Effect of manganese substitution on the physicochemical properties and catalytic toluene oxidation activities of Mg-Al layered double hydroxides
Effect of manganese substitution on the physicochemical properties and catalytic toluene oxidation activities of Mg-Al layered double hydroxides
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DOI：
10.1016/s1387-1811(99)00123-7
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发表时间：
1999-12
影响因子：
5.2
通讯作者：
S. Velu;Neepa Shah;T. Jyothi;S. Sivasanker
中科院分区：
文献类型：
--
作者：
S. Velu;Neepa Shah;T. Jyothi;S. Sivasanker
采用共沉淀法合成了一系列（Mg+Mn）/Al原子比为0.3、Mg：Mn原子比为3：0 ~ 0：3的镁锰铝三元类水滑石层状双氢氧化物（LDHS）。采用粉末X射线衍射、傅里叶变换红外光谱、电子顺磁共振（EPR）光谱、紫外-可见漫反射光谱（UV-Vis DRS）和热重分析等技术研究了Mn在MgAl-LDH中的掺入。得到对应于LDH的单相，其组成为Mg：Mn：Al=2.2：0.8：1。Mn含量的进一步增加导致形成LDH、Mn（OH）2和MnCO 3相的混合物。UV-Vis DRS结果表明，Mn ~（2+）部分氧化为Mn ~（3+），EPR和UV-Vis DRS结果表明，Mn ~（2+）/Mn ~（3+）处于畸变的八面体环境中。在723 K下煅烧导致形成结晶不良的MgO样固溶体。即使在空气中在非常高的温度（1473 K）下煅烧，Mn也保持其+2氧化态（EPR活性）。当Mn含量较高时，在723 K时，除了生成类MgO相外，还生成了Mn 3 O 4和MnAl 2 O 4等尖晶石相。以过氧化叔丁基为氧化剂，考察了所合成样品在甲苯液相氧化反应中的催化活性。苯甲醛和苯甲酸是主要产物，还有少量苯甲醇。发现在类似的反应条件下，没有LDH相的化合物对甲苯的氧化活性较低。发现溶剂对催化活性和产物选择性有深刻的影响。
A series of Mg–Mn–Al ternary hydrotalcite-like layered double hydroxides (LDHs) with (Mg+Mn)/Al atomic ratios of ∼3 and Mg:Mn atomic ratios ranging from 3:0 to 0:3 were synthesized by a co-precipitation method. The incorporation of Mn in the MgAl-LDH was investigated by employing various techniques, such as powder X-ray diffraction, Fourier transform infrared spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, UV–visible diffuse-reflectance spectroscopy (UV–Vis DRS) and thermogravimetry. A single phase corresponding to LDH was obtained up to the composition Mg:Mn:Al=2.2:0.8:1. A further increase in the Mn content resulted in the formation of a mixture of LDH, Mn(OH)2and MnCO3phases. Partial oxidation of Mn2+to Mn3+took place during synthesis, as evidenced by UV–Vis DRS. According to the EPR and UV–Vis DRS results, the Mn2+/Mn3+ions were present in a (distorted) octahedral environment. Calcination at 723K resulted in the formation of a poorly crystalline MgO-like solid solution. Mn retained its +2 oxidation state (EPR active) even upon calcination at very high temperature (1473K) in air. At a higher Mn content, in addition to the MgO-like phase, spinel phases such as Mn3O4and MnAl2O4were also obtained at 723K. The catalytic activity of the as-synthesized samples was tested in the liquid-phase oxidation of toluene using tert-butyl hydroperoxide as the oxidant. Benzaldehyde and benzoic acid were the major products with minor amounts of benzyl alcohol. The compound without an LDH phase was found to be less active for the oxidation of toluene under similar reaction conditions. Solvents were found to have a profound influence on the catalytic activity and the product selectivity.