Isobutane polyoxometalates

Figure 2.64 Structure of polyoxometalate catalysts used for the conversion of isobutane into methacrylic acid.

Modification of redox and catalytic properties of Keggin-type, Sb-doped P/Mo polyoxometalates in the selective oxidation of isobutane to methacrylic acid control of preparation conditions... 

The current industrial production of methylmethacrylate by the acetone-cyanohydrin process suffers from a number of drawbacks, which make it environmentally unfriendly. In particular, it makes use of a very toxic reactant (HCN) and intermediate (acetone cyanohydrin), and coproduces large amounts of impure ammonium sulphate, contaminated with organic compounds. Among the several alternative synthetic routes which have been proposed, particularly interesting from both the practical and scientific points of view is the single-step oxidation of isobutane to methacrylic acid, intermediate in the synthesis of methylmethacrylate. Several industrial companies have studied this reaction (and the selective oxidation of propane to acrylic acid, as well), and it has been established that the most active and selective catalysts are those which are based on Keggin-type polyoxometalates (POM s), containing phosphorus and molybdenum as the main components [1-18]. 

Mansuy D, Bartoli JF, Battioni P, Lyon DK, Finke RG (1991) Highly oxidation resistant inorganic-porphyrin analog polyoxometalate oxidation catalysts. J Am Chem Soc 113 7222-7226 Langpape M, Millet JCM (2000) Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid Part I. An experimental study on isobutane oxidation catalysts. J Mol Catal A Gen 200 89-101... 

Methacryhc acid is a major chemical conunodity produced annually via a multistep route that does, unfortunately, produce many environmentally undesirable by-products.Methacrylic acid, however, can also be direcily formed by oxidation of isobutane over polyoxometalate Keggin catalysts (e.g. H3PM012O40 and its vanadium-substituted analogs H3+xPMoi2-xVx04o) and such a green process has been investigated by several researchers. This possibility has resulted in many oxidation studies with polyoxometalate catalysts over the past two decades. 

Systems based on Keggin-type polyoxometalates (POMs) (phosphomolybdic acid derivatives) have been the most effective and widely used catalysts in gas-phase isobutane selective oxidation research. This Keggin POM catalyst application was hrst claimed in 1981 by Rohm and Haas, and after several catalyst mod-ihcations has been patented by other companies such as Asahi Kasei, Sumitomo, Mitshubishi, etc. However, in addition to a Keggin phosphomolybdate, vanadium must also be present in the anion structure in order to obtain a catalyst effective in the selective oxidation of isobutane. Moreover, the addition of - -3 metal cations to the phosphomolybdate anion increases the thermal stability of the catalyst, significantly minimizing its deactivation. " ... 

Cavani, R, Mezzogori, R., Pigamo, A., et al. (2001). Improved Catalytic Performance of Keggin-Type Polyoxometalates in the Oxidation of Isobutane to Methacryhc Acid under Hydrocarbon-Lean Conditions Using Antimony-Doped Catalysts, Chem. Eng. J., 82, pp. 33-42. 

Polyoxometalates with the required redox properties can also be used in a straightforward manner as autooxidation catalysts. In this way the trisubstituted Keggin compound, [M3(H20)3PW9037] (M = Fe" and Cr ") and [Fe2M(H20)3PW9037 (M = Ni", Co , Mn and Zn ) were used in the autooxidation of alkanes such as propane and isobutane to acetone and tert-butyl alcohol [61]. Later [Fe2. Ni(0Ac)3PW9037] was prepared and used to oxidize alkanes such as adamantane. 

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