Note: Descriptions are shown in the official language in which they were submitted.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-1-
Title: Electrochemical Process For Oxidation Of Alkanes To Alkenes
FIELD OF THE INVENTION
This invention relates to an electrochemical process for oxidation of an
alkane to the corresponding alkene using an electrochemical cell that has a
proton-conducting medium. The process of the invention is for the selective
production of alkenes while generating electricity and water.
BACKGROUND OF THE INVENTION
An alkane can be converted to the corresponding alkene by
several processes, including partial oxidation and thermal cracking.
According to these processes, for example, propane may be converted to
propylene. Other alkanes can also be similarly converted to a corresponding
alkene, for example: butane to one or more of 1-butene and 2-butene, and
ethyl benzene to styrene.
Propane can be chemically oxidized to a mixture of products including
propylene by reaction with a limited amount of oxygen. Catalysts are known
for the activation of propane. When a mixture of propane and a limited amount
of oxygen is passed over a catalyst a mixture of products is formed, including
propylene, other hydrocarbon products, and oxides of carbon. It is very
difficult to oxidize propane selectively to propylene. Typically, when propane
is
heated to a high temperature, typically several hundreds of degrees Celsius,
the propane is cracked to form a mixture containing hydrogen, propylene,
ethane, methane, ethylene, and higher hydrocarbons. The cracking process
consumes energy. Further, the cracking process is not highly selective to
propylene, and typically operates at low conversion. It is therefore necessary
to separate the products of a catalytic oxidation reaction to obtain propylene
in
a commercially saleable or useable form (e.g., with other reaction products of
the cracking process removed of reduced on a volume percent). Further, the
heat gerierated by the oxidation reaction is recoverable only as process
energy and not as high-grade energy such as electricity.
When a fuel is oxidized in a fuel cell, the products are the oxidation
products from the fuel and electrical energy. Oxide ion conducting solid
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-2-
membranes are used in solid oxide fuel cells (a "SOFC"). In such cells, a
source of oxygen is fed to a cathode catalyst where the oxygen combines with
electrons to form oxide ions. The oxide ions pass through the solid membrane
from the cathode to the anode. At a catalytic anode in a SOFC, oxide ions
react with a fuel to generate oxidation products and electrons. When the fuel
is propane, the oxidation products are usually oxides of carbon. Thus an oxide
ion conducting SOFC can be designed to use propane as fuel. Mazanec et al.
in United States Patent 4,933,054 which issued in 1990, describe an
electrochemical process using oxide ion conducting SOFC at temperatures in
the range of about 500°C to about 950°C for electrochemical
oxidative
dehydrogenation of saturated hydrocarbons. The saturated hydrocarbons
have from 2 to 6 carbon atoms, and include propane, and are converted to the
corresponding unsaturated hydrocarbons, including propylene. Michaels and
Vayenas, in Journal of Catalysis, Volume 85, 477-487 (1984), describe
electrochemical oxidative dehydrogenation of ethyl benzene to styrene in the
vapor phase using SOFC operated at high temperatures (e.g. above 650 °
Celcius).
Proton conducting solids are known, including polymer electrolyte
membranes ("PEM"). PEM are used in H2-O2 fuel cells, an example of which
is as described by Fuglevand et al. in United States Patent 6,030,718. The
hydrogen used as fuel in PEM fuel cells can be generated in several ways.
Propane can be reformed to generate a hydrogen containing fuel for a fuel
girl
cell, and can be used as a coolant for a fuel cell. For example, Ziaka and
Vasileiadis in United States Patent 6,090,312, issued in 2000, disclose
reforming reactions of light hydrocarbons having from 1 to 4 carbon atoms to
generate hydrogen for use as fuel in a fuel cell. Nakagaki et al. in United
States Patent 6,099,983, issued in 2000, discloses reforming of propane to
generate a hydrogen containing gas that is used as fuel in a fuel cell, in
which
the reformed hydrogen containing gas also serves as coolant for the fuel cell.
Each of the above examples uses propane as a source of hydrogen to be
used as fuel, and does not use propane as fuel.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-3-
SUMMARY OF THE INVENTION
One aspect of the present invention relates to operating an alkane fuel
cell with a proton conducting medium that converts an alkane to at least one
corresponding alkene at low temperatures and preferably low pressures.
Another aspect of the present invention relates to operating an alkane
fuel cell with a proton conducting medium that converts an alkane to the
corresponding alkene with a high degree of selectivity.
In accordance with one embodiment of this invention, there is provided
an electrochemical process for oxidation of an alkane to a corresponding
alkene using an electrochemical cell having an anode chamber having an
anode and a cathode chamber having a cathode, the anode chamber and the
cathode chamber separated at least in part by a proton conducting medium,
said process comprising:
(a)providing at least one alkane to the anode chamber;
(b) providing an oxygen containing gas to the cathode chamber;
(c) passing protons through the said medium from the anode chamber
to the cathode chamber
whereby at least a portion of the alkane is converted to a corresponding
alkene.
In one embodiment, the anode comprises at least one metal
catalyst active for activation of the alkane and the anode and cathode are in
electrical contact with each other and the process comprises producing
electrons during the conversion of the alkane to the alkene and the catalytic
cathode comprises at least one metal catalyst active for combination of
oxygen with protons and electrons to form water.
In another embodiment, the process further comprises
maintaining the electrochemical cell at a temperature and a pressure that
maintains the moisture of said medium.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-4-
In another embodiment, the process further comprises providing
the alkane in a gaseous state.
In another embodiment, the alkane is selected from the group
consisting of propane, a mixture of propane and at least one inert gas, a
mixture of propane and at least one inert liquid, and a mixture of
hydrocarbons containing propane, and the process comprises producing
propylene as the corresponding alkene.
In another embodiment, the alkane is selected from the group
consisting of butane, a mixture of butane and at least one inert gas, a
mixture
of butane and at least one inert liquid, and a mixture of hydrocarbons
containing butane, and the process comprises producing at least one of 1-
butene and 2-butene as the corresponding alkene.
In another embodiment, the alkane is selected from the group
consisting of a mixture of ethyl benzene and at least one inert gas, a mixture
of ethyl benzene and at least one inert liquid, and a mixture of hydrocarbons
containing ethyl benzene, and the process comprises producing styrene as
the corresponding alkene.
In another embodiment, the oxygen containing gas is selected
from a group consisting of oxygen, a mixture of oxygen and at least one inert
gas, and air and the process further comprises combining protons which have
passed through the medium and oxygen to produce water.
In another embodiment, the process is operated at a
temperature of at least about 50°C.
In another embodiment, the process is operated at a
temperature in the range of about 50°C to about 155°C.
In another embodiment, the process is operated at a
temperature in the range of about 50°C to about 100°C.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
_5_
In another embodiment, the process is operated at a pressure of
at least atmospheric pressure and below a pressure at which one or more of
the alkane and the alkene will condense to form a liquid phase.
In another embodiment, the pressure is maintained sufficiently
high so as to maintain moistness of the proton-conducting medium.
In another embodiment, the process is operated at a pressure of
at least atmospheric pressure and below a pressure at which one or more of
propane and propylene will condense to form a liquid phase, the pressure
being sufficiently high so as to maintain the moistness of the proton
conducting medium at the operating temperature.
In another embodiment, process is operated at a pressure of at
least atmospheric pressure and below a pressure at which one or more of
butane, and at least one of 1-butene and 2-butene will condense to form a
liquid phase, the pressure being sufficiently high so as to maintain the
moistness of the proton conducting medium at the operating temperature.
In another embodiment, the process is operated at a pressure of
at least atmospheric pressure and below a pressure at which one or more of
ethyl benzene, and styrene will condense to form a liquid phase, the pressure
being sufficiently high so as to maintain the moistness of the proton
conducting medium at the operating temperature.
In another embodiment, the process is operated at a pressure in
the range of about 0.5 atm to about 10 atm.
In another embodiment, the process is operated at about
atmospheric pressure.
In accordance with another aspect of the instant invention, there
is provided an electrochemical apparatus for oxidation of an alkane to a
corresponding alkene comprising:
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-6-
(a) an anode chamber having an anode, the anode comprising a
metal catalyst active for activation of the alkane;
(b) a cathode chamber having a cathode, the cathode
comprising a metal catalyst active for combination of a
proton acceptor with protons; and,
(c) a proton conducting medium positioned in fluid flow
communication with both the anode chamber and the
cathode chamber.
In one embodiment, the proton acceptor comprises oxygen.
In another embodiment, the proton acceptor is a gas selected
from a group consisting of oxygen, a mixture of oxygen and at least one inert
gas, and oxygen is combined with protons that have passed through the
medium and oxygen to produce water.
In another embodiment, the alkane is gaseous.
In another embodiment, the alkane is a linear molecule or a
linear substituent of a cyclic or aromatic molecule.
In another embodiment, the alkane has a carbon chain length of
from 2 to 6 carbon atoms.
In another embodiment, the proton conducting medium is a solid
perfluorosulphonic acid proton conducting membrane.
In another embodiment, the catalytic anode and the catalytic
cathode separately are formed of compressed carbon powder loaded with
metal catalyst, the metal catalyst of the catalytic anode being selected from
metal catalysts active for activation of an alkane, and the metal catalyst of
the
catalytic cathode being selected from metal catalysts active for combination
of
oxygen with protons and electrons to form water.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-7-
In another embodiment, the alkane comprises propane and the
catalytic anode and the catalytic cathode separately are formed of carbon
cloth loaded with metal catalyst, the metal catalyst of the catalytic anode
being selected from metal catalysts active for activation of propane, and the
metal catalyst of the catalytic cathode being selected from metal catalysts
active for combination of oxygen with protons and electrons to form water.
In another embodiment, the alkane comprises butane and the
catalytic anode and the catalytic cathode separately are formed of nickel
mesh impregnated with metal catalyst, the metal catalyst of the catalytic
anode being selected from metal catalysts active for activation of propane,
and the metal catalyst of the catalytic cathode being selected from metal
catalysts active for combination of oxygen with protons and electrons to form
water.
In another embodiment, the metal catalyst active for activation of
alkane is selected from the group consisting of platinum, palladium, silver,
nickel, cobalt, gold, bismuth, manganese, vanadium, ruthenium, copper, zinc,
chromium, iron or indium oxide-stannous oxide mixtures, or any mixtures
thereof.
In another embodiment, the metal catalyst active for activation of
the alkane is selected from the group consisting of nickel, cobalt or a
mixture
of nickel and cobalt.
In another embodiment, the metal catalyst for activation of
alkane is selected from the group consisting of platinum, palladium or a
mixture of platinum and palladium.
In another embodiment, the metal catalyst active for
combination of oxygen with protons and electrons to form water is selected
from the group consisting of nickel, cobalt, gold, bismuth, manganese,
vanadium, ruthenium, copper, zinc, chromium, iron or indium oxide-stannous
oxide mixtures, or any mixtures thereof.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
_$_
In another embodiment, the metal catalyst active for
combination of oxygen with protons and electrons to form water is selected
from the group consisting of nickel, cobalt or a mixture of nickel and cobalt.
In another embodiment, the metal catalyst active for
combination of oxygen with protons and electrons to form water is selected
from the group consisting~of platinum, palladium or a mixture of platinum and
palladium.
In another embodiment, the apparatus is operated at a
temperature of at least about 50°C.
In another embodiment, the apparatus is operated at a
temperature in the range of about 50°C to about 155°C.
In another embodiment, the apparatus is operated at a
temperature in the range of about 50°C to about 100°C.
In another embodiment, the process is operated at a pressure of
at least atmospheric pressure and below a pressure at which one or more of
the alkane and the alkene will condense to form a liquid phase.
In another embodiment, the pressure is maintained sufficiently high so
as to maintain moistness of the proton-conducting medium.
Accordingly, one advantage according to one aspect of the
instant invention is that the conversion of alkanes to alkenes may be
conducted at temperatures below 200°C.
Another advantage of the invention is that according to one aspect, the
process may be used to operate a propane fuel cell that converts propane
with a high degree of selectivity to propylene.
Another advantage of the invention is that according to one aspect, the
process may be used to oxidize propane selectively to propylene at a
temperature lower than a temperature of operation of a SOFC and even at a
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
_g_
temperature below the boiling point of water, and thereby recover water as
liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
These and the other features of the invention will become more
apparent from the following description in which reference is made to the
appended drawings, wherein:
Figure 1 is a schematic diagram of a cell for the electrochemical
oxidation of an alkane to the corresponding alkene.
Figure 2 is a diagram showing the electrochemical reactions
comprising the electrochemical oxidation of propane to propylene and the
electrochemical reduction of oxygen to water in the fuel cell of Figure 1.
Figure 3 is a diagram comprising three plots of the relationship
between current and potential for operation of a laboratory scale version of
the cell for the electrochemical oxidation of propane to propylene illustrated
in
Figure 2, using different compositions of the catalytic anode at different
operating temperatures and pressures
Figure 4 is a diagram showing the relationship between current and
potential for operation of a laboratory scale version of the cell illustrated
in
Figure 1 for the electrochemical oxidation of butane to butene at atmospheric
pressure and 90°C.
Figure 5 is a diagram showing the relationship between current and
potential for operation of a laboratory scale version of the cell illustrated
in
Figure 1 for the electrochemical oxidation of ethyl benzene to styrene at
atmospheric pressure and 90°C.
DETAILED DESCRIPTION OF THE INVENTION
The electrochemical cell and process for electrochemical
oxidation of an alkane to one or more corresponding alkenes will now be
described with reference to FIGURES 1 through 5. The process of this
invention is applicable to any alkane. The alkane may be a linear alkane (e.g.
propane and butane) or a cyclic or aromatic alkane (e.g. cyclohexane, ethyl
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-10-
benzene). The linear alkane may have a straight chain or may be branched.
Preferably, the alkane is a linear alkane or a substituted cyclic or a
substituted
aromatic alkane (e.g. tetrahydronaphthalene). In the case of a substituted
cyclic or a substituted aromatic alkane, the reaction is preferably targeted
at
converting the substituant carbon chain and not the cyclic or aromatic portion
of the molecule. For example, the organic feedstock may be 2-methyl alkane
so that, e.g., 2-methylhexane could be converted to 2-methyl-1-hexene plus
the 2-methyl-2-hexene isomer. More preferably, the alkane is a linear alkane.
Most preferably, the alkane is a linear, non-branched alkane. The alkane may
have a chain length of from 2 to 12 carbon atoms and preferably from 2 to 6
carbon atoms.
The alkane may be in any form that can flow so as to flow through the
electrochemical cell. The alkane may be a liquid or a gas. If the alkane is a
light hydrocarbon, then the organic feedstock is preferably in the form of a
gas
since otherwise an elevated pressure would be required to cause the
hydrocarbon to be in its liquid form. Similarly, if the organic feedstock is a
mid-
range hydrocarbon (e.g. decane) the hydrocarbon is preferably in the form of
a liquid since otherwise a relatively high temperature would be required to
use
the hydrocarbon in its gaseous state.
According to this process, one or more bonds in the alkane is
converted to an alkene. Thus this process may be used to convert propane to
propylene or to convert butane to 1-butene and/or 2-butene. Preferably, the
alkene generated by the process of the present invention is reagent grade
alkene. The reaction has a high degree of selectivity. By this it is meant
that a
high percentage, and preferably substantially all, of the alkane that is
consumed by the process is converted to the corresponding alkene (or if there
is more than one corresponding alkene, then to one or more of the
corresponding alkenes) and that only a minor portion, and preferably
essentially none, of the corresponding alkyne is produced. Unlike catalytic
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-11-
cracking, the carbon chain length of the alkane in the feedstock is
essentially
not altered.
For purpose of illustration only, the process of the present invention will
be described for the electrochemical conversion of propane to propylene.
Application of the process for the conversion of alkane anode feeds, including
propane, butane and ethyl benzene separately, will then be illustrated by a
series of non-limiting examples.
The process of this invention may be conducted in any electrochemical
cell that has at least one anode and at least one cathode that are each in
ionic
contact with a proton-conducting medium. In one embodiment, the
electrochemical cell used in the invention has one anode, one cathode and a
proton-conducting medium.
In the embodiment of Figure 1 electrochemical cell 10 has a body 11
enclosing an anode chamber 12 on one side 14 of a proton conducting
medium 16, and a cathode chamber 18 on another side 20 of said medium
16. Anode chamber 12 has a first inlet 22 and a first outlet 24. Cathode
chamber 18 has a second inlet 26 and a second outlet 28. Body 11 and the
proton conducting medium 16 are electrically insulated from each other by
insulators 29. A first current collector 40 is in electrical contact with
catalytic
anode 32. A second current collector 42 is in electrical contact with
catalytic
cathode 34. It will be appreciated that cell 10 may be of a variety of
configurations provided that protons are produced at least one anode
chamber and transmitted to at least one cathode chamber through a proton
conducting medium.
In one embodiment, catalytic anode 32, catalytic cathode 34 and solid
proton conducting membrane 30, which functions as the proton-conducting
medium, are assembled as a membrane electrode assembly (MEA). However
it will be appreciated that each of these elements may be separately
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-12-
assembled into a cell and need not be positioned adjacent each other as
shown in Figure 1.
The proton-conducting medium can be made from any material that
can transfer protons from catalytic anode 32 to catalytic cathode 34.
Commercially available proton conducting materials suitable for membrane 30
include perfluorosulphonic acid polymer available under the trademark
NAFIONT"" (Du Pont de Nemours and Company), Gore SelectT"" (W.L. Gore
and Associates) and ion-exchange amide systems. Membrane materials that
may be used as the proton conducting medium 16 include membranes of
modified perfuorinated sulphonic acid polymer, polyhydrocarbon sulphonic
acid and composites of two or more kinds of proton exchange membranes
can be used. Membranes of polyethylene and polypropylene sulphonic acid,
polystyrene sulphonic acid and other polyhydrocarbon-based sulphonic acids
(such as membranes made by RAI Corporation, USA) may also be used
depending on the temperature and duration of fuel cell operation. Composite
membranes consisting of two or more types of proton-conducting cation-
exchange polymers with differing acid equivalent weights, or varied chemical
composition (such as modified acid group or polymer backbone), or varying
water contents, or differing types and extent of cross-linking (such as cross
linked by multivalent cations e.g., AI3+, Mgz+ and the like) may be used.
Catalytic anode 32 comprises or consists essentially of a first metal
catalyst 36. First metal catalyst 36 is selected from metal catalysts active
for
activation of an alkane, for example propane. Activation of an alkane is
defined as catalyzing the oxidative dehydrogenation of alkanes to the
corresponding alkenes. Examples of metals and metal oxides useful in
activating an alkane include platinum, palladium, silver, nickel, cobalt,
gold,
bismuth, manganese, vanadium, ruthenium, copper, zinc, chromium, iron or
indium oxide-stannous oxide mixtures, or any mixtures of said metals and
metal oxides. In a preferred embodiment, the first metal catalyst 36 of the
catalytic anode 32 is selected from nickel, cobalt, platinum, palladium, a
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-13-
mixture of platinum and palladium, or mixtures thereof. In a more preferred
embodiment, the first metal catalyst 36 of the catalytic anode 32 is selected
from platinum, palladium or a mixture of platinum and palladium. Generally,
the selection of the first metal catalyst may be based on the activity of the
metal for thermo-catalytic processes with the selected alkane.
Catalytic cathode 34 may comprise or consist essentially of second
metal catalyst 38. Second metal catalyst 38 is selected from metal catalysts
active for combining oxygen with protons and electrons to form water.
Examples of metals and metal oxides useful in combining oxygen with protons
and electrons to form water include platinum, palladium, silver, nickel,
cobalt,
gold, bismuth, manganese, vanadium, ruthenium, copper, zinc, chromium,
iron or indium oxide-stannous oxide mixtures, or any mixtures of said metals
and metal oxides. In a preferred embodiment, the second metal catalyst 38 of
the catalytic cathode 34 is selected from nickel, cobalt, platinum, palladium,
a
mixture of platinum and palladium, a mixture of nickel and cobalt or mixtures
thereof. In a more preferred embodiment, the second metal catalyst 38 of the
catalytic cathode 34 is selected from platinum, palladium or a mixture of
platinum and palladium.
Anode 32 may be constructed from first metal catalyst 36. Alternately,
anode 32 may comprise a support loaded with a first metal catalyst 36. Thus,
the catalytic metal may be plated on or other wise associated with a support.
The support may be inert to the reaction (i.e. it does not electrochemically
affect the reaction in anode chamber 12 or at least does not deleteriously
affect the reaction). Alternatively, the support may be a first catalytic
metal.
Similarly, cathode 34 may be constructed from second metal catalyst 38 or
may comprise a support loaded with a second metal catalyst 38. For
example, the support of catalytic anode 32 or catalytic cathode 34 may be
formed from compressed carbon powder onto which has been deposited
(loaded) metal catalyst 36 or 38 respectively, carbon cloths supporting metal
catalyst 36 and 38 respectively or nickel mesh impregnated with metal
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
- 14-
catalyst 36 and 38 respectively. The catalytic anode 32 and catalytic cathode
34 can be assembled according to established methods, for example, by the
methods described in United States Patent 6,294,068 granted to Petrovic et
al. on September 25, 2001.
In the process of the present invention, the alkane is provided to anode
chamber by any method known in the art and may be removed therefrom by
any method known in the art. Preferably, the alkane is gaseous since the
viscosity of the liquid to flow through anode chamber 12 is less than if the
hydrocarbon feed was in the liquid state. For example, in the embodiment of
Figure 1, an anode stream containing propane is fed through first inlet 22
into
anode chamber 12- and exits anode chamber 12 through first outlet 24, as
indicated by arrows 44. Anode chamber 12 may include one or more inlets 22
and one or more outlets 24.
The alkane may be fed alone through one or more first inlets 22 info
anode chamber 12, or as a mixture containing an alkane diluted with one or
more inert gases such as nitrogen, helium, neon, argon, krypton, xenon or
any other gas, including steam that does not deleteriously interfere with the
oxidative dehydrogenation of the alkane. The alkane can also be mixed with
other hydrocarbon gases, including a mixture with methane. It will be
appreciated that the hydrocarbon feedstock fed to treated in anode chamber
12 may comprise a mixture of alkanes so as to obtain a mixture of the
corresponding alkenes. In such a case, each different alkane may not be
converted to the same degree. The alkane or alkanes may also be fed
through first inlet 22 into anode chamber 12 by themselves, or with an inert
carrier liquid (e.g. nitrogen or argon) or with a pure (compressed) liquid
(e.g.
hexane) that can be used at the temperatures at which electrochemical cell 10
operates.
A cathode stream which does not deleteriously interfere with the
reaction in cathode chamber 18 is fed through one or more second inlets 26
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
_ _ .» o v t p V 7
-15-
into cathode chamber 18 and exits cathode chamber 18 through one or more
second outlets 28, as indicated by arrows 46.1n one embodiment, the cathode
stream may consist of or include any proton acceptor (a medium that will
accept the protons which are transferred to cathode chamber 12). The
medium may be a liquid or a gas and is preferably a gas. The gas may be a
halogen such as chlorine that when combined with protons would form HCI.
Preferably, the cathode stream consists of or includes oxygen. The stream
may contain oxygen alone, air or a mixture of oxygen and at least one inert
gas. The inert gas may be selected from nitrogen, helium, neon, argon,
krypton, xenon or any other gas that does not interfere with the reduction of
oxygen to water. It will be recognized that the direction of arrows 44 and the
direction of arrows 46 is shown for purposes of illustration only, and are not
to
be construed as indicating that the anode stream and the cathode stream
must necessarily flow in the same direction across respectively first side 14
and second side 20 of medium 16. For example, the flow may be counter
current or concurrently through chambers 12, 18.
The process of the present invention comprises an anode reaction and
a cathode reaction. Referring to Figure 1, in anode chamber 12, an alkane is
activated to form the corresponding alkene, protons, and electrons. Protons
pass through proton conducting medium 16 from anode chamber 12 to
cathode chamber 18. Electrons are collected at catalytic anode 32 by first
current collector 40 and are conveyed to catalytic cathode 34 by second
current collector 42. First current collector 40 and second current collector
42
may be directly connected to each other or may be connected via an external
electrical load (not illustrated) by electrical leads 64. If employed, the
load
(which may be a fixed or variable resistance controller) is used to control
the
electrical flow to cathode chamber 18. The leads are insulated to prevent
electrical contact with body 11. Electrons combine with protons and oxygen
at active sites of the second metal catalyst 38 of the catalytic cathode 34 to
form water. The overall process is the oxidation of an alkane to the
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-16-
corresponding alkene, and the formation of water from oxygen, protons and
electrons.
In one embodiment, with reference to Figure 2, propane 50 (C3H$) is
activated at active sites of first metal catalyst 36 of catalytic anode 32 to
form
propylene 52 (C3H6), protons 54 (H+), and electrons 56 (e ) according to
Equation 1. Protons 54 pass through proton conducting membrane 30 from
catalytic anode 32 to catalytic cathode 34 as indicated by arrow 58. Protons
54 combine with electrons 56 and oxygen 60 (02) at active sites of second
metal catalyst 38 of catalytic cathode 34 to form water 62 (H20) according to
.
Equation 2. The overall process is oxidation of propane 50 to propylene 52
and the formation of water 62 from oxygen 60, protons 54 and electrons 56,
according to Equation 3. Electrons 56 are collected at catalytic anode 32 by
first current collector 40 and are conveyed to catalytic cathode 34 by second
current collector 42. First current collector 40 and second current collector
42
are connected to an external electrical load (not illustrated) by electrical
leads
64, as shown in Figure 1, the leads being insulated to prevent electrical
contact with body 11.
C3H$ -~ C3H6 + 2 H+ + 2e [1]
02 + 4 H+ + 4 a -3 2 H20 [2]
2 C3Hs + 02 -~ 2 C3H6 + 2 H20 [3]
The process of the present invention may be conducted at a
temperature below about 200°C. The lower range of the temperature is
premised upon the reaction kinetics. As the temperature is reduced, the
reaction rate decreases. Preferably, the process is conducted at a
temperature from about 20°C to about 155°C, preferably from
about 50°C to
about 155°C and most preferably from about 50°C to about
100°C. If the
temperature is below about 50°C, then the reaction proceeds relatively
slowly.
The process may be conducted below this temperature if the rate of reaction
is acceptable. If the process is conducted at a temperature below about
100°C
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-17-
(i.e. below the boiling point of water at atmospheric pressure), then the
resultant water will be liquid. If the process is conducted above atmospheric
pressure, then, liquid water may be produced at an even higher temperature.
The process may be operated at higher temperatures provided that cell 10
maintains it structural integrity at those temperatures. The process of the
present invention may also be operated wherein the temperature in the anode
chamber 12 can be different than the temperature in the cathode chamber 18.
Preferably the temperature in anode chamber 12 is substantially the same as
the temperature in cathode chamber 18. The use of a lower operating
temperature permits better control of the feedstock and the product and a
reduced level of side reactions. A lower temperature may increase the
longevity of some membranes.
For example, in one embodiment, when cell 10 uses NAFIONT"" as
proton conducting membrane 30, the oxidation of propane to propylene may
be conducted at a temperature in the range of about 65°C to about
95°C.
When the temperature is below 65°C the rate of the reaction is
slow. When
the temperature of the reaction is above 95°C it is necessary to
operate cell
10 at a pressure greater than atmospheric pressure to ensure that NAFIONT""
membrane 30 does not lose structural water, and thereby remains moist and
maintains proton-conducting capability.
Optionally, means may be provided for humidifying one or both of
chambers 12, 18, such as by humidifying anode chamber stream 44 before
first inlet 22 and/or cathode chamber stream 46 before second inlet 26 to
provide sufficient water to prevent membrane 16 from drying out at higher
operating temperatures. However, membranes that can conduct protons
without having to remain moist, may also serve as the proton conducting
medium 16 in the present invention and reduce or eliminate the need to
humidify any of the streams.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
- 18-
The process of the present invention may be conducted at a pressure
from about 0.1 atmospheres (atm) to about 100 atm, preferably from about
0.5 atm to about 10 atm and more preferably from about 1 atm to about 5 atm.
Further, preferably the pressure is at least atmospheric pressure, thereby
providing for a high concentration of alkane at catalytic anode 32 and a high
concentration of oxygen at catalytic cathode 34.
The process of the present invention is preferably operated at a
pressure below a pressure at which the alkane to be oxidized, the
corresponding alkene or a combination of the alkane and the alkene would
condense to form a liquid phase at the temperature of the reaction.
Accordingly, the shorter the carbon chain, the higher the preferred pressure
may be.
The pressure in the anode chamber 12 can be different than the
pressure in the cathode chamber 18. Preferably the pressure in anode
chamber 12 is substantially the same as the pressure in cathode chamber 18,
thereby reducing or essentially reducing stress on proton conducting medium
16 and crossover. Crossover is a process where the alkane feed permeates
through proton conducting medium 16 and combines with oxygen on the
catalytic surface of the cathode. Crossover lowers the efficiency of cell 10,
reduces performance and generates heat in the fuel cell. Factors which
lower the occurrence of crossover include a lower flow rate of the alkane, a
lower concentration of the alkane, operation of the electrochemical cell at a
lower temperature, and minimizing the access of the alkane to the proton
conducting medium, such as by the design of the anode with hydrophobic and
hydrophilic regions. It is appreciated by those skilled in the art that an
amount
of crossover may occur without deleteriously affecting the commercial
usability of the electrochemical cell in the present invention. Unbalanced
pressures can lead to rupture of the membrane and possible crossover.
However, it will be appreciated that stress on the proton conducting medium
16 during the operation of the cell as a result of a difference in pressure
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-19-
between the anode chamber and the cathode chamber can be reduced or
essentially reduced by structurally reinforcing the proton conducting medium
16.
As the pressure is increased, the concentration of alkane at the
catalyst sites is increased and the flux is increased. Further, at increased
pressures, the propensity for water to evaporate is reduced, and hence this
may improve membrane life and activity.
Operation of cell 10 to oxidize propane to propylene will now be
illustrated with reference to Figures 1 through 3 by three non-limiting
examples. Further examples will illustrate operation of the cell to oxidize
each
of butane in Example 4, and ethyl benzene in Example 5. The data are in
each case for operation of an unoptimized cell 10 design and unoptimized
operating parameters. It will be recognized that improved performance of cell
10 may be obtained under different operating conditions using amendments to
the design for cell 10 without departing from the spirit of the present
invention.
Example 1:
Laboratory test equipment was constructed including laboratory scale
cell 10. Laboratory scale cell 10 had a MEA 16 having an effective surface
area of approximately 1 square centimeter for catalytic anode 32. Catalytic
anode 32 comprised compressed Teflonized carbon powder loaded with
platinum as first metal catalyst 36. Catalytic cathode 34 comprised the same
material as catalytic anode 32. The anode chamber stream comprised a
mixture of propane (10% by volume) diluted with nitrogen as an inert diluent.
Oxygen was fed into cathode chamber 18. Open circuit potentials up to 555
millivolts were obtained when cell 10 was operated at.atmospheric pressure
and at temperatures in the range 50°C to 95°C. A series of
resistances
ranging from 0.1 ohms to 1,000,000 ohms was applied as an external circuit
load across electrical leads 64 of cell 10. Referring to Figure 3, it was
found
that the current and the potential provided by the cell varied with the load,
as
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-20-
illustrated by line 70 for operation of cell 10 at atmospheric pressure and at
a
temperature of 85°C. For example, for a load of 1.0 ohm the potential
was
found to be 24 millivolts and the current was 24 milliamps. Samples of the
anode chamber effluent from first outlet 24 were collected into a gas
collection
cell for use in an infrared spectrometer. Propylene was detected in the
infrared spectrum of the anode chamber effluent in amounts corresponding
closely to the amounts expected from the current generated by laboratory
scale cell 10. Conversion of the alkane to alkene was about 4 - 5%. N o
propyne was detected. However, higher conversion rates could be obtained
by reducing the flow rate of the hydrocarbon feedstock. Thus the selectivity
to
propylene as opposed to propyne by electrochemical oxidation of a mixture
containing 10% propane was high. The alkene in the effluent from the
process could be separated from the alkane in the effluent by means of the
different boiling points of the compounder.
Example 2:
Pure propane was fed as anode chamber feed to laboratory cell 10
having the same catalytic anode 32 and the same catalytic cathode 34 as
were used in Example 1. Oxygen was the cathode chamber feed. The
operating pressure in both of anode chamber 12 and cathode chamber 18
was 44 Asia. (about 4 atm) and the temperature of cell 10 was 135°C.
The
open circuit potential generated was 464 millivolts. When an electrical load
was connected across electrical leads 64, it was found that the current and
the potential provided by the cell varied with the load, as illustrated by
line 72
in Figure 3. When the external circuit load was 1.0 ohm, the potential
generated was 42 millivolts and the current generated was 42 milliamps.
Propylene was detected in the infrared spectrum of the anode chamber
effluent in amounts corresponding closely to the amounts expected from the
current generated by laboratory scale cell 10. Conversion was about 4 - 5%.
No propyne was detected. Thus, the oxidation of propane to propylene as
opposed to propyne was selective.
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-21 -
Example 3:
Pure propane was fed as anode chamber feed to laboratory cell
having a catalytic anode 32 comprising compressed Teflonized carbon
5 powder loaded with palladium as first metal catalyst 36 and the same
catalytic
cathode 34 as was used in Example 1 and Example 2. Oxygen was the
cathode chamber feed. The operating pressure in both of anode chamber 12
and cathode chamber 18 was 44 psia and the temperature of cell 10 was
135°C. The open circuit potential generated was 353 millivolts. When an
10 electrical load was connected across electrical leads 64, it was found that
the
current and the potential provided by the cell varied with the load, as
illustrated by line 74 in Figure 3. When the external circuit load was 1.0
ohm,
the potential generated was 12 millivolts and the current generated was 12
milliamps. Propylene was detected in the infrared spectrum of the anode
chamber effluent in amounts corresponding closely to the amounts expected
from the current generated by laboratory scale cell 10. Conversion was about
4 - 5%. No propyne was detected. Thus, the oxidation of propane to
propylene as opposed to propyne was selective.
Example 4:
Pure butane was fed as anode chamber feed to laboratory cell
10 having a catalytic anode 32 comprising compressed Teflonized carbon
powder loaded with 4.2% platinum and supported on carbon cloth as first
metal catalyst 36. The catalytic cathode was a similar catalytic cathode 34 to
that used in Examples 1 through 3. The electrodes had a catalyst layer that
was 0.5 - 1 mm thick. Oxygen was the cathode chamber feed. The
operating pressure in both of anode chamber 12 and cathode chamber 18
was atmospheric pressure and the temperature of cell 10 was 88°C. The
open circuit potential generated was 655 millivolts after operating for 2
hours,
which decreased to 644 millivolts after a further 2 hours. When an electrical
load was connected across electrical leads 64, it was found that the current
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-22-
and the potential provided by the cell varied with the load. When the external
circuit load was 1.0 ohm, the potential generated was 305 millivolts and the
current generate was 305 milliamps after operating for 2 hours. Figure 4 is a
diagram showing the relationship between current and potential for operation
of a laboratory scale version of the cell illustrated in Figure 1 for the
electrochemical oxidation of butane to butene at atmospheric pressure and
90°C. wherein the catalyst layer was at least 2 mm thick.
When the active catalyst of the catalytic anode was 2.4%
platinum and laboratory cell 10 was operated at 88°C, the initial open
circuit
potential was 494 millivolts, which decreased to 429 milllivolts after
operating
for 2 hours.
When the active catalyst of the catalytic anode was 9.1
palladium, the open circuit potential was 368 millivolts at an operating
temperature of 90°C. Laboratory cell 10 was shut down after 3 hours,
and
then restarted after a further 19 hours. The open circuit potential then was
255
millivolts at an operating temperature of 82°C and 392 millivolts at
102°C.
When a thick layer (i.e. over 1 mm) of catalyst was used, the
performance was reduced, as illustrated in Figure 4, and the maximum power
density was 11 milliwatts per square centimeter. No butyne was observed.
Example 5:
Nitrogen gas was passed through liquid ethyl benzene at 90°C
to form a feed gas stream. The gas stream comprising ethyl benzene in
nitrogen as an inert carrier gas was fed at 15 milliliters per minute as anode
chamber feed to laboratory cell 10 having a catalytic anode 32 comprising
compressed Teflonized carbon powder loaded with palladium as first metal
catalyst 36 and the same catalytic cathode 34 as was used in Examples 1
through 3. The electrodes had a catalyst layer that was 0.5 - 1 mm thick.
Oxygen was the cathode chamber feed. The operating pressure in both of
CA 02428200 2003-05-07
WO 02/38832 PCT/CA01/01603
-23-
anode chamber 12 and cathode chamber 18 was atmospheric pressure and
the temperature of cell 10 was 90°C. The open circuit potential
generated
was 460 millivolts. When an electrical load was connected across electrical
leads 64, it was found that the current and the potential provided by the cell
varied with the load, as illustrated for one experiment in Figure 5 wherein
the
catalyst layer was over 2 mm thick. When the external circuit load was 1.0
ohm, the potential generated was 25 millivolts and the current generated was
25 milliamps. No ethynyl benzene was observed.
