Integrative CO2 Capture and hydrogenation to methanol with reusable catalyst and amine: Toward a carbon neutral methanol economy

Article abstract of DOI:10.1021/jacs.7b12183

Herein we report an efficient and recyclable system for tandem CO₂ capture and hydrogenation to methanol. After capture in an aqueous amine solution, CO₂ is hydrogenated in high yield to CH₃OH (>90%) in a biphasic 2-MTHF/water system, which also allows for easy separation and recycling of the amine and catalyst for multiple reaction cycles. Between cycles, the produced methanol can be conveniently removed in vacuo. Employing this strategy, catalyst Ru-MACHO-BH and polyamine PEHA were recycled three times with 87% of the methanol producibility of the first cycle retained, along with 95% of catalyst activity after four cycles. CO₂ from dilute sources such as air can also be converted to CH₃OH using this route. We postulate that the CO₂ capture and hydrogenation to methanol system presented here could be an important step toward the implementation of the carbon neutral methanol economy concept.

Full text of DOI:10.1021/jacs.7b12183

Communication
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
pubs.acs.org/JACS
Integrative CO Capture and Hydrogenation to Methanol with
2
Reusable Catalyst and Amine: Toward a Carbon Neutral Methanol
Economy
Sayan Kar, Raktim Sen, Alain Goeppert, 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
7
condition in the presence of dimethylamine (Figure 1). The
presence of an amine provides the opportunity to capture and
ABSTRACT: Herein we report an efficient and recyclable
system for tandem CO capture and hydrogenation to
2
methanol. After capture in an aqueous amine solution,
CO is hydrogenated in high yield to CH OH (>90%) in a
2
3
biphasic 2-MTHF/water system, which also allows for easy
separation and recycling of the amine and catalyst for
multiple reaction cycles. Between cycles, the produced
methanol can be conveniently removed in vacuo. Employ-
ing this strategy, catalyst Ru-MACHO-BH and polyamine
PEHA were recycled three times with 87% of the methanol
producibility of the first cycle retained, along with 95% of
Figure 1. Amine assisted CO hydrogenation to CH OH.
2
3
hydrogenate CO in tandem, as was demonstrated by our
2
catalyst activity after four cycles. CO from dilute sources
2
8
group. Since Sanford et al.’s initial report, multiple studies have
such as air can also be converted to CH OH using this
3
been published using various metal complexes for amine
route. We postulate that the CO capture and hydro-
2
8,9
assisted hydrogenation of CO to CH OH. However, the
2
3
genation to methanol system presented here could be an
important step toward the implementation of the carbon
neutral methanol economy concept.
integration of CO capture with subsequent hydrogenation to
2
CH OH with easy recycling of the active elements had not
3
been explored.
Most of the reported metal complexes catalyzing CO2
hydrogenation to CH OH are soluble in organic solvents,
whereas the capturing amines are soluble in water. Aqueous
3
he rise of atmospheric CO concentration and associated
global warming have prompted researchers to develop
2
T
solutions of amines have long been utilized for scrubbing CO
from industrial gas streams. Water is a desirable solvent due
2
strategies for capturing CO from both emission point sources
10
2
1
and diffuse sources like ambient air. Whereas the captured
to its benign nature and ability to enhance the amines’ CO2
absorption capacity. Thus, a biphasic system was envisioned,
where after the hydrogenation step, the amine and catalyst can
be easily separated and recycled from the aqueous and organic
CO can be sequestered underground in geological formations,
2
a more sustainable approach is to utilize the CO to produce
2
2
fuels and other value-added products. CH OH in particular
3
can be used as a fuel, fuel additive or precursor in organic
layer, respectively (Figure 2). The formed CH OH can be
3
3
synthesis. The utilization of CO to produce CH OH through
2
3
extracted through distillation. Similar biphasic systems were
hydrogenation, followed by the use of CH OH as fuel results in
3
an overall carbon neutral cycle, and represents an area of
4
interest in the context of carbon footprint reduction.
Development of integrated CO2 capture and utilization
(
CCU) systems, wherein the captured CO can be directly
2
converted to value-added products (in this case CH OH), is an
3
area of enormous interest as it can bypass the otherwise
intermediary and energy intensive desorption and compression
steps to produce pure CO . For practical implementation,
2
recycling of the catalyst and capture material is essential to keep
the entire process cost-effective. Traditional catalysts for CO2
hydrogenation to CH OH are heterogeneous and require high
temperatures and pressures. Over the past decade, however,
significant advances were made in both indirect and direct one-
3
5
Figure 2. Schematic representation of biphasic CO2 to methanol
system with recyclable catalyst and amine.
pot homogeneous catalytic CO to CH OH synthesis under
2
3
6
much milder conditions. In 2015, Sanford et al. demonstrated
Received: November 17, 2017
a direct CO hydrogenation system to CH OH under basic
2
3
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b12183
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
demonstrated recently by us and Leitner et al. for integrated
Following the capture, the formed aqueous solutions were
11
CO capture and conversion to formate salts.
hydrogenated at 145 °C in the presence of homogeneous
2
For the CO capture, amines with low vapor pressures are
catalysts and 70 bar of H , after addition of 5 mL 2-MTHF
2
2
desirable to avoid atmospheric amine contamination. High
boiling polyamines were therefore selected for capture, along
with two ethanolamines (Table 1). Among various polyamines,
(Table 2). When the catalyst Ru-MACHO-BH (C-1) (10
μmol) was used along with PEHA as the capture material, 47%
CH OH yield was observed (5.2 mmol) after 72 h (entry 1).
3
No concomitant CO/CH formation was observed through GC
analysis of the reaction gas mixture. H and C NMR revealed
4
a
1
13
Table 1. CO Capture by Aqueous Amine Solutions
2
that 14% of formed CH OH was present in the upper organic
3
layer, whereas the remaining CH OH along with PEHA,
3
formamide and formate intermediates, was in the bottom
aqueous layer (Figure S6 and S7). The catalyst remained in the
organic layer as observed by 31P NMR (Figure S5), indicating
the possibility of easy catalyst separation from the biphasic
mixture. Increasing the catalyst loading to 20 μmol increased
the methanol yield to 79% (8.7 mmol) (entry 2). Next, various
amine solutions after CO2 capture (from Table 1) were
hydrogenated to identify the most promising amine for an
integrated CO capture/hydrogenation system. Switching from
2
PEHA to BPEI800 or BPEI25k decreased both the amounts of
CH OH formed (4.5 and 5.2 mmol, respectively) and the
hydrogenation yield (45% and 50%) (entry 3 and 4). For
3
b
c
d
Entry
Amine
$/kg
CO (mmol)/g
CO /N
2
2
1
2
3
PEHA
105 (S)
11.0
10.2
10.4
6.2
0.43
0.46
0.47
0.36
0.25
0.28
0.71
0.53
LPEI2.5k, LPEI100k, and PAA10k, CH OH yields decreased
3
BPEI800
BPEI25k
PAA10k
LPEI2.5k
LPEI100k
MEA
352 (S)
drastically to 0.9, 0.9, and 0.1 mmol, respectively (entry 5−
320 (S)
7). Surprisingly, with MEA, no CH OH was formed (entry 8),
3
e
4
7533 (P)
56,000 (P)
24,500 (P)
35 (S)
but increased amounts of formamide and formate intermediates
were observed. We surmise that in the presence of primary
amines, such as PAA10k and MEA, the second hydrogenation
step of formamide to methanol becomes more challenging.
Indeed, when DEEDA, a secondary analogue of MEA, was
used, methanol was obtained in 46% yield after 72 h (3.3
mmol; entry 9). Thus, among various amines, PEHA was the
f
5
f
5.5
6
6.1
7
8
11.7
7.2
DEEDA
820 (T)
a
Capture conditions: Amine (1g), water (3 mL), stirring (800 rpm),
rt. Aqueous amine solutions stirred in CO atmosphere at a constant
pressure of 1 psi. Captured CO2 amounts calculated through
gravimetric analysis. Calculations error ± 5%. Prices from Sigma-
Aldrich (S), Polysciences (P) or TCI America (T), as of Nov 14, 2017.
CO captured per gram of amine. mols of CO captured per mol of
nitrogen. Commercial 15 wt % PAA aqueous solution used directly.
2
most efficient for the overall CO capture and conversion to
2
b
CH OH. The low vapor pressure and easy availability of
3
inexpensive PEHA (Table 1) make it promising for a large scale
CCU process.
c
d
2
2
e
1
0k
f
Next, known hydrogenation catalysts were screened to
investigate their efficacy. Ru-MACHO (C-2), expectedly, was
almost equally effective to Ru-MACHO-BH, in the presence of
an additional base K PO (entry 10). The P-substituent in the
10 mL water, capture at 70 °C
3
4
pentaethylenehexamine (PEHA), and branched polyethyleni-
PNP ligand heavily influenced the CH
RuHClPNP (CO) (C-3), a meager 5% methanol yield was
observed (entry 11). Complex MnBrPNP (CO)
recently reported by us to catalyze sequential CO
genation to methanol, was only capable of producing CH OH
3
OH yield. With
iPr
mines (BPEI) were found efficient for CO capture. The
2
iPr
aqueous PEHA solution captured 11.0 mmol of CO per g of
2
(C-4),
2
PEHA after 4 h, corresponding to 0.43 mol of CO captured
2
hydro-
2
13
per mol of amino group (CO /N), (Table 1, entry 1). The C
3
2
NMR of the CO loaded aqueous PEHA solution revealed the
in 5% yield (0.5 mmol; entry 12). No methanol formed with
2
iPr
presence of carbamate and carbonate/bicarbonate (Figure S2).
FeHBrPNP (CO) (C-5) (entry 13). Accumulation of
Similarly, BPEI800 and BPEI25k captured 0.46 and 0.47 CO /N,
formamide and formate intermediates in the case of C-3 to
C-5 suggests lower activities of these catalysts for the effective
2
respectively (10.2 and 10.4 mmol of CO /g, respectively)
2
(
entry 2−3). CO capture by aqueous PAA solution was
hydrogenation of formamides and formates to CH
the present conditions.
3
OH under
2
10k
slower, as after 4 h only 6.2 mmol/g of CO was captured,
2
corresponding to 0.36 CO /N (entry 4). Linear polyethyleni-
The most suitable organic solvent for the biphasic hydro-
genation system was subsequently explored. A slight decrease in
methanol formation was observed when switching from 2-
MTHF to cyclopentyl methyl ether (CPME) or p-xylene (52%
and 57%, respectively) (entry 14 and 15). The reason behind
the decrease in methanol yield with more hydrophobic solvents
is not entirely clear, but is most probably a combination of
different catalyst/H /CO /CH OH solubility in different
2
mines (LPEI2.5k, LPEI100k) displayed limited solubility in water
at room temperature, making them less convenient for aqueous
CO2 capture. With more water (10 mL), and a higher
temperature (70 °C), LPEI2.5k and LPEI100k captured 5.5 and
6
.1 mmol of CO /g, respectively. Monoethanolamine (MEA),
2
which has long been used industrially for scrubbing CO and
2
H S from flue gases, was the most effective for CO capture
2
2
2
2
3
both by mass and efficiency of amine utilization (11.7 mmol/g;
.71 CO /N) (entry 7). Similarly, 7.2 mmol/g CO (0.53
organic solvents (see SI). Also, the higher hydrophobicity of
these solvents compared to 2-MTHF resulted in an
accumulation of the produced methanol exclusively in the
aqueous layer (Figure S13). However, the lower solubility of C-
0
2
2
CO /N) was captured by diethanolethylenediamine (DEEDA)
2
(entry 8).
B
DOI: 10.1021/jacs.7b12183
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 2. Tandem Homogeneous Hydrogenation of CO Captured by Aqueous Amine Solutions
2
b
b
b
b
c
Entry
Amine
Captured CO (mmol)
Catalyst (μmol)
Formate (%)
Formamide (%)
MeOH (mmol)
Yield (%)
PMeOH
TON
2
1
2
3
4
5
6
7
8
PEHA
PEHA
BPEI800
BPEI25k
LPEI2.5k
LPEI100k
PAA10k
MEA
11.0
11.0
10.2
10.4
5.5
C-1(10)
C-1(20)
C-1(20)
C-1(20)
C-1(20)
C-1(20)
C-1(20)
C-1(20)
C-1(20)
C-2(20)
C-3(20)
C-4(20)
C-5(20)
C-1(20)
C-1(20)
C-1(50)
C-1(50)
11
5
10
2
5.2
8.7
4.5
5.2
0.9
0.9
0.1
0
47
79
45
50
16
15
2
0.16
0.17
0.18
0.13
0.11
0.11
0
520
435
225
260
45
16
10
30
21
32
26
15
6
13
7
15
44
38
17
6
6.1
45
6.2
5
11.7
7.2
0
nd
0
9
DEEDA
PEHA
PEHA
PEHA
PEHA
PEHA
PEHA
PEHA
PEHA
3.3
7.4
0.5
0.5
0.0
5.7
6.3
10.4
4.8
46
67
5
0.14
0.15
0
165
370
25
d
10
11
12
13
14
15
16
17
11.0
11.0
11.0
11.0
11.0
11.0
11.0
5.4
13
20
19
18
11
6
d
d
d
e
15
18
20
12
9
5
0
25
0
nd
0
52
57
95
89
0
285
315
208
96
f
0
g
h
3
0
0.17
0.11
5
0
a
Reaction conditions: Solutions from Table 1 (as specified) were hydrogenated after adding organic solvent and catalyst. 2-MTHF (5 mL), H (70
2
b
1
bar), 145 °C, 72 h. Yields based on H NMR with 1,3,5-trimethoxybenzene (TMB) and imidazole (Im) as internal standards for organic and
aqueous layer, respectively. P
solvent. P-xylene used as organic solvent. H (80 bar). CO captured from simulated air (CO concentration: 408 ppm) with 0.79 g PEHA. Yield
c
d
e
= methanol in organic layer/methanol in aqueous layer. K PO (1 mmol) added. CPME used as organic
3 4
g h
MeOH
f
2
2
2
calculations error ± 5%. TON = mols of methanol formed per mol of catalyst. nd = nondefinable
1
in these solvents at room temperature caused some of the
catalyst to precipitate out from the solution during workup,
making its complete recycling challenging. Hence, 2-MTHF
was identified as the most convenient solvent for repeated
capture and utilization studies. Using 2-MTHF, a CH OH yield
3
as high as 95% was obtained with a higher C-1 loading of 50
μmol (entry 16). Finally, CO from air was also captured and
2
hydrogenated to CH OH in high yields following this protocol
3
(
entry 17).
With the optimized selection of amine (PEHA), catalyst (C-
), and organic solvent (2-MTHF), two recycling studies were
Figure 3. Methanol formation with catalyst recycling (A) and catalyst
and amine recycling (B). Reaction conditions: After capture with 1 g
1
conducted. In a first study, only the catalyst was recovered from
the organic layer and reused for successive hydrogenation
cycles (see SI). After four cycles, 95% of C-1’s catalytic
efficiency of the initial cycle was retained, with a total of 40.5
PEHA in 3 mL water, H (80 bar), C-1 (50 μmol), 2-MTHF (10 mL),
2
1
145 °C, 72 h. Methanol yields calculated from H NMR with Ph−CH
3
and Im (A)/ t-BuOH (B) as internal standards for organic and
aqueous layer, respectively. Error in yield calculations ±5%.
mmol of CH OH formed, demonstrating the high recyclability
3
of the catalyst, enabled by this biphasic system (Figure 3A). In a
second study (Figure 3B), both the catalyst and capturing
reaction and the loss of amine while transferring PEHA
solution between glassware.
amine were recovered and reused. 89% of the CO capture
2
In conclusion, a tandem system for CO capture (even from
2
efficiency of the amine was retained in the third cycle, along
with 87% of methanol productivity. The slight loss in capture is
most probably due to the presence of formate species after the
air) in aqueous amine solution and subsequent hydrogenation
to methanol is described where the catalyst and amine can be
recycled multiple times without significant loss in effectiveness.
C
DOI: 10.1021/jacs.7b12183
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Among the catalysts tested, a well-defined and commercially
available complex, Ru-MACHO-BH (C-1) was found most
effective, whereas, among various amines, high boiling poly-
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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/jacs.7b12183.
■
S
(
Chem., Int. Ed. 2015, 54, 6186−6189. (b) Khusnutdinova, J. R.; Garg,
J. A.; Milstein, D. ACS Catal. 2015, 5, 2416−2422. (c) Kar, S.;
Goeppert, A.; Kothandaraman, J.; Prakash, G. K. S. ACS Catal. 2017,
General information and experimental details (PDF)
7
, 6347−6351. (d) Ribeiro, A. P. C.; Martins, L. M. D. R. S.;
Pombeiro, A. J. L. Green Chem. 2017, 19, 4811−4815. (e) Everett, M.;
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Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.; Beller, M.; Laurenczy, G.
Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.7b00182. (g) Kar, S.;
Kothandaraman, J.; Goeppert, A.; Prakash, G. K. S. J. CO2 Util. 2018,
AUTHOR INFORMATION
Corresponding Author
gprakash@usc.edu
ORCID
■
*
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b) Oyenekan, B. A.; Rochelle, G. T. Ind. Eng. Chem. Res. 2006, 45,
G. K. Surya Prakash: 0000-0002-6350-8325
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
Support of our work by the Loker Hydrocarbon Research
Institute, USC is gratefully acknowledged.
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DOI: 10.1021/jacs.7b12183
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