A study of the effect of cesium loading on the phase transformation of iron in iron phosphate over the oxidative dehydro
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A study of the effect of cesium loading on the phase transformation of iron in iron phosphate over the oxidative dehydrogenation reactions Faiza B. Khan · Venkata D. B. C. Dasireddy · K. Bharuth-Ram · H. Masenda · H. B. Friedrich
© Springer International Publishing Switzerland 2014
Abstract A phase specific iron orthophosphate catalyst, FePO4 , was synthesized and promoted with cesium. This catalyst was subjected to oxidative dehydrogenation reactions to form an alkyl methacrylate. The phases of the catalyst, before and after the reactions, were studied as a function of different cesium loading. M¨ossbauer spectra of the catalysts show the change of the catalyst precursor FePO4 , to i) the tridymite-like phase, ii) the reduced form, iron(II) pyrophosphate and Fe2 P2 O7, and iii) the α-phase of iron phosphate which is governed by the temperatures of oxidation. X-ray diffraction and M¨ossbauer measurements on the spent catalyst show a transformation of the catalyst to a mixture of phases. Keywords Cesium · Iron phosphates · M¨ossbauer spectroscopy · X-ray diffraction
1 Introduction The use of promoters for FePO4 catalysts has been studied extensively over the past few decades for oxidative dehydrogenation reactions (ODH) involving the conversion of isobutyric acid (IBA) to methyacryclic acid (MAA) [1, 2]. With use of alkali promoters such as
Proceedings of the 5th Joint International Conference on Hyperfine Interactions and International Symposium on Nuclear Quadrupole Interactions (HFI/NQI 2014) Canberra, Australia, 21–26 September 2014 F. B. Khan · V. D. B. C. Dasireddy () · K. Bharuth-Ram · H. B. Friedrich School of Chemistry and Physics, University of KwaZulu-Natal, Durban, South Africa e-mail: [email protected] K. Bharuth-Ram Durban University of Technology, Durban 4000, South Africa H. Masenda School of Physics, University of the Witwatersrand, Johannesburg 2050, South Africa
F. B. Khan et al.
K, Rb and Cs, the catalysts showed enhanced activities in the ODH reactions [3, 4]. Further studies on the effect of cesium showed that the presence of cesium ions during the preparation of the FePO4 catalyst favoured the formation of the tridymite-like FePO4 phase over the stable quartz type FePO4 structure, provided that an excess of phosphorous was used during the synthesis procedure [5, 6]. Using a P/Fe ratio up to an optimum of 1.32 resulted in an almost pure (95 %) tridymite-like FePO4 phase [2, 5]. Cesium was found to exist in the form of CsFeP2 O7 which showed poor catalytic selectivity in the ODH of IBA to MAA; however, mixtures of CsFeP2 O7 and Fe2 P2 O7 did exhibit a synergistic effect at low CsFeP2 O7 content [4, 5]. The addition of water during the reaction conditions to form alkyl methacrylate was found to enhance the formation of the active and selective α-phase. However, the use of the organic co-feed, both independently or as a mixture with water [7], allowed for the formation of additional phases and enhanced the occurrence of the Fe2 P2 O7 phase while reducing the occurrence of the α-phase of
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