Generation of acid sites on silica-supported rare earth oxide catalysts: Structural characterization and catalysis for α-pinene isomerization
作者:Takashi Yamamoto、Takahiro Matsuyama、Tsunehiro Tanaka、Takuzo Funabiki、Satohiro Yoshida
DOI:10.1039/a900742c
日期:——
Silica-supported rare earth oxide catalysts (Ln/SiO2; Ln=La, Ce, Pr, Sm, Eu, Tb, Yb and Y), loading amounts of which were 3.4 mmol g (support)-1, were characterized by α-pinene isomerization, temperature-programmed desorption (TPD), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), thermogravimetric-differential thermal analysis (TG-DTA) and Raman spectroscopy. In the lanthanoid series, the catalytic activity increased with atomic number from 57La to 70Yb, except for Ce. All the Ln/SiO2 catalysts, except for Ce, were amorphous. On the surface of the catalyst, LnâOâSi and LnâOâLn linkages formed, the ratio of which varied with the loaded element. The ratio of LnâOâSi linkage increases with stronger affinity among LnOn units and SiO4 tetrahedra, and the affinity depends on the size of Ln3+. With increasing ratio of LnâOâSi to LnâOâLn linkage, the catalytic activity increases. Silica-supported yttrium oxide catalyst, the trivalent ion radius of which is quite similar to that of ytterbium, exhibited the same activity as that of Yb/SiO2. Raman spectroscopic characterization revealed that excess loading of Yb atoms on SiO2-support block YbâOâSi linkage to form Yb2O3 fine particles. When Yb/SiO2 was pretreated at 1273 K, fine ytterbium silicate crystallites formed. LnâOâSi linkage without a long-ranged ordering structure was the active site for α-pinene isomerization.
Introduction
以二氧化硅为载体的稀土氧化物催化剂(Ln/SiO2;Ln=La、Ce、Pr、Sm、Eu、Tb、Yb 和 Y),其负载量为 3.通过δ-蒎烯异构化、温度编程解吸 (TPD)、傅里叶变换红外 (FTIR)、X 射线衍射 (XRD)、X 射线吸收精细结构 (XAFS)、热重-差热分析 (TG-DTA) 和拉曼光谱对这些催化剂进行了表征。在镧系元素系列中,除铈外,催化活性随原子序数从 57La 到 70Yb 的增加而增加。除 Ce 外,所有 Ln/SiO2 催化剂都是无定形的。催化剂表面形成了 LnâOâSi 和 LnâOâLn 连接,其比例随负载元素的不同而变化。LnâOâSi 连接的比例随着 LnOn 单元和 SiO4 四面体之间亲和力的增强而增加,亲和力取决于 Ln3+ 的大小。随着 LnâOâSi 与 LnâOâLn 连接比例的增加,催化活性也随之增加。二氧化硅支撑的氧化钇催化剂(其三价离子半径与镱的三价离子半径十分相似)表现出与 Yb/SiO2 催化剂相同的活性。拉曼光谱表征显示,过量的镱原子负载在 SiO2-support 上会阻断 YbâOâSi 连接,形成 Yb2O3 微粒。在 1273 K 下预处理 Yb/SiO2 时,形成了细小的硅酸镱晶体。没有长范围有序结构的 LnâOâSi 连接是δ-蒎烯异构化的活性位点。
简介