MXPA99008455A

MXPA99008455A - Selective electrocatalytic oxidation of hydrocarb

Info

Publication number: MXPA99008455A
Application number: MXPA/A/1999/008455A
Authority: MX
Inventors: Stochniol Guido; Duda Mark; Kuhnle Adolf
Original assignee: Creavis Gesellschaft Für Technologie Und
Priority date: 1998-09-14
Filing date: 1999-09-14
Publication date: 2000-12-06

Abstract

A process for the electrochemical oxidation of organic compounds, in which, as the anode material, mixed oxide of the general formula MoaBibX1cX2dX3eX4fX5g0h (I) is used with X1 = V, Nb, Cr, W, Ta, Ga, Ce and / or La , X2 = Li, La, K, Rb, Cs, Cu, Ag, Au, Pd and / or Pt, X3 = Fe, Co, Ni and / or Zn, X4 = Sn, Pb, Sb and / or Te, X5 = Ti, Zr, Si and / or Al, where a = 0 to 3, b = 0 to 3, with the proviso that a + d (0.15, c = 0 to 12.5, d = 0 to 5, e = 0 to 1.5, f = 0 to 1, and g = 0 to

Description

SELECTIVE ELECTROCATALYTIC OXIDATION OF HYDROCARBONS The invention relates to an electrochemical process for the selective preparation of partially oxidized organic compounds. Until now, the direct selective oxidation of organic compounds was only possible in a few cases, since partially oxidized products are generally more reactive than the educts used, which leads to complete oxidation with the formation of carbon dioxide. Especially . The problem of the direct oxidation of alkanes had not been satisfactorily solved so far. Exclusively maleic anhydride, starting from n-butane, can be obtained by direct oxidation, the stabilization of the product of oxidation through the formation of rings having a decisive role. Many attempts to perform the partial direct oxidation of non-reactive organic compounds are concentrated in the development of new heterogeneous catalysts, the performance of the partially oxidized product often being not technically relevant. In contrast, partial electrochemical oxidation was given less attention. In this field, on the contrary, the use of the total oxidation of compounds suitable for the production of electrical energy in cells of combustible substance, occupied the preponderant place of the development work. In US 4 329 208 an example of the electrochemical oxidation of organic compounds is described, based on the oxidation of ethyl to ethylene oxide. This oxidation takes place in an anode consisting of silver, or a silver alloy, by means of a fixed electrolyte system of zirconium oxides. Another method for the electrochemical oxidation of organic compounds is disclosed in US Pat. No. 4,661,422. In this case, the hydrocarbons are oxidized at a metal / metal oxide anode in a salt melt as an electrolyte. The salt melt contains carbonate, nitrate or sulphate salts, - the cathode is made of mixed oxides of metals of groups I B, II B, III A, V B, VI B, VII B and V III of the periodic table. Takehira et al., In Catalysis Today 1995, 25, 371, investigated the partial oxidation of propene in a configuration similar to that of fuel substance cells. As an electrolyte, they used Zr02 stabilized with Y. As an anode material they used Au, which carried a mixed oxide of Mo-Bi as a catalyst, and as an Ag cathode material. The reaction temperature was 475 ° C.

The yield of the respectively desired oxidation product is usually so low that none of these processes has technical relevance. Also here, the problem of the total oxidation of the organic substrate to carbon dioxide has not been solved. In addition, the electrolyte acts as an "oxygen pump", that is, the oxygen necessary for oxidation is reduced at the cathode, to then migrate through the electrolyte in ionic form to the anode. In the anode space there is only the substrate to be oxidized and, eventually, an inert gas. The oxygen supply to the anode space does not lead to an increase in the yield of the desired oxidation product. It is also disadvantageous that the reaction temperature is determined by the conductivity of oxygen ions of the electrolyte. The electrolytes used have a sufficient conductivity only at temperatures that are clearly above the optimum temperatures for these oxidation reactions, which surely explains in part the low selectivity of the inspected procedures. In particular, processes using a salt melt as an electrolyte inevitably have such high reaction temperatures (up to 750 ° C) that it is almost impossible to avoid decomposition of the products. Procedures of this type are not suitable for obtaining thermally unstable compounds (for example, Michael systems). The discovery of the NEMCA (Non Faradaic Electrochemical Modification of Catalytic Activity) effect opened the possibility of developing more cost-effective electrochemical procedures. Vayenas et al. Describe in "Studies in Surface Science and Catalysis", R.K. Graselli, S.T. Oyama, A.M. Gaffney, J.E. Lyones (Editors), 110, 77 (1997) and Science (1994), 264, 1563, an electrochemical process which is based on a conductive film of porous (oxide) metal on a solid electrolyte, such as, for example, Zr02 stabilized with Y. In this case, a gas-tight separation of the anode and cathode space is no longer required, and the oxidation agent can be fed into the anode space. However, it was observed that the main product of the oxidation, carbon dioxide, still resulted from the total oxidation of the substrate and that the selectivity for a desired partially oxidized product was very low, even with low yields. The objective of the present invention was, therefore, the development of an electrochemical process for the partial oxidation of organic compounds. Surprisingly, it was found that organic compounds can be oxidized electrochemically in a highly targeted manner, when the anode material contains mixed oxides of the MoaBibX1cX2dX3eX4fX? G0ll type. Therefore, the object of the present invention is a method for the electrochemical oxidation of organic compounds, wherein a mixed oxide of the general formula is used as the anode material. with X1 = V, Nb, Cr, W, Ta, Ga, Ce and / or La, X2 = Li, La, K, Rb, Cs, Cu, Ag, Au, Pd and / or Pt, X3 = Fe, Co , Ni and / or Zn, X4 = Sn, Pb, Sb and / or Te, X5 = Ti, Zr, Si and / or Al, where a = 0 to 3, b = 0 to 3, c = 0 to 12.5, d = 0 to 5, e = 0 to 1.5, f = 0 to 1, and g = 0 to 25, with the proviso that a + d > 0.15. The number of oxygen atoms h is determined by the valence and frequency of the different oxygen elements of the formula (I).

The term "mixed oxides" in the sense of the present invention includes multimetal oxide masses such as metal oxides which occur in a contiguous manner. In any case, depending on the stoichiometry and the temperature treatment of the mixed oxides, phase precipitation is possible. Mixed oxides of the aforementioned type are known from another technical field and are used, for example, as heterogeneous catalysts for gas phase reactions. The preparation and use of these compounds can be found, for example, in EP 0 417 723. The use of mixed oxides of this type as heterogeneous catalysts is carried out in chemical reactions that take place without current; the use as an anode material in electrochemical processes is not known. The anode may consist wholly or partly of the mixed oxides of the formula I. Usually, an existing electrode, for example of platinum, can also be equipped with a surface of these mixed oxides. In practice, it has been proven that a film of the mixed oxides on the electrolyte was first applied by means of the screen printing technique and joined by a malleability stage. An example of this technique is found in JP 09 239 956. The electrocatalytic layer conveniently has a rough surface with BET surfaces of 5 to 20 m2 / g. The organic compound to be oxidized, optionally mixed with oxygen and / or an inert gas, such as nitrogen, is passed through an anode as above. The educts can be fed as gases or liquids, however, at the reaction temperatures according to the invention, a gaseous feed has been demonstrated. With the aid of the process according to the invention, a plurality of organic compounds, such as aromatics, aliphatics, olefins or alicycles can be oxidized. The present invention is particularly suitable for the oxidation of ethane, propane, ethyl, acetylene, propene, benzene, butane, butadiene, butene, cyclohexane, octane, octene, cyclododecane or cyclododecene. In the process according to the invention, a solid which carries oxygen ions, especially a metal oxide, can be used as the electrolyte. In a particular embodiment of the present invention, a perovskite of the general formula is used as the oxygen ion conducting solid.

LaiXskX7? MgmOr (II) with X6 = Ca, Sr, Ba, X7 = Ga, Al, i = 0.3 to 0.9, k = 0.1 to 0.7 with the proviso that i + k = 0.9 to 10, 1 = 0.3 to 0.9, m = 0.1 to 0.7 , with the proviso that l + m = the ll The number of oxygen atoms n is determined by the valence and frequency of the different oxygen elements of the formula (II). Metal oxides suitable as solids that conduct oxygen ions are also, for example, Zr02 stabilized by CaO, Sc03, Y203 or Yb203, or Ce02 stabilized by La203 / T203, Yb203 or Gd203 - In another embodiment of the present invention, as solid that conducts oxygen ions can be used pyrochlore compounds of the general formula (lloXp) 2 (X qX r) 20. (III) with Ln = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, X8 = Mg, Ca or Sr, X9 = Ti or Zr, X10 = Fe , Al, Se, Ga or Y, or = 0.4 to 1, p = 0 to 0.6, q = 0.4 to 1, r = 0 to 0.6. The number of oxygen atoms s is determined by the valence and frequency of the different oxygen elements of the formula (III). These compounds can be obtained, for example, by "drip pyrolysis" [P. Gordes and collaborators, Den. J. Mater. Sci. (1995), 30 (4), 1053-8] or methods of decomposition [for example: N. Dhas et al., India J. Mater. Chem. (1993), 3 (12), 1289-1294, or D. Fumo et al., Por. Mater Res. Bull. (1997), 32 (10), 1459-1470]. In the process according to the invention, a metal, preferably silver or platinum, can be used as the cathode. Also, as cathode one or more metal oxides or a mixed metal oxide can be used. Also, as a cathode, perovskites of the general formula can be used (IV) with X11 = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, X12 = Ca, Sr, Ba, X13 = Mn, Fe, X14 = Cr, Co, Ni, Al, t = 0.5 to 1, u = 0 to 0.5, v = 0.1 to 0.4, with the proviso that t + u + v < 1, w = 0.6 to 1.1, x = 0 to 0.7, with the proviso that w + x > 1. The conditions with respect to the sums of t, u and v (< 1) as well as w and x (> 1) constitute certain embodiments of the present invention. In other modalities, t + u + v can be between 0.85 and 1. Likewise, w + x can be between 1 and 1.1. The number of oxygen atoms (3 ± y) is determined by the valence and frequency of the different oxygen elements. In DE-PS 197 02 619 Cl, the production of non-stoichiometric perovskites of the formula LwMxMnyCoz03 is described as a cathode material for high temperature combustible substance cells. However, the combustible substance cells were developed for another technical process, the production of electrical energy with the total oxidation of a substrate. The electrochemical oxidation of organic compounds according to the process according to the invention is carried out at elevated temperatures, preferably at 200 to 750 ° C, especially preferred at 250 to 550 ° C. The use of high pressures is also possible; pressures of 1 to 100 bar, preferably 1 to 10 bar, can be applied. In the method according to the invention, on the one hand, at the cathode oxygen is converted to an ionic form and carried to the anode through the electrolyte, and on the other, it is activated in such a manner at the anode, which has place a reaction with the passing organic compound. In the above, the oxygen supply can also be carried out through a porous solid electrolyte, not gas tight. The gas flow in the anode space, in addition to the organic compound to be oxidized and oxygen, may also contain an inert gas. Figure 1 shows an exemplary configuration of a device for carrying out the method according to the invention. The cathode B and the anode C are placed in the electrolyte A that conducts oxygen ions. In the above, a compound that conducts current must be selected for the materials, for example, by means of the maleability. The two electrodes receive current from the voltage source D. The gas flows are brought to and from the electrodes through an external gas guide G and an internal gas guide F, the cell being hermetically closed out through the gasket E of the external gas guide G. The educt and oxygen are carried as gas flow a) to the anode C; the resulting product gas c) is discharged either by the gas flow pressure a) or by a corresponding depression. The gas flow b) on the cathode side may consist of air, oxygen or other gas mixture containing oxygen and is discharged through the gas flows d), enriched with oxygen. The arrangement in the space of the anode, the cathode and the electrolyte is not limited to flat plates or continuous layers. It is also possible to use tubular reactors for the process according to the invention. In this case, either, anode and cathode materials are placed in an electrolyte tube or a carrier tube is provided with inert material (eg, Al203) with the corresponding layers. The necessary current supplies must be adjusted accordingly. Also, the anode or cathode layers may be applied as woven or structured surface layers with regular depressions or elevations. It is intended that the following examples illustrate the invention in more detail without limiting its scope. And emplos; 1. Catalytic Oxidation of Propene (Comparative Example) A porous catalytic film with a BET surface area of 16 m2 / g is applied by screen printing and subsequent malleability on a Lao.8Sro.2Gao.85Mgo.15O2 electrolyte film. As counter electrode it is vacuum metallized Pt. The reaction temperature is 400 ° C. A mixture of 5% propene, 5% oxygen and 90% nitrogen is passed at 2 1 / h through the anode. The air is passed through the cathode with the same flow. Results: Anode material (catalytic film) Formation of acrolein [mmol / hxg] M0O3 0.05 M? 3Bi1.25FeCo2Ca0.025K0.o25 ?? 0.25 M? Bl0.25 l2.07Fe0.49Sl2.5Ko .0125Na0.0375 ?? 0.45 2. Electrocatalytic oxidation of propene under Faraday conditions A porous catalytic film with a BET surface area of 16 m / g is applied by screen printing and subsequent malleability on a Lao.8Sro.2Gao.85Mgo.15O2 electrolyte film. As counter electrode it is vacuum metallized Pt. The reaction temperature is 400 ° C. A mixture of 5% propene and 95% nitrogen is passed at 2 1 / h at the anode. The air is passed through the cathode with the same flow. The applied voltage for the control of the flow of oxygen ions is 1 V. Results: Anode material (catalytic film) Acrolein formation [mmol / hxg] M0O3 0.75 M03BÍ! 25FeC? 2Cao.o25K0.o25 ?? 0.45 3. Electrocatalytic oxidation of propene with oxygen in the educt stream A porous catalytic film with a BET surface area of 16 m2 / g is applied by screen printing and subsequent malleability on a Lao.8Sro.2Gao.85Mgo.15O2 electrolyte film. As counter electrode it is vacuum metallized Pt. The reaction temperature is 400 ° C. A mixture of 5% propene, 5% oxygen and 90% nitrogen is passed at 2 1 / h through the anode. The air is passed through the cathode with the same flow. The applied voltage for the control of oxygen ion flow is 0-3 V. Under these conditions, the electrochemical reactor works under Faraday conditions. Results: Anode material (catalytic film) Acrolein formation [mmol / hxg] Mo03 2.5 M? 3Bi1.2sFeCo2Ca0.025 _o.o25 ?? 1.1 M? 3BÍ0.25NÍ2.07Fe0.49Yes2.5Ko.ol25Na? .0375 ?? 3.2

Claims

1. A process for the electrochemical oxidation of organic compounds, characterized in that a mixed oxide of the general formula is used as the anode material MoaBibX1cX2dX3e ¡X fX c Oh (I) with X1 = V, Nb, Cr, W, Ta, Ga, Ce and / or La, X2 = Li, La, K, Rb, Cs, Cu, Ag, Au, Pd and / or Pt, X3 = Fe, Co , Ni and / or Zn, X4 = Sn, Pb, Sb and / or Te, X5 = Ti, Zr, Si and / or Al, where a = 0 to 3, b = 0 to 3, c = 0 to 12.5, d = 0 to 5, e = 0 to 1.5, f = 0 to 1, and g = 0 to 25, with the proviso that a + d > 0.15.

2. A process according to claim 1, characterized in that a conductive solid that conducts oxygen ions is used as the electrolyte.

3. A process according to claim 2, characterized in that the oxygen ion conducting solid contains a metal oxide.

4. A process according to claim 3, characterized in that a perovskite of the general formula is used as a solid that conducts oxygen ions. LaiX6kX71MgmOE (II) with X = Ca, Sr, Ba, X 'Ga, Al, i = 0.3 to 0.9, k = 0.1 to 0.7 with the proviso that i + k = 0.9 to l.0, 1 = 0.3 to 0.9, m = 0.1 to 0.7, with the proviso that l + m = the ll

5. A process according to claim 2, characterized in that Zr02 stabilized by CaO, Sc03, Y203 or Yb203, or Ce02 stabilized by La203, Y203, Yb203 or Gd203 is used as the oxygen ion conducting solid.

6. A process according to claim 2, characterized in that pyrochlore compounds of the general formula are used as the oxygen ion conducting solid. (Ln0X8P) 2 (X9qX10r) 20s (III) with Ln = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, X8 = Mg, Ca or Sr, X9 = Ti or Zr, X10 = Fe , Al, Se, Ga or Y, or = 0.4 to 1, p = 0 to 0.6, q = 0.4 to 1, r = 0 to 0.6.

7. A method according to any of claims 1 to 6, characterized in that a metal is used as the cathode.

8. A method according to claim 7, characterized in that silver or platinum is used as the cathode.

9. A process according to any of claims 1 to 6, characterized in that one or more metal oxides or a mixed oxide of metal is used as the cathode.

10. A process according to claim 9, characterized in that a perovskite of the general formula is used as the cathode. with X11 = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu, X = Ca, Sr, Ba, X13 = Mn, Fe, X14 = Cr, Co , Ni, Al, t = 0.5 to 1, u = 0 to 0.5, v = 0.1 to 0.4, with the proviso that t + u + v < 1, w = 0.6 to 1.1, x = 0 to 0.7, with the proviso that w + x > 1.

11. A process according to any of claims 1 to 10, characterized in that the electrochemical oxidation is carried out at temperatures of 200 to 750 ° C.

12. A process according to claim 11, characterized in that the electrochemical oxidation is carried out at temperatures of 250 to 500 ° C.

13. A method according to any of claims 1 to 12, characterized in that the electrochemical oxidation is carried out at pressures from 1 to 100 bar.

14. A method according to claim 13, characterized in that the electrochemical oxidation is carried out at pressures of 1 to 10 bar.