Generation of acid sites on silica-supported rare earth oxide catalysts: Structural characterization and catalysis for α-pinene isomerization

Abstract

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, Ln–O–Si and Ln–O–Ln linkages formed, the ratio of which varied with the loaded element. The ratio of Ln–O–Si linkage increases with stronger affinity among LnO_n units and SiO₄ tetrahedra, and the affinity depends on the size of Ln³⁺. With increasing ratio of Ln–O–Si to Ln–O–Ln 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 Yb–O–Si linkage to form Yb₂O₃ fine particles. When Yb/SiO₂ was pretreated at 1273 K, fine ytterbium silicate crystallites formed. Ln–O–Si linkage without a long-ranged ordering structure was the active site for α-pinene isomerization.

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