysts are stable to metal leaching, show a truly heterogeneous nature of the catalysis, can be easily recovered by filtration, regenerated by washing and evacuation, and then reused several times without loss of the catalytic performance.
在本工作中，我们基于Venturello配合物[PO{WO(O)}]（PW）以及无氮或氮掺杂的碳纳米管（CNTs或N-CNTs）制备了用于烯烃环氧化和硫醚与过氧化氢水溶液进行硫氧化反应的多相催化剂。制备了含有5-15 wt.% PW且酸度不同的催化剂PW/CNTs和PW/N-CNTs（1.8 at.% N），并通过元素分析、N吸附、红外光谱、高分辨透射电子显微镜（HR-TEM）和高角度环形暗场扫描透射电子显微镜（HAADF-STEM）对其进行了表征。在HAADF模式下通过STEM研究表明，PW在CNTs表面呈准分子分散状态。在固定化过程中添加酸并非确保PW在CNTs表面实现位点隔离和强结合所必需的，但它能增加PW的负载量，并影响催化活性和产物选择性。在温和条件（25-50°C）下，在基于H₂O₂的两种模型底物环辛烯和苯甲硫醚的氧化反应中评估了负载型PW催化剂的催化性能。在以乙腈或碳酸二甲酯为溶剂、将PW固定在无氮CNTs上时，在活性和选择性方面获得了最佳结果。只要通过仔细控制PW固定化过程中添加的酸量来调节催化剂活性和选择性之间的平衡，催化剂PW/CNTs就可用于多种烯烃和硫醚的选择性氧化。在PW/CNTs催化剂上进行环氧化反应所实现的选择性、转化率和周转频率与文献中报道的基于PW的均相体系相近。红外光谱证实了Venturello结构在催化反应使用后得以保留。所制备的催化剂对金属浸出稳定，显示出真正的多相催化性质，可通过过滤轻松回收，通过洗涤和抽空进行再生，然后可重复使用多次而不损失催化性能。
