Continuous-flow hydrogenation of carbon dioxide to pure formic acid using an integrated scCO2 process with immobilized catalyst and base

Article abstract of DOI:10.1002/anie.201203185

Dual role for CO2: Pure formic acid can be obtained continuously by hydrogenation of CO2 in a single processing unit (see scheme). An immobilized ruthenium organometallic catalyst and a nonvolatile base in an ionic liquid (IL) are combined with supercritical CO2 as both reactant and extractive phase.

Full text of DOI:10.1002/anie.201203185

Angewandte
Chemie
DOI: 10.1002/anie.201203185
CO2 Hydrogenation
Continuous-Flow Hydrogenation of Carbon Dioxide to Pure Formic
Acid using an Integrated scCO2 Process with Immobilized Catalyst and
Base**
Sebastian Wesselbaum, Ulrich Hintermair, and Walter Leitner*
The use of carbon dioxide as carbon resource provides an
attractive option in the increasing efforts to reduce the carbon
footprint at the interface between the energy and the
chemical sector.[1] The hydrogenation of CO2 to formic acid
(HCO2H) has been studied intensively in this context.[2] This
transformation offers direct access to chemical products
based on waste material from the energetic use of fossil
fuels, opens a possible route to convert CO2 into CO, and is
discussed as a potential option for hydrogen storage. Homo-
geneous catalysts for the addition of hydrogen to CO2 have
been developed since the mid-seventies, and remarkable
progress in terms of catalyst activity has been achieved.[2]
However, all currently known systems produce salts, adducts,
or derivatives of formic acid because the formation of pure
formic acid from CO2 and H2 is strongly disfavored by entropy
shifting the equilibrium far to the left (Scheme 1).[2a,c] Thus,
multistep downstream processes are required for removal of
the primary products from the catalyst to avoid back-reaction,
and their subsequent cleavage to obtain pure formic acid.[3,4]
Integration of molecular catalyst development with new
reaction engineering concepts is needed to address this
limitation of CO2 hydrogenation, because free HCO2H is
eventually required for most applications.
Early patents by BP and some recent studies describe
multistep processes where the initially formed azeotropic
adduct of HCO2H and NEt3 is transformed into a thermally
cleavable salt by exchange of the base.[3] Multiphasic reaction
systems have also been investigated to separate the adduct
from the catalyst, but these still necessitate downstream
purification.[4] BASF recently presented a new process using
NHex3 as product-stabilizing base with a ruthenium catalyst
in protic media.[4b,d] The resulting HCO2H/NHex3 adduct can
be separated and cleaved into free formic acid and amine by
distillation. Zhang et al. proposed a process in which an
immobilized ruthenium catalyst was used in water together
with an ionic liquid (IL) containing a tertiary amino group as
nonvolatile base.[5] After reaction, the solid catalyst is
separated by filtration, and the formic acid can be distilled
out from the basic IL. Yasaka et al. proposed the use of
imidazolium-based ILs with formate anions to stabilize
HCO2H.[6]
Herein we present a new concept that allows the
continuous-flow hydrogenation of supercritical CO2 (scCO2)
with integrated product separation from an immobilized
catalyst and stabilizing base to produce pure HCO2H in
a single processing unit.[7]
Scheme 1. Thermodynamics of CO2 hydrogenation to formic acid in
the absence and presence of base.[2c]
The concept is based on a biphasic reaction system
consisting of scCO2 as mobile phase and an IL as stationary
phase[8] containing the catalyst and a nonvolatile base
(Scheme 2). The use of scCO2 as both reactant and extractive
phase affords continuous removal of product from the
[*] S. Wesselbaum, Dr. U. Hintermair, Prof. Dr. W. Leitner
Institut fꢀr Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 1, 52074 Aachen (Germany)
E-mail: leitner@itmc.rwth-aachen.de
Homepage: http://www.itmc.rwth-aachen.de
Prof. Dr. W. Leitner
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim an der Ruhr (Germany)
Dr. U. Hintermair
Department of Chemistry, Yale University
225 Prospect Street, New Haven, CT 06520 (USA)
[**] Part of this work was performed with support of the project
CO2RRECT (01RC1006B) funded by the German Ministry of
Education and Research (BMBF) and the Government of North
Rhine-Westphalia in the research network SusChemSys (005-1112-
0002). Generous donation of QuadraPure by Johnson Matthey and
fruitful discussion with Dr. Giancarlo Franciꢁ is gratefully acknowl-
edged.
Scheme 2. Process for the direct continuous-flow hydrogenation of
CO2 to free formic acid based on a two-phase system with scCO2 as
extractive mobile phase and an ionic liquid (IL) as stationary phase
containing the catalyst and the stabilizing base.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203185.
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reactor, thereby causing the reaction equilibrium to readjust
in the reactive phase. Facile recovery of pure HCO2H can
then be achieved by simple decompression of the CO2 flow
downstream.[9] The negligible vapor pressure of the compo-
nents in the IL phase renders them virtually insoluble in
scCO2,[9] and pure HCO2H that is free of any cross-contam-
ination can be obtained without interrupting the working
catalyst inside the reactor.
We then focused on the integration of a nonvolatile base
for intermittent product stabilization. Following previous
developments,[5,6] we synthesized a set of scCO2-insoluble
organic salts containing amine functionalities (5–9) or intrin-
sic anion basicity (10) to stabilize HCO2H (Scheme 3; see the
First we identified suitable combinations of transition
metal catalysts and ionic liquid matrices in batch experiments,
starting with the standard IL 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide (EMIM NTf2) and NEt3 as
base for the stabilization of HCO2H under 100 bar total
pressure of H2/CO2 (1:1) at a reaction temperature of T=
508C (see the Supporting Information). The precursor com-
plexes were combined with monosulfonated triphenylphos-
phine (TPPMS) in metal-to-ligand ratios of between 1:3 and
1:3.5. The sodium cation of TPPMS was exchanged for
tetrabutylphosphonium (PBu4) or 1-ethyl-3-methylimidazo-
lium (EMIM) counterions to match the solvent properties of
the IL.[8] Selected results are summarized in Table 1.
Scheme 3. Nonvolatile bases tested as stationary phases for CO2
hydrogenation using catalyst 4 (PS=polystyrene).
Supporting Information). Only
compounds 5 and 9 were room-
temperature ILs, but 8 and 10
liquefied under reaction conditions.
6 and 7 had to be used as 1:1 molar
mixtures with the low-melting salt
5.
Table 1: Catalyst optimization for use in ILs with NEt3.[a]
Entry Cat.
no.
Precursor
Ligand
Additive
(equiv)
t [h] TON[b]
TOF[c] Formic
[hÀ1
]
acid/
amine[d]
1
1
[Rh(cod)acac]
EMIM
–
2
52
26 0.05
TPPMS
Catalyst system 4 led to forma-
tion of HCO2H under standard
2
3
4
5
6
7
2
2
3
3
4
5
[Ru(cod)Cl2]n
[Ru(cod)Cl2]n
[Ru(cod)(methallyl)2] PBu4TPPMS
[Ru(cod)(methallyl)2] PBu4TPPMS
[Ru(cod)(methallyl)2] PBu4TPPMS EMIMCl(2) 0.5 516
[Ru(cod)(methallyl)2] PBu4TPPMS EMIMCl(10) 0.5 545
PBu4TPPMS
PBu4TPPMS
–
–
–
–
0.5 425
16 850
0.5 314
16 1003
850 0.43
43 0.85
627 0.34
63 1.00
1032 0.52
1089 0.55
conditions
(p(CO2) = p(H2) =
50 bar, T= 508C) in all of the basic
IL phases without additional NEt3
(Table 2). Among the amine-func-
tionalized cations, the imidazolium-
based structures 5–7 gave the high-
est formic acid concentrations
(Table 2, entries 1–3). Variation of
the anions revealed a consistent
increase in TONs and TOFs follow-
[a] Conditions: T=508C, p(H2)=p(CO2)=50 bar, V(IL)=2.89 mL, V(NEt3)=1.11 mL, V(auto-
clave)=12–13 mL, c(cat.)=2 mmolmLÀ1; [b] TON=mol(HCO2H) per mol(cat.); [c] TOF=TON hÀ1
[d] mol(HCO2H) per mol(amine) after 20 h.
;
The ruthenium-based in situ catalyst systems [{Ru-
(cod)Cl2}n]/PBu4TPPMS (1:3 molar ratio; 2) and [Ru-
Table 2: Evaluation of nonvolatile bases as stationary phases for CO2
hydrogenation using catalyst 4.[a]
(cod)(methallyl)2]/PBu4TPPMS (1:3.5; 3) showed much
higher activities than the Rh-based [Rh(cod)acac]/EMIM
TPPMS (1:3) system 1 (Table 1, entries 1–5). With catalyst 3,
the amount of formic acid formed after 16 h corresponded to
a HCO2H/NEt3 ratio of 1:1 (Table 1, entry 5). Superior
activity in terms of turnover frequency (TOF) was obtained
with the chloride-containing precursor in system 2 as com-
pared to 3 (Table 1, entries 2 and 4). The deliberate addition
of chloride (2 equiv of EMIM Cl per Ru) to the catalyst
system 3 also improved the TOF by more than 50% from
627 hÀ1 to 1032 hÀ1 (Table 1, entries 4 and 6). Larger amounts
of chloride did not afford significant improvement (Table 1,
entry 7). The activity of this IL-adjusted system 4 compares
well with those of similar Ru/PAr3 catalysts in organic
solvents or water.[2] It thus provides a suitable basis for
further process development and was used in all of the
following experiments.
[c]
Entry
Nonvolatile
base
TON[b]
TOFini
Formic acid/
amine[d]
[hÀ1
]
1
2
3
4
5
6
7
5
8
9
73
25
27
126
599
1968
583
>23
>8
>12
0.05
0.02
0.03
0.10
0.54
–
5/6[e]
5/7[e]
10
>36
>314
>295[f]
>234
11[g]
0.47[h]
[a] Conditions: T=508C, p(H2)=p(CO2)=50 bar, V(nonvolatile
base)=1.0 mL, V(autoclave)=12–13 mL, c(cat.)=2 mmolmLÀ1
[b] TON=mol(HCO2H) per mol(cat.) after 20 h reaction time;
;
[c] TOF=TON hÀ1 after 2 h reaction time; [d] mol(HCO2H) per mol-
(amine) after 20 h; [e] 1:1 molar ratio; [f] after 1 h reaction time; [g] 0.5 g
basic resin in 1.0 mL EMIM NTf2; [h] calculated for a theoretical amine
capacity of the resin of 5 mmolgÀ1
.
2
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ing the order NTf2À < OTfÀ < HCO2 that parallel their
HCO2H from the amine-functionalized IL 5 proceeded more
À
increasing basicities, dielectric constants, and Kamlet–Taft
parameters b (Table 2, entries 1, 4, and 5).[10] Using the amine-
free IL 10 with a formate anion also resulted in high TONs
and TOFs (Table 2, entry 6). Notably, the TOF of 314 hÀ1
obtained in the 1:1 molar mixture of ILs 5 and 7 is one order
of magnitude higher than the highest activity previously
reported in ILs without addition of nonionic amines. The high
formic acid-to-amine ratios of up to 1:2 indicate that high
HCO2H loadings can be achieved in these media, a beneficial
property for potential extraction processes.
Immobilizing the stabilizing base on the surface of a solid
support was also shown to be effective for the first time. A
polystyrene resin functionalized with diethylamine groups
(QuadraPure-DMA, Johnson Matthey) suspended in non-
functionalized EMIM NTf2 (11) gave high initial TOFs above
200 hÀ1, together with a high formic acid-to-amine ratio of
approximately 1:2.
effectively, and after 95 h an extraction efficiency of 84% was
obtained, corresponding to initial extraction rates of at least
3.5 mghÀ1. Up to 94% mass recovery was reached after
longer times. Pure HCO2H without any cross-contamination
was extracted with scCO2 from all of the bases according to
1H NMR spectroscopic analysis of the extracts.
Based on these data, the IL-adjusted catalyst system 4 was
combined with selected nonvolatile bases as the stationary
phase for the continuous-flow CO2 hydrogenation process
with integrated product extraction. A high-pressure steel
autoclave (V= 10 mL) equipped with a window was charged
with a solution of catalyst 4 in the amine-functionalized IL 5
(V= 1.0 mL, c(cat.) = 2 mmolmlÀ1), heated to 508C and
pressurized to 200 bar with CO2. The continuous reaction
was started by passing the supercritical CO2/H2 flow (V-
(CO2) = 200 mLN minÀ1, V(H2) = 20 mLN minÀ1) through the
reactor under vigorous stirring. Extracts were collected by
passing the exiting stream through cooling traps filled with
water, which were periodically changed and analyzed by
1H NMR. The error in the determination of the HCO2H
concentration was determined to be in the range 2–10% by
calibration measurements (see the Supporting Information).
Already in the first sample after 5 h, a HCO2H concentration
corresponding to a TON of 28 was detected. The cumulative
TONs over time are depicted in Figure 2. The TON of
HCO2H produced after 211 h was 358. Pronounced catalyst
The extraction of free HCO2H from these nonvolatile
bases with scCO2 was assessed in a custom-built continuous-
flow setup with precise control over pressure, temperature,
and flow.[11] The solubility of HCO2H in scCO2 was deter-
mined to be at least 49 gLÀ1 at 508C and 100 bar. Figure 1
shows selected examples for the extraction efficiency (mass of
Figure 1. Extraction efficiency (mass of extracted HCO2H/mass of
initially added HCO2H) during extractions of HCO2H with scCO2 from
immobilized bases. Lines are only provided as a guide for the eye.
Conditions: T=508C, p=200 bar, V(IL)=1.0 mL, V-
Figure 2. Cumulative turnover numbers for catalyst 4 in IL 5 (ꢂ) and in
(CO2)=200 mLN minÀ1, V(H2)=20 mLN minÀ1, V(autoclave)=10 mL; 5
*
QuadraPure-DMA/IL (11, ) with respect to time. Lines are only
!
(ꢂ): m(HCO2H)=159 mg, acid/amine=1.2; 10 ( ): m-
provided as a guide for the eye. Conditions: T=508C, p=200 bar,
*
(HCO2H)=375 mg, acid/formate=1.3, after 26 h T=708C; 11 ( ):
V(IL)=1.0 mL, V(CO2)=200 mLN minÀ1, V(H2)=20 mLN minÀ1, V(auto-
m(resin)=0.5 g, m(HCO2H)=130 mg, acid/amine=1.1 calculated for
clave)=10 mL, c(cat.)=2 mmolmLÀ1
.
a theoretical amine capacity of the resin of 5 mmolgÀ1
.
deactivation was observed after 70 h in this experiment, which
could also be visually followed by color changes from bright
yellow to orange. The H NMR spectrum of IL 5 recovered
extracted HCO2H/mass of initially added HCO2H) over time
at initial formic acid-to-base ratios between 1.1–1.3 and
comparable scCO2/H2 flow values (see the Supporting
Information for details). Extraction from the formate IL 10
was sluggish, reaching an extraction efficiency of only 14%
after 74 h at an average rate of 0.9 mghÀ1. Increasing the
temperature from 508C to 708C did not significantly improve
the extraction rate. HCO2H extraction from the IL-suspended
resin QuadraPure-DMA 11 gave an extraction efficiency of
24% after 91 h at an average rate of 0.4 mghÀ1. Recovery of
1
from the reactor after 211 h on-stream was virtually
unchanged (see the Supporting Information), indicating
a reasonable stability of the IL under sustained reaction
conditions.
When the reactor was charged with a solution of catalyst 4
in EMIM NTf2 (V= 1.0 mL, c(cat.) = 2 mmolmLÀ1) with
QuadraPure-DMA 11 (0.5 g) suspended therein, the contin-
uous reaction conducted under the same conditions showed
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a slightly slower initial rate, but reached higher TONs of 485
after 190 h. As judged from the more constant slope of the
TON curve, catalyst stability was significantly higher with the
heterogeneous QuadraPure-DMA base, and the initial activ-
ity was largely retained even after 200 h time on-stream.
1H NMR analysis of the extracts confirmed that pure HCO2H
was extracted in both cases, and no components of the
stabilizing medium were entrained (see the Supporting
Information). Thus, separation and continuous removal of
pure HCO2H with scCO2 flow was effectively realized.
In summary, we have successfully demonstrated a new
process that allows the continuous hydrogenation of CO2 to
pure formic acid in a single process unit. This was achieved
through an integrated design and development of the macro-
scale process, the mesoscale separation strategy, and the
molecular-scale catalyst system. Careful adjustment of all
components of the stationary phase led to systems with high
activities (initial TOFs > 314 hÀ1) matching or even surpassing
the performance of similar ruthenium catalysts in conven-
tional solvents. Together with the high formic acid loadings
achieved under equilibrium conditions, these data indicate
that the rates of formic acid extraction are currently the
limiting factor under continuous-flow conditions. Therefore,
the key design parameter for future optimization of this
approach appears to be the development of matrices that
combine a sufficient stabilizing effect for formic acid during
formation with a favorable partitioning of HCO2H into the
scCO2 phase.
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Received: April 25, 2012
Revised: May 31, 2012
Published online: && &&, &&&&
Keywords: carbon dioxide fixation · formic acid ·
.
homogeneous catalysis · hydrogenation · supercritical fluids
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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Angewandte
Chemie
Communications
CO2 Hydrogenation
Dual role for CO2: Pure formic acid can be
obtained continuously by hydrogenation
of CO2 in a single processing unit (see
scheme). An immobilized ruthenium or-
ganometallic catalyst and a nonvolatile
base in an ionic liquid (IL) are combined
with supercritical CO2 as both reactant
and extractive phase.
S. Wesselbaum, U. Hintermair,
W. Leitner*
&&&& — &&&&
Continuous-Flow Hydrogenation of
Carbon Dioxide to Pure Formic Acid using
an Integrated scCO2 Process with
Immobilized Catalyst and Base
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!