Abstract
A new homogeneous catalyst system has been developed for the oxidation of olefins to carbonyls — ethylene to acetaldehyde and higher olefins to ketones. The catalyst system was first developed for the oxidation of ethylene to acetaldehyde in Wacker-type acetaldehyde plants. The aqueous catalyst solution has three key components. A palladium(II) catalyst oxidizes the olefin to the carbonyl, which is analogous to the Wacker system but with only a fraction of its palladium. Keggin phospho-molybdovanadates of the general formula PMo(12-x)Vx O40 (3+x)- provide a dioxygen-reversible vanadium(V)/ vanadium(IV) redox agent for palladium(O) reoxidation, which is analogous to the copper(II)/copper(I) chlorides in the Wacker system. Chloride at centimolar concentrations, lacking in earlier reported palladium and polyoxometalate catalyst systems, is essential to maintain stable palladium(II) catalyst activity. Kinetic characterization and reaction engineering provided ethylene and oxygen reaction rates comparable to those obtained with the Wacker catalyst. A new, efficient method of preparing aqueous phosphomolybdovanadate solutions was developed for laboratory and large-scale production. This paper describes the catalyst system and its reactions with emphasis on the polyoxometalate chemistry.
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References and Notes
International Applications published under the Patent Cooperation Treaty, Publication Numbers WO 91/13681, WO 91/13853, and WO 91/13853, and WO 91/13854, published 19 September, 1991.
J. H. Grate, D. R. Hamm, and S. Mahajan: in Catalysis of Organic Reactions; J. Kosak and T. Johnson (eds.), Marcel Dekker: (in press).
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(a) For metallic Pd°, the formal potential for Pd2+/Pd0 is 0.99 V NHE, while for PdCl42-7Pd0 (1 M HC1) it is 0.62 V NHE. The standard potential for Cu2+/Cu+ is only 0.16 V NHE. However, copper(I) in the Wacker solution is predominantly CuCl2-, while copper(II) is predominantly Cu2+ and CuCl+. This chloride complexation raises the copper(II/I) potential, so that copper(II) in the Wacker solution can oxidize even metallic Pd0 to PdCl42-.
Copper(I) is poorly soluble as CuCl, but with sufficient chloride is solubilized as CuCl2-. Even so, CuCl precipitation limits the useful redox span of the Wacker system to < 50% of the copper reduced.
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Decimolar phosphomolybdovanadate solutions have high ionic strength and accurate measurement of their pH, -log(a+), is not straightforward, especially for pH < 1. We measure p[H+], -log[H+], by calibration with solutions of AyH(4-y)PMo11VO40 with the same P concentration and A+ countercation as the solution to be measured. PMo11VO404-is completely unprotonated even in concentrated solutions of its free acid, H4PMonVO40. Accordingly, solutions of AyH(4-y)PMO11VO40, have [H+] = (4-y)[P]. Details of the p[H+] measurement can be found in our patent applications (see [1]).
M. T. Pope: Heteropoly and Isopoly Oxometalates; Springer-Verlag, New York (1983).
(a) L. Pettersson: in this volume.
L. Pettersson, I. Andersson, A. Söderlund, and J. H. Grate: (unpublished results).
These positional isomer counts are for the predominant α-Keggin isomers, and ignore enantiomers. β-Keggin isomers are also observed in our solutions, in lesser amounts [8].
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We have adapted this method to prepare just slightly neutralized phosphomolybdovanadic acid solutions with x > 3, for example {Na1.2H4.8PMo9O40} and {Li 1.15H5.85 PMo8V4O40}. The unreacted V2O5 is quantitatively recovered, dissolved with minimal alkali.carbonate, and the resulting alkali vanadate solution is returned to the phosphomolybdovanadic acid solution [1].
12(a). V2O5 is less costly and usually more pure than commercial NaVO3.
In laboratory preparations, we typically add some hydrogen peroxide to the NaVO3 solution to oxidize the vanadium(IV) usually present in V2O5. The vanadate catalytically decomposes any excess hydrogen peroxide,
Details and examples of the synthetic method are in patent applications [1].
The masses of the starting materials and the volume of the resulting solution can be measured more accurately and precisely than the elements can be analyzed in the resulting solution.
Although the reaction equation as written suggests the acidity of the catalyst solution may increase with vanadium(V) reduction it does not. Catalyst solutions are typically less acidic after vanadium(V) reduction, at least once fully equilibrated. For example, 0.30 {Na3H3PMo9V3O40} has p[H+] = 0.45, but after its full reduction and equilibration, the solution has p[H+] > 2. Evidently, after vanadium(V) is reduced to vanadium(IV), the polyoxoanion species undergo reequilibrations that consume hydrogen ions. Dissociation of VIVO2+ and reconstitution of Keggin ions having lower vanadium content, hydrogen ion consuming processes (see Equation 12), have been confirmed [8].
In certain circumstances, we observed ethylene oxidation greater than the vanadium(V) capacity of the solution, which must be ascribed to molybdenum(VI) reduction. (Nominal {H3PMo12O40} solutions with palladium(II) catalysts oxidize ethylene.) These circumstances include: (a) Very acidic solutions, such as free phosphomolybdovanadic acid solutions having p[H+] < 0, for example, 0.3 M {H5PMo10V2O40}. Highly acidic conditions favor VO2+ dissociation from vanadium(IV)-phosphomolybdovanadates, which may leave vanadium-free polyoxoanions in which molybdenum(VI) is reducible. (b) Low vanadium-content phosphomolybdovanadate solutions, with average x < 2, for example 0.30 M {Li2.67H1.33PMo11VO40}. After PMo11VO404-has its single vanadium reduced, it may still be oxidizing to palladium(O). (c) High chloride concentrations, > 0.1 M. Chloride complexation to palladium(II) lowers the palladium(0/II) potential, perhaps so palladium(O) is oxidized by molybdenum(VI) even in vanadium(IV)-phosphomolybdovanadates.
Ref. [5a] Example 6.
The ethylene reaction rate decreases with increasing phosphomolybdovanadate concentration. We attribute this to decreasing ethylene solubility with increasing salt concentration.
L. I. Elding: Inorg. Chim. Acta 6, 647 (1972).
Palladium(II) diacetate is protonolyzed to Pd(H2O)42+ and acetic acid in these acidic solutions.
Mechanistic discussions of the Wacker reaction often invoke copper(II) chlorides coordinating palladium(O) through bridging chlorides, with resulting transfer of these chlorides to the oxidized palladium product. Such an inner-sphere mechanism does not appear possible with phosphomolybdovanadates as palladium(O) oxidants. We have considered whether the pervanadyl cation, VO2+, always present in small equilibrium concentrations in our phosphomolybdovanadate solutions, may be responsible for palladium(O) oxidation and, as a cation, may complex chloride and enable such a chloride-bridged transition state. 51V-NMR studies on pervandyl solutions showed no evidence of any chloride complexation with even 3.0 M chloride ion [8b].
Although the reaction equation as written suggests the acidity of the catalyst solution may decrease with vanadium(IV) oxidation, it does not. Solutions are typically more acidic after vanadium(IV) oxidation, at least when comparing fully equilibrated solutions. See note 14.
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Grate, J.H., Hamm, D.R., Mahajan, S. (1994). Palladium and Phosphomolybdovanadate Catalyzed Olefin Oxidation to Carbonyls. In: Pope, M.T., Müller, A. (eds) Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity. Topics in Molecular Organization and Engineering, vol 10. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-0920-8_21
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