Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
2000-12-22
2003-07-15
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
C252S500000, C252S509000, C252S513000, C252S521100, C501S097200, C501S097300, C501S098200, C423S210000, C423S583000, C423S600000, C423S608000, C096S004000, C096S007000, C505S701000, C505S702000, C505S785000, C505S779000, C422S198000
Reexamination Certificate
active
06592782
ABSTRACT:
BACKGROUND OF THE INVENTION
Strong incentives exist for the development of efficient processes for the separation of oxygen from gas mixtures, such as air. Low-cost production would enhance the availability of pure oxygen for a variety of industrial applications including its use in high efficiency combustion processes. There is significant potential for the application of solid state catalytic membranes to oxygen separation. This technology is presently limited by the ceramic materials that are available. New ceramic materials that exhibit higher oxygen flux and improved mechanical and chemical stability in long term operation for use in membrane reactors are of significant interest in the art.
SUMMARY OF THE INVENTION
This invention relates to mixed metal oxide materials that are particularly useful for the manufacture of catalytic membranes for gas-phase oxygen separation processes. Oxygen-deficient oxides of this invention are derived from brownmillerite materials which have the general structure A
2
B
2
O
5
. The materials of this invention maintain high oxygen anion conductivities at relatively low membrane operating conditions ranging from about 700° C. to 900° C. The metal elements at the B-site in the brownmillerite structure are selected to provide mixed ion- and electron-conducting materials and, particularly, to provide materials that conduct oxygen anions and electrons. The materials of this invention have the general formula:
A
x
A′
x′
A″
2-(x+x′)
B
y
B′
y′
B″
2-(y+y′)
O
5+z
where:
x and x′ are greater than 0;
y and y′ are greater than 0;
x+x′ is less than or equal to 2;
y+y′ is less than or equal to 2;
z is a number that makes the metal oxide charge neutral;
A is an element selected from the lanthanide elements and yttrium;
A′ is an element selected from the Group IIA elements;
A″ is an element selected from the f block lanthanides, Be, Mg, Ca, Sr, Ba and Ra;
B is an element selected from the group consisting of Al, Ga, In or mixtures thereof; and
B′ and B″ are different elements and are independently selected from the group of elements Mg or the d-block transition elements.
The lanthanide metals include the f block lanthanide metals: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Yttrium has properties similar to the f block lanthanide metals and is also included herein in the definition of lanthanide metals. A is preferably La or Gd, with La more preferred. Group IIA metal elements of the Periodic Table are Be, Mg, Ca, Sr, Ba, and Ra. The preferred Group IIA elements for the A′ element of the materials of this invention are Ca, Sr and Ba and Sr is most preferred. The more preferred B elements are Ga and Al, with Ga more preferred. The d block transition elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Preferred B′ and B″ elements are Mg, Fe and Co, with Fe and Co being more preferred as B′ and B″, respectively.
Mixed metal oxides in which B′ and B″ are Fe and Co are particularly preferred for membranes having high oxygen flux rates.
Mixed metal oxides in which B′ and B″ are Fe and Mg are also preferred for membranes having high oxygen flux rates.
The value of z in the above formula depends upon the values of x, x′, y and y′ and the oxidation states of the A, A′, A″, B, B′ and B″ elements. The value of z is such that the mixed metal oxide material is charge neutral. In preferred materials, 0<z<1.
Preferred stoichiometries for materials of this invention of the above formula are those in which x is about 0.1 to about 0.6, and x′ is about 1.4 to about 1.9, and where in addition x+x′ is about equal to 2. When x+x′ is equal to 2, the mixed metal oxide contains only A and A′ metals. More preferred are materials in which x is about 0.2 to about 0.5 and x′ is about 1.5 to about 1.8. Also preferred are those materials of the above formula where y is about 0.3 to about 0.9 and y′ is about 0.6 to about 1.8. More preferred materials have y equal to about 0.5 to about 0.8 and y′ equal to about 1.0 to about 1.4. Preferred materials have y+y′ equal to about 1.4 up to about 2.0. More preferred materials have y+y′ equal to about 1.6 to about 1.9.
In specific embodiments, mixed metal oxide of this invention include those of the above formula wherein B″ is Fe and:
(a) B″ is Co;
(b) B″ is Co and 1.4≦y+y′<2.0;
(c) B″ is Co and 1.6≦y+y′ ≦1.9;
(d) B″ is Co, 1.6≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(e) B″ is Mg;
(f) B″ is Mg and 1.5≦y+y′≦2.0;
(g) B″ is Mg and 1.7≦y+y′≦1.9;
(h) B″ is Mg, 1.7≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(i) B″ is Mg, y+y′=18, A is La, A″ is Sr and x+x′ is 2;
(j) B is Ga, B″ is Co, 1.6≦y+y′≦1.9, A is La, A″ is Sr and x+x is 2;
(k) B is Ga, B″ is Mg, 1.7≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(l) B is Ga, y is 0.3 to 0.9, B″ is Co, 1.6≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(m) B is Ga, y is 0.3 to 0.9, B″ is Mg, 1.7≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(n) B is Al, B″ is Co, 1.6≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(o) B is Al, B″ is Mg, 1.7≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2;
(p) B is Al, y is 0.3 to 0.9, B″ is Co, 1.6≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2; or
(q) B is Al ,y is 0.3 to 0.9, B″ is Mg, 1.7≦y+y′≦1.9, A is La, A″ is Sr and x+x′ is 2.
Electronically- and ionically-conducting membranes employed in the oxygen-separation reactors of this invention comprise mixed metal oxides of the above formula. Substantially gas-impermeable membranes having both electronic and ionic conductivity are formed by initially preparing mixed metal oxide powders by repeatedly calcining and milling the powders of individual metal oxides or the corresponding carbonates (or other metal precursors) in the desired stoichiometric ratios. The resulting mixed metal oxide is then pressed and sintered into dense membranes of various shapes, including disks and open-one-ended tubes. These membranes are then employed to construct catalytic membrane reactors, particularly for oxygen separation processes. The purity of the product oxygen produced in reactors of this invention, which can be stored or used in other chemical processes, is generally greater than about 90% and preferably greater than about 99%.
The presence of the mixed metal oxide of desired stoichiometry (as in the given formulas) in a repeatedly calcined and milled mixed metal oxide can be assessed by X-ray diffraction studies. Further, the presence of distinct phases of metal oxides or other metal species that may be present in the mixed metal oxides materials of this invention can be detected by X-ray diffraction techniques by the observation of peaks not assignable with the predominate mixed metal oxide of desired stoichiometry. The level of distinct phase material that can be detected depends upon the resolution and sensitivity of the X-ray diffractometer employed and upon the identity and number of the distinct phases present. It is believed that greater than about 4% by weight of another phase can be detected by the X-ray diffraction method employed (FIGS.
1
-
6
).
A catalytic reactor of this invention comprises an oxidation zone and a reduction zone separated by the substantially gas-impermeable catalytic membrane which comprises the electronically and ionically conducting mixed metal oxides of the above formula. Once in the
MacKay Richard
Sammells Anthony F.
Schwartz Michael
Eltron Research, Inc.
Greenlee Winner and Sullivan P.C.
Kopec Mark
Vijayakumar Kallambella
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