Hydrogenation of Carbon Dioxide to C2–C4 Hydrocarbons Catalyzed by Pd(PtBu3)2–FeCl2 with Ionic Liquid as Cocatalyst

Article abstract of DOI:10.1002/cssc.201901820

Direct hydrogenation of CO₂ to C₂₊ hydrocarbons is very interesting, but achieving this transformation below 200 °C is challenging and seldom reported. Herein, a homogeneous catalytic system was developed composed of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF₆]), Pd(PtBu₃)₂, FeCl₂, and the ligand 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) for hydrogenation of CO₂ under mild conditions, which resulted in C₂–C₄ hydrocarbons in selectivities up to 98.3 C-mol % at 180 °C. The combination of [BMIm][PF₆]) with Xantphos endowed the Pd–Fe catalysts with the ability of activating CO₂ and H₂ simultaneously via [HPd(PtBu₃)(BMIm-COO)(BMIm)(PF₆)Fe]⁺ species, thus catalyzing the formation of C₂–C₄ hydrocarbons through CO₂ hydrogenation. In addition, this catalytic system is stable and recyclable, which may have promising applications.

Full text of DOI:10.1002/cssc.201901820

Accepted Article
Title: Hydrogenation of Carbon Dioxide to C₂-C₄ Hydrocarbons
Catalyzed by Pd(PtBu₃)₂-FeCl₂ with Ionic Liquid as a Cocatalyst
Authors: Zhimin Liu, Huan Wang, Yanfei Zhao, Yunyan Wu, Ruipeng
Li, Hongye Zhang, Bo Yu, Fengtao Zhang, Junfeng Xiang,
and Zhenpeng Wang
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To be cited as: ChemSusChem 10.1002/cssc.201901820
Link to VoR: http://dx.doi.org/10.1002/cssc.201901820
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10.1002/cssc.201901820
ChemSusChem
COMMUNICATION
Hydrogenation of Carbon Dioxide to C2-C4 Hydrocarbons
Catalyzed by Pd(PtBu3)2-FeCl2 with Ionic Liquid as a Cocatalyst
Huan Wang,[a, b], ‡ Yanfei Zhao,[a], ‡ Yunyan Wu,[a, b] Ruipeng Li,[a, b] Hongye Zhang,[a] Bo Yu,[a] Fengtao
Zhang, [a, b] Junfeng Xiang, [a] Zhenpeng Wang[a] and Zhimin Liu*[a,b,c]
Abstract: Direct hydrogenation of CO2 to C2+ hydrocarbons is very
interesting, but achieving this transformation below 200 °C is
challenging and seldom reported. Herein, we report a homogeneous
catalytic system composed of ionic liquid, 1-butyl-3-methylimidazol-
ium hexafluorophosphate ([BMIm][PF6]), Pd(PtBu3)2, FeCl2 and
ligand 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos)
for hydrogenation of CO2 under mild conditions, which can realize
the CO2 hydrogenation to C2-C4 hydrocarbons in selectivity up to
98.3 C-mol% at 180 °C. The combination of [BMIm][PF6] with
Xantphos endows the Pd-Fe catalysts to be capable of activating
CO2 and H2 simultaneously via forming [HPd(PtBu3)(BMIm-
COO)(BMIm)(PF6)Fe]+ species, thus catalyzing the formation of C2-
C4 hydrocarbons from CO2 hydrogenation. In addition, this catalytic
system is stable and recyclable, which may have promising
applications.
on the CO2 hydrogenation to C2−C4 hydrocarbons over
homogeneous catalysts in literature survey. The
a
hydrogenation of carbon monoxide (CO) to light hydrocarbons
has been widely investigated using homogeneous and
heterogeneous catalysts.[6] Ir4(CO)12 was reported to be capable
of catalyzing the hydrogenation CO into methane (CH4), ethane
and trace amounts of propane and isobutene in molten NaCl-
2AlCl3 mixture, and AlCl3 was found to promote the carbon chain
growth through catalyzing the formation of CH3Cl and CH3CH2Cl
intermediates.[6b-c]
Ionic liquids (ILs) are a kind of salts composed totally of
organic cations and inorganic/organic anions with high thermal
and chemical stability, extremely low vapor pressure,
nonflammability and tunable properties.[7] Their unique features
allow them to be widely applied in many fields, especially in CO2
transformation as effective media.[8] For example, imidazolium-
based ILs combined with Ru catalysts realized the CO2
hydrogenation to formic acid,[8a] CO,[8b] and CH4,[8c] respectively.
The ILs generally played multiple roles in the reaction process
with activating CO2, stabilizing intermediates, and/or mediating
the activity of the metal catalysts. Though much progress has
been made, the IL-mediated CO2 hydrogenation to
hydrocarbons has been reported rarely in a literature survey.[5, 8c]
Carbon dioxide (CO2) is a nontoxic, abundant, easily available,
and renewable C1 resource, [1] and its hydrogenation into fuels
and chemicals is of great significance for the sustainable
development. C2-C4 hydrocarbons are the crucial chemical
feedstocks for synthesizing value-added products such as
detergents, polymer materials and drugs.[2] The selective
hydrogenation of CO2 to C2-C4 hydrocarbons is a promising way,
but it imposes major technological challenges due to
thermodynamic stability and kinetic inertness of CO2 and suffers
from low selectivity towards to C2-C4 hydrocarbons.[3] So far,
some heterogeneous catalysts have been developed for
hydrogenation of CO2 to C2-C4 hydrocarbons.[4] For instance,
iron catalysts combined with K, Na, Ru, Zr promoters could
realize this kind of transformation through reverse water gas
reaction (RWGS) and Fischer–Tropsch synthesis (FTS) at 300–
[4b, 4d-f]
400 °C.
Co–Fe bimetallic catalysts could catalyze the
reaction at lower temperature (e.g. 270 °C), achieving C2+
hydrocarbons selectivity up to 69%.[4a] In most cases, the CO2
hydrogenation to C2−C4 hydrocarbons was performed at
temperature higher than 250 °C, and that at temperature lower
than 200 °C was seldom reported.[5] To date, there is no report
Scheme 1. Synthesis of hydrocarbons from CO2 hydrogenation.
Herein, we report the IL-promoted CO2 hydrogenation to C2-C4
hydrocarbons over Pd(PtBu3)2-FeCl2 homogeneous catalysts in
the presence of ligand, 4,5-bis(diphenylphosphino)-9,9-dimethy-
lxanthene (Xantphos) at relatively low temperature (Scheme 1).
In this protocol, the combination of 1-butyl-3-metylimidazolium
hexafluorophosphate ([BMIm][PF6]) with Xantphos modified the
electronic states of Pd and Fe catalysts via forming active
species including [HPd(Xanphos)]+, [Pd(Xanphos)(BMIm-COO)]+,
[Pd(Xanphos)Cl]+ and [HPd(PtBu3)(BMIm-COO)(BMIm)(PF6)F-
e]+), which realized the activation of CO2 and H2 simultaneously,
thus accomplishing the CO2 hydrogenation to C2-C4
hydrocarbons (including ethane, propane and n-butane) at
180 °C. With a gas mixture (H2/CO2 = 2/1) at 9 MPa, a yield of
1.08 C-mmol was achieved, together with the selectivity towards
C2-C4 hydrocarbons up to 98.3 C-mol%, much higher than those
obtained over the catalysts reported in literature.[4] In addition,
[a]
H. Wang, Dr. Y. Zhao, Y. Wu, R. Li, Dr. H. Zhang, Dr. B. Yu, F.
Zhang, Dr. J. Xiang, Dr. Z. Wang and Prof. Dr. Z. Liu
Beijing National Laboratory for Molecular Sciences, Key Laboratory
of Colloid, Interface and Thermodynamics, CAS Research/Education
Center for Excellence in Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
E-mail: liuzm@iccas.ac.cn
[b]
[c]
[‡]
H. Wang, Y. Wu, R. Li, F. Zhang and Prof. Dr. Z. Liu
University of Chinese Academy of Sciences, Beijing 100049, China.
Physical Science Laboratory, Huairou National Comprehensive Science
Center
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901820
ChemSusChem
COMMUNICATION
the catalytic system was stable and reusable.
itions. However, no product was obtained (Table 1, entry 5),
indicating that DMI inhibited the catalytic activity of Pd-Fe
catalyst in this reaction system. Though [EMIm][PF6] was
effective for the CO2 hydrogenation, the yield and the selectivity
of C2-C4 hydrocarbons were much lower than those achieved in
[BMIm][PF6]. The above results indicated that the cooperation of
cation and anion of the IL was responsible for their activity to
promote the CO2 hydrogenation to C2-C4 hydrocarbons.
Various ILs and solvents were examined in the hydrogenation
of CO2 catalyzed by Pd(PtBu3)2 and FeCl2 in the presence of
Xantphos ligand, and the results are listed in Table 1 and Table
S1. It is clear that the ILs significantly influenced the reaction.
[BMIm][PF6], [EMIm][PF6] and [BMIm][Cl] were effective for CO2
hydrogenation, and [BMIm][PF6] afforded C2-C4 hydrocarbons
(including ethane, propane and n-butane) as the main products
with the selectivity up to 98.3 C-mol%, while [BMIm][Cl] offered
methanol as the main product accompanied with CH4 (Table 1,
entries 1-3, figures S1-S3). The differences in selectivity
observed between the [BMIm][Cl]- and [BMIm][PF6]-based
systems may be ascribed to the differences in their interactions
with water. The hydrophobicity of [BMIm][PF6] can render the
water formed in the reduction of CO2 to be expelled from the
catalytic active sites, which may be favorable to the C-C
coupling to form C2+ products.[5] While for hydrophilic [BMIm][Cl],
the formed water dissolves in the reaction system, which may
influence the reduction of CO2, preferentially forming methanol.
[BMIm][NTf2] was ineffective for the CO2 hydrogenation without
product detectable (Table 1, entry 4). For comparison, a weak
Lewis base solvent, 1,3-dimethyl-2-imidazolidinone (DMI) that
was used in CO2 hydrogenation to acetic acid, higher alcohols
and other hydrocarbons,[8d,9] were tested under the similar cond-
To explore the roles of other components in the CO2
hydrogenation, several control experiments were performed, and
the results were listed in Table 1 (Entries 6-10). It was indicated
that all Pd(PtBu3)2, FeCl2, Xantphos ligand and [BMIm][PF6]
were necessary for the production of C2-C4 hydrocarbons from
CO2 hydrogenation. Without Xantphos ligand or FeCl2, the CO2
hydrogenation in IL produced CO and CH4 with small
productivity, while the reaction did not occur in the absence of
Pd catalyst. For comparison, Pd/C or other bidentate phosphine
ligands including dppp (1,3-bis(diphenyphosphino)propane) and
dppb (1,4-bis(diphenylphosphino)butane) were examined as well,
however, only trace amount of CO were detected (Table S2,
entries 1-3). From the above results, it can be deduced that
each component of the designed catalytic system played vital
roles in the formation of C2-C4 hydrocarbons. Furthermore, the
catalytic system was treated with H2 or N2 without CO2 under the
similar conditions, respectively, and no products were detected
in both cases (Table 1, entries 11 and 12), which excluded the
possibility of the C2-C4 hydrocarbons originated from the organic
components of the catalytic system. Thermogravimetric analysis
(TGA) results indicated that [BMIm][PF6] was stable in air up to
Table 1. The hydrogenation of CO2 to hydrocarbons over different catalytic
systems.[a]
Entry
IL
Product selectivity
(C-mol%)
Yield[l]
(C-mmol)
CO
0.7
4.94
0
CH4
C2-C4
MeOH
o
390 C (Figure S4). 19F and 31P NMR of catalytic system before
1
[BMIm][PF6]
[EMIm][PF6]
[BMIm][Cl]
[BMIm][NTf2]
DMI
1
98.3
90
0
0
0
77
0
0
0
0
0
0
0
0
0
0
0
1.08
0.09
0.28
0
and after reaction also indicated that [PF6]- was stable under the
experimental conditions (Figure S5). In addition, five catalytic
cycles of CO2 hydrogenation were conducted under the same
conditions, and the yield of C2-C4 hydrocarbons almost remained
unchanged at the fifth cycle, indicating that the catalytic system
was stable and recyclable (Figure S6). IR of the remaining
catalytic solution indicated that no formates and oxalates were
2
5.6
23
0
3
4
0
0
5[b]
6[c]
7[d]
8[e]
9[f]
0
0
0
0
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
[BMIm][PF6]
28
64
0
72
36
0
0
0.02
0.05
0
0
0
0
0
0
0
10[g]
11[h]
12[i]
13[j]
14[k]
57
0
43
0
0
0.04
0
0
0
0
0
0
-
39.1
-
60.9
0
0.11
0
0
[a] Reaction conditions: Pd(PtBu3)2 (40 μmol), FeCl2 (40 μmol), Xantphos (80
μmol), IL (0.5 mL), CO2 (3 MPa) and total (9 MPa) at room temperature, 180
oC, 12h. [b] DMI (0.5 mL). [c] Without Xantphos. [d] Without FeCl2. [e] Without
Pd(PtBu3)2. [f] Without Pd(PtBu3)2 and Xantphos. [g] Without FeCl2 and
Xantphos. [h] Only H2 (9 MPa) was used as the reactant. [i] Only N2 (3 MPa)
was used. [j] CO (3 MPa) and total (9 MPa). [k] CH4 (3 MPa) and total (9 MPa).
[l] Yield denotes the total.
Figure 1. (A) XPS spectra of Pd 3d: a) Pd(PtBu3)2; b) Pd-L; c) Pd-L-Fe. (B)
XPS spectra of Cl 2p. (C) XPS spectrum of Pd 3d: Pd-L-Fe-F. (D) Fe L-edge
soft XANES spectra: e) FeCl2; f) Fe-L; g) Pd-L-Fe; h) Pd-L-Fe-F.
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10.1002/cssc.201901820
ChemSusChem
COMMUNICATION
produced in the reaction process (Figure S7).
(Figure S8, k).[13] However, the L3 edge of Fe-L-Pd (Figure 1D, g)
shows two main peaks at 708.1 and 709.5 eV, revealing that Fe
was in two different chemical environments. While, in the spectr-
um of Pd-L-Fe-F the intensity ratio between the second and the
first peaks reached up to 66.7/33.3, which suggested the
complexed chemical interactions among Fe, Pd, Xantphos and
[BMIm][PF6] (Figure1D, h).
From the above results, it is obvious that Pd(PtBu3)2 was the
main catalyst for the CO2 hydrogenation, and other components
including Xantphos, FeCl2 and [BMIm][PF6] were necessary for
the production of C2-C4 hydrocarbons in high selectivity. In order
to explore the influences of every component on the activity of
Pd(PtBu3)2 in the reaction process, three samples made of
Pd(PtBu3)2 with Xantphos ligand (denoted as Pd-L), with
Xantphos and FeCl2 (denoted as Pd-L-Fe), and with Xantphos,
FeCl2 and [BMIm][PF6] (denoted as Pd-L-Fe-F) were,
respectively examined by means of X-ray photoelectron
spectroscopy (XPS). In the Pd 3d spectrum of Pd(PtBu3)2
(Figure S8 a), two pairs of peaks with binding energies (BEs) at
335.24, 340.49 and 337.19, 342.38 eV, are assigned to Pd0 and
Pd2+.[10] Compared to that of Pd(PtBu3)2, the spectra of Pd-L, Pd-
L-Fe and Pd-L-Fe-F changed accordingly, which indicate that all
Xantphos, FeCl2 and [BMIm][PF6] can influence the electronic
environment and coordination of Pd (Figure 1 A and C, figure S8
b-c). Figure 1A illustrates that the BEs of Pd0 and Pd2+ of Pd-L
and Pd-L-Fe shifted to smaller values in sequence compared to
those of Pd(PtBu3)2. Furthermore, the P 2p (Figure S8 d-g) and
Cl 2p XPS spectra (Figure 1B) also revealed that the P atoms of
Xantphos and PF6- and Cl- from FeCl2 coordinated with the Pd
and Fe atoms. The above results suggest that Pd(PtBu3)2 was
activated by Xantphos and Cl-, and thus apt to coordinate with
the imidazole ring of [BMIm][PF6], supported by the N1s XPS
analysis (Figure S8 h and i). As illustrated in Figure 1C, the Pd
3d spectrum of Pd-L-Fe-F can be deconvoluted into three pairs
of peaks, which are assigned to 3d5/2 and 3d3/2 signals for Pd0
(335.15 and 340.43 eV), Pd+(336.76 and 342.26 eV), and
Pd2+(337.72 and 343.0 eV), respectively.[10] Here, it is worthy
pointing out that the Pd+ species appeared in Pd-L-Fe-F, which
may result from the Pd-π coordination between imidazole ring
and the Pd catalyst.[11a] In addition, a signal of Pd5/2 was
observed at 336.76 eV, which was corresponded to Pd-F bond
(Figure 1C and Figure S8 j).[11b] The XPS analysis indicates that
the modification of Pd(PtBu3)2 with [BMIm][PF6] and Xantphos
altered the electronic state of Pd species, and Pd-L-Fe-F may
show activity different from Pd(PtBu3)2.
To elucidate ionic species in the reaction process, the reaction
solution was analyzed by means of ESI-MS. As shown in Figure
2
and Figure S9, Pd species including [HPd(Xanphos)]+,
[Pd(Xanphos)(BMIm-COO)]+ and [Pd(Xanphos)Cl]+ were
detected in the ESI-MS(+) spectrum, suggesting that under the
experimental conditions Pd species were mainly in the forms of
Pd0, Pd+, Pd2+ complexed with Xantphos, carbene species
([BMIm-COO]) and Cl-, although much larger amount of PF6
-
than Cl- was present in the catalytic system. This is consistent
with the results of XPS analysis. Moreover, the Pd-Fe bimetal
species, [HPd(PtBu3)(BMIm-COO)(BMIm)(PF6)Fe]+, were also
detected. These results indicated that the in situ generated
complex was mainly in the form of Pd0- and Fe2+-complexed with
-
PtBu3, [BMIm-COO], [BMIm]+ and PF6 , which could activate CO2
and H2 simultaneously[14] and may be responsible for catalyzing
the CO2 hydrogenation to C2-C4 hydrocarbons at relatively low
temperature (e.g., 180 oC). From the above findings, it can be
deduced that the high activity of the catalytic system was
attributed to the synergetic effect from the active metal species
stabilized by [BMIm][PF6] combined with the Xantphos ligand.
Scheme 2. Proposed mechanism of the reaction over the Pd-Fe-ILs catalytic
system.
To explore the possible reaction pathway of CO2
hydrogenation to C2-C4 hydrocarbons, the hydrogenation of CO
and CH4 were performed over the same catalytic system under
the similar conditions, respectively. It was revealed that C2-C4
hydrocarbons could be obtained from CO hydrogenation with
low activity and selectivity, while no product was detected in the
case of CH4 (Table 1, entries 13 and 14). Consequently, it can
be deduced that the formation of C2-C4 hydrocarbons from CO2
hydrogenation may be undergo the CO pathway. Moreover, the
reaction solution of CO2 hydrogenation in [BMIm][PF6] was
Figure 2. HR-ESI-MS spectrum of the reaction solution. Reaction conditions:
Pd(PtBu3)2 (80 μmol), FeCl2 (80 μmol), Xantphos (160 μmol), [BMIm][PF6] (2
mmol), CO2 (3 MPa) and total (9 MPa) at room temperature, 180 C, 3h.
The Fe L-edge soft XANES spectra of FeCl2 together with Fe-
L (composed of FeCl2 and Xantphos), Pd-L-Fe and Pd-L-Fe-F
were collected, as shown in Figure 1D (e and f). The distinct
splitting of L3-edge with a small post-peak (~709.9eV) and a
main peak (~707.9eV) indicated the presence of only Fe2+ form
in FeCl2,[12] consistent with the results of Fe 2p XPS analysis
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10.1002/cssc.201901820
ChemSusChem
COMMUNICATION
examined by GC-MS (Figure S10). Trace amounts of CH3Cl and
CH3CH2Cl were detected, which are possible intermediates to
[3] a) J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Xu,
Nat Commun. 2017, 8, 15174; b) R. W. Dorner, D. R. Hardy, F.
W. Williams, H. D. Willauer, Energy Environ. Sci. 2010, 3, 884-
890.
form CH4, ethane, propane and n-butane.[6c] Based on the
experimental results and previous reports,[6c],
a possible
[15]
reaction pathway is proposed as shown in Scheme 2. Firstly, CO
is generated via Pd* catalyzed CO2 hydrogenation (Step 1), [16]
which is then coordinated with Pd* to form the Pd*CO complex
(Step 2). The HPd*CHO complex may be formed via the
hydrogenation of Pd*CO complex (Step 3), which further reacts
with H2 and Cl- to convert into CH3Pd*Cl promoted by Fe* (Step
4). Subsequently, CO inserts into the CH3-Pd* bond, which
easily occurs,[17] generating CH3COPd*Cl (Step 5). Followed by
hydrogenation, CH3CH3 is formed (Step 6). The generation of
propane and n-butane from CH3CH2Cl maybe follow the similar
steps. In this work, the Fe catalyst mainly promotes the
formation of CH3Cl and CH3CH2Cl intermediates,[6c] while the Pd
catalyst is responsible for hydrogenation (Table 1, entry 8).
[BMIm][PF6] and ligand (Xantphos) play vital roles in modifying
the electronic states of Pd and Fe catalysts and stabilizing the
metal-based active species (Table 1, entries 1 and 3).
In summary, [BMIm][PF6] could promote the selective
hydrogenation of CO2 over the Pd-Fe homogeneous catalysts,
yielding C2-C4 hydrocarbons including ethane, propane and n-
butane in selectivity up to 98.3 C-mol% and yield as high as 1.08
C-mmol at 180 C. The catalytic system could be recycled and
reused for at least 5 times without activity loss. The IL and
Xantphos ligand complexed with Pd and Fe to form active
species, which could activate CO2 and H2 simultaneously,
further catalyzing the CO2 hydrogenation to C2-C4 hydrocarbons.
We believe that this kind of catalytic system may find promising
applications in the transformation of CO2 to C2+ alcohols/hydroc-
arbons under mild conditions. Further work is under way in our
group.
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Acknowledgements
The work was supported by National Key Research and
Development Program of China (2018YFB0605801)，National
Natural Science Foundation of China (21890761, 21533011,
21773266), Chinese Academy of Sciences (QYZDY-SSW-
SLH013-2), and Beijing Municipal Science
Commission (Z181100004218004).
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Keywords: ionic liquid • CO2 hydrogenation • Pd–Fe homogeneous
catalysis • C2-C4 hydrocarbons synthesis
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10.1002/cssc.201901820
ChemSusChem
COMMUNICATION
Entry for the Table of Contents
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COMMUNICATION
Huan Wang,[a, b],+ Yanfei Zhao,[a],+
The [BMIm][PF6]-based Pd(PtBu3)2-
FeCl2 homogeneous catalytic system
can realize CO2 hydrogenation to C2-C4
hydrocarbons under mild conditions
(e.g., 180 oC).
Yunyan Wu,[a, b] Ruipeng Li,[a, b] Hongye
Zhang,[a] Bo Yu,[a] Fengtao Zhang, [a, b]
Junfeng Xiang, [a]Zhenpeng Wang,[a] and
Zhimin Liu [a, b],
*
Page No. – Page No.
Hydrogenation of Carbon Dioxide to C2-
C4 Hydrocarbons Catalyzed by
Pd(PtBu3)2-FeCl2 with Ionic Liquid as a
Cocatalyst
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