Conversion of CO2 from air into formate using amines and phosphorus-nitrogen PN3P-Ru(ii) pincer complexes

Article abstract of DOI:10.1039/c8gc02186d

Well-defined ruthenium(ii) PN³P pincer complexes were developed for the hydrogenation of carbon dioxide. Excellent product selectivity and catalytic activity with TOF (turnover frequency) and TON (turnover number) up to 13000 h^-1 and 33000, respectively, in a THF/H₂O biphasic system were achieved. Notably, effective conversion of carbon dioxide from air into formate was conducted in the presence of an amine, allowing easy product separation and catalyst recycling.

Full text of DOI:10.1039/c8gc02186d

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Conversion of CO2 from Air into Formate Using Amines and
Phosphorus-Nitrogen PN3P-Ru(II) Pincer Complexes
Chao Guana+, Yupeng Pana+, Eleanor Pei Ling Anga, Jinsong Hua, Changguang Yaoa, Mei-Hui Huanga,
Huaifeng Lia, Zhiping Laia, Kuo-Wei Huanga*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: Well-defined ruthenium (II) PN3P pincer complexes were
developed for the hydrogenation of carbon dioxide. Excellent
product selectivity and catalytic activity with TOF (turnover
frenquency) and TON (turnover number) up to 13,000 h-1 and
33,000, respectively, in a THF/H2O biphasic system were achieved.
Notably, effective conversion of carbon dioxide from air into
formate was conducted in the presence of an amine, allowing easy
product separation and catalyst recycling.
et al. reported a Ru complex, [Ru(Acriphos)(PPh3)(Cl)(PhCO2)], for
the CO2 hydrogenation to FA, affording an average TOF of 1,000 h-1
in the presence of DMSO/H2O at 60 °C and 120 bar of H2/CO2
(2:1).[12d] Later, Peng and Zhang et al. reported a rigid N,N,N-pincer
Ru complex for CO2 hydrogenation with TOF of 63 h-1 at 130 °C and
30 bar of H2.[12b] However, the synthesis of most of these
compounds involves the use of costly ligands (Fig. 1).[8e] Very few
examples have been reported to convert a low concentration of CO2
into formate. Olah, Prakash, and coworkers have demonstrated
hydrogenation of CO2 from air to methanol and formate using a
commercial catalyst.[14] However, there is still a need for more
economical and efficient approaches for such processes for future
practical applications. To realize this goal, diversified approaches to
offer more examples for the better understanding on the catalyst
structure/activity correlation will be invaluable. Our group has
recently developed a series of complexes based on a new class of
PN3-pincer ligands, and demonstrated that these complexes show
enhanced stability in the presence of water and air and reactivities
towards hydrogenation and dehydrogenation reactions.[15] Herein,
we report the synthesis of an economical PN3P-pincer ruthenium
complex to convert CO2 captured from air (409 ppm) with excellent
product selectivity and catalytic activity to formate.
Meeting increasing global energy demands by comsuming
carbon-based fossil fuels has led to the carbon dioxide (CO2) levels
in the atmosphere rising from the preindustrial 280 ppm to the
current 409 ppm.[1] One attractive approach to reducing CO2 levels
is carbon capture and recycling (CCR), which utilizes CO2 as an
important C1 source for the synthesis of value-added chemicals or
liquid fuels.[2] Several important products, such as formic acid (FA)
or formate,[3] methane,[4] formate esters,[5] methanol,[6] or
gasoline,[7] can be obtained from hydrogenation of CO2. Of these,
FA is considered a promising energy carrier with an energy density
of 1.77kW·h/L.[8]
On this basis, the catalytic hydrogenation of CO2 into
FA/formate has gained significant interest in recent years and a
variety of transition-metal complexes have been employed, such as
those based on iron,[9] cobalt,[10] ruthenium,[11] iridium[12] and
rhodium.[13] Nozaki et al. set the record for CO2 hydrogenation
activity in 2009 using
(PiPr2)2C5H3N]IrH3 in
a
PNP-pincer iridium complex [2,6-
a
THF/H2O binary solvent medium. This
catalyst gave a turnover frequency (TOF) of up to 150,000 h-1 at
200 °C and 80 bar. However, this result was obtained in a very
dilute concentration of catalyst with a low conversion.[12a] In 2014,
Pidko et al. demonstrated that a Ru-PNP pincer complex could
hydrogenate CO2 in DMF in the presence of a DBU base with a
remarkable TOF of up to 1,100,000 h-1 in 2 min.[11b] In 2016, Leitner
Figure 1. State-of-the-art catalytic systems for CO2 hydrogenation.
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Complex 1 was synthesized using the Ru metal precursor monometallic species 3 was obtained. Pale yellow crystals of 3,
RuHCl(CO)(PPh3)3 and
PN3P ligand derived from 2,6- suitable for X-ray crystallography, were obtained from
diaminopyridine and chlorodiphenylphosphine (N,N'- concentrated THF solution in a glovebox. The structure of 3 was
a
a
bis(diphenylphosphino)-2,6-diaminopyridine) (Scheme 1). By slow characterized by a PPh3 coordinated to the Ru center with a CO,
evaporation, colorless single crystals of 1’ (Fig. 2), suitable for X-ray hydride and pincer-type PN3P ligand (Fig. 3). In contrast to 1’, the
crystallography, were obtained from the solution. The molecular C1-N1(1.377(5) Å) and C2-N3 (1.341(5) Å) bond lengths of the two
structure of 1 (M·Cl·Cl) contains two Ru(II) centers and three PN3P arms in the pincer ligand were very different in 3, as were the P1-
ligands. As indicated in 1’ (M·Cl·HCO3), during crystal growth one of N1-C1 angle (121.2(3) °) and the P2-N3-C2 angle (115.7(3) °),
the chloride ions was readily replaced by a bicarbonate ion, indicating that the pyridine ring in 3 underwent dearomatization
presumably due to the absorption of CO2 and moisture from air. (Fig. 3). Consistently, the different chemical shifts in the 31P{1H}
The other chloride ion was held in the crystal cell by the formation NMR of 3 corresponded to the different chemical environments of
of N-H…Cl hydrogen bonds (Fig. 2).
phosphine at the two arms of the pincer ligand (Fig. S10).
2+
2Cl
PPh2
Ph
P
2
N
H
CO
N
N
H
OC
Ph2
Ph2
RuHCl(CO)(PPh3)3
P
P
HN
NH
HN
PPh2
N
NH
Ru
Ru
THF, 65oC, 2 hours
PPh2Ph2P
1
N
N
H
H
PPh2
N
N
H
H
PPh2
Ph
P
2
N
H
CO
N
N
Ph2
H
Ph2
P
KOtBu
P
PPh3
THF
HN
N
N
H
PPh2
OC
HN
NH
Ru
H
Ru
H
2 hours
P
Ru
Ph2
Ph3P
PPh2Ph2P
2
N
N
N
N
CO
3
Scheme 1. Preparation of complex 1, 2 and 3.
Figure 3. (a) Molecular structure of 3 with thermal ellipsoids set at
50% probability. The H atoms were omitted for clarity. Selected
bond lengths (Å): Ru1–P2 2.3156(11) Ru1–P1 2.3153(11), C1-
N1(1.377(5) C2-N3 (1.341(5). Selected bond angles (°): P1-N1-C1
121.2(3), P2-N3-C2 115.7(3).
Each Ru center of 1’ was symmetric with respect to the plane of the
pyridine ring in the pincer ligand to which it was coordinated,
indicated by the almost equal C1-N1 (1.378(6) Å) and C2-N3
(1.375(6) Å) bond lengths of the two pincer arms. Moreover, the
P1-N1-C1 angle (120.1(3) °) and P2-N3-C2 angle (121.4(3) °) were
almost the same, revealing the aromatized nature of 1’ (Fig. 2).
The catalytic activity of the dearomatized Ru-PN3P pincer
complex 2 was tested for CO2 hydrogenation in a THF/H2O solution
at 110 °C (Table 1). Under the base-free condition, very low
reactivity was observed even after a prolonged reaction time (Entry
1). Subsequently, various bases were used, including NaOH, Na2CO3
and NaHCO3. Compared to the base-free results, the catalytic
performance of 2 was greatly improved in the presence of a base
(Entry 2-5). These observations suggest that the base played an
important role in the catalytic activity. A maximum average TOF of
740 h-1 was achieved using Na2CO3 as the base and excess CO2
(Entry 3). When the reaction was carried out under the same
conditions in the absence of CO2, a negligible amount of formate
product was obtained[17] (Entry 4), indicating the importance of CO2
as the source of the formate production. In all cases, the liquid
composition of the product was confirmed by 1H NMR and 13C NMR
(Fig. S12-S13), and GC analysis of gas mixture after reaction showed
no detectable amount of CO. To our delight, the catalyst showed
excellent selectivity for CO2 hydrogenation to formate. Interestingly,
when NaHCO3 was used instead, only H2 was needed to produce
formate, indicating that NaHCO3 can serve as both the source of CO2
and the base; the catalyst also afforded a moderate TOF and TON
(Entry 5). In general, however, higher catalytic activity was obtained
when both CO2 and H2 were present (Entry 2-3), since a bicarbonate
makes the hydrogenation process more efficient to carry out.[18]
Hence, in subsequent studies, hydrogenation of bicarbonates were
further examined.
Figure 2. Molecular structure of 1’ (M·Cl·HCO3) with thermal
ellipsoids set at 50% probability. The H atoms were omitted for
clarity. Selected bond lengths (Å): Ru1–P2 2.2961(12) Ru1–P1
2.2821(12), Ru1–P3 2.4232(10), C1-N1 (1.378(6), C2-N3 (1.375(6),
Selected bond angles (°): P1-N1-C1 120.1(3), P2-N3-C2 121.4(3).
The dearomatized complex 2 was achieved by the treatment
of complex 1 with potassium t-butoxide in THF according to a
similar procedure previously reported by us (Scheme 1).[16] Upon
coordination of 2 with a neutral PPh3 ligand, a dearomatized
2 | J. Name., 2012, 00, 1-3
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Table 1. Hydrogenation of CO2, carbonate and bicarbonate in the
presence of complex 2*.
Based on the feature of aromatization-dearomatization of
t
complex 2 and our previous mechanistic studies of Bu-PN3P Ru-
catalyzed FA dehydrogenation[16a]
, we proposed a plausible
TOF
mechanism involving metal-ligand cooperation[19] (Figure 5). The
initial dissociation of a PN3P ligand from the bimetallic ruthenium
complex 2, gives two coordinately unsaturated 16-electron complex
P(H )/P(CO )
NMR
Time Formate
(h) (mmol)
2
2
Entry
Base
yield TON
(%)
-1
(h )
(psi)
A
as the catalytically active species. Upon hydrogenation, A
undergoes rearomatization of the pyridine ring and a bihydride
1
2
/
400/400
400/400
17
0.089
9.9
/
/
12
0.7
-
species B is formed. HCO3 could dissociate into CO2 and OH-,[20]
serving as the CO2 source. Insertion of CO2 into the Ru-H bond of B
then affords a Ru-formate complex C, similar to the intermediate
NaOH
Na CO
2.25
1338 600
1662 740
t
formed in the Bu case. Finally, elimination of the formate product
in the presence of OH- can regenerate the catalytically active A. To
support A is a plausible active species, further investigation was
carried out using the monometallic 3 in the reaction. In the
presence of additional PPh3, there was indeed a significant drop in
catalytic activity (Fig. S19), strongly suggesting that the dissociation
of the phosphine ligand is needed to achieve a catalytically active
species, i.e. A.
3
4
5
400/200
400/0
2.25
12.3
/
/
2
3
Na CO
2.25 trace
2.25 7.14
/
/
2
3
NaHCO
400/0
69 965 430
3
o
*General conditions: T=110 C, base (10.4 mmol), THF/H2O (1:1, 20
mL). n(2)=3.7 µmol, internal standard DMSO (1.4 mmol). TOF is an
average value and calculated according to the reaction time.
PPh2
Ph
P
2
N
H
CO
N
N
PN3P
Ph2
H
Ph2
P
P
HN
N
N
OC
HN
NH
Ru
H
Ru
H
P
Ru
PPh2
Ph2
To illustrate the possible macro-process taking place during
bicarbonate hydrogenation, an image of the reaction with three
phases, gas, liquid 1 (THF) and liquid 2 (THF/H2O), is shown in Fig. 4.
It was rationalized that complex 2 (or the active catalytic species
derived from 2) would prefer to stay in the organic THF phase
where the catalytic reaction occurred, because of the presence of
phenyl groups that made it relatively hydrophobic. Hydrogen and
bicarbonate then diffused into the THF phase to react to yield
formate, which was desolved back into the THF/H2O phase.
PPh2Ph2P
N
N
H
CO
N
N
Active species A
Bimetallic 2
HCOO- + H2O
A
HN
N
N
P
Ru
PPh2
Ph2
H
CO
OH-
H2
-
HCO3
HN
N
NH
H
P
Ru
PPh2
Ph2
HCOO
CO
B
C
CO2
HN
N
NH
H
P
Ru
PPh2
Ph2
H
CO
Figure 5. Proposed mechanism for hydrogenation of bicarbonate
using complex 2.
With the possible macro-process involving three phases and
the plausible mechanism of bicarbonate hydrogenation in hand, we
further developed a model for CO2 capture and utilization.[21] First,
we sought to capture CO2 from air using various bases to create
bicarbonate, which could then be hydrogenated into formate using
our Ru-PN3P pincer complex 2. A series of commercially available
bases were screened to evaluate their capacity for CO2 capture by
bubbling air through their aqueous solution (Table S1, Fig. S20). It
was observed that the CO2 absorption abilities of the inorganic
bases were too low even after a prolonged period of time (over 50
Figure 4. The macro-process of bicarbonate hydrogenation.
The catalytic performance of 2 was also optimized by varying
different parameters, such as the reaction temperature, catalyst
loading, bicarbonate concentration and pressure of H2 (Fig. S14-17).
The relationship between TON and the reaction time for the
bicarbonate hydrogenation under the optimized reaction conditions
(130 °C, catalyst loading of 0.1µmol, 1600 psi H2) is shown in Fig.
S18. TON increased as a function of the reaction time, and a
maximum TON of 33,000 was achieved after approximately 50
hours.
hours), except for NaOH (Table S1, Entry 1). However,
a
quantitative 13C NMR spectroscopy analysis of the salt formed in
the aqueous NaOH solution after CO2 absorption showed that only
carbonate had formed, not suitable for our intended hydrogenation
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conditions (Table 1, Entry 4). Gratifyingly, the organic bases, such as Figure 6. Recyclability test of complex 2. Reaction conditions: 13
amines, showed much better CO2 absorption potential.[22] All the mmol PMDTA+ 10 mL H2O (CO2 captured in each cycle = 15 mmol),
amines chosen for this study captured CO2 from air or a gas cylinder 10 mL 2-MeTHF, T=130 °C, Time=12 h, n(2)=20 μmol.
(1 bar) to give carbamate, bicarbonate or both (Table S1, Entry 4-9,
Fig. S21).[22c] Among the monoamines (Table S1, Entry 4-6), MEA,
which has been used industrially as a CO2 absorber[23], exhibited the Conclusions
highest absorption efficiency (17%), which was more than or almost
In summary, the PN3P bimetallic Ru complex 2 serves as a
double that of AMP (7%) and MDEA (9%). When the CO2 absorption
catalyst precusor with excellent selectivity and catalytic activity
time was shortened to 24 hours, TEPA, NTEA and PMDTA still
with TOF and TON up to 13,000 h-1 and 33,000, respectively. Our
showed outstanding CO2 absorption potential with absorption
catalytic system allowed both product separation and catalyst
efficiencies of 38%, 59% and 39%, respectively, when the source of
recycling, offering the advantages of both homogeneous and
CO2 was from a gas cylinder (1 bar) instead of air. It is noteworthy
heterogeneous systems.
that only tertiary amine PMDTA (efficiency 39% (1.16 eq)) and
MDEA (9%) captured and converted CO2 into solely bicarbonate
(Table S1; Fig. S22).[22a] Hence, PMDTA amine was the optimal
Conflicts of interest
choice for CO2.
There are no conflicts to declare.
The final aqueous solution of the PMDTA amine, obtained
after CO2 capture from air and salt formation, was then subjected
to hydrogenation in the presence of our Ru-PN3P pincer complex
2.[21] An average TOF of 990 h-1 (yield 71%) was achieved at 130 °C
Acknowledgements
with a H2 pressure of 1600 psi. (Fig. S23). In addition, recyclability
This work is supported by King Abdullah University of Science
and Technology (KAUST).
studies were also performed in a 2-MeTHF/H2O biphasic system
using PMDTA as the CO2 absorber. The low miscibility of 2-MeTHF
with water allows a clean organic-water phase separation.[24] In this
way, both product separation and catalyst recovery could be easily
realized since the formate formed in the aqueous phase and the
catalyst was in the organic phase. In the recyclability test, the
organic layer, containing the catalyst, could be reused for five
consecutive cycles in the hydrogenation of CO2 captured from air in
the presence of PMDTA without a significant reduction in catalytic
activity (Figure 6). After five consecutive cycles, the resting state of
the catalyst is confirmed to be complex 2 (Fig. S24) and only 4.1 %
of the catalyst was leached into the aqueous phase as confirmed by
Inductively Coupled Plasma Optical Emission Spectrometry (ICP-
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