Two component-three dimensional catalysis

Details Two component-three dimensional catalysis Two component-three dimensional catalysis

1997-10-29

2002-03-12

Barry, Chester T. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C422S198000, C422S222000, C210S500250, C210S510100, C502S004000, C502S060000, C502S303000, C502S327000

Reexamination Certificate

active

06355093

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to catalytic partial and full oxidation of hydrocarbons and related reduced species using catalytic membrane reactors. Reactors containing gas-impermeable, solid state membranes with an adherent catalyst layer in combination with fixed (or packed)-bed catalyst are disclosed. Membrane materials, catalyst layers and packed-bed catalysts are selected to achieve a desired selective oxidation reaction. Catalytic membrane reactions include, among others, the partial oxidation of methane or natural gas to synthesis gas.
BACKGROUND OF THE INVENTION
Catalytic membrane reactors using solid state membranes for the oxidation or decomposition of various chemical species have been studied and used previously. One potentially valuable use of such reactors is in the production of synthesis gas. See, for example, Cable et al. EP patent application 90305684.4 (published Nov. 28, 1990) and Mazanec et al. U.S. Pat. No. 5,306,411. Synthesis gas, a mixture of CO and H
2
, is widely used as a feedstock in the chemical industry for production of bulk chemicals such as methanol and liquid fuel oxygenates. For most efficient use in the synthesis of methanol, the ratio of H
2
:CO in synthesis gas should be adjusted to 2:1.
In a catalytic membrane reactor that facilitates oxidation/reduction reactions, a catalytic membrane separates an oxygen-containing gas from a reactant gas which is to be oxidized. Oxygen (O
2
) or other oxygen-containing species (for example, NO
x
or SO
x
) are reduced at one face of the membrane to oxygen anions that are then transported across the membrane to its other face in contact with the reactant gas.
Materials for membranes in catalytic membrane reactors must be conductors of oxygen anions, and the materials must be chemically and mechanically stable at the high operating temperatures and under the harsh conditions required for reactor operation. In addition, provision must be made in the reactor for electronic conduction to maintain membrane charge neutrality. Electronic conductivity in a reactor is necessary to maintain charge neutrality permitting anion conduction through the membrane. Electron conduction can be achieved by adding an external circuit to a reactor which allows for current flow. See: U.S. Pat. Nos. 4,793,904, 4,802,958 and 4,933,054 (all of Mazanec et al.).
Electronic conductivity can also be achieved by doping oxygen-anion conducting materials with a metal ion, as illustrated by U.S. Pat. Nos. 4,791,079 and 4,827,071 (both of Hazbun), to generate dual (electrons and oxygen anions) conducting materials. The disadvantage of this approach is that the dopant metal ions can act as traps for migrating oxygen anions, inhibiting the ionic conductivity of the membrane.
Dual conducting mixtures can also be prepared by mixing an oxygen-conducting material with an electronically-conducting material to form a composite, multi-component, non-single phase material. Problems associated with this method include possible deterioration of conductivity due to reactivity between the different components of the mixture and possible mechanical instability, if the components have different thermal expansion properties.
The preferred method for obtaining electronic conductivity is to use membrane materials which inherently possess this property.
As described in U.S. patent applications (Parent and Grandparent to this case), mixed conducting metal oxides possessing the brownmillerite crystallographic structure can be used to prepare gas-impermeable ceramic membranes for use in membrane reactors for spontaneously separating oxygen from a gas, e.g., from air, on their reducing surface and mediating transfer of this oxygen as oxygen anions to the oxidation surface of the membrane where they can participate in a selected oxidative chemical process. For example, natural gas (predominantly methane) can be spontaneously converted to synthesis gas, a mixture of carbon monoxide (CO) and hydrogen (H
2
) which is useful as a feedstock for preparation of liquid fuels.
The reaction to form synthesis gas is a partial oxidation that is written:
CH
4
+O
2−
→CO+2H
2
+2e
−
FIG. 1
illustrates schematically how this reaction would occur ideally in a ceramic membrane reactor. The membrane of
FIG. 1
illustrated as having a reduction catalyst on the reduction surface and a partial oxidation catalyst on the membrane oxidation surface.
FIG. 1
illustrates that molecular oxygen (O
2
) is reduced at the reducing surface of the membrane to form oxygen anions (O
2−
) which are conducted across the membrane (due to the presence of an oxygen gradient). O
2−
at the oxidizing surface of the membrane reacts with methane to give the partial oxidation product CO and H
2
with H
2
:CO ratio of 2:1.
A problem that occurs with ceramic membrane reactors is that the membrane material itself can be catalytically active toward oxygen anion changing the nature of the oxygen species that are available for reaction at the membrane surface. For example, the membrane material may catalyze reoxidation of oxygen anions to molecular oxygen. The membrane then serves to deliver molecular oxygen to the oxidation zone of the reactor. The presence of molecular oxygen can significantly affect the selectivity of a given reaction. For example, reaction of methane with molecular oxygen leads to deep oxidation of methane generating CO
2
:
CH
4
+2O
2
→CO
2
+2H
2
O.
A membrane that exhibits no substantial reactivity toward oxygen anions, yet retains ionic and electronic conductivity, i.e. a membrane that is not inherently catalytically active toward oxygen, would provide for better reaction selectivity in a membrane reactor. In this case, reactivity could be determined by choice of an adherent catalyst layer on the oxidation surface of the membrane. By appropriate choice of the adherent catalyst layer a high degree of selectivity for a desired oxidation reaction should be achievable.
The use of a membrane material which has minimal catalytic activity towards oxygen separates the oxygen transport properties of the membrane from the catalytic activity. This will allow fine tuning of catalytic activity by catalyst layer choice, in particular it will allow control of the surface oxygen species allowing selection among a variety of oxygen species at the membrane surface O
2−
, O
2
−
(superoxide), O. (radical), peroxo (O
2
2−
), etc.
SUMMARY OF THE INVENTION
This invention provides a catalytic membrane reactor for partial or full oxidation of reduced species, particularly of hydrocarbons. The reactor comprises a gas-impermeable membrane which exhibits ion conductivity. The membrane is also provided with electronic conduction to maintain membrane charge neutrality. Electronic conduction can be provided by an external circuit or the membrane material can itself be an electronic conductor. The reactor has an oxidation zone and a reduction zone separated by the membrane which itself has an oxidation surface exposed to the oxidation zone and a reduction surface exposed to the reduction zone. The oxidation surface of the membrane is, at least in part, covered with an adherent catalyst layer. The reduction surface of the membrane is optionally provided with an oxygen reduction catalyst. The reactor is also optionally provided with a three-dimensional catalyst in the oxidation zone of the reactor in close contact with the adherent layer on the oxidation surface of the membrane.
Preferred membranes of this invention are single phase mixed ionic and electronic conducting ceramics. In this case no external electric circuit is required to maintain membrane charge neutrality. To facilitate selective oxidation, preferred membranes are those that exhibit minimal catalytic activity for oxidation of oxygen anions, e.g., are minimally active for reoxidation of oxygen anions to molecular oxygen, on transport of oxygen anion through the membrane. These membranes deliver minimal amounts of molecular oxygen to the oxidation surface

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