Hydrogenation of carbon dioxide in the presence of rhodium catalysts

Article abstract of DOI:10.1007/s11172-005-0151-5

The results of CO₂ hydrogenation in the presence of the Wilkinson complexes, viz., RhCl₃ and acacRh(CO)₂, at room temperature and excess PPh₃ are presented. The influence of different ions on the catalytic properties of the Rh complexes was studied. Methanol and methyl formate are formed along with formic acid in the presence of an inorganic salt. Ions that are the most active in the formation of formic acid are the least active in methanol formation.

Full text of DOI:10.1007/s11172-005-0151-5

2542
Russian Chemical Bulletin, International Edition, Vol. 53, No. 11, pp. 2542—2545, November, 2004
Hydrogenation of carbon dioxide in the presence of rhodium catalysts
b
a
†
N. V. Kolesnichenko,a N. N. Ezhova, E. V. Kremleva, and E. V. Slivinskii
ꢀ
aA. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences,
29 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 485 0922. Eꢀmail: nvk@ips.ac.ru
bInstitute of High Temperatures, Russian Academy of Sciences,
13/19 ul. Izhorskaya, 127412 Moscow, Russian Federation.
Fax: +7 (095) 230 0922
The results of СО2 hydrogenation in the presence of the Wilkinson complexes, viz., RhCl3
and acacRh(CO)2, at room temperature and excess PPh3 are presented. The influence of
different ions on the catalytic properties of the Rh complexes was studied. Methanol and
methyl formate are formed along with formic acid in the presence of an inorganic salt. Ions that
are the most active in the formation of formic acid are the least active in methanol formation.
Key words: hydrogenation, carbon dioxide, formic acid, methanol, triphenylphosphine.
1
It has previously1 been established that carbon dioxide
genation. Formic acid was detected by Н NMR. The concenꢀ
tration of formic acid obtained as an adduct HCOOH•NEt3 was
calculated from the amount of amine used. The content
of methanol and methyl formate was determined by GLC
on a Chromꢀ5 chromatograph (capillary column 50 m, phase
PEG20M, flameꢀionization detector, carrier gas nitrogen,
80 °С). Dimethylformamide was used as an internal standard.
The concentration of introduced salts was 10–2 mol L–1. In
experiments with variable molar ratio of NEt3 to methanol, the
overall concentration of methanol and triethylamine was kept
constant and taken on the basis of the NЕt3 concentration usuꢀ
ally used in this reaction.
Complexes RhCl(PPh3)3 and acacRh(CO)2 and ligand
ethriolphosphite (ЕТРО)5 were synthesized according to known
procedures. Commercially available (Fluka) RhCl3, PPh3, and
DMSO were used as received. Triethylamine was distilled beꢀ
fore use.
undergoes a rapid hydrogenation to formic acid in the
presence of the Wilkinson complex with excess PPh3 at
room temperature and low pressures. Hydride rhodium
complexes were found to be catalytically active species in
this reaction. The deactivation of RhCl(PPh3)3 is caused
by the elimination of triphenylphosphine from the coorꢀ
dination sphere. This process is suppressed when the reꢀ
action occurs in excess PPh3 or in the presence of HCl
and KNO3.2 To get further insight into the nature of
catalytically active complexes and to find methods for
their stabilization, it seemed of interest to study the influꢀ
ence of various additives on the catalytic properties of the
Wilkinson complex in this reaction. In this work, we reꢀ
port on the results of studying the influence of the nature
of inorganic salts on stability of the Wilkinson complex in
carbon dioxide hydrogenation to formic acid. The effect
of different phosphorusꢀcontaining additives on the seꢀ
lectivity of the process is also discussed.
3
4
Results and Discussion
As shown previously,2 the hydrogenation of СО2 to
formic acid in the presence of the Wilkinson complex
with an additive of KNO3 occurs with a higher rate than
that with HCl addition. Therefore, the influence of difꢀ
ferent cations of nitrate salts on the catalytic properties of
the Wilkinson complex was studied. The replacement
of К+ by lithuim cations decreases the reaction rate
(Fig. 1). However, in the presence of Li+, the yield of
formic acid after 22 h is higher than that in the case of the
Wilkinson complex without salt additives. In the presence
of ammonium nitrate, the reaction rate is low, and after
22 h the reaction still proceeds. In the case of Cr3+ and
Al3+ ions, an induction period appears; however, the reꢀ
action rate increases sharply after 16 h. Thus, in all cases,
the addition of salts results in the stabilization of catalytiꢀ
Experimental
Hydrogenation of carbon dioxide was conducted in a stainless
steel autoclave (150 mL) with an electromagnetic stirrer. Before
the reaction, the rhodium complexes ([Rh] = 10–2 gꢀat L–1) and
additives used (PPh3, inorganic salts, methanol) were stirred in
a mixture consisting of dimethyl sulfoxide (DMSO) and triethyꢀ
lamine (NEt3) under Н2. After this, СО2 (4 MPa) was introꢀ
duced into the reactor following by hydrogen to bring the presꢀ
sure in the reactor to 6 MPa. This moment was considered
to be the onset of the reaction. The reaction was carried out
at 25 °С for 22 h. The liquid product was sampled during hydroꢀ
† Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2436—2439, November, 2004.
1066ꢀ5285/04/5311ꢀ2542 © 2004 Springer Science+Business Media, Inc.

Rhodiumꢀcatalyzed hydrogenation of CO2
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 11, November, 2004 2543
CHCOOH/mol L–1
Table 2. Influence of the nature of the anion on the catalytic
activity of the RhCl(PPh3)3+3PPh3 system in СО2 hydrogenaꢀ
tion
1.2
1.0
0.8
0.6
0.4
0.2
1
Salt
[HCOOH]
[MeOH]
mol L–1
[MF]
—
KI
KNO3
K2SO4
K2CO3
К3PO4
0.950
0.950
1.250
0.530
0.690
0.800
—
—
2
3
0.040
0.025
0.015
0.010
0.039
0.020
0.010
0.005
0.008
0.010
4
5
6
The nature of anions also exerts a significant effect on
the formation of formic acid and methanol (Table 2). In
the presence of potassium nitrate, the yield of formic acid
increased. Phosphate and iodide anions do not virtually
affect the formation of formic acid, but methanol is also
formed in their presence. The addition of sulfate and
carbonate ions decreases the yield of formic acid. At the
same time, the yield of methanol increases in the presꢀ
ence of these salts. The order of decreasing activity in
formic acid formation for anions is
4
8
12
16
20 t/h
Fig. 1. Influence of cations of nitrate salts on the yield of formic
acid in the presence of the RhCl(PPh3)3 +3PPh3 catalytic
system: 1, KNO3; 2, without addition of salts; 3, LiNO3;
4, Al(NO3)3; 5, Cr(NO3)3; 6, NH4NO3.
cally active complexes, and the reaction rate increases
significantly only in the case of KNO3.
NO3– > I– > PO43– > CO32– > SO4
2–
In the presence of salts, methanol and methyl formate
are formed along with formic acid (Table 1). The highest
yield of formic acid was obtained when using К+ and Li+,
whereas the highest yield of methanol is achieved in the
presence of Al+ and Cr+. Based on their activity in formic
acid formation, the cations can be arranged in the followꢀ
ing series (see Table 1):
In the case of methanol formation, the order of deꢀ
creasing activity is
–
2–
I– > NO3 > PO43– > SO42– > CO3
The formation of methanol at room temperature is an
unusual result because, as known,6,7 high pressures and
elevated temperatures are needed to form methanol from
carbon dioxide. To explore the possibility of increasing
the methanol yield and to get insight into the nature of an
active site, we studied the influence of different ligands on
the yield of methanol in the presence of various rhodium
precursors of the catalyst of СО2 hydrogenation (Table 3).
In the case of RhCl3 modified with PPh3, only formic
acid is formed. When salt KNO3 is added, the yield of
formic acid decreases, and methanol and methyl formate
are formed. If the reaction is carried out without triꢀ
phenylphosphine in the presence of KNO3 only, no forꢀ
mic acid is formed, and the yield of methanol increases
significantly. The highest yield of methanol is achieved
when the reaction is carried out in the presence of RhCl3
and OPPh3, and no formic acid is formed in this case.
When аcacRh(CO)2 was used, the highest yield of methaꢀ
nol was obtained on unmodified аcacRh(CO)2 in the presꢀ
ence of KNO3. Nearly no methanol forms in the absence
of NЕt3. Interesting results were obtained when ethriolꢀ
phosphite with an enhanced withdrawing power was used.
Unlike PPh3, the modification of аcacRh(CO)2 with
ЕТРО does not result in the formation of formic acid but
+
К+ > Li+ > Al3+ ≈ Cr3+ > NH4
,
while for the formation of methanol the series is
+
Cr3+ > Al3+ > Li+ > К+ > NH4
.
It can be concluded that cations that are the most
active in formic acid formation are the least active in the
formation of methanol.
Table 1. Influence of the nature of the cation on the catalytic
properties of the RhCl(PPh3)3+3PPh3 system in СО2 hydrogeꢀ
nation
Salt
[HCOOH]
[MeOH]
mol L–1
[MF]*
—
0.950
1.250
0.970
0.640
0.640
0.520
—
—
KNO3
0.025
0.045
0.092
0.124
0.006
0.010
0.020
0.026
0.010
0.004
LiNO3
Al(NO3)3
Cr(NO3)3
NH4NO3
* MF is methyl formate.

2544
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 11, November, 2004
Kolesnichenko et al.
Table 3. Hydrogenation of СО2 in the presence of the rhodium
catalysts
formation. The presence of triethylamine in the reaction
medium is needed for the formation of both formic acid
and methanol.
Evidently, methanol and formic acid form on differꢀ
ent catalytically active sites. The influence of ions on
their stability is selective; however, the presence of a salt
in the reaction medium exerts a stabilizing effect on the
formation of both formic acid and methanol.
Methyl formate can form through the esterification of
formic acid with methanol. To reveal the role of methaꢀ
nol in the hydrogenation of СО2, we studied the influence
of the СН3ОН concentration on the distribution of the
reaction products. The addition of a minor amount of
methanol instead of DMSO insignificantly increases the
yield of formic acid and sharply increases the yield of
methyl formate (Fig. 2, а). However, the further increase
Ligand
(P/Rh = 6)
[HCOOH]
[MeOH]
mol L–1
[MF]
RhCl3
PPh3*
0.500
—
—
PPh3
Without ligand
OPPh3*
0.390
—
—
0.004
0.040
0.210
0.002
0.001
0.003
acacRh(CO)2
PPh3*
0.500
—
—
PPh3**
0.420
—
—
—
0.004
0.457
Traces
0.032
0.064
0.177
0.002
0.014
—
Traces
0.002
0.158
Without ligand
Without ligand
OPPh3*
ETPO
ETPO*
—
—
RhCl(PPh3)3
0.950
1.250
—
[HCOOH]/mol L–1
[MF]•102/mol L–1
1.6
PPh3*
PPh3
OPPh3
OPPh3
—
—
3.5
a
0.025
0.155
0.030
0.010
0.015
Traces
1
1.4
1.2
1.0
0.8
0.6
0.4
0.2
3.0
2
—
2.5
2.0
1.5
1.0
0.5
* In the absence of KNO3.
** In the absence of NEt3.
a large amount of methanol is formed. The yield of methaꢀ
nol decreases sharply when KNO3 is added. For the
Wilkinson complex, the highest yield of methanol was
obtained in the presence of KNO3 and OPPh3.
For the catalytic system of RhCl3 and OPPh3, the
replacement of KNO3 by other salts decreases the yield of
methanol (Table 4). Similar results were obtained for
аcacRh(CO)2.
The presence of a salt in the reaction medium stabiꢀ
lizes a catalytically active complex to form methanol and
methyl formate. As a rule, salts that manifest activity in
the formation of formic acid are inactive in the formation
of methanol. A necessary condition for the formation of
НСООН is the presence of РРh3 in the reaction medium,
while the presence of РРh3 is not necessary for methanol
0
0.76
2.28
6.84
MeOH : DMSO
[HCOOH]/mol L–1
0.8
[MF]•102/mol L–1
1.8
b
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Table 4. Hydrogenation СО2 to formic acid in the presence of
different salts
Compound
RhCl3
Ligand
(P/Rh = 6)
Additive
[МеOH]
[MF]
mol L–1
OPPh3
OPPh3
OPPh3
—
—
—
Na3PO4
K2CO3
KI
K2CO3
KI
0.008
0.007
0.002
0.030
0.070
0.030
0.012
0.002
—
0011
0.016
—
0.04
0.15
0.36
0.58
NEt3 : MeOH
Fig. 2. Plots of the concentrations of formic acid and methanol
vs. molar ratios СН3ОН : DMSO (a) and NH3 : СН3ОН (b) in
СО2 hydrogenation in the presence of the catalytic system
RhCl(PPh3)3+3PPh3: 1, НСООН; 2, methyl formate.
acacRh(CO)2
K2CO3
Na3PO4
0.016
0.014
OPPh3

Rhodiumꢀcatalyzed hydrogenation of CO2
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 11, November, 2004 2545
in the methanol concentration results in an increase in
the yield of formic acid and a decrease in the yield of
methyl formate. The decrease in the methyl formate yield
is apparently related to the fact that esterification is not
the only way to form methyl formate under the conditions
of СО2 hydrogenation. The highest yield of the acid is
achieved at the ratio MeOH : DMSO = 6.8. Thus, methaꢀ
nol is an esterifying agent and also promotes the hydrogeꢀ
nation of СО2 to formic acid. The hydrogenation of СО2
is thermodynamically unfavorable and can occur in a high
yield only in the presence of tertiary amines.8 Methanol
binds formic acid to form methyl formate and thus iniꢀ
tiates СО2 hydrogenation in the same manner as NЕt3.
The influence of the NЕt3 : МеОН ratio on the yield
of reaction products was studied. An increase in the NЕt3
to methanol molar ratio to 0.15 (see Fig. 2, с) increases
the yield of all reaction products; however, the further
increase in this ratio decreases the yields of both formic
acid and methyl formate. Carbon dioxide is not hydrogeꢀ
nated without NЕt3 in the presence of DMSO as a solvent
(Table 3), and neither formic acid, nor methyl formate
were found in the reaction products. The partial replaceꢀ
ment of NЕt3 by methanol considerably decreases the
yield of formic acid.
Thus, under conditions of СО2 hydrogenation, formic
acid can form in a high yield in the presence of NЕt3 only,
which is a component of a catalytically active complex.
References
1. N. N. Ezhova, N. V. Kolesnichenko, A. V. Bulygin, E. V.
Slivinskii, and S. Khan, Izv. Akad. Nauk, Ser. Khim., 2002,
2008 [Russ. Chem. Bull., Int. Ed., 2002, 51, 2165].
2. N. N. Ezhova, N. V. Kolesnichenko, A. V. Bulygin, E. V.
Kremleva, M. P. Filatova, and E. V. Slivinskii, Neftekhimiya,
2004, 44, 27 [Petroleum Chemistry, 2004, 44 (Engl. Transl.)].
3. D. N. Lawson, J. A. Osborn, and G. Wilkinson, J. Chem.
Soc. (A), 1996, 1728.
4. K. C. Dewhirst, W. Keim, and C. A. Reilly, Inorg. Chem.,
1968, 7, 24.
5. U. S. Varshavsky and T. G. Cherkasov, J. Inorg. Chem.,
1967, 6, 1709.
6. G. C. Chinchen, P. J. Denny, D. J. Parker, M. S. Spenser,
and D. Whan, Appl. Catal., 1987, 30, 333.
7. K. C. Waugh, Catal. Today, 1992, 15, 51.
8. W. Leitner, E. Dinjus, and F. Gassner, J. Organomet. Chem.,
1994, 475, 257.
Received June 5, 2004;
in revised form October 14, 2004