Pudukudy Manoj, Jia Qingming, Dong Yanan, Yue Zhongxiao, Shan Shaoyun
Faculty of Chemical Engineering, Kunming University of Science and Technology Kunming 650500 Yunnan People's Republic of China
RSC Adv. 2019 Oct 11;9(56):32517-32534. doi: 10.1039/c9ra04685b. eCollection 2019 Oct 10.
A series of magnetically separable rGO/FeO nanocomposites with various amounts of graphene oxide were successfully prepared by a simple ultrasonication assisted precipitation combined with a solvothermal method and their catalytic activity was evaluated for the selective liquid phase oxidation of cyclohexene using hydrogen peroxide as a green oxidant. The prepared materials were characterized using XRD, FTIR, FESEM, TEM, HRTEM, BET/BJH, XPS and VSM analysis. The presence of well crystallized FeO as the active iron species was seen in the crystal studies of the nanocomposites. The electron microscopy analysis indicated the fine surface dispersion of spherical FeO nanoparticles on the thin surface layers of partially-reduced graphene oxide (rGO) nanosheets. The decoration of FeO nanospheres on thin rGO layers was clearly observable in all of the nanocomposites. The XPS analysis was performed to evaluate the chemical states of the elements present in the samples. The surface area of the nanocomposites was increased significantly by increasing the amount of GO and the pore structures were effectively tuned by the amount of rGO in the nanocomposites. The magnetic saturation values of the nanocomposites were found to be sufficient for their efficient magnetic separation. The catalytic activity results show that the cyclohexene conversion reached 75.3% with a highest 1,2-cyclohexane diol selectivity of 81% over 5% rGO incorporated nanocomposite using HO as the oxidant and acetonitrile as the solvent at 70 °C for 6 h. The reaction conditions were further optimized by changing the variables and a possible reaction mechanism was proposed. The enhanced catalytic activity of the nanocomposites for cyclohexene oxidation could be attributed to the fast accomplishment of the Fe/Fe redox cycle in the composites due the sacrificial role of rGO and its synergistic effect with FeO, originating from the conjugated network of π-electrons in its surface structure. The rapid and easy separation of the magnetic nanocomposites from the reaction mixture using an external magnet makes the present catalysts highly efficient for the reaction. Moreover, the catalyst retained its activity for five repeated runs without any drastic drop in the reactant conversion and product selectivity.
通过简单的超声辅助沉淀结合溶剂热法成功制备了一系列含有不同量氧化石墨烯的磁性可分离rGO/FeO纳米复合材料，并以过氧化氢作为绿色氧化剂，对其催化环己烯选择性液相氧化的活性进行了评估。使用XRD、FTIR、FESEM、TEM、HRTEM、BET/BJH、XPS和VSM分析对制备的材料进行了表征。在纳米复合材料的晶体研究中发现存在结晶良好的FeO作为活性铁物种。电子显微镜分析表明球形FeO纳米颗粒在部分还原的氧化石墨烯（rGO）纳米片的薄表面层上有良好的表面分散。在所有纳米复合材料中都能清楚地观察到FeO纳米球在薄rGO层上的修饰。进行XPS分析以评估样品中存在的元素的化学状态。通过增加GO的量，纳米复合材料的表面积显著增加，并且通过纳米复合材料中rGO的量有效地调节了孔结构。发现纳米复合材料的磁饱和值足以实现其高效磁分离。催化活性结果表明，在70℃下以HO为氧化剂、乙腈为溶剂反应6小时，含5%rGO的纳米复合材料上，环己烯转化率达到75.3%，1,2-环己二醇的最高选择性为81%。通过改变变量进一步优化了反应条件，并提出了可能的反应机理。纳米复合材料对环己烯氧化的催化活性增强可归因于rGO的牺牲作用及其与FeO的协同效应，使得复合材料中Fe/Fe氧化还原循环快速完成，这源于其表面结构中π电子的共轭网络。使用外部磁铁从反应混合物中快速、容易地分离磁性纳米复合材料，使得本催化剂对该反应具有高效性。此外，该催化剂在五次重复运行中保持其活性，反应物转化率和产物选择性没有任何急剧下降。
