du Gaz Naturel, Faculté de Chimie, USTHB, BP32, El-Alia, 16111Bab-Ezzouar,. Alger ... [2] D. R. Park H. Kim, J. C. Jung, S. H. Lee, I. K. Song,. Catal. Comm. 9 (2008). 293â298. [3] J. Randall, D. K. Lyon, R.J. Domaille, R.G. Finke, Inorg. Synth.
Liquid-phase oxidation of cyclohexanol to adipic acid catalysed by Dawson-type polyoxometalates C. Rabia, M. Moudjahed, L. Dermeche, S. Benadji, T. Mazari; Laboratoire de Chimie du Gaz Naturel, Faculté de Chimie, USTHB, BP32, El-Alia, 16111Bab-Ezzouar, Alger, Algérie.
1. Introduction The liquid-phase oxidation of cyclohexanol to adipic acid catalysed by Dawson-type polyoxometalates could be a cleaner alternative to the current industrial process based on the reaction of HN0 3 with a mixture consisting of cyclohexanol and cyclohexanone. The drawback of this process is the formation of NOx effluents (NO, NO 2
and
N 2 O)
that
constitutes
a
major
source
of
polluting
species.
Polyoxometalates (POMs) constitute a diverse class of inorganic oxo-metal clusters composed of early transition metals in their highest oxidation state [1, 2]. They have tunable acidic and redox properties that can be controlled by changing the constituent elements. The aim of this work was to prepare a series of Dawson-type POMs, as potassium
salts, namely K 6 P 2W 18-x Mo x O 62 (x= 0, 5, 6), in order to examine the
effect of the chemical composition on their catalytic performances on liquid-phase oxidation of cyclohexanol to adipic acid in the presence of aqueous hydrogen peroxide (30%).
2. Experimental K 6 P 2W 18 O 62 was prepared according to the literature data [3]. Mixed W/Mo/V polyoxometalates [P 2 W 18-(x+y) Mo x V y O 62 ]n- (x = 5, 6 and y = 0, 1) were obtained from the hexavacant anion [H 2 P 2W 12 O 56 ]12- according to the method described by Contant et al. [3]. The materials are noted P 2W 18 , P 2 W 12 Mo 6 and P 2W 12 Mo 5 V. The used process, in the catalytic test, is the refluxing method of a mixture of cyclohexanol and POM, where into one drop of hydrogen peroxide (30%) was added whenever the POM showed a colour change. The reaction time was determined to
be a time in which an appropriate amount of hydrogen peroxide in the dropping funnel had been consumed that corresponds to total oxidation of cyclohexanol. The reaction was carried at 90°C together cyclohexanol (0.05Mol) and with an amount of POM soluble in the reaction medium. The resultant aqueous mixture was cooled at 0°C overnight. Adipic acid, one of oxidation products, was isolated as crystals and identified by IR spectroscopy and melting point (151°C).
3. Results The formation of heteropolyanions has been identified by four characteristic IR bands in 1100–770 cm-1 range with several shoulders or small bands for substituted heteropolyanions. In 31P NMR spectroscopy as already reported by Contant et al. [3], the chemical shift values depend on the composition and on the relative positions of the W, Mo and V atoms in the framework. For P 2W 18 and P 2W 12 Mo 6 only one 31P line is observed at -12.51 and -9.51 ppm respectively in agreement with the equivalence of the two half-anions. P 2W 12 Mo 5 V gave two peaks at -9.73 and -10.01 ppm in agreement with two unequivalent half units.
The reaction of cyclohexanol did not proceed in the absence of any catalyst, indicating that they are catalytic in nature. P 2W 18 is not very active, adipic acid is observed only in trace amount. The effect of the catalyst amount (0.032-0.250 g) on the formation of adipic acid was investigated on P 2W 12 Mo 6 catalyst. The adipic acid yield increases from ca. 16 to ca. 59% with increasing of the mass. The replacement of 1 W atom of P 2W 12 Mo 6 Dawson unit by 1 V atom decreases the adipic acid yield. 4. Conclusion The clean synthesis of adipic acid can be obtained from liquid-phase oxidation of cyclohexanol in the presence of hydrogen peroxide and Dawson-type POMs as catalyst. The best catalytic performance was obtained with P 2W 12 Mo 6 . References [1] F. Cavani, R. Mezzogori, A. Trovarelli, J. Mol. Catal. A, 204–205 (2003) 599–607. [2] D. R. Park H. Kim, J. C. Jung, S. H. Lee, I. K. Song,. Catal. Comm. 9 (2008) 293–298. [3] J. Randall, D. K. Lyon, R.J. Domaille, R.G. Finke, Inorg. Synth. 32 (1998) 242. [4] R. Contant, M. Abbessi, R. Thouvenot, G. Hervé, Inorg. Chem. 43 (2004) 35973604.
