Homogeneous catalytic oxidation of light alkanes: C-C bond cleavage under mild conditions

Article abstract of DOI:10.1134/S0023158410050071

The combined oxidation of CO and C₂-C₄ alkanes (associated petroleum gas and natural gas components) under the action of oxygen in trifluoroacetic acid solutions in the presence of rhodium and copper chlorides was accompanied by the oxidative degradation of C-C bonds in a hydrocarbon chain with the formation of carbonyl compounds, alcohols, and esters. For butane and isobutane, the reaction path with C-C bond cleavage was predominant. The buildup curves of isobutane oxidation products (both with the retention and with the degradation of the chain) were S-shaped and characterized by the same induction period; they did not pass through a maximum. A reaction scheme was proposed to reflect the main special features of the mechanism of transformations occurring in the O₂/Rh/Cu/Cl^- oxidation system.

Full text of DOI:10.1134/S0023158410050071

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2010, Vol. 51, No. 5, pp. 666–671. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.G. Chepaikin, A.P. Bezruchenko, G.N. Menchikova, N.I. Moiseeva, A.E. Gekhman, 2010, published in Kinetika i Kataliz, 2010, Vol. 51, No. 5,
pp. 691 696.
⎯
Homogeneous Catalytic Oxidation of Light Alkanes:
C–C Bond Cleavage under Mild Conditions
E. G. Chepaikina, A. P. Bezruchenkoa, G. N. Menchikovaa, N. I. Moiseevab, and A. E. Gekhmanb
aInstitute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
bKurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 117907 Russia
eꢀmail: moiseeva@igic.ras.ru
Received November 9, 2009
Abstract—The combined oxidation of CO and C2–C4 alkanes (associated petroleum gas and natural gas
components) under the action of oxygen in trifluoroacetic acid solutions in the presence of rhodium and copꢀ
per chlorides was accompanied by the oxidative degradation of C–C bonds in a hydrocarbon chain with the
formation of carbonyl compounds, alcohols, and esters. For butane and isobutane, the reaction path with C–
C bond cleavage was predominant. The buildup curves of isobutane oxidation products (both with the retenꢀ
tion and with the degradation of the chain) were Sꢀshaped and characterized by the same induction period;
they did not pass through a maximum. A reaction scheme was proposed to reflect the main special features
of the mechanism of transformations occurring in the O2/Rh/Cu/Cl– oxidation system.
DOI: 10.1134/S0023158410050071
INTRODUCTION
The interaction of hydrogen peroxide with Rh(III)
results in the formation of a corresponding peroxo
complex, and it is believed that the peroxo complexes
of rhodium in the highest oxidation states can oxidize
methane [15].
It is likely that the role of copper consists in not
only the generation of hydrogen peroxide but also the
stabilization of an active intermediate [13].
In this study, we found that, in the course of the
oxidative conversions of С2–С4 alkanes (associated
petroleum gas and natural gas components) under
conditions of their combined oxidation with CO in the
presence of metal complex catalytic systems, the subꢀ
strates can enter into reactions that lead to C–C bond
cleavage.
The literature on the activation and functionalizaꢀ
tion of C–H bonds in saturated hydrocarbons under
mild conditions with the participation of metal comꢀ
plexes is extensive [1–5]. At the same time, there are
only a few publications concerning the participation of
metal complexes in C–C bond cleavage [6–8]. A
promising approach to the activation and oxidation of
saturated hydrocarbons consists in a search for cataꢀ
lytic systems analogous to biocatalysts [9], in which a
reducing agent is present along with oxygen. Homogeꢀ
neous catalytic systems consisting of rhodium chloride
and cocatalysts (iodine, copper, or iron compounds
and alkali metal chlorides) exhibited high activity and
stability in the course of the oxidative functionalizaꢀ
tion of methane under the action of molecular oxygen
and carbon monoxide, which served as a reducing
agent [10–15]. The oxidation of CO under the action
of Rh(III)
EXPERIMENTAL
The following chemicals were used: RhCl3 ·
(H2O) (34.5 wt % Rh), NaCl, CuO (chemically
Rh(I) + CO2 + 2H+,
n
Rh(III) + CO + H2O
pure), CF3COOH (chemically pure), D2O (99.9%),
CF3COOD (99.9%), CO (99.9%), O2 (99.5%), ethane
(99.5%), propane (99.2%), butane (99.9%), isobutane
(99.5%), and twiceꢀdistilled water. The catalytic
experiments were performed in a fluoroplasticꢀlined
autoclave in accordance with a published procedure
[10–12].
resulted in the formation of Rh(I), which, in turn, can
reduce Cu(II) to Cu(I):
Rh(I) + 2Cu(II) + 2H+
Rh(III) + 2Cu(I).
Thus, copper in the test system can serve as a reoxꢀ
idant for catalyst metal. Moreover, the intermediate
formation of hydrogen peroxide or other peroxo derivꢀ
atives on the oxidation of Cu(I) in these systems is
hypothesized:
The qualitative and quantitative analysis of oxidaꢀ
1
tion products was performed by H NMR spectrosꢀ
copy with the use of an external standard (DMSO in
D2O, capillary) on Bruker AꢀC200 P (200 MHz) and
Tesla BS 587 A (80 MHz) instruments, by gas chromaꢀ
2Cu(I) + 2H+ + 1/2O2
2Cu(I) + 2H+ + O2
2Cu(II) + H2O;
2Cu(II) + H2O2.
666

HOMOGENEOUS CATALYTIC OXIDATION OF LIGHT ALKANES
667
Transformations of C1–С4 alkanes in the presence of the RhCl3 (2.5 × 10–3 mol/l) + Cu(OAcf 2
)
(5 × 10–2 mol/l) + NaCl
(7.5 × 10–3 mol/l) catalytic system in a solution of CF3COOH–H2O (2.5 ml) with m
= 0.6
H2O
Reaction products, mol/l
Parent alkane
H4a
C2 Hb
0.10
0.20
0.30
0.16
0.21
0.15
0.23
0.07
0.32
0.24
0.05d
0.02
0.03
0.19
C
0.84
0.09
0.18
6
3 Hb
8
0.24
0.27
0.16
0.01
0.01
tr
0.15
C
n
ꢀС4Hb
0.03
0.04
0.02
10
isoꢀС4Hb
0.39
0.02
10
a
b
c
d
Pressure, MPa: CH , 6.0; He, 3.6; O , 0.56; and CO, 1.84. Temperature: 95°C. Reaction time: 2 h.
4
2
Pressure, MPa: He, 3.6; O , 0.8; and CO, 1.6. Alkane condensed: ~12 mmol. Temperature: 80°C.
2
Reaction paths of formic acid formation will be discussed elsewhere.
Acetic acid results from the oxidative carbonylation of methane.
tography (Kristallolyuks 4000M chromatograph detected upon the oxidation of С2–С4 alkanes in the
O2/Rh/Cu/Cl– system. This was evidenced by the
presence of methyl trifluoroacetate (AcfOMe), as well
as alcohols, ketones, and carboxylic acids whose
hydrocarbon chains were shorter than that in the parꢀ
ent alkane, in the reaction mixture.
equipped with Separon CHN and Separon SDA colꢀ
umns of size 2 m mm .125–0.200 mm) and gas
chromatography–mass spectrometry (GC–MS)
(Automass 150 instrument with a CPꢀSil column of
×
3
×
0
size 25 m
The experiments were performed at the concenꢀ
trations [RhCl3] = 2.5
10–3 mol/l, [Cu(II)] =
10–2 mol/l, and [NaCl] = 7.5 10–3 mol/l in
the CF3COOD(CF3COOH)–D2O(H2О) reaction
medium, where the mole fraction of water was
×
0.15 mm
×
1.2
µ
m).
The ratio between alkane oxidation products with
the retention of the carbon skeleton and oxidative degꢀ
radation products depends on the length and branchꢀ
ing of the carbon chain of the hydrocarbon. Thus,
upon the oxidation of ethane, hydroxylation prodꢀ
ucts—ethanol and ethyl trifluoroacetate (AcfOEt)—
and a product of the deep oxidation of ethane (acetic
acid) were predominant in reaction solutions (Scheme 1,
the table). The concentration of methyl trifluoroaceꢀ
tate was much lower than those of the above oxygenꢀ
ates. In propane oxidation products (Scheme 2, the
table), the following oxygenꢀcontaining compounds
with three carbon atoms in the chains were predomiꢀ
nant: propanol and isopropanol, their esters, acetone,
and propionic acid. Oxidative degradation products
(methyl trifluoroacetate, ethyl trifluoroacetate, and
acetic acid) were present in much smaller amounts.
×
5
×
×
m
H2O
(or
) = 0.6, the liquid phase volume was 2.5 cm3,
mD O
2
and the gas phase volume was 31 cm3. Alkanes were
introduced into a cooled reactor by recondensation.
The component pressures were the following, MPa:
He, 3.6; O2, 0.56; and CO, 1.84. The table specifies the
experimental conditions.
RESULTS AND DISCUSSION
In an aqueous solution of trifluoroacetic acid
(AcfOH) in the presence of rhodium chloride, potasꢀ
sium iodide or copper (iron) chloride, and sodium
chloride, methane under the action of oxygen and carꢀ
Upon the oxidation of nꢀbutane, the yield of ethyl
bon monoxide was converted into methanol, methyl trifluoroacetate, methyl trifluoroacetate, and other
trifluoroacetate, formic acid, and acetic acid [10–14]. oxidative degradation products was much higher than
Both expected oxidation products, in which the carꢀ the yield of expected oxygenates—the butyl esters of
bon skeleton was retained (alcohols or their esters, trifluoroacetic acid and methyl ethyl ketone
1
(Scheme 3, the table). Upon the oxidation of isobuꢀ
ketones, and carboxylic acids), and the oxidative degꢀ
2
radation products of saturated hydrocarbons were tane, only small amounts of isobutyl trifluoroacetate
1
2
Aldehydes were present in only trace amounts, probably,
because of rapid oxidation.
The amount of tertꢀbutyl trifluoroacetate was even smaller; howꢀ
ever, it was reliably identified by GC–MS analysis.
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668
CHEPAIKIN et al.
CH3 CH3
HCOOH
–
CH3COOH
O
O
CF3 C O CH3
CF3 C O C2H5
Scheme 1. Ethane oxidation products.
O
HC
OOH
CH3–CH2–CH3
CH3 C CH3
CH3CH2COOH
O
O
CF3 C O CH3
CF3 C O CH2CH2CH3
O
CH3
CH3COOH
CF3 C O CH
O
CH3
CF3 C O C2H5
Scheme 2. Propane oxidation products.
O
HC
OOH
CH3–CH2–CH2–CH3
CH3 C C2H5
O
CF3 C O (CH2)3CH3
CF3 C O CH3
O
C2H5
CF3 C O CH
CH3COOH
CH3
C2H5
O
O
CF3 C O CH2CH2CH3
CF3 C O C2H5
CH3CH2COOH
Scheme 3.
O
n
ꢀButane oxidation products.
CH3 CH CH3
CH3
O
HCOOH
CH3
C
CF3O
O CH2 CH
O
CH3
CH3
C
CF3O
CH3
C
OCH3
O
O
CF O
3C
O
CH3CHCH3
Scheme 4. Isobutane oxidation products.
were detected in reaction solutions; the other compoꢀ same induction period (see the figure) and did not pass
nents were oxidative degradation products (Scheme 4,
the table).
through a maximum. This suggests that the substrate
hydroxylation product and oxidative degradation
products (methyl trifluoroacetate and isopropyl trifluꢀ
oroacetate) were simultaneously formed in substrate
The kinetic curves of the buildup of all the isobuꢀ
tane oxidation products (with both retained and
degraded chains) were Sꢀshaped; they exhibited the reactions with an active oxidizing agent through a
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HOMOGENEOUS CATALYTIC OXIDATION OF LIGHT ALKANES
669
common intermediate because the formation of at period was due to the formation of an active intermeꢀ
least two different active oxidizing agents at the same diate responsible for the oxidation of hydrocarbons.
rate would otherwise be hypothesized. In terms of the
proposed model, the occurrence of an induction
Previously, it was found that rhodium peroxo comꢀ
plex ( ) participates in the hydroxylation of methane
in the test system [15]:
I
#
CH3
O
O
2H+
H2O2
–2H+
+CH4
–CH3OH
Rhn
O
Rhn
O
Rhn=O
Rhn , where 2 < n < 4.
–H2O
Rhn
H
I
The key step of this reaction is the insertion of an alkane) to weaken the C–C bond. Thus, the radical
oxygen atom into a C–H bond of the alkane. It is cation can react both with C–C bond cleavage and
believed that this complex is also responsible for the with the retention of the molecular hydrocarbon skelꢀ
hydroxylation of С2–С4 alkanes. However, the formaꢀ eton. In resulting ion–radical pair II, a positive charge
tion of carbonyl compounds or compounds with a and an unpaired electron are localized at different carꢀ
shortened hydrocarbon chain cannot be explained in bon atoms. Thus, it is likely that, after proton eliminaꢀ
terms of this mechanism.
tion, complex II undergoes oxidation through the
intermediate formation of radical III to form a carboꢀ
nyl compound (Scheme 5).
The high electronegativity of the Rhn=О leaving
group of the complex allows us to hypothesize that
At the same time, II can react with a nucleophilic
particle and molecular oxygen; in this case, the direcꢀ
tion of a nucleophilic attack of water or carboxylic acid
depends on the predominant localization of a positive
charge, whereas the function of molecular oxygen is
hydrogen atom abstraction from a carbon atom at
which an unpaired electron is mainly localized. In this
case, a C–C bond is cleaved (Scheme 5).
It is likely that the oxidation of alkanes in the
O2/Rh/Cu/Cl– catalytic system occurs via several
reaction paths; this fact can explain the variety of
products.
complex (I), which is shown in Eq. (I) as X–O, can
transfer not only an oxygen atom but also an oxygen
radical cation (in this case, the oxidation number of
rhodium decreases by unity) to the substrate with the
formation of a tight ion pair without the escape of the
free radical cation into the bulk:
C+
.
C
.
X−
,
(I)
O X
OH
H
I
II
where X = Rhn=O
.
It is reasonable to hypothesize the mechanism of
Upon the formation of a radical cation, an electron C–C bond cleavage in alkanes. Thus, for example,
is removed from the highest occupied molecular ethane and propane conversions were observed at
orbital (from a σꢀbonding orbital in the case of an 180°С in the course of a stoichiometric reaction of
R
R
+ [X−O]
H
C C
C C
H
H
OH
R
R
.
.
.
+
C
+
C
C
H
C
H
X−
OH
OH
II
HY, O2
–H+
R
O
–HO2
OH
H
O2
–HO•2
.
–H+
+
R C
C C
Y C
C C
O
H
R
or
III
.
+
C O2
RC
O
X–O is an oxidant, Y = OH or AcO
Scheme 5. Hypothetical mechanism of oxidative C–C bond degradation.
KINETICS AND CATALYSIS Vol. 51
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670
CHEPAIKIN et al.
cobalt(III) trifluoroacetate or a catalytic reaction of CF3COOC2H5 from propane and CF3COOCH3 and
cobalt(II) trifluoroacetate in the presence of oxygen CF3COO iso C3H7 from isobutane evidence of the
ꢀ
ꢀ
(4 MPa) [6]. It is believed that the process begins with participation of reactive radical cations in the reaction
electron abstraction from a hydrocarbon. The resultꢀ because, in terms of the reaction scheme under conꢀ
ing radical cation decomposes with C–C bond cleavꢀ sideration, the resulting radical cation in the presence
age to form organic cations and radicals. A carbonꢀ of oxygen reacts in accordance with Scheme 6.
centered radical is converted into a cation under the
action of cobalt(III). The cations react with trifluoroꢀ
with an active intermediate that is responsible for the
A tight cation–radical pair results from interaction
insertion of О+ into an alkane C–H bond (reaction (I)).
Undoubtedly, the methylene group is more reactive
than the methyl group; however, the number of hydroꢀ
gen atoms in the methylene group of a propane moleꢀ
cule is smaller than that in methyl groups by a factor of
3. Thus, we can hypothesize that intermediate cation–
radical pairs with the participation of СН2 and СН3
groups are formed in commensurable amounts to proꢀ
vide the commensurable yields of methanol and ethaꢀ
nol and/or their esters.
acetic acid to form esters, which were detected as
product constituents. The initial formation of radical
cations was also hypothesized in a discussion of the
results of experiments in which hydrocarbons were
oxidized at C–C bonds in a rhodium–iodide–chloꢀ
ride system [7]. However, Lin et al. [7] gave preference
to a molecular mechanism with the participation of
the inner coordination sphere of rhodium. Interesting
hydrocarbon transformations were observed on peroxꢀ
ide oxidation in the presence of vanadium complexes
[8]. Only branched alkanes underwent oxidation at the
R–СH3 bond. Stolyarov with coauthors [6, 8]
explained the oxidative cleavage of C–C bonds by the
considerable weakening of these bonds in radical catꢀ
ions. A molecular mechanism of C–C bond cleavage
was established in the stoichiometric oxidation of
cyclohexane on an OsO4 catalyst in water. It is likely
that, at pH 12, the C–C bond was cleaved at cisꢀoxo
groups of osmium tetraoxide according to a [3+2]
mechanism [16].
i
i
The resulting peroxide radicals
and
CH3O2
HO2
cannot initiate and run a chain process because of
rapid disproportionation and reduction to a hydroperꢀ
oxide in the presence of copper(I) ions in the former
and latter cases, respectively.
The С4 hydrocarbons are more prone to oxidative
degradation at C–C bonds than ethane and propane;
this is likely due to the better stabilization of intermeꢀ
diate radical cation species. On the other hand, the
Thus, we can hypothesize that, in the presence of process can begin with the activation of a C–H bond
the rhodium–copper–chloride catalytic system, C–C and the formation of primary and secondary alcohols,
bonds are activated as a result of electron trapping at which are subsequently oxidized to acids and ketones,
the electrophilic fragment of a catalytically active as found previously. We found that acids and ketones
intermediate (rhodium peroxo complex) [15]. are oxidized at C–C bonds under the same conditions
Approximately the same yields of CF3COOCH3 and and with the formation of the same products as those
CH3
H3C
CH2
X–O
+
.
CH3
H3C
+
.
CH2
CH
H3C
CH2
OH
OH
–
–
O , Y
2
O , Y
2
•
2
•
2
•
•
2
–HO
–CH O
3
–HO
–HO
2
CH2O
CH3–CHO
CH3 CH Y
CH3CH2CHY
CH3CH2Y
CH3Y
OH
OH
–HY
–HY
CH3–CHO
CH3CH2CHO
X–O is an oxidant, Y = OH or AcO
Scheme 6. Hypothetical mechanism of propane oxidation with C–C bond degradation.
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HOMOGENEOUS CATALYTIC OXIDATION OF LIGHT ALKANES
×
102, mol/l
671
ACKNOWLEDGMENTS
Concentration
22
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 09ꢀ03ꢀ00387) and
the Council of the President of the Russian Federation
for Support for Young Scientists and Leading Scienꢀ
tific Schools (grant no. NSh 1733.2008.3).
1
20
18
16
14
12
10
8
2
3
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CF3COOCH3, CF3COOC2H5, and CH3COOH
,
.
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2010