Manganese-Catalyzed Sequential Hydrogenation of CO2 to Methanol via Formamide

Article abstract of DOI:10.1021/acscatal.7b02066

Mn(I)-PNP pincer catalyzed sequential one-pot homogeneous CO₂ hydrogenation to CH₃OH by molecular H₂ is demonstrated. The hydrogenation consists of two parts - N-formylation of an amine utilizing CO₂ and H₂, and subsequent formamide reduction to CH₃OH, regenerating the amine in the process. A reported air-stable and well-defined Mn-PNP pincer complex was found active for the catalysis of both steps. CH₃OH yields up to 84% and 71% (w.r.t amine) were obtained, when benzylamine and morpholine were used as amines, respectively; and a TON of up to 36 was observed. In our opinion, this study represents an important development in the nascent field of base-metal-catalyzed homogeneous CO₂ hydrogenation to CH₃OH.

Full text of DOI:10.1021/acscatal.7b02066

Letter
pubs.acs.org/acscatalysis
Manganese-Catalyzed Sequential Hydrogenation of CO to Methanol
2
via Formamide
Sayan Kar, Alain Goeppert, Jotheeswari Kothandaraman, and G. K. Surya Prakash*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los
Angeles, California 90089-1661, United States
*
S Supporting Information
ABSTRACT: Mn(I)-PNP pincer catalyzed sequential one-pot homoge-
neous CO hydrogenation to CH OH by molecular H is demonstrated.
2
3
2
The hydrogenation consists of two partsN-formylation of an amine
utilizing CO and H , and subsequent formamide reduction to CH OH,
2
2
3
regenerating the amine in the process. A reported air-stable and well-
defined Mn-PNP pincer complex was found active for the catalysis of both
steps. CH OH yields up to 84% and 71% (w.r.t amine) were obtained,
3
when benzylamine and morpholine were used as amines, respectively; and
a TON of up to 36 was observed. In our opinion, this study represents an
important development in the nascent field of base-metal-catalyzed
homogeneous CO hydrogenation to CH OH.
2
3
KEYWORDS: homogeneous hydrogenation, CO capture and utilization, Mn-pincer catalyst, methanol economy,
2
CO reduction to methanol
2
he rapid increase in atmospheric CO level due to human
Scheme 1. CO Hydrogenation to CH OH via Formamide
2 3
2
T
activities over the last three centuries has prompted the
development of carbon capture processes. While most capture
efforts have been focused on concentrated anthropogenic CO2
sources like flue gases and various industrial activities, capture
1
from diffuse sources like air has also gained attention. Once
captured, the CO can be used as a feedstock for the synthesis
2
facilitate the installation of small CCU units in remote areas,
isolated from large carbon emission sources.
of value-added products such as formic acid, methanol,
2
methane, and higher hydrocarbons. Among these, methanol
The development of first-row transition-metal-based catalysts
is important because of the high abundance and low cost of
such metals compared to more widely studied catalysts based
on noble metals such as Pt, Ru, and Ir. It should be noted
however that the cost and accessibility of the ligand should also
be taken into consideration while designing such catalysts.
Other crucial and most desirable features include high activity,
high selectivity, and high durability, along with the ease of
recyclability. Despite the growing attention that base-metal
catalysis has received in recent years, surprisingly, only one
study involving base-metal catalysts has been reported for the
represents a particularly attractive product as it can be used as a
fuel (in direct methanol fuel cells, internal combustion engines,
etc.), drop-in fuel additive, C1 source in organic trans-
formations, or as a chemical precursor for the production of
3
higher hydrocarbons. Industrially, CH OH is produced from
3
synthesis gas (CO, H , and CO ) at high temperature and
2
2
4
pressure over Cu/ZnO/Al O -based heterogeneous catalysts.
2
3
CH OH can also be produced from CO hydrogenation with
3
2
5
similar heterogeneous catalysts. In the last 5 years,
homogeneous catalysts, mainly based on noble metals like Ru
6
7
and Ir, have also been reported for direct and indirect CO
2
important process of CO2 to CH OH reduction under
reduction to CH OH with molecular H at much lower
3
3
2
9
8
homogeneous conditions. That study, published by the Beller
temperatures and pressures.
group in early 2017, employed an in situ generated Co-based
One of the promising pathways for CO2 to CH OH
3
catalyst to reduce CO to CH OH with a TON of up to 78.
reduction is via a formamide intermediate in the presence of
2
3
6e
Mn-based catalysts have been explored extensively for C−H
an amine, first developed by the Sanford group (Scheme 1).
10
activation. In 2016, Beller et al. reported the catalytic
The presence of an amine in the system provides a unique
opportunity for its integration with carbon capture processes as
was demonstrated by our group by converting directly CO
Received: June 23, 2017
Revised: August 14, 2017
2
6f
captured from air into CH OH. Such an integrated carbon
3
capture and utilization process from diffused sources can
©
XXXX American Chemical Society
6347
DOI: 10.1021/acscatal.7b02066
ACS Catal. 2017, 7, 6347−6351

ACS Catalysis
Letter
a
hydrogenation of aldehydes, ketones, and nitriles in the
Table 1. Optimization of N-Formylation of 1
11a
presence of Mn(I) complexes. Since then, several studies
have successfully employed Mn(I) complexes for formic acid
1
1b
11c
decomposition,
N-alkylation,
aqueous methanol refor-
1
1e
11f
ming, N-formylation through CH OH dehydrogenation,
3
11d,g
11h
ester hydrogenation,
amide hydrogenation,
aminomethylation,
alcohol
and transfer hydro-
Based on some of these results, we wondered
1
1i
11j
deoxygenation,
1
1k
genation.
whether such catalysts based on inexpensive Mn(I) would be
able to reduce CO to CH OH. During the course of this study,
2
3
two articles have surfaced, reporting successful CO reduction
2
12
to formate salts and formamide. In the present study, we
show for the first time that CO can be sequentially reduced in
2
the same pot by 3 eq. of H to CH OH in the presence of
b
2
3
entry
P (bar)
T (°C)
time (h)
solv.
THF
yield (%)
13
Mn(I) catalyst C-1 and an amine (Scheme 2, Table 1).
A
1
2
3
4
60
60
60
60
60
70
60
60
60
60
60
60
120
110
100
110
110
110
110
110
110
110
110
110
24
24
24
36
24
24
36
36
24
24
24
36
65
77
53
92
93
90
66
65
43
32
95
53
maximum TON of 36 was achieved, which is comparable to the
TON reported earlier with Co-based homogeneous catalysts.
THF
THF
THF
THF
THF
THF
Scheme 2. Mn(I)-Catalyzed Reductions
c
5
6
d
7
8
Ph−CH3
CPME
Cy
c
9
c
c
c
,
,
,
e
e
f
1
1
1
0
1
2
THF
THF
a
Reaction conditions: morpholine (2 mmol), cat. (0.5 mol %), K PO
3
4
b
(2.5 mol %), solvent (2 mL), CO : H (1:1). Yields were determined
2 2
1
w.r.t amine from H NMR spectra (N−CHO peaks) using 1,3,5-
trimethoxybenzene (TMB) as an internal standard. Morpholine (1
mmol), cat. (2 mol %), K PO (10 mol %). Catalyst C-2 (0.5 mol %)
was used. t-BuOK (10 mol %) was used instead of K PO . Set up in
c
d
3
4
e
f
3
4
benchtop condition, Na CO (10 mol %) was used as t-BuOK and
2
3
K PO are too hygroscopic to store under an open atmosphere. NMR
yield calculations error = ± 5%.
In the initial stage of this study, we investigated the
effectiveness of Mn-pincer complexes C-1 and C-2 for the
3
4
reduction of CO to formamide in the presence of morpholine
2
1
4,15
(1) (Table 1).
Such Mn(I)-PNP pincer catalysts were
compared to entry 5). Upon opening the reaction vessel, a
brownish yellow solution was observed instead of a yellow
solution, generally obtained when reactions were performed
under inert atmosphere, indicating a probable catalyst
decomposition taking place with the reaction set up in air.
Among various amines, benzylamines reacted to afford
formamide products in high yields. For example, benzylamine
(2) and N-methylbenzylamine (3) were converted to the
corresponding −NCHO products, 2a and 3a, after 24 h at 2
mol % catalyst loading and a CO :H (1:1) pressure of 60 bar
previously reported, as mentioned earlier, as active for
aldehyde, ketone and nitrile reduction, formic acid decom-
11a,b,e
position, and for CH OH dehydrogenation.
Satisfyingly,
3
catalyst C-1 was also able to reduce CO in the presence of 1,
2
K PO4 and molecular H2 at 120 °C in THF, and the
3
corresponding product, 4-formylmorpholine (1a), was ob-
1
served through H NMR in 65% yield after 24 h (Table 1, entry
1
). Decreasing the reaction temperature to 110 °C increased
the yield to 77% (entry 2). However, decreasing the
2
2
temperature further was found to produce less 1a after 24 h
in 93% and 94% yields, respectively (Table 2, entry 2−3). On
the other hand, alkyl amines, such as primary amylamine (4)
and secondary N,N′-dimethylethylenediamine (5) produced
moderate N-formyl yields even at higher pressures and catalyst
loadings (entry 4−6).
(
entry 3). A higher yield was observed with a longer reaction
time (entry 4), increasing catalyst loading (entry 5), or when
the reaction was performed at a higher pressure of 70 bar (entry
Cy
6
). Catalyst C-2, MnBrPNP (CO) was less effective than C-1
2
for this reduction (entry 7). Among solvents, THF was found
to be the most suitable, while less polar solvents such as
toluene, cyclopentyl methyl ether (CPME), and cyclohexane
provided lower yields (entry 8−10). t-BuOK was found to be
similarly effective as K PO as an initial activator of catalyst C-1
Having explored the first reaction step in this sequential
reduction process, we proceeded to investigate the feasibility of
in situ reduction of the generated formamides. A study on
catalyst C-1’s effectiveness for formamide reduction showed
that at 70 bar of H pressure, 1a can be reduced to morpholine
3
4
2
(
entry 11). Further, catalyst C-1 was reported in the literature
and CH OH with a 64% CH OH yield (TON = 128) at 0.5
3
3
as being air stable even after 25 days, leading us to wonder
whether the reaction could be performed in the absence of an
inert atmosphere. Selective N-formylation products were
observed (conversion 59%, yield 53%) even when the reaction
was set up without using a glovebox with commercially bought
chemicals (see SI), but the yield was lower (Table 1, entry 12;
mol % catalyst loading in THF after 24 h (Table 3, entry 1).
Expectedly, when we subjected the in situ formamide from the
reaction mixture from Table 1, entry 4 (T1, E4) to 70 bar H2
pressure at 150 °C for 36 h, reduction of the in situ formed 1a
was indeed observed with 11% of CH OH formation (TON =
3
22) (Table 3, entry 2), although the TON was lower. 78% of
6
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Letter
†
Table 2. N-Formylation of Various Amines
of CO : H (1:1) in the presence of t-BuOK (0.3 mmol) and 1
2
2
(
0.2 mmol) in THF-d (1 mL) at 110 °C, a yellow solution was
8
31
obtained after 6 h. The P NMR spectrum of the solution
showed the presence of a major peak at 87.4 ppm along with a
minor peak due to catalyst C-1 at 82.3 ppm (Figure S9B). In
1
the H NMR, one N−H triplet peak was observed at 7.1 ppm
(
J = 11.6 Hz) along with a singlet at 8.4 ppm (Figure S9A).
Product 1a was also observed as a sharp singlet at 7.98 ppm.
1
31
The new H and P peaks were assigned to the complex
iPr
Mn(OOCH)PNP (CO) (C-3) (Figure S1), which is formed
2
in the reaction mixture through CO insertion into in situ
2
iPr
formed complex MnHNP (CO) (C-4) (Scheme 3, step 1).
2
Scheme 3. A Plausible Mechanism
†
Reaction conditions: amine (1 mmol), C-1 (2 mol %), t-BuOK (10
a
mol %), THF (2 mL), CO : H (1:1), 110 °C Yields were
determined w.r.t amines from H NMR spectra (N−CHO peaks)
using TMB as an internal standard. Amine (0.5 mmol) t-BuOK (20
2
2
Interestingly, the complex C-4 itself was not observed in the
1
1
31
solution in either H or P NMR spectra, which can be due to
b
its high reactivity. Complex C-3 was found to be active for the
mol %). NMR yield calculations error = ± 5%.
reduction of pure 1a to 1 and CH OH (Table S1, entry 2). We
3
surmise that the complex C-3, formed during initial N-
formylation step, gets decarboxylated during subsequent
unreduced 1a was also observed. An increase in the catalyst
loading and H pressure to 2 mol % and 80 bar, respectively,
formamide reduction step at high H pressure (70 bar) to
form back active catalytic species C-4, which in turn reduces
2
2
increased both CH OH yield and TON (= 36), with only 6% of
3
the in situ formed formamide (Scheme 3, step 2). Similar
observations were recently reported by Gonsalvi et al. with
12b
another manganese-PNP complex.
The presence of t-
butoxide manganese species (C-8) (Figure S1) was also
detected after the hydrogenation of in situ formed formamides
(Figure S11−15).
Unfortunately, attempts of direct CH OH synthesis in one
3
8
4% (TON = 21) was obtained when 4 mol % C-1 was used
step with low CO and high H pressure failed to produce
2
2
(
entry 5). Notably, no N−CH side-products were observed
CH OH in the presence of C-1. When the reactor vessel was
3
3
1
13
through H and C NMR, and no trace of CO was detected in
GC during hydrogenation of the in situ formed formamides.
Next, mechanistic studies were performed to gain an
understanding of the active catalytic species formed in the
reaction mixture. When 40 μmol of C-1 was treated with 60 bar
charged with 5 bar CO and 85 bar H and heated at 150 °C for
2
2
24 h in the presence of morpholine and C-1, only a minute
amount (5%) of formamide was formed with no observable
CH OH. When a stepwise temperature increase procedure was
3
followed with an initial 24 h heating at 110 °C to allow the
a
Table 3. Hydrogenation of In Situ-Formed Formamides
b
b
b,c
entry
formamide source
P (bar)
time (h)
N−CHO (%)
amine (%)
CH OH (%)
TON
3
1
2
3
4
ex situ
T1, E4
T2, E1
T2, E2
T2, E2
70
70
80
80
80
24
36
36
24
36
trace
78
6
94
15
83
#
64
11
71
46
84
128
22
36
23
21
52
5
d
5
#
a
Reaction conditions: reaction mixtures from Table 1 or 2 (as specified) were subjected to H pressure after the release of previous CO :H (1:1)
2
2
2
b
1
gas mixture. T = 150 °C. Amount of formamide, amine, and CH OH were calculated w.r.t. amines from H NMR spectra with TMB as internal
standard. Lower CH OH yields are due to losses, while releasing gases after reaction. 4 mol % of C-1 loading. benzyl CH peak overlapped with
3
c
d
#
3
2
THF peaks making quantification challenging. T1, E4 signifies Table 1, entry 4. NMR yield calculations error = ± 5%. TON = mol of CH OH
3
formed per mol of C-1.
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ACS Catalysis
Letter
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subsequent formamide hydrogenation, 38% of 1a was observed
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3
We suggest that under these conditions (5 bar CO , 85 bar H ),
2
2
the rates of both N-formylation and formamide reduction
becomes sluggish (Table S2).
Finally, we decided to scale up the reaction with 10 mmol
benzylamine at a low C-1 loading (0.1 mol %) to observe the
extent of catalytic efficiency of C-1. The first reduction step to
formamide proceeded with 84% yield (TONN−CHO = 840) after
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̈
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16
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3
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3
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b02066.
■
*
S
(
7) Indirect CH OH formation from HCOOH disproportionation:
3
(
a) Miller, A. J. M.; Heinekey, D. M.; Mayer, J. M.; Goldberg, K. I.
Angew. Chem., Int. Ed. 2013, 52, 3981−3984. (b) Savourey, S.; Lefevre,
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014, 4, 1498−1512. (b) Wang, W. H.; Himeda, Y.; Muckerman, J. T.;
̀
General information and experimental details (PDF)
́
2
AUTHOR INFORMATION
Corresponding Author
E-mail: gprakash@usc.edu.
■
(
2
*
Manbeck, G. F.; Fujita, E. Chem. Rev. 2015, 115, 12936−12973.
(9) Schneidewind, J.; Adam, R.; Baumann, W.; Jackstell, R.; Beller, M.
Angew. Chem., Int. Ed. 2017, 56, 1890−1893.
ORCID
G. K. Surya Prakash: 0000-0002-6350-8325
Notes
The authors declare no competing financial interest.
(10) (a) Zhou, B.; Chen, H.; Wang, C. J. Am. Chem. Soc. 2013, 135,
1
1
(
264−1267. (b) Liu, W.; Groves, J. T. Acc. Chem. Res. 2015, 48,
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ACKNOWLEDGMENTS
Support of our work by the Loker Hydrocarbon Research
Institute, USC is gratefully acknowledged.
■
A.; Baumann, W.; Ludwig, R.; Junge, K.; Beller, M. J. Am. Chem. Soc.
2016, 138, 8809−8814. (b) Tondreau, A. M.; Boncella, J. M.
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16) A TON
of 2150 was observed by our group with Ru-
MeOH
Macho-BH catalyst for homogeneous CO hydrogenation to CH OH
2
3
(
ref 6f).
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DOI: 10.1021/acscatal.7b02066
ACS Catal. 2017, 7, 6347−6351