Catalytic hydrogenation of cyclic carbonates: A practical approach from CO2 and epoxides to methanol and diols

Article abstract of DOI:10.1002/anie.201207781

As an economical, safe and renewable carbon resource, CO₂ turns out to be an attractive C1 building block for making organic chemicals, materials, and carbohydrates.[1] From the viewpoint of synthetic chemistry,[2] the utilization of CO₂ as a feedstock for the production of industrial products may be an option for the recycling of carbon.[3] On the other hand, the transformation of chemically stable CO₂ represents a grand challenge in exploring new concepts and opportunities for the academic and industrial development of catalytic processes.[4] The catalytic hydrogenation of CO₂ to produce liquid fuels such as formic acid (HCO ₂H)[5] or methanol[6] is a promising solution to emerging global energy problems. Methanol, in particular, is not only one of the most versatile and popular chemical commodities in the world, with an estimated global demand of around 48 million metric tons in 2010, but is also considered as the key to weaning the world off oil in the future.[6e, f] Although the production of methanol has already been industrialized by the hydrogenation of CO with a copper/zinc-based heterogeneous catalyst at high temperatures (250-300°C) and high pressures (50-100 atm),[6e, 7] the development of a practical catalytic system for the hydrogenation of CO₂ into methanol still remains a challenge, as high activation energy barriers have to be overcome for the cleavage of the C=O bonds of CO₂, albeit with favorable thermodynamics.[8] Heterogeneous catalysis for the hydrogenation of CO ₂ into CH3OH has been extensively investigated, and Cu/Zn-based multi-component catalyst was found to be highly selective with a long life, but under relatively harsh reaction conditions (250 °C, 50 atm).[3b, 6d] Therefore, the production of methanol from CO₂ by direct hydrogenation under mild conditions is still a great challenge for both academia and industry.

Full text of DOI:10.1002/anie.201207781

Angewandte
Chemie
DOI: 10.1002/anie.201207781
Homogeneous Catalysis
Catalytic Hydrogenation of Cyclic Carbonates: A Practical Approach
from CO2 and Epoxides to Methanol and Diols**
Zhaobin Han, Liangce Rong, Jiang Wu, Lei Zhang, Zheng Wang, and Kuiling Ding*
As an economical, safe and renewable carbon resource, CO2
turns out to be an attractive C1 building block for making
organic chemicals, materials, and carbohydrates.[1] From the
viewpoint of synthetic chemistry,[2] the utilization of CO2 as
a feedstock for the production of industrial products may be
an option for the recycling of carbon.[3] On the other hand, the
transformation of chemically stable CO2 represents a grand
challenge in exploring new concepts and opportunities for the
academic and industrial development of catalytic processes.[4]
The catalytic hydrogenation of CO2 to produce liquid fuels
such as formic acid (HCO2H)[5] or methanol[6] is a promising
solution to emerging global energy problems. Methanol, in
particular, is not only one of the most versatile and popular
chemical commodities in the world, with an estimated global
demand of around 48 million metric tons in 2010, but is also
considered as the key to weaning the world off oil in the
future.[6e,f] Although the production of methanol has already
been industrialized by the hydrogenation of CO with
a copper/zinc-based heterogeneous catalyst at high temper-
atures (250–3008C) and high pressures (50–100 atm),[6e,7] the
development of a practical catalytic system for the hydro-
genation of CO2 into methanol still remains a challenge, as
high activation energy barriers have to be overcome for the
hydrogenation of carbonates or carbamates using pincer-
type RuII catalysts, with the general structure [(PNN)Ru-
(CO)(H)], under mild conditions.[10] In particular, the hydro-
genation of dimethyl carbonate has established a bridge from
CO2 to methanol,[11] as dimethyl carbonate can be produced
from CO2 and methanol.[6e] However, from an economic point
of view, this process is less effective in cost, as the starting
dimethyl carbonate (US$ 1000/ton) is much more expensive
than the product methanol (US$ 400/ton). Direct hydrogena-
tion of CO2 to methanol under homogeneous conditions using
either combined RuII complexes and Lewis acid or a single
RuII complex have also been reported recently by Sanford
and Huff,[12a] and Klankermayer, Leitner, and co-workers,
respectively.[12b] However, the catalytic efficiency of these
processes still remain unsatisfactory. Therefore, the develop-
ment of a new reaction pathway and a practical catalyst
system for the production of methanol from CO2 is in urgent
demand. Herein, we report our preliminary results on the use
of a type of readily available (PNP)RuII catalyst (1) for the
homogeneous hydrogenation of cyclic carbonates from read-
ily available CO2 and epoxides (Scheme 1b), to give methanol
and the corresponding diols with excellent catalytic efficiency.
=
cleavage of the C O bonds of CO2, albeit with favorable
thermodynamics.[8] Heterogeneous catalysis for the hydro-
genation of CO2 into CH3OH has been extensively inves-
tigated, and Cu/Zn-based multi-component catalyst was
found to be highly selective with a long life, but under
relatively harsh reaction conditions (2508C, 50 atm).[3b,6d]
Therefore, the production of methanol from CO2 by direct
hydrogenation under mild conditions is still a great challenge
for both academia and industry.[6a–b,9]
Milstein and coworkers recently developed an indirect
approach from CO2 to methanol through homogeneous
Scheme 1. a) Shell omega process for the production of EG, and b) the
present approach to produce EG and methanol.
[*] Dr. Z. B. Han, L. C. Rong, J. Wu, Dr. L. Zhang, Dr. Z. Wang,
Prof. Dr. K. Ding
Considering the difficulties in the production of dimethyl
carbonate from CO2, we decided to use ethylene carbonate as
the starting material for a hydrogenation study, as the
insertion of CO2 into ethylene oxide to produce ethylene
carbonate is thermodynamically favorable and the process
has been well developed[4,9] as the key step in the “omega
process” (Scheme 1a) for the production of ethylene glycol
(EG).[13] On the other hand, pincer complexes have been
extensively studied and have proven to be highly active in the
catalytic hydrogenation of various amides and esters, as well
as the reverse processes.[14] We initiated our investigation by
using pincer-type RuII complexes as catalysts to check the
feasibility of catalyzing the hydrogenation of cyclic carbo-
nates. A preliminary screening of various reaction parameters
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P. R. China)
E-mail: kding@mail.sioc.ac.cn
[**] We thank the NSFC (21121062 and 91127041), the Major Basic
Research Development Program of China (2010CB833300), the
Science & Technology Commission of Shanghai Municipality, and
CAS for financial support of this work. We also thank Prof. Xiao-Bing
Lu (Dalian University of Technology) for generously providing
a sample of poly(propylene carbonate), and Professors Li-Xin Dai
and Pierre Dixneuf for their valuable discussions and comments on
this manuscript.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207781.
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for the hydrogenation of 4-methyl-1,3-dioxolan-2-one in the
presence of a catalytic amount of RuII complex 1a,[14h]
including the solvent, substrate concentration, tBuOK con-
centration, hydrogen pressure, and the reaction temperature,
clearly established the feasibility of this catalysis (for details,
see the Supporting Information, Tables S1–S5). Subsequently,
we examined the catalytic activity of a variety of RuII and IrIII
complexes (1 and 2) in the hydrogenation of ethylene
carbonate. The reactions were conducted in THF at 1408C
under 50 atm of H2 in the presence of the catalytic RuII or IrIII
complex and the base tBuOK (substrate/catalyst/tBuOK =
1000–100000:1:1), and the results are shown in Table 1.
the reaction time to 1 h, which afforded both methanol and
EG in > 99% yield (entry 7). At a catalyst (1a) loading of
0.02 mol%, similar results were obtained in 10 h (entry 8).
When the catalyst loading was decreased to 0.01 mol%, the
reaction could be completed in 48 h to give methanol and EG
in > 99% yield (entry 9). When the catalyst loading was
further decreased to 0.001 mol%, the reaction still proceeded
smoothly at 60 atm of H2 with a 89% conversion of ethylene
carbonate in 72 h, to afford methanol and EG in 84% and
87% yields, respectively (with a TON of 87000 and a TOF of
1200 hÀ1; entry 10). This result demonstrates the high effi-
ciency and good stability of the catalyst.
The application of the optimized catalyst 1a was then
extended to the hydrogenation of a variety of cyclic carbo-
nates (Table 2). Heating a solution of propylene carbonate
(2.92 g, 28.6 mmol) and H2 (50 atm) with a catalytic amount of
complex 1a (3.4 mg, 0.0057 mmol, 0.02 mol%) at 1408C in
THF (20 mL) in a 125 mL autoclave for 10 h selectively
produced methanol and propylene glycol in quantitative
yields (entry 1). Under similar conditions, other cyclic carbo-
nates, including 5-membered 1,3-dioxolan-2-ones with one,
two, or even four substitutents, as well as a six-membered
carbonate, can also be efficiently and selectively hydro-
genated into methanol and their corresponding diols. The
steric hindrance of the substituents at the backbone of cyclic
carbonates has a significant impact on the activity of the
reaction, as longer reaction time and/or higher catalyst
loading are required to reach a complete conversion of
sterically more demanding substrates (entries 2–10 vs. 1). The
cyclic carbonate with a six-membered ring, 1,3-dioxan-2-one,
was also selectively hydrogenated in the presence of
0.05 mol% of 1a to afford methanol and 1,3-propanediol in
quantitative yield (entry 11).
In another unprecedented reaction, complex 1a was
found to catalyze the hydrogenation of polycarbonate into
methanol and the corresponding diol, thus realizing a hydro-
genative degradation of polycarbonate. As shown in
Scheme 2, a sample of poly(propylene carbonate) (PPC;
2.69 g) with a weight-average molecular weight (Mw) of
100698 (Mw/Mn = 1.77, > 99% carbonate linkages), which
had been prepared by an alternative copolymerization of
propylene oxide and CO2,[15] was readily depolymerized by
hydrogenation in the presence of 1a (15.8 mg, 0.1 mol%) to
afford methanol and 1,2-propyleneglycol in high yields (both
99%). Therefore, the present catalytic system may provide
a potential approach to the use of recovered waste poly-
carbonate as a resource.
Table 1: Hydrogenation of ethylene carbonate in the presence of RuII and
IrIII catalysts.[a]
Entry Cat.
(mol%)
t [h] Conv.
[%][b]
Yield of EG
[%][b]
Yield of MeOH
[%][b]
1
2
3
4
5
6
7
8
9
1a (0.1)
1b (0.1)
1c (0.1)
1d (0.1)
1e (0.1)
2 (0.1)
0.5 >99
>99
74
15
76
24
>99
45 (18)[c]
13
0.5
0.5
0.5
0.5
1
74
16
76
24
11
48 (20)[c]
22
10
1
>99
98
>99
84
1a (0.05)
1a (0.02)
1a (0.01)
1
>99
>99
>99
>99
87
10 >99
48 >99
10[d] 1a (0.001)
72
89
[a] Standard reaction conditions: substrate (28.6 mmol), 1 equiv of
KOtBu to the RuII or IrIII complex, THF (20 mL), in a 125 mL autoclave.
[b] Determined by GC analysis using p-xylene (50 mL) as the internal
standard. [c] The data in parentheses are the GC yields of methyl formate
(HCOOMe). [d] 60 atm of H2 was applied.
It was found that at a catalyst loading of 0.1 mol%, RuII
complexes 1a–e are active for the hydrogenation, simulta-
neously affording methanol and EG in various yields
(entries 1–5), whereas Ir complex 2 is much less effective
(entry 6). In the reaction with 1b and 1d some amount of
methyl formate is also formed along with the expected
alcohol products (entries 2 and 4). This is probably due to
their lower activity for the hydrogenation of the 2-hydrox-
yethylformate intermediate, which may undergo a fast ester
exchange with methanol to give methyl formate. Among the
Ru catalyst series (1a–e), complex 1a, which has a readily
available and air-stable HN(CH2CH2PPh2)2 ligand, turned out
to be optimal in terms of catalytic performance (entries 2–5
vs. 1).
Following the optimized reaction conditions mentioned
above, a deuterium-labeling study was carried out by using D2
instead of H2 to get insight into the mechanistic aspects of the
catalysis. As shown in Scheme 3a, in the 1a-catalyzed
deuteration of ethylene carbonate, methanol is produced
with only 87% deuterium content at the methyl group, as
1
determined by H NMR analysis of its benzoate ester, which
indicates a partial transfer of hydrogen atoms from the
ethylene backbone of the carbonate to the carbon of
methanol. This is also confirmed by a significant deuterium
substitution at the carbon atoms of EG (49% deuteration),
which implicates the catalytic activity of complex 1a in both
With the optimized catalyst in hand, we subsequently
turned our attention to improving the catalyst turnover
numbers (entries 7–10). Complete conversion of the substrate
was realized in the presence of 0.05 mol% of 1a by extending
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Table 2: Hydrogenation of various cyclic carbonates in the presence of
RuII complex 1a.[a]
Entry Substrate
1a
[mol%]
t [h] Yield of diol
Yield of MeOH
[%][c]
[%][b]
1
2
3
4
5
6
7
8
9
0.02
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
10 99[c]
99
99
10 96[c]
Scheme 3. Deuterium labeling studies of the hydrogenation.
the dehydrogenation of EG and the hydrogenation of the
resultant 2-hydroxyacetaldehyde under the experimental
conditions. In fact, the treatment of EG with D2 (50 atm) in
the presence of 1a (0.1 mol%) and KOtBu (0.1 mol%) at
1408C for 0.5 h results in the formation of EG with a total of
81% deuterium incorporation at the ethylene carbon atoms
(Scheme 3b), thus supporting the above rationale. To prevent
D/H exchange in the preparation of deuterated methanol,
tetramethyl ethylene carbonate was employed as a substrate
instead of ethylene carbonate. As expected, deuterated
methanol can be prepared in 99% yield with a > 99%
deuterium incorporation at the methyl group (Scheme 3c).
Therefore, the present catalytic system has provided a facile
and practical method for the preparation of deuterated
methanol.
On the other hand, the 1a-catalyzed hydrogenation of an
optically active (> 99% ee) (R)-propylene carbonate under
the optimized conditions afforded racemic 1,2-propylenegly-
col (Scheme 4a) in addition to methanol, which implies that
the initially formed optically active 1,2-propylene glycol
might undergo racemization under the experimental condi-
tions, as a result of hydrogenation/dehydrogenation equilib-
rium under the reaction conditions. In fact, treatment of
optically active (R)-1,2-propyleneglycol (> 95% ee) with 1a
(0.1 mol%) and KOtBu (0.1 mol%) in the presence or
absence of H2 (50 atm) at 1408C for 1 h afforded racemic
4
4
4
4
4
99
99
98
99
97
>99
>99
>99
>99
>99
98
10 99
12 97
20 96
>99
10
11
0.1
95
99
0.05
2
99[c]
[a] Standard reaction conditions: substrate (28.6 mmol), 1 equiv of
KOtBu to 1a, THF (20 mL), in a 125 mL autoclave. [b] Yield of the
isolated diol products, except for entries 1, 2, and 11. [c] Determined by
GC analysis using p-xylene (50 mL) as the internal standard.
Scheme 2. Hydrogenative depolymerization of PPC.
Scheme 4. Stereochemistry studies of the hydrogenation.
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1,2-propylene glycol (Scheme 4b,c), but with the co-gener-
ation of hydroxy acetone (25%) as minor product in the
absence of H2 (Scheme 4c), which further confirms the dual
function of 1a in the dehydrogenation of alcohols and
hydrogenation of aldehydes or ketones under basic condi-
tions, and is consistent with the observations in the deute-
rium-labeling study.
Based on these observations, we proposed the reaction
mechanism shown in Scheme 5. First, catalyst 1a is activated
by the elimination of HCl in the presence of a base to afford
lytic activity from the NH-containing catalyst (1) to the
analogous N-Me-derived catalyst 3 indicates that the NH
moiety is critically important for the catalytic activity, and that
the NH proton should be intimately involved in the catalytic
event by cooperation with the metal center, as in the scenarios
in many other related catalytic hydrogenation systems.[14,18]
As shown in Scheme 5, three independent elementary
steps (steps 3a–c) related to INT2 may operate in the overall
transformation from ethylene carbonate to methanol and EG.
This transformation begins with the initial reduction of
ethylene carbonate to 1,3-dioxolan-2-ol (4; a lactol form),
which stays in an equilibrium with 2-hydroxyethylformate (4’)
in solution (step 3a), followed by the further reduction of
2-hydroxyethylformate (4’) to 2-(hydroxymethoxy)ethanol
(5) (step 3b), which is a hemiacetal in equilibrium with
formaldehyde and glycol. The transformation is finally
accomplished by the reduction of formaldehyde to methanol
(step 3c). In fact, the direct hydrogenation of paraformalde-
hyde, which can decompose into formaldehyde in situ through
heating, in the presence of 1a (0.1 mol%) proceeded to
completion in THF (2 h) to give methanol in 98% yield
(Scheme 6a). This result indicates the feasibility of step 3c in
Scheme 6. Hydrogenation of formaldehyde (a) and a mixture of
hydroxyhexyl formates with 1-hexene carbonate (b) catalyzed by 1a.
Scheme 5. Proposed mechanism for the (PNP)RuII-catalyzed hydroge-
nation of cyclic carbonates.
the 16eÀ amido Ru complex INT1 (step 1).[16] Then, a mole-
cule of H2 is expected to undergo heterolytic cleavage by
reacting with INT1 via TS1 to give the 18eÀ Ru dihydride
INT2 (step 2).[17] Finally, nucleophilic addition of Ru dihy-
dride INT2 to the carbonyl group of a carbonate, formate, or
formaldehyde gives the corresponding reduced products 4 or
4’ (step 3a), 5 or formaldehyde (step 3b), and methanol
(step 3c), respectively, and to regenerate 16eÀ amido Ru
complex INT1 (step 3),[16] which enters into the next catalytic
cycle. The addition of Ru dihydride INT2 to the carbonyl
group is proposed to proceed by the hydrogen-bonding
activation of the carbonyl group in the substrate with the
proton of an NH group via a six-membered transition state
(TS2).
An experiment to clarify the function of NH group in
ligand 1 in the catalytic cycle was carried out by examining the
catalytic performance of an analogous RuII complex 3
(Scheme 5), containing the PNP ligand with an N-Me group.
It was found that no reaction occurs with complex 3 under
otherwise identical conditions. The dramatic switch of cata-
the transformation into methanol. On the other hand, the
high chemoselectivity for the formation of methanol, even
with incomplete conversion of ethylene carbonate (as shown
in Table 1, entry 10), implies that the reactivity of the
intermediates, such as alkyl formate (4’) and formaldehyde,
should be much higher than that of the starting ethylene
carbonate. To compare the rate of step 3a with that of step 3b
(Scheme 5), the hydrogenation of an equimolar mixture of
1-hexene carbonate and hydroxyhexyl formates, containing
a 2:1 isomeric mixture of 2-hydroxyhexyl formate (A) and
1-hydroxyhexan-2-yl formate (B) (both can be viewed as
model molecules of 4’), was carried out in the presence of
1a (0.1 mol%) at 1408C and 50 atm of H2 with stirring
(Scheme 6b). Over 90% conversion of 2-hydroxyhexyl for-
mate and 1-hydroxyhexan-2-yl formate to methanol (72%)
and hexane-1,2-diol (95%) with the co-generation of meth-
ylformate (22%) was observed in 0.5 h. In contrast, 1-hexene
carbonate was largely recovered (95%) without obvious
conversion. This experiment suggested that the initial hydro-
genation of cyclic carbonate (step 3a) should be the most
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Chen, C. K. Schauer, T. J. Meyer, M. Brookhart, J. Am. Chem.
difficult one among the sequential hydrogenation steps 3a–c
shown in Scheme 5.
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In conclusion, we have developed a highly efficient
catalytic hydrogenation of cyclic carbonates for the prepara-
tion of methanol with the cogeneration of the corresponding
diols by using (PNP) RuII pincer complexes as the catalysts
under relatively mild conditions. This process has provided
a facile approach for the simultaneous production of two
important bulk chemicals, methanol and EG, from ethylene
carbonate, which is industrially available by reacting ethylene
oxide with CO2. The coupling of the present catalytic system
with the process of ethylene carbonate production in the
omega process is expected to establish a new bridge from CO2
and ethylene oxide to methanol and EG. Apart from the clean
production of diol, a big bonus of the present protocol is the
efficient chemical utilization of CO2, which represents a dis-
tinct advantage in terms of sustainability over the omega
process, which gives back CO2. Moreover, this catalytic
system has also provided a potential process for the utilization
of waste poly(propylene carbonate) as a resource to afford
1,2-propylene diol and methanol through hydrogenative
depolymerization, and a convenient method for the prepara-
tion of deuterated methanol from CO2 and D2. A possible
catalytic mechanism is proposed, in which the NH moiety of
the ligand is demonstrated to be critically important in
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[13] The omega process (the only advanced mono EG process) is
a process in which ethylene oxide is reacted with CO2 to first
afford ethylene carbonate, followed by catalytic hydrolysis of the
carbonate to selectively produce mono EG, which is an
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Received: September 26, 2012
Published online: November 14, 2012
Keywords: carbon dioxide · ethylene glycol · hydrogenation ·
.
methanol · ruthenium
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