Uncommon oxidative transformations of acetic and propionic acids

Article abstract of DOI:10.1007/s11172-011-0007-0

The oxidative decarbonylation of acetic and propionic acids with the formation of the corresponding alcohol and alkyl carboxylate is observed in the Rh^III/Cu^I,II/Cl^- catalytic system in the presence of O₂ and CO. The decarbonylation of propionic acid in a deuterated solvent results in the substitution of hydrogen atoms by deuterium in the alkyl part of the products to form CH₂DCOOD (CHD ₂COOH) and CHD₂COOD (CD₃COOH). The subsequent decarbonylation of deuterated acetic acids affords the corresponding deuteromethanols detected as esters with propionic and deuteroacetic acids. The substitution of the hydrogen atom by deuterium in the alkyl part of molecules of the products of oxidative decarbonylation of propionic acid, when the reaction is carried out in a deuterated solvent, indicates that propionic acid behaves as saturated hydrocarbon and blocks the oxidation of poorly soluble methane. Unlike propionic acid, acetic acid enters only the oxidative decarbonylation reaction and does not block methane oxidation.

Full text of DOI:10.1007/s11172-011-0007-0

Russian Chemical Bulletin, International Edition, Vol. 60, No. 1, pp. 49—54, January, 2011
49
Uncommon oxidative transformations of acetic and propionic acids
Е. G. Chepaikin,а А. P. Bezruchenko,а G. N. Menchikova,а N. I. Моiseevа,b А. Е. Gekhmаn,b and I. I. Моisееvb
аInstitute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences,
8 ul. Institutskaya, 142432 Chernogolovka, Моscow Region, Russian Federation.
Fax: +7 (496) 524 6222. Eꢀmail: echep@ism.ac.ru
bN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Моscow, Russian Federation.
Fax: +7 (495) 955 4835. Eꢀmail: gekhman@igic.ras.ru
The oxidative decarbonylation of acetic and propionic acids with the formation of the
corresponding alcohol and alkyl carboxylate is observed in the RhIII/CuI,II/Cl– catalytic sysꢀ
tem in the presence of О2 and CO. The decarbonylation of propionic acid in a deuterated
solvent results in the substitution of hydrogen atoms by deuterium in the alkyl part of the
products to form CH2DCOOD (CHD2COOH) and CHD2COOD (CD3COOH). The subseꢀ
quent decarbonylation of deuterated acetic acids affords the corresponding deuteromethanols
detected as esters with propionic and deuteroacetic acids. The substitution of the hydrogen
atom by deuterium in the alkyl part of molecules of the products of oxidative decarbonylation of
propionic acid, when the reaction is carried out in a deuterated solvent, indicates that propionic
acid behaves as saturated hydrocarbon and blocks the oxidation of poorly soluble methane.
Unlike propionic acid, acetic acid enters only the oxidative decarbonylation reaction and does
not block methane oxidation.
Key words: methane, acetic and propionic acids, oxidation, rhodium and copper compounds.
at the temperature of liquid nitrogen and then evaporated at
–(60—50) °С to remove heavy admixtures, for instance, CCl4.
Catalytic experiments were carried out in a 34ꢀcm3 Fluoroꢀ
plastꢀlined stainless steel reactor using the earlier described proꢀ
cedure.3,4 In a typical experiment on methane oxidation, an
autoclave was loaded with 6.25•10–3 mmole of RhCl3 and
18.75•10–3 mmole of NaCl as solutions (0.25 g) in D2O;
Interest in chemistry of carboxylic acids is generated
by their role of biomass treatment aimed at producing
diesel fuel. The presence of the carboxyl group decreases
the energy content of the fuel. Therefore, one of the probꢀ
lems, namely, removal of the carboxyl group from fatty
acids of lipids, can be solved by the decarbonylation of the
acid followed by the dehydration of the alcohol.1 In the
present work, the decarbonylation of propionic and acetic
acids under mild conditions (95 °С) was studied. It has
previously2 been found that the decarbonylation of stearic
acid on the rhodium catalysts occurs only at 240—280 °С.
[RhCl3] = 2.5•10–3 mol L–1 and [NaCl] = 7.5•10–3 mol L–1
.
Then a 0.96•10–3 М D2SO4 solution (0.25 g) in D2O, 2.27 g of
CD3COOD, and 10 mg of CuO were added. The autoclave was
fed with the following gases: methane (6.0 MPa), oxygen
(0.56 MPa), and carbon monoxide (1.84 MPa). The time of the
experiment was 2 h. In a typical experiment on the oxidation of
propionic acid, the autoclave was loaded with CuO (10 mg) and
a solution (25 mL) prepared as follows: H2O (1.4 mL), a solution
of H2SO4 in H2O ((0.56 mL, 3.9 mol L–1), and C2H5COOH
(17 mL) were added to an aqueous solution (1.0 mL) containing
RhCl3 (5•10–2 mol L–1) and NaCl (0.15 mol L–1). The gases
were fed in the following order: helium (4 MPa), oxygen
(0.56 MPa), carbon monoxide (1.84 MPa), and again helium to
a total pressure of 8.4 MPa. The duration of the experiment was 2 h.
The gas phase was analyzed by GC using the known proceꢀ
dure,3,4 and the liquid phase was analyzed by GC (flame ionizaꢀ
tion detector, N2) on a Chromꢀ5 chromatograph with glass
packed columns (inner diameter 2.5 mm). To determine the
oxidation products of acetic acid, Separon CHN was used
(0.115—0.200 mm, 2 m, 130 °С, dioxane as an internal stanꢀ
dard), and propionic acid was determined on Separon SDA
(2.4 m, 140 °С, heptane as an internal standard). The products
Experimental
The following reagents and materials were used:
RhCl3•(H2O)n (34.5 wt.% Rh), NaCl, CuO, NaCl, H2SO4, and
HClO4 (reagent grade), as well as H2O (bidistillate). Acetic acid
(reagent grade) was distilled, C2H5COOH (Merck) was used as
received, heptane (standard) was distilled off, and dioxane (reꢀ
agent grade) was refluxed for 2 h above metallic sodium and then
distilled. Methane (99.8%), carbon monoxide (99.9%), oxygen
(99.9%), helium (trade mark A), nitrogen (special purity grade),
hydrogen (99.0%), and deuterated compounds D2O (99.9 at.%),
CD3COOD (99.5 at.%), and D2SO4 (98.0 at.%) were used withꢀ
out purification; and CD4 (98.2 at.%) was condensed twice
* Dedicated to Academician S. N. Khadzhiev on the occasion of
his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 49—53, January, 2011.
1066ꢀ5285/11/6001ꢀ49 © 2011 Springer Science+Business Media, Inc.

50
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 1, January, 2011
Chepaikin et al.
of oxidation of CH4 in CD3COOD/D2O were analyzed by
1H NMR on Bruker АСꢀ200 Р (200 MHz) and Tesla BSꢀ587ꢀA
(80 MHz) instruments. An admixture of CHD2COOD in
CD3COOD was used as an internal standard for quantitative
measurements.
sary component of the catalytic system in aqueous trifluoꢀ
roacetic acid is CO.3—5 The initial activity of these sysꢀ
tems at 95 °С and pressures (MPa, in parentheses) of СН4
(6.0), СО (1.84), and О2 (0.56) reaches 100—150 moles of
СН4 (mole of Rh)–1 h–1. The oxidation of СН4 in the
RhIII/CuI,II/Cl– system in a solution of CF3COOH (see
Ref. 4) affords the following products:
The initial ratio CH4/CD4 was determined by mass spectroꢀ
metry on an MIꢀ1201 instrument with a needle valve as a leak
(pressure in the forecylinder 10–1 Torr, electron impact 70 eV,
ionization current 0.2 mA). The GC/MS experiment was carꢀ
ried out on an Automass 150 instrument (Delsi—Nermag) with
a DN200 gas liquid chromatograph (Delsi), chromatographic
column CPSilꢀ5 (25 m × 0.15 mm), df = 1.2 μm (Chromopack),
He as a carrier gas, constant pressure regime 1.2 bar, flow splitꢀ
ting injector 1 : 50, Tinj = 220 °C, temperatureꢀprogrammed colꢀ
(1)
umn Tinit = 40 °C (4 min), Tfin = 250 °C (4 min), 10 °C min–1
.
The replacement of the solvent by an aqueous solution
of acetic acid (both with and without additives of sulfuric
acid*) results in methane oxidation (Table 1, entries 1—3)
and also (unexpectedly) the decarbonylation of acetic acid
by reaction (2).
The ionization regime was as follows: electron impact, 70 eV,
and resolution at least 2.0 М (М = m/z) in the range m/z 18—131.
For the analysis of the isotopic composition, the spectra were
recorded in the scanning mode in the range 15—100 m/z with
scanning duration 100 ms. The Lucy ver. 2.0 and AMDIS
ver. 2.62 program packages were used for processing GC/MS
experiments. For sampling, the reaction solution was placed in
a tube sealed with a chromatographic diaphragm. The tube was
heated to ∼60 °C, and then a gas sample was taken above the
solution using a chromatographic syringe heated to ∼50 °C.
A gas sample (50—100 μL) was injected into the chromatograph.
2 MeC(O)OH + CO + O2
MeC(O)OMe + 2 CO2 + H2O
(2)
Free radicals make no substantial contribution to methꢀ
ane oxidation under our experimental conditions.6 In the
absence of CO, neither methane nor the acid are involved
in the reactions.
Results and Discussion
Homogeneous systems RhIII/I–/Cl–, RhIII/CuI,II/Cl–,
and RhIII/FeII,III/Cl– manifest high catalytic activity in
the direct oxidation of methane with oxygen. The necesꢀ
* Sulfuric acid was added to the solution in some experiments
with acetic and propionic acids in order to mimic CF3COOH.
Table 1. Catalytic oxidative functionalization of methane in aqueous acetic and propionic acidsa
Entry
Organic
acid
Water
(C/mol L–1
Concentration of H2SO4
/mol L–1
Methaneb
Productsc
)
1d
2
CH3COOH
CD3COOD
CH3COOH
CH3COOH
C2H5COOH
C2H5COOH
C2H5COOH
C2H5COOH
H2O
(8.0)
D2O
(17.1)
H2O
(16.0)
Н2О
(7.0)
H2O
(7.0)
H2O
(7.0)
H2O
(16.0)
H2O
(7.0)
0.20
—
CH4 : CD4 = 0.76
CH3COOCH3—CH3COOCD3
(78 : 1)
CD3COOCD3—CD3COOCH3
(1.83 : 1)
CH3COOCH3—CH3COOCD3
(17.5 : 1)
СH3COOCH3 (3.1•10–2 mol L–1
)
CH4
CD4
0e
3
4
—
0.20
0.10
0.11
—
5
6
7
8
CH4
0e
C2H5COOCH3—C2H5COOC2H5
(1.9 : 1)
C2H5COOCH3—C2H5COOC2H5
(1.7 : 1)
CD4
CD4
C2H5COOCH3, C2H5COOC2H5
0.10
C2H5COOCH3, C2H5COOC2H5
a [RhCl3] = 2.5•10–3 mol L–1, [NaCl] = 7.5•10–3 mol L–1, [CuI,II] = 5•10–2 mol L–1, T = 95 °C, PO = 0.56 MPa, РСО = 1.84 MPa, 2 h.
2
b СН4 (6.0 MPa), CD4 (4.0 MPa) + He (2.0 MPa).
c The quantitative composition and the ratio were determined by the GC/MS method.
d The reaction time is 1 h.
e Methane was replaced by helium.

Uncommon oxidative transformations of acids
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 1, January, 2011
51
It was found that under the experimental conditions
used the amount of methyl acetate exceeds the amount of
the loaded catalyst (rhodium) by at least an order of magꢀ
nitude. Methyl acetate is formed both in the presence
of methane (see Table 1, entries 1—3) and without it
(see Table 1, entry 4). Thus, methane is not the single
source of methanol and its ester. At least a portion of these
products is formed from a component of the solvent, viz.,
acetic acid.
presence of the palladium complexes is due to the protoꢀ
nation of the hydroxyl group coordinatively bound in the
πꢀpalladium complex.7 On the one hand, an increase in
the water concentration favors the oxidation of CO (reacꢀ
tion (4)) and, on the other hand, it decreases the yield of
the decarbonylation products of propionic acid, ethanol
and ethyl propionate (Fig. 2, curves 2 and 7).
CO + 0.5 O2
CO2
(4)
Methyl propionate, ethyl propionate, ethyl chloride,
and acetic and formic acids were identified in an aqueous
solution of propionic acid acidified with sulfuric acid both
in the presence and in the absence of methane in the cataꢀ
lytic system RhIII/CuI,II/Cl–. When methane is replaced
by helium, methyl propionate and ethyl propionate are
formed in almost the same ratio (see Table 1, entries 5 and 6).
No transformations of acetic or propionic acid are obꢀ
served in the experiments, where helium, helium + oxyꢀ
gen, and helium + carbon monoxide were used as the gas
phase. Thus, the necessary condition for the decarbonylaꢀ
tion of the studied carboxylic acids (reaction (3)) is the
presence of both CO and О2.
This indirectly confirms the existence of the same inꢀ
termediate in the oxidative decarbonylation of carboxylic
acids and CO oxidation.
The oxidation of CD4 in propionic acid (with and withꢀ
out sulfuric acid additives) affords no deuterated prodꢀ
ucts* (see Table 1, entries 7 and 8). The absence of deuꢀ
terosubstituted products of the oxidation of CD4 in the
studied system indicates that methane does not participate
in the formation of methyl propionate. This means that,
unlike solutions of acetic acid, no methane oxidation
occurs in solutions of propionic acid. All identified prodꢀ
ucts with the carbon chain shorter than that of propionic
acid are formed by the oxidative decarbonylation of proꢀ
pionic acid.
2 RC(O)OH + CO + O2
The formation of acetic acid, methanol, and methyl
propionate from propionic acid cannot be considered as
RC(O)OR + 2 CO2 + H2O
(3)
R = Me, Et
* The analytical procedures used in the work would make it
possible to determine deuterated methyl propionate in the conꢀ
Additives of sulfuric acid decrease the probability of
the nucleophilic attack of water or reduce the concentraꢀ
tion of the rhodium or copper oxo complexes and increase
the yield of decarbonylation products (Fig. 1, curves 1 and 3).
Similar inhibition of olefin oxidation by the acid in the
centration down to 10–3 mol L–1
.
C•103/mol L–1
140
120
100
80
C•102/mol L–1
6
10
8
5
4
5
6
60
4
3
7
2
40
4
2
0
3
20
1
2
1
4
6
8
10
12 14 [H2O]/mol L–1
Fig. 2. Influence of the H2O concentration on the yield of the
oxidation products of propionic acid: C2H5OH (1),
C2H5COOC2H5 (2), CH3OH (3), CH3COOH (4),
C2H5COOCH3 (5), CH3OH + C2H5COOCH3 (6), and
C2H5OH + C2H5COOC2H5 (7). Small amounts of methyl aceꢀ
tate are also observed (omitted). Conditions: [RhCl3] =
= 2.5•10–3 mol L–1, [CuII] = 5.0•10–2 mol L–1, [NaCl] =
= 7.5•10–3 mol L–1, [H2SO4] = 0.1 mol L–1, PHe = 6 MPa,
0
0.05 0.10 0.15 0.20 0.25 0.30 [H2SO4]/mol L–1
Fig. 1. Yield of the products of propionic acid transformation vs
concentration of H2SO4: EtOH (1), MeOH (2), EtCOOC2H5 (3),
MeCOOH (4), and EtCOOCH3 (5). Small amounts of methyl
acetate are also observed (omitted). Conditions: [RhCl3] =
2.5•10–3 mol L–1, [CuII] = 5.0•10–2 mol L–1, [NaCl] =
= 7.5•10–3 mol L–1, [H2O] = 7.0 mol L–1, PHe = 6 MPa,
=
PCO = 1.84 MPa, PO = 0.56 MPa, Т = 95 °С, 2 h.
PCO = 1.84 MPa, PO = 0.56 MPa, Т = 95 °С, 2 h.
2
2

52
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 1, January, 2011
Chepaikin et al.
Irel (%)
acid and C2D5OD affords no CD3COОН. Therefore, acetic
acid is not a product of ethanol oxidation, which is formed
due to the oxidative decarbonylation of propionic acid.
Under conditions described above, deuterosubstituted
acetic acids containing one and two D atoms in the methꢀ
yl group were found among the oxidation products of proꢀ
pionic acid in the presence of D2SO4 (0.2 mol L–1) and
D2O (7.0 mol L–1) (see Fig. 1). This is indicated by the
signals with m/z 62 (CH2DCOOD or CHD2COOH) and
63 (CHD2COOD or CD3COOH), whose intensity is comꢀ
parable with that of the signal of the molecular ion of
acetic acid (m/z 60) in the mass spectrum* (Fig. 3, a)
averaged by the chromatographic peak at 6.7—7.1 min
(Fig. 4). In addition, the intensities of the signals with m/z
44 (CO2 and CH2DCO) and 45 (COOH and CHD2COO)
in the mass spectrum of acetic acid obtained by the oxidaꢀ
tion of propionic acid are substantially higher than the
intensities of the same signals in the reference spectrum of
CH3COOH (Fig. 3, b).
a
45
100
50
43
62
64
30
18
19
57
60
28
27
46
38
53
51
49
47
34
42
24
59
55
32 36
14 19 24 29 34 39 44 49 54 59 64 m/z
Irel (%)
b
43
45
100
50
44
60
61
15
16
18
42
29
31
14
41
46
25
14 19 24 29 34 39 44 49 54 59 m/z
Fig. 3. Mass spectra of the chromatographic peak at 6.7—7.1 min
(a) and the reference spectrum of acetic acid (b).
→
→
the successive transformation C2H5COOH C2H5OH
→
* The mass spectrum was recorded in each point (with a freꢀ
quency of ∼5 s–1) of the chosen region of the chromatogram.
The final mass spectrum is weighed average.
CH3COOH and the further decarbonylation of
CH3COOH, since the combined oxidation of propionic
I (arb. units)
3
98.9
74.1
2.45
1.89
1
49.4
1.34
2
0.78
24.7
2.80 3.30 3.79 4.29 4.78 5.28 5.77 6.27 6.76 7.25 7.75 t/min
2.67 3.18
3.69 4.20 4.71 5.22
5.73
6.24 6.74 7.25 7.76
8.27 8.78
t/min
Fig. 4. Chromatogram of the oxidation products of propionic acid in the presence of D2SO4 (0.2 mol L–1) and D2O (7.0 mol L–1):
methyl acetate (1), acetic acid (2), and methyl propionate (3). Conditions: [RhCl3] = 2.5•10–3 mol L–1, [CuII] = 5.0•10–2 mol L–1
,
[NaCl] = 7.5•10–3 mol L–1, PHe = 6 MPa, PCO = 1.84, PO = 0.56 MPa, Т = 95 °С, 2 h. Rectangles designate regions of averaging the
chromatographic signals of the product and background; I 2is the total ionic current.

Uncommon oxidative transformations of acids
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 1, January, 2011
53
Irel (%)
presence of atoms D in both the acetyl and methoxy group
of the molecule (see Fig. 6).
a
57
100
In the absence of oxygen, only the hydrogen atom of
the carboxyl group in acetic and propionic acids is reꢀ
placed by deuterium, while no H—D exchange occurs in
the presence of methyl acetate and methyl propionate addꢀ
ed to the system. Based on all these facts, the deuterosubꢀ
stituted acetic acid derivatives are formed by the oxidation
of propionic acid in the deuterated solvent, which involves
the methyl groups.
29
52
50
45
88
59
74
75
27
18
15
55
55
31 37
35
64
67
42
71
48
25
45
65
85
m/z
Irel (%)
100
b
29
57
The presence of methyl propionates deuterosubstituted
at the methoxy group in the reaction products suggests
that deuterosubstituted methanols are formed due to the
oxidative decarbonylation of the acetic acid derivatives
deuterosubstituted at the methyl group involving CO and
О2. If this assumption is valid, the ratio of peak intensities
corresponding to the ions DH2CO+ and H3CO+ of methyl
acetate should coincide with the ratio of peak intensities
50
27
25
59
88
85 m/z
31
37
45
42
55
55
15
35
45
65
75
Fig. 5. Mass spectra of the chromatographic peak at 8.46 min (a)
and the reference spectrum of methyl propionate (b).
of the ions C2H5COOCH2D+ and C2H5COOCH3
+
•
•
in the averaged mass spectrum of methyl propionate
(for diꢀ and trideuterosubstituted esters, the comparison of
the ratios of ion intensities is incorrect because of
the kinetic isotopic effect of methoxy group formaꢀ
The cluster of the molecular ion of methyl propionate
found in the reaction products (peak at 8.46 min in the
chromatogram, see Fig. 4) consists of ions with m/z 88,
89, and 90 with the comparable intensity. In the region of
m/z 29—32 (fragmentation ion C2H5+), the spectrum of
the reaction product almost coincides with the tabulated
spectrum of methyl propionate (Fig. 5, b). Both these
facts indicate that the methyl propionates formed contain
deuterium atoms only in the methoxy groups.
The mass spectrum of methyl acetate found among the
oxidation products of propionic acid exhibits ions with
m/z 43, 44, 45, and 46 (Fig. 6, a) in the cluster of the
acetyl ion, indicating the deuteration of the acetyl moiety
of the ester molecule. The cluster of the molecular ion
contains ions from [М]+• to [М + 6]+•, which shows the
tion). In experiment IDH CO+/IH CO = 0.48 and
+
2
3
IC H COOCH D+/IC H COOCH + = 0.46 were obtained.
5
2
3
2 Thus, it 2was foun5d in the present work that the oxidaꢀ
tive decarbonylation of acetic and propionic acids to methꢀ
anol (methyl acetate) and ethanol (ethyl propionate), resꢀ
pectively, occurs during the combined oxidation of carbꢀ
oxylic acids and CO in the presence of the rhodium cataꢀ
lyst. The unusual substitution of deuterium for hydrogen
atom in the alkyl part of molecules of the products of
propionic acid oxidative carbonylation was also observed.
Propionic acid, whose methyl group is separated from
the electronꢀacceptor carboxyl group, behaves similarꢀ
ly to unsaturated hydrocarbon and competes with poorly
soluble methane in the reaction with the reactive interꢀ
mediate. The formation of the deuterosubstituted prodꢀ
ucts with the carbon chain smaller than that in the initial
acid is due to the oxidation of the methyl group. Unlike
propionic acid, acetic acid is involved only in the oxidaꢀ
tive decarbonylation reaction and does not block methane
oxidation.
a
Irel (%)
43
100
50
29
59
56
18
74
80
80
25 32
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 09ꢀ03ꢀ00387)
and the Council on Grants at the President of the Russian
Federation (Program for State Support of Leading
Scientific Schools of the Russian Federation, Grant
NShꢀ65264.2010.3).
10
20
30
40 50
60
70
m/z
Irel (%)
b
43
100
50
74
76
59
56
15
10
29
25
33
30
References
20
40
50
60 70
80
m/z
Fig. 6. Mass spectra of the chromatographic peak at 4.67 min (a)
and the reference spectrum of methyl acetate (b).
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Received November 18, 2010;
in revised form December 21, 2010