In situ hydrogenation of captured CO2 to formate with polyethyleneimine and Rh/monophosphine system

Article abstract of DOI:10.1039/c3gc41265b

CO₂ in the air can be efficiently captured with simultaneous activation by PEI (polyethyleneimine) to form ammonium carbamate and/or carbonate species. Thus, the in situ hydrogenation of captured CO₂ into energy-storage materials rather than going through the desorption of conventional CCS (carbon capture and storage) runs better in comparison with equivalent gaseous CO₂, thus validating this potential application of CCU (carbon capture and utilization) for supplying renewable energy. PEI ₆₀₀ as an effective carbon absorbent in this study could also be assumed to serve as both ligand and base to promote the catalytic hydrogenation of captured CO₂, consequently acting as a 'hinge base' to combine capture and hydrogenation processes. The pathway was studied by NMR and in situ FT-IR spectroscopy under CO₂ pressure. This protocol could open up great potential in transforming the captured CO₂ from waste to fuel-related products.

Full text of DOI:10.1039/c3gc41265b

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In situ hydrogenation of captured CO2 to formate with
polyethyleneimine and Rh/monophosphine system†
Cite this: Green Chem., 2013, 15, 2825
Yu-Nong Li, Liang-Nian He,* An-Hua Liu, Xian-Dong Lang, Zhen-Zhen Yang, Bing Yu
and Chao-Ran Luan
CO2 in the air can be efficiently captured with simultaneous activation by PEI (polyethyleneimine) to
form ammonium carbamate and/or carbonate species. Thus, the in situ hydrogenation of captured CO2
into energy-storage materials rather than going through the desorption of conventional CCS (carbon
capture and storage) runs better in comparison with equivalent gaseous CO2, thus validating this poten-
tial application of CCU (carbon capture and utilization) for supplying renewable energy. PEI600 as an
effective carbon absorbent in this study could also be assumed to serve as both ligand and base to
promote the catalytic hydrogenation of captured CO2, consequently acting as a ‘hinge base’ to combine
capture and hydrogenation processes. The pathway was studied by NMR and in situ FT-IR spectroscopy
under CO2 pressure. This protocol could open up great potential in transforming the captured CO2 from
waste to fuel-related products.
Received 30th June 2013,
Accepted 13th August 2013
DOI: 10.1039/c3gc41265b
www.rsc.org/greenchem
required to overcome the thermodynamic limitation. In this
aspect, inorganic bases are often used for aqueous systems.7
Introduction
Carbon capture and sequestration (CCS) is being explored as a Nitrogen-containing bases like NH(CH3)2, trialkylamines,8
potential option to mitigate CO2 accumulation.1 Unfortunately, imidazoles, quinolines,9 and amino-functionalized ionic
extensive energy input into the desorption/compression liquids,10 commonly work well and lead to the formation of
process is a crucial barrier to realize practical CCS. On the a “base–formic acid” adduct, or formate, or formamide.11
other hand, as a nontoxic, abundant, renewable C1 building In addition, CO2 hydrogenation is usually performed at high
block, catalytic transformation of CO2 would be promising for CO2 pressure and temperature.
the production of fuels and value-added chemicals.2 However,
We envisage that the utilization of PEI, e.g. PEI600 (Mw =
the reactions involving CO2 are commonly carried out at high 600 Da, Scheme S1, see ESI†), for CO2 hydrogenation could be
pressure, which may not be economically suitable and also more attractive due to its strong basicity, non-volatility and
poses safety concerns. In this context, we have proposed good stability. In particular, PEI is proved to be an excellent
carbon capture and utilization (CCU) for the formation of CCS absorbent which can absorb CO2 directly from the air.
useful chemicals as an alternative approach to addressing the Notably, three types of amines (i.e. primary, secondary and ter-
energy penalty problem in CCS.3 Moreover, CO2 reduction as a tiary amine) could improve capture efficiency.12 Notably, the
utilization approach could be an option for energy storage.4 amino group in the repeating unit may also serve as a ligand
Recently, Olah and co-workers have initiated a methanol to alter the reactivity of the transition metal catalyst. Further-
economy strategy for supplying renewable energy in a carbon more, captured CO2 as an activated species is more easily
neutral cycle.5
converted to value-added chemicals via forming C–O/C–N
Hydrogenation of CO2 to formic acid and its derivatives has bond.3a–c Herein, we would like to validate the strategy of CO2
been appealing for suitable liquid hydrogen sources due to capture with simultaneous activation and in situ hydrogen-
easy storage and transportation.6 The presence of a base is ation of the captured CO2 to produce fuel-related products
such as formic acid derivatives by the combined PEI600 with
RhCl3·3H2O/CyPPh2 system.
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai
University, Tianjin, 300071, People’s Republic of China. E-mail: heln@nankai.edu.cn;
Fax: (+86)-22-2350-3878; Tel: (+86)-22-2350-3878
Results and discussion
†Electronic supplementary information (ESI) available: General experimental
methods, experimental procedures and results, characterization for absorption
CO2 is supposed to be absorbed by PEI600 via the formation of
the ammonium carbamate 1 (Scheme 1a).12 On the other
and hydrogenation systems, and copies of NMR, in situ FTIR, GC-MS spectra.
See DOI: 10.1039/c3gc41265b
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a
Table 1 Hydrogenation of gaseous CO2 by RhCl3·3H2O/PPh3
pKa of the conjugated
Entry
Base
acidd
T (°C)
TONb
1c
2
3
4
5
6
7
8
9
10
11
12c
13c
14c
PEI600
MEA
DEA
∼10e
60
60
60
60
60
60
60
60
60
60
60
100
80
40
465
35
9.50f
7.77–9.50f
10.98 f
8.36f
123
107
220
207
109
310
62
n.d.
21
n.d.
103
222
Et2NH
Morpholine
TEA
TMEDA
DABCO
HMTA
nBu4Br
KOH
7.77f
Scheme 1 CO2 capture with PEI.
9.42g
8.72g
4.89g
hand, the ammonium alkylcarbonate 2 pathway could also be
preferential in the presence of alcohol (Scheme 1b).9 The
capture mechanism is further investigated by in situ FT-IR
under CO2 pressure and NMR techniques.
13h, −9i
15.7 (31.2)j
PEI600
PEI600
PEI600
In the 13C NMR spectrum of CO2 capture by PEI600 in
methanol, additional signals at 164.01 and 159.51 ppm can be
assigned to CvO bands in ammonium carbamate 1 and
ammonium methylcarbonate 2 (R = Me), respectively (Fig. S1,
see ESI†).
Likewise, an upfield shift of carbons in PEI600 is observed
upon CO2 sorption. In situ FT-IR shows that the CvO stretch-
ing vibration of the ammonium carbamate at 1705, 1694,
1676 cm−1 gradually increases upon CO2 uptake (Fig. S2, ESI†).
N–H asymmetric and symmetric deformation bands at
1592–1586, 1552–1523 cm−1, and N–H bending vibration of
NHCOO− at 1496 cm−1 suggest the formation of the carbamate
1. The characteristic peak of CvO in ammonium methylcarbo-
nate 2 at 1570 cm−1 presumably supports path b. Finally,
because of the lack of a band at 835 cm−1, we could exclude
the formation of bicarbonate anion.13
a Conditions: RhCl3·3H2O (0.01 mmol, 2.6 mg), PPh3 (0.1 mmol,
26 mg), base (3.5 mmol), methanol (3 mL), CO2 (4 MPa), H2 (4 MPa),
16 h. b Refers to HCOO−BH+, determined by 1H NMR using 1,1,2,2-
tetrachloromethane as internal standard, TON = turnover number:
moles of formate per mole of Rh catalyst. c PEI600 (0.5 mmol, 0.3 g).
d pKa values in water, and values in DMSO are given in brackets. e Ref.
14a. f Ref. 14b. g Ref. 14c. h Ref. 14d. i Ref. 14e. j Ref. 9.
Table 2 Screening phosphine ligandsa
Entry
Ligand
TONb
Entry
14
Ligand
TONb
24
1c
2
3
—
7
465
138
PPh3
PnBu3
4
5
6
PCy3
(2-Furyl)3P
PPhEt2
32
0
447
15
13
Subsequently, hydrogenation of gaseous CO2 was initiated
by employing a concise and well-documented RhCl3·3H2O/
PPh3/base catalytic system in methanol. PEI600 showed excel-
lent performance at 60 °C, better than typical bases9,14 used in
previous reports,15 including primary amine (monoethanol-
amine), secondary amine (diethanolamine, Et2NH and
morpholine), tertiary amine (triethanolamine, DABCO (triethy-
lenediamine), TMEDA (N,N,N′,N′-tetramethylethylenediamine),
HMTA (hexamethylenetetramine)), quaternary ammonium salt
(nBu4Br) and inorganic base e.g. KOH (Table 1, entry 1 vs.
entries 2–11). Accordingly, base strength and solubility limit-
ation could have an impact on the reaction outcome.9 More-
over, increasing the reaction temperature would cause the
catalytic activity to sharply drop (entries 1, 12, 13), whereas the
temperature below 40 °C also gave an inferior result (entry 14
vs. 1). That is probably because hydrogenation of CO2 to
HCOOH is an exothermic reaction,16 thus, increasing temp-
erature is not beneficial to the reaction. Meanwhile, higher
temperature could also be detrimental to PEI600 and the cata-
7
8
9
PPhCy2
(o-Tolyl)PPh2
(p-Tolyl)PPh2
80
24
398
16
17
18
108
n.d.
14
10
PPh2Cy
PPh2Cy
PPh2Cy
481
542
852
11c
12c,d
13
n.d.
a Conditions: RhCl3·3H2O (0.01 mmol, 2.6 mg), ligand (0.1 mmol),
PEI600 (0.3 g), methanol (3 mL), CO2 (4 MPa), H2 (4 MPa), 60 °C, 16 h.
b Refers to HCOO−BH+, determined by 1H NMR. Formic acid was
separated by distillation and the characterization charts are given in
the ESI.† c RhCl3·3H2O (0.005 mmol, 1.3 mg), ligand (0.05 mmol,
13 mg). d 32 h.
The results stimulated us to further study the phosphine
lyst system leading to more side reactions at the expense of ligand effect. CO2 hydrogenation almost did not occur in the
product selectivity. Hence, when the temperature was absence of phosphine ligand (entry 1, Table 2). Electron-donat-
increased to 60 and 100 °C, the yields sharply dropped. On the ing phosphine ligands were firstly screened because they could
other hand, higher temperature could facilitate the reaction. favor the in situ formation of the catalytic species.17 In this
Therefore proper temperature is needed to run the reaction. study, PPh3 gave a better result than PnBu3, PCy3, (2-furyl)3P
Indeed, 40 °C gave an inferior result and 60 °C showed the (entry 2 vs. entries 3–5). CyPPh2 showed the highest activity
best result.
among the alkylphenylphosphines, tentatively being attributed
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to its appropriate steric and electric effect (entry 10 vs. entries
Furthermore, direct hydrogenation of several ammonium
6–9). To our delight, a TON of up to 852 can be obtained carbamates derived from CO2, e.g. DETA+CO2 (DETA = diethy-
−
through roughly optimizing the reaction conditions lenetriamine), ammonium carbonates such as [DBNH]
t
(entries 11–12). The sterically hindered ligand Bu-XPhos was [OCO2(C2H4O)3H], [DBNH] [OCO2CH2OH] (DBN = 1,5-diaza
inactive (entry 13), while other bidentate and multidentate bicycle[4.3.0]non-5-ene), was also successfully performed
ligands such as BINAP, pentaphenyltriphosphan, pyphos, (entries 2–6, Table 3). It is worth mentioning that a TON of
tBuDavePhos, and terPy exhibited inferior activity in compari- 726 can be attained (entry 6). To a certain extent, increasing
son with the monodentate ligands (entries 14–18). This is pre- the amount of the absorbent and the base gave better results
sumably because the structural flexibility of less hindered (entry 3 vs. 2, 6 vs. 5). On the other hand, the ammonium car-
monophosphines could facilitate the ligand exchange to offer bonate showed a slightly higher activity than the ammonium
free sites for coordination with metal. In addition, other metal carbamate by using an equal amount of absorbent (entry 5 vs.
salts were also examined with PEI600/CyPPh2/methanol system. 3, Table 3). In addition, aniline bicarbonate can somehow be
RhCl3·3H2O gave the best results among RuCl3·6H2O, PdCl2, hydrogenated under the present conditions although there is
Rh(acac)3, NiCl2·6H2O and Fe(BF4)2·6H2O (Table S1, ESI†).
solubility limitation (entry 7). Notably, the captured CO2 can
We next investigated hydrogenation of the captured facilitate the hydrogenation and thus gave a higher reaction
CO2 using PEI600/RhCl3·3H2O/CyPPh2 system with non-volatile rate and better results (entry 8 vs. 1) in comparison with equi-
ethylene glycol as a solvent. PEI600 in ethylene glycol can valent free gaseous CO2, probably implying CO2 activation
absorb gaseous CO2 smoothly to afford the PEI-CO2 adducts upon capture with PEI (Fig. S4, ESI†).
i.e. 1 and 2 (Scheme 1) with a capacity of 0.159 g CO2/0.3 g
In situ FT-IR under CO2 pressure and NMR study were per-
PEI600. Then, the hydrogenation reaction proceeded with the formed to gain deeper insight into the one-pot process for CO2
absorbed mixture after the consecutive introduction of capture and subsequent hydrogenation of the captured CO2.
RhCl3·3H2O, CyPPh2 and hydrogen. Formate with regard to the The appearance of the carbonyl peak at 1676 cm−1 supports
PEI-CO2 adduct was obtained in 37% yield (Table 3, entry 1). the formation of ammonium carbamate upon uptaking CO2
To the best of our knowledge, this is the first example of the in (Fig. 1). Intensity changes at 1950, 2000–2079 cm−1, and
situ hydrogenation of captured CO2 into an energy-storage characteristic peaks at 1579 and 1353 cm−1 suggest the gen-
material rather than going through the desorption step of con- eration of the formato-Rh complex during the hydrogenation
ventional CCS.
step. Broad absorptions from 1776 to 1810 cm−1 could be sug-
gestive of formic acid and derivatives, indicating the target
product formation.18
a
Table 3 Hydrogenation of the captured CO2
Furthermore, the 1H NMR spectrum also exhibits character-
istic signals19 of Rh–H species at −17.76 (d, JRh–H = 108 Hz)
and −19.18 ppm (d, JRh–H = 92 Hz) (Fig. 2). The cyclohexyl
protons shift upfield upon the addition of PEI600, illustrating
that PEI600 could weaken the coordination between Rh and
CyPPh2 (Fig. S5, ESI†). 31P NMR study implies ligand exchange
of phosphine with PEI600 (Fig. S6, ESI†). Furthermore, N–H
peaks in in situ FT-IR move to the lower frequency region upon
Captured CO2
(mmol)
Yieldc
(%)
Entry
Substrate
TONb
1d
2e
PEI600-CO2/glycol
3.6
5.6
260
126
37
12
3f
4g
5h
9.7
5.5
5.5
291
117
500
15
11
46
6i
6.6
0.6
726
22
55
19
7j
8k
CO2 (gas)
3.6
168
23
a Conditions: RhCl3·3H2O (0.005 mmol, 1.3 mg), CyPPh2 (0.05 mmol,
13 mg), methanol (2 mL), H2 (4 MPa), 60 °C, 16 h. b Refers to
HCOO−BH+, determined by 1H NMR. c Relative to the amount of
captured CO2. d PEI600 (0.5 mmol)/glycol (18 mmol). e DETA
(2.9 mmol)/glycol (18 mmol). f DETA (5 mmol)/glycol (31 mmol). g DBN
(5 mmol)/triethylene glycol (6 mmol).3a h DBN (5 mmol)/glycol
(6 mmol). i DBN (6 mmol)/glycol (7.2 mmol). j Aniline/H2O (5/
56 mmol). k CO2 (3.6 mmol, 0.2 MPa) with fresh PEI600 (0.5 mmol,
0.3 g), glycol (18 mmol, 1 mL).
Fig. 1 Monitoring of the reaction process by in situ FT-IR under CO2 pressure.
Conditions: RhCl3·3H2O (0.03 mmol, 7.9 mg), CyPPh2 (0.3 mmol, 80 mg), PEI600
(1.5 mmol, 0.9 g), methanol (9 mL), H2 (4 MPa), CO2 (4 MPa), 60 °C, 13 h. The
spectrum of PEI600 was subtracted. 2340 cm−1 can be assigned to the gaseous
CO2 peak.
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Fig. 2 1H NMR spectrum of the reaction mixture showing the formation of the
Rh–H species (400 MHz, CDCl3). Reaction conditions: RhCl3·3H2O (0.01 mmol,
2.6 mg), CyPPh2 (0.1 mmol, 26 mg), PEI600 (0.5 mmol, 0.3 g), methanol (2 mL),
H2 (4 MPa), CO2 (4 MPa), 60 °C.
Scheme 2 Proposed pathways of carbon capture and subsequent hydrogen-
ation of the captured CO2.
the formation of the intermediate [Rh(η2-OCHO)] 4. The intra-
proton transfer along with the cleavage of the C–N or C–O
bond, and subsequent hydride transfer to the carbonyl group
could result in the formation of complex 5. Finally, oxidative
addition of H2 to complex 5 gives the dihydride intermediate
6, and HCOO−BH+ is produced via reductive elimination with
regenerating the Rh–hydride species 3 to complete the catalytic
cycle.
Conclusions
In summary, the present PEI/RhCl3·3H2O/CyPPh2 sequential
protocol provides a successful example for CO2 capture and
subsequent hydrogenation of the absorbed CO2, validating the
CCU strategy to supply energy products as a part of a carbon
neutral cycle. CO2 is likely captured by PEI to form the carba-
mate and/or carbonate species, which can sequentially be
hydrogenated to formate. The reaction was studied by NMR
and in situ FT-IR spectroscopy under CO2 pressure. This
process could avoid energy input for the desorption step, and
could open up great potential for transforming the captured
CO2 from waste to fuel-related products.
Fig. 3 In situ FT-IR spectrum of the mixture in the absence of CO2 and hydro-
gen. Conditions: RhCl3·3H2O (0.02 mmol, 5.2 mg), CyPPh2 (0.2 mmol, 53 mg),
PEI600 (1 mmol, 0.6 g), methanol (6 mL), 1.5 h. The spectrum of the mixture of
CyPPh2 and CH3OH was subtracted. Peaks a and b correspond to the stretching
and bending vibrations of N–H, respectively.
the introduction of RhCl3·3H2O, indicating the possible
coordination of PEI600 with Rh (Fig. 3).20
Acknowledgements
Based on the experimental results in this study and recent
reports21 on the hydrogenation of CO2 derivatives including
carbamates, carbonates, ureas, and amides, the proposed
pathway for the combined process of carbon capture with
in situ hydrogenation is shown in Scheme 2. Although gaseous
CO2 can be hydrogenated to formic acid via a well documental
mechanism, the present process likely proceeds through initial
capture and subsequent hydrogenation of the captured
species, i.e. ammonium carbamate 1 and ammonium methyl-
carbonate 2. Decoordination of PEI from Rh–hydride species 3
in situ formed from RhCl3, phosphine and PEI, could provide
two free sites for the coordination of 1 or 2 with Rh leading to
This work was financially supported by the National Natural
Sciences Foundation of China (No. 21172125, 21121002), the
“111” Project of the Ministry of Education of China
(No. B06005), and Tianjin Co-Innovation Center of Chemical
Science and Engineering.
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