Keggin-Type Heteropoly Salts as Bifunctional Catalysts in Aerobic Baeyer-Villiger Oxidation.

Katarzyna Pamin,Jan Połtowicz,Joanna Kryściak-Czerwenka,Ewa M Serwicka,Robert Karcz,Mateusz Prończuk

doi:10.3390/ma11071208

Abstract

The cobalt, manganese, and iron salts of tungstophosphoric or molybdophosphoric acid with growing content of metals were applied for the first time as catalysts in the Baeyer-Villiger (BV) oxidation of cyclohexanone to ε-caprolactone with molecular oxygen. The catalysts were characterized with Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), and ethanol decomposition reaction. Introduction of transition metals into the heteropoly structure increases the activity of resulting heteropoly salts in comparison with parent heteropolyacids. It was shown that the most active catalysts are salts of the heteropoly salts with one metal atom introduced and one proton left (HMPX) type, (where M = Co, Fe, Mn, and X = W, Mo) with the metal to proton ratio equal one. Among all of the studied catalysts, the highest catalytic activity was observed for HCoPW. The effect indicates that both the acidic and redox properties are required to achieve the best performance. The Baeyer-Villiger (BV) oxidation mechanism proposed identifies the participation of heteropoly compounds in three steps of the investigated reaction: oxidation of aldehyde to peracid (redox function), activation of carbonyl group (Lewis acidity), and decomposition of the Criegee adduct to ε-caprolactone (Brønsted acidity).

Highlights

Compounds have attracted great interest as catalysts because their acidic and redox functions can be tuned at the molecular level by appropriate choice of the constituting elements.The most widely studied heteropoly compounds are based on Keggin anions
We describe the application of cobalt, manganese, and iron as compensating cations in the Keggin type of heteropolyanions to the liquid-phase BV oxidation of cyclohexanone to ε-caprolactone in the oxygen-aldehyde system
In view of the known hydrolytic lability of Keggin anions, X-ray fluorescence (XRF) and Fourier transform infrared spectroscopy (FTIR) have been used as chief tools to ascertain the chemical identity of the catalyst, some of the data having been discussed in our previous works [21,24]

Summary

Introduction

Compounds have attracted great interest as catalysts because their acidic and redox functions can be tuned at the molecular level by appropriate choice of the constituting elements. The most widely studied heteropoly compounds are based on Keggin anions. A typical structure of Keggin anion is represented by formula (XM12 O40 )n− , where X denotes the central heteroatom contained in a tetrahedron, and M is the addenda atom, usually Mo(VI) or W(VI), placed in a MO6 octahedron [1]. Modification of heteropolyacids with selected transition metal ions makes them effective catalysts of oxidation reactions [2]. The addition of such dopants offers means of controlling the catalysts multifunctionality, by the modification of their acid-base and redox properties. Polyoxometalates are stable over a wide range of conditions and are resistant to oxidative degradation, which ensures an extended lifetime in oxidation reactions

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Conclusion