A highly catalytic bimetallic system for the low-temperature selective oxidation of methane and lower alkanes with dioxygen as the oxidant

Article abstract of DOI:10.1021/ja964371k

This report describes a highly catalytic bimetallic system for the low temperature selective oxidation of methane, ethane, and butane with dioxygen as the oxidant. The catalytic system consists of a mixture of copper chloride and metallic palladium and operates in a 3:1 mixture (v/v) of trifluoroacetic acid and water in the presence of dioxygen and carbon monoxide. Methane was selectively converted to methanol. The dependence of the reaction rate on the partial pressure of methane was measured, and saturation kinetics was observed. The dependence of the rate on the partial pressure of carbon monoxide was measured at two different pressures of methane, and a first- order dependence on the partial pressure of carbon monoxide was observed in both instances. The activation parameters for the overall reaction were obtained under the reaction conditions when the rate was first order in both methane and carbon monoxide. The values obtained were A = 2 x 10⁴ s^-1 and E(a) = 15.3 kcal mol^-1. Both C-H and C-C cleavage products were observed for ethane and n-butane, resulting in the formation of methanol, ethanol, and acetic acid. The rate of formation of methanol from methane with the bimetallic system was ca. 65 x 10^-4 M/min at 145-150 °C. This rate may be compared to the rate of formation of acetic acid from methanol in the benchmark Monsanto process: 380 x 10^-4 M/min at 180 °C.

Full text of DOI:10.1021/ja964371k

6048
J. Am. Chem. Soc. 1997, 119, 6048-6053
A Highly Catalytic Bimetallic System for the Low-Temperature
Selective Oxidation of Methane and Lower Alkanes with
Dioxygen as the Oxidant
Minren Lin, Terrence Hogan, and Ayusman Sen*
Contribution from the Department of Chemistry, The PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802
X
ReceiVed December 20, 1996
Abstract: This report describes a highly catalytic bimetallic system for the low temperature selective oxidation of
methane, ethane, and butane with dioxygen as the oxidant. The catalytic system consists of a mixture of copper
chloride and metallic palladium and operates in a 3:1 mixture (v/v) of trifluoroacetic acid and water in the presence
of dioxygen and carbon monoxide. Methane was selectively converted to methanol. The dependence of the reaction
rate on the partial pressure of methane was measured, and saturation kinetics was observed. The dependence of the
rate on the partial pressure of carbon monoxide was measured at two different pressures of methane, and a first-
order dependence on the partial pressure of carbon monoxide was observed in both instances. The activation parameters
for the overall reaction were obtained under the reaction conditions when the rate was first order in both methane
4
-1
-1
and carbon monoxide. The values obtained were A ) 2 × 10 s and Ea ) 15.3 kcal mol . Both C-H and C-C
cleavage products were observed for ethane and n-butane, resulting in the formation of methanol, ethanol, and acetic
acid. The rate of formation of methanol from methane with the bimetallic system was ca. 65 × 10 M/min at
45-150 °C. This rate may be compared to the rate of formation of acetic acid from methanol in the benchmark
-
4
1
-
4
Monsanto process: 380 × 10 M/min at 180 °C.
Methane is the least reactive and the most abundant member
conditions.5,6 Additionally, catalytic C-C cleavage is the key
step in petroleum cracking, and a “one-pot” system for both
cracking and subsequent oxidation of the light residues is of
great interest in the context of the need for oxygenates in clean-
of the hydrocarbon family. Ethane comes second in both
categories. Together, their known reserves equal that of
petroleum.1 Thus, the selective oxidative functionalization of
these alkanes to more useful chemical products is of great
7
burning gasoline.
2
practical interest. For example, one of the highest volume
We previously reported a catalytic system for methane and
ethane functionalization that consisted of RhCl3 along with
functionalized organics produced commercially is methanol
9
3
-
-
whose 1995 U.S. production was 11.3 × 10 lbs. The current
several equivalents of Cl and I ions dissolved in an acidic
5
technology for the conversion of alkanes to methanol involves
solvent. This Rh-based catalytic system is superior to other
4
a multi-step process: (a) the high temperature steam reforming
reported systems for the low temperature functionalization of
8
of alkanes to a mixture of H2 and CO and (b) the high
temperature conversion of the mixture of H2 and CO to
methanol. Clearly, the direct, low temperature conversion to
methanol would be far more attractive from an economical
standpoint. Of particular interest would be the formation of
the same end product from different starting alkanes, thus
obviating the need to separate the alkanes. For example, natural
gas is principally methane with 5-10% ethane. A system that
converts both methane and ethane to the same C1 product, such
as methanol, would not require the prior separation of the
alkanes. Of course, the formation of C1 products from ethane
and higher alkanes would require a catalytic cleavage and
oxidation of C-C bonds which has little precedence under mild
methane and ethane with dioxygen; nevertheless, it has several
drawbacks from a practical standpoint. First, rhodium is
relatively expensive and the efficient postseparation of the
catalyst poses a problem because of the homogeneous reaction
system. More importantly, in the reaction of methane, while
methanol could be made the principal product by the appropriate
(5) Lin, M.; Hogan, T. E.; Sen, A. J. Am. Chem. Soc. 1996, 118, 4574.
(6) Examples of noncatalytic C-C cleavage by metal species under mild
conditions: (a) Perthuisot, C.; Jones, W. D. J. Am. Chem. Soc. 1994, 116,
3647. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. (c) Chiang, H. J.; Liu, C.-S. J. Organomet. Chem. 1993, 458,
167. (d) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.; Ben-David,
Y.; Milstein, D. Nature 1994, 370, 42. (e) Gozin, M.; Weisman, A.; Ben-
David, Y.; Milstein, D. Nature 1993, 364, 699. (f) Rondon, D.; Chaudret,
B.; He, X.-D.; Labroue, D. J. Am. Chem. Soc. 1991, 113, 5671. (g) Hartwig,
J. F.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1989, 111, 2717.
(h) Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110,
976. (i) Crabtree, R. H.; Dion, R. P.; Gibboni, D. J.; McGrath, D. V.; Holt,
E. M. J. Am. Chem. Soc. 1986, 108, 7222. (j) Suggs, J. W.; Jun, C.-H. J.
Am. Chem. Soc. 1986, 108, 4679. (k) Watson, P. L.; Roe, D. C. J. Am.
Chem. Soc. 1982, 104, 6471.
X
Abstract published in AdVance ACS Abstracts, June 15, 1997.
(1) (a) Axelrod, M. G.; Gaffney, A. M.; Pitchai, R.; Sofranko, J. A. In
Natural Gas ConVersion II; Curry-Hyde, H. E.; Howe, R. F., Eds.;
Elsevier: Amsterdam, 1994; p 93. (b) Masters, C. D.; Root, D. H.; Attanasi,
E. D. Science 1991, 253, 146. (c) Starr, C.; Searl, M. F.; Alpert, S. Science
1
992, 256, 981.
2) Recent reviews: (a) Olah, G. A.; Moln a´ r, A. Hydrocarbon Chemistry;
Wiley: New York, 1995. (b) Crabtree, R. H. Chem. ReV. 1995, 95, 987.
c) Labinger, J. A. Fuel Process. Technol. 1995, 42, 325. (d) Fierro, J. L.
(
(7) Unzelman, G. H. Oil Gas J. 1991, 89, 44, 62.
(8) (a) Yamanaka, I.; Soma, M.; Otsuka, K. Chem. Lett. 1996, 565. (b)
Nishiguchi, T.; Nakata, K.; Takaki, K.; Fujiwara, Y. Chem. Lett. 1992, 1141.
(c) Goldstein, A. S.; Drago, R. S. J. Chem. Soc., Chem. Commun. 1991,
21. (d) Kao, L.-C.; Hutson, A. C.; Sen, A. J. Am. Chem. Soc. 1991, 113,
700. Examples at higher temperatures (∼200 °C): (e) Vargaftik, M. N.;
Stolarov, I. P.; Moiseev, I. I. J. Chem. Soc., Chem. Commun. 1990, 1049.
(f) Periana, R. A.; Taube, D. J.; Evitt, E. R.; L o¨ ffler, D. G.; Wentrcek, P.
R.; Voss, G.; Masuda, T. Science 1993, 259, 340. Also: Snyder, J. C.;
Grosse, A. V. U.S. Patent 2,493,038, 1950.
(
G. Catal. Lett. 1993, 22, 67. (e) Srivastava, R. D.; Zhou, P.; Stiegel, G. J.;
Rao, V. U. S.; Cinquegrane, G. Catal. (London) 1992, 9, 183.
(
3) Chem. Eng. News 1996, April 8, p 17.
(4) (a) Cheng, W.-H.; Kung, H. H., Eds.; Methanol Production and Use;
Marcel Dekker: New York, 1994. (b) Wade, L. E.; Gengelbach, R. B.;
Trumbley, J. L.; Hallbauer, W. L. In Kirk-Othmer Encyclopedia of Chemical
Technology; Wiley: New York, 1981; Vol. 15, p 398.
S0002-7863(96)04371-5 CCC: $14.00 © 1997 American Chemical Society

SelectiVe Oxidation of Methane and Lower Alkanes
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6049
Scheme 1
The final step involved the metal-catalyzed oxidation of the
alkane by hydrogen peroxide.
The following pieces of evidence support the mechanism
9
13
shown in Scheme 1. The use of CO resulted in the formation
1
3
13
of CO2 as the only C-containing product. Water was also
necessary since no oxidation was observed in a dry trifluoro-
acetic acid-nitromethane mixture. The above observations
were consistent with a metal-catalyzed water gas shift reaction.
It was possible to bypass this first catalytic reaction by replacing
carbon monoxide with dihydrogen. In the latter instance it was
possible to run the reaction in a non-aqueous medium such as
a dry trifluoroacetic acid-nitromethane mixture.
That hydrogen peroxide (or its equivalent) was formed in
this system, starting with carbon monoxide and dioxygen in the
presence of water, was verified by running the reaction in the
absence of alkane. Titration of the resultant solution with a
standard solution of KMnO4 indicated a hydrogen peroxide
concentration of 0.002 M. In an independent experiment we
observed the slow Pd-catalyzed oxidation of ethane to ethanol,
acetic acid, and formic acid by hydrogen peroxide at 30 °C. A
lower reaction temperature was chosen in order to avoid the
observed metal-catalyzed decomposition of hydrogen peroxide
to water and dioxygen. It is this latter undesirable reaction that
made carbon monoxide a more effective coreductant than
dihydrogen. Starting with carbon monoxide, hydrogen peroxide
was formed at a low steady rate through the first two catalytic
reactions and was used efficiently for alkane oxidation. On the
other hand, starting with dihydrogen, hydrogen peroxide was
formed rapidly (as evidenced by a relatively rapid drop in gas
pressure), but most of it underwent subsequent metal-catalyzed
decomposition at the reaction temperature (85-100 °C).
choice of the reaction medium, there was always a significant
amount of acetic acid formed as a coproduct. This was, of
5
course, undesirable if the goal was the production of methanol
only. Given these disadvantages of the Rh-based catalytic
system, we sought a new catalytic system that (a) was based
on less expensive metals, (b) was insoluble in the reaction
medium and could be separated by simple filtration, and (c)
converted methane to methanol, only.
Results and Discussion
We previously discovered9,10 a Pd-based catalytic system for
the oxidation of methane and ethane. In this system, metallic
palladium was found to catalyze the oxidation of methane and
ethane by dioxygen in aqueous medium at 70-110 °C in the
presence of carbon monoxide. Formic acid was the observed
oxidation product from methane while acetic acid, together with
some formic acid, was formed from ethane. No alkane
oxidation was observed in the absence of added carbon
monoxide. The essential role of carbon monoxide in achieving
While acetic acid was formed in good yields from ethane,
the analogous formation of formic acid from methane proceeded
only in low yields because of the general instability of the latter
acid under the reaction conditions. Since formic acid is a much
less desirable product from methane than is methanol, we have
now examined the possibility of halting the oxidation of methane
at the methanol stage.
“
difficult” alkane oxidation was shown by a competition
12
12
13
12
experiment between CH3 CH3 and CH3 CH2OH, in both
the presence and absence of carbon monoxide. In the absence
of added carbon monoxide, only CH3 CO2H was formed by
the oxidation of CH3 CH2OH. When carbon monoxide was
11
13
12
1
3
12
12
12
added, almost half of the products were derived from CH3 CH3.
1
2
12
Simply changing the solvent in the Pd-based catalytic system
from water to a mixture of water and either perfluorobutyric
acid or trifluoroacetic acid (some water was necessary for the
reaction, see Scheme 1) had no significant effect on product
composition: formic acid was still the principal product from
methane. However, the addition of CuCl2 to the reaction
mixture had a dramatic effect. Methanol and its ester now
became the preferred products, with Wirtually no acetic acid
and little formic acid being formed (Figure 1)! Typical
reaction conditions and product yields are shown in Table 1.
High pressure stainless steel reactors equipped with glass liners
The products from this reaction were
CH3 CH2OH,
1
2
12
13
12
CH3 CO2H, and CH3 CO2H. Thus, the more inert ethane
was oxidized only in the presence of added carbon monoxide.
The requirement of a coreductant (carbon monoxide) makes the
overall reaction formally analogous to the monooxygenases in
which only one of the two oxygen atoms in the dioxygen
molecule is used for substrate oxidation.1
2
Studies indicated that the overall transformation encompassed
9
three catalytic steps in tandem (Scheme 1). The first was the
water gas shift reaction involving the oxidation of carbon
monoxide to carbon dioxide with the simultaneous formation
of dihydrogen. The second catalytic step involved the combina-
1
were employed. The products were quantified by H-NMR
13
spectroscopy with use of an external standard consisting of a
capillary tube containing 1 µL of DMSO in 60 µL of D2O. Table
tion of dihydrogen with dioxygen to yield hydrogen peroxide.
(
(
(
(
9) Lin, M.; Sen, A. J. Am. Chem. Soc. 1992, 114, 7307.
10) Sen, A.; Lin, M. U.S. Patent 5,393,922, 1995.
11) Hogan, T.; Simpson, R.; Lin, M.; Sen, A. Catal. Lett. 1996, 40, 95.
12) Reviews on monooxygenases and their synthetic analogs: (a)
1
also shows that as little as 1 mg of 5% Pd on carbon (60
µmol of surface Pd atoms/g of catalyst, as determined from
dihydrogen chemisorption studies) was effective; no reaction
was observed when Pd was omitted. Since methyl trifluoro-
acetate is both volatile and easily hydrolyzed back to the acid
and methanol, it should be possible to design a system where
the acid is recycled and methanol is the end product.
Valentine, J. S. in Bioinorganic Chemistry; Bertini, I., Gray, H. B., Lippard,
S. J., Valentine, J. S., Eds.; University Science Books: Mill Valley, CA,
1994; p 253. (b) Que, L. in Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel
Dekker: New York, 1993; p 347. (c) Mansuy, D.; Battioni, P. In
Bioinorganic Catalysis; Reedijk, J., Ed.; Marcel Dekker: New York, 1993;
p 395. (d) Groh, S. E.; Nelson, M. J. In SelectiVe Hydrocarbon ActiVation;
Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; VCH:
New York, 1990; p 305. (e) Ortiz de Montellano, P. R. In Cytochrome
P-450: Structure, Mechanism and Biochemistry; Ortiz de Montellano, P.
R., Ed.; Plenum: New York, 1986; p 217. (f) Groves, J. T. J. Chem. Ed.
Table 2 shows the effect of varying the concentration of
CuCl2. Within the range studied, the amount of Cu(II) ion
present did not appear to make a significant difference in the
rate of product formation, although in its absence formic acid
became the preferred product (see above). The fate of CuCl2
in the course of the reaction was briefly examined. As expected,
1
1
985, 62, 928. (g) Guegerich, F. P.; Macdonald, T. L. Acc. Chem. Res.
984, 17, 9.
(13) Gosser, L. W. U.S. Patent 4,681,751, 1987.

6050 J. Am. Chem. Soc., Vol. 119, No. 26, 1997
Lin et al.
-
Table 3. The Cu(II) + Cl /[Pd/C] Catalyst System in H
2
O/
CF CO
3
2
H for Methane Oxidationa
NaCl (mmol)
time (h)
CH
3
OH (+ ester) (M)
0.00
0.05
0.10
0.15
42b
48
48
48
0.02
0.42
0.45
0.42
a
Standard reaction conditions: Cu(CF
C], 3.0 mg; H O, 1.0 mL; CF CO H, 3.0 mL; CH
psi; O , 100 psi; 85-95 °C. The solution was homogeneous after
3
CO
2
)
2
, 0.1 mmol; [5% Pd/
2
3
b
2
4
, 900 psi; CO, 200
2
reaction.
M); H2O, 0.5 mL; CF3CO2H, 1.5 mL; CH4, 800 psi; CO, 150
psi; O2, 75 psi; 90-95 °C. The comparable yields of product
obtained from the two reactions in 1 h also indicated that there
was no significant induction period during which Cu(II) was
being reduced to Cu(I) in the first reaction. Whether CuCl
remains in the solid state through the course of the reaction
cannot be addressed unequivocally since it is quite possible that
under high pressure of CO, soluble Cu(I)-carbonyl complexes
Figure 1. 1H-NMR spectrum obtained with the following reaction
conditions: CuCl , 0.1 mmol; 5% Pd/C, 1.0 mg; H O, 1.0 mL;
CF CO H, 3.0 mL; CH , 900 psi; CO, 200 psi; O , 100 psi; 85-95
C; 90 h.
2
2
1
4
were being formed.
3
2
4
2
°
The recycling of catalyst was carried out in one experiment.
Following the oxidation of methane under the standard reaction
conditions, the reactor was opened and the precipitated CuCl,
together with metallic Pd, was filtered off and used in a second
methane oxidation reaction. The yield obtained in the latter
reaction was comparable to that in the first.
Table 1. The CuCl
Methane Oxidation
2
/[Pd/C] Catalyst System in H
2
O/CF
3
CO
2
H for
a
time (h)
[5% Pd/C] (mg)
0.0
CH
3
OH (+ester) (M)
2
0
tr
1
6
2.0
1.0
6
2.0
1.0
.0
.0
0.12
0.16
0.07
0.48
0.46
0.22
0.75
0.69
0.39
-
As Table 3 illustrates, the presence of Cl ion was essential
for the conversion of methane to methanol and its ester to occur.
1
1
1
-
An alternative way to add both Cl ion and metallic Pd to the
5
9
0
0
.0
system was by starting with K2PdCl4 which was rapidly reduced
-
to the metallic Pd by CO with the simultaneous release of Cl
ion. A direct comparison between K2PdCl4 and 5% Pd/C in
the oxidation of ethane revealed similar product distributions
and yields (see below).
The effect of varying the halide ion in the copper salt
employed was examined. With the following conditionssCu
salt, 0.06 mmol; 5% Pd/C, 20 mg; H2O, 1.0 mL; CF3CO2H,
3.0 mL; CH4, 1000 psi; CO, 100 psi; O2, 100 psi; temperature,
85-95 °Csthe yields of methanol and its ester in 20 h were
6
2.0
.0
a
Standard reaction conditions: CuCl
2
2
, 0.1 mmol; H O, 1.0 mL;
CF
3
CO H, 3.0 mL; CH , 900 psi; CO, 200 psi; O
2
4
2
, 100 psi; 85-95 °C.
Table 2. The CuCl
Methane Oxidation
2
/[Pd/C] Catalyst System in H
2
O/CF
3
CO
2
H for
a
0
.27, 0. 12, and 0.04 M for CuCl2, CuBr2, and CuI, respectively.
time (h)
CuCl
2
(mmol)
CH
3
OH (+ester) (M)
-
-
-
Thus, the yield decreased in the order Cl > Br > I .
While many of the catalytic studies were carried out with
5% Pd on carbon, Pd black was also effective for the reaction.
Additionally, control experiments indicated that carbon itself
did not catalyze methane oxidation in the absence of metallic
Pd.
18
50
90
0.05
0.16
0.12
0.10
0.47
0.48
0.48
0.73
0.75
0.63
b
0
0
.10
.15
0.05
0
0
.10
.15
0.05
The relative rates of methane oxidation to methanol derivative
versus further oxidation of methanol were examined by carrying
0
0
.10
.15
1
2
13
out the oxidation of CH4 in the presence of CH3OH. A 3:1
mixture (v/v) of trifluoroacetic acid and water was made 0.19
a
Standard reaction conditions: [5% Pd/C], 1.0 mg; H
CO H, 3.0 mL; CH , 900 psi; CO, 200 psi; O , 100 psi; 85-95 °C.
For 20 h.
2
O, 1.0 mL;
CF
3
2
4
2
1
3
b
M in CH3OH (most of which was converted to the methyl
ester) and the methane functionalization reaction was run under
conditions identical with those shown in Figure 1. The methanol
and its derivatives observed after 48 h were as follows:
CF3CO2 CH3 + CH3OH (0.05 M), and CF3CO2 CH3 +
CH3OH (0.88 M). The data indicated that the rate of methanol
the initial reaction mixture was blue due to the presence of Cu(II)
ion in solution. However, when the reactor was opened after
the partial oxidation of methane was allowed to proceed for
several hours, the solution was found to be colorless and a white
precipitate was found at the bottom of the glass liner employed.
Upon exposure to air this precipitate dissolved to regenerate
the blue solution. An elemental analysis of the white precipitate
revealed it to be CuCl. Presumably, the Cu(II) salt added
initially to the reaction mixture was reduced by CO in the course
of the reaction. Essentially identical amounts of methanol and
its derivative (0.09 M) were formed in 1 h starting with either
1
3
13
12
12
formation was at least 6.5 times greater than the overoxidation
of methanol. The unusually high selectivity toward methanol
formation is striking. Given that the C-H bond energy of
methane is 10 kcal/mol higher than that in methanol, any
oxidation procedure that involved hydrogen atom abstraction
from the substrate C-H bond would have resulted in up to six
(14) (a) Jagner, S.; Håkansson, M. Inorg. Chem. 1990, 29, 5241. (b)
-
CuCl2 (0.0074 M) or CuCl + Cl (0.0074 M each) under the
Busch, M. A.; Franklin, T. C. Inorg. Chem. 1979, 18, 521. (c) Scott, A. F.;
Wilkening, L. L.; Rubin, B. Inorg. Chem. 1969, 8, 2533.
-
5
following reaction conditions: K2PdCl4, 0.02 mg (3.0 × 10

SelectiVe Oxidation of Methane and Lower Alkanes
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6051
Table 4. Effect of Temperature on Methane Oxidation with the
a
2 2 4 2 3 2
CuCl /K PdCl Catalyst System in H O/CF CO H
turnover/min
CH
(+ester) (M)
3
OH
based
on Cu
based
on Pd
temp (°C)
time (min)
8
9
5-90
360
270
225
90
85
85
0.34
0.42
0.58
0.30
0.45
0.55
0.13
0.21
0.35
0.45
0.72
0.88
31.5
52.2
86.2
110.8
176.1
216.7
5-100
1
1
1
1
10-115
25-130
40-145
45-150
a
-3
Standard reaction conditions: CuCl
2
, 2.0 mg (7.4 × 10 M);
3 2
-
5
K
2
PdCl
4
, 0.02 mg (3.0 × 10 M); H
2
O, 0.5 mL; CF CO H, 1.5 mL;
CH , 1000 psi; CO, 200 psi; O
4
2
, 100 psi.
Figure 3. Rate dependence on the partial pressure of carbon monoxide
at 300 psi CH
0
4
(4) and 600 psi CH
.015 mmol; K PdCl , 0.06 µmol; H
, 300 (4) or 600 (O) psi; CO + N
4
(0). Reaction conditions: CuCl
O, 0.5 mL; CF CO
, 900 (4) or 600 (0) psi; O
2
2
,
2
4
2
3
2
H, 1.5 mL;
CH
4
2
,
1
00 psi; 77-85 °C; 1 h.
Figure 2. Rate dependence on the partial pressure of methane. Reaction
conditions: CuCl , 0.015 mmol; K PdCl , 0.06 µmol; H O, 0.5 mL;
CF CO H, 1.5 mL; CO, 200 psi; O , 100 psi; CH + N , 1000 psi;
7-85 °C; 1 h.
2
2
4
2
3
2
2
4
2
7
orders of magnitude higher rate for methanol overoxidation
when compared to its rate of formation from methane (however,
see below). Note that at 900 psi of pressure, the concentration
of methane in pure water is 0.045 M. While the solubility of
methane in a 6:1 mixture (v/v) of perfluorobutyric acid and water
is expected to be higher, it is not likely to significantly exceed
.19 M, the concentration of added 1 CH3OH. The activity of
3
Figure 4. Arrhenius plot for the oxidation of methane. Reaction
conditions: CuCl , 0.015 mmol; K PdCl , 0.06 µmol; H O, 0.5 mL;
CF CO H, 1.5 mL; CH , 500 psi; CO, 200 psi; O , 100 psi; N , 500
psi; 1 h.
0
2
2
4
2
methane at constant pressure should not, of course, vary with
solvent composition.
3
2
4
2
2
The most interesting aspect of the chemistry described above
was the significant increase in the rate of methane to methanol
conversion with an increase in reaction temperature. This is
illustrated in Table 4. It should be noted that because of the
experimental setup, it required 20-40 min for the reaction
mixture to reach the indicated temperature. Thus the actual rate
of reaction at a given temperature was invariably higher than
that shown in Table 4.
The dependence of the reaction rate on the partial pressures
of methane and carbon monoxide was examined. In order to
avoid the lag time associated with the stainless steel bomb and
the reaction mixture reaching the designated reaction temper-
ature, the bomb and its contents (except for the reactant gases)
were preheated. The reaction was then initiated by adding the
reactant gases (see Experimental Section for details). Saturation
kinetics was observed for methane (Figure 2). Thus, a first-
order dependence on methane partial pressure was obtained up
to a pressure of 700 psi. However, the reaction rate became
zero order in methane when its partial pressure exceeded 700
psi. The dependence of the rate on the partial pressure of carbon
monoxide was measured at two different pressures of methane
(Figure 3). A first-order dependence on the partial pressure of
carbon monoxide was observed in both instances.
We have obtained the activation parameters for the overall
reaction under the reaction condition when the rate was first
order in both methane and carbon monoxide. The Arrhenius
plot obtained is shown in Figure 4 and gave the following
4
-1
-1
values: A ) 2 × 10 s ; Ea ) 15.3 kcalmol . For comparison,
the activation energy for hydrogen abstraction from methane
in the oxidative coupling reaction catalyzed by Li/MgO varies
-
1 15
from 25 to 50 kcalmol .
The ability of metal salts, other than CuCl2, as cocatalysts in
halting the functionalization of methane at the methanol stage
(15) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970.

6052 J. Am. Chem. Soc., Vol. 119, No. 26, 1997
Lin et al.
Table 5. Fe(II)/[Pd/C] and Fe(III)/[Pd/C] Catalyst Systems in
they are rapidly trapped by O2 (however, a non-radical mech-
anism for Fenton chemistry has also been suggested recently ).
a
H
2
O/CF
3
CO
2
H for Methane Oxidation
17
cocatalyst
time (h)
48
CH
3
OH (+ester) (M)
Thus, the rate of methanol formation was at least 6.5 times
greater than the overoxidation of methanol. Given that the C-H
bond energy of methane is 10 kcal/mol higher than that in
methanol, any oxidation procedure that involved hydrogen atom
abstraction from the substrate C-H bond would have resulted
in up to six orders of magnitude higher rate for methanol
overoxidation when compared to its rate of formation from
methane. However, this argument does not take into account
the increase in the C-H bond energy when methanol is
converted to the ester (the following C-H bond-energy data
illustrate the point: H-CH3, 104 kcal/mol; H-CH2OH, 94 kcal/
FeCl
2
3
0.12
0.11
0.07
0.16
84
FeCl
48
84
a
Standard reaction conditions: [5% Pd/C], 3.0 mg; cocatalyst, 0.10
mmol; H O, 1.0 mL; CF CO H, 3.0 mL; CH , 900 psi; CO, 200 psi;
, 100 psi; 85-95 °C.
2
3
2
4
O
2
18
mol; H-CH2OCOC6H5, 100.2 kcal/mol ). A further evidence
against the participation of free radicals was that acetic acid
was not formed from methane even though carbon monoxide
was present in the reaction mixture. Note that carbon monoxide
is a trapping agent for alkyl radicals (e.g., rate constant in water
•
19
6
-1 -1
for trapping of CH3 by CO: 2 × 10 L mol s ) and we
have observed the formation of acetic acid in good yields from
methyl radical and carbon monoxide in water under oxidizing
2
0
conditions.
Nevertheless, the lack of carbon monoxide
trapping does not rule out radical formation and recombination
within a solvent cage.
We now turn to the catalytic C-C cleavage reactions. From
the data given above on ethane oxidation, the ratio of products
derived from initial C-H cleavage versus C-C cleavage was
approximately 1, on a per bond basis. No methanol was
observed when ethane was omitted from the reaction mixture,
indicating that methanol was not being formed from carbon
monoxide (e.g., by hydrogenation). Experiments with ethanol,
acetaldehyde, and acetic acid as substrates indicated that all three
formed methanol in varying degrees (acetaldehyde > ethanol
Figure 5. 1H-NMR spectrum obtained with the following reaction
conditions: CuCl , 0.1 mmol; 5% Pd/C, 1.0 mg; H O, 1.0 mL;
CF CO H, 3.0 mL; C , 400 psi; CO, 150 psi; O , 75 psi; N , 400
psi; 85-95 °C; 90 h.
2
2
3
2
H
2 6
2
2
was briefly examined. As shown in Table 5, methanol and its
ester were also the dominant products in the presence of either
FeCl2 or FeCl3, but the rate of methane conversion was lower
compared to that observed in the presence of CuCl2.
We have also briefly examined ethane and n-butane as
substrates in the CuCl2/Pd catalytic system. As expected, the
rate of ethane conversion was significantly higher than methane
conversion with the products being ethanol (and its ester),
methanol (and its ester), and acetic acid (see Figure 5). Thus,
.
acetic acid). However, control experiments involving
1
3
12
13
12
CH3CH2OH and C2H6, as well as CH3CO2H and C2H6,
showed that in the presence of the alkane both ethanol and acetic
acid were unreactive, with all the products formed being derived
from ethane.
For n-butane, virtually all of the observed products were
derived by C-C cleavage. Note that the formation of methanol
from ethane and ethanol from n-butane would appear to rule
out metal-assisted cleavage of vicinal diols as the source of C-C
cleavage products.
0
.75 M ethanol and its ester, 0.30 M methanol and its ester,
and 0.14 M acetic acid were formed in 16 h under the following
reaction conditions: CuCl2, 0.1 mmol; [5% Pd/C], 3.0-5.0 mg;
H2O, 1.0 mL; CF3CO2H, 3.0 mL; C2H6, 400 psi; CO, 150 psi;
O2, 75 psi; N2, 400 psi; temperature 85-95 °C. For comparison,
a similar product distribution was obtained when K2PdCl4 was
used in place of 5% Pd/C. For example, 0.72 M ethanol and
its ester, 0.22 M methanol and its ester, and 0.15 M acetic acid
were formed in 18 h under the following reaction conditions:
Finally, it is instructive to compare our reaction rate with
that in the “Monsanto system” for the carbonylation of
methanolsan enormously successful solution-phase commercial
2
1
process. In a 3:1 mixture (v/v) of trifluoroacetic acid and
water, the rate of formation of methanol from methane with
-
4
use of the bimetallic system was ca. 65 × 10 M/min at 145-
1
50 °C. In the Monsanto process, the rate of formation of acetic
-
4
22
-
3
-5
acid from methanol is 380 × 10 M/min at 180 °C. Thus,
the rate is 6 times faster in the latter system. However, the
substrate concentration in the Monsanto system is typically 5
M. On the other hand, at the 1000 psi of pressure that we
employ, the aqueous concentration of methane is approximately
CuCl2, 2.0 mg (7.4 × 10 M); K2PdCl4, 0.02 mg (3.0 × 10
M); H2O, 0.5 mL; CF3CO2H, 1.5 mL; C2H6, 400 psi; CO, 150
psi; O2, 75 psi; N2, 400 psi; temperature 90-95 °C. In the
case of n-butane, 0.14 M ethanol and its ester, 0.14 M methanol
and its ester, and 0.15 M acetic acid were formed in 20 h under
the following reaction conditions: CuCl2, 2.0 mg (7.4 × 10
M); K2PdCl4, 0.02 mg (3.0 × 10 M); H2O, 0.5 mL; CF3CO2H,
.5 mL; C4H10, 30 psi; CO, 150 psi; O2, 75 psi; N2, 800 psi;
-
3
(17) Sawyer, D. T.; Sobkowiak, A.; Matsushita, T. Acc. Chem. Res. 1996,
-5
29, 409.
(18) CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC
1
Press: Boca Raton, 1991; pp 9-114.
temperature 85-95 °C.
(19) Bakac, A.; Espenson, J. H. J. Chem. Soc., Chem. Commun. 1991,
16
Unlike the “classical” Fenton-type systems, free alkyl
1497.
(20) Lin, M.; Sen, A. J. Chem. Soc., Chem. Commun. 1992, 892.
21) (a) Acetic Acid and Its DeriVatiVes; Agreda, V. H., Zoeller, J. R.,
radicals appear not to be intermediates in our system unless
(
(
16) Reviews: (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed
Oxidations of Organic Compounds; Academic: New York, 1981; p 35,
71, 329. (b) Edwards, J. O.; Curci R. In Catalytic Oxidations with Hydrogen
Eds.; Marcel Dekker: New York, 1993. (b) Wagner, F. S. In Kirk-Othmer
Encyclopedia of Chemical Technology; Wiley: New York, 1978; Vol. 1, p
124. (c) Forster, D. AdV. Organomet. Chem. 1979, 17, 255.
(22) Hjortkjaer, J.; Jensen, V. W. Ind. Eng. Chem., Prod. Res. DeV. 1976,
15, 46.
1
Peroxide as Oxidant; Strukul, G., Ed.; Kluwer Academic: Dordrecht, 1992;
p 97.

SelectiVe Oxidation of Methane and Lower Alkanes
.05 M. If the solution concentration (more precisely, the
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6053
0
products was further confirmed by GC/MS and/or by comparison with
authentic samples.
CAUTION: Appropriate precautions should be taken while work-
ing with gases under high pressures. Particular attention should be
paid to flammability limits of gas mixtures.
activity) of methane in a trifluoroacetic acid/water mixture is
similar, then under similar substrate concentrations our rate of
methane conversion is significantly higher than the rate of
methanol conversion in the Monsanto system (this does not take
into account the higher reaction temperature employed for the
latter). Our results, therefore, strongly suggest that the low
temperature direct partial oxidation of methane and lower
alkanes by dioxygen is commercially feasible.
A. Cu/Pd Based Catalytic System. 1. Oxidation of Methane.
In a typical reaction, CuCl
1.0 mg) were added to 1.0 mL of H
CF CO H was then added slowly. The glass container was then placed
in a high pressure bomb that was then sealed. The bomb was purged
and pressurized to 200 psi with CO, to 1200 psi with CH , then to
300 psi with O . The contents were stirred at 80-85 °C for 20 h
2
(13.5 mg, 0.1 mmol) and 5% Pd on carbon
(
2
O in a glass container; 3 mL of
3
2
4
Conclusion
1
2
following which the reactor was cooled and the pressure slowly released.
The reaction mixture was then directly analyzed by NMR spectroscopy.
We have discovered a highly catalytic system that simulta-
neously cleaves C-H and C-C bonds of simple alkanes, as
well as activates dioxygen, under mild conditions. For ethane
and n-butane, products derived from C-C cleavage competed
with or dominated over those derived from C-H cleavage on
a per bond basis.
2
. Oxidation of Ethane. The reaction was conducted as described
in procedure A.1, except that 150 psi of CO, 400 psi of C , 75 psi
of O , and 400 psi of N were added to the bomb.
2
H
6
2
2
B. Fe/Pd Based Catalytic System: Oxidation of Methane. The
reactions were conducted as described in procedure A.1, except that
The present bimetallic system is similar to the previously
described homogeneous Rh-based system5 in its ability to
activate both dioxygen and the alkane and requiring a coreduc-
tant (carbon monoxide). However, there are important differ-
ences in reactivity. First, in the case of methane, the bimetallic
system specifically forms methanol whereas the latter system
produced both methanol and acetic acid (albeit in varying
amounts) under all reaction conditions. Second, while both
systems gave C-C cleavage products from ethane and n-butane,
only the former also yielded C-C cleavage products from the
corresponding alcohols.
FeCl
the system.
3 2 2
(0.1 mmol) or FeCl (0.1 mmol) instead of CuCl was added to
C. Determination of Activation Energy. To a glass liner was
added 0.5 mL of 0.03 M CuCl (aq) solution, 1.5 mL of CF CO H, and
2
3
2
10 µL of 0.006 M K
placed in a 125-mL high-pressure bomb that was then sealed. The
bomb was pressurized to 500 psi with N and preheated to reaction
temperature. The reactor was then pressurized to 1000 psi with CH
to 1200 psi with CO, and finally to 1300 psi with O . The contents
were stirred at the reaction temperature for 1 h following which the
reactor was cooled and the pressure slowly released. The reaction
mixture was then directly analyzed by NMR spectroscopy.
D. Rate Dependence on Methane Pressure. The reactions were
conducted as described in procedure C, except that the reactor was
charged with 100 to 1000 psi of CH
preheated to reaction temperature for 1 h. The reactor was then
pressurized with 200 psi of CO and with 100 psi of O . The contents
2 4
PdCl (aq) solution. The glass liner was then
2
4
,
2
On a more general theme, the requirement of a coreductant
(
carbon monoxide) by the above two catalytic systems is striking
12
and resembles the monooxygenases.
How general is this
4 2
and then to 1000 psi with N and
requirement for a coreductant (e.g., CO or H2) in achieving
difficult” catalytic hydrocarbon oxidations by dioxygen? There
“
2
have been other recent publications on catalytic systems for the
were stirred at 77-85 °C for 1 h following which the reactor was cooled
for 1 h, and the pressure slowly released. The reaction mixtures were
then directly analyzed by NMR spectroscopy.
E. Rate Dependence on Carbon Monoxide Pressure. 1. At 300
psi of Methane. The reactions were conducted as described in
oxidation of hydrocarbons, including olefins and aromatics, that
also call for either CO or H2 as the coreductant.23 While, from
a practical standpoint, it is more desirable for both oxygen atoms
of O2 to be used for substrate oxidation, there appears to be no
known catalytic system that operates as an artificial “dioxy-
genase” under mild conditions toward “difficult” substrates, such
as those possessing unactivated primary C-H bonds.
procedure C, except that the reactor was charged with 300 psi of CH
and 400 to 800 psi of N . The reactor was then preheated to reaction
temperature for 1 h. To the hot reactor was then added 100 to 500 psi
of CO (total pressure of N + CO ) 900 psi) and 100 psi of O . The
4
2
2
2
contents were stirred at 77-85 °C for 1 h following which the reactor
was cooled for 1 h, and the pressure slowly released. The reaction
mixtures were then directly analyzed by NMR spectroscopy.
Experimental Section
General. The following chemicals were used as received: 5% Pd/
2
. At 600 psi of Methane. The reactions were conducted as
carbon (Johnson Matthey); CuCl
FeCl , FeCl , NaCl, H , and K
dioxygen, and carbon monoxide (Matheson); CH
2
, CuCl, CuBr
2 3 2 2
, CuI, Cu(CF CO ) ,
described in procedure C, except that the reactor was charged with
600 psi of CH4 and 100 to 500 psi of N2. The reactor was then
preheated to reaction temperature for 1 h. To the hot reactor was then
3
2
2
O
2
2 2 8
S O
(Aldrich); methane, ethane,
13
13
3
CH
(Cambridge Isotope). For the 5% Pd/carbon
employed, dihydrogen chemisorption studies indicated the presence of
0 µmol of surface Pd atoms/g of sample. Reactions under pressure
were carried out in Parr general purpose bombs with glass liners.
2 3
OH, CH OH,
1
3
13
2 2 4
D O, CO, and C H
added 100 to 500 psi of CO (total pressure of N
2
+ CO ) 600 psi)
and 100 psi of O . The contents were stirred at 77-85 °C for 1 h
2
6
following which the reactor was cooled for 1 h, and the pressure slowly
released. The reaction mixtures were then directly analyzed by NMR
spectroscopy.
1
13
Reaction products were identified by their H- and C-NMR spectra
recorded on a Bruker AM 300 FT-NMR spectrometer with the solvent
resonance at the appropriate frequency or an external standard consisting
of a capillary tube containing 1 µL of DMSO in 60 µL of D
lock, reference, and as an integration standard. The identity of the
2
O for
Acknowledgment. This research was funded by the National
Science Foundation and by the CANMET Consortium on the
Conversion of Natural Gas. We thank Professor M. A. Vannice
for the determination of surface metal atoms in supported
palladium through dihydrogen chemisorption studies.
(23) Representative examples: (a) Otake, M. Chemtech 1995, 36. (b)
Miyake, T.; Hamada, M.; Sasaki, Y.; Oguri, M. Appl. Catal. A: General
1
1
995, 131, 33. (c) Teranishi, T.; Toshima, N. J. Chem. Soc., Dalton Trans.
995, 979. (d) Wang, Y.; Otsuka, K. J. Catal. 1995, 155, 256. (e) Benvenuto,
M. A.; Sen, A. J. Chem. Soc., Chem. Commun. 1993, 970.
JA964371K