Silica-supported ytterbium oxide characterized by spectroscopic methods and acid-catalyzed reactions

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Abstract

Acid–base properties of silica-supported ytterbium oxide catalyst, loading amounts of which were 25 μmol–8.4 mmol Yb g(SiO2)−1, were investigated by TPD experiment and α-pinene isomerization. The relations among acid properties, structures, and catalytic properties are discussed. The catalytic activity depends on loading amounts of ytterbium and pre-treatment temperatures, and reached the highest when loading amount was 3.4 mmol g(SiO2)−1 and pre-treatment temperature of 1073 K. No crystalline phase was detected with XRD technique. By XAFS spectroscopy, Yb atoms adjacent to Yb were found to be absent, but the presence of Si atoms adjacent to Yb was observed in all the catalysts. XRD and XAFS analyses revealed that Yb atoms are supported on SiO2 in a highly dispersed form as YbO6 octahedra. Solid acidity is concluded to be due to strong interaction between a YbO6 octahedron and a SiO4 tetrahedron, and the specific activity per Yb–Si unit was independent of loading amounts. When loading amounts of Yb were excess to form two-dimensional ytterbium oxide overlayer, total turn over numbers were drastically reduced.

Introduction

The catalysis of rare-earth oxide is well-known, and many reactions have been reported 1, 2, 3. Rare-earth oxide is known as a solid base catalyst, but there are few reports about catalysis over supported rare-earth compound. As a general use, rare-earth elements are added to catalyst supports to enhance their thermal stability [4]. Binary oxides of SiO2–La2O3 and SiO2–Y2O3 were predicted to exhibit acidity, according to the hypothesis proposed by Tanabe et al. 5, 6. Shen et al. [7]investigated acid–base property of europium oxides supported on silica and alumina by microcalorimetry and IR spectra of adsorption of ammonia and carbon dioxide. They observed a generation of base sites on alumina-supported europium oxide, whereas a generation of acid sites was observed on silica-supported europium oxide. Connell and Dumesic [8]reported the generation of Lewis and Brønsted acid sites on Sc/SiO2 with IR spectra of adsorbed pyridine. However, catalytic reactions have not been performed in these works mentioned above. Recently, Soled et al. [9]reported the catalysis by rare-earth oxide loaded on silica–aluminas, acid strength of which was the same level as halide-treated aluminas. However, their aim was to control the acidity of silica–alumina by reduction of acid strength. Therefore, the relations among acid–base properties, structures, and catalytic properties of supported rare-earth oxide have been left unclear.

In the series of rare-earth elements, we paid attention to ytterbium. Kobayashi and Hachiya [10]reported that lanthanide trifluoromethanesulfonate acts as a water-tolerant Lewis acid, and the ytterbium triflate exhibited the highest activities in lanthanide triflates. Imamura et al. 11, 12, 13and Baba et al. 14, 15, 16prepared various kinds of supported Yb-amide complexes, which were prepared by impregnation of support-materials with a liquid ammonia solution of Yb metal, followed by evacuation. They reported that supported Yb(III)-amide complexes successively changed to Yb(II)-imide and finally to Yb(III)-nitride as ramping evacuation temperatures. The catalytic properties exhibited by these complexes were quite different from each other. In the case of Y-type zeolite supported Yb complex, Yb(II)-imide complex promoted base-catalyzed reactions 14, 16. In the case of Yb-dosed SiO2, the atomic array of Si–O–Yb–NH2 was responsible for hydrogenation property [12].

However, catalysis by supported ytterbium oxide has not been reported. We prepared silica-supported ytterbium oxide catalysts with various loading amounts of Yb by conventional impregnation method. In this work, we report acid–base property of silica-supported ytterbium oxide catalysts and the results of characterization of their structure. For the model reaction for acid–base property, α-pinene isomerization was utilized.

Section snippets

Experimental

SiO2 gel was synthesized from tetraethyl orthosilicate (Nacalai tesque, EP-grade, once distilled) by hydrolysis in a water–ethanol mixture at boiling point followed by calcination at 773 K for 5 h in a dry air stream. Before calcination, a dried sample was ground to powder under 100 mesh. Silica-supported ytterbium oxide catalyst was prepared by impregnation of aqueous solution of ytterbium nitrate (Mitsuwa, 99.9%) to SiO2 gel at 353 K. The sample was dried at 363 K for 12 h followed by

Catalysis

It is known that α-pinene isomerization was one of the excellent test reactions for acid–base catalyst 18, 19, 20, and the products of α-pinene isomerization can be classified to three groups as shown in Scheme. 1. The first group is β-pinene. The second group consists of bicyclic products (camphene, α-fenchene, etc.). The last group is composed of monocyclic products (limonene, terpinolene, α-terpinene, γ-terpinene, etc.). Over solid base catalysts such as MgO and SrO, only equilibrium between

Conclusion

Silica-supported ytterbium oxide catalyst exhibit a solid acidity and catalyzes α-pinene isomerization. The selectivity is independent of loading amounts and pre-treatment temperature. The highest activity is exhibited when pre-treatment is performed at 1073 K. Catalytic activity per Yb atom is constant when loading amounts are below 5.8 mmol g(SiO2)−1. Ytterbium atoms are widely spread on SiO2 surface as a YbO6 octahedron, and the YbO6 octahedron strongly interacts with SiO4 tetrahedron. Local

Acknowledgements

This work has been performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 95G201), and SPring-8 Users Office (Japan Synchrotron Radiation Research Institute) (Proposal No. 1997B0071-NX -np). We are indebted to Prof. M. Nomura (KEK-PF) in carrying out the X-ray measurements. We thank Drs. S. Emura (Osaka University), Y. Nishihata (JAERI), H. Tanida (RIKEN), and T. Uruga (SPring-8) for making the XAFS beam of SPring-8 BL01B1 available. This work was partially

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