Investigation of 12-Tungstophosphoric Acid Supported on Ce 0.5 Zr 0.5 O 2 Solid Solution
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Investigation of 12-Tungstophosphoric Acid Supported on Ce0.5Zr0.5O2 Solid Solution G. Ranga Rao Æ T. Rajkumar
Received: 1 June 2007 / Accepted: 13 September 2007 / Published online: 2 October 2007 Springer Science+Business Media, LLC. 2007
Abstract The nature of the Keggin ions of tungstophosphoric acid interacting with Ce0.5Zr0.5O2 solid solution has been investigated. The vibrational study shows additional IR features at 1051 and 957 cm–1 which are correlated to the primary Keggin anions interacting with Lewis sites involving Ce4+ and Zr4+ ions, and thus affecting the P–O and W=Oterminal bonds. The IR study indicates the formation of interfacial Ce4+–O–W and Zr4+–O–W bonds. The chemisorbed Keggin molecular layers on Ce0.5Zr0.5O2 show activity towards conversion of acetophenone to styrene by Meerwein–Ponndorf–Verley reduction followed by dehydration. The activity is correlated with the relative intensities of IR peaks at 1051 and 957 cm–1 of the perturbed Keggin molecular layers. Keywords Ce0.5Zr0.5O2 solid solution Tungstophosphoric acid Keggin molecular layers FTIR Styrene
1 Introduction In recent years, CexZr1–xO2 solid solutions have been extensively investigated for variety of applications [1–6] and their catalytic properties are tested for number of vapour and liquid phase reactions [3–9]. It has been recognized that ceria is a crucial promoting component due to its strong redox as well as mild acid–base bifunctional properties. The CexZr1–xO2 solid solutions are of particular
G. Ranga Rao (&) T. Rajkumar National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India e-mail: [email protected]
importance due to the incorporation of Zr4+ ions in the cubic lattice of ceria and show surface O2– ? Ce3+ and O2– ? Ce4+ charge transfer transitions in UV region, and the combination and overtone bands of surface hydroxyl species in NIR region [2]. This is an evidence for the presence of surface O2––Ce3+, O2––Ce4+ sites and –OH groups attached primarily to Zr4+ ions giving rise to both acid and base properties [10]. The surface structural changes and non-stoichiometry introduced by Zr4+ incorporation can lead to surface acid–base pair sites. These mild acid–base pairs formed by coordinatively unsaturated Zr4+, Ce3+, Ce4+ (Lewis acidic) and oxygen ions (Lewis basic) on CexZr1–xO2 solid solutions assist various molecules including water to adsorb dissociatively or undissociatively. It is, therefore, interesting to use these materials as supports for studying the interaction and the stability of heteropoly acids (HPA). HPAs are heteropolyoxometalates consisting of metal-oxygen octahedra and PO4 tetrahedron as basic structural units. The most useful HPAs in catalysis are the Keggin-type with general formula [XM12O40]x–8 where X is the central heteroatom (Si4+, P5+, etc), x its oxidation state and M is the metal ion such as W6+, Mo6+. The negative charge on XW12 O3 40 unit can be compensated by hydrogen ions or by external cations with which the
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