Hydrogenation of CO2 to formic acid in the presence of the Wilkinson complex

Article abstract of DOI:10.1023/A:1022162713837

Formic acid was synthesized in a high yield at room temperature in the presence of the Wilkinson complex and an excess of PPh₃. The catalytic properties of the rhodium complex depend strongly on the reaction conditions. The mechanism of the rhodium catalyst deactivation was studied by the kinetic method and ³¹P NMR spectroscopy. The methods for the stabilization of the rhodium catalyst were found.

Full text of DOI:10.1023/A:1022162713837

Russian Chemical Bulletin, International Edition, Vol. 51, No. 12, pp. 2165—2169, December, 2002
2165
Hydrogenation of CO2 to formic acid in the presence of the Wilkinson complex
b
b
c
N. N. Ezhova,a N. V. Kolesnichenko,b A. V. Bulygin, E. V. Slivinskii , and S. Han
ꢀ
аInstitute of High Temperatures, Russian Academy of Sciences,
13/19 ul. Izhorskaya, 127412 Moscow, Russian Federation.
Fax: +7 (095) 485 0922
bA. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,
29 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 230 2224
cKorean Institute of Science and Technology,
130ꢀ650 Seoul, Korea.
Fax: (+82 2) 958 5229
Formic acid was synthesized in a high yield at room temperature in the presence of the
Wilkinson complex and an excess of PPh3. The catalytic properties of the rhodium complex
depend strongly on the reaction conditions. The mechanism of the rhodium catalyst deactivaꢀ
tion was studied by the kinetic method and 31Р NMR spectroscopy. The methods for the
stabilization of the rhodium catalyst were found.
Key words: hydrogenation, carbon dioxide, formic acid, Wilkinson complex, triphenylꢀ
phosphine, 31Р NMR spectroscopy.
atmosphere of one of the following gases: air, argon, СО2, or Н2.
Hydrogenation to formic acid is an attractive method
for СО2 fixation because only 1 mole of H2 is needed for
the formation of formic acid and the reaction occurs withꢀ
out oxygen loss to CO2. The process is thermodynamiꢀ
cally unfavorable1 but can result in a high yield in the
presence of tertiary amines binding the acid
Then Н2 and СО2 were consecutively introduced into the reacꢀ
tor until a specific pressure was achieved. This moment was
considered as the beginning of the reaction. Samples of the
liquid product, which was spectroscopically analyzed, were taken
during the reaction. Formic acid was detected by 1Н NMR
spectroscopy, and products of transformation of triphenylꢀ
phosphine and triphenylphosphineꢀrhodium complexes were
detected by 31Р NMR spectroscopy. Dimethylformamide (1Н)
and triphenyl phosphate (31Р) were used as internal standards.
Products precipitated from the solution were molded with KBr
to form pellets and analyzed by IR spectroscopy. NMR and IR
spectra were recorded on Varian (200, 300, and 600 MHz) and
MIDAC FTIR instruments, respectively.
CO2 + H2 + Et3N → HCOOH•NEt3.
Two main approaches to this reaction are presently
known. Formic acid is obtained under supercritical condiꢀ
tions in the presence of the ruthenium complexes.2 In
solutions of the rhodium complexes modified by the
R2P(CH2)nPR2 bidentate phosphine ligands, the process
is carried out at room temperature and a low pressure.3
This is precisely the direction of studies which presently
seems most promising from the viewpoint of utilization of
carbon dioxide and a decrease in its emission. In this work,
the results of studying СО2 hydrogenation in the presence
of the Wilkinson complex (RhCl(PPh3)3) are presented.
The conditions under which the reaction occurs with a
high yield were found, and the routes of possible deactivaꢀ
tion of catalytically active complexes were studied.
The efficiency of the catalyst operation was estimated from
two parameters: the achieved concentration of formic acid and
the turnover number of the catalyst (TON), which is equal to the
ratio of the number of moles of the product formed per mole of
the catalyst used.
The RhCl(PPh3)3,4 HRh(PPh3)4,5 Rh(acac)CO2,6
8
HRh(CO)(PPh3)3,7 and [RhCl(CO)2]2 complexes and the
ligand etriolphosphite (ЕТРО)9 were synthesized using known
procedures. Commercial reagents (Fluka) RhCl3, PPh3, 2,2´ꢀdiꢀ
pyridyl (2,2´ꢀbipy), bis(1,4ꢀdiphenylphosphino)butane (dppb),
bis(1,2ꢀdiphenylphosphino)ethane (dppe), OPPh3, PBun3, and
DMSO were used without additional purification. Triethylamine
was distilled before use.
Experimental
Results and Discussion
The hydrogenation of СО2 was carried out at 25 °С in a
stainless steel autoclave (150 mL) with an electromagnetic stirꢀ
rer. Before the reaction, the rhodium complex with the ligands
was dissolved upon stirring in a solvent—NEt3 mixture in an
The results of СО2 hydrogenation to formic acid using
various ligands are presented in Table 1. Formic acid is
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2008—2012, December, 2002.
1066ꢀ5285/02/5112ꢀ2165 $27.00 © 2002 Plenum Publishing Corporation

2166
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002
Ezhova et al.
Table 1. Hydrogenation of СО2 to HCOOH•NEt3 in the presꢀ
ence of the Rh complexes modified by various ligands
Table 2. Hydrogenation of СО2 in the presence of the Wilkinson
complexa
Entry Rh complex
Ligand TON [HCOOH•NEt3]
Entry Solvent T/°C Mediꢀ [HCOOH] [MeOH] [MF]b
/mol L–1
umc
mol L–1
1
2
3
4
5
6
7
8
Rh(acac) (CO)2
[RhCl(CO)2]2
RhCl3
—
—
—
—
—
—
0
0
0
40
60
330
0
0
30
60
0
0
0
0.04
0.06
0.33
0
1
2
3
4
5
6
7
8
Heptane
Benzene
THF
25
25
25
25
25
25
25
25
19
40
25
Ar
Ar
Ar
0
0
0
0
0
0.09
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
HRh(PPh3)4
HRh(CO)(PPh3)3
RhCl(PPh3)3
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
[RhCl(CO)2]2
HRh(PPh3)4
RhCl3
DMSO
DMSO
DMSO
DMSO
DMSOd
DMSO
DMSO
Н2О
Ar
0.87
0.55
0.40
0.96
0.26
0.70
0.20
Traces
СO2
Air
Н2
Н2
Н2
Н2
Н2
2,2ꢀbipy
OPPh3
Dppe
Dppb
PPNCl
ЕТРО
0
9
0.03
0.06
0
8´
8″
9
10
11
12
13
14
15
16
17
18
19
20
21
0
0
0.01
0
10
DMSO—
—MeОН 25
(50 wt.%)
PBun
200
500
440
470
500
540
950
480
60
0.20
0.50
0.44
0.47
0.50
0.54
0.95
0.48
0.06
3
Н2
Н2
1.50
2.50
—
—
0.12
0.22
PPh3
PPh3
PPh3
PPh3
11
MeОН
25
a Reaction conditions: RhCl(PPh3)3 + 3 PPh3, 25 °С, [Rh] =
HRh(CO)(PPh3)3 PPh3
1•10–3 gꢀat. L–1, pCO = 40 atm, pH2 = 20 atm, 20 h.
2
RhCl(PPh3)3
RhCl(PPh3)3
HRh(PPh3)4
PPh3
OPPh3
OPPh3
b MF is methyl formate.
c The medium of catalyst formation.
d [Rh] = 1•10–4 gꢀat. L–1
.
Note. Hydrogenation conditions: pCO = 40 atm, pH = 20 atm,
2
2
[Rh] = 1•10–3 gꢀat. L–1, [NEt3] = 1.45 mol L–1, P : Rh = 6,
DMSO, 20 h.
complete replacement of DMSO by methanol increases
sharply the yields of formic acid and MF (entry 11).
The activity of the catalyst changes, depending on the
gaseous atmosphere used during the dissolution of the
Wilkinson complex in DMSO. The dissolution of the cataꢀ
lytic system in the presence of an oxidative atmosphere
(in air or under the СО2 pressure) decreases the yield of
formic acid. The yield of the acid increases when argon or
dihydrogen is used. The highest yield of formic acid is
achieved when the catalyst is formed in a hydrogen atmoꢀ
sphere.
not formed in the presence of the rhodium complexes
without a phosphine ligand (see Table 1, entries 1—3).
For example, Rh(acac)(CO)2, nonmodified or modified
by 2,2´ꢀbipy (entry 7) or OPPh3 (entry 8), is inactive
in СО2 hydrogenation and gains activity only after
the addition of phosphine (entries 9, 10, 13, and 14 ).
[RhCl(CO)2]2 and RhCl3 behave similarly (entries 2, 3,
15, and 17).
All Rh complexes containing phosphine in the coorꢀ
dination sphere are active (see Table 1, entries 4—6). The
Wilkinson complex in the presence of PPh3 exhibits the
highest activity (entry 19).
The catalytic properties of the Wilkinson complex are
strongly affected by the conditions of formation of cataꢀ
lytically active sites (solvent, gaseous medium) and reacꢀ
tion conditions.
As can be seen in Table 2, in such solvents as heptane,
benzene, and THF, the Wilkinson complex is inactive
(entries 1—3), and in a DMSO solution (entries 4—8),
the hydrogenation of СО2 to formic acid occurs. When
DMSO is replaced by water, the reaction is almost comꢀ
pletely suppressed (entry 9). The addition of methanol
(50 wt.%) to DMSO (entry 10) results in a considerable
increase in the yield of formic acid and in the appearance
of a significant amount of methyl formate (MF). The
The yield of formic acid depends strongly on the temꢀ
perature of hydrogenation (see Table 2, entries 8, 8´,
and 8″). It increases with the temperature increase from
19 to 25 °С but diminishes with the further temperature
rise (to 40 °С).
The P : Rh ratio also has a substantial effect on the
yield of formic acid. When the P : Rh ratio changes from 0
to 6 (Fig. 1, curve 1), the concentration of formic acid
increases drastically. The further increase in this ratio
somewhat decreases the yield of the acid. The lowest acꢀ
tivity of the system is observed at P : Rh = 3, and the
catalyst is deactivated after 4 h. Noteworthy that the plot
of the change of the formic acid concentration vs. P : Rh
ratio remains unchanged with a decrease in the Rh conꢀ
centration by an order of magnitude. However, in this
case, the maximum is more pronounced and shifts toward
higher P : Rh values (see Fig. 1, curve 2).

CO2 hydrogenation to formic acid
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2167
TON
Table 3. Products of the reaction of RhCl(PPh3)3 with the comꢀ
ponents of the reaction medium of СО2 hydrogenation and charꢀ
acteristics of their 31P NMR spectra
2500
2
2000
1500
1000
500
Reagent
Product
δ *
JRh—P Refs.
/Hz
P
Benzene
DMSO
RhCl(PPh3)3
37.89
54.61
144
191
125
183
154
10
**
**
**
11
1
RhCl(PPh3)2(DMSO) 20.0
RhCl(PPh3)(DMSO)2 37.6
RhCl(PPh3)2(NEt3)
NEt3,
46.6
DMSO
NEt3, DMSO,
air, H2O
H2, DMSO
H2, NEt3,
DMSO
CO2, DMSO
CO2, NEt3,
DMSO
OPPh3
32.5
—
**
0
3
6
9
12
15
P : Rh
H2RhCl(PPh3)3
RhCl(PPh3)2(NEt3)
Rh
RhCl(CO2)(PPh3)2
Rh(CO)2(OPPh3)4***
OPPh3
H2RhCl(PPh3)3
RhCl(PPh3)2(NEt3)
OPPh3
Rh(CO)2(OPPh3)4***
Rh
44.5, 26.1 90
12
11
**
13
**
**
**
**
**
**
**
Fig. 1. Turnover number of the catalyst (TON) as a funcꢀ
tion of the P : Rh ratio (pCO = 40 atm, pH = 20 atm,
[NEt3] = 1.45 mol L–1, DMSO, 20 h) at [Rh] = 1•10–3 (1) and
1•10–4 gꢀat. L–1 (2).
46.6
—
35.53
—
32.5
44.5, 26.1 90
154
—
152
—
2
2
—
The maximum concentration and yield of formic acid
were achieved at 30 atm of CO2 and 20 atm of H2
(TON = 1000) (Fig. 2). At pCO = 40 atm and pH
50 atm, TON = 600.
These results suggest that for the studied interval of
parameters the acid is formed with the highest yield at
CO2,H2,
DMSO,
NEt3
46.6
32.5
—
154
—
—
=
2
2
—
—
* Relative to the signal for P from free PPh3.
** This work.
25 °С, pCO = 30—40 atm, pH2 = 20 atm, and P : Rh = 6,
2
and the most active catalyst is formed under a hydrogen
atmosphere in polar solvents (DMSO and MeOH). Unꢀ
der these conditions, within 20 h TON ∼1000 in DMSO
and TON ∼2715 (acid and MF) in MeOH were achieved.
To obtain an information on the nature of the cataꢀ
lytically active site of СО2 hydrogenation to formic acid,
we studied the interaction of the Wilkinson complexes
*** IR, ν/cm–1: 2038, 1919 (ν(CO)); 1085, 1182, 1431, 1475
(ν(P=O)); 694, 723, 743 (δ(Ph). Found (%): C, 69.0; H, 5.0.
Calculated (%): C, 69.8; H, 4.7.
with the components of the reaction medium (benzene,
DMSO, NЕt3, air, Н2, and СО2) by 31Р NMR spectrosꢀ
copy (Table 3). Solid products were analyzed by IR specꢀ
troscopy.
The data in Table 3 show that the interaction of
RhCl(PPh3)3 with DMSO results in the elimination of
PPh3 from the coordination sphere of rhodium: DMSO
rapidly displaces the first phosphine molecule, and then
the second molecule is slowly displaced. This is accompaꢀ
nied by the appearance of signals attributed to the
RhCl(PPh3)2(DMSO) and RhCl(PPh3)(DMSO)2 comꢀ
plexes. In the absence of DMSO, when benzene is used as
a solvent, no elimination of PPh3 occurs.
C/mol L–1
1.0
0.8
1
2
0.6
The reaction of RhCl(PPh3)3 with Et3N in DMSO is
also accompanied by the elimination of PPh3 from the
coordination sphere to form the RhCl(PPh3)2(NЕt3) comꢀ
plex. This complex rapidly decomposes in air and in the
presence of moisture traces: PPh3 is rapidly oxidized to
ОPPh3. It was established by special experiments that
PPh3 is oxidized only in the presence of both oxygen and
moisture present in air.
The reactions of the Wilkinson complex with Н2
and СО2 in DMSO afford the Н2RhCl(PPh3)3 and
RhCl(CO2)(PPh3)2 complexes, respectively. In the presꢀ
ence of amine, Н2RhCl(PPh3)3 is transformed into
0.4
0.2
0
10
20
30
40
50
p/atm
Fig. 2. Influence of the CO2 pressure (1) (pH = 20 atm) and H2
2
pressure (2) (pCO = 40 atm) on the yield of formic acid (C)
([Rh] = 1•10–3 gꢀ2at. L–1, [NEt3] = 1.45 mol L–1, DMSO, 20 h).

2168
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002
Ezhova et al.
60
50
40
30
20
10
0
–10
δ
7.15 31.82
6.96
21.14
5.71
40.57
Fig. 3. 31Р NMR spectrum of the catalytic solution of СО2 hydrogenation.
C•105/mol L–1
RhCl(PPh3)2(NЕt3), whereas it is reduced to metallic
350
rhodium in the presence of amine in a hydrogen atmoꢀ
sphere. In the presence of triethylamine in a CO2 atmoꢀ
sphere, the RhCl(CO2)(PPh3)2 complex rapidly decomꢀ
poses to form ОPPh3 and a yellow precipitate with the
Rh(CO)2(ОPPh3)4 composition. As found by special exꢀ
periments, СО2 is the oxidant of PPh3 to OPPh3. The
elimination of PPh3 from RhCl(PPh3)3 and decomposiꢀ
tion of the RhCl(PPh3)2(NЕt3), Н2RhCl(PPh3)3, and
RhCl(CO2)(PPh3)2 complexes in the presence of Et3N
are accelerated with temperature.
When RhCl(PPh3)3 reacts simultaneously with
DMSO, NЕt3, Н2, and СО2, СО2 is hydrogenated to
formic acid. The RhCl(PPh3)2(NЕt3) and Н2RhCl(PPh3)3
complexes, PPh3, and OPPh3 were found in the hydrogeꢀ
nation product. The most characteristic 31P NMR specꢀ
trum of the catalytic solution is presented in Fig. 3. It
contains signals corresponding to the Н2RhCl(PPh3)3
1
300
250
200
150
100
50
2
3
4
0
4
B
8
12
16
t/h
A
C
D
C•10/mol L–1
20
5
15
10
5
(δP 44.5, JRh—P
= 90 Hz and δP 26.1) and
RhCl(PPh3)2(NЕt3) (δP 46.6, JRh—P = 154 Hz) complexes
and signals assigned to triphenylphosphine (δP 0) and
OPPh3 (δP 32.5). An internal standard was added to a
catalytic solution for the quantitative determination of
concentrations of these phosphorusꢀcontaining comꢀ
pounds. Figure 4 shows the changes in the concentrations
of formic acid, PPh3, OPPh3, and rhodium complexes
detected during CO2 hydrogenation in samples taken in
the time moments corresponding to points A, B, C, and D.
In point А the acid has not yet formed. The 31Р NMR
spectrum of the sample contains signals attributed to PPh3,
OPPh3, and Н2RhCl(PPh3)3 (a portion of this complex
exists in the insoluble form as a yellow powder).
0
4
B
8
12
16
t/h
A
C
D
Fig. 4. Concentration plots obtained from the 31Р and 1Н NMR
spectra of the catalytic solution during RhCl(PPh3)3ꢀcatalyzed
СО2 hydrogenation to formic acid ([Rh] = 1•10–2 gꢀat. L–1
P : Rh = 6, [NEt3] = 1.45 mol L–1, DMSO): PPh3 (1);
OPPh3 (2); H2PhCl(PPh3)3 (3); RhCl(PPh3)2(NEt3) (4 ), and
HCOOH (5 ).
,

CO2 hydrogenation to formic acid
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 12, December, 2002 2169
C•102/mol L–1
The presented results allow a relation between the
formation of formic acid and the presence of the
RhCl(PPh3)2(NЕt3) complex to be established, namely,
the beginning and end of СО2 hydrogenation coincide
with the appearance and disappearance of this complex,
respectively. Based on the data obtained, we can suggest
that the RhCl(PPh3)2(NЕt3) complex is a precursor of the
catalytically active complex.
1
1.2
0.6
2
4
The RhCl(PPh3)2(NЕt3) complex decomposes during
hydrogenation to yield free PPh3, the product of its oxiꢀ
dation OPPh3, metallic rhodium, and Rh(CO)2(ОPPh3)4.
Taking into account the data in Table 3, we can say that
the formation of metallic rhodium is favored by the presꢀ
ence of hydrogen and triethylamine.
3
0
5
10
15
t/h
Fig. 5. Changes in the concentrations of OPPh3 (1, 2) and
RhCl(PPh3)2(NEt3) (3, 4) in time at different P : Rh ratios equal
to 6 (1, 3) and 9 (2, 4); RhCl(PPh3)3, [Rh] = 1•10–2 gꢀat. L–1
[NEt3] = 1.45 mol L–1, DMSO.
,
The decomposition of RhCl(PPh3)2(NЕt3) and oxiꢀ
dation of PPh3 to OPPh3 (Fig. 5) are inhibited in the
presence of an excess of phosphine. Metallic rhodium is
not formed, and the yield of formic acid increases signifiꢀ
cantly (Fig. 6).
C/mol L–1
1
0.8
0.6
References
1. Carbon Dioxide Chemistry: Environmental Issues, Eds. J. Paul
and C.ꢀM. Pradies, The Royal Society of Chemistry, Lonꢀ
don, 1994, 25.
2. P. G. Jessop, T. Ikariya, and R. Noyori, Letters to Nature,
1994, 368.
0.4
2
3. W. Leitner, E. Dinjus, and F. Gassner, J. Organomet. Chem.,
1994, 475.
0.2
4. D. N. Lawson, J. A. Osborn, and G. Wilkinson, J. Chem.
Soc. (A), 1966, 1728.
5. K. C. Dewhirst, W. Keim, and C. A. Reilly, Inorg. Chem.,
1968, 7, 24.
0
5
10
15
t/h
6. Yu. S. Varshavskii and T. G. Cherkasov, Zh. Neorg. Khim.,
1967, 12, 1709 [J. Inorg. Chem. USSR, 1967, 12 (Engl.
Transl.)].
7. P. S. Hallman, D. Evans, J. A. Osborn, and J. Wilkinson,
Chem. Commun., 1967, 305.
Fig. 6. Hydrogenation of СО2 in the presence of the Wilkinson
complex: RhCl(PPh3)3 + 6 PPh3 (1), RhCl(PPh3)3 (2); [Rh] =
1•10–3 gꢀat. L–1, pCO = 40 atm, pH = 20 atm, [NEt3] =
1.45 mol L–1, DMSO, 220 h.
2
8. Y. Fukumoto, N. Chatani, and S. Murai, J. Org. Chem.,
1993, 58, 4187.
The formation of formic acid starts in point В. The
spectra exhibited additional signals assigned to
9. U. S. Varshavsky and T. G. Cherkasov, J. Inorg. Chem.,
RhCl(PPh3)2(NЕt3).
The
concentration
of
1967, 6, 1709.
10. G. Halpern, Inorg. Chim. Acta, 1981, 50, 11.
11. B. T. Heaton, J. A. Iggo, C. Jacob, J. Nadarajah, M. A.
Fontaine, R. Messere, and A. F. Noels, J. Chem. Soc. (A),
Dalton Trans., 1994, 19, 2875.
12. P. Meakin, J. P. Jessop, and C. A. Tolman, J. Am. Chem.
Soc., 1972, 94, 3240.
Н2RhCl(PPh3)3 decreased, and that of PPh3 increased.
In point С the hydrogenation of СО2 occurs intensely;
the spectra contain signals characteristic of
RhCl(PPh3)2(NЕt3), PPh3, and OPPh3.
The hydrogenation process has stopped in point D,
despite an excess of amine in the reaction medium. The
spectrum does not exhibit signals from the phosphineꢀ
rhodium complexes. The concentrations of PPh3 and
OPPh3 increased significantly.
13. M. Aresta and C. F. Noble, Inorg. Chim. Acta, 1977, 24, 49.
Received November 2, 2001;
in revised form April 23, 2002