Abstract
The mono-lacunary phosphotungstic ammonium salt (NH4)7PW11O39 and its two substituted compounds, (NH4)5ZrPW11O39 and (NH4)5(ZrO2)PW11O39, were synthesized and their catalytic application in the ammoximation of cyclohexanone is presented. Mono-lacunary phosphotungstic ammonium salts were synthesized using phosphotungstic acid and transition metal salts at a pH of ∼4.0–5.0. The products exhibited a typical Keggin-structure, which was verified by Fourier transform-infrared spectroscopy, ultraviolet–visible spectroscopy, thermogravimetric-differential scanning calorimetry, X-ray diffractometry, and cyclic voltammetry. All mono-lacunary phosphotungstic ammonium salts showed preferable catalytic activity with good recyclability towards the ammoximation of cyclohexanone. However, an obvious reduction in catalytic activity was observed in the second catalytic cycle. (NH4)7PW11O39 exhibited the best catalytic performance with 87.9% cyclohexanone conversion and 98.6% cyclohexanone oxime selectivity compared with Zr2+- and ZrO22+-substituted mono-lacunary phosphotungstic ammonium salts. Fourier transform-infrared spectroscopy and X-ray diffractometry characterization showed that the primary Keggin-type catalyst structure remained intact during ammoximation of the cyclohexanone reaction, whereas the secondary structure was destroyed to some extent.
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