Oxidative functionalization of methane in the presence of a homogeneous rhodium-copper-chloride catalytic system: Transformation of acetic and propionic acids as solvent components

Article abstract of DOI:10.1134/S0965544111020022

The oxidative functionalization of methane (O2, CO, 95°C, Rh ^III/Cu^I, ^II/Cl^- catalytic system) was studied in an aqueous acetic or propionic acid medium. It was shown that oxidative decarbonylation of carboxylic acids takes place along with methanol and methyl carboxylate formation.

Full text of DOI:10.1134/S0965544111020022

ISSN 0965ꢀ5441, Petroleum Chemistry, 2011, Vol. 51, No. 2, pp. 133–142. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.G. Chepaikin, A.P. Bezruchenko, GN. Menchikova, N.I. Moiseeva, A.E. Gekhman, I.I. Moiseev, 2011, published in Neftekhimiya, 2011, Vol. 51, No. 2,
pp. 144–153.
Oxidative Functionalization of Methane in the Presence
of a Homogeneous Rhodium–Copper–Chloride Catalytic System:
Transformation of Acetic and Propionic Acids as Solvent
Components
E. G. Chepaikina, A. P. Bezruchenkoa, G. N. Menchikovaa, N. I. Moiseevab,
A. E. Gekhmanb, and I. I. Moiseevb
a Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Institutskaya ul., 8,
Chernogolovka, Moscow Region, 142434 Russia
Eꢀmail: echep@ism.ac.ru
b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
Eꢀmail: gekhman@igis.ras.ru
Received October 25, 2010
Abstract—The oxidative functionalization of methane (O2, CO, 95°
C, RhIII/CuI, II/Cl– catalytic system)
was studied in an aqueous acetic or propionic acid medium. It was shown that oxidative decarbonylation of
carboxylic acids takes place along with methanol and methyl carboxylate formation.
DOI: 10.1134/S0965544111020022
The oxidative catalytic functionalization of satuꢀ and CH3COOD) and the results of experiments with
rated hydrocarbons, especially methane, is one of the 18Oꢀlabeled H2O, CF3COOH, and O2, a reaction
most important problems of petroleum chemistry and scheme that includes the activation of methane on
natural gas chemistry (e.g., see [1–7]). Gasꢀphase oxoꢀ [11, 13, 14] and peroxoꢀ [16] rhodium complexes
methane oxidation on heterogeneous catalysts does has been proposed. The presence of water [7]
not provide satisfactory selectivity for the desired enhances the efficiency of the RhIII/I–/Cl– and
functionalized, oxygenꢀcontaining products because RhIII/CuII/Cl– catalytic systems. To learn more about
of their deep oxidation [8]. This is why the developꢀ these systems, in this work we studied the behavior of
ment of catalysts operating in the liquid phase is promꢀ propionic and acetic acids under the action of O2 and
ising [9]. Unexpectedly, the Pd/C–CoCl2 catalytic CO in the RhIII/CuI, II/Cl– catalytic system.
system turned out to be efficient in the functionalizaꢀ
tion of C1–C4 hydrocarbons by the action of O2 and
CO in trifluoroacetic acid [10], which is a reactionꢀ
stopping agent [9].
EXPERIMENTAL
Materials
Carbon monoxide (CO) is an essential coreactant
in the RhIII/I–/Cl–, RhIII/CuI, II/Cl–, and RhIII/FeII,
III/Cl– – homogeneous systems for direct methane
oxidation with dioxygen in aqueous trifluoroacetic
acid, to give methanol (or trifluoromethyl acetate) and
formic acid [11–17].
The chemicals RhCl3 · (H2O)n (Rh 34.5 wt %),
NaCl, CuO, H2SO4, and HClO4 were of the reagent
grade; H2O was doubly distilled; reagentꢀgrade
СH3COOH was distilled; C2H5COOH (Merck) was
used as received; nꢀheptane primary reference fuel was
purified by rectification; and reagentꢀgrade dioxane
was refluxed over sodium metal over 2 h and then disꢀ
tilled. The gases used had the following purity: CH4
(99.8%), CO (99.9%), O2 (99.9%), grade A He, speꢀ
cial purity grade nitrogen, and H2 (99.0%). The deuꢀ
terated compounds D2O (99.9 at %), CD3COOD
CH3OH (CH3OCCF3)
CF COOH
3
.
+ CO2
CH4 + CO + O2
O
HCOOH
Scheme 1.
The catalytic activity of these systems at 95 С and (99.5 at %), and D2SO4 (98.0 at %) were used without
°
CH4, CO, and O2 gas pressures of 6.0, 1.84, and further purification; CD4 (98.2 at %) was condensed
0.56 MPa, respectively, attains 100–150 mol CH4/mol twice at a liquid nitrogenꢀtemperature and then evapꢀ
Rh h). On the basis of analysis of the kinetic isotope orated at –60 to –50°C for the removal of heavy
effect (kH/kD) for methane and the solvent (CF3COOD impurities, e.g., CCl4.
133

134
CHEPAIKIN et al.
Catalytic experiments were carried out in a stainꢀ again until reaching a total pressure of 8.4 MPa. The
less steel reactor of a 34ꢀcm3 capacity lined with fluoꢀ duration of the run was 2 h.
roplastic. The contact of the liquid phase with the
metal was completely eliminated, and the vapor–gas
Analysis
phase had minimal contact with the metal in the
unheated part of the reactor. An RhCl3 solution in H2O
or D2O, NaCl, and CuO were loaded into the reactor,
The gas phase was analyzed by GC (thermal conꢀ
ductivity detector, He) in a model 3700 chromatoꢀ
graph (Khromatograf experimental plant, Moscow) at
then
a
solvent
(
CH3COOH, C2H5COOH
,
or
СD3COOD) was added. The water concentration was
brought to the required value by adding an H2SO4/H2O
or D2SO4/D2O solution. To ensure the accurate comꢀ
ponent concentrations and water/carboxylic acid
ratio, the catalytic system was prepared by weighing all
components, including liquid materials, in a special
vessel. The total volume of the liquid phase was 2.5 or
5 cm3. A fluoroplastic disk was placed into the reactor
for better stirring, and the reactor was sealed and conꢀ
nected to a gasꢀmixing device equipped with a 10ꢀMPa
pressure gauge with a scale factor of 0.04 MPa. The
55
(0.2–0.3 mm, 3 m
of О2, N2, СН4, and CO, and a 2 m
°
С
. A column packed with molecular sieves
mm) was used for determination
2.5 mm Porapak
5Å
×
3
×
Q (0.115–0.200 mm) column was used for the СО2
analysis.
The liquid phase was analyzed by GC (flame ionꢀ
ization detector, N2) on a Chromꢀ5 chromatograph
with glass columns (2.5 mm id). Acetic acid was deterꢀ
mined on a 2ꢀm Separon CHN (0.115–0.200 mm)
column at 130
dard, and propionic acid was determined on a Separon
SDA column (2.4 m) at 140 , with ꢀheptane as an
°
С, with dioxane as an internal stanꢀ
reactor was fed with СН4 (6 MPa), or a CH4
– CD4
°С
n
(6 MPa) mixture, or CD4 (4 MPa); О2 (0.56 MPa); and
CO (1.84 MPa) in the given sequence. Helium
(6 MPa) was supplied instead of methane in some
runs. The introduction of helium was needed in order
to maintain the CO and О2 partial pressures at a conꢀ
stant level for the correct comparison of the experiꢀ
mental results in the presence and absence of methꢀ
ane. In addition, this was necessary for safety reasons
to prevent explosion. For the gas ratio to be varied, it is
necessary first to calculate gas compositions to avoid
the formation of explosive mixtures. The reactor jacket
was connected to a water thermostat preliminary
heated up to the experimental temperature. Experiꢀ
ments were carried out with intense agitation of the gas
and liquid phases with the use of a shaker. When the
internal standard. The products of CH4 oxidation in
CD3COOD/D2O were analyzed by H NMR on a
Brucker ACꢀ200 P spectrometer (200 MHz) and a
Tesla BS 587 A spectrometer (80 MHz). The admixꢀ
ture of CHD2COODin CD3COOD was used as an
internal standard in quantitative measurements.
1
The initial CH4/CD4 ratio was measured by mass
spectrometry with an MIꢀ1201 instrument equipped
with a needle leak valve, a prevacuum vessel pressure
of 10–1 torr, an electron ionization energy of 70 eV, and
an ionization current of 0.2 mA. The GC/MS deterꢀ
mination was conducted on an Automass 150 instruꢀ
ment (DelsiꢀNermag) with a DN200 gasꢀliquid chroꢀ
matograph (Delsi), using a CPSil chromatographic
column of 5 25 m
×
0.15 mm, df = 1.2
µ
m
run was over, the reactor was quickly cooled to 12 С,
°
(Chrompack); He as the carrier gas in the constant
pressure (1.2 bar) mode; split injection at a split ratio
and the gas phase was sampled for analysis. The liquid
phase was quantitatively recondensed in a vacuum into
a liquid nitrogenꢀcooled receiver and analyzed by
chromatography and H1 NMR.
of 1 : 50 and 50.
ature programming:
(4 min), 10 C/min. In the electron ionization mode,
T
inj = 220
°
C
; and the following temperꢀ
Tin = 40
°C
(4 min), rf = 250°C
T
°
In a typical methane oxidation run, the autoclave
resolution was at least 2.0 M (where M = m/z) in the
range of m/z 18–131. To analyze the isotopic compoꢀ
sition, the spectra were recorded in the scanning mode
over the range of m/z 15–100, at a scanning time of
100 ms. The software suites Lucy ver.2.0 and AMDIS
ver. 2.62 were used for the processing of the GC/MS
results. To sample the reaction solution, it was placed
in a test tube sealed with a membrane, heated up to
was charged with RhCl3 (6.25
18.75
10–3 mmol) as a solution (0.25 g) in
D2O [RhCl3] = 2.5
10–3 mol l–1 and [NaCl] =
7.5
10⎯ 3 mol l–1). Then, 0.25 g of D2SO4 solution in
D2O ([D2SO4] = 0.96
10–3 mol l–1), 2.27 g of
×
10–3 mmol) and NaCl
(
×
(
×
×
×
CD3COOD, and 10 mg CuO were added. The CH4
(6.0 MPa), O2 (0.56 MPa), and CO (1.84 MPa) gases
were supplied to the autoclave. The duration of the run
was 2 h.
~60
°
C
, and headꢀspaced with a chromatographic
syringe heated to ~50
injected into the chromatograph was 50–100
°C
. The amount of the gas phase
µ
l.
In a typical propionic acid oxidation run, the autoꢀ
clave was charged with 10 mg of CuO and 2.5 ml of the
solution prepared as follows: to 1.0 ml of aqueous soluꢀ
RESULTS AND DISCUSSION
tion containing [RhCl3] = 5
×
10–2 mol l–1 and
[NaCl] = 0.15 mol l–1, 1.4 ml of H2O, 0.56 ml of H2SO4
It was found that the oxidation of methane does not
in H2O (3.9 mol l–1), and 17 ml of C2H5COOH were proceed in dioxane, tetrahydrofuran, and acetonitrile
added. The gases were supplied in the sequence of He in the presence of RhIII/CuI,II/Cl– both in pure solꢀ
(4 MPa), О2 (0.56 MPa), CO (1.84 MPa), and He vents and with addition of water or H2SO4.
PETROLEUM CHEMISTRY Vol. 51
No. 2
2011

OXIDATIVE FUNCTIONALIZATION OF METHANE IN THE PRESENCE
135
4.5
4.0
3.5
7
3
6
5
4
3
2
3.0
2.5
2.0
1.5
1.0
0.5
3
2
2
1
1
1
0
5
10
D O, mol l
20
15
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
–1
–1
2
D SO , mol l
2
4
Fig. 2. The product yield in the methane oxidation as a
Fig. 1. The product yield in the methane oxidation as a
function of D O concentration in CD COOD at
function of D SO concentration in CD COOD at the
2
3
2
4
3
ꢀ1
mole fraction of water
m
D O = 0.6: (
1
) CH , (
2
) CH OD,
[D SO ] = 0.19 mol l : (
1
) CH OD, (
2
) CH COOD, and
2
4
3
2
4
3
3
ꢀ3
–1
ꢀ3
–1
II
and (
3
) CH OC(O)CD ; [RhCl ] 2.5
×
10 mol l
,
(
3
) CD COOCH ; [RhCl ] 2.5
×
10 mol l , [Cu ] =
3
3
3
3
3
3
II
ꢀ2
–1
ꢀ3
–1
–2
–1
ꢀ3
–1
[Cu ] = 5.0
×
10 mol l , [NaCl] = 7.5
×
10 mol l
,
5.0 ×10 mol l , [NaCl] = 7.5 ×10 mol l , CH = 6 MPa,
4
CH = 6 MPa, O = 0.56 MPa,CO = 1.84 MPa, T= 95°C, 2 h.
4
2
O = 0.56 MPa, CO = 1.84 MPa, T = 95°C, 2 h.
2
In the CD3COOD/D2O medium, the СН4 oxidaꢀ tate and methyl chloride, respectively [13, 17]. Thus,
tion in the RhIII/CuI,II/Cl– system affords the prodꢀ
ucts shown in Scheme 2:
the mechanism of acetic acid formation during the
oxidative carbonylation of methane (Scheme 1, Fig. 1)
must differ from that of the formation of acetic acid
from methanol and CO catalyzed by rhodium iodide
compounds (Monsanto process) [18].
The acidity of the medium has an effect on the
composition and the yield of the methane oxidation
products: as [D2SO4] is increased, methyl acetate
becomes the major product, the methanol yield
decreases, and a small maximum in the acetic acid
yield is observed (Fig. 1). If the methanol yield
dropped as a result of its D2SO4ꢀcatalyzed esterificaꢀ
tion, the total concentration of methyl acetate and
residual methanol would be constant. However, this
was not the case. Consequently, methane hydroxylaꢀ
tion or acetoxylation are significantly accelerated by
CH3OD
CH3OCCD3
CD COOD/D O
3
2
CH4 + CO + O2
O
HCOOD
Scheme 2.
CH3COOD
To activate dioxygen in the rhodium–copper–
chloride catalytic system, a reducing agent is required.
When CO is used as a reductant, acetic acid is found in
the reaction products and the intensive catalytic oxiꢀ
dation of CO to СО2 simultaneously occurs
(reaction (1))
III
II
–
Rh /Cu /Cl
CO + 1/2O2
CO2.
(1)
the action of D2SO4
.
At the given concentration of deuterosulfuric acid
Free radicals do not contribute to methane oxidaꢀ
tion under the conditions of our experiments [16].
Methanol is the major product in the absence of
[
D2SO4] = 0.19 mol l–1, the decrease in the D2O conꢀ
centration to 5 mol l–1 slightly increases the yield of
acetic acid and significantly increases the total yield of
methyl acetate and methanol. However, there is no
oxidation in glacial acetic acid. These two observaꢀ
tions support the hypothesis that the reaction with
methane yielding the products observed involves the
nucleophilic attack of the water or acetic acid moleꢀ
cule. Within the range of 0–5 mol l–1, there is an optiꢀ
mal water concentration at which the yield of methyl
acetate attains the maximum.
1
sulfuric acid Unexpectedly, acetic acid without the
nondeuterated methyl group was found in the prodꢀ
ucts. This product may be formed if methanol gives
acetic acid in the reaction with CO [18]. However,
methanol and methyl iodide do not interact with CO
in the presence of O2 under our experimental condiꢀ
2
tions , and are converted into methyl trideuteroaceꢀ
1
Sulfuric acid was added to the solution in certain runs with aceꢀ
The amount of methyl acetate formed under the
experimental conditions exceeds by at least an order of
magnitude the amount of the catalyst (rhodium)
loaded both in the presence (table, run 5) and in the
tic or propionic acid, in order to increase the acidity to the level
corresponding to CF COOH.
3
2
–2
–1
–2
–1
RhCl = 1.5
×
10 mol l , KI = 2.5
× 10 mol l , DCl =
3
–1
0.12 mol l , CD COOD–D O.
3
2
PETROLEUM CHEMISTRY Vol. 51
No. 2
2011

136
CHEPAIKIN et al.
Catalytic oxidative functionalization of methane in aqueous acetic and propionic acids. [RhCl3] = 2.5
×
10–3 mol l–1,
[NaCl] = 7.5
No.
×
10–3 mol l–1, [Cu I, II] = 5
Water,
×
10–2 mol l–1,
T
= 95
°
C, PO = 0.56 MPa,
P
CO = 1.84 MPa, 2 h
2
Inorganic
Organic acid
Methanea)
5
Productsb)
6
mol l–1
3
acid, mol l–1
4
1
1
2
CH3COOH
H2O
8.0
H2SO4
0.2
CH4 : CD4 = 0.76 CH3COOCH3 : CH3COOCD3 = 78
2c)
3
CH3COOH
CD3COOD
CH3COOH
CH3COOH
CH3COOH
C2H5COOH
C2H5COOH
C2H5COOH
C2H5COOH
C2H5COOH
H2O
8.0
D2O
17.1
H2O
16.0
H2O
7.7
H2O
7.0
H2O
16.0
H2O
16.0
H2O
7.0
H2SO4
0.2
CH4 : CD4 = 0.81 CH3COOCH3 : CH3COOCD3 = 67
0
CH4
CD4
CH4
0d)
CD3COOCD3 : CD3COOCH3 = 1.83
CH3COOCH3 : CH3COOCD3 = 17.5
4
0
5
H2SO4
0.2
H2SO4
0.2
HClO4
0.22
CH3COOCH3 (5.5
CH3COOCH3 (3.1
×
×
10–2 M)
10–2 M)
6
7
CH4 : CD4 = 0.14 C2H5COOCH3, C2H5COOC2H5,
8
–
CD4
CD4
CH4
0d)
C2H5COOCH3, C2H5COOC2H5
C2H5COOCH3, C2H5COOC2H5
C2H5COOCH3 : C2H5COOC2H5 = 1.9
C2H5COOCH3 : C2H5COOC2H5 = 1.7
9
H2SO4
0.1
H2SO4
0.1
H2SO4
0.11
10
11
H2O
7.0
H2O
7.0
Notes: a) CH (6.0 MPa), CD (4.0 MPa) + He (2.0 MPa); b) the qualitative composition and the ratio were determined by GC/MS; c) reacꢀ
4
4
tion time, 1 h; d) methane is substituted by helium.
absence of methanol (table, run 6). Thus, methane is propionic acid yields deuterated products (table, runs
not the only source of methanol and its ester. At least a
portion of these products is formed from acetic acid,
the solvent component. This makes it impossible to
7–9). Nevertheless, not only methyl propionate
expected as a methane oxidation product, but also
ethyl propionate was formed in these runs, which, in
principle, cannot result from nonradical oxidation of
methane. The absence of deuterated methyl propiꢀ
measure the isotopic kinetic effect kCH /kCD by the
4
4
method of competitive reactions; i.e., in the joint oxiꢀ
dation of methane and tetradeuteromethane. For
example, in the oxidation of the CH4 : CD4 = 0.76
mixture, the light and heavy esters are formed in acetic
acid in a ratio of ~70, which cannot be explained
(table, runs 1, 2).
4
onate in the reaction products cannot be accounted
for by the fact that the CD4 oxidation rate in propionic
acid (as in the case of acetic acid) is sixfold lower than
the rate of CН4 oxidation. Moreover, when methane is
replaced with helium, the same products are formed
and almost in the same ratios (table, runs 10, 11). All
these observations indicate that methane is not subꢀ
jected to oxidation in propionic acid, unlike the cases
of acetic acid, and does not participate in the formaꢀ
tion of methyl propionate. Being far from the carboxꢀ
ylic group, the methyl group of propionic acid probaꢀ
bly behaves as a saturated hydrocarbon and competes
with sparingly soluble methane in the reaction of the
active intermediate.
Note that in the CD3COOD/D2O system,
HCOOD, CH3COOD, CH3OD, and CH3OC(O)CD3
can be formed from CH4 only. Taking this circumꢀ
stance into account, we can approximately estimate
the KIE for methane in acetic acid as ((kH/kD) ~ 6)
~
from the ratio of ~6 of the products of CH4 and CD4
oxidation in CD3COOD and CH3OOH, respectively
3
(table, runs 3, 4).
Unexpectedly, it turned out that neither the oxidaꢀ
tion of CD4 nor the joint oxidation of СH4 and CD4 in
3
4
It is assumed that the reaction is firstꢀorder in methane and
The analytical procedures used in this work allowed for the deterꢀ
–3
KIE = 1 for the solvent, as in the case of CF COOH(D) [14].
mination of methyl propionate in a concentration of 10 mol/l.
3
PETROLEUM CHEMISTRY Vol. 51
No. 2
2011

OXIDATIVE FUNCTIONALIZATION OF METHANE IN THE PRESENCE
137
100
90
80
70
60
50
10
5
9
5
4
8
7
6
4
40
30
20
10
0
5
4
3
3
3
2
1
2
1
0
2
1
2
4
6
8
10
12
14
–1
H O, mol l
2
0
0.05 0.10 0.15 0.20 0.25 0.30
H2SO4, mol l–1
Fig. 3. The product yield in the propionic acid oxidation as
a function of H O concentration in the absence of H SO :
2
2
4
(
1) C H COOC H , (2) C H COOCH , (3) C H OH,
2 5 2 5 2 5 3 2 5
–3
–1
(
4
) CH COOH, and (5) CH OH; [RhCl ] 2.5 ×10 mol l ,
3 3 3
Fig. 5. The product yield in the propionic acid oxidation as
function of H SO concentration: C H OH,
II
–2
–1
–3
–1
[Cu ] = 5.0
He = 6 MPa, CO = 1.84 MPa, O = 0.56 MPa,
×
10 mol l , [NaCl] = 7.5
×
10 mol l
,
a
(1)
2
4
2 5
T
= 95°C, 2 h.
(
2
) CH OH, (
3
) C H COOC H , (
4
) CH COOH, and
2
3
2
5
2
5
3
ꢀ3
–1
II
(
5
) C H COOCH ; [RhCl ] 2.5
×
10 mol l , [Cu ] =
2
5
3
3
–2
–1
–3
–1
5.0
×
10 mol l , [NaCl] = 7.5
×
10 mol l , [H O] =
2
Methyl propionate, ethanol, ethyl propionate, and
acetic and formic acids were found in the products of
propionic acid oxidation (Figs. 3–5).
–1
7.0 mol l , He = 6 MPa, CO = 1.84 MPa, O = 0.56 MPa,
T
2
= 95°C, 2 h.
For interpretation of the mechanism of the transꢀ
formation of carboxylic acids, it is important that no
transformation products of acetic and propionic acids
have been found in an He, He + О2, or He + CO
atmosphere. Thus, the decarbonylation of these acids
requires the presence of both CO and О2
:
2 RC(O)OH + CO + O2
RC(O)OR + 2CO2 + H2O
R = CH3, C2H5.
(2)
140
6
Although the mechanism of the reaction under
study is complex, some preliminary conclusions can
be made. The stability of propionic acid in the absence
of CO indicate the participation of CO in decarbonyꢀ
lation. The stoichiometric oxidation of carbon monꢀ
oxide with rhodium(III) compounds involves the
intermediate formation of the rhodium(III) complex
with CO [19]. Therefore, the oxidative decarbonylaꢀ
tion of carboxylic acids and the oxidation of CO can be
considered coupled reactions with the same intermeꢀ
diate. The rhodium(III) complex with CO and carꢀ
boxylic acid can be this intermediate. Most of the
amount of CO (~90%) is oxidized to СО2 according to
reaction (1) under our experimental conditions. Carꢀ
bon dioxide can be formed from rhodium(III) carbonyl
120
100
5
80
4
60
3
40
7
2
1
20
0
2
4
6
8
10 12 14 16 18
–1
H O, mol l
2
Fig. 4. The product yield in the propionic acid oxidation as
function of H O concentration: C H OH,
acetate (1) via two routes. One is the nucleophilic attack
a
(
(1)
2
2 5
of water on coordinated CO and the reduction of rhodꢀ
ium(III) to rhodium(I) (Scheme 3, reaction (3)). The
other involves the intrasphere attack of the coordiꢀ
nated carboxylate anion on coordinated CO and the
2) C H COOC H , (3) CH OH, (4) CH COOH,
) C H COOCH , (6) CH OH + C H COOCH , and
2 5 2 5 3 3
(5
2 5 3 3 2 5 3
–3
1
(7) C H OH + C H COOC H ; [RhCl ] 2.5 × ,
10 mol l⎯
2 5 2 5 2 5 3
II
ꢀ2
–1
–3
1
[Cu ] = 5.0
×
10 mol l , [NaCl] = 7.5
×
10 mol l⎯
,
intermediate formation of complex 2 (Scheme 3,
reaction (4)). Both reactions are well known in the
coordination chemistry of palladium [20].
–1
[H SO ] = 0.1 mol l , He = 6 MPa, CO = 1.84 MPa,
2
4
O = 0.56 MPa,
T
= 95°C, 2 h.
2
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138
CHEPAIKIN et al.
salts [21]. Indeed, the admixtures of sulfuric acid
increase the yield of the decarbonylation products by
decreasing the fraction of hydroxo complexes (Fig. 3;
Ln RhI + CO + 2HX
(3)
2
CO
Ln RhIIIX2
OC
R
H O
2
curves
1
,
3
; Fig. 4, curves
1, 2, 7).
O
1
R
OC
O
RhIIIX2
Compounds of type
acyl complex of type
mation route:
2
can eliminate СО2 to give the
[20]. This is another СО2 forꢀ
Ln
3
(4)
C O C R
O
O
2
RhIIIX2
Ln
RhIIIX2 + CO
Ln
2
X = OH–, Cl–, RCOO–
(5)
C O C R
C R
Scheme 3.
O
O
O
2
3
An increase in the water content should promote
reaction (3). Indeed, the yield of the decarbonylation
products decreases with an increase in the water conꢀ
Complex
3 is similar to the intermediate in the
Monsanto process [18], the key stage of which is the
reversible intrasphere insertion of CO in the Rh–C
bond. We assume that complex
its isomer , which interacts with a nucleophilic agent
(water or carboxylic acid) to give the corresponding
centration (Fig. 4; curves 1, 2, 7), thereby confirming
indirectly the truth of the assumption on which
Scheme 3 is based. It is likely that the coordinated
hydroxyl ion, rather than the free water molecule,
attacks the coordinated CO molecule in reaction (3),
3 is in equilibrium with
4
as in the case of oxidation of olefins with palladium(II) alcohol or ester:
CO
H O or
CO
2
RhIIIX2
R'COOH
Ln RhIIIX2
I
Ln
or R'COOR
or RX
.
Ln Rh X + HX + ROH
C R
R
O
3
4
The suggested scheme agrees with the fact of the oxidative decarbonylation of propionic acid confirms
formation of ethanol and ethyl propionate from propiꢀ this statement. Moreover, the yields of methanol +
onic acid or methanol and methyl acetate from acetic methyl propionate and ethanol + ethyl propionate
acid.
Within the framework of this scheme, the role of
copper reduces to the reoxidation of rhodium(I) in the
catalytic cycle with respect to copper (reactions (6)
and (7)):
depend on the concentrations of H2O and H2SO4 in
different manners: the [methanol + methyl propiꢀ
onate] concentration increases, whereas [ethanol +
ethyl propionate] decreases, with an increase in the
water concentration (Figs. 3, 4); with an increase in
the concentration of H2SO4, [methanol + methyl proꢀ
pionate] passes through a maximum, and [ethanol +
ethyl propionate] does not change (Fig. 5). This leads
to the conclusion that the formation of methanol and
ethanol (and their esters) involves different active
intermediates.
2CuII + RhI
2CuI + O2 + 2H+
2CuI + RhIII,
2CuII + H2O.
(6)
(7)
½
Hydrogen peroxide can appear as a result of the
reactions of both Rh(I) and Cu(I):
RhI + O2 + 2H+
RhIII + H2O2,
(8)
In the reactions of propionic acid, acetic acid,
methanol, and methyl propionate are not formed in
the sequence of transformations C2H5COOH
2CuI + O2 + 2H+
2CuII + H2O2.
(8a)
The formation of hydrogen peroxide was observed
during the oxidation of CuI to CuII with molecular
oxygen [22].
The considered reaction sequence does not contraꢀ
dict the experimental results, but it does not cover all
reactions occurring in the RhIII/CuII/Cl–/propionic
C2H5OH
CH3COOH and the subsequent decarꢀ
bonylation of CH3COOH according to the mechaꢀ
nism given above, because the joint oxidation of
C2D5OD and propionic acid yields only deuteroethyl
propionate (C2H5COOC2D5), rather than acetic acid
acid system. The fact of the methanol formation in the deuterosubstituted, at the methyl group.
PETROLEUM CHEMISTRY Vol. 51
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OXIDATIVE FUNCTIONALIZATION OF METHANE IN THE PRESENCE
139
In the products of propionic acid oxidation in the 43, 44, 45, and 46 were found in the cluster of acetyl
deuterated solvent, deuterosubstituted acetic acids ions, indicating the deuteration of the acetyl moiety of
•
•
were found, which contain one or two D atoms in the the ester molecule. Ions from М+ to (М + 6)+ were
methyl group. This is indicated by signals at m/z 62 found in the cluster of the molecular ion, thereby indiꢀ
(CH2DCOOD or CHD2COOH) and m/z 63 cating the presence of D atoms both in the acetyl and
(CHD2COOD or CD3COOH) comparable in intenꢀ methoxy groups of the molecule (Fig. 6c).
sity with the signal of the molecular ion of acetic acid
m/z) 60) in the mass spectrum averaged over the chroꢀ
matographic peak. Moreover, the signal intensities at
44 (CO2 and CH2DCO) and 45 (COOH and
CHD2COO) in the mass spectrum of acetic acid
obtained in the oxidation of propionic acid are subꢀ
stantially higher than the intensities of these signals in
the standard spectrum of CH3COOH (Fig. 6a).
In the absence of dioxygen, only the carboxylic
hydrogen atoms of acetic and propionic acids are
exchanged by deuterium. The H–D exchange was not
observed in the methyl acetate and methyl propionate
introduced into the solution. All these facts cannot be
rationalized within the framework of the mechanism
suggested above. According to this mechanism, the
shortening of the carboxylic acid carbon chain occurs
(
m
/
z
m/z
The methyl propionate molecular ion cluster found as a result of oxidative decarbonylation. The formation
in the reaction products consists of ions having 88, of deuterosubstituted acetic acids during the oxidation
m/z
89, and 90 with comparable intensities. The spectrum of propionic acid in D2O can indicate the existence of
of the reaction product practically coincides with the the route that involves the methyl or the methylene
tabulated spectrum of methyl propionate in the range group.
of m/z
29–32 (C2H5+ fragment ion) (Fig. 6b). These
two observations indicate that methyl propionates difꢀ
fering by the depth of deuteration contain the deuteꢀ
rium atoms only in the methoxy group.
We cannot rule out that the coordinated propionate
ion can be subjected to the innerꢀsphere oxidation by
the action of the peroxo group coordinated at the
rhodium atom [16]. The decarboxylation of the interꢀ
In the mass spectrum of methyl acetate found in mediate malonate in the protonꢀdonating medium
the products of propionic acid oxidation, ions at gives CDH2COOD
m/z
:
O
C
O
C
O
C
O
C
H O
2
H2C
H2C
H
O
Rh
H2C
H2C
O
H2C
HC
O
O
H O
2
O
Rh
H2C
H2C
2
…
–H O
2
Rh
O
Rh
O
O O
O O
OH
OH
O
O
C
O
C
C
D O
2
H2C
HO C
O
O
H2C
DO C
O
O
H2DC
O
–CO
2
H2DCCOOD.
HDO
Rh
O
Rh
O
Rh
O
The possibility of the oxidation of propionic acid in aqueous CF3COOH. The formation of CD2HCOO–
and CD3COO groups can be accounted for by the
exchange of the methylene hydrogen atoms of malonic
acid for deuterium [24].
into acetic acid via the intermediate formation of 3ꢀ
oxypropionic and malonic acids agrees with the pubꢀ
lished data [23] on the oxidation of labeled
CH3CH213COOH; acetic acid CH313COOH is formed by
The presence of methyl propionate deuterated in
the action of O2 and CO in the presence of Pd/C–CuCl2 the methoxy group in the reaction products suggests
Fig. 6. Chromatoꢀmass spectra of the products of propionic acid oxidation in the presence of D SO (0.2 M) and D O (7 M):
2
4
2
(A) the experimental peak of acetic acid (a) and the standard spectrum of acetic acid (b), (B) the experimental peak of methyl
propionate (a) and the standard spectrum of methyl propionate (b), and (C) the experimental peak of methyl acetate (a) and the
–3
–1
II
–2
–1
–3
–1
standard spectrum of methyl acetate (b); [RhCl ] = 2.5
×
10 mol l , [Cu ] = 5.0
× 10 mol l , [NaCl] = 7.5 × 10 mol l ,
3
–1
[H O] = 7.0 mol l , He = 6 MPa, CO = 1.84 MPa, O = 0.56 MPa,
T
= 95°C, 2 h.
2
2
PETROLEUM CHEMISTRY Vol. 51
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140
CHEPAIKIN et al.
100
50
(a)
43
?
60
46
47
18
19
19
28
62
64
42
30
32
27
38
34 36
34
24
24
49
49
53
51
55
59
57
0
А
39
54
29
59
64
(b)
14
44
43
100
45
О
60
50
15
42
41
ОН
29
29
14
44
16
31
18
19
25
24
46
61
0
14
34
39
44
54
64
49
59
57
100
(a)
29
?
50
88
59
27
55
45
31
42
64
18
67
37
35
52
74
71
48
0
25
75
85
15
45
55
65
B
29
100
57
(b)
О
50
27
59
О
88
31
45
55
42
37
35
0
85
25
75
15
45
65
55
43
100
(a)
?
50
29
74
18
20
59
32
25
80
80
56
0
30
50
70
10
60
90
C
40
43
100
(b)
О
50
0
74
76
О
59
29
15
33
25
56
10
50
60
90
20
80
30
70
40
PETROLEUM CHEMISTRY Vol. 51
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OXIDATIVE FUNCTIONALIZATION OF METHANE IN THE PRESENCE
141
that deuterosubstituted methanols are formed as a terosubstituted in the methyl group according to the
result of oxidative decarboxylation of acetic acids deuꢀ mechanism with participation of CO described above:
O
C
O
C
O
C
O
C
O
C
H
H
H
D C
D
H
D C
D
D
H
D O
2
–CO
2
CO
C
O
C
O
O
O
Rh
O
H O
2
HO C
O
Rh
O
DO C
O
Rh
O
Rh
O
–CO
2
O
O
C
C
CO
CO
H
D C
D
H
CH2D
CHD2
O
C
O
C H COOH
5
CH DCOOD
2
2
C H C OCHD +Rh
C22H5C OCHD 22 +Rh
Rh
D C Rh
5
O
O
O
D
O
O
O
If this assumption is true, the ratio of the ion intenꢀ
sities corresponding to DH2CO+ and H3CO+ in the
averaged mass spectrum of methyl acetate should
coincide with the ratio of the ion intensities of
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PETROLEUM CHEMISTRY Vol. 51
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2011