Catalytic oxidation of hydrocarbons of natural and oil gas

Article abstract of DOI:10.1134/S0965544110050026

Alkane oxidation by O₂ and CO in the presence of Rh-, Pd-, and Pt-containing catalytic systems leads to the product of C-H bond oxidation and the products of C-C bond oxidative destruction. A deuterated methyl group in acetic acid is observed in the oxidation of n-propane in a deuterium-donor medium. The possible mechanisms of alkane C₂-C₄ conversion are proposed.

Full text of DOI:10.1134/S0965544110050026

ISSN 0965ꢀ5441, Petroleum Chemistry, 2010, Vol. 50, No. 5, pp. 332–337. © 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 Neftekhimiya, 2010, Vol. 50, No. 5, pp. 346–350.
Catalytic Oxidation of Hydrocarbons of Natural and Oil Gas
E. G. Chepaikina, A. P. Bezruchenkoa, G. N. Menchikovaa, N. I. Moiseevab, and A. E. Gekhmanb
a Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, Russia
eꢀmail: grig@ism.ac.ru; echep@ism.ac.ru
b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
Received March 1, 2010
Abstract—Alkane oxidation by O2 and CO in the presence of Rhꢀ, Pdꢀ, and Ptꢀcontaining catalytic systems
leads to the product of C–H bond oxidation and the products of C–C bond oxidative destruction. A deuterꢀ
ated methyl group in acetic acid is observed in the oxidation of nꢀpropane in a deuteriumꢀdonor medium.
The possible mechanisms of alkane C2–C4 conversion are proposed.
DOI: 10.1134/S0965544110050026
INTRODUCTION
The goal of this work is to elucidate the main
aspects of the mechanism of the catalytic oxidation of
С2–С4 alkanes in the presence of Rhꢀ, Ptꢀ, and Pdꢀ
containing catalytic systems.
The development of oneꢀstage methods for the
synthesis of the main oxygenates from natural and oil
gas is an important task of petroleum chemistry. The
C–H bond activation and functionalization in satuꢀ
rated hydrocarbons under mild conditions in the presꢀ
ence of metal complexes has been discussed in [1–6].
The participation of metal complexes in the cleavage
of the C–C bond has also been reported [7–9].
Because of the high temperatures, conventional heterꢀ
ogeneous catalysis does not allow alkanes to be selecꢀ
tively oxidized into highꢀtech products [10]. It is
promising to consider the activation of saturated
hydrocarbons analogous to biocatalytic ones [11]. For
such systems, a reducing agent should be present
together with an oxidant. The oxidation of this agent
generates active intermediates that are capable of
selectively oxidizing alkanes. Thus, systems consisting
of rhodium chloride and cocatalysts—compounds of
iodine, copper (or iron), and alkali metal chlorides—
are active in the coupled oxidation of methane and
carbon monoxide by molecular oxygen in an aqueous
solution of trifluoroacetic acid [12–16].
EXPERIMENTAL
The following reagents were used in this work:
RhCl3 ⋅ (H2O)n (34.5 wt % Rh), PdCl2 (59.5%), PdBr2
(39.6%), K2PdBr4 (21.0%), NaCl, CuO (chemically
pure), CF3COOH (chemically pure), D2O (99.9%),
CF3COOD (99.9%), and doubleꢀdistilled Н2О. Gases:
helium (grade A), N2 (special purity grade), CO
(99.9%), O2 (99.5%), methane (99.8%), ethane
(99.5%), propane (99.2%), butane (99.9%), and isobꢀ
utane (99.5%). Under experimental conditions, CuO
was rapidly dissolved in the reaction mixture to form
trifluoroacetate. Catalytic experiments were carried
out in a constantꢀtemperature shaken stainless steel
reactor lined with fluoroplastics.
In a typical run of methane oxidation, a reactor was
charged with 10.0 mg CuO and a 2.5ꢀml solution preꢀ
pared by mixing 0.5 ml of an aqueous solution of
RhCl3 (0.05 M) and NaCl (0.15 M), 1.4 ml H2O, and
8.1 ml СF3COOH. To maintain the accuracy of the
concentration, the contact solution was prepared by
weighing all components, including the liquids, in a
special container. The reactor was sealed and attached
to the unit for gas mixing equipped by a standard presꢀ
sure gage (with a maximum of 10 MPa and a division
value of 0.04 MPa). The following gases were supplied:
CH4 (6.00 MPa), O2 (0.56 MPa), and CO (1.84 MPa).
The autoclave jacket was attached to an ultrathermoꢀ
Carbon monoxide oxidation by oxygen in the cataꢀ
lytic system of platinum metal salt–halogenide can be
described by the following reactions:
Mn + CO + H2O
Mn–2 + 2Cu(II) + 2H+
2Cu(I) + 2H+ +
O2
2Cu(I) + 2H+ + O2
Mn – 2 + CO2 + 2H+,
(1)
(2)
(3)
(4)
Mn + 2Cu(I),
2Cu(II) + H2O,
2Cu(II) + H2O2.
½
stat preliminarily heated to 80°С. Experiments were
In these reactions, the intermediate formation of carried out for 2 h under conditions of intense stirring
hydrogen peroxide and other peroxo derivatives of using a shaker. After each run, the reactor was cooled
rhodium in the high oxidation states is possible, which to 10–12°С, the gas was discharged into an evacuated
presumably oxidize methane [17]. vessel, and its composition was analyzed by gas chroꢀ
332

CATALYTIC OXIDATION OF HYDROCARBONS OF NATURAL AND OIL GAS
333
matography. The gas composition (mol %) was CH4 = using AꢀC200 P, Bruker 200 MHz, and BC 587 A Tesla
66.1, CO = 11.8, O2 = 3.0, and CO2 = 8.7. To analyze 80 MHz instruments; by gas chromatography (Kristalꢀ
the liquid phase, 1.0 ml was withdrawn and 20
dioxane was added as an internal GC standard. The SDA 2 m
composition of the liquid phase was as follows chtomatography coupled with mass spectrometry
(M): CF3COOCH3 = 0.1, HCOOH = 0.04, and (Automass 150, column CPꢀSil 5 0.15 mm 25
CH3COOH = 0.03. Experiments on C2–C4 alkane 1.2 m). Isotopeꢀsubstituted products were detected
oxidation were carried out in an analogous way, but using the mass spectrum averaged by the chromatoꢀ
alkanes (12 mmol) were initially condensed from a graphic peak of acetic acid ( 44, 45, 46, Fig. 1),
μl of lyuks 4000M, columns Separon CHN and Separon
×
3
mm 0.125–200 mm); and by gas
×
m ×
μ
m/z
calibrated gas burette in an intermediate metering methyl trifluoroacetate, and ethyl trifluoroacetate.
device and then added to the reactor under helium
pressure. Helium was also introduced to prevent the
formation of explosive mixtures and to make the analꢀ
RESULTS AND DISCUSSION
ysis of the gas phase more convenient. Then, O2
In the oxidative functionalization of C1–C4 alkanes
(0.56 MPa) and CO were supplied to the reactor. under the action of O2 and CO, catalytic systems were
Alcohols were oxidized under the same conditions as studied that contained Rh, Pd, and Pt halides and Cu
alkanes. Under reaction conditions, alcohols conꢀ compounds. In aqueous trifluoroacetic acid, methane
verted into the thrifluoroacetatic acid esters to an is converted according to the following equations
extent of 90%. After the runs, the reactor was cooled to (Table 1):
10–12°С. The gas was collected in an evacuated vessel
and its composition was analyzed by gas chromatograꢀ
phy using a Model 3700 chromatograph with zeolite
CH4 + CO + O2
CH4 + CO + O2
CH4 + 2CO + 5/2O2
The oxidation of C2–C4 alkanes affects both C–H
CH3OH (CF3COOCH3) + CO2
CH3COOH + CO2
HCOOH + 2CO2 + H2O. (7)
,
(5)
,
(6)
5A (2 m
umns. The quantitative and qualitative analyses of the
× 3 mm) and Porapak Q (2 m × 3 mm) colꢀ
1
alkane oxidation products were carried by H NMR and C–C bonds as illustrated in Scheme 1 using an
with an external standard (DMSO in D2O, capillary) example of propane.
CF3COOC3H7 + CF3COOC3H7i
O
+ CO
2
C3H6
CH3C(O)CH3 + C2H5COOH
CF3COOC2H5 + CH3COOH + CF3COOCH3 + HCOOH
Scheme 1.
In the catalytic system containing Pd and Pt chloꢀ one may assume that they are not the products of
alkane oxidation.
rides, to prevent the formation of metal blacks, ligands
such as ꢀbipyridyl (bipy), 1,10ꢀphenathroline
α,α
The qualitative composition of the alkane oxidaꢀ
(phen) that are hard to oxidize were introduced into tion products in all of the catalytic systems studied is
the same, but the yield is higher in the presence of
Rh(III) compounds. Therefore, here we will only conꢀ
sider the catalytic system RhCl3/Cu(II)/Cl– . The ratio
between the products of alkane oxidation with and
without oxidative destruction depends on the length of
a chain and on the degree of alkane branching. Thus,
in ethane oxidation (Table 2), the hydroxylation prodꢀ
ucts (ethanol and ethyl trifluoroacetate) and the prodꢀ
ucts of deeper oxidation (acetic acid) dominate in the
reaction solutions. The concentration of methyl trifluꢀ
oroacetate is substantially lower than the concentraꢀ
tion of the above oxygenates. The major propane oxiꢀ
dation products (Table 3) are oxygenꢀcontaining comꢀ
pounds with three carbon atoms in a chain, propanol,
isopropanol, their esters, acetone, and propionic acid.
the system. These ligands help to preserve the lowꢀ
valence compounds of Pd and Pt in the solution. Palꢀ
ladiumꢀcontaining systems were stable at <85°С, and
platinumꢀcontaining systems partially underwent the
reductive destruction to metallic platinum. The cataꢀ
lytic activities of the Pd(II)–bipy and Pd(II)–phen
systems were approximately the same (Table 1).
Under reaction conditions, the product of methꢀ
ane oxidation, methanol, is converted into formic
acid, which is rather stable and is accumulated in the
reaction solutions. The rate of methyl trifluoroacetate
oxidation is much lower, and esterification, therefore,
prevents methanol from further oxidation to a large
degree. Furthermore, in the absence of alkanes,
HCOOH and АсfOCH3 were not found. Therefore, The amounts of products of oxidative destruction
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CHEPAIKIN et al.
(а) 43
45
100
50
0
60
O
OH
15
29
18
24
23
55
53
13
33
43
45
63
(mainlib) Acetic acid
(b)
100
50
43
18
28
60
55
23
24
51
36
34
0
14
44
56
64
(History) Manual Component (6.910 min) in D:\SPA\
(a) The mass spectrum of acetic acid and (b) the mass spectrum averaged over the chromatographic peak for acetic acid from the
reaction solutions of nꢀpropane oxidation in the medium of Ac OD/D O.
f 2
(methyl trifluoroacetate, ethyl trifluoroacetate, CF3COOCH3 and CF3COOC2H5 in propane oxidaꢀ
and acetic acid) are much lower. In the oxidation of
ꢀbutane, the yield of ethyl trifluoroacetate, methyl
tion and CF3COOCH3 and CF3COOꢀ
tane oxidation.
iꢀC3H7 in isobuꢀ
n
trifluoroacetate, and other products of oxidative
destruction is substantially higher than the amount of
oxygenates without C–C bond cleavage: butyl trifluoꢀ
roacetates and methyl ethyl ketone (Table 4). The
The oxidative destruction of alkanes may also begin
with alkane hydroxylation with the formation of priꢀ
mary and secondary alcohols (nꢀpropanol and isoproꢀ
products of isobutane oxidation contain small panol in the case of propane oxidation), which are
amounts of isobutyl trifluoroacetate and the others are
the products of oxidative destruction, mostly methyl
trifluoroacetate and isopropyl trifluoroacetate. In the
oxidation of all alkanes, formic acid is also formed in
substantial amounts.
then converted into acids and ketones, as has been
found. These compounds can be oxidized with the
cleavage of C
⎯C bonds under the same conditions and
form the same products as alkanes do (Table 5).
Acetic acid and methanol may be formed in nꢀproꢀ
pane oxidation via the product of isopropanol oxidaꢀ
tion (acetone) by the Baeyer–Villiger reaction
One cannot exclude that alkane oxidation with
C⎯C bond cleavage occurs via the mechanism of radꢀ
ical cation O+· transfer from the active oxidant to the
substrate [7, 9]. The participation of radical species in
alkane oxidation in the Rh–Cu–chloride system is
(Scheme 2, pathway
A) with the participation of
hydrogen peroxide formed by reaction 4 in CO oxidaꢀ
evident from the approximately equal yields of tion.
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CATALYTIC OXIDATION OF HYDROCARBONS OF NATURAL AND OIL GAS
335
CH3–CH2–CH3
A
OH(OAcf)
CH3–CH2–CH2OH(OAcf)
CH3–CH2COOH
CH3–CH–CH3
O
CO
D+
B
C
CH3 C CH3
O
O
DO–CH2CH2COOD
[H2O2], H+
Rh C O C C2H5
– CO2
CH3COOCH3
DOOC–CH2–COOD
– CO2
O
H2O
CF3COOH
CH2D–COOD
C2H5OH(OAcf)
Rh C C2H5
– CO
CH3OH + AcOH CH3OOCCF3 + AcOH
HCOOH
Rh–C2H5
Scheme 2. Pathways of the coupled oxidation of propane and CO by molecular oxygen in the
RhCl /Cu(II)/Cl catalytic system.
3
Under reaction conditions, nꢀpropanol is oxidized roacetate). In the oxidation of propionic acid in
AcfOD–D2O, deuterated ethanol was not formed,
which does not contradict the proposed mechanism.
Methanol (methyl trifluoroacetate) is formed analoꢀ
gously in acetic acid oxidation.
The oxidation of propionic acid in AcfOD–D2O
solutions leads to acetic acid with a deuteriumꢀsubstiꢀ
tuted methyl group (see figure). Because in the oxidaꢀ
to propionic acid, which then is converted via two
pathways: into ethanol and acetic acid. According to
[18], the conversion of propionic acid into ethanol
(ethyl trifluoroacetate) involves CO insertion into a
rhodium–propionate bond and further decarboxylaꢀ
tion (Scheme 2, pathway
B). After the decarboxylation
of the propyl complex to form the ethyl complex, its
hydrolysis (acidolysis) leads to ethanol (ethyl trifluoꢀ tion systems studied alkyl groups do not exchange
Table 1. Oxidative functionalization of methane in the presence of Rh, Pd, and Ptꢀcontaining catalytic systems (PCH = 6.0 MPa,
4
PO = 0.56 MPa, PCO = 1.6 MPa, 80°C, m
= 0.6, Cu(OAcf)2 = 5 × 10–2 M, 2 h)
H2O
2
Catalytic system
ligand,
Products, M ×
103
No.
platinum metal
NaCl, M
×
103
CH3OAcf
HCOOH
CH3COOH
compound, M × 103
M ×
103
1
2
3
4
5
RhCl3 (2.5)
–
7.5
–
–
–
–
102
71
66
23
21
40
20
20
10
10
30
Na2PdCl4 (5.0)
Na2PdCl4 (5.0)
Na2PtCl4 (5.0)a)
bipy (5.0)
phen (5.0)
bipy (5.0)
phen (5.0)
traces
traces
traces
traces
Na2PtCl4 (5.0)a)
a)
Note: Pt black precipitation was observed.
Table 2. Oxidation of ethane in the presence of Rhꢀ and Pdꢀcontaining systems (12 mmol of ethane was condensed; PHe = 3.6 MPa,
PO = 0.8 MPa, PCO =1.6 MPa, 80°C, m
= 0.5, Cu(OAcf)2 = 5 × 10–2 M, 2 h)
H2O
2
Products, M ×
103
Platinum metal c
Ligand
,
NaCl,
No.
ompound, M × 103
M
×
10
M
×
103
MeOAcf
EtOAcf
AcOH
1
2
RhCl3 (2.5)
Na2PdCl4 (5.0)
–
7.5
–
120
35
53
11
10
2
bipy (5.0)
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336
CHEPAIKIN et al.
Table 3. Oxidation of propane in the presence of Rhꢀ and Pdꢀcontaining catalytic systems (12 mmol of propane was condensed;
P
He = 3.6 MPa, P = 0.8 MPa, PCO =1.6 MPa, 80°C, m
= 0.5, 2 h; Cu(OAcf)2 = 5 × 10–2 M)
H2O
O2
Products,
M × 103
Platinum metal
Ligand
,
NaCl,
compound,
No.
M × 103
M × 103
CH3C(O)
CH3
M
×
103
MeOAcf
EtOAcf
AcOH PrꢀiꢀOAcf PrOAcf
1
2
3
4
RhCl3 (2.5)
–
7.5
–
65
35
130
27
20
560
70
310
35
200
73
Na2PdCl4 (5.0) bipy (5.0)
K2PdBr4 (5.0) bipy (5.0)
–
traces
traces
traces
traces
traces
traces
10
traces
<10
30
PdBr4 (5.0)
bipy (5.0)
–
10
40
Table 4. Oxidation of nꢀbutane (12 mmol of nꢀbutane was condensed; PHe = 3.6 MPa, P = 0.8 MPa, PCO = 1.6 MPa, 80°C,
O2
mH O = 0.5, 2 h, 80°C, Cu(OAcf)2 = 5 × 10–2 M)
2
Catalytic system
Products, M ×
103
platinum metal
No.
ligand,
103
NaCl,
compound,
M
×
M
×
103
M × 103
1
2
3
RhCl3 (2.5)
–
7.5
–
150
15
90
11
7
10
8
15
5
10
15
4
90
19
20
Na2PdCl4 (5.0) bipy (5.0)
PdBr2 (5.0) bipy (5.0)
7
9
–
12
3
1
2
Table 5. Oxidation of compounds that are intermediate products in propane oxidation (conditions are as in Table 3, no. 1;
RhCl3 = 2.5 × 10–3 M; Cu(OAcf)2 = 5 × 10–2 M)
Products, M
AcOH
×
103
No.
Substrate, M
AcfOCH3
AcfOC2H5
acetone
C2H5COOH
10
1
2
3
4
5
n
ꢀC3H7OHa) (1.06)
40
30
50
30
20
30
10
–
80
40
–
–
C2H5COOH (1.04)
isoꢀC3H7OHb) (0.99)
Acetone (0.98)
–
230
–
–
–
–
250
CH3COOH (0.98)
–
–
deuterium with the medium, a possible mechanism of anism with the intermediate formation of radical catꢀ
ions based on alkanes and the mechanism of consecuꢀ
tive oxidation via alcohols, ketones, and acids.
deuterium inclusion in the methyl group of acetic acid
is the oxidation of propionic acid to 3ꢀoxyacetic acid
and then to malonic acid. The decarboxylation of deuꢀ
terated malonic acid DOOCH2COOD leads to the forꢀ
1
ACKNOWLEDGMENTS
mation of CH2DCOOD (Scheme 2, pathway C) .
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 09ꢀ03ꢀ00387), the
Presidential Program for the Support of Leading Sciꢀ
entific Schools (NShꢀ1733.2008.3), and the Program
of the Presidium of the Russian Academy of Sciences
no. 19.
Thus, at least two mechanisms of alkane converꢀ
sion with C–C bonds involved are possible: the mechꢀ
1
CHD COOD formation can be explained by deuterium
2
exchange between the methylene group of malonic acid and the
medium.
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