Indirect CO2 Methanation: Hydrogenolysis of Cyclic Carbonates Catalyzed by Ru-Modified Zeolite Produces Methane and Diols

Article abstract of DOI:10.1002/anie.201811086

We report a ruthenium-modified zeolite which efficiently transforms propylene carbonate to propylene glycol and methane, under solvent-free conditions. The catalyst achieved high product selectivity and no significant ageing effect was observed after multiple cycles. The resulting liquid product (water-containing glycol) can be directly used as anti-freeze solution and the gas phase can directly be used as an energy carrier in the form of H₂-enriched methane. This process efficiently bridges energy storage and an important chemical synthesis under sustainable (CO₂ consuming) conditions.

Full text of DOI:10.1002/anie.201811086

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Accepted Article
Title: Indirect CO2 methanation: hydrogenolysis of cyclic carbonates
catalyzed by Ru-modified zeolite produces methane and diols
Authors: Wei-Tse Lee, Antoine Philippe van Muyden, Felix Daniel
Bobbink, Zhangjun Huang, and Paul J. Dyson
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811086
Angew. Chem. 10.1002/ange.201811086
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Angewandte Chemie International Edition
10.1002/anie.201811086
COMMUNICATION
Indirect CO
2
methanation: hydrogenolysis of cyclic carbonates
catalyzed by Ru-modified zeolite produces methane and diols
+
+
Wei-Tse Lee , Antoine P. van Muyden , Felix D. Bobbink, Zhangjun Huang, Paul J. Dyson*
Abstract: We report a ruthenium-modified zeolite which efficiently
transforms propylene carbonate to propylene glycol and methane,
under solvent-free conditions. The catalyst achieves high product
selectivity and no significant ageing effect was observed after multiple
cycles. The resulting liquid product (water-containing glycol) can be
directly used as anti-freeze solution and the gas phase can directly be
center (manganese) have been reported that transform organic
carbonates into diols and methanol in high yields (see Table
S2).[26–28]
A limitation of the aforementioned approaches is that the
methanol and glycol-products must be separated prior to further
utilization, which is an energy intensive step. Hence, we decided
to develop a catalytic process that converts cyclic carbonates
directly into methane and diols, producing value-added gaseous-
and liquid-phases that do not require further separation. To
achieve this goal, we relied on a well-established type of
heterogeneous catalyst (Ru nanoclusters stabilized by zeolites).
These types of catalysts were previously found to display
used as an energy carrier in the form of H
process efficiently bridges energy storage and an important chemical
synthesis under sustainable (CO consuming) conditions.
2
-enriched methane. This
2
The utilization of carbon dioxide (CO
abundant (35 GT/year from human emissions) C
2
) as a non-toxic and
feedstock has
1
excellent properties and long-term stability, for example in the
attracted significant attention in the transition towards a more
sustainable chemical industry.[1,2] Value-added products, and in
[29–33]
2 x
hydrogenation of aromatics and CO or in NO absorption.
The catalyst is robust and operates under solvent-free reaction
conditions to afford hydrogen-enriched natural gas (HENG)[
and propylene glycol (PG) water mixture, a ready-to-use anti-
freeze solution.
particular energy-related chemicals derived from CO
coveted owing to the escalating demand of global fuel supply.
However, the high stability of CO
hinders its efficient utilization.[5–
2
, are
34,35]
[
3,4]
2
7
]
Methanation is a typical example of direct CO
2
transformation
(
Figure 1a) with the process requiring high temperatures (250-
3
00 °C) and high pressures (50-100 atm) and a catalyst.[8,9]
into methanol[10–12] or formic
under relatively mild conditions have also been
developed.
Indirect methanation methods employing “relay molecules”
convert CO to CH via distinct steps (Figure 1b, typically below
00 °C). Organic carbonates are suitable relay molecules due to
their low-toxicity and facile synthesis from CO via the
cycloaddition of CO
into epoxides.[15–18] In the second step the
corresponding diol and methane are produced, with the diol being
Catalysts that directly reduce CO
2
[
13,14]
acid
2
4
2
2
2
a
value-added product compared to the epoxide starting
Figure 1. a) Direct and b) Indirect CO utilization pathways.
2
material.[
19,20]
Industrially, the production of ethylene glycol is
with
ethylene oxide, followed by hydrolysis of the carbonate to
Results and discussions
achieved via the OMEGA process via the reaction of CO
2
Based on previous studies, we evaluated both Cu and Ru
nanoparticles (NPs) immobilized on a faujasite type x zeolite
framework (FAU-zeolite). Despite Cu being one of the most
[
21]
generate the glycol and regenerate CO
be obtained by coupling the OMEGA process with a reduction
step, thus simultaneously valorizing CO
In a pioneering study, Milstein reported the hydrogenolysis of
dimethyl carbonate (DMC) into methanol employing
2
.
Additional value can
2
prominent metals for CO -related reactions, the Cu catalyst
2
.
showed very limited reactivity under the experimental reaction
conditions (see Table S4, SI), presumably due to the large size of
the particles.[23–25,36] In contrast, the Ru catalyst showed promising
activity and was investigated further.
a
homogeneous Ru(II)-pincer catalyst (see Table S1 for the
reaction conditions).[22] Heterogeneous catalysts able to convert
cyclic organic carbonates with high selectivity have also been
reported by other groups (Table S1).[23–25] Very recently, three
We employed a cation exchange procedure (Figure 2) followed
by reduction with excess NaBH4(aq).[31] This way, the reduced
metal ions aggregate on the Ru atoms present into the zeolite
framework resulting in embedded NPs (to produce the solid phase
examples of homogeneous catalysts based on a non-noble metal
catalyst termed Ru-FAU
,
Figure 2c).[31,37]
[
a]
W.-T. Lee, A. P. van Muyden, Dr. F. D. Bobbink, Dr. Z. Huang, Prof.
Dr. P. J. Dyson Institut des Sciences et Ingénieuries Chimique,
École Polytechnique Fédérale de Lausanne (EPFL), 1015
Lausanne, Switzerland. Email: pjd@epfl.ch
+
: equal contribution
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.

Angewandte Chemie International Edition
10.1002/anie.201811086
COMMUNICATION
2
adsorption and a surface area of 214.45 m /g. This decrease in
adsorption indirectly confirms that Ru NPs partially fill the FAU-
zeolite pores and has been observed in similar types of supported
metal catalysts.[
38]
Powder X-ray diffraction (powder-XRD) further demonstrates the
crystalline nature of the Ru-FAU catalyst (Figure S9b) and
showed that the native structure of the FAU support is
conserved. Broadening of the peaks combined with a decrease
in peak intensity suggests that the different crystalline frameworks
are entangled and slightly amorphous.[39,40]
Figure 2. Illustration of the synthesis of solid phase Ru catalyst a) FAU-zeolite
[31]
type X); b) Ion exchange of Na by Ru3+; c) Ru-FAU.
+
(
X-ray photoelectron spectroscopy (XPS) revealed the presence
of Ru species at 4.4%±0.1% on the surface of the Ru-FAU
catalyst. (Ru(3p) was selected to represent the elemental
concentration, see SI for details). As expected, aluminum silicate
is the major component of both solids (Table S2). Note the signal
of Na (1s, 1071.50 eV) was chosen as the calibration due to the
lack of suitable C(1s) species.[41] Mg2+ ions are also present in the
+
2+
support material (Figure S8). The native cations (Na and Mg )
are observed by XPS before and after ion exchange indicating
that the exchange process is not quantitative under the conditions
used herein (Table S2). The cation loading was further examined
by means of ICP-OES analysis with 7.3 wt% of Ru, 8.2 wt% of Na
and 0.9 wt% of Mg determined.
The Ru-FAU catalyst was initially studied using propylene
carbonate (PC) as the substrate with the optimization results
compiled in Table S3. Propylene glycol (PG) and water are the
major liquid products with traces of 2-propanol (PrOH) and
ethanol (EtOH) detected. At 150 and 160 °C conversion is low
Figure 3. a
FAU catalyst; b
of the Ru-FAU catalyst, yellow circle: crystalline Ru nano-structures; c
image of the Ru-FAU catalyst; c2, , c ) STEM elemental mapping of the Ru-
FAU catalyst: Ru, Si, Al, respectively.
1
) SEM image of the FAU-zeolite support; a
2
) SEM image of the Ru-
) HRTEM image
) HAADF
i
1
) HRTEM image of the FAU-zeolite support; b
2
1
(
13% and 40% respectively, see Figure 4, left), whereas at higher
c
3
4
temperatures (≥ 170 °C) conversions were near-quantitative (97 -
1
9
9%, see Figures S12-S13 for representative H NMR spectrum).
At 180 °C, however, the PG product is converted into gaseous
products lowering the yield (Table S3, entry 4). Consequently,
The FAU-zeolite (in the absence of Ru NPs) has a particle
diameter ranging from 1.5 to 3.0 µm (Figure S1, SI), which
remains essentially the same in the Ru-FAU catalyst (cf. Figure
170 °C (Table S3 entry 3) was selected as the optimum reaction
temperature for subsequent studies. This temperature is similar
to that used in other reports describing the hydrogenation of cyclic
carbonate.[
3
a
1
vs. a
protrude from the FAU-zeolite surface with diameters typically
ranging from 10 to 20 nm (see Figure 3b and 3b for a
2
). HRTEM images reveal that crystalline Ru NPs
36]
1
2
comparison between the FAU-zeolite and the Ru-FAU catalyst,
and Figures S2-S3 for further details).
The main gaseous product is methane with traces of propane
(
2
2%) and CO (1%) detected (Figure 4, right, and Figure S10).
CO results from the hydrolysis of the carbonate by the water
produced during the reaction. To exclude that adsorbed water into
the zeolite framework leads to the hydrolysis of PC the reaction
2
The protruding Ru NPs were further analyzed by high-angle
annular dark-field scanning transmission electron microscopy
(
HADDF-STEM, Figure 3) together with STEM-energy-dispersive
X-ray spectrometry (STEM-EDX, Figures S5-S7). Strong Ru
signals were associated with these structures (Figure 3c and c ),
whereas Si and Al are absent within the protruding Ru
nanostructure (Figure 3c and c ). Mapping indicates that the
2 2
was performed under a N atmosphere and CO and PG were not
detected. Presumably, the acidic site present in the Ru-FAU
catalyst is also able to promote reductive dehydration,
rationalizing the formation of propane and the absence of MeOH,
in sharp contrast with previous reports that produce MeOH
selectively.[
1
2
3
4
2
pores of the FAU-zeolite are partially filled with Ru (Figure 3c ),
firmly embedding the Ru NPs on the support.
26,36]
The influence of hydrogen pressure was studied with the
conversion of PC ranging from 84% at 40 bar to 98% at 80 bar.
Hydrogen pressure strongly impacts on the selectivity of gaseous
Brunauer–Emmett–Teller (BET) analysis showed that the porosity
of the FAU-zeolite decreases upon formation of the Ru NPs. The
2
FAU-zeolite displays a surface area of 423.32 m /g (Figure S9a)
products, with less CO
2
formed at higher hydrogen pressures
and the Ru-FAU catalyst has a similar adsorption curve, i.e. the
(
Table S3, entries 5-8). This indicates that either hydrolysis is
adsorption of a microporous material, but with decreased N
2
2
suppressed or CO is reduced via a Sabatier-type pathway under
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Angewandte Chemie International Edition
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COMMUNICATION
the reaction conditions.34 In this respect, a reaction using CO
as
As a comparison, a commercial Ru/C was tested, which led to
69% conversion of PC and produced PG and methane (Table S4,
entry 4), but upon recycling the activity decreased markedly and
PG was obtained in a lower yield (Table S4, entry 5). The FAU-
2
substrate demonstrated that Ru-FAU is also able to catalyze the
methanation of CO at temperatures well below those typically
2
used in the Sabatier process (Table 1, entry 4).[42,43] Remarkably,
higher hydrogen pressure also favored the conversion of PG to
gaseous products resulting in a lower yield of PG (Table S3, entry
3
zeolite or RuCl alone are able to convert PC, but are not selective
towards glycol and methane (Table S4 entries 7-8) No conversion
was achieved by these three catalysts under nitrogen, confirming
that the potential presence of water in the catalyst does not
participate in the hydrolysis of the cyclic carbonate.
8). Consequently, 70 bar of H
2
was chosen as the optimal applied
pressure.
In summary, Ru modified FAU-zeolites efficiently catalyze the
indirect methanation of CO via the hydrogenolysis of PC. The
2
reaction results in the production of valuable gaseous energy
carrier, i.e. hydrogen enriched methane, together with a liquid
phase composed of PG and water that can be directly used as
antifreeze solution following further dilution with water. The
catalyst is easily recovered and recycled. This process represents
a feasible extension to OMEGA process, bridging chemical
production with efficient energy storage and, at the same time,
2
valorizing CO and with the added advantage of no separation of
the products required.
Figure 4. Hydrogenolysis of propylene carbonate (PC) to propylene glycol (PG)
and methane. Reaction conditions: PC (1021 mg, 10.0 mmol), Ru-FAU (100
Experimental Section
mg; Ru loading = 0.072 mmol), H
2
(70 bar), 18.5 h. Right: reaction temperature:
1
70 °C. * Conversions and yields were determined by 1H NMR spectroscopy
Experimental details are provided in the Supporting Information.
6
with dibromomethane as an internal standard in d -acetone.
Acknowledgements
The substrate scope was extended to ethylene carbonate (EC)
and a non-cyclic carbonate, i.e dimethyl carbonate (DMC), which
are converted in 99% and 91%, respectively, under the optimized
reaction conditions (Table 1, entries 1-2). When using DMC, only
traces of MeOH were present implying that MeOH is also
transformed into methane (confirmed in Table 1, entry 3). As
expected, methane and ethylene glycol (EG) were concomitantly
generated in the case of EC.
We thank the EPFL, Swiss National Science Foundation and the
Swiss Competence Center for Energy Research (SCCER) on Heat
and Electricity Storage for financial support. Daniel Sun, Alexander
Stephens, José Bila, Pierre Mettraux, and Pascal Schouwink are
thanked for help with the BET, XPS and XRD analysis.
Keywords: Sustainable chemistry, heterogeneous catalysis,
ruthenium nanoparticles, hydrogenolysis, carbon dioxide, organic
carbonates.
Table 1. Conversion of different substrates by the Ru-FAU
catalyst.
Gas prod. [%]
Entry
Substrate Conv. [%] Liquid prod. [%]
CH
4
– CO
2
References
1
2
3
DMC
EC
91
99
MeOH, 2
EG, 62
EtOH, 1
None
68 – 0
30 – 1
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56
87 – 0
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a
4
CO
2
None
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Reaction conditions: Substrate (10 mmol), Ru-FAU(100 mg; Ru loading = 0.072
mmol), H (20 bar), H (70 bar), 200 ºC;
(70 bar), 170 °C, 18.5 h. a CO
Conversions and yields were determined by 1H NMR spectroscopy with
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COMMUNICATION
Entry for the Table of Contents
TOC TEXT: A nanocrystalline ruthenium catalyst embedded into a zeolite catalyses the hydrogenolysis of cyclic
carbonates into glycols and methane. The catalyst displays unprecedented selectivity towards methane, and the
reaction may be considered as an extension of the OMEGA process, in which both a valuable liquid and gaseous
product are formed.
COMMUNICATION
Wei-Tse Lee, Antoine P. van Muyden,
Felix D. Bobbink, Huang Zhangjun, Paul
J. Dyson*
Page No. – Page No.
2
Indirect CO methanation:
hydrogenolysis of cyclic carbonates
catalyzed by Ru-modified zeolite
produces methane and diols
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