Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2002-05-23
2004-05-04
Nguyen, Cam N. (Department: 1754)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S538000, C562S542000, C562S416000, C558S308000, C548S476000, C549S248000, C502S004000, C502S302000, C502S303000, C502S304000, C502S324000, C502S340000, C502S341000
Reexamination Certificate
active
06730808
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the field of heterogeneous catalysis. More particularly, the present invention relates to oxidation reactions carried out over a heterogeneous catalyst mounted on a mixed conductive membrane. Such membranes possess electron conductivity as well as selective conductivity for oxygen ions.
BACKGROUND ART
Oxygen permeable membranes whereby oxygen may be selectively transported through the membranes are well known. Such membranes make it possible to separate oxygen from oxygen-containing mixtures with other, preferably gaseous, elements. The selectivity is due to the migration of O
2−
ions through a layer composed of certain ceramic materials. Before the oxygen passes through the membrane, electrons are transferred to the oxygen on one side of the membrane. After migration through the membrane to the other side, the oxide ions recombine to form oxygen molecules while the electrons migrate in the opposite direction. The ceramic materials mentioned are for instance oxidic compounds of transition metals. These have to have crystallographic vacancies in the lattice for oxygen ion transport to take place.
These lattice vacancies can be produced for example by replacing metal ions in a given oxide with ions of some other valency. This creates in a relatively simple manner O
2−
vacancies via which the oxide ions are transported to the other end of the membrane.
The best-known oxygen selective ceramic material is ZrO
2
, where oxide ion conductivity is generated by partly replacing the tetravalent zirconium with trivalent yttrium or divalent calcium ions. Using such a material as an oxygen selective membrane provides a material enabling the transport of O
2−
ions. The material has no electron conductivity. Such a material is accordingly in itself not capable of transporting oxygen. If oxygen is to be transported through a membrane composed of such a material, an external circuit has to be applied to electrodes mounted on both sides of the membrane in order that charge equalization may be ensured.
To eliminate the need to apply external circuitry, it has been proposed to produce membranes that possess electron conductivity as well as oxide ion conductivity. This is generally achieved on adding a second component possessing good electron conductivity to the ceramic membrane. Palladium, platinum, silver or gold or conductive oxidic compounds of these or other metals are frequently used for this purpose. This gives rise to dual phase membranes. These are disclosed for example in EP-A-399 833. EP-A-399 833 describes inter alia electron conducting as well as oxygen selective membranes comprising a mixture of from 25 to 99% by volume of yttrium-doped zirconia and from 1 to 75% by volume of platinum. Such membranes are then used in oxidation reactions in which oxygen is selectively extracted from mixtures by the membrane and fed to the reaction.
A further development of the above-described membranes is mixed conducting membranes. These are made of not two different materials or phases but a single material capable of selectively conducting not only oxide ions but also electrons. These conductor properties are to be found in only a very limited number of materials. The materials used in general are certain perovskites which are generally doped with other cations to increase the conductivity properties and the thermal stability.
There are a number of publications describing such materials and mixed conducting membranes produced therewith.
U.S. Pat. No. 4,791,079 and U.S. Pat. No. 4,827,071 describe mixed conducting, oxide ion selective membranes that are comprised of zirconia doped with a metal of group VB, VIB or titanium dioxide. The preferred ceramic material used is zirconia doped with yttria and titania. The membrane supports a porous material containing an oxidation catalyst for hydrocarbons. Depending on the nature of this catalyst, the membranes thus obtained are useful in various oxidation reactions, for example the production of ethylene oxide or propylene oxide from the respective olefin, the oxidative dehydrogenation of monoolefins or the oxidative coupling of methane or other alkanes.
EP-A-663 232 describes a mixed conducting, oxide ion selective membrane comprised of a metal oxide comprising a plurality of different metal ions and of a catalyst applied thereto. The oxide preferably comprises bismuth, barium, vanadium, molybdenum, cerium, ruthenium, manganese, cobalt, rhodium or praseodymium ions. The catalyst is preferably formed of a metal selected from the group consisting of platinum, palladium, gold and silver. The membrane catalysts thus obtained are useful for various oxidations, for example the conversion of methane to syngas, but also the conversion of oxides of, for example, sulfur or nitrogen into the respective elements, for example the conversion of sulfur oxides into sulfur and oxygen.
WO98/41394 teaches an oxide ion selective membrane of the general formula ABO
3-&dgr;
, where A is one or more of calcium, strontium, barium, yttrium and lanthanum, B is one or more of chromium, manganese, iron, cobalt, nickel and copper, and &dgr; ranges from 0 to 0.5.
The membrane has a catalyst on both sides, namely a catalyst for activating oxygen on one side and a hydrocarbon partial oxidation catalyst on the other. A catalyst of such a composition is useful for example for selectively carrying out the partial oxidation of methane to carbon monoxide and hydrogen.
WO 99/21649 describes a reactor membrane comprised of a mixed conducting, oxide ion selective membrane coated with the oxidation catalyst. The membrane is an oxidic compound of a plurality of metals selected from the group consisting of lanthanides, yttrium, 3d transition metals and group 13 metals. A multiplicity of oxidation catalysts, both oxidic and metallic, that can be applied to such membranes are disclosed.
However, all the oxidation reactions mentioned in the above-cited references are generally carried out at high temperatures, generally about 800 to 1000° C. This is because prior art mixed conducting, oxide ion selective membranes need such high temperatures to provide an oxygen permeance sufficient for industrial application. Below the temperature range mentioned, the oxygen diffusion coefficient of the above-disclosed materials is so small that insufficient oxygen migrates through the membrane. The use of these mixed conducting membranes in oxidation reactions is thus generally restricted to oxidation reactions that proceed in the temperature range mentioned. It would be desirable, however, to have oxidation catalysts on mixed conducting, oxide ion selective membranes that can also be used at relatively low temperatures.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide such a reactor membrane. It shall retain the previously known advantages of oxidation catalysts applied to mixed conducting membranes. This is in particular the elimination of the need for an air separation in those cases where the particular oxidation reaction has to be carried out with pure oxygen. In addition, the selectivity of the oxidation is frequently enhanced by making the oxygen available as lattice oxygen at the catalyst/reaction gas phase boundary. Unwanted secondary reactions taking place homogeneously in the gas phase are suppressed.
We have found that this object is achieved by a reactor membrane comprising a selective oxidation catalyst on a mixed conducting, oxide ion selective ceramic membrane of the composition (Sr
1-x
Ca
x
)
1-y
A
y
Mn
1-z
B
z
O
3-&dgr;
where
A is Ba, Pb, Na, K, Y, an element of the lanthanide group or a combination thereof,
B is Mg, Al, G, In, Sn, an element of the 3d or 4d series or a combination thereof,
x is from 0.2 to 0.8,
y is from 0 to 0.4,
z is from 0 to 0.6, and
&dgr; is a number, dependent on x, y or z, that renders the composition charge neutral.
This invention further provides for the use of the above-described reactor membrane in oxidation reactions of hydrocarbons using oxygen.
The reactor membrane of
Bitterlich Stefan
Hibst Hartmut
Pippardt Ute
Tenten Andreas
Voigt Ingolf
BASF - Aktiengesellschaft
Nguyen Cam N.
Weinkauf Keil B.
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