Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol

Article abstract of DOI:10.1021/ja208760j

This communication demonstrates the homogeneous hydrogenation of CO ₂ to CH₃OH via cascade catalysis. Three different homogeneous catalysts, (PMe₃)₄Ru(Cl)(OAc), Sc(OTf) ₃, and (PNN)Ru(CO)(H), operate in sequence to promote this transformation.

Full text of DOI:10.1021/ja208760j

COMMUNICATION
pubs.acs.org/JACS
Cascade Catalysis for the Homogeneous Hydrogenation of CO to
2
Methanol
Chelsea A. Huff and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S Supporting Information
b
Scheme 1. Proposed Sequence for Converting CO + H to
CH OH + H O
3 2
2
2
ABSTRACT: This communication demonstrates the homo-
geneous hydrogenation of CO to CH OH via cascade
2
3
catalysis. Three different homogeneous catalysts, (PMe ) Ru-
3
4
(
Cl)(OAc), Sc(OTf) , and (PNN)Ru(CO)(H), operate in
3
sequence to promote this transformation.
he global demand for energy is increasing rapidly as a result
of population and economic growth. Currently, the vast
majority of energy usage involves the combustion of nonrenew-
T
able fossil fuels, which is leading to unsustainable increases in
1
anthropogenic emissions of CO . As a result, the identification
2
an alternative catalyst. The major challenge for this approach
is to identify a series of catalysts that are compatible with one
another, operate effectively under the same reaction condi-
tions, and are not poisoned by catalytic intermediates and/
or products. This communication demonstrates the viability
of this approach for the hydrogenation of CO to CH OH
of carbon-neutral alternatives to fossil feedstocks is a critical goal
2
for the scientific community. One attractive approach would involve
3
the hydrogenation of CO (captured from the atmosphere)
2
4
with H (ideally derived from a renewable source) to produce
methanol. This overall process would be carbon-neutral, and the
product is both a potential gasoline replacement and a starting
2
5
2
3
5
using (PMe ) Ru(Cl)(OAc), Sc(OTf) , ROH, and (PNN)Ru-
3
4
3
material for the synthesis of important platform chemicals,
(
CO)(H) as catalysts.
We first targeted a cascade catalysis sequence involving:
6
including ethylene and propylene.
Previous work in this area has focused on developing single
catalysts that promote the multistep sequence of reduction
(
a) hydrogenation of CO to formic acid, (b) esterification to
2
generate a formate ester, and (c) hydrogenation of the ester to
release methanol (Scheme 1). Steps (a) and (b) have been
demonstrated previously with several different homogeneous
reactions required to transform CO into CH OH. For example,
2
3
Cu-based heterogeneous catalysts for the reaction of CO with
2
7,8
16À19
H to form CH OH and H O have been reported. However,
2
3
2
catalysts (albeit at high CO pressures).
In contrast, when
2
these systems suffer from the significant disadvantage that they
require high operating temperatures (200À250 °C), which limits
the theoretical yield of the entropically disfavored reduction
we began this work, homogeneous hydrogenation of formate
20
esters (step c) had little precedent in the literature. Thus, we
first sought to establish whether Ru complexes C-1ÀC-3 (which
are known to catalyze the hydrogenation of alkyl esters and
9
products. In addition, rational tuning of the reactivity and
9
21À23
selectivity of heterogeneous catalysts remains challenging. For
amides to alcohols)
could promote the reaction of H with
2
these reasons, significant recent work has aimed at the develop-
methyl formate to afford 2 equiv of CH OH. Gratifyingly, C-1
3
ment of homogeneous catalysts for the low(er) temperature
and C-2 both showed high activity for this transformation at
10À13
conversion of CO to methanol.
Several such systems that
2
135 °C under 5 bar H (Table 1, entries 1 and 2). Notably, similar
2
2
4
operate at room temperature and contain tunable supporting
results were disclosed in a very recent publication by Milstein.
11a,12b
ligands have been reported.
However, current catalysts gen-
A key requirement for the cascade catalysis sequence in
Scheme 1 is that the formate ester hydrogenation catalyst be
erally remian limited by the requirement for impractical and
8,11,12
expensive hydrogen sources such as boranes and hydrosilanes.
We sought to address these challenges by exploiting cascade
compatible with CO . Initial studies suggested that this might be
2
problematic. For example, the C-1-catalyzed hydrogenation of
1
4,15
catalysis
for the homogeneous catalytic reduction of CO2
methyl formate in the presence of 5 bar H /30 bar CO produced
2
2
with H to produce CH OH. This approach involves the use of a
2
3
only a 17% yield of methanol, versus a yield of 98% under other-
series of different homogeneous catalysts operating in a single
wise analogous conditions but without CO (Table 1, entries
2
vessel to promote the various steps of the CO reduction
2
4 and 1, respectively). Furthermore, treatment of C-2 with 1 atm
sequence. Importantly, our strategy precludes the requirement
of isolating thermodynamically disfavored and/or chemically
CO at 25 °C resulted in the appearance of at least nine new Ru
2
1
hydride species (as determined by H NMR spectroscopic
unstable intermediates (e.g., HCO H or HCOH, respectively).
2
Furthermore, it offers the distinct advantage that the rate and selec-
tivity of each step can potentially be tuned by simply substituting
Received: September 16, 2011
Published: October 26, 2011
r 2011 American Chemical Society
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dx.doi.org/10.1021/ja208760j
|J. Am. Chem. Soc. 2011, 133, 18122–18125

Journal of the American Chemical Society
COMMUNICATION
Table 1. Hydrogenation of HCO CH in the Presence of
CO2
Table 3. Conversion of CO to HCO CH : Lewis/Brønsted
Acid Catalysts
2
3
2
2
3
a
a
entry
catalyst A/B
TON
1
2
3
4
5
A-1/B-1
A-2/B-1
A-3/B-1
A-1/B-2
A-2/B-2
A-3/B-2
11
b
10
entry catalyst P 2
H
:PCO2
conv. of HCO
2
CH
3
3
yield of CH OH
13
1
2
3
4
5
6
7
C-1
C-2
C-3
C-1
C-1
C-1
C-2
5:0
5:0
5:0
5:35
98%
99%
100%
54%
85%
97%
43%
98%
40
102%
16
5
c
d
<3%
6
17%
76%
97%
16%
a
Conditions: 0.0126 mmol of catalysts A and B, 2 mL of CH
35 °C.
3
OH, 16 h,
20:20
30:10
30:10
1
1
7À19
a
Conditions: 0.01 mmol of catalyst C, 1 mmol of methyl formate, 1 mL
of dioxane. Pressures in bar. 1 mmol of KOtBu was added under
otherwise identical conditions. The major organic product was methyl
A number of catalysts (e.g., A-1, A-2, and A-3 in Table 2)
are known to convert CO , H , and CH OH to HCO CH .
However, these reactions typically require high CO pressures
b
c
2
2
3
2
3
d
2
tert-butyl ether.
(
1
90À125 bar) and an excess of CO relative to H (CO :H =
2
2
2
2
17À19
.5:1 to 3:1).
Thus, it was necessary to establish that they
would operate effectively under the optimal conditions for ester
Table 2. Conversion of CO to HCO CH : Thermal Ester-
ification with and without NEt3
2
2
3
hydrogenation (30 bar H /10 bar CO ). As shown in Table 2,
a
2
2
under neutral thermal esterification conditions (0.0126 mmol of
catalyst A in 2 mL of CH OH, 135 °C, 16 h), catalysts A-1ÀA-3
3
formed modest quantities of HCO CH (entries 1À3). The
2
3
turnover numbers (TONs) in these systems could be improved
by the addition of triethylamine, a base that is commonly used to
provide a thermodynamic driving force for the first step of the
1
7À19
hydrogenation/esterification sequence (entries 4À6).
alysts A-2 and A-3 performed best, providing 21 turnovers after
6 h. However, ester formation was slow under these conditions,
and A-2/NEt and A-3/NEt each afforded only two turnovers
Cat-
1
3
3
after 1 h (Table S3 in the Supporting Information).
25
26
It is well-known that both Brønsted and Lewis acid
catalysts can accelerate the esterification of carboxylic acids with
alcohols. Thus, we hypothesized that such catalysts might also
entry
catalyst A/additive
TON
prove advantageous for the formation HCO CH from CO .
1
2
3
A-1/none
A-2/none
A-3/none
3
2
3
2
Gratifyingly, the combination of (PMe ) Ru(Cl)(OAc) (A-1)
1
3 4
and Sc(OTf) (B-2) provided significantly enhanced TONs of
3
10
b
HCO CH relative to the thermal and/or base-promoted reactions
2
3
4
A-1/NEt
A-2/NEt
A-3/NEt
3
3
3
18
(
TON = 40 vs 3 and 18, respectively; Table 3, entry 4). This A-1/
b
5
21
21
B-2 cascade reaction was also significantly faster than the NEt -
promoted esterification, with a TON of 32 after 1 h at 135 °C.
3
b
6
27
a
b
Conditions: 0.0126 mmol of catalyst A, 2 mL of CH
.2 mL of NEt was added under otherwise identical conditions.
3
OH, 16 h, 135 °C.
With conditions established for steps aÀc at 30 bar H /10 bar
2
0
3
CO , we next examined combining the catalysts to achieve the
2
desired cascade catalytic reduction of CO to MeOH. A variety of
2
analysis of the crude mixture; see Figure S6 in the Supporting
A/B/C catalyst combinations were examined (see Table S4 in
the Supporting Information) under the following conditions: 30
bar CO /10 bar H , 0.0126 mmol of each of the catalysts A, B,
Information), indicating the potential for CO to divert the
2
proposed catalytically active species. We reasoned that increasing
2
2
the partial pressure of H in the gas mixture would enhance the rate
of the desired hydrogenation process relative to catalyst inhibition/
and C, and 2 mL of CD OH. We were pleased to find that the use
2
3
of A-1/B-2/C-1 afforded 2.5 turnovers of CH OH along with 34
3
deactivation pathways. Gratifyingly, moving to a 30 bar H /10 bar
turnovers of HCO CH (Scheme 2). This result provides a
proof-of-principle demonstration of the viability of cascade
2
2
3
CO mixture resulted in a 97% yield of CH OH (entry 6).
2
3
With the conditions for step c in hand, we next focused on
steps a and b of the proposed cascade reduction sequence.
catalysis for effecting this transformation. However, the yield of
CH OH was significantly lower than would be expected on the
3
1
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dx.doi.org/10.1021/ja208760j |J. Am. Chem. Soc. 2011, 133, 18122–18125

Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Cascade Catalytic Conversion of CO to CH OH
’ AUTHOR INFORMATION
2
3
with Catalysts A-1, B-2, C-1, and CD OH
3
Corresponding Author
mssanfor@umich.edu
’
ACKNOWLEDGMENT
C.A.H. thanks the NSF for a Graduate Research Fellowship
and the University of Michigan Graduate School for a Rackham
Merit Fellowship. In addition, we thank the NSF Center for
Enabling New Technologies through Catalysis for support of this
work. Finally, we thank Professor Karen Goldberg and Kate Allen
for helpful discussions.
Scheme 3. Transfer Approach to Cascade Catalytic CO2
Hydrogenation
’
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3
28
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3
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2
2
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2
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(
’
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Journal of the American Chemical Society
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1
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