U.S. patent application number 12/604629 was filed with the patent office on 2010-02-18 for oxidation catalyst and process.
This patent application is currently assigned to MONSANTO TECHNOLOGY LLC. Invention is credited to James P. Coleman, Martin P. McGrath.
Application Number | 20100041546 12/604629 |
Document ID | / |
Family ID | 27734707 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100041546 |
Kind Code |
A1 |
Coleman; James P. ; et
al. |
February 18, 2010 |
OXIDATION CATALYST AND PROCESS
Abstract
An oxidation catalyst is prepared by pyrolyzing a source of iron
and a source of nitrogen on a carbon support. Preferably, a noble
metal is deposited over the modified support which comprises iron
and nitrogen bound to the carbon support. The catalyst is effective
for oxidation reactions such as the oxidative cleavage of tertiary
amines to produce secondary amines, especially the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)-glycine.
Inventors: |
Coleman; James P.; (Maryland
Heights, MO) ; McGrath; Martin P.; (North Andover,
MA) |
Correspondence
Address: |
SENNIGER POWERS LLP (MTC)
100 NORTH BROADWAY, 17TH FLOOR
ST LOUIS
MO
63102
US
|
Assignee: |
MONSANTO TECHNOLOGY LLC
St. Louis
MO
|
Family ID: |
27734707 |
Appl. No.: |
12/604629 |
Filed: |
October 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11274555 |
Nov 15, 2005 |
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12604629 |
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10366947 |
Feb 14, 2003 |
7129373 |
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11274555 |
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60356916 |
Feb 14, 2002 |
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Current U.S.
Class: |
502/177 ;
502/200 |
Current CPC
Class: |
B01J 35/023 20130101;
B01J 31/1805 20130101; B01J 2531/842 20130101; B01J 23/8913
20130101; B01J 37/0205 20130101; Y02P 20/50 20151101; Y02P 20/588
20151101; B01J 23/626 20130101; B01J 31/1616 20130101; B01J 35/0033
20130101; C07F 9/3813 20130101; B01J 23/8906 20130101; B01J 35/10
20130101; B01J 23/74 20130101; B01J 35/1033 20130101; B01J 37/082
20130101; B01J 31/183 20130101; B01J 37/0203 20130101; B01J 23/89
20130101; C07C 209/68 20130101; B01J 21/18 20130101; B01J 2531/845
20130101 |
Class at
Publication: |
502/177 ;
502/200 |
International
Class: |
B01J 27/22 20060101
B01J027/22; B01J 27/24 20060101 B01J027/24 |
Claims
1. An oxidation catalyst comprising a particulate carbon support
having a transition metal/nitrogen composition thereon, said
transition metal being selected from the group consisting of iron
and cobalt, wherein said transition metal/nitrogen composition
comprises an iron or cobalt nitride, an iron or cobalt
nitride-carbide, or combinations thereof, and the nitrogen of said
transition metal/nitrogen composition constitutes from about 1% to
about 5% by weight of said catalyst, wherein said catalyst further
comprises a noble metal at a surface of said particulate carbon
support having a transition metal/nitrogen composition thereon.
2. An oxidation catalyst as set forth in claim 1 comprising said
transition metal/nitrogen composition in such proportion that said
Fe, Co or the sum of (Fe+Co) of said transition metal/nitrogen
composition constitutes at least about 0.1% by weight of said
catalyst.
3. An oxidation catalyst as set forth in claim 2 wherein said Fe,
Co or the sum of (Fe+Co) of said transition metal/nitrogen
composition constitutes from about 0.1% to about 10% by weight of
said catalyst.
4. An oxidation catalyst as set forth in claim 3 wherein said Fe,
Co or the sum of (Fe+Co) of said transition metal/nitrogen
composition constitutes from about 0.25% to about 7% by weight of
said catalyst.
5. An oxidation catalyst as set forth in claim 4 wherein said Fe,
Co or the sum of (Fe+Co) of said transition metal/nitrogen
composition constitutes from about 0.5% to about 5% by weight of
said catalyst.
6. An oxidation catalyst as set forth in claim 1 wherein the atomic
ratio of transition metal to nitrogen in said transition
metal/nitrogen composition is from about 1:4 to about 3:1.
7. An oxidation catalyst as set forth in claim 1 wherein said
transition metal/nitrogen composition comprises an iron or cobalt
nitride.
8. An oxidation catalyst as set forth in claim 7 wherein said
transition metal/nitrogen composition comprises .xi.-Fe.sub.3N.
9. An oxidation catalyst as set forth in claim 8 wherein said
transition metal/nitrogen composition further comprises an iron
species selected from the group consisting of iron oxides, iron
carbides, and metallic iron.
10. An oxidation catalyst as set forth in claim 7 wherein said
transition metal/nitrogen composition comprises an iron nitride and
superparamagnetic iron.
11. An oxidation catalyst as set forth in claim 10 wherein said
transition metal/nitrogen composition as determined from Mossbauer
spectra comprises from about 30% to about 70% by weight
.xi.-Fe.sub.3N and from about 5% to about 20% by weight
superparamagnetic iron.
12. An oxidation catalyst as set forth in claim 11 wherein said
transition metal/nitrogen composition as determined from Mossbauer
spectra further comprises an additional iron species selected from
.alpha.-iron, isolated iron atoms and mixtures thereof.
13. An oxidation catalyst as set forth in claim 12 wherein said
transition metal/nitrogen composition as determined from Mossbauer
spectra comprises from about 15% to about 25% by weight
.alpha.-iron, and from about 10% to about 20% by weight isolated
iron atoms.
14. An oxidation catalyst as set forth in claim 1 wherein said
transition metal/nitrogen composition comprises an iron or cobalt
nitride-carbide.
15. An oxidation catalyst as set forth in claim 1 wherein said
transition metal/nitrogen composition is fixed to the carbon
support.
16. An oxidation catalyst as set forth in claim 1 wherein said
transition metal/nitrogen composition comprises an active phase for
the catalysis of a redox reaction.
17. An oxidation catalyst as set forth in claim 16 wherein said
active phase is effective for catalyzing the reduction of molecular
oxygen.
18. An oxidation catalyst as set forth in claim 1 wherein at least
about 95% of the particles of said carbon support are from about 2
to about 300 .mu.m in their largest dimension.
19. An oxidation catalyst as set forth in claim 18 wherein at least
about 98% of the particles of said carbon support are from about 2
to about 200 .mu.m in their largest dimension.
20. An oxidation catalyst as set forth in claim 19 wherein about
99% of the particles of said carbon support are from about 2 to
about 150 .mu.m in their largest dimension.
21. An oxidation catalyst as set forth in claim 20 wherein about
95% of the particles of said carbon support are from about 3 to
about 100 .mu.m in their largest dimension.
22. An oxidation catalyst as set forth in claim 1 wherein said
carbon support has a BET surface area of from about 500 to about
2100 m.sup.2/g.
23. An oxidation catalyst as set forth in claim 22 wherein said
carbon support has a BET surface area of from about 750 to about
2100 m.sup.2/g.
24. An oxidation catalyst as set forth in claim 23 wherein said
carbon support has a BET surface area of from about 750 to about
1750 m.sup.2/g.
25. An oxidation catalyst as set forth in claim 1 wherein said
particulate carbon support is porous and said transition
metal/nitrogen composition is substantially evenly distributed
throughout the carbon particle.
26. An oxidation catalyst as set forth in claim 1 wherein said
carbon support has a pore volume of from about 0.1 to about 2.5 ml
per gram of catalyst.
27. An oxidation catalyst as set forth in claim 26 wherein said
carbon support has a pore volume of from about 0.2 to about 2.0 ml
per gram of catalyst.
28. An oxidation catalyst as set forth in claim 27 wherein said
carbon support has a pore volume of from about 0.4 to about 1.7 ml
per gram of catalyst.
29. An oxidation catalyst as set forth in claim 1 wherein said
noble metal is selected from the group consisting of platinum,
palladium, rhodium, iridium, osmium, ruthenium, and combinations
thereof.
30. An oxidation catalyst comprising a particulate carbon support
having a transition metal/nitrogen composition thereon, said
transition metal being selected from the group consisting of iron
and cobalt, wherein said transition metal/nitrogen composition
comprises an iron or cobalt nitride, an iron or cobalt
nitride-carbide, or combinations thereof, the nitrogen of said
transition metal/nitrogen composition constitutes from about 0.1%
to about 7% by weight of said catalyst, and said particulate carbon
support has a BET surface area of from about 500 to about 2100
m.sup.2/g, wherein said catalyst further comprises a noble metal at
a surface of said particulate carbon support having a transition
metal/nitrogen composition thereon.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/274,555, filed Nov. 15, 2005, which is a
divisional of U.S. patent application Ser. No. 10/366,947, filed
Feb. 14, 2003, now U.S. Pat. No. 7,129,373, which claims the
benefit of U.S. Provisional Application Ser. No. 60/356,916, filed
Feb. 14, 2002, the entire disclosures of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] This invention is directed to redox reaction catalysts, and
more particularly to improved catalysts effective for the reduction
of molecular oxygen in the conduct of oxidation reactions. The
invention more particularly relates to the preparation of secondary
amines by catalytic oxidative cleavage of tertiary amines, e.g.,
the preparation of N-(phosphonomethyl)glycine by catalytic
oxidation of N-(phosphonomethyl)iminodiacetic acid.
[0003] N-(phosphonomethyl)glycine (known in the agricultural
chemical industry as "glyphosate") is described in Franz, U.S. Pat.
No. 3,799,758. N-(phosphonomethyl)glycine and its salts are
conveniently applied as a post-emergent herbicide in an aqueous
formulation. It is a highly effective and commercially important
broad-spectrum herbicide useful in killing or controlling the
growth of a wide variety of plants, including germinating seeds,
emerging seedlings, maturing and established woody and herbaceous
vegetation, and aquatic plants.
[0004] Various methods for making N-(phosphonomethyl)glycine are
known in the art. Franz, U.S. Pat. No. 3,950,402, teaches that
N-(phosphonomethyl)glycine may be prepared by the liquid phase
oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid
(sometimes referred to as "PMIDA") with oxygen in the presence of a
catalyst comprising a noble metal deposited on the surface of an
activated carbon support:
##STR00001##
Other by-products also may form, such as formic acid, which is
formed by the oxidation of the formaldehyde by-product; and
aminomethylphosphonic acid, which is formed by the oxidation of
N-(phosphonomethyl)glycine. Even though the Franz method produces
an acceptable yield and purity of N-(phosphonomethyl)glycine, high
losses of the costly noble metal into the reaction solution (i.e.,
"leaching") result because under the oxidation conditions of the
reaction, some of the noble metal is oxidized into a more soluble
form and both N-(phosphonomethyl)iminodiacetic acid and
N-(phosphonomethyl)glycine act as ligands which solubilize the
noble metal.
[0005] In U.S. Pat. No. 3,969,398, Hershman teaches that activated
carbon alone, without the presence of a noble metal, may be used to
effect the oxidative cleavage of N-(phosphonomethyl)iminodiacetic
acid to form N-(phosphonomethyl)glycine. In U.S. Pat. No.
4,624,937, Chou further teaches that the activity of the carbon
catalyst taught by Hershman may be increased by removing the oxides
from the surface of the carbon catalyst before using it in the
oxidation reaction. See also, U.S. Pat. No. 4,696,772, which
provides a separate discussion by Chou regarding increasing the
activity of the carbon catalyst by removing oxides from the surface
of the carbon catalyst. Although these processes obviously do not
suffer from noble metal leaching, they do tend to produce greater
concentrations of formaldehyde by-product when used to effect the
oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid. This
formaldehyde by-product is undesirable because it reacts with
N-(phosphonomethyl)glycine to produce unwanted by-products (mainly
N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as
"NMG") which reduce the N-(phosphonomethyl)glycine yield. In
addition, the formaldehyde by-product itself is undesirable because
of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
[0006] Optimally, therefore, it has been suggested that the
formaldehyde be simultaneously oxidized to carbon dioxide and water
as the N-(phosphonomethyl)iminodiacetic acid is oxidized to
N-(phosphonomethyl)glycine in a single reactor, thus giving the
following reaction:
##STR00002##
As the above teachings suggest, such a process requires the
presence of both carbon (which primarily effects the oxidation of
N-(phosphonomethyl)iminodiacetic acid to form
N-(phosphonomethyl)glycine and formaldehyde) and a noble metal
(which primarily effects the oxidation of formaldehyde to form
carbon dioxide and water). Like Franz, Ramon et al. (U.S. Pat. No.
5,179,228) teach using a noble metal deposited on the surface of a
carbon support. To reduce the problem of leaching (which Ramon et
al. report to be as great as 30% noble metal loss per cycle),
however, Ramon et al. teach flushing the reaction mixture with
nitrogen under pressure after the oxidation reaction is completed
to cause re-deposition of the noble metal onto the surface of the
carbon support. According to Ramon et al., nitrogen flushing
reduces the noble metal loss to less than 1%.
[0007] Using a different approach, Felthouse (U.S. Pat. No.
4,582,650) teaches using two catalysts: (i) an activated carbon to
effect the oxidation of N-(phosphonomethyl)iminodiacetic acid into
N-(phosphonomethyl)glycine, and (ii) a co-catalyst to concurrently
effect the oxidation of formaldehyde to carbon dioxide and water.
The co-catalyst consists of an aluminosilicate support having a
noble metal located within its pores. The pores are sized to
exclude N-(phosphonomethyl)glycine and thereby prevent the noble
metal of the co-catalyst from being poisoned by
N-(phosphonomethyl)glycine. According to Felthouse, use of these
two catalysts together allows for the simultaneous oxidation of
N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine
and of formaldehyde to carbon dioxide and water. This approach,
however, suffers from several disadvantages: (1) it is difficult to
recover the costly noble metal from the aluminosilicate support for
re-use; (2) it is difficult to design the two catalysts so that the
rates between them are matched; and (3) the carbon support, which
has no noble metal deposited on its surface, tends to deactivate at
a rate which can exceed 10% per cycle.
[0008] Ebner et al., in U.S. Pat. No. 6,417,133, describe a deeply
reduced noble metal on carbon catalyst which is characterized by a
CO desorption of less than 1.2 mmole/g, preferably less than 0.5
mmole/g, when a dry sample of the catalyst, after being heated at a
temperature of about 500.degree. C. for about 1 hour in a hydrogen
atmosphere and before being exposed to an oxidant following the
heating in the hydrogen atmosphere, is heated in a helium
atmosphere from about 20.degree. to about 900.degree. C. at a rate
of about 10.degree. C. per minute, and then at about 900.degree. C.
for about 30 minutes. The catalyst is further characterized as
having a ratio of carbon atoms to oxygen atoms of at least about
20:1, preferably at least about 30:1, at the surface as measured by
x-ray photoelectron spectroscopy after the catalyst is heated at a
temperature of about 500.degree. C. for about 1 hour in a hydrogen
atmosphere and before the catalyst is exposed to an oxidant
following the heating in the hydrogen atmosphere.
[0009] The catalysts of U.S. Pat. No. 6,417,133 have proven to be
highly advantageous and effective catalysts for the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine, and for the further oxidation of
by-product formaldehyde and formic acid, and without excessive
leaching of noble metal from the carbon support. It has further
been discovered that these catalysts are effective in the operation
of a continuous process for the production of
N-(phosphonomethyl)glycine by oxidation of
N-(phosphonomethyl)iminodiacetic acid.
[0010] The advent of continuous processes for the oxidation of
N-(phosphonomethyl)iminodiacetic acid has created an opportunity
for further improvements in productivity through the development of
catalysts which accelerate the rate of oxidation of
N-(phosphonomethyl)iminodiacetic acid and/or formaldehyde beyond
the rates achievable with the catalysts of U.S. Pat. No. 6,417,133.
Since the productivity of a continuous oxidation reactor is not
constrained by the turnaround cycle of a batch reactor, any
improvement in reaction kinetics translates directly into an
increase in the rate of product output per unit reactor volume.
[0011] Carbon and noble metal sites on the catalysts of U.S. Pat.
No. 6,417,133 are highly effective for transfer of electrons in the
oxidation of N-(phosphonomethyl)iminodiacetic acid, and the noble
metal sites are especially effective for this purpose in the
oxidation of formaldehyde and formic acid. However, the
productivity of these reactions could be enhanced if the catalyst
were more effective for transfer of electrons in the concomitant
reduction of molecular oxygen, which can be a rate limiting step in
the overall catalytic reaction between molecular oxygen and the
N-(phosphonomethyl)iminodiacetic acid, formaldehyde, and formic
acid substrates.
SUMMARY OF THE INVENTION
[0012] Among the several objects of the present invention,
therefore, may be noted the provision of an effective oxidation
catalyst; the provision of such a catalyst which promotes reduction
of molecular oxygen in the course of oxidation reactions; the
provision of such a catalyst which is effective for the conversion
of tertiary amines to secondary amines by oxidative cleavage; the
provision of such a catalyst which is effective for the preparation
of secondary amines in high productivity; the provision of such a
catalyst which is effective for the oxidation of the tertiary
amine, N-substituted N-(phosphonomethyl)glycine, to the secondary
amine, N-(phosphonomethyl)glycine; the provision of such a catalyst
which is particularly effective for the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine; the provision of such a catalyst which
is effective for the preparation of N-(phosphonomethyl)glycine in
high productivity; the provision of such a catalyst which is
effective for the preparation of N-(phosphonomethyl)glycine in high
yield based on N-(phosphonomethyl)iminodiacetic acid; and the
provision of such a catalyst which is effective for the further
oxidation of by-product C.sub.1 compounds produced in the oxidation
of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0013] Further objects of the invention include the provision of a
novel catalytic oxidation process for the conversion of tertiary
amines to secondary amines, and more particularly, the conversion
of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine; and the provision of such a process
which is effective for the preparation of
N-(phosphonomethyl)glycine in high productivity and high yield; the
provision of such a process which produces a
N-(phosphonomethyl)glycine product of high quality with
commercially acceptable maximum concentrations of by-products; and
the provision of such a process in which C.sub.1 by-products of
N-(phosphonomethyl)iminodiacetic acid oxidation are also
effectively oxidized.
[0014] Briefly therefore, an embodiment of the invention is
directed to an oxidation catalyst comprising a noble metal
deposited over a modified carbon support. The modified carbon
support comprises carbon having a transition metal and nitrogen
thereon, wherein the transition metal is selected from the group
consisting of iron and cobalt.
[0015] Further, another embodiment of the invention is directed to
a process for the preparation of a redox catalyst. The process
comprises pyrolyzing a source of iron or cobalt and a source of
nitrogen on a carbon support surface to provide a modified carbon
support comprising iron or cobalt and nitrogen thereon. Thereafter,
a noble metal is deposited on the modified carbon support.
[0016] Still further, another embodiment of the invention is
directed to a process for the oxidation of an organic substrate.
The process comprises contacting an organic substrate with an
oxidizing agent in the presence of a oxidation catalyst comprising
a noble metal deposited over a modified carbon support. The
modified carbon support has a transition metal and nitrogen
thereon. The transition metal is selected from the group consisting
of iron and cobalt.
[0017] Still further, another embodiment of the invention is
directed to a process for the oxidation of an organic substrate.
The process comprises contacting an organic substrate with an
oxidizing agent in the presence of a catalyst comprising a modified
carbon support. The modified carbon support has a transition
metal/nitrogen composition thereon and the transition metal of the
catalyst is selected from the group consisting of iron and cobalt.
The process is further characterized in that the catalyst comprises
the transition metal/nitrogen composition in such proportion that
the Fe, Co or the sum of (Fe+Co) of the transition metal/nitrogen
composition constitutes at least about 0.1% by weight of the
catalyst, and the nitrogen of the transition metal/nitrogen
composition constitutes at least about 0.1% by weight of the
catalyst.
[0018] Other features of the invention will be in part apparent and
in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows cyclic voltammograms for the reduction of
molecular oxygen as obtained in accordance with the procedure of
Example 1 comparing a particulate carbon catalyst prepared in
accordance with Chou, U.S. Pat. No. 4,696,772, and designated MC-10
with a particulate carbon catalyst of the invention prepared by
pyrolysis of FeTPP on MC-10 and designated FeTPP/MC-10.
[0020] FIG. 2 shows cyclic voltammograms for the reduction of
molecular oxygen as obtained in accordance with the procedure of
Example 2 comparing a particulate carbon support sold under the
trade designation CP-117 (Engelhard Corp., Iselin, N.J.) with a
particulate carbon catalyst of the invention prepared by pyrolysis
of FeTPP on a CP-117 carbon support and designated
FeTPP/CP-117.
[0021] FIG. 3 shows a cyclic voltammogram for the reduction of
molecular oxygen as obtained in accordance with the procedure of
Example 3 for a catalyst designated PtFe/FeTPP/CP-117 prepared by
depositing platinum and an iron promoter over a FeTPP/CP-117
modified carbon support prepared in accordance with Example 2.
[0022] FIG. 4 sets forth the comparative profiles of ORP and %
CO.sub.2 in the off-gas for the oxidation runs conducted in Example
4 using the FeTPP/MC-10 modified particulate carbon catalyst
prepared in Example 1 and unmodified MC-10 particulate carbon
catalyst to catalyze the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0023] FIG. 5 plots profiles of impurities as determined by high
performance liquid chromatography (HPLC) analysis of samples of
reaction mixture taken during the course of the comparative
oxidation runs of Example 4.
[0024] FIG. 6 sets forth the comparative profiles of ORP and %
CO.sub.2 in the off-gas for the oxidation runs conducted in Example
5 using the FeTPP/CP-117 modified particulate carbon catalyst
prepared in Example 2 and unmodified CP-117 particulate carbon
catalyst to catalyze the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0025] FIG. 7 constitutes an overlay of the ORP and % CO.sub.2
profiles of FIG. 4 and those of FIG. 6.
[0026] FIG. 8 is an overlay of the plot of the impurities profiles
of the comparative oxidation runs of Example 4, as taken from FIG.
5, with the impurities profiles of the comparative oxidation runs
of Example 5.
[0027] FIG. 9 sets forth the comparative profiles of ORP and %
CO.sub.2 in the off-gas for the oxidation runs conducted in Example
6 using the CoTMPP/MC-10 and CoTMPP/CP-117 modified particulate
carbon catalysts and unmodified MC-10 carbon catalyst to catalyze
the oxidation of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0028] FIG. 10 sets forth the comparative profiles of % CO.sub.2 in
the off-gas for the oxidation runs conducted in Example 7 using the
TPP/CP-117 and FeTPP/CP-117 modified particulate carbon catalysts
and unmodified CP-117 particulate carbon catalyst to catalyze the
oxidation of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0029] FIG. 11 shows cyclic voltammograms for the reduction of
molecular oxygen as obtained in accordance with the procedure of
Example 8 for a hydrochloric acid washed FeTPP-117/CP-117 modified
particulate carbon catalyst.
[0030] FIG. 12 shows cyclic voltammograms for the reduction of
molecular oxygen as obtained in accordance with the procedure of
Example 9 for a hydrochloric acid washed PtFe/FeTPP-117/CP-117
modified particulate carbon catalyst.
[0031] FIG. 13 is a schematic illustration of the equivalent anodic
and cathodic half cell reactions believed to occur at the surface
of the catalyst in the non-electrolytic oxidation of
N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine
by catalytic reaction with molecular oxygen in accordance with a
preferred process of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] In accordance with the invention, it has been discovered
that the oxidation of tertiary amines to secondary amines can be
effectively promoted by a catalyst comprising a carbon body having
bound thereto a composition comprising a transition metal and
nitrogen. Such catalysts are prepared, e.g., by the pyrolysis of an
Fe or Co co-ordination compound on the surface of a particulate
carbon support, wherein the co-ordination ligands comprise
nitrogen, more particularly, coordinated nitrogen atoms. The
catalysts of the invention have been found particularly useful in
oxidation reactions conducted in an aqueous reaction medium, such
as the oxidation of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine, especially where the catalyst further
comprises a noble metal at the surface of the carbon, wherein the
transition metal/nitrogen composition on carbon constitutes a
modified carbon support for the noble metal. Although the present
invention is not limited to or dependent on a particular theory, it
is believed that the Fe/N or Co/N composition on the carbon
catalyst promotes the reduction of molecular oxygen in the course
of the oxidation of a substrate wherein electrons transferred from
the substrate are combined with protons and molecular oxygen to
ultimately form water. It further appears that the Fe/N or Co/N
composition serves as an active phase which promotes the reduction
of oxygen by supply of electrons removed from the substrate in the
oxidation thereof.
[0033] Thus, for example, in the oxidation of methane:
CH.sub.4+2H.sub.2O.fwdarw.CO.sub.2+4H.sup.++4e.sup.-
O.sub.2+4H.sup.++4e.sup.-.fwdarw.2H.sub.2O
The function of the catalyst of the invention can be analogized to
a short circuited fuel cell. Active Fe/N or Co/N sites on the
catalyst are believed to function as efficient cathodic sites at
which electrons are supplied in the reduction of molecular oxygen.
Noble metal and carbon sites are believed to randomly function as
either anodic sites in which electrons are transferred to the
catalyst body from the substrate to be oxidized, or cathodic sites
at which electrons are supplied in the reduction of oxygen.
Electron transfer through the conductive carbon body between anodic
and cathodic sites completes the circuit.
[0034] The catalyst of the invention has been found highly
effective for the preparation of N-(phosphonomethyl)glycine by the
catalytic oxidation of N-(phosphonomethyl)iminodiacetic acid with
molecular oxygen. By comparison with catalysts previously available
to the art, the catalyst of the invention significantly enhances
rate of oxidation of N-(phosphonomethyl)iminodiacetic acid. It is
also effective for oxidation of C.sub.1 by-products (e.g.,
formaldehyde and formic acid), and enhances the reaction rate
constants in these reactions as well. It thus appears that
productivity of a process for the preparation of
N-(phosphonomethyl)glycine can be materially enhanced by use of the
catalyst of the invention. It has further been found that this
catalyst can be used to produce N-(phosphonomethyl)glycine in high
yield and high quality, without significant over-oxidation to
aminomethylphosphonic acid or formation of
N-methyl-N(phosphonomethyl)glycine.
[0035] The carbon support for the catalyst can assume a variety of
forms. In one embodiment of this invention, the support is a
monolithic support. Suitable monolithic supports may have numerous
different shapes. A monolithic support may be, for example, in the
form of a screen, a honeycomb, or in the form of a reactor
impeller.
[0036] In a particularly preferred embodiment, the carbon support
is in the form of particulates. Because particulate supports are
especially preferred, most of the following discussion focuses on
embodiments which use a particulate support. It should be
recognized, however, that this invention is not limited to the use
of particulate supports.
[0037] Suitable particulate supports may have a wide variety of
shapes. For example, such supports may be in the form of pellets,
granules and powders. These particulate supports may be used in a
reactor system as free particles, or, alternatively, may be bound
to a structure in the reactor system, such as a screen or an
impeller. Preferably, the support is in the form of a powder.
Granular supports may be preferred where the catalyst is used in a
fixed bed reactor, e.g., of the type described in coassigned U.S.
Publication No. US-2002-0068836-A1, which is expressly incorporated
herein by reference. U.S. Publication No. US-2002-0068836-A1 is
directed to continuous processes for the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine.
[0038] Typically, a support which is in particulate form comprises
a broad size distribution of particles. For powders, preferably at
least about 95% of the particles are from about 2 to about 300
.mu.m in their largest dimension, more preferably at least about
98% of the particles are from about 2 to about 200 .mu.m in their
largest dimension, and most preferably about 99% of the particles
are from about 2 to about 150 .mu.m in their largest dimension with
about 95% of the particles being from about 3 to about 100 .mu.m in
their largest dimension. Particles being greater than about 200
.mu.m in their largest dimension tend to fracture into super-fine
particles (i.e., less than 2 .mu.m in their largest dimension),
which are difficult to recover.
[0039] A variety of carbon supports can be used in the catalyst of
the invention, including graphitic carbon. However, the specific
surface area of the carbon support, measured by the
Brunauer-Emmett-Teller (BET) method using N.sub.2, is preferably
from about 10 to about 3,000 m.sup.2/g (surface area of carbon
support per gram of carbon support), more preferably from about 500
to about 2,100 m.sup.2/g, and still more preferably from about 750
to about 2,100 m.sup.2/g. In some embodiments, the most preferred
specific area is from about 750 to about 1,750 m.sup.2/g.
[0040] The pore volume of the carbon support may vary widely. Using
the measurement method described in Example of U.S. Pat. No.
6,417,133, the pore volume preferably is from about 0.1 to about
2.5 ml/g (pore volume per gram of catalyst), more preferably from
about 0.2 to about 2.0 ml/g, and most preferably from about 0.4 to
about 1.7 ml/g. Catalysts comprising supports with pore volumes
greater than about 2.5 ml/g tend to fracture easily. On the other
hand, catalysts comprising supports having pore volumes less than
0.1 ml/g tend to have small surface areas and therefore low
activity.
[0041] Carbon supports for use in the present invention are
commercially available from a number of sources. The following is a
listing of some of the activated carbons which may be used with
this invention: Darco G-60 Spec and Darco X (ICI-America,
Wilmington, Del.); Norit SG Extra, Norit EN4, Norit EXW, Norit A,
Norit Ultra-C, Norit ACX, and Norit 4.times.14 mesh (Amer. Norit
Co., Inc., Jacksonville, Fla.); Gl-9615, VG-8408, VG-8590, NB-9377,
XZ, NW, and JV (Barnebey-Cheney, Columbus, Ohio); BL Pulv., PWA
Pulv., Calgon C 450, and PCB Fines (Pittsburgh Activated Carbon,
Div. of Calgon Corporation, Pittsburgh, Pa.); P-100 (No. Amer.
Carbon, Inc., Columbus, Ohio); Nuchar CN, Nuchar C-1000 N, Nuchar
C-190 A, Nuchar C-115 A, and Nuchar SA-30 (Westvaco Corp., Carbon
Department, Covington, Va.); Code 1551 (Baker and Adamson, Division
of Allied Amer. Norit Co., Inc., Jacksonville, Fla.); Grade 235,
Grade 337, Grade 517, and Grade 256 (Witco Chemical Corp.,
Activated Carbon Div., New York, N.Y.); Columbia SXAC (Union
Carbide New York, N.Y.) and CP-117 (Engelhard Corp., Iselin,
N.J.).
[0042] The catalysts are preferably prepared by first depositing a
co-ordination compound comprising an Fe or Co salt and ligands
containing nitrogen. Any of a wide variety of ligands may be used.
The main requirement for the ligand is that it be subject to
pyrolysis to yield nitrogen that is bound to the carbon surface to
provide a transition metal/nitrogen composition which contributes
to adsorption and/or reduction of molecular oxygen. It is also
desirable that the ligand have an affinity, in fact preferably an
appreciable solubility, in the liquid medium from which the
co-ordination complex is deposited on a carbon body as described
below.
[0043] Preferred ligands for the co-ordination compound comprise
porphyrins or porphyrin derivatives such as tetraphenyl porphyrin.
Generally, other exemplary nitrogen-containing organic ligands
comprise five or six membered heterocyclic rings comprising
nitrogen. Suitable ligands useful in the preparation of the
catalyst include polyacrylonitrile, phthalocyanines, pyrrole,
substituted pyrroles, polypyrroles, pyridine, substituted
pyridines, bipyridyls, phthalocyanines, imidazole, substituted
imadazoles, pyrimidine, substituted pyrimidines, acetonitrile,
o-phenylenediamines, bipyridines, salen ligands,
p-phenylenediamines, and cyclams. The ligands are preferably bound
to an Fe or Co salt such as FeCl.sub.2, FeCl.sub.3, FeSO.sub.4,
Fe(OAc).sub.3, CoCl.sub.2, CoBr.sub.3, CO.sub.2 (SO.sub.4).sub.3,
Fe pentacarbonyl, dicobalt octacarbonyl, and the like. Although the
oxidative state of the transition metal ion is not critical, it is
believed that a co-ordination complex comprising a transition metal
ion of relatively low oxidation state, e.g., Fe.sup.+2 may be
effective to reduce a species on the carbon surface, leading to a
stronger bond between the metal and the surface.
[0044] To deposit the co-ordination compound on the carbon support,
a suspension is prepared comprising a particulate carbon support
and the co-ordination compound in a suitable medium, and preferably
agitated for a time sufficient for adsorption of the co-ordination
compound on the carbon surface. For example, a suspension may be
prepared in an organic polar solvent such as acetone, acetonitrile,
ethanol, methanol, tetrahydrofuran, methyl ethyl ketone, methyl
isobutyl ketone, i-propanol, or dimethylformamide containing
particulate carbon in a proportion of from about 5 to about 20
grams/L and an Fe co-ordination compound such as an
5,10,15,20-tetraphenyl-21H,23H-porphine iron (III) halide in a
proportion of from about 1 to about 10 grams/L, with the carbon and
iron co-ordination complex in such relative proportions that the
weight ratio of Fe/C is in the range of from about 0.001 to about
0.1, preferably from about 0.002 to about 0.07, most preferably
from about 0.005 to about 0.05. Such a suspension may be stirred
under ambient conditions until adsorption of the co-ordination
compound on the carbon surface has been accomplished. Under ambient
conditions in acetone, for example, adsorption typically requires
at least about 10 hours, more typically from about 24 to about 72
hours, most typically from about 36 to about 48 hours. After
adsorption is accomplished the solids are filtered from the medium
and dried, conveniently under vacuum.
(5,10,15,20-tetraphenyl-21H,23H-porphine iron salts are sometimes
referred to hereinafter as "FeTPP" which is intended to be generic
to both ferric and ferrous salts and with counteranions other than
halides. In the working examples, however, it is intended to refer
specifically to the FeCl.sub.3 salt).
[0045] The catalyst precursor obtained from the adsorption step is
pyrolyzed to produce the Fe/N or Co/N on carbon redox catalyst.
Pyrolysis is conducted under an inert or reducing atmosphere at a
temperature of at least about 600.degree. C., preferably from about
6000 to about 1000.degree. C., most preferably about 800.degree. C.
The particulate carbon bearing the Fe or Co complex and its
pyrolysis products is preferably subjected to such pyrolysis
conditions for a period of at least about 60 minutes, more
preferably from about 80 to about 150 minutes, even more preferably
from about 100 to about 140 minutes.
[0046] Upon pyrolysis, metal and nitrogen may be chemically bound
to the support or trapped in pores, film or other surface
microstructure which effectively fixes both metal and nitrogen to
the surface. Where metal and nitrogen are chemically bound to the
carbon surface, each may be directly bound to the carbon support,
or nitrogen may be bound to Fe or Co that is bound to the support,
or vice versa. Whatever the exact chemical structure, the
composition of bound metal and nitrogen is believed to function as
a transition metal active phase that promotes reduction of oxygen.
Generally, it is understood that the metal and nitrogen are present
in the form of a complex comprising metal nitrides, metal carbides,
metal oxides, nitride-carbides, etc. Whether serving as an active
phase and/or promoting activity at other sites, the transition
metal/nitrogen composition obtained by pyrolysis may comprise one
or more of such species. Regardless of the precise surface/activity
relationships, the pyrolytic formation of this composition results
in the creation of active sites which have been demonstrated to be
more effective for oxygen reduction than the carbon support alone.
Where a porous, particulate carbon support is used, it has been
found that the bound transition metal/nitrogen composition is
substantially evenly distributed throughout the carbon particle,
not merely concentrated at the geometric surface. The surface of
the carbon body is substantially devoid of any discrete transition
metal particles having a principal dimension greater than about 5
.mu.m.
[0047] The transition metal/nitrogen composition preferably
comprises at least about 0.2%, preferably from about 0.4% to about
15% by weight of the catalyst. Iron, cobalt, or (Fe+Co) of the
transition metal/nitrogen active phase is bound to the carbon
support in a proportion of at least about 0.1% by weight, more
preferably from about 0.1% to about 10% by weight, more preferably
from about 0.25% to about 7% by weight, most preferably from about
0.5% to about 5% by weight, basis the carbon support as so
modified. On the same basis, nitrogen of the transition
metal/nitrogen composition is bound to the carbon in a proportion
of about 0.01% by weight, preferably from about 0.01% to about 10%
by weight, more preferably from about 0.1% to about 7% by weight,
most preferably from about 1% to about 5% by weight. Generally, the
ratio of Fe, Co or (Fe+Co) to N in the transition metal/nitrogen
complex is from about 1:4 and about 3:1.
[0048] Analysis of the transition metal/nitrogen complex by
Mossbauer spectroscopy indicates a complex structure of the
transition metal/nitrogen composition formed on pyrolysis. In the
case of the pyrolysis of a co-ordination compound comprising an
iron salt with organic nitrogen containing ligands such as FeTPP on
a particulate carbon support, a typical overall composition of the
transition metal/nitrogen complex appears to be substantially the
following:
TABLE-US-00001 .xi.-Fe.sub.3N-like nitride 30-70 wt. %
superparamagnetic iron 5-20 wt. % .alpha.-iron 15-25 wt. % isolated
iron atoms 10-20 wt. %
However, other species including iron carbides and other iron
nitrides may also be present. Similar compositions are believed to
be formed upon pyrolysis of co-ordination compounds comprising
cobalt and an organic nitrogen containing ligand. While sometimes
referred to herein as an "active phase," the product of pyrolysis
may actually comprise plural solid phases in a highly complex
microstructure. Whatever its exact makeup, the product of the
pyrolysis is referred to herein as the "transition metal/nitrogen
composition," or specifically as the "Fe/N" or "Co/N"
composition.
[0049] Optionally, the transition metal/nitrogen composition on
carbon can be subjected to an acid wash before use as a catalyst
for an oxidation reaction. Acid washing of the catalyst, e.g., with
0.2% by weight hydrochloric acid, has been found to remove a
substantial fraction of iron from the catalyst surface, but does
not have a proportionate effect on catalyst activity.
[0050] Other methods may be used in preparing the Fe/N or Co/N on
carbon catalysts of the present invention. For example, a source of
Fe or Co, such as a salt, oxide or hydroxide, can be pyrolyzed on a
carbon support in the presence of a nitrogen source. Preferably,
the Fe or Co salt, oxide or hydroxide is subjected to a reduction
step prior to contact with the nitrogen source, or simultaneously
therewith. Reduction is typically conducted at a temperature in the
range of from about 400.degree. to about 1000.degree. C.,
preferably from about 500.degree. to about 900.degree. C., most
preferably about 600.degree. C., in the presence of reducing gas
such as hydrogen. After reduction, the Fe/C precursor is pyrolyzed
in the presence of a nitrogen source. The nitrogen source need not
be initially coordinated to the Fe or Co, though co-ordination
bonding may arise incident to contact of the Fe or Co salt with the
nitrogen source under pyrolysis conditions. Advantageously, a vapor
phase nitrogen source is maintained in contact with the Fe/C
precursor during the pyrolysis. Suitable vapor phase nitrogen
sources include compounds selected from the group consisting of
ammonia, volatile amines, and volatile nitrites. Preferred vapor
phase nitrogen sources include compounds selected from the group
consisting of ammonia, ethylenediamine, isopropylamine,
dimethylamine, acetonitrile and propionitrile.
[0051] Pyrolysis is conducted at a temperature of from about
400.degree. to about 1200.degree. C., preferably from about
600.degree. to about 1100.degree. C., most preferably about
1000.degree. C. In this embodiment of the invention, it is
important to maintain an adequate supply of the vapor phase
nitrogen source in contact with the carbon support to replenish
that which has reacted during pyrolysis. An adequate supply of the
nitrogen source can be assured, and mass transfer of nitrogen to
the carbon surface promoted, by passing a stream of the vapor phase
nitrogen source through the pyrolysis zone while maintaining the
zone under pyrolysis conditions and substantially free of oxidizing
gases.
[0052] Still other methods can be used for producing the Fe/N or
Co/N on carbon catalyst. For example, an iron or cobalt salt
comprising a nitrogen containing anion may be deposited on a carbon
support and thereafter pyrolyzed. Salts that can be used in such a
process include cyanides and amino acid salts. However, to provide
an adequate supply of nitrogen, pyrolysis should be conducted under
an atmosphere in which a substantial partial pressure of the
nitrogen source is maintained. Otherwise, the volatilization of
nitrogen compounds from the carbon surface under pyrolysis
conditions will not leave a sufficient source of nitrogen on the
surface to create the concentration of active sites that are
desired for the catalytic reduction of oxygen. In yet another
method, the metal and nitrogen can be sputtered onto a carbon
surface, or metal, nitrogen and carbon can be sputtered onto an
inert support.
[0053] Catalysts comprising a transition metal/nitrogen composition
bound to a carbon support have been demonstrated to exhibit a
substantially enhanced efficacy for the reduction of molecular
oxygen as compared to the carbon support alone. This may be
demonstrated by subjecting the catalyst to cyclic voltammetric
reduction of oxygen. For example, when cyclic voltammetry in the
reduction of molecular oxygen is conducted in an electrolytic
medium consisting of 0.1M H.sub.3PO.sub.4, a catalyst prepared from
Fe(III)/tetraphenylporphyrin on carbon typically exhibits an
increased reduction current relative to the untreated carbon
support under reference conditions wherein the catalyst serves as
an electrode that is cycled in the range of +0.5 to +0.1 volts vs.
an Ag/AgCl electrode.
[0054] For use in reactions such as the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine, the redox catalyst of this invention
preferably further comprises a noble metal deposited over the Fe/N
and/or Co/N modified carbon support. In this case, formation of the
Fe/N and/or Co/N composition is not only believed to provide active
sites for oxygen reduction, but further provides a modified carbon
support for a further active phase comprising noble metal that
catalyzes transfer of electrons from an organic substrate to be
oxidized. It has been found that Fe/N and/or Co/N on carbon, or in
fact a carbon catalyst alone, is effective for the selective
oxidative cleavage of one of the two carboxymethyl substituents of
the N-(phosphonomethyl)iminodiacetic acid substrate, wherein carbon
sites are believed to catalyze electron transfer from the substrate
molecule. However, neither the unmodified nor the modified carbon
is an entirely satisfactory catalyst for the further oxidation of
both the C.sub.1 by-products of the oxidative cleavage, i.e.,
formaldehyde and formic acid. Carbon alone has very little activity
in catalyzing oxidation of C.sub.1 compounds. The Fe/N or Co/N
modified carbon catalyst of the invention has demonstrated some
improvement for this purpose over carbon alone, but may still not
be as active as may be desired. Unless the formaldehyde is
effectively removed from the reaction zone, it tends to react with
N-(phosphonomethyl)glycine to produce an undesired by-product,
N-methyl-N-(phosphonomethyl)glycine ("NMG"), thereby reducing
yields, reducing productivity, and compromising product
quality.
[0055] Where the catalyst comprises a noble metal over a modified
support comprising Fe/N and/or Co/N on carbon, it has not only been
proven effective for the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine, but has also been demonstrated to be
highly effective in the oxidation of organic compounds that are not
readily amenable to oxidation in the presence of the Fe/N or Co/N
on carbon alone, notably the formaldehyde produced as a by-product
of the oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid.
Novel catalysts comprising a noble metal over Fe/N/carbon or
Co/N/carbon have further been demonstrated to promote the oxidation
of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine at rates and productivity much enhanced
over the rates found attainable with the otherwise highly desirable
catalysts described in U.S. Pat. No. 6,417,133. However, the
relative rate of over-oxidation, i.e., oxidation of product
N-(phosphonomethyl)glycine to aminomethylphosphonic acid, is low,
so the oxidation reaction mixture has a low aminomethylphosphonic
acid as well as a low N-methyl-N-(phosphonomethyl)glycine content.
Thus high yields and a high quality N-(phosphonomethyl)glycine
product may be obtained.
[0056] In preparation of the novel catalysts of the invention, it
is important that the Fe/N and/or Co/N active phase be deposited
before deposition of the noble metal, and before deposition of any
promoter that is alloyed with or associated with the noble metal.
If the noble metal is deposited first, the above described methods
for depositing the Fe/N or Co/N active phase tend to poison or
otherwise deactivate the noble metal phase.
[0057] The noble metal is preferably a platinum group metal such as
platinum, palladium, rhodium, iridium, osmium, ruthenium or
combinations thereof. Because platinum is for many purposes the
most preferred noble metal, the following discussion is directed
primarily to embodiments using platinum. It should be understood,
however, that the same discussion is generally applicable to the
other noble metals and combinations thereof. It also should be
understood that the term "noble metal" as used herein means the
noble metal in its elemental state as well as the noble metal in
any of its various oxidation states.
[0058] As described in U.S. Pat. No. 6,417,133, oxygen-containing
functional groups (e.g., carboxylic acids, ethers, alcohols,
aldehydes, lactones, ketones, esters, amine oxides, and amides) at
the surface of the support increase noble metal leaching and
potentially increase noble metal sintering during liquid phase
oxidation reactions, thus reducing the ability of the catalyst to
oxidize oxidizable substrates, particularly formaldehyde during the
N-(phosphonomethyl)iminodiacetic acid oxidation. As used herein, an
oxygen-containing functional group is "at the surface of the carbon
support" if it is bound to an atom of the carbon of the support and
is able to chemically or physically interact with compositions
within the reaction mixture or with metal atoms deposited on a
modified support.
[0059] Many of the oxygen-containing functional groups that reduce
noble metal resistance to leaching and sintering and reduce the
activity of the catalyst desorb from the carbon support as carbon
monoxide when the catalyst is heated at a high temperature (e.g.,
900.degree. C.) in an inert atmosphere (e.g., helium or argon).
Thus, measuring the amount of CO desorption from a fresh catalyst
(i.e., a catalyst that has not previously been used in a liquid
phase oxidation reaction) under high temperatures is one method
that may be used to analyze the surface of the catalyst to predict
noble metal retention and maintenance of catalyst activity. One way
to measure CO desorption is by using thermogravimetric analysis
with in-line mass spectroscopy ("TGA-MS"). Preferably, no more than
about 1.2 mmole of carbon monoxide per gram of catalyst desorb from
the catalyst when a dry, fresh sample of the catalyst in a helium
atmosphere is subjected to a temperature which is increased from
about 20.degree. to about 900.degree. C. at about 10.degree. C. per
minute, and then held constant at about 900.degree. C. for about 30
minutes. More preferably, no more than about 0.7 mmole of carbon
monoxide per gram of fresh catalyst desorb under those conditions,
even more preferably no more than about 0.5 mmole of carbon
monoxide per gram of fresh catalyst desorb, and most preferably no
more than about 0.3 mmole of carbon monoxide per gram of fresh
catalyst desorb. A catalyst is considered "dry" when the catalyst
has a moisture content of less than about 1% by weight. Typically,
a catalyst may be dried by placing it into a N.sub.2 purged vacuum
of about 25 inches of Hg and a temperature of about 120.degree. C.
for about 16 hours.
[0060] Measuring the number of oxygen atoms at the surface of a
fresh catalyst support is another method which may be used to
analyze the catalyst to predict noble metal retention and
maintenance of catalytic activity. Using, for example, x-ray
photoelectron spectroscopy, a surface layer of the support which is
about 50 .ANG. in thickness is analyzed. Presently available
equipment used for x-ray photoelectron spectroscopy typically is
accurate to within .+-.20%. Typically, a ratio of carbon atoms to
oxygen atoms at the surface (as measured by presently available
equipment for x-ray photoelectron spectroscopy) of at least about
20:1 (carbon atoms:oxygen atoms) is suitable. Preferably, however,
the ratio is at least about 30:1, more preferably at least about
40:1, even more preferably at least about 50:1, and most preferably
at least about 60:1. In addition, the ratio of oxygen atoms to
metal atoms at the surface (again, as measured by presently
available equipment for x-ray photoelectron spectroscopy)
preferably is less than about 8:1 (oxygen atoms:metal atoms). More
preferably, the ratio is less than about 7:1, even more preferably
less than about 6:1, and most preferably less than about 5:1.
[0061] The concentration of the noble metal deposited at the
surface of the modified carbon support may vary within wide limits.
Preferably, it is in the range of from about 0.5% to about 20% by
weight ([mass of noble metal/total mass of catalyst].times.100%),
more preferably from about 2.5% to about 10% by weight, and most
preferably from about 3% to about 7.5% by weight. If concentrations
less than 0.5% by weight are used, the catalyst may be ineffective
for the oxidation of certain substrates, e.g., by-product
formaldehyde from the oxidation of N-(phosphonomethyl)iminodiacetic
acid. On the other hand, at concentrations greater than about 20%
by weight, layers and clumps of noble metal tend to form. Thus,
there are fewer surface noble metal atoms per total amount of noble
metal used. This tends to reduce the activity of the catalyst and
is an uneconomical use of the costly noble metal.
[0062] The dispersion of the noble metal at the surface of the
modified carbon support preferably is such that the concentration
of surface noble metal atoms is from about 10 to about 400
.mu.mole/g (.mu.mole of surface noble metal atoms per gram of
catalyst), more preferably, from about 10 to about 150 .mu.mole/g,
and most preferably from about 15 to about 100 .mu.mole/g. This may
be determined, for example, by measuring chemisorption of H.sub.2
or CO using a Micromeritics ASAP 2010C (Micromeritics, Norcross,
Ga.) or an Altamira AMI100 (Zeton Altamira, Pittsburgh, Pa.).
[0063] Preferably, the noble metal is at the surface of the
modified carbon support in the form of metal particles. At least
about 90% (number density) of the noble metal particles at the
surface of the modified support are preferably from about 0.5 to
about 35 nm in their largest dimension, more preferably from about
1 to about 20 nm in their largest dimension, and most preferably
from about 1.5 to about 10 nm in their largest dimension. In a
particularly preferred embodiment, at least about 80% of the noble
metal particles at the surface of the modified support are from
about 1 to about 15 nm in their largest dimension, more preferably
from about 1.5 to about 10 nm in their largest dimension, and most
preferably from about 1.5 to about 7 nm in their largest dimension.
If the noble metal particles are too small, there tends to be an
increased amount of leaching when the catalyst is used in an
environment that tends to solubilize noble metals, as is the case
when oxidizing N-(phosphonomethyl)iminodiacetic acid to form
N-(phosphonomethyl)glycine. On the other hand, as the particle size
increases, there tends to be fewer noble metal surface atoms per
total amount of noble metal used. As discussed above, this tends to
reduce the activity of the catalyst and is an uneconomical use of
the noble metal.
[0064] In addition to the noble metal, at least one promoter may be
at the surface of the modified carbon support. Although the
promoter typically is deposited onto the surface of the modified
carbon support, other sources of promoter may be used (e.g., the
carbon support itself may naturally contain a promoter). A promoter
tends to increase catalyst selectivity, activity, and/or stability.
A promoter additionally may reduce noble metal leaching.
[0065] The promoter may, for example, be an additional noble
metal(s) at the surface of the support. For example, ruthenium and
palladium have been found to act as promoters on a catalyst
comprising platinum deposited at a carbon support surface. The
promoter(s) alternatively may be, for example, a metal selected
from the group consisting of tin (Sn), cadmium (Cd), magnesium
(Mg), manganese (Mn), nickel (Ni), aluminum (Al), cobalt (Co),
bismuth (Bi), lead (Pb), titanium (Ti), antimony (Sb), selenium
(Se), iron (Fe), rhenium (Re), zinc (Zn), cerium (Ce), zirconium
(Zr), tellurium (Te), and germanium (Ge). Preferably, the promoter
is selected from the group consisting of bismuth, iron, tin,
tellurium and titanium. In a particularly preferred embodiment, the
promoter is tin. In another particularly preferred embodiment, the
promoter is iron. In an additional preferred embodiment, the
promoter is titanium. In a further particularly preferred
embodiment, the catalyst comprises both iron and tin. Use of iron,
tin, or both generally (1) reduces noble metal leaching for a
catalyst used over several cycles, and (2) tends to increase and/or
maintain the activity of the catalyst when the catalyst is used to
effect the oxidation of N-(phosphonomethyl)iminodiacetic acid.
Catalysts comprising iron generally are most preferred because they
tend to have the greatest activity and stability with respect to
formaldehyde and formic acid oxidation.
[0066] It will be understood that the promoter is metal that is
alloyed or associated with the noble metal active phase. Iron or
cobalt present in or on the carbon support as part of the pyrolytic
Fe/N or Co/N composition is not understood to function as a
promoter of the noble metal for transfer of electrons from the
substrate to be oxidized, but primarily to constitute or provide
sites for transfer of electrons to an O.sub.2 molecule to be
reduced. As described in detail below, the noble metal with which
the promoter is alloyed or associated is typically present in the
form of crystallites on the carbon surface, whereas the Fe or Co of
the transition metal/nitrogen composition is more intimately
associated with or bound to the carbon atoms of the support, and
more uniformly distributed throughout the catalyst particle.
[0067] In one preferred embodiment, the promoter is more easily
oxidized than the noble metal. A promoter is "more easily oxidized"
if it has a lower first ionization potential than the noble metal.
First ionization potentials for the elements are widely known in
the art and may be found, for example, in the CRC Handbook of
Chemistry and Physics (CRC Press, Inc., Boca Raton, Fla.).
[0068] The amount of promoter at the surface of the modified carbon
support (whether associated with the carbon surface itself, metal,
or a combination thereof) may vary within wide limits depending on,
for example, the noble metal and promoter used. Typically, the
weight percentage of the promoter is at least about 0.05% ([mass of
promoter/total mass of the catalyst].times.100%). The weight
percent of the promoter preferably is from about 0.05 to about 10%,
more preferably from about 0.1 to about 10%, still more preferably
from about 0.1 to about 2%, and most preferably from about 0.2 to
about 1.5%. When the promoter is tin, the weight percent most
preferably is from about 0.5 to about 1.5%. Promoter weight
percentages less than 0.05% generally do not promote the activity
of the catalyst over an extended period of time. On the other hand,
concentrations of promoter greater than about 10% by weight tend to
decrease the activity of the catalyst.
[0069] The molar ratio of noble metal to promoter may also vary
widely, depending on, for example, the noble metal and promoter
used. Preferably, the ratio is from about 1000:1 to about 0.01:1;
more preferably from about 150:1 to about 0.05:1; still more
preferably from about 50:1 to about 0.05:1; and most preferably
from about 10:1 to about 0.05:1. For example, a preferred catalyst
comprising platinum and iron has a molar ratio of platinum to iron
of about 3:1.
[0070] In a particularly preferred embodiment of this invention,
the noble metal (e.g., Pt) is alloyed with at least one promoter
(e.g., Sn, Fe, or both) to form alloyed metal particles. A catalyst
comprising a noble metal alloyed with at least one promoter tends
to have all the advantages discussed above with respect to
catalysts comprising a promoter. It has been found in accordance
with this invention, however, that catalysts comprising a noble
metal alloyed with at least one promoter tend to exhibit greater
resistance to promoter leaching and further stability from cycle to
cycle with respect to formaldehyde and formic acid oxidation.
[0071] The term "alloy" encompasses any metal particle comprising a
noble metal and at least one promoter, irrespective of the precise
manner in which the noble metal and promoter atoms are disposed
within the particle (although it is generally preferable to have a
portion of the noble metal atoms at the surface of the alloyed
metal particle). The alloy may be, for example, any of the
following:
[0072] 1. An intermetallic compound. An intermetallic compound is a
compound comprising a noble metal and a promoter (e.g.,
Pt.sub.3Sn).
[0073] 2. A substitutional alloy. A substitutional alloy has a
single, continuous phase, irrespective of the concentrations of the
noble metal and promoter atoms. Typically, a substitutional alloy
contains noble metal and promoter atoms which are similar in size
(e.g., platinum or platinum and palladium). Substitutional alloys
are also referred to as "monophasic alloys."
[0074] 3. A multiphasic alloy. A multiphasic alloy is an alloy that
contains at least two discrete phases. Such an alloy may contain,
for example Pt.sub.3Sn in one phase, and tin dissolved in platinum
in a separate phase.
[0075] 4. A segregated alloy. A segregated alloy is a metal
particle wherein the particle stoichiometry varies with distance
from the surface of the metal particle.
[0076] 5. An interstitial alloy. An interstitial alloy is a metal
particle wherein the noble metal and promoter atoms are combined
with non-metal atoms, such as boron, carbon, silicon, nitrogen,
phosphorus, etc.
[0077] Preferably, at least about 80% (number density) of the
alloyed metal particles are from about 0.5 to about 35 nm in their
largest dimension, more preferably from about 1 to about 20 nm in
their largest dimension, still more preferably from about 1 to
about 15 nm in their largest dimension, and most preferably from
about 1.5 to about 7 nm in their largest dimension.
[0078] The alloyed metal particles need not have a uniform
composition; the compositions may vary from particle to particle,
or even within the particles themselves. In addition, the catalyst
may further comprise particles consisting of the noble metal alone
or the promoter alone. Nevertheless, it is preferred that the
composition of metal particles be substantially uniform from
particle to particle and within each particle, and that the number
of noble metal atoms in intimate contact with promoter atoms be
maximized. It is also preferred, although not essential, that the
majority of noble metal atoms be alloyed with a promoter, and more
preferred that substantially all of the noble metal atoms be
alloyed with a promoter. It is further preferred, although not
essential, that the alloyed metal particles be uniformly
distributed at the surface of the carbon support.
[0079] Regardless of whether the promoter is alloyed to the noble
metal, it is presently believed that the promoter tends to become
oxidized if the catalyst is exposed to an oxidant over a period of
time. For example, an elemental tin promoter tends to oxidize to
form Sn(II)O, and Sn(II)O tends to oxidize to form Sn(IV)O.sub.2.
This oxidation may occur, for example, if the catalyst is exposed
to air for more than about 1 hour. Although such promoter oxidation
has not been observed to have a significant detrimental effect on
noble metal leaching, noble metal sintering, catalyst activity, or
catalyst stability, it does make analyzing the concentration of
detrimental oxygen-containing functional groups at the surface of
the carbon support more difficult. For example, as discussed above,
the concentration of detrimental oxygen-containing functional
groups (i.e., oxygen-containing functional groups that reduce noble
metal resistance to leaching and sintering, and reduce the activity
of the catalyst) may be determined by measuring (using, for
example, TGA-MS) the amount of CO that desorbs from the catalyst
under high temperatures in an inert atmosphere. However, it is
presently believed that when an oxidized promoter is present at the
surface, the oxygen atoms from the oxidized promoter tend to react
with carbon atoms of the support at high temperatures in an inert
atmosphere to produce CO, thereby creating the illusion of more
detrimental oxygen-containing functional groups at the surface of
the support than actually exist. Such oxygen atoms of an oxidized
promoter also can interfere with obtaining a reliable prediction of
noble metal leaching, noble metal sintering, and catalyst activity
from the simple measurement (via, for example, x-ray photoelectron
spectroscopy) of oxygen atoms at the catalyst surface.
[0080] Thus, when the catalyst comprises at least one promoter
which has been exposed to an oxidant and thereby has been oxidized
(e.g., when the catalyst has been exposed to air for more than
about 1 hour), it is preferred that the promoter first be
substantially reduced (thereby removing the oxygen atoms of the
oxidized promoter from the surface of the catalyst) before
attempting to measure the amount of detrimental oxygen-containing
functional groups at the surface of the carbon support. This
reduction preferably is achieved by heating the catalyst to a
temperature of about 500.degree. C. for about 1 hour in an
atmosphere consisting essentially of H.sub.2. The measurement of
detrimental oxygen-containing functional groups at the surface
preferably is performed (a) after this reduction, and (b) before
the surface is exposed to an oxidant following the reduction. Most
preferably, the measurement is taken immediately after the
reduction.
[0081] The preferred concentration of metal particles at the
surface of the modified support depends, for example, on the size
of the metal particles, the specific surface area of the carbon
support, and the concentration of noble metal on the catalyst. It
is presently believed that, in general, the preferred concentration
of metal particles is roughly from about 3 to about 1,500
particles/.mu.m.sup.2 (i.e., number of metal particles per
.mu.m.sup.2 of surface of carbon support), particularly where: (a)
at least about 80% (number density) of the metal particles are from
about 1.5 to about 7 nm in their largest dimension, (b) the carbon
support has a specific surface area of from about 750 to about 2100
m.sup.2/g (i.e., m.sup.2 of surface of support per gram of modified
carbon support), and (c) the concentration of noble metal at the
carbon support surface is from about 1% to about 10% by weight
([mass of noble metal/total mass of catalyst].times.100%). In more
preferred embodiments, narrower ranges of metal particle
concentrations and noble metal concentrations are desired. In one
such embodiment, the concentration of metal particles is from about
15 to about 800 particles/.mu.m.sup.2, and the concentration of
noble metal at the carbon support surface is from about 2% to about
10% by weight. In an even more preferred embodiment, the
concentration of metal particles is from about 15 to about 600
particles/.mu.m.sup.2, and the concentration of noble metal at the
support surface is from about 2% to about 7.5% by weight. In the
most preferred embodiment, the concentration of the metal particles
is from about 15 to about 400 particles/.mu.m.sup.2, and the
concentration of noble metal at the support surface is about 5% by
weight. The concentration of metal particles at the surface of the
modified carbon support may be measured using methods known in the
art.
[0082] Methods used to deposit the noble metal and/or promoter over
the modified carbon support are generally known in the art and
further described in U.S. Pat. No. 6,417,133, the text of which is
expressly incorporated herein by reference. For example, suitable
methods for deposition of the noble metal and/or promoter include
liquid phase methods such as reaction deposition techniques (e.g.,
deposition via reduction of the metal compounds, and deposition via
hydrolysis of the metal compounds), ion exchange techniques, excess
solution impregnation, and incipient wetness impregnation; vapor
phase methods such as physical deposition and chemical deposition;
precipitation; electrochemical deposition; and electroless
deposition. See generally, Cameron et al., "Carbons as Supports for
Precious Metal Catalysts," Catalysis Today, 7, 113-137 (1990).
[0083] In a preferred embodiment, the modified carbon support
surface is reduced after deposition of the noble metal as described
in U.S. Pat. No. 6,417,133 (incorporated herein by reference) to
produce a deeply reduced catalyst characterized by the
above-described CO desorption and C/O surface ratio parameters. In
particular, the surface of the catalyst is reduced, for example, by
heating the surface at a temperature of at least about 400.degree.
C. It is especially preferred to conduct this heating in a
non-oxidizing environment (e.g., nitrogen, argon, or helium), even
more preferably while exposing the catalyst to a reducing
environment (e.g., a gas phase reducing agent such as H.sub.2,
ammonia or carbon monoxide). Preferably, the surface is heated at a
temperature of at least about 500.degree. C., more preferably from
about 550.degree. to about 1200.degree. C., and even more
preferably from about 550.degree. to about 900.degree. C.
[0084] It is important to note that, in embodiments of the
invention wherein the surface of the modified carbon support is
reduced after noble metal deposition, it may be possible to prepare
the modified carbon support at a lower temperature than as
described above. For example, when the modified carbon support is
prepared by first depositing a co-ordination compound comprising an
Fe or Co salt and ligands containing nitrogen as described above,
it may be possible to fix the Fe/N or Co/N active phase on the
carbon support by pyrolysis at lower temperatures and/or for
shorter durations than as described above. Thus, high temperature
treatment in reducing the catalyst surface after deposition of the
noble metal and/or promoter, as described in U.S. Pat. No.
6,417,133, serves as a further pyrolysis to provide the Fe/N or
Co/N active phase and the noble metal active phase of the
catalyst.
[0085] The catalysts of the invention can be used in a wide variety
of redox reactions. In certain applications as referred to above,
it is highly preferred that the catalyst include a noble metal
phase. However, the modified carbon bodies comprising a transition
metal and nitrogen are also effective for the oxidation of a wide
variety of organic substrates even in the absence of a noble metal.
For example, the modified carbon bodies may serve for example as
oxidation catalysts in various commercial oxidation processes such
as the partial oxidation of hydrocarbons to produce aldehydes,
ketones and carboxylic acids. In particulate or agglomerated form,
the modified carbon can constitute a fixed or fluid bed for gas
phase oxidations; in slurry form, the modified carbon can serve to
catalyze liquid phase oxidations. The novel catalysts of the
invention, preferably comprising a noble metal active phase over a
support comprising carbon and a transition metal/nitrogen
composition, are advantageously used in gas phase reactions such as
benzyl alcohol to benzaldehyde, glucose to gluconic acid, and
various carbohydrate oxidations. Examples of liquid phase reactions
in which the noble metal-bearing catalysts can be used include the
oxidation of alcohols and polyols to form aldehydes, ketones, and
acids (e.g., the oxidation of 2-propanol to form acetone, and the
oxidation of glycerol to form glyceraldehyde, dihydroxyacetone, or
glyceric acid); the oxidation of aldehydes to form acids (e.g., the
oxidation of formaldehyde to form formic acid, and the oxidation of
furfural to form 2-furan carboxylic acid); the oxidation of
tertiary amines to form secondary amines (e.g., the oxidation of
nitrilotriacetic acid ("NTA") to form iminodiacetic acid ("IDA"));
the oxidation of secondary amines to form primary amines (e.g., the
oxidation of IDA to form glycine); and the oxidation of various
acids (e.g., formic acid or acetic acid) to form carbon dioxide and
water.
[0086] In a particularly preferred application, the novel noble
metal over Fe/N/carbon or Co/N/carbon catalysts of the invention
are used in the catalytic oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine. In this reaction, the noble-metal
bearing catalysts of the present invention have been demonstrated
to substantially enhance the rate of conversion as compared to the
otherwise highly preferred catalysts described in U.S. Pat. No.
6,417,133.
[0087] As noted above, the oxidation of
N-(phosphonomethyl)iminodiacetic acid results not only in the
formation of N-(phosphonomethyl)glycine but also the C.sub.1
by-products formaldehyde and formic acid. As the C.sub.1
by-products are formed, formaldehyde and formic acid are preferably
further oxidized to carbon monoxide, thereby to avoid excessive
formation of by-product N-methyl-N-(phosphonomethyl)glycine. The
overall reactions involved in the process are accomplished via
complementary oxidation and reduction steps which may be summarized
as follows:
##STR00003##
The oxidation of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine is believed to occur at both carbon
sites and noble metal sites of the catalyst, while the oxidation of
formaldehyde to formic acid takes place primarily on the noble
metal. The complementary reduction of oxygen, which drives the
oxidation reactions, is believed to take place not only at the
noble metal sites of the catalyst but also on the carbon surface
carrying finely dispersed pyrolyzed Fe/N compounds that may also
include bound carbon. This is illustrated schematically in FIG. 13
which shows the oxidation of N-(phosphonomethyl)iminodiacetic acid
to N-(phosphonomethyl)glycine at both Pt and other sites, the
oxidation of formaldehyde to formic acid and the further oxidation
of formic acid to CO.sub.2 and water which takes place primarily at
the Pt sites, and the reduction of oxygen which is understood to
take place predominantly at Pt sites or sites of an active Fe/N
phase that had been produced by pyrolysis of an FeTPP complex on
the carbon support during the preparation of the catalyst. The
reactions are balanced by transfer of electrons from oxidation to
reduction sites through the bulk of the catalyst body. The presence
of the pyrolyzed Fe/N compounds is believed to enhance the reaction
by supplementing the oxygen reduction capability of the
catalyst.
[0088] To begin the N-(phosphonomethyl)iminodiacetic acid oxidation
reaction, it is preferable to charge the reactor with the
N-(phosphonomethyl)iminodiacetic acid reagent (i.e.,
N-(phosphonomethyl)iminodiacetic acid or a salt thereof), catalyst,
and a solvent in the presence of oxygen. The solvent is most
preferably water, although other solvents (e.g., glacial acetic
acid) are suitable as well.
[0089] The reaction may be carried out in a wide variety of batch,
semi-batch, and continuous reactor systems. The configuration of
the reactor is not critical. Suitable conventional reactor
configurations include, for example, stirred tank reactors, fixed
bed reactors, trickle bed reactors, fluidized bed reactors, bubble
flow reactors, plug flow reactors, and parallel flow reactors. A
further discussion of suitable reactor systems, particularly
continuous reactor systems, may be found in U.S. Publication No.
US-2002-0068836-A1, which is hereby incorporated by reference in
its entirety.
[0090] When conducted in a continuous reactor system, the residence
time in the reaction zone can vary widely depending on the specific
catalyst and conditions employed. Typically, the residence time can
vary over the range of from about 3 to about 120 minutes.
Preferably, the residence time is from about 5 to about 90 minutes,
and more preferably from about 5 to about 60 minutes. When
conducted in a batch reactor, the reaction time typically varies
over the range of from about 15 to about 120 minutes. Preferably,
the reaction time is from about 20 to about 90 minutes, and more
preferably from about 30 to about 60 minutes.
[0091] In a broad sense, the oxidation reaction may be practiced in
accordance with the present invention at a wide range of
temperatures, and at pressures ranging from sub-atmospheric to
super-atmospheric. Use of mild conditions (e.g., room temperature
and atmospheric pressure) have obvious commercial advantages in
that less expensive equipment may be used. However, operating at
higher temperatures and super-atmospheric pressures, while
increasing plant costs, tends to improve phase transfer between the
liquid and gas phase and increase the
N-(phosphonomethyl)iminodiacetic acid oxidation reaction rate.
[0092] Preferably, the N-(phosphonomethyl)iminodiacetic acid
reaction is conducted at a temperature of from about 20.degree. to
about 180.degree. C., more preferably from about 50.degree. to
about 140.degree. C., and most preferably from about 80.degree. to
about 110.degree. C. At temperatures greater than about 180.degree.
C., the raw materials tend to begin to slowly decompose.
[0093] The pressure used during the
N-(phosphonomethyl)iminodiacetic acid oxidation generally depends
on the temperature used. Preferably, the pressure is sufficient to
prevent the reaction mixture from boiling. If an oxygen-containing
gas is used as the oxygen source, the pressure also preferably is
adequate to cause the oxygen to dissolve into the reaction mixture
at a rate sufficient such that the N-(phosphonomethyl)iminodiacetic
acid oxidation is not limited due to an inadequate oxygen supply.
The pressure preferably is at least equal to atmospheric pressure.
More preferably, the oxygen partial pressure is from about 30 to
about 500 psig, and most preferably from about 30 to about 130
psig.
[0094] The catalyst concentration preferably is from about 0.1% to
about 10% by weight ([mass of catalyst/total reaction
mass].times.100%). More preferably, the catalyst concentration
preferably is from about 0.2% to about 5% by weight, and most
preferably from about 0.3% to about 1.5% by weight. Concentrations
greater than about 10% by weight are difficult to filter. On the
other hand, concentrations less than about 0.1% by weight tend to
produce unacceptably low reaction rates.
[0095] The concentration of N-(phosphonomethyl)iminodiacetic acid
reagent in the feed stream is not critical. Use of a saturated
solution of N-(phosphonomethyl)iminodiacetic acid reagent in water
is preferred, although for ease of operation, the process is also
operable at lesser or greater N-(phosphonomethyl)iminodiacetic acid
reagent concentrations in the feed stream. If the catalyst is
present in the reaction mixture in a finely divided form, it is
preferred to use a concentration of reactants such that the
N-(phosphonomethyl)glycine product remains in solution so that the
catalyst can be recovered for re-use, for example, by filtration.
On the other hand, greater concentrations tend to increase reactor
through-put.
[0096] It should be recognized that, relative to many
commonly-practiced commercial processes, this invention allows for
greater temperatures and N-(phosphonomethyl)iminodiacetic acid
reagent concentrations to be used to prepare
N-(phosphonomethyl)glycine while minimizing by-product formation.
In the commonly practiced commercial processes using a carbon-only
catalyst, it is economically beneficial to minimize the formation
of the NMG by-product formed by the reaction of
N-(phosphonomethyl)glycine with the formaldehyde by-product. With
these processes and catalysts, temperatures of from about
60.degree. to about 90.degree. C. and
N-(phosphonomethyl)iminodiacetic acid reagent concentrations below
about 9.0% by weight ([mass of N-(phosphonomethyl)iminodiacetic
acid reagent/total reaction mass].times.100%) typically are used to
achieve cost effective yields and to minimize the generation of
waste. At these temperatures, the maximum
N-(phosphonomethyl)glycine solubility typically is less than 6.5%.
However, with the oxidation catalyst and reaction process of this
invention, the loss of noble metal from the catalyst and catalyst
deactivation have been minimized and the formaldehyde is more
effectively oxidized, thereby allowing for reaction temperatures as
high as 180.degree. C. or greater with
N-(phosphonomethyl)iminodiacetic acid reagent solutions and
slurries of the N-(phosphonomethyl)iminodiacetic acid reagent. The
use of higher temperatures and reactor concentrations permits
reactor throughput to be increased, reduces the amount of water
that must be removed before isolation of the solid
N-(phosphonomethyl)glycine, and reduces the cost of manufacturing
N-(phosphonomethyl)glycine. This invention thus provides economic
benefits over many commonly-practiced commercial processes.
[0097] Normally, a N-(phosphonomethyl)iminodiacetic acid reagent
concentration of up to about 50% by weight ([mass of
N-(phosphonomethyl)iminodiacetic acid reagent/total reaction
mass].times.100%) may be used (especially at a reaction temperature
of from about 20.degree. to about 180.degree. C.). Preferably, a
N-(phosphonomethyl)iminodiacetic acid reagent concentration of up
to about 25% by weight is used (particularly at a reaction
temperature of from about 60.degree. to about 150.degree. C.). More
preferably, a N-(phosphonomethyl)iminodiacetic acid reagent
concentration of from about 12% to about 18% by weight is used
(particularly at a reaction temperature of from about 100.degree.
to about 130.degree. C.). N-(phosphonomethyl)iminodiacetic acid
reagent concentrations below 12% by weight may be used, but their
use is less economical because less N-(phosphonomethyl)glycine
product is produced in each reactor cycle and more water must be
removed and energy used per unit of N-(phosphonomethyl)glycine
product produced. Lower temperatures (i.e., temperatures less than
100.degree. C.) often tend to be less advantageous because the
solubility of the N-(phosphonomethyl)iminodiacetic acid reagent and
N-(phosphonomethyl)glycine product are both reduced at such
temperatures.
[0098] The oxygen source for the N-(phosphonomethyl)iminodiacetic
acid oxidation reaction may be any oxygen-containing gas or a
liquid comprising dissolved oxygen. Preferably, the oxygen source
is an oxygen-containing gas. As used herein, an "oxygen-containing
gas" is any gaseous mixture comprising molecular oxygen which
optionally may comprise one or more diluents which are non-reactive
with the oxygen or with the reactant or product under the reaction
conditions. Examples of such gases are air, pure molecular oxygen,
or molecular oxygen diluted with helium, argon, nitrogen, or other
non-oxidizing gases. For economic reasons, the oxygen source most
preferably is air or pure molecular oxygen.
[0099] The oxygen may be introduced by any conventional means into
the reaction medium in a manner which maintains the dissolved
oxygen concentration in the reaction mixture at the desired level.
If an oxygen-containing gas is used, it preferably is introduced
into the reaction medium in a manner which maximizes the contact of
the gas with the reaction solution. Such contact may be obtained,
for example, by dispersing the gas through a diffuser such as a
porous frit or by stirring, shaking, or other methods known to
those skilled in the art.
[0100] The oxygen feed rate preferably is such that the
N-(phosphonomethyl)iminodiacetic acid oxidation reaction rate is
not limited by oxygen supply. If the dissolved oxygen concentration
is too high, however, the catalyst surface tends to become
detrimentally oxidized, which, in turn, tends to lead to more
leaching and decreased formaldehyde activity (which, in turn, leads
to more NMG being produced).
[0101] Generally, it is preferred to use an oxygen feed rate such
that at least about 40% of the oxygen is utilized. More preferably,
the oxygen feed rate is such that at least about 60% of the oxygen
is utilized. Even more preferably, the oxygen feed rate is such
that at least about 80% of the oxygen is utilized. Most preferably,
the rate is such that at least about 90% of the oxygen is utilized.
As used herein, the percentage of oxygen utilized equals: (the
total oxygen consumption rate/oxygen feed rate).times.100%. The
term "total oxygen consumption rate" means the sum of: (i) the
oxygen consumption rate ("R.sub.i") of the oxidation reaction of
the N-(phosphonomethyl)iminodiacetic acid reagent to form the
N-(phosphonomethyl)glycine product and formaldehyde, (ii) the
oxygen consumption rate ("R.sub.ii") of the oxidation reaction of
formaldehyde to form formic acid, and (iii) the oxygen consumption
rate ("R.sub.iii") of the oxidation reaction of formic acid to form
carbon dioxide and water.
[0102] In one embodiment of this invention, oxygen is fed into the
reactor as described above until the bulk of
N-(phosphonomethyl)iminodiacetic acid reagent has been oxidized,
and then a reduced oxygen feed rate is used. This reduced feed rate
preferably is used after about 75% of the
N-(phosphonomethyl)iminodiacetic acid reagent has been consumed.
More preferably, the reduced feed rate is used after about 80% of
the N-(phosphonomethyl)iminodiacetic acid reagent has been
consumed. The reduced feed rate may be achieved by purging the
reactor with air, preferably at a volumetric feed rate which is no
greater than the volumetric rate at which the pure molecular oxygen
was fed before the air purge. The reduced oxygen feed rate
preferably is maintained for a period of from about 2 to about 40
minutes, more preferably from about 5 to about 20 minutes, and most
preferably from about 5 to about 15 minutes. While the oxygen is
being fed at the reduced rate, the temperature preferably is
maintained at the same temperature or at a temperature less than
the temperature at which the reaction was conducted before the air
purge. Likewise, the pressure is maintained at the same or at a
pressure less than the pressure at which the reaction was conducted
before the air purge. Use of a reduced oxygen feed rate near the
end of the N-(phosphonomethyl)iminodiacetic acid reaction tends to
reduce the amount of residual formaldehyde present in the reaction
solution without producing detrimental amounts of
aminomethylphosphonic acid by oxidizing the
N-(phosphonomethyl)glycine product.
[0103] Reduced losses of noble metal may be observed with this
invention if a sacrificial reducing agent is maintained or
introduced into the reaction solution. Suitable reducing agents
include formaldehyde, formic acid, and acetaldehyde. Most
preferably, formic acid, formaldehyde, or mixtures thereof are
used. Experiments conducted in accordance with this invention
indicate that if small amounts of formic acid, formaldehyde, or a
combination thereof are added to the reaction solution, the
catalyst will preferentially effect the oxidation of the formic
acid or formaldehyde before it effects the oxidation of the
N-(phosphonomethyl)iminodiacetic acid reagent, and subsequently
will be more active in effecting the oxidation of formic acid and
formaldehyde during the N-(phosphonomethyl)iminodiacetic acid
oxidation. Preferably from about 0.01% to about 5.0% by weight
([mass of formic acid, formaldehyde, or a combination thereof/total
reaction mass].times.100%) of sacrificial reducing agent is added,
more preferably from about 0.01% to about 3.0% by weight of
sacrificial reducing agent is added, and most preferably from about
0.01% to about 1.0% by weight of sacrificial reducing agent is
added.
[0104] In one preferred embodiment, unreacted formaldehyde and
formic acid are recycled back into the reaction mixture for use in
subsequent cycles. In this instance, the recycle stream also may be
used to solubilize the N-(phosphonomethyl)iminodiacetic acid
reagent in the subsequent cycles.
[0105] Typically, the concentration of N-(phosphonomethyl)glycine
in the product mixture may be as high as 40% by weight or more.
Preferably, the N-(phosphonomethyl)glycine concentration is from
about 5% to about 40% by weight, more preferably from about 8% to
about 30% by weight, and still more preferably from about 9% to
about 15% by weight. Concentrations of formaldehyde in the product
mixture are typically less than about 0.5% by weight, more
preferably less than about 0.3% by weight, and still more
preferably less than about 0.15% by weight.
[0106] Following the oxidation, the catalyst preferably is
subsequently separated from the product mixture by filtration. The
N-(phosphonomethyl)glycine product may then be isolated by
precipitation, for example, by evaporation of a portion of the
water and cooling.
[0107] It should be recognized that the catalyst of this invention
has the ability to be reused over several cycles, depending on how
oxidized its surface becomes with use. Even after the catalyst
becomes heavily oxidized, it may be reused by being reactivated. To
reactivate a catalyst having a heavily oxidized surface, the
surface preferably is first washed to remove the organics from the
surface. It then preferably is reduced in the same manner that a
catalyst is reduced after the noble metal is deposited onto the
surface of the support, as described above.
[0108] In addition to their function as catalysts for the oxidation
of N-(phosphonomethyl)iminodiacetic acid and other substrates with
molecular oxygen, the catalysts of the invention may be used as
electrode materials in electrocatalytic reactions, that can occur
on carbon at less than or equal to about 0.6V vs. an Ag/AgCl
electrode, including the electrocatalytic oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine, and other liquid phase oxidation
substrates as described hereinabove. In the oxidation of
N-(phosphonomethyl)iminodiacetic acid, it is highly desirable if
not essential that the catalyst include a noble metal deposited
over a support modified by the presence of a transition
metal/nitrogen composition of the type herein described. For
certain other substrates the modified carbon may in itself function
as an effective catalyst for the electrolytic reaction. The high
dispersion of the active sites on the carbon surface provides very
high reactivity and promotes rapid catalytic reactions.
[0109] The modified carbon supports and catalysts of the present
invention having a transition metal/nitrogen composition thereon
can also be used as cathode materials in a fuel cell. In operation,
electrical energy is generated by supplying fuel in contact with an
anode of the fuel cell and molecular oxygen in contact with a
cathode thereof. The cathode comprises carbon bodies, a carbon
monolith, or other carbon support modified to provide the aforesaid
transition metal/nitrogen composition thereon. The fuel gives up
electrons at the anode while oxygen is reduced at the cathode by
electrons flowing through the circuit to the anode. A potential is
generated between the electrodes effective for providing electrical
energy to a load in the circuit such as a lamp, a motor, etc.,
between the anode and the cathode.
[0110] A carbon support modified by the presence of the transition
metal/nitrogen composition may serve other functions as well, e.g.,
as an adsorbent for the removal of oxygen and/or hydrogen peroxide
from an aqueous solution or another fluid matrix.
[0111] The following examples are intended to further illustrate
and explain the present invention. This invention, therefore,
should not be limited to any of the details in these examples.
Example 1
[0112] A particulate carbon catalyst designated MC-10 (8 g)
prepared in accordance with Chou, U.S. Pat. No. 4,696,772, and iron
(III) chloride complexed with tetraphenylporphyrin (FeTPP) (2 g)
were stirred into acetone (400 ml) with continued stirring for 48
hours. The solids were separated from the slurry by filtration and
the filtered solids pyrolyzed at 800.degree. C. for 2 hours under a
constant flow of argon. Metal analysis of the pyrolysis product
revealed that the Fe content of the solids was 1.1% by weight.
[0113] A sample (5 mg) of the Fe/N/carbon pyrolysis product
designated FeTPP/MC-10 was suspended in a solution of 0.1 M
orthophosphoric acid (100 ml) at 70.degree. C. and the suspension
subjected to cyclic voltammetry in the reduction of molecular
oxygen using a Model 273A potentiostat/galvanostat (Princeton
Applied Research, Oak Ridge, Tenn.). The applied potential was
varied from 0.5 to 0.1 volts vs. an Ag/AgCl electrode immersed in
the suspension. The cyclic voltammetry cell included a second
electrode comprising a carbon cloth against which the suspended
FeTPP/MC-10 particulates were held by circulating the solution of
orthophosphoric acid through the cloth. Oxygen was bubbled into the
suspension so that it gently contacted the carbon cloth electrode.
As a control, a sample of unmodified MC-10 particulate carbon
catalyst was subjected to cyclic voltammetry under identical
conditions. The resulting voltammograms are set forth in FIG.
1.
[0114] In evaluating these results, it may be noted that, with 5 mg
catalyst, a current of 105 mA is equivalent to a complete 6
electron oxidation of N-(phosphonomethyl)iminodiacetic acid at a
rate 30 g N-(phosphonomethyl)iminodiacetic acid substrate/hour-gram
catalyst.
[0115] Since the MC-10 particulate carbon catalyst has been used
commercially for the oxidation of N-(phosphonomethyl)iminodiacetic
acid to N-(phosphonomethyl)glycine, it provides a reasonable
control for evaluating the oxygen reduction capability of the
FeTPP/MC-10 catalyst of the invention prepared by pyrolysis of
FeTPP on MC-10. While the cyclic voltammograms reflect some oxygen
reduction capability of the MC-10 catalyst, radical improvement is
obtained with the FeTPP/MC-10 catalyst produced in accordance with
the invention.
Example 2
[0116] A second Fe/N/carbon catalyst in accordance with the present
invention was prepared as described in Example 1 except that a
particulate carbon support sold under the trade designation CP-117
(Engelhard Corp., Iselin, N.J.) was substituted for the MC-10
particulate carbon catalyst. The modified CP-117 carbon designated
FeTPP/CP-117 was subjected to cyclic voltammetry in the reduction
of molecular oxygen in the manner described in Example 1. Again a
control was run using unmodified CP-117. The results are set forth
in FIG. 2. Note that although the unmodified CP-117 carbon support
is much less active with respect to oxygen reduction than is the
MC-10 particulate carbon catalyst, FeTPP/CP-117 resulting from
treatment of CP-117 with FeTPP and pyrolysis is nevertheless quite
effective for the purpose.
Example 3
[0117] A third Fe/N/carbon catalyst was prepared by depositing
platinum (5% by weight) and an iron promoter (0.5% by weight) on
the FeTPP/CP-117 modified carbon prepared in Example 2. Deposition
of the Pt active phase and the Fe promoter was carried out in the
manner described in U.S. Pat. No. 6,417,133. This catalyst,
designated PtFe/FeTPP/CP-117, was also subjected to cyclic
voltammetry in the reduction of molecular oxygen in the manner
described in Example 1. The results are shown in FIG. 3.
Example 4
[0118] Comparative oxidation runs were conducted in which the
FeTPP/MC-10 modified particulate carbon catalyst prepared in
Example 1 and unmodified MC-10 particulate carbon catalyst were
separately tested in catalyzing the oxidation of
N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine. In each oxidation run, a 12% by weight
solution of N-(phosphonomethyl)iminodiacetic acid (60 g) in water
(440 ml) was charged to a 1 liter Parr reactor together with
catalyst at a loading of 0.25% (1.25 g). The mixture was heated to
100.degree. C. and potential observed with an ORP probe. A flow of
molecular oxygen gas was introduced into the mixture and the
concentration of CO.sub.2 in the reactor off-gas was also measured
to determine the rate of oxidation. Periodic samples of the
reaction mixture were taken and analyzed by high performance liquid
chromatography (HPLC) for N-(phosphonomethyl)iminodiacetic acid
(PMIDA), N-(phosphonomethyl)glycine (glyphosate), formaldehyde
(CH.sub.2O), formic acid (HCO.sub.2H) and various impurities,
including aminomethylphosphonic acid
(AMPA)+N-methylaminomethylphosphonic acid (MAMPA),
N-methyl-N-(phosphonomethyl)glycine (NMG), phosphate ion (PO.sub.4)
and imino-bis-(methylene)-bis-phosphonic acid (iminobis). The
results of the HPLC analyses are set forth below in Table 1 for the
oxidation run using the unmodified MC-10 particulate carbon
catalyst, and in Table 2 for the oxidation run using the
FeTPP/MC-10 modified carbon catalyst. Comparative ORP and %
CO.sub.2 profiles or illustrated in FIG. 4, and comparative
impurity profiles plotted in FIG. 5.
TABLE-US-00002 TABLE 1 HPLC Analysis of PMIDA Oxidation Using
Unmodified MC-10 Particulate Carbon Catalyst Experiment 1A 1B 1C
Sample Time 10 36 42 (minutes) GLYPHOSATE % 1.853 6.13 6.321 PMIDA
% 4.508 0.71 0.00222 CH.sub.2O % 0.286 0.876 0.931 HCO.sub.2H %
0.075 0.443 0.501 AMPA/MAMPA % 0.060 0.119 0.272 NMG % 0.003 0.075
0.060 PO.sub.4 % 0.010 0.020 0.025 IMINOBIS % 0.028 0.037 0.037
TABLE-US-00003 TABLE 2 HPLC Analysis of PMIDA Oxidation Using
FeTPP/MC-10 Modified Particulate Carbon Catalyst Experiment 2A 2B
2C 2D Sample Time Pre- 8 16 22 (minutes) heatup GLYPHOSATE % 0.299
3.449 6.79 6.141 PMIDA % 0.6244 4.414 0.348 0.00328 CH.sub.2O %
0.016 0.623 1.215 1.287 HCO.sub.2H % 0.025 0.062 0.162 0.174
AMPA/MAMPA % 0.010 0.006 0.032 0.438 NMG % 0.002 0.005 0.043 0.039
PO.sub.4 % DBNQ 0.003 0.006 0.011 IMINOBIS % 0.003 0.038 0.041
0.048
[0119] Based on the CO.sub.2 evolution curve of FIG. 4, the rate of
N-(phosphonomethyl)iminodiacetic acid oxidation was approximately
twice as fast for the FeTPP/MC-10 modified carbon catalyst than for
the unmodified MC-10 carbon catalyst. This is confirmed by the data
of Tables 1 and 2. The data of Tables 1 and 2 also demonstrate that
the FeTPP/MC-10 modified carbon catalyst also changed the
formaldehyde to formic acid ratios significantly. Although the
FeTPP/MC-10 modified catalyst produced higher formaldehyde levels
than the unmodified MC-10 carbon catalyst, the amount of formic
acid produced was only about half that produced using the
unmodified MC-10 carbon catalyst. The level of
N-methyl-N-(phosphonomethyl)glycine impurity was lower using the
modified FeTPP/MC-10 carbon catalyst, possibly due to the low
formic acid to formaldehyde ratio and the shorter run time.
Example 5
[0120] Comparative oxidation runs were conducted in the manner
described in Example 4 except that the catalysts used were the
FeTPP/CP-117 modified particulate carbon catalyst prepared in
Example 2 and unmodified CP-117. HPLC analysis results are shown in
Tables 3 and 4 below. Comparative CO.sub.2 and ORP profiles are
illustrated in FIG. 6. FIG. 7 constitutes an overlay of the data
plotted in FIG. 4 from Example 4 and FIG. 6. FIG. 8 is an overlay
of the plot of the impurities profiles of the comparative oxidation
runs of Example 4, as taken from FIG. 5, with the impurities
profiles of the comparative oxidation runs of this Example.
TABLE-US-00004 TABLE 3 HPLC Analysis of PMIDA Oxidation Using
Unmodified CP-117 Particulate Carbon Catalyst Experiment 3A 3B 3C
Sample Time 12 182 210 (minutes) GLYPHOSATE % 0.821 6.147 5.834
PMIDA % *Too 0.04963 0.00457 Conc. CH.sub.2O % 0.092 0.158 0.150
HCO.sub.2H % 0.047 1.410 1.411 AMPA/MAMPA % 0.004 0.100 0.139 NMG %
0.002 0.232 0.213 PO.sub.4 % 0.004 0.087 0.088 IMINOBIS % 0.018
0.033 0.032
TABLE-US-00005 TABLE 4 HPLC Analysis of PMIDA Oxidation Using
FeTPP/CP-117 Modified Particulate Carbon Catalyst Experiment 4A 4B
4C Sample Time 10 15 19 (minutes) GLYPHOSATE % 5.639 6.401 5.73
PMIDA % 1.442 0.02187 0.00226 CH.sub.2O % 0.899 1.219 1.285
HCO.sub.2H % 0.096 0.109 0.109 AMPA/MAMPA % 0.030 0.434 0.815 NMG %
0.019 0.025 0.016 PO.sub.4 % 0.003 0.004 0.006 IMINOBIS % 0.048
0.052 0.053
[0121] As shown in FIG. 8 and in Table 4, the impurities profiles
for the FeTPP/CP-117 modified carbon catalyst are very similar to
those for the FeTPP/MC-10 modified carbon catalyst, especially with
regard to formaldehyde, formic acid and
N-methyl-N-(phosphonomethyl)glycine. A difference does in appear in
the levels of aminomethylphosphonic acid
(AMPA)+N-methylaminomethylphosphonic acid (MAMPA) in the reaction
solution. It appears that the oxidation reaction using the
FeTPP/CP-117 modified carbon catalyst was so fast that the
N-(phosphonomethyl)glycine product was exposed to oxidation before
the reaction cycle was terminated.
Example 6
[0122] Transition metal/N on carbon catalyst were prepared by
treatment of MC-10 and CP-117 carbons with a co-ordination complex
and pyrolysis substantially in the manner described as described in
Examples 1 and 2 except that cobalt rather than iron was used as
the transition metal and tetramethoxyphenyl porphyrin (TMPP) served
as the ligand in the coordination complex. Comparative
N-(phosphonomethyl)iminodiacetic acid oxidation runs were conducted
using separately the modified carbon catalyst, designated
CoTMPP/MC-10 and CoTMPP/CP-117, and unmodified MC-10 particulate
carbon catalyst substantially in the manner as described in
Examples 4 and 5. Comparative ORP and % CO.sub.2 in the off gas
profiles are plotted in FIG. 9.
Example 7
[0123] A modified CP-117 carbon catalyst was prepared in the manner
described in Example 2 except that only TPP, not a transition
metal/TPP complex, was deposited on the carbon surface prior to
pyrolysis at 800.degree. C. Comparative
N-(phosphonomethyl)iminodiacetic acid oxidation runs were conducted
comparing the performance of this catalyst, designated TPP/CP-117,
with the FeTPP/CP-117 modified catalyst of Example 2 and a control
in which unmodified CP-117 particulate carbon catalyst was used.
These runs were conducted substantially in the manner as described
in Example 4. Comparative % CO.sub.2 in the off gas profiles are
plotted in FIG. 10.
Example 8
[0124] An FeTPP/CP-117 modified catalyst in accordance with the
present invention was prepared substantially in the manner
described in Example 2 above. The catalyst was washed in 0.2% by
weight hydrochloric acid (HCl). Metal analyses conducted before and
after the acid wash indicated a metal loss of 62 to 75%. Subsequent
to acid washing, the FeTPP/CP-117 modified catalyst was subjected
to cyclic voltammetry in the reduction of molecular oxygen using
the method described in Example 1 at a pH of 1.91. The
voltammograms resulting from repeated sweeps are set forth in FIG.
11. The electrochemical data show good current for oxygen reduction
for the acid washed catalyst, between 175 and 275 mA. While the
acid wash resulted in a substantial loss of Fe, the acid washed
FeTPP/CP-117 modified catalyst did not suffer a proportionate loss
in activity for oxygen reduction when compared to the
electrochemical data generated by subjecting the FeTPP/CP-117
modified catalyst to cyclic voltammetry before acid washing.
Example 9
[0125] An iron-promoted platinum on carbon catalyst containing 5%
by weight Pt and 0.5% by weight Fe promoter was prepared by
depositing Pt and Fe onto an FeTPP/CP-117 modified carbon prepared
in the manner of Example 2. The catalyst was washed in 0.2% by
weight hydrochloric acid (HCl). The metal loadings changed from
5.0% Pt and 1.9% Fe before acid washing to 3.7% Pt and 1.2% Fe
after acid washing, again indicating a substantial loss of metal.
Subsequent to acid washing, the PtFe/FeTPP/CP-117 modified catalyst
was subjected to cyclic voltammetry in the reduction of molecular
oxygen using the method described in Example 1 at a pH of 1.92. The
voltammograms resulting from repeated sweeps are set forth in FIG.
12. The electrochemical data show a modest O.sub.2 reduction
current of 50 mA for the first two cycles that rise to 175 mA in
subsequent cycles.
Example 10
[0126] MC-10 particulate carbon catalyst prepared in accordance
with Chou, U.S. Pat. No. 4,696,772, was modified by providing an
Fe/N composition on the carbon bodies in accordance with the method
generally described in Example 1. Using the noble metal on carbon
catalyst preparation method described hereinabove and in U.S. Pat.
No. 6,417,133, platinum and an iron promoter were deposited on the
modified MC-10 carbon support. The Fe-promoted Pt on Fe/N modified
carbon catalyst contained about 5% by weight noble metal and about
0.5% by weight Fe promoter. After Pt and Fe deposition, the
catalyst was pyrolyzed at 800.degree. C. in 7% H.sub.2 in Argon for
2 hours.
[0127] A portion of the pyrolyzed Pt/Fe on modified carbon catalyst
(1.38 g) and an aqueous solution (448 ml) containing
N-(phosphonomethyl)iminodiacetic acid (PMIDA) (50 g; 10% by weight)
was charged to a 1 liter reactor. The charge mixture was heated to
100.degree. C. under nitrogen, after which molecular oxygen gas was
introduced into the mixture at a rate of 2.111 g moles/kg-hr. The
reaction mass was sampled at 6 minutes, 10 minutes, 15 minutes, 19
minutes and 22 minutes after the start of oxygen flow. Each of
these samples was analyzed for N-(phosphonomethyl)glycine
(glyphosate), N-(phosphonomethyl)iminodiacetic acid (PMIDA),
formaldehyde (CH.sub.2O), formic acid (HCO.sub.2H),
N-methyl-N-phosphonomethyl(glycine) (NMG), phosphate ion (PO.sub.4)
and aminomethylphosphonic acid (AMPA)+N-methylaminomethylphosphonic
acid (MAMPA).
[0128] From the analytical data, computations were made of the
total C.sub.1 compounds generated between successive pairs of
samples, total C.sub.1 compounds oxidized between samples, formic
acid oxidized between samples, formaldehyde oxidized between
samples, total oxygen demand between samples, average rate of
oxygen consumption between samples, average rate of oxidation of
N-(phosphonomethyl)iminodiacetic acid between samples, average rate
of formaldehyde oxidation between samples, average proportion of
CO.sub.2 in the off gas from the reactor during operation between
samples, integrated average Arrhenius constant for oxidation of
formaldehyde between samples, and integrated average Arrhenius
constant for oxidation of formic acid between samples. The results
are set forth in Table 5, which also includes a computation of the
phosphorus and nitrogen material balance closure between the second
(10 minute) and third (15 minute) samples, and between the third
and fourth (19 minute) samples.
TABLE-US-00006 TABLE 5 0-Smp 1 Smp 1-2 Smp 2-3 Smp 3-4 Smp 4-5
Averages for 6 min 10 min 15 min 19 min 22 min Smp 5-6 min Smp 6-7
min Up to ~EP 19 min C.sub.1's ox./C.sub.1's gen (%) 43.01 36.03
59.32 89.33 88086.24 -- -- 56.34 HCO.sub.2H ox/CH.sub.2O ox (%)
65.79 62.38 76.33 89.13 141.31 -- -- 75.56 Total O.sub.2 req'd 0.80
0.52 0.83 0.83 0.29 -- -- 0.78 (gmoles/hr) rO.sub.2 (gmoles/kgm-hr)
1.60 1.04 1.65 1.66 0.58 -- -- 1.56 r(PMIDA) 1.53 1.08 1.39 1.14
0.00 -- 0.00 1.36 (gmoles/kgm-hr) r(CH.sub.2O) (gmoles/kgm-hr) 1.00
0.62 1.08 1.14 0.46 -- -- 1.01 r(HCO.sub.2H) 0.6 0.39 0.83 1.02
0.66 -- -- 0.76 (gmoles/kgm-hr) CO.sub.2 Calc (% in off 81.11%
57.94% 82.96% 82.65% 30.12% -- -- 79.52% gas) k(CH.sub.2O) (1/hr)
28.1928 8.4980 10.7479 10.4666 4.9688 -- -- 9.2529 k(HCO.sub.2H)
(1/hr) 28.6996 9.6943 12.8734 13.2620 8.9875 -- -- 9.7643 P&N
balance (Out/In) 97.5989 97.8332 95.2785 95.3597
Example 11
[0129] Using a catalyst prepared in the manner described in Example
10, oxidation of N-(phosphonomethyl)iminodiacetic acid to
N-(phosphonomethyl)glycine was conducted in the manner also
described in Example 10 except that the aqueous charge solution
contained N-(phosphonomethyl)iminodiacetic acid in the amount of 60
g (12% by weight) rather than 50 g (10% by weight). Sampling and
calculations were carried out in the manner described in Example
10, providing the results summarized in Table 6.
TABLE-US-00007 TABLE 6 0-Smp 1 Smp 1-2 Smp 2-3 Smp 3-4 Smp 4-5 Up
to ~EP Averages for 5 min 10 min 15 min 20 min 27 min Smp 5-6 min
Smp 6-7 min 24 min C.sub.1's ox./C.sub.1's 31.48 39.66 57.10 84.30
267.18 -- -- 56.19 gen (%) HCO.sub.2H ox/CH.sub.2O ox 60.48 61.14
72.65 90.84 113.71 -- -- 75.68 (%) Total O.sub.2 req'd 0.72 0.73
0.87 0.95 0.40 -- -- 0.74 (gmoles/hr) r(O.sub.2) 1.45 1.47 1.75
1.90 0.81 -- -- 1.49 (gmoles/kgm-hr) r(PMIDA) 1.57 1.43 1.48 1.37
0.26 -- 0.00 1.29 (gmoles/kgm-hr) r(CH.sub.2O) 0.82 0.93 1.16 1.27
0.61 -- -- 0.96 (gmoles/kgm-hr) r(HCO.sub.2H) 0.50 0.57 0.84 1.15
0.69 -- -- 0.73 (gmoles/kgm-hr) CO.sub.2 Calc (% in 75.67% 75.71%
86.47% 92.20% 42.15% -- -- 76.49% off gas) k(CH.sub.2O) (1/hr)
19.4494 10.2145 9.4947 9.2816 5.4317 -- -- 7.2092 k(HCO.sub.2H)
(1/hr) 27.4077 12.0496 11.2915 12.7952 7.9788 -- -- 7.7793 P&N
balance 87.4567 101.2419 99.0351 97.4716 (Out/In)
Example 12
[0130] N-(phosphonomethyl)iminodiacetic acid was oxidized to
N-(phosphonomethyl)glycine using the process as described in
Example 11. The catalyst was prepared in the manner described in
Example 10, except that final pyrolysis was conducted at
850.degree. C. rather than 800.degree. C. Sampling and calculations
were carried out in the manner described in Example 10, providing
the results summarized in Table 7.
TABLE-US-00008 TABLE 7 0-Smp 1 Smp 1-2 Smp 2-3 Smp 3-4 Smp 4-5 Up
to ~EP Averages for 5 min 10 min 15 min 20 min 25 min Smp 5-6 min
Smp 6-7 min 22 min C.sub.1's ox./C.sub.1's 21.28 26.68 51.57 68.05
962.54 -- -- 44.72 gen (%) HCO.sub.2H ox/CH.sub.2O ox 48.12 45.27
68.92 79.36 137.62 -- -- 65.95 (%) Total O.sub.2 req'd 0.69 0.76
0.90 0.81 0.36 -- -- 0.75 (gmoles/hr) r(O.sub.2) 1.38 1.53 1.81
1.62 0.72 -- -- 1.50 (gmoles/kgm-hr) r(PMIDA) 1.66 1.65 1.60 1.27
0.08 -- 0.00 1.41 (gmoles/kgm-hr) r(CH.sub.2O) 0.73 0.97 1.19 1.09
0.55 -- -- 0.96 (gmoles/kgm-hr) r(HCO.sub.2H) 0.35 0.44 0.82 0.87
0.75 -- -- 0.63 (gmoles/kgm-hr) CO.sub.2 Calc (% in 73.29% 78.28%
88.95% 81.35% 37.37% -- -- 76.96% off gas) k(CH.sub.2O) (1/hr)
14.1961 8.4482 7.6563 6.1607 3.4994 -- -- 5.7473 k(HCO.sub.2H)
(1/hr) 16.6133 7.1583 8.5058 7.2455 6.5757 -- -- 5.2814 P&N
balance 102.0112 99.3144 98.3704 (Out/In)
Comparative Example A
[0131] For purposes of comparison another oxidation run was
conducted in the manner described in Example 11 except that the
catalyst used was unmodified MC-10 particulate carbon catalyst
prepared in accordance with Chou, U.S. Pat. No. 4,696,772. Sampling
and calculations were carried out in the manner described in
Example 10, providing the results summarized in Table 8.
TABLE-US-00009 TABLE 8 Smp Smp Smp Smp Smp Averages 0-Smp 1 1-2 2-3
3-4 4-5 5-6 Smp Up to for 5 min 10 min 20 min 26 min 32 min 40 min
6-7 min ~EP 34 min C.sub.1's 18.80 16.61 42.87 49.98 94.09 205.16
-- 43.91 ox./C.sub.1's gen (%) HCO.sub.2H 40.14 28.08 48.85 54.31
88.91 105.68 -- 55.75 ox/CH.sub.2O ox (%) Total O.sub.2 0.47 0.53
0.54 0.50 0.47 0.28 -- 0.51 req'd (gmoles/hr) r(O.sub.2) 0.94 1.06
1.08 0.99 0.95 0.56 -- 1.02 (gmoles/kgm-hr) r(PMIDA) 1.13 1.21 0.94
0.82 0.63 0.22 0.00 0.91 (gmoles/kgm-hr) r(CH.sub.2O) 0.53 0.71
0.82 0.75 0.67 0.42 -- 0.72 (gmoles/kgm-hr) r(HCO.sub.2H) 0.21 0.20
0.40 0.41 0.59 0.44 -- 0.40 (gmoles/kgm-hr) CO.sub.2 Calc 53.43%
57.32% 56.59% 52.36% 51.23% 29.73% -- 54.56% (% in off gas)
k(CH.sub.2O) 15.7754 9.1884 7.9078 6.5684 5.8229 4.4041 -- 6.5437
(1/hr) k(HCO.sub.2H) 12.0122 3.6380 3.4582 2.5195 3.3144 2.4715 --
2.2232 (1/hr) P&N 99.7028 99.2844 100.0640 97.6328 balance
(Out/In)
[0132] Based on comparison of the results of this Example with the
results achieved in the catalytic oxidation reactions of Examples
10 to 12, it is apparent that the catalyst of the invention is more
effective for the reduction of oxygen than an otherwise identical
catalyst wherein the carbon support has not been modified to
provide a transition metal/nitrogen composition on the carbon. It
may further be concluded that the catalyst of the invention
comprises sites formed by the pyrolytic treatment that are
especially effective for the reduction of oxygen, and that such
sites comprise nitrogen, a transition metal, or a combination of
nitrogen and transition metal. In the aggregate, these formed
catalytic reduction sites are more active for the reduction of
oxygen than the catalytic reduction sites of the carbon support
prior to modification in accordance with the present invention.
Indeed the modified catalyst comprises a population of reduction
sites that are each more active for the reduction of oxygen than
substantially any sites of the carbon support prior to
modification.
[0133] The present invention is not limited to the above
embodiments and can be variously modified. The above description of
the preferred embodiments, including the Examples, is intended only
to acquaint others skilled in the art with the invention, its
principles, and its practical application so that others skilled in
the art may adapt and apply the invention in its numerous forms, as
may be best suited to the requirements of a particular use.
[0134] With reference to the use of the word(s) comprise or
comprises or comprising in this entire specification (including the
claims below), unless the context requires otherwise, those words
are used on the basis and clear understanding that they are to be
interpreted inclusively, rather than exclusively, and that each of
those words is intended to be so interpreted in construing this
entire specification.
* * * * *