U.S. patent application number 09/866636 was filed with the patent office on 2002-02-07 for electrochemical cell for the oxidation of organic compounds, and electrocatalytic oxidation process.
Invention is credited to Duda, Mark, Kuehnle, Adolf, Stochniol, Guido, Tanger, Uwe, Zanthoff, Horst-Werner.
Application Number | 20020014417 09/866636 |
Document ID | / |
Family ID | 7644202 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014417 |
Kind Code |
A1 |
Kuehnle, Adolf ; et
al. |
February 7, 2002 |
Electrochemical cell for the oxidation of organic compounds, and
electrocatalytic oxidation process
Abstract
An electrochemical cell comprising a negative electrode, a solid
electrolyte which conducts oxygen ions, and a positive electrode
comprising zeolites, mordenites, silicates, phosphates or
mixed-metal oxides having a pore size of less than 200 nm can be
used in a process for partially oxidizing organic compounds, for
example alkanes, olefins or aromatic compounds.
Inventors: |
Kuehnle, Adolf; (Marl,
DE) ; Duda, Mark; (Ludwigshafen, DE) ;
Stochniol, Guido; (Gelnhausen, DE) ; Tanger, Uwe;
(Bochum, DE) ; Zanthoff, Horst-Werner; (Essen,
DE) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Family ID: |
7644202 |
Appl. No.: |
09/866636 |
Filed: |
May 30, 2001 |
Current U.S.
Class: |
205/437 ;
204/252; 204/283; 204/290.01; 204/291; 204/292; 204/295 |
Current CPC
Class: |
C25B 9/00 20130101; C25B
3/23 20210101; Y02E 60/50 20130101 |
Class at
Publication: |
205/437 ;
204/252; 204/291; 204/290.01; 204/283; 204/295; 204/292 |
International
Class: |
C25B 003/02; C25B
009/10; C25B 011/03; C25B 011/04; C25B 013/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2000 |
DE |
100 26 940.0 |
Claims
What is claimed as new and is intended to be secured by Letters
Patent is:
1. An electrochemical cell comprising a negative electrode, a solid
electrolyte which conducts oxygen ions, a positive electrode
comprising a positive electrode material comprising at least one
material selected from the group consisting of a zeolite, a
mordenite, a silicate, a phosphate, and a mixed-metal oxide, and a
power source connected to the positive and negative electrode,
wherein said positive electrode material has a pore size of less
than 200 nm.
2. The electrochemical cell of claim 1, wherein the positive
electrode material is a coating.
3. The electrochemical cell of claim 1, wherein the negative
electrode and positive electrode are disposed on opposing surfaces
of the solid electrolyte.
4. The electrochemical cell of claim 1, wherein the positive
electrode material comprises a compound of formula
IV(El.sub.2O.sub.e).sub..alpha.(-
Fe.sub.2O.sub.3).sub..beta.SiO.sub.2 (IV)wherein El=an element from
group IA, IIA, IIIA, IVA or VA or IIIB, IVB, VB, VIB, VIIB or VIII
of the Periodic Table of the Elements .alpha.=from 0 to 0.1,
.beta.=from 0 to 0.1, with the proviso that .alpha. and .beta. are
not simultaneously 0.
5. The electrochemical cell of claim 1, wherein the pore size of
the positive electrode material is less than 20 nm.
6. The electrochemical cell of claim 1, wherein the solid
electrolyte which conducts oxygen ions comprises cerium oxide
(CeO.sub.2) or cerium oxide (CeO.sub.2) stabilized with at least
one compound selected from the group consisting of lanthanum oxide
(La.sub.2O.sub.3), yttrium oxide (Y.sub.2O.sub.3), ytterbium oxide
(Yb.sub.2O.sub.3) and gadolinium oxide (Gd.sub.2O.sub.3).
7. The electrochemical cell of claim 1, wherein the solid
electrolyte which conducts oxygen ions comprises zirconium oxide
(ZrO.sub.2) or zirconium oxide (ZrO.sub.2) stabilized with at least
one compound selected from the group consisting of calcium oxide
(CaO), scandium oxide (Sc.sub.2O.sub.3), yttrium oxide
(Y.sub.2O.sub.3) and ytterbium oxide (Yb.sub.2O.sub.3).
8. The electrochemical cell of claim 1, wherein the solid
electrolyte which conducts oxygen ions comprises a metal or metal
oxide.
9. The electrochemical cell of claim 1, wherein the solid
electrolyte which conducts oxygen ions comprises a perovskite of
formula IILn.sub.aX.sup.1.sub.bX.sup.2.sub.cX.sup.3.sub.dO.sub.e
(II)wherein Ln=La Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb and/or Lu X.sup.1=Ca, Sr, Ba and/or Mg X.sup.2=Ga, Al, Mn, Ti,
Nb, Y, W and/or Zr X.sup.3=Fe, Co, Ni and/or Cu a=0.1 to 0.9, b=0.1
to 0.9, c=0 to 0.9, d=0 to 0.9 with the proviso that a+b=0.3 to
1.5.
10. The electrochemical cell of claim 1, wherein the solid
electrolyte which conducts oxygen ions comprises a pyrochloro
compound of formula
III(Ln.sub.fX.sup.4.sub.g).sub.2(X.sup.5.sub.hX.sup.6.sub.i).sub.2O.sub.k
(III)wherein Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb and/or Lu X.sup.4=Na, Mg, Ca and/or Sr, X.sup.5=Ti, Nb, Ta
and/or Zr, X.sup.6=Fe, Al, Sc, Ga and/or Y, f=0.2 to 1.2, g=0 to
0.8, h=0.2 to 1.2, i=0 to 0.8.
11. An electrochemical cell of claim 1, wherein the positive
electrode material comprises a mixed oxide of formula
VA.sub.lB.sub.mX.sup.7.sub.nX-
.sup.8.sub.oX.sup.9.sub.pX.sup.10.sub.qX.sup.11.sub.rX.sup.12.sub.sO.sub.t
(V)wherein A and B are an element from Group IA, IIA and/or VA
and/or Group IVB, VB, VIB, VIIB, or VIII of the Periodic Table of
the Elements X.sup.7=V, Nb, Cr, W, Ta, Ga and/or Ce X.sup.8=Li, Na,
K, Rb, Cs, Be, Mg, Ca, Sr and/or Ba, X.sup.9=La, Ce, Pr, Nd, Pm,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Cu, Ag, Au, Pd and/or Pt,
X.sup.10=Fe, Co, Ni and/or Zn, X.sup.11=Sn,Pb,Sb and/or Te,
X.sup.12=Ti, Zr, Si and/or Al, where l=0.001 to 30, m=0.001 to 20,
n=0 to 15, o=0.001 to 10, p=0 to 10, q=0to 40, r=0 to 10, and s=0
to 80, with the proviso that l+m is .gtoreq.0.01 and l+o is
.gtoreq.0.005.
12. The electrochemical cell of claim 1, wherein the positive
electrode further comprises a mixture of the solid electrolyte
which conducts oxygen ions and the positive electrode material.
13. The electrochemical cell of claim 1, wherein the positive
electrode further comprises a mixture of an electrically conductive
metal and the positive electrode material.
14. The electrochemical cell of claim 1, wherein a metal foil
having a maximum thickness of 200 .mu.m is disposed between the
solid electrolyte which conducts oxygen ions and the positive
electrode.
15. The electrochemical cell of claim 14, wherein the metal foil
comprises at least one metal selected from the group consisting of
Cu, Au, Ag, Pt, Pd and Ir.
16. The electrochemical cell of claim 1, wherein the negative
electrode comprises a metal.
17. The electrochemical cell of claim 1, wherein the negative
electrode comprises at least one metal selected from the group
consisting of Cu, Au, Ag, Pt, Pd, and Ir.
18. The electrochemical cell of claim 1, wherein the negative
electrode comprises one or more metal oxides or a metal mixed
oxide.
19. The electrochemical cell of claim 1, wherein the negative
electrode comprises a perovskite of the general formula
ILa.sub.uX.sup.13.sub.vX.su-
p.14.sub.wX.sup.15.sub.xX.sup.16.sub.yO.sub.3.+-.z (I)wherein
X.sup.13=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or
Lu, X.sup.14=Ca, Sr, Ba and/or Mg, X.sup.15=Mn, Fe, Ti, Ga, Mn
and/or Zr, X.sup.16=Co, Ni, Cu, Al an d/or Cr u=0 to 0.2, v=0 to
1.0, w=0.01 to 0.8, with the proviso that u+v+w is .ltoreq.1.5, and
x=0.2 to 1.3, y=0 to 0.9, with the proviso that x+y is
.gtoreq.0.3.
20. A process for the selective oxidation of an organic compound in
an electrochemical cell, comprising: passing the organic compound
over the surface of a positive electrode comprising a positive
electrode material comprising at least one material selected from
the group consisting of a zeolite, a mordenite, a silicate, a
phosphate, and a mixed-metal oxide having a pore size of less than
200 nm, thereby partially oxidizing the organic compound, passing
an oxygen- or N.sub.2O-containing gas over a negative electrode,
wherein a solid electrolyte which conducts oxygen ions is disposed
between the positive and negative electrodes, and the positive and
negative electrodes are connected to a power source.
21. The process of claim 20, wherein the positive electrode
material comprises a compound of formula
IV(El.sub.2O.sub.e).sub..alpha.(Fe.sub.2O-
.sub.3).sub..beta.SiO.sub.2 (IV)wherein El=an element from group
IA, IIA, IIIA, IVA or VA or IIIB, IVB, VB, VIB, VIIB or VIII of the
Periodic Table of the Elements .alpha.=from 0 to 0.1 .beta.=from 0
to 0.1, with the proviso that .alpha. and .beta. are not
simultaneously 0.
22. The process of claim 20, wherein the pore size of the positive
electrode material is less than 20 nm.
23. The process of claim 20, wherein the solid electrolyte which
conducts oxygen ions comprises cerium oxide (CeO.sub.2) or cerium
oxide (CeO.sub.2) stabilized with at least one stabilizer selected
from the group consisting of lanthanum oxide (La.sub.2O.sub.3),
yttrium oxide (Y.sub.2O.sub.3), ytterbium oxide (Yb.sub.2O.sub.3)
and gadolinium oxide (Gd.sub.2O.sub.3).
24. The process of claim 20, wherein the solid electrolyte which
conducts oxygen ions comprises zirconium oxide (ZrO.sub.2) or
zirconium oxide (ZrO.sub.2) stabilized with at least one stabilizer
selected from the group consisting of calcium oxide (CaO), scandium
oxide (Sc.sub.2O.sub.3), yttrium oxide (Y.sub.2O.sub.3) and
ytterbium oxide (Yb.sub.2O.sub.3).
25. The process of claim 20, wherein the solid electrolyte which
conducts oxygen ions comprises a metal or metal oxide.
26. The process of claim 20, wherein the solid electrolyte which
conducts oxygen ions comprises a perovskite of formula
IILn.sub.aX.sup.1.sub.bX.su- p.2.sub.cX.sup.3.sub.dO.sub.e
(II)wherein Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb and/or Lu X.sup.1=Ca, Sr, Ba and/or Mg X.sup.2=Ga, Al, Mn, Ti,
Nb, Y, W and/or Zr X.sup.3=Fe, Co, Ni and/or Cu a=0.1 to 0.9, b=0.1
to 0.9, c=0 to 0.9, d=0 to 0.9 with the proviso that a+b=0.3 to
1.5.
27. The process of claim 20, wherein the solid electrolyte which
conducts oxygen ions is a pyrochloro compound of formula
III(Ln.sub.fX.sup.4.sub.g-
).sub.2(X.sup.5.sub.hX.sup.6.sub.i).sub.2O.sub.k (III)wherein
Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu
X.sup.4=Na, Mg, Ca and/or Sr, X.sup.5=Ti, Nb, Ta and/or Zr,
X.sup.6=Fe, Al, Sc, Ga and/or Y, f=0.2to 1.2, g=0 to 0.8, h=0.2 to
1.2, i=0 to 0.8.
28. The process of claim 20, wherein the positive electrode
material comprises a mixed oxide of formula
VA.sub.lB.sub.mX.sup.7.sub.nX.sup.8.su-
b.oX.sup.9.sub.pX.sup.10.sub.qX.sup.11.sub.rX.sup.12.sub.sO.sub.t
(V)wherein A and B are an element from Group IA, IIA and/or VA
and/or Group IVB, VB, VIB, VIIB, or VIII of the Periodic Table of
the Elements X.sup.7=V, Nb, Cr, W, Ta, Ga and/or Ce X.sup.8=Li, Na,
K, Rb, Cs, Be, Mg, Ca, Sr and/or Ba, X.sup.9=La, Ce, Pr, Nd, Pm,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Cu, Ag, Au, Pd and/or Pt,
X.sup.10=Fe, Co, Ni and/or Zn, X.sup.11=Sn,Pb,Sb and/or Te,
X.sup.12=Ti, Zr, Si and/or Al, where l=0.001 to 30, m=0.001 to 20,
n=0 to 15, o=0.001 to 10, p=0 to 10 q=0 to 40 r=0 to 10 and s=0 to
80, with the proviso that l+m is .gtoreq.0.01 and l+o is
.gtoreq.0.005.
29. The process of claim 20, wherein the positive electrode further
comprises a mixture of the solid electrolyte which conducts oxygen
ions and the positive electrode material.
30. The process of claim 20, wherein the positive electrode further
comprises a mixture of an electrically conductive metal and the
positive electrode material.
31. The process of claim 20, wherein a metal foil having a maximum
thickness of 200 .mu.m is disposed between the solid electrolyte
which conducts oxygen ions and the positive electrode.
32. The process of claim 31, wherein the metal foil comprises at
least one metal selected from the group consisting of Cu, Au, Ag,
Pt, Pd and Ir.
33. The process of claim 20, wherein the negative electrode
comprises a metal.
34. The process of claim 20, wherein the negative electrode
comprises at least one metal selected from the group consisting of
Cu, Au, Ag, Pt, Pd, and Ir.
35. The process of claim 20, wherein the negative electrode
comprises one or more metal oxides or a mixed metal oxide.
36. The process of claim 20, wherein the negative electrode
comprises a perovskite of formula
ILa.sub.uX.sup.13.sub.vX.sup.14.sub.wX.sup.15.sub.x-
X.sup.16.sub.yO.sub.3.+-.z (I)wherein X.sup.13=Ce, Pr, Nd, Pm, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and/or Lu, X.sup.14=Ca, Sr, Ba
and/or Mg, X.sup.15=Mn, Fe, Ti, Ga, Mn and/or Zr, X.sup.16=Co, Ni,
Cu, Al and/or Cr u=0 to 1.2, v=0 to 1.0 w=0.01 to 0.8, with the
proviso that u+v+w is .ltoreq.1.5 x=0.2 to 1.3, y=0 to 0.9, with
the proviso that x+y is .gtoreq.0.3.
37. An electrochemical cell comprising a negative electrode means,
a positive electrode means, an electrolyte means capable of
conducting oxygen ions therebetween, and means for applying a
voltage to the positive and negative electrode means.
38. A process for the selective oxidation of an organic compound in
an electrochemical cell, comprising: passing the organic compound
over the surface of a positive electrode comprising a positive
electrode material, whereby the organic compound is partially
oxidized, passing an oxygen- or N.sub.2O-containing gas over a
negative electrode, whereby the oxygen or N.sub.2O is converted to
oxygen ions, wherein the oxygen ions are conducted in an ion
conductive means disposed between the positive and negative
electrodes, and the positive and negative electrodes are connected
to a means for applying a voltage to the positive and negative
electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to an electrochemical cell and to an
electrochemical, catalytic oxidation process for preparing
selectively oxidized organic compounds.
[0003] 2. Discussion of the Background
[0004] The direct selective oxidation of organic compounds has
until now rarely been possible because the partially oxidized
product produced thereby is usually more reactive than the starting
materials employed, resulting in complete oxidation of the starting
materials to form, ultimately, carbon dioxide. An exception to this
usual rule is the direct oxidation of n-butane to provide maleic
anhydride, but in this case, stabilization of the oxidation product
by cyclization plays a crucial role. In particular, the problem of
how to directly oxidize alkanes and aromatic compounds has not been
satisfactorily resolved to-date.
[0005] Many attempts have been made to develop novel heterogeneous
catalysts for the partial direct oxidation of unreactive organic
compounds. However, the yield of the partially oxidized product
using such catalysts is usually too low to be industrially
useful.
[0006] By contrast, little attention has been paid to partially
oxidizing organic compounds by electrochemical methods. On the
contrary, the forefront of development work in this area is in the
area of total oxidation of suitable compounds in fuel cells for the
production of electrical energy.
[0007] U.S. Pat. No. 4,329,208 describes the electrochemical
oxidation of ethene to ethylene oxide using a positive electrode
consisting of silver or a silver alloy and a solid electrolyte
system comprising zirconium oxides.
[0008] U.S. Pat. No. 4,661,422 describes another process for
electrochemically oxidizing organic compounds, in which
hydrocarbons were oxidized at a metal/metal oxide positive
electrode using a molten salt electrolyte. The molten salt
comprises carbonate, nitrate or sulfate salts, and the negative
electrode is made of mixed oxides of metals from groups IB, IIB,
IIIA, VB, VIB, VIIB and VIIIB of the Periodic Table.
[0009] Takehira et al., in Catalysis Today, 1995, 25, 371, have
investigated the partial oxidation of propene in an apparatus
similar to fuel cells, using a Y-stabilized ZrO.sub.2 electrolyte,
a Au supported Mo/Bi mixed catalyst positive electrode material,
and a Ag negative electrode material. The reaction temperature was
475.degree. C.
[0010] In each case, the yield of the desired oxidation product is
generally so low that none of these processes are industrially
useful, and in addition, the problem of the total oxidation of the
organic substrate to carbon dioxide has not yet been solved. In
addition, the electrolyte acts as "oxygen pump", i.e. the oxygen
needed for the oxidation is reduced at the negative electrode and
then migrates in ionic form through the electrolyte to the positive
electrode. The positive electrode space contains only the substrate
to be oxidized and possibly an inert gas. Even if oxygen is fed
into the positive electrode space, the yield of the desired
oxidation product does not increase.
[0011] Another disadvantage of the above-described electrochemical
partial oxidations is that the reaction temperature is determined
by the oxygen conductivity of the electrolyte. The electrolytes
employed only have sufficient oxygen conductivity at temperatures
significantly above the optimum temperatures for such oxidation
reactions, which undoubtedly, at least in part, explains why such
process show low selectivity.
[0012] In particular, processes which employ molten salt
electrolytes invariably have such high reaction temperatures (up to
750.degree. C.) that it is difficult to avoid decomposing the
oxidation products. In addition, such processes are not suitable
for preparing thermally unstable compounds (for example Michael
systems).
[0013] The discovery of the NEMCA (Non Faradaic Electrochemical
Modification of Catalytic Activity) effect opens up the possibility
of developing more economical electrochemical processes. Vayenas et
al. in "Studies in Surface Science and Catalysis", R. K. Grasselli,
S. T. Oyama, A. M. Gaffney, J. E. Lyons (Editors), 110, 77 (1997)
and Science (1994), 264, 1563, describe an electrochemical process
based on a conductive, porous metal oxide film on a solid
electrolyte, such as, for example, ZrO.sub.2 stabilized by Y. In
this process, gas-tight separation of the positive electrode and
negative electrode spaces is not required, and the oxidant can be
fed concomitantly into the positive electrode space. However,
carbon dioxide is still produced by the total oxidation of the
substrate, and the selectivity of the oxidation reactions to
produce a desired, partially oxidized product is very low, even at
low conversions.
SUMMARY OF THE INVENTION
[0014] The object of the present invention is to provide an
electrochemical cell for selectively oxidizing organic compounds
and a process for selectively oxidizing organic compounds using
this cell.
[0015] Surprisingly, it has been found that an electrochemical cell
consisting of a negative electrode, a solid electrolyte which
conducts oxygen ions, and a positive electrode coated or comprising
a catalytically active compound is highly suitable for the
selective oxidation of organic substrates.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a diagram of an exemplary electrochemical cell
according to the present invention.
[0017] FIG. 2 is a diagram of a test apparatus for optimizing
operating conditions of an electrochemical cell according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] In a first embodiment, the present invention provides an
electrochemical cell consisting of a negative electrode, a solid
electrolyte which conducts oxygen ions, and a positive electrode
comprising a positive electrode material comprising zeolites,
mordenites, silicates, phosphates or mixed metal oxides with a pore
size of below 200 nm. In a second embodiment, the positive
electrode material of the electrochemical cell is a coating on the
solid electrolyte. In a third embodiment, the positive electrode
material of the electrochemical cell is coated on a conventional
metal electrode. In a fourth embodiment, the positive electrode of
the electrochemical cell further comprises a mixture of the solid
electrolyte positive electrode material. In a fifth embodiment, the
positive electrode of the electrochemical cell further comprises a
mixture of an electrically conductive metal and the positive
electrode material. In a sixth embodiment, an organic compound is
partially or selectively oxidized using an electrochemical cell
according to the present invention.
[0019] The negative electrode of the electrochemical cell of the
present invention may be a metal, such as copper, gold, silver,
platinum or iridium, or mixtures of these metals, one or more metal
oxides or a mixed metal oxide. For the purposes of the present
invention, the term "mixed oxides" includes multi-metal oxide
materials as well as mixtures of metal oxides. Multi-metal oxide
materials are metal oxides in which two or more metals occupy
lattice sites in the oxide structure. In any case, phase
transitions are possible, depending on the stoichiometry of the
mixed oxide and how they are heat treated.
[0020] In addition, the negative electrode may comprise perovskites
of the general formula I
La.sub.uX.sup.13.sub.VX.sup.14.sub.wX.sup.15.sub.xX.sup.16.sub.yO.sub.3.+--
.z (I)
[0021] where
[0022] X.sup.13=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb
and/or Lu,
[0023] X.sup.14=Ca, Sr, Ba and/or Mg,
[0024] X.sup.15=Mn, Fe, Ti, Ga, Mn and/or Zr,
[0025] X.sup.16=Co, Ni, Cu. Al and/or Cr
[0026] u=0 to 1.2,
[0027] v=0 to 1.0
[0028] w=0.01 to 0.8,
[0029] with the proviso that u+v+w is .ltoreq.1.5, and
[0030] x=0.2to 1.3,
[0031] y=0 to 0.9,
[0032] with the proviso that x+y is .gtoreq.0.3.
[0033] The number of oxygen atoms (3.+-.z) is defined by the
valency and amount of the elements other than oxygen in the
oxide.
[0034] German patent 197 02 619 C1 describes the preparation of
negative electrode materials for high-temperature fuel cells
composed of non-stoichiometric perovskites of the formula
L.sub..alpha.M.sub..beta.Mn- .sub.xCo.sub..delta.O.sub.3. However,
fuel cells are designed for the production of electrical energy by
means of total oxidation of a substrate, rather than the partial
oxidation of a substrate (e.g., an organic compound).
[0035] The composition of the solid electrolyte which conducts
oxygen ions is has a significant effect on the conductivity of the
electrochemical cell. The conductivity of the solid electrolyte may
be drastically increased depending on both the composition and
geometry of the electrolyte, in particular the layer thickness of
the solid electrolyte. Preferred electrolyte layer thicknesses are
less than 200 .mu.m, more preferably less than 150 .mu.m, most
preferably less than 60 .mu.m.
[0036] The solid electrolyte which conducts oxygen ions in the
electrochemical cell of the present invention may, for example,
consist of cerium oxide (CeO.sub.2), or cerium oxide stabilized
with any of lanthanum oxide (La.sub.2O.sub.3), yttrium oxide
(Y.sub.2O.sub.3), ytterbium oxide (Yb.sub.2O.sub.3) and/or
gadolinium oxide (Gd.sub.2O.sub.3). It is furthermore possible to
employ a solid electrolyte consisting of zirconium oxide
(ZrO.sub.2), or zirconium oxide stabilized with any of calcium
oxide (CaO), scandium oxide (Sc.sub.2O.sub.3), yttrium oxide
(Y.sub.2O.sub.3) and/or ytterbium oxide (Yb.sub.2O.sub.3). In the
simplest embodiment, the solid electrolyte which conducts oxygen
ions contains a metal or metal oxide or complex mixed-metal oxides,
as shown below.
[0037] In addition to mixing metal powders or flakes, for example
copper, silver, gold, platinum, palladium and iridium, in the solid
electrolyte, it is also possible to arrange a metal foil,
preferably an oxygen-permeable metal foil, between the electrolyte
and the positive electrode. Metal foils of this type may consist of
a metal of high electrical conductivity, such as copper, silver,
gold, platinum, palladium, iridium or a mixture of these metals.
The thickness of these metal foils should be less than 200 .mu.m,
preferably less than 100 .mu.m, more preferably less than 50
.mu.m.
[0038] The layer thickness of the solid electrolyte may be modified
by various methods or the conductivity may be improved by, for
example, by CVD (i.e., chemical vapor deposition), PVD (i.e.,
physical vapor deposition), spin rotation or MOD spin casting
(Swider, Karen Elizabeth, Univ. Pennsylvania, Philadelphia, Pa.,
USA. Avail. Univ. Microfilms Int., Order No. DA9308667. (1992); 242
PP: From: Diss. Abstr. Int. B 1993, 53 (11), 5927), tape casting
(Plucknett, Kevin P.; Caceres, Carlos H.; Wilkinson, David S.;
Department of Materials Science Engineering, McMaster University,
Hamilton, ON, Can.; J. Am. Ceram. Soc. (1994), 77(8), 2137-44),
slip casting (Forthmann, R.; Blass, G.; Buchkremer, H. -P.
Forschungszentrum Julich GmbH, Julich, Germany. Editor(s): Sarton,
L. A. J. L.; Zeedijk, H. B.; Mater., Funct. Des., Process. Eur.
Conf. Adv. Mater. Processes Appl., 5th (1997), 3 3/271-31274.
Publisher: Netherlands Society for Materials Science; Zwijndrecht,
Neth.) or especially by the MOCVD method (i.e., metal-organic
chemical vapor deposition). This method allows membrane
thicknesses, or in the present case, electrolyte layer thicknesses,
of between 1 .mu.m and 50 .mu.m to be obtained through
decomposition on a porous substrate. The method has been described
by O. Gorbenko, A. Kaul, A. Molodyk, V. Fuflygin, M. Novozhilov, A.
Bosak, U. Krause, G. Wahl in "MOCVD of perovskites with metallic
conductivity", Journal of Alloys and Compounds, 251 (1997),
337-341. All of the above-noted references describing various
methods of preparing thin layers are herein incorporated by
reference. Experimental problems, such as stoichiometric deviation,
loss of alkali metal and alkaline earth metal compounds in the film
and the formation of cracked products can be solved through a
suitable choice of the starting stoichiometry and through the
choice of the decomposition conditions.
[0039] Suitable substrates for these membrane or electrolyte layers
are aluminum oxide (Al.sub.2O.sub.3), lanthanum calcium manganate
(La.sub.1-xCa.sub.xMnO.sub.3) and calcium stabilized zirconium
(Zr.sub.1-xCa.sub.xO.sub.2).
[0040] In a particular embodiment of the present invention, the
solid which conducts oxygen ions is a perovskite of the general
formula II
Ln.sub.aX.sup.1.sub.bX.sup.2.sub.cX.sup.3.sub.dO.sub.e (II)
[0041] where
[0042] Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb
and/or Lu
[0043] X.sup.1=Ca, Sr, Ba and/or Mg
[0044] X.sup.2=Ga, Al, Mn, Ti, Nb, Y, W and/or Zr
[0045] X.sup.3=Fe, Co, Ni and/or Cu
[0046] a=0.1 to 0.9,
[0047] b=0.1 to 0.9,
[0048] c=0 to 0.9,
[0049] d=0 to 0.9
[0050] with the proviso that a+b=0.3 to 1.5.
[0051] The number of oxygen atoms e is determined by the valency
and amounts of the elements other than oxygen in this formula.
[0052] In a further embodiment, the solid which conducts oxygen
ions is a pyrochloro compound of the general formula III
(Ln.sub.fX.sup.4.sub.g).sub.2(X.sup.5.sub.hX.sup.6.sub.i).sub.2O.sub.k
(III)
[0053] where
[0054] Ln=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb
and/or Lu
[0055] X.sup.4=Na, Mg, Ca and/or Sr,
[0056] X.sup.5=Ti, Nb, Ta and/or Zr,
[0057] X.sup.6=Fe, Al, Sc, Ga and/or Y,
[0058] f=0.2 to 1.2,
[0059] g=0 to 0.8,
[0060] h=0.2 to 1.2,
[0061] i=0to 0.8.
[0062] The number of oxygen atoms k is again determined by the
valency and amount of the elements other than oxygen in this
formula.
[0063] These compounds can be prepared, for example, by a sol-gel
process (Shag Zonping; Sheng, Shishan; Chen, Hengrong; Li, Lin;
Pan, Xiulian; Xiong Guoxing; State Key Laboratory of Catalysis,
Dalian Institute of Chemical Physics, Chinese Academy of Science,
Dalian, Peop. Rep. China. Gongneng Cailiao (1998), 29 (Suppl),
1091-1093, 1096), spray drying (Sizgek, E.; Bartlett, J. R.;
Brungs, M. P. Materials Division, Australian Nuclear Science and
Technology Organisation, Menai, Australia. J. Sol-Gel Sci. Technol.
(1998), 13 (1/2/3), 1011-1016 or drip pyrolysis (P. Gordes et al.,
Den. J. Mate, Sci. (1995), 30 (4), 1053-8) or decomposition methods
(for example: N. Dhas et al., India J. Mater. Chem. (1993), 3 (12),
1289-1294, or D. Fumo et al., Port. Mater Res. Bull. (1997), 32
(10), 1459-1470), each of which is herein incorporated by
reference.
[0064] The positive electrode of the electrochemical cell of the
present invention may consist entirely or in part of mixed oxides
of formula V (described hereinafter) or consist of a conventional
metal electrode, for example platinum, gold, silver, copper,
iridium, palladium or alloys of these metals, coated with an
undoped or metal-doped zeolites, such as, for example, mordenites,
ZSM-5 or ZSM-11. Titanium silicates, such as, for example, TS-1,
are also suitable for the positive electrode coating. Catalytically
active materials of this type are described comprehensively in
"Technische Katalyse" (Industrial Catalysis) (Jens Hagen), VCH
Verlagsgesellschaft mbH, D-6951 Weinheim, 243-267 (1996), herein
incorporated by reference. Amorphous microporous mixed-metal
oxides, as described, for example, in DE 195 06 843, herein
incorporated by reference, are also suitable as coating materials
for the positive electrode. In addition, catalysts of the
composition disclosed in DE 198 54 615, herein incorporated by
reference, metal-doped zeolites, silicates, aluminum silicates and
aluminum phosphates, are also suitable coating materials for the
positive electrode. In each case, it is essential that the pore
size of these positive electrode coating materials is less than 200
nm, preferably less than 100-nm, in particular less than 20 nm,
preferably less than 10 nm, very particularly less than 1 nm.
[0065] Preferred positive electrode materials for the
electrochemical cells of the present invention may consist of a
compound of formula IV
(El.sub.2O.sub.e).sub..alpha.(Fe.sub.2O.sub.3).sub..beta.SiO.sub.2
(IV)
[0066] where
[0067] El=an element from group IA, IIA, IIIA, IVA or VA or IIIB,
IVB, VB, VIB, VIIB or VIII of the Periodic Table of the
Elements
[0068] .alpha.=from 0 to 0.1
[0069] .beta.=from 0 to 0.1,
[0070] with the proviso that .alpha. and .beta. are not
simultaneously 0.
[0071] e is determined by the valency and frequency of the elements
other than oxygen in formula IV. In a preferred embodiment,
.alpha.=from 0 to 7-10.sup.-2 and .beta.=from 0.1 to 10.sup.-5.
[0072] In practice, a film of the positive electrode materials, in
particular the mixed oxides, may be first applied to the solid
electrolyte, for example by screen printing, and then bonding the
mixed oxide positive electrode material directly to the solid
electrolyte by heating. An example of this technique is given in JP
09 239 956, herein incorporated by reference.
[0073] The positive electrode materials may also be sintered onto a
conventional metal electrode in amounts of at most 300% by weight
(based on the weight of the conventional metal electrode), in
particular at most 200% by weight, preferably at most 100% by
weight, at temperatures of at most 1500.degree. C., in particular
at temperatures of at most 900.degree. C., preferably at
temperatures of at most 750.degree. C. The heat stability of these
coatings is critical. Thus, for example, zeolites are substantially
stable even at temperatures of above 1000.degree. C. However, the
coatings can also be grown onto the surface of the conventional
metal electrode and allowed to crystallize, for example by
crystallizing the desired mesoporous, microporous or nanoporous
catalytically active layer onto a conventional metal electrode.
This procedure may be analogous to the crystallization of, for
example, a zeolite layer on the surface of a support (e.g., U.S.
Pat No. 4,800,187, incorporated herein by reference).
Alternatively, as described above, the positive electrode materials
of the present invention may be coated directly onto the solid
electrolyte.
[0074] An additional possible method for crystallizing, for
example, a zeolite layer on the surface of a mesoporous support or,
in the present case, on the surface of a conventional metal
electrode or positive electrode is the crystallization of zeolite
grafts within the pores of a monolithic support (U.S. Pat No.
4,699,892, FR 94-5562 and WO 95/29751, each of which is herein
incorporated by reference).
[0075] A suitable positive electrode material or positive electrode
coating, i.e. catalytically active coating, is also a mixed oxide
of the general formula V
A.sub.lB.sub.mX.sup.7.sub.nX.sup.8.sub.oX.sup.9.sub.pX.sup.10.sub.qX.sup.1-
1.sub.rX.sup.12.sub.sO.sub.t (V)
[0076] where
[0077] A and B are an element from Group IA, IIA and/or VA and/or
Group IVB, VB, VIB, VIIB, or VIII of the Periodic Table of the
Elements
[0078] X.sup.7=V, Nb, Cr, W, Ta, Ga and/or Ce
[0079] X.sup.8=Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and/or Ba,
[0080] X.sup.9=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, Lu, Cu, Ag, Au, Pd and/or Pt,
[0081] X.sup.10=Fe, Co, Ni and/or Zn,
[0082] X.sup.11=Sn, Pb, Sb and/or Te,
[0083] X.sup.12=Ti, Zr, Si and/or Al, where
[0084] l=0.001 to 30,
[0085] m=0.001 to 20,
[0086] n=0 to 15,
[0087] o=0.001 to 10,
[0088] p=0 to 10,
[0089] q=0to 40
[0090] r=0 to 10 and
[0091] s=0 to 80, with the proviso that l+m is .gtoreq.0.01 and l+o
is .gtoreq.0.005.
[0092] The number of oxygen atoms t is determined by the valency
and amount of the elements other than oxygen in this formula.
[0093] As described above, and for the purposes of the present
invention, the term "mixed oxides" includes multi-metal oxide
materials as well as mixtures of metal oxides, where multi-metal
oxide materials are metal oxides in which two or more metals occupy
lattice sites in the oxide structure. In any case, phase transition
are possible, depending on the stoichiometry of the mixed oxides,
and how they are heat treated.
[0094] Mixed oxides as described above are known from other
technical areas and may be employed, for example, as heterogeneous
catalysts for gas-phase reactions. The preparation and use of such
compounds is described, for example, in EP 0 417 723, herein
incorporated by reference. While it is known that mixed oxides of
this type may be used as heterogeneous catalysts in
non-electrolytic chemical reactions, the use of such mixed oxides
as a positive electrode material in electrochemical processes,
according to the present invention, is not described in the
literature.
[0095] Inter alia, the following mixed oxides, for example, may be
suitable for the electrochemical process of the present
invention:
[0096] a) Mo.sub.9.57Bi.sub.0.86Fe.sub.6.4Co.sub.3.2K.sub.0.05
oxide
[0097] b) Mo.sub.12Bi.sub.0.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1
oxide
[0098] c) Mo.sub.12Bi.sub.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1
oxide
[0099] d)
Mo.sub.12Bi.sub.0.98Ni.sub.8.29Fe.sub.1.95Si.sub.10K.sub.0.05Na.-
sub.0.15 oxide
[0100] e)
Mo.sub.12Bi.sub.4Si.sub.36Ni.sub.3Co.sub.1.9Cu.sub.0.26Fe.sub.0.- 1
oxide
[0101] f)
Mo.sub.12Bi.sub.4Si.sub.36Ni.sub.3Fe.sub.1.9Cu.sub.0.26Co.sub.0.- 1
oxide
[0102] Empirical formulae data for said mixed oxides may also be
described using smaller indices in the empirical formula. For
example, the empirical formula of mixed oxide c) , i.e.,
Mo.sub.12Bi.sub.5Fe.sub.4Co.s- ub.8Ca.sub.0.1K.sub.0.1 oxide can
also be divided by the number 12 to give the empirical formula:
Mo Bi.sub.0.416Fe.sub.0.33Co.sub.0.66Ca.sub.0.083K.sub.0.083
oxide
[0103] Both empirical formulae describe the same mixed oxide. This
means that in many cases the oxide composition may be definitively
described only by the ratio of the components present therein.
[0104] The positive electrode may be composed entirely or only
partly of the mixed oxides of the formula V. In general, a
conventional metal electrode, for example made of platinum, gold,
silver, copper, iridium, palladium or alloys of these metals, may
also be provided with a surface comprising these mixed oxides. It
is likewise possible for the catalytically active mesoporous,
microporous or nanoporous positive electrode coating to be sintered
or allowed to crystallize directly onto a metallic electrode.
[0105] It is likewise possible to mechanically mix a conductive
metal, such as, for example, copper, silver, gold, platinum,
palladium or iridium, with the positive electrode material--for
example if mixed oxides are present--in order to improve the
conductivity of the coating material. However, since pure metals
may chemically react during the sintering process, the solid
electrolytes of formula II or III, preferably a perovskite or
cerium oxide, more preferably a stabilized perovskite or cerium
oxide, is usually mechanically mixed with the positive electrode
material in order to increase the conductivity of the positive
electrode.
[0106] The electrochemical cells and processes of the present
invention allow the partial or selective oxidation of organic
compounds, for example saturated organic compounds, such as ethane,
propane, isobutane, butane, hexane, pentane, cyclohexane,
vinylcyclohexane, octane and cyclododecane, or compounds containing
triple bonds, such as, for example, ethyne, propyne or butyne. In
addition, these hydrocarbons may also be used as starting materials
to prepare synthesis gas, i.e. hydrogen plus carbon monoxide using
the electrochemical cell and process of the present invention.
[0107] Furthermore, olefins or compounds containing double bonds
and aromatic compounds, such as, for example, ethene, propene,
butene, isobutene, hexene, cyclohexene, vinylcyclohexene,
cyclooctene, cyclododecene, butadiene, isoprene, pentadiene,
hexadiene, cyclooctadiene, cyclodedecatriene and benzene, toluene,
ortho-xylene, meta-xylene, para-xylene, cumene, cumylbenzene,
cyclododecylbenzene, 2-n-butylbenzene, ethylbenzene,
tert-butylbenzene, tert-butyltoluene, methoxytoluene and
phenoxytoluene, or derivatives thereof, may be oxidized to the
corresponding epoxides, hydroxyl compounds, aldehydes and ketones,
and carboxylic acids using the electrochemical cell and process of
the present invention.
[0108] Alcohols, ketones and aldehydes may be converted selectively
into the corresponding aldehydes or carboxylic acids, with chain
cleavage where appropriate. Examples thereof include
trimethylcyclohexanol (from trimethylcyclohexane), cyclododecanol
(from cyclododecane), acrolein (from propane or propene),
methacrolein (from isobutane or isobutene) and cyclododecanone
(from cyclododecane). Thus, by partial or selective oxidation, we
mean that the organic compound used as a starting material in the
process of the present invention is not completely oxidized to
carbon dioxide, but rather is oxidized to an intermediate
level.
[0109] In the electrochemical cell of the present invention, oxygen
is taken up by the negative electrode and passed to the positive
electrode through the solid electrolyte. The negative electrode can
also be exposed to a stream of air or an oxygen-containing off-gas
stream. It is important that this gas stream contains a gas which
can be ionized, such as oxygen, which can then migrate through the
solid electrolyte to the positive electrode.
[0110] The organic compound to be oxidized (i.e., the organic
starting material) can be mixed with air and/or oxygen and/or an
inert gas, such as, for example, nitrogen, and passed over the
positive electrode. The organic starting material may be introduced
in gaseous or liquid form, but introducing the organic starting
material in the gaseous form is preferred, and has provided good
results at the reaction temperatures of the present invention.
Thus, it benzene or benzene derivatives, together with dinitrogen
monoxide (i.e., N.sub.2O) and/or air or oxygen containing
dinitrogen monoxide or another off-gas containing dinitrogen
monoxide, for example from the synthesis of adipic acid, may be
oxidized in the electrochemical cell of the present invention,
thereby forming phenol or derivatives thereof.
[0111] The present invention also relates to a process for
electrochemically oxidizing organic compounds, in which organic
compounds are introduced into the positive electrode side of an
electrochemical cell consisting of a negative electrode, a solid
electrolyte which conducts oxygen ions, and a positive electrode
which has a coating of zeolites, mordenites, silicates, phosphates
or mixed-metal oxides having a pore size of less than 200 nm, and a
gas containing oxygen or N.sub.2O is introduced into the negative
electrode side.
[0112] The electrochemical oxidation of organic compounds by the
process of the present invention or in the electrochemical cell of
the present invention is carried out at elevated temperatures,
preferably from 100 to 650.degree. C., particularly preferably from
200 to 550.degree. C.
[0113] The process of the present invention may also be carried out
at elevated pressures, for example at pressures of up to at most
100 bar, preferably from 1 to 20 bar, more preferably at about 10
bar.
[0114] In the process and electrochemical cell of the present
invention, oxygen is ionized at the negative electrode and moves
through the negative electrode to the positive electrode. The
oxygen is activated at the positive electrode in such a way that it
reacts with the organic compound passed over the positive
electrode. The oxygen feed stream can also move through a porous,
non-gas-tight solid electrolyte. The gas stream in the positive
electrode space may also contain an inert gas in addition to the
organic compound substrate (i.e., the compound to be oxidized) and
oxygen.
[0115] An example of the structure of an electrochemical cell of
the present invention which may be used to carrying out the process
of the present invention is shown in FIG. 1.
[0116] The negative electrode K and the positive electrode A are
joined to the electrolyte E which conducts oxygen ions, for example
by a heat treatment, so that a current-conducting connection is
provided therebetween. The two electrodes are supplied with current
via a voltage source S.
[0117] The starting material and oxygen are introduced as gas
stream a) to the positive electrode A, and the resultant product
gas b) is transported away either by the positive pressure of the
gas stream a) or by means of a corresponding reduced pressure. The
gas stream c) on the negative electrode side of the electrochemical
cell may comprise air, oxygen or another oxygen-containing gas
mixture, and after passing over the negative electrode, is depleted
in oxygen.
[0118] The positive electrode space is sealed against the
electrodes by gold foils D. The oxygen feed stream (at a pressure
of about 10 kPa) is introduced to the electrochemical cell via the
porous element O.
[0119] The electrochemical cell of the present invention may have
any spatial arrangement of positive electrode, negative electrode
and electrolyte, and is not restricted to flat plates or continuous
layers. For example, the electrochemical cell of the present
invention may be a tubular reactor, in which case, positive
electrode and negative electrode materials are applied to a tube
made from the electrolyte, and connected appropriately to a voltage
source. Furthermore, the positive electrode or negative electrode
layer may have the topography of a fabric or structured surface
layer with regular recesses or elevations.
[0120] The experimental arrangement shown diagrammatically in FIG.
2 may be used to determine or optimize the effects of the
temperature, the gas flow rate, gas residence time and the current
strength used in the process of the present invention, on the
performance of the electrochemical cell. The current strength
required generally depends on the size or product capacity of the
electrochemical cell and is generally between 0 and 10 mA,
preferably from 10 to 20 mA. A current source S which operates at a
current strength of from -100 to +100 mA, can be programmed for
different flow rates. The excess voltage at the positive electrode
A (V.sub.A) the excess voltage at the negative electrode K
(V.sub.K), the measurement-cell voltage (V.sub.cell) and the
voltage V.sub.ref of the reference electrodes RA and RK (which are
usually made of platinum) may be monitored.
[0121] The current strengths described above relate to those used
in a pilot plant scale electrochemical process, such as, for
example, those shown in FIG. 2. When greater production capacities
are desired, correspondingly higher current strengths are
necessary. Industrial scale current strengths can be determined
readily by simple preliminary experiments.
[0122] In FIG. 2, E denotes the solid electrolyte which conducts
oxygen ions, a) denotes the starting-material gas stream
(benzene.fwdarw.phenol)- , and c) denotes oxygen or an
oxygen-containing gas.
[0123] Having generally described this invention, a further
understanding can be obtained by reference to certain specific
examples which are provided herein for purposes of illustration
only, and are not intended to be limiting unless otherwise
specified.
EXAMPLES
[0124] Step a): Preparation of the solid electrolyte which conducts
oxygen ions
[0125] aa) either commercially available cerium oxide (Indec b.v.)
is used; or
[0126] ab) a suspension containing a binder (for example 16 g of
ethylcellulose, Merck) and a solvent (e.g., 422 g of terpineol;
i.e. p-menth-1-en-8-ol) is prepared with the desired compounds, for
example, by mixing for 24 hours using a ball mill or bead mill
(Netzsch). This suspension is cast as a thick film, which, after
removal or evaporation of the solvent, provides a crude film of the
electrolyte. The crude electrolyte film may then be cut to the
desired size.
[0127] Depending on the electrolyte used
(La.sub.0.8Sr.sub.0.2Ga.sub.0.85M- g.sub.0.15O.sub.2 and CeO.sub.2
were used in the present experiment), it may be advantageous to use
different layer thicknesses. For example, when the electrolyte is
La.sub.0.8Sr.sub.0.2Ga.sub.0.85Mg.sub.0.15O.sub.2, a layer
thickness of about 80 .mu.m was used, while when the electrolyte
was CeO.sub.2 a layer thickness of 200 .mu.m was used. This
electrolyte film may be sintered between two porous aluminum oxide
plates at a temperature of 1500.degree. C. for 6 hours (e.g., when
the electrolyte film is
La.sub.0.8Sr.sub.0.2Ga.sub.0.85Mg.sub.0.15O.sub.2) or 1300.degree.
C. and 8 hours (e.g., when the electrolyte film is CeO.sub.2).
[0128] Step b): Production of the negative electrode
[0129] ba) The negative electrode was either a platinum layer
vapor-deposited onto the electrolyte; or
[0130] bb) as described in detail under c).
[0131] The composition of the negative electrode powder was
La.sub.0.6Sr.sub.0.4Fe.sub.0.8Co.sub.0.2O.sub.3 (Rhone Poulenc),
sintered for 1 hour at 1100.degree. C.
[0132] Step c): Production of the positive electrode/catalytic
layer
[0133] The desired molar ratios of the elements comprising the
positive electrode/catalytic layer (except for molybdenum), in the
form of their respective nitrate salts (Merck), were dissolved in
about 50.degree. C. warm water and stirred with a stainless steel
paddle stirrer. Molybdenum, in the form of ammonium heptamolybdate
tetrahydrate (NH.sub.4).sub.6Mo.sub.7O.sub.24.times.4H.sub.2O (for
example from H. C. Starck), was initially dissolved separately.
[0134] The molar ratios of the elements are shown, for example, in
the table for Examples 1 to 4 or by the formula IV.
[0135] The ammonium heptamolybdate, in an amount corresponding to
the desired molar ratio, was then poured into the nitrate salt
solution of the other elements, and stirred. A product initially
precipitated, and then re-dissolved upon continued stirring, then
gelled after a short time.
[0136] The gel was subsequently dried at 110.degree. C. in a stream
of air and calcined at 450.degree. C. The resultant material was
subsequently ground or used directly for the preparation of the
catalytic paste.
[0137] The desired catalytically active compounds were converted
into a paste with binder and solvent using a ball mill or a bead
mill. This paste was then screen-printed onto the electrolyte.
After removal or evaporation of the solvent, the entire cell was
sintered at a temperature of 400.degree. C., thereby providing a
catalytically active layer with a layer thickness of 50 .mu.m,
using the following detailed process:
[0138] The catalytically active paste was prepared by mechanically
mixing (e.g., by stirring) the catalyst powder, i.e., for example,
the mixed-metal oxide powder, the zeolite and optional additives,
for example with a cellulose-based binder.
[0139] The binder was prepared by mixing for 20 minutes, using a
propeller stirrer, 16 g of ethylcellulose (Merck) in 422 g of
terpineol (i.e., p-menth-1-en-8-ol). 32 g of catalyst and optional
additives which increase the conductivity were then initially
mixed, into 22 g of the binder, by hand, with a spatula, then
further mixed using a 3-roll mill (Netzsch). The paste produced
thereby was then collected in a 50 ml bottle.
[0140] This paste was then printed onto the electrolyte layer using
a screen-printing device (DEK) using a screen with a mesh size of
53. Finally, the catalytically active layer was sintered at
400.degree. C. for one hour.
[0141] 1. Catalytic Oxidation of Benzene (Comparative Example)
[0142] Various porous catalytic films having a BET surface area of
16 m.sup.2/g were screen printed on a 200 .mu.m electrolyte film of
CeO.sub.2, then heat treated (see following table). Pt was then
vapor-deposited on the electrolyte film as a counter-electrode. The
reaction temperature was 450.degree. C. A mixture of 5% benzene, 5%
oxygen and 90% nitrogen was passed over the positive electrode at a
rate of 2 l/h. Air was passed over the negative electrode at the
same rate. No current was applied to the electrodes of the cell,
and oxygen transfer from the negative electrode to the positive
electrode was either substantially suppressed or not evident.
[0143] Results
1 Positive electrode material (catalytic film) Phenol formation
[mmol/h*g] MoO.sub.3 0
Mo.sub.9.57Bi.sub.0.86Fe.sub.6.4Co.sub.3.2K.sub.0.05 oxide 0
Mo.sub.12Bi.sub.0.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0 plus
10% by weight of CeO.sub.2 plus 30% by weight of zeolite ZSM-5
Mo.sub.12Bi.sub.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0 plus
40% by weight of zeolite ZSM-5
[0144] 2. Electrocatalytic Oxidation of Benzene (According to the
Present Invention)
[0145] Various porous catalytic films (see following table) having
a BET surface area of 16 m.sup.2/g were screen printed on a 200
.mu.m electrolyte film of CeO.sub.2, then heat treated. Pt was then
vapor-deposited on the electrolyte film as a counter-electrode. The
reaction temperature was 450.degree. C. A mixture of 5% benzene, 5%
oxygen and 90% nitrogen was then passed over the positive electrode
at a rate of 2 l/h. Air was passed over the negative electrode at
the same rate. The applied voltage on the cell for controlling the
oxygen ion flow was 1 V.
[0146] Results
2 Positive electrode material (catalytic film) Phenol formation
[mmol/h*g] MoO.sub.2 0
Mo.sub.9.57Bi.sub.0.86Fe.sub.6.4Co.sub.3.2K.sub.0.05 oxide 0
Mo.sub.12Bi.sub.0.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0.35
plus 10% by weight of CeO.sub.2 plus 30% by weight of zeolite ZSM-5
Mo.sub.12Bi.sub.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0 plus
40% by weight of zeolite ZSM-5
[0147] 3. Catalytic Oxidation of Benzene Using Dinitrogen Monoxide
in the Starting-material Stream (Comparative Example)
[0148] Various porous catalytic films (see following table) having
a BET surface area of 18 m.sup.2/g were screen printed on a 80
.mu.m electrolyte film of
La.sub.0.8Sr.sub.0.2Ga.sub.0.85Mg.sub.0.15O.sub.2, the heat
treated. Pt was then vapor-deposited on the electrolyte film as a
counter-electrode. The reaction temperature was 395.degree. C. A
mixture of 5% benzene, 5% oxygen, 3% dinitrogen monoxide and 87%
nitrogen was then passed over the positive electrode at a rate of 2
l/h. Air was passed over the negative electrode at the same rate.
No current was applied to the electrodes of the cell, and oxygen
transfer from the negative electrode to the positive electrode was
either substantially suppressed or not evident.
[0149] Results
3 Positive electrode material (catalytic film) Phenol formation
[mmol/h*g] MoO.sub.3 0
Mo.sub.9.57Bi.sub.0.86Fe.sub.6.4Co.sub.3.2K.sub.0.05 oxide 0
Mo.sub.12Bi.sub.0.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0.5
plus 10% by weight of CeO.sub.2 plus 30% by weight of zeolite ZSM-5
Mo.sub.12Bi.sub.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0.10
plus 40% by weight of zeolite ZSM-5
[0150] 4. Electrocatalytic Oxidation of Benzene Using Dinitrogen
Monoxide in the Starting-material Stream (According to the Present
Invention)
[0151] Various porous catalytic films (see following table) having
a BET surface area of 18 m.sup.2/g were screen printed on a 80
.mu.m electrolyte film of
La.sub.0.8Sr.sub.0.2Ga.sub.0.85Mg.sub.0.15O.sub.2, then heat
treated. Pt was then vapor-deposited on the electrolyte film as a
counter-electrode. The reaction temperature was 395.degree. C. A
mixture of 5% benzene, 5% oxygen, 3% dinitrogen monoxide and 87%
nitrogen was passed over the positive electrode at a rate of 2 l/h.
Air was passed over the negative electrode at the same rate. The
applied voltage on the cell for controlling the oxygen ion flow was
1 V.
[0152] Results
4 Positive electrode material (catalytic film) Phenol formation
[mmol/h*g] MoO.sub.3 0
Mo.sub.9.57Bi.sub.0.86Fe.sub.6.4Co.sub.3.2K.sub.0.05 oxide 0
Mo.sub.12Bi.sub.0.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 2.10
plus 10% by weight of CeO.sub.2 plus 30% by weight of zeolite ZSM-5
Mo.sub.12Bi.sub.5Fe.sub.4Co.sub.8Ca.sub.0.1K.sub.0.1 oxide 0.95
plus 40% by weight of zeolite ZSM-5
[0153] The priority document of the present application, German
patent application 100 26 940.0 filed May 30, 2000, is incorporated
herein by reference.
[0154] Obviously, numerous modifications and variations on the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
* * * * *