Shape-Selective Zeolites Promote Ethylene Formation from Syngas via a Ketene Intermediate

Article abstract of DOI:10.1002/anie.201801397

Syngas conversion by Fischer–Tropsch synthesis (FTS) is characterized by a wide distribution of hydrocarbon products ranging from one to a few carbon atoms. Reported here is that the product selectivity is effectively steered toward ethylene by employing

Full text of DOI:10.1002/anie.201801397

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Accepted Article
Title: Shape-Selective Zeolites Promote Ethylene Formation from
Syngas via a Ketene Intermediate
Authors: Feng jiao, Xiulian Pan, Ke Gong, Yuxiang Chen, Gen Li, and
Xinhe Bao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201801397
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10.1002/anie.201801397
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Shape-Selective Zeolites Promote Ethylene Formation from
Syngas via a Ketene Intermediate
Feng Jiao,‡ Xiulian Pan,‡* Ke Gong, Yuxiang Chen, Gen Li, Xinhe Bao*
years with efforts mainly focusing on modification of Fe- and Co-
based FTS catalysts.[2] Despite of significant progress recently,[2c,
2e, 3] the C2= selectivity remains low and the highest C2 selectivity
Abstract: Syngas conversion via Fischer-Tropsch synthesis (FTS)
is characterized by a wide distribution of hydrocarbon products
ranging from one to a few ten carbon atoms. We report here that
selectivity can be effectively steered toward ethylene employing the
oxide-zeolite (OX-ZEO) catalyst concept with ZnCrOx-mordenite
(MOR). The selectivity of ethylene alone among hydrocarbons
reaches as high as 73% at 26% CO conversion, in stark contrast to
a maximum 30% predicted for C2 hydrocarbons by the Anderson-
Schultz-Flory (ASF) model in FTS. Ethylene selectivity is also
significantly higher than those obtained in any other direct syngas
conversion or the multi-step via methanol-to-olefin process.
Selective site blocking experiments reveal that this highly selective
pathway is realized over the catalytic sites within the 8-membered
ring (8MR) side pockets of MOR via ketene as an intermediate. The
12MR channels are not at all selective for ethylene. This study
provides substantive evidence for a new type of syngas chemistry
with ketene as the key reaction intermediate that enables
extraordinary ethylene selectivity within the OX-ZEO catalyst
framework.
o
(including C2= and ethane C2 ) is predicted to be 30% by the ASF
model (Figure S1).
A previous study demonstrated that a selectivity of ethane as
high as 83% can be obtained by applying ZSM-5 impregnated
with Cr, Zn, and Al, but no ethylene was detected which is
obviously a much more valuable C2 product.[4] A series of
bifunctional catalysts were reported giving C2-C4 paraffins as
products but no ethylene.[5] Arakawa et al. reported an ethylene
selectivity of 45% together with 33% CH4 at CO conversion of 6%
by sequential conversion within two catalyst beds, i.e. a Rh-
based catalyst and H-silicate.[6]
We recently reported that partially reducible oxides such as
ZnCrOx and MnOx in combination with a mesoporous SAPO-34
(MSAPO) zeolite can separate CO activation and C-C coupling
onto two active sites.[3a, 7] This OX-ZEO catalyst concept has
enabled synthesis of mixed light olefins directly from syngas with
a selectivity of 80% among hydrocarbons at CO conversion of
17%.[3a] Thereafter, Cheng et al. reported a selectivity of 70% to
mixed light olefins over a similar oxide-zeolite combination at CO
conversion of 10%.[3b] However, in that study,[3a] the ethylene
selectivity was only 23%. More importantly, the chemistry
leading to the “surprisingly high selectivity”[8] to light olefins
remains controversial, particularly the actual intermediates being
either methanol or ketene.[3, 9]
=
Ethylene (C2 ) is an important basic building block for
production of a wide range of plastics, solvents and cosmetics.
Its worldwide demand exceeds that of most other organic
compounds. Ethylene is traditionally produced by steam
cracking of naphtha. Alternatively, it can be synthesized by C–C
coupling of C1 molecules e.g. via the methanol-to-olefins (MTO)
process or directly from CO via Fischer-Tropsch synthesis (FTS).
=
=
The multi-step via MTO technology gives mixed olefins (C2 -C4 )
as products and its selectivity is as high as 80-90%, but that of
C2= is usually only 40-50%. The C2= selectivity can be increased
up to 60% using small cavity zeolites but at the expense of a
much shorter catalyst lifetime.[1] In comparison, direct conversion
of syngas to ethylene is attractive because it involves fewer
operation units. This has been under extensive study for over 50
[*]
F. Jiao, Prof. Dr. X. Pan, K. Gong, Y. Chen, G. Li, Prof. Dr. X. Bao
State Key Laboratory of Catalysis, National Laboratory for Clean
Energy, 2011-Collaborative Innovation Center of Chemistry for
Energy Materials
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023
Figure 1. Syngas conversion over the composite catalyst ZnCrOx-MOR#1.
(A) Hydrocarbon distribution at 360
̊C; (B) Performance at different
temperatures. Reaction conditions: ZnCrOx/MOR#1 = 1/1 (mass ratio), H2/CO
= 2.5/1 (vol.), 2.5 MPa and space velocity = 1857 ml/gcat·h.
Here we report evidence for ketene as the key intermediate in
a highly selective reaction pathway toward ethylene, which
becomes accessible by employing mordenite zeolite (MOR) in
the OX-ZEO concept. Figure 1A shows that the composite
ZnCrOx-MOR#1 with a 1:1 mass ratio catalyzes syngas
E-mail: panxl@dicp.ac.cn
xhbao@dicp.ac.cn
F. Jiao, K. Gong, Y. Chen, G. Li
University of Chinese Academy of Sciences, Beijing 100049
conversion to hydrocarbons at 360 ̊C, H2/CO = 2.5/1 and 2.5
‡
These authors contributed equally.
MPa. Remarkably, the selectivity of ethylene alone (excluding
CO2) reaches as high as 83%, which is much higher than the
23% obtained over ZnCrOx-MSAPO (Figure S2).[3a] Such an
ethylene selectivity is unprecedented in any other direct syngas
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201801397
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conversion processes[2, 10] and also higher than that of MTO
processes reported so far.[1, 11] Furthermore, the selectivity of the
byproduct CH4 is only 5%. Figure 1B shows that CO conversion
belong to the OH groups vibrating in the 12MR channels.[13-14]
Many studies have demonstrated that the H+ species in the 8MR
pockets can be preferentially exchanged by Na+.[13, 14c, 14d] Figure
S3 shows that the LF band progressively attenuates with
increasing Na+ concentration and eventually disappears while
the TF and HF bands are retained. It validates that the 8MR sites
can be selectively shielded in this way (Table S1).
increases with the temperature and reaches 11% at 390 ̊C while
=
C2 selectivity still remains at 71%. As proposed previously,[3a]
the OX-ZEO concept affords a bifunctionality, where CO is
activated over oxides, and subsequent C-C coupling is
effectuated by zeolites. Still, the extraordinary selectivity toward
ethylene over ZnCrOx-MOR is surprising considering the
predominant unidirectional 12MR channels of MOR (7.0 × 6.5 Å),
which has been widely employed in reactions involving long
chain hydrocarbons such as isomerization and hydrocarbon
cracking.[12] Yet, MOR possesses another smaller channel
formed by elliptical 8MR (5.7 × 2.6 Å) windows running in parallel
as the 12MR channels, which are interconnected by the 8MR
openings (with an aperture of 4.8 × 3.4 Å), so called side pockets
(Figure S2).[13] The properties of Brønsted acid sites, from which
the catalytic activities usually originate, differ strongly in these
channels due to their specific local environments. The infrared
(IR) spectra in Figure S3 show characteristic antisymmetric
stretch modes of these sites in the range of 3550-3650 cm-1.[13-
14] The 3582-3590 cm-1 band is attributed to the OH sites within
the 8MR side pockets (denoted as LF band), while the bands at
3600-3610 cm-1 (HF band) and 3617-3625 cm-1 (TF band)
Figure 2A demonstrates that the rate of ethylene formation
=
(expressed in mmol C2 per gram catalyst per hour) increases
monotonically with the number of H+ sites in the 8MR pockets.
They are almost linearly correlated in the range of 0–0.20 mmol
=
H+/gcat (Figure S4A). Above 0.20 mmol H+/gcat, C2 formation
slows down due to the mismatched activities in the tandem
catalysis: the intermediates generated over the oxide are
insufficient for the subsequent C-C coupling over MOR. When
more active sites are provided by increasing the mass ratio of,
=
e.g. ZnCrOx/MOR#1, from 1/1 up to 4/1, C2 formation rate is
=
enhanced from 0.76 to 1.09 mmol C2 /(gcat·h). Similar
enhancement is observed for ZnCrOx-MOR#2 and ZnCrOx-
=
MOR#5. In contrast, there is no clear correlation of the C2
formation activity with the number of H+ sites within the 12MR
channels (Figure S4B). These results indicate that the acid sites
within the 8MR are likely the active sites for ethylene formation.
This is further corroborated by the sample MOR#14 whose 8MR
acid sites are largely shielded by Na+ leaving only the 12MR H+
sites available. In combination with ZnCrOx, this composite gives
a wide distribution of hydrocarbons in syngas conversion (Figure
S5), indicating that the 12MR sites are not selective for ethylene
formation.
Further experiments with the 12MR acid sites selectively
shielded point in the same direction. For example, 29% acidic
sites of MOR#2 are located within the 12MR channel, which can
be selectively shielded by pyridine (MOR#2-py, Figure S6)
because this molecule is too large to enter the 8MR channels.[14a,
14b]
Thus only the 8MR H+ sites are accessible for the reaction
with this MOR#2-py. As a result, C2= selectivity increases to 73%
compared to 62% over the unmodified MOR#2 (at reaction time
of ~20 h, Figure 2B). Interestingly, the overall CO conversion is
also enhanced from 18% to 26%, which, however, is not
understood yet. These results clearly demonstrate that the
reaction proceeds along different pathways over the 8MR and
12MR sites, thus leading to significantly different product
distributions.
Our previous study suggested that syngas conversion via
methanol may not be the dominating pathway over ZnCrOx-
MSAPO but ketene likely plays an important role.[3a] Ketene was
also ever suggested to be an important intermediate in several
syngas conversion processes.[15] Theoretical modelling by Wang
et al. demonstrated that ketene can be effectively converted to
light olefins within the pores of SAPO-34,[9] but methanol was
also proposed to be the key intermediate in other studies.[3b] In
order to assess the likelihood of ketene and methanol as the
intermediates in the selective conversion of syngas to ethylene
over ZnCrOx-MOR catalysts, we fed both directly as educts to
MOR zeolites and studied their adsorption and conversion since
adsorption is the first step of the reaction. Solid state NMR using
129Xe as a probe molecule (Figure 3A) shows that 129Xe adsorbs
in both the 8MR and 12MR channels giving two signals at 245-
208 and 176-162 ppm, respectively.[16] The intensity ratio reflects
the relative concentration of 129Xe in these two channels. When
MOR is first exposed to ketene followed by 129Xe, the intensity
ratio of the 8MR 129Xe signal to that of 12MR diminishes
Figure 2. Formation rate of ethylene in syngas conversion. (A) As a
function of the number of the 8MR acid sites over composite catalysts ZnCrOx-
MOR#n (with n representing the sample number from 1 to 17) at 375
̊C; (B)
ZnCrOx-MOR#2-py, with MOR#2-py denoting pyridine modified MOR#2 (filled
symbols) in comparison to the unmodified MOR#2 (open symbols) at 360
Reaction conditions: H2/CO = 1, 2.5 MPa, 1857 ml/gcat·h .
̊C.
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10.1002/anie.201801397
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Figure 3. 129Xe NMR spectra of the preferential adsorption of ketene and methanol in different channels of MOR. (A) Exposure to 129Xe; (B) Exposure to
ketene followed by 129Xe and (C) Exposure to methanol followed by 129Xe.
obviously (Figure 3B) compared to the spectrum in Figure 3A. It
indicates that more ketene molecules have entered and
occupied the 8MR than the 12MR channels. In contrast,
exposure to methanol and then 129Xe results in a significantly
lowered intensity ratio of the 12MR signal to that of the 8MR one
(Figure 3C), suggesting that more methanol molecules have
occupied the 12MR channels than the 8MR side pockets. These
results demonstrate that ketene is more favored to adsorb over
the 8MR sites whereas methanol prefers the 12MR sites. The
DFT calculations presented by Boronat et al. also showed that
the 12MR sites were much more favored for methanol adsorption,
as the adsorption energies were 6-12 kcal/mol higher than those
over the 8MR sites.[17] In addition, those authors showed that the
T4-O44 position in the 12MR channel (corresponding to HF O10
in this study) exhibited a lowest reaction energy for the process
of Z-H + CH3OH → Z-CH3 + H2O.[17] Furthermore, Rasmussen
et al. predicted that ketene adsorbed stronger over the 8MR sites
than that over the 12-MR ones.[18] Thus, ketene conversion may
be facilitated over the 8MR sites whereas methanol conversion
may be more favored over the 12MR sites.
methanol conversion over MOR zeolites in comparison to
syngas conversion over the corresponding composite catalysts.
One sees that syngas conversion over ZnCrOx-MOR#2-py
(Figure 4A) gives a similar product distribution profile as that of
ketene conversion over MOR#2-py (Figure 4B), where only the
8MR sites are accessible. But in ketene conversion the ethylene
selectivity is higher (reaching 89%). Such a high selectivity
toward ethylene is retained under different space velocities of
ketene and reaction temperatures (Figure S7). In contrast,
methanol conversion over MOR#2-py gives
a
much
widerproduct distribution with C3 hydrocarbons being the most
dominant (selectivity 48%) and C2 only 31% (Figure 4C). In
comparison, over MOR#14 with the 8MR sites blocked and only
the 12MR sites being accessible, ketene is converted non-
selectively to a wider range of hydrocarbons (Figure 4E), again
similar to syngas conversion over ZnCrOx-MOR#14 (Figure 4D).
Interestingly, for methanol conversion these blocked 8MR sites
do not impede its conversion as the initial conversion reaches
almost 100% (Figure S8). However, it produces mainly higher
hydrocarbons with C3+ selectivity 86% and C2 only 12% (Figure
4F). In addition, it deactivates fairly fast within 75 min, consistent
with previous (D)MTO studies.[19] The relative distribution of
Figure 4 shows the product distribution profiles of ketene and
Figure 4. Hydrocarbon distributions in the conversion of syngas, ketene and methanol over different sites of MOR zeolites at 375 ̊C. (A), (B) and (C) MOR#2-py
with only the 8MR acid sites accessible; (D), (E) and (F) MOR#14 with only the 12MR acid sites accessible; (G), (H) and (I) MOR#3 with both the 8MR and 12MR
acid sites available; (A), (D) and (G) syngas over ZnCrOx-MOR; (B), (E) and (H) ketene conversion over MOR; (C), (F) and (I) methanol conversion over MOR.
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=
=
C3 /C2 also agrees with the previously reported MTO catalyzed
by other zeolites.[1, 20] Furthermore, a previous syngas conversion
study combining a methanol synthesis catalyst (Pd/SiO2) and
MOR also showed favored formation of longer aliphatic
hydrocarbons (C3+) and aromatics.[21] Note that varying methanol
partial pressures (0.05 – 5.0 kPa), space velocities (0.03 – 5.27
gCH3OH/gzeolite·h) and reaction temperatures, methanol conversion
changes between 80-100% and the hydrocarbon product
distribution changes slightly but remains non-selective, with C2 no
more than 36%, no matter over MOR#2-py, MOR#14 or MOR#3
(Figure S8 and S9). It suggests that methanol can be convered to
hydrocarbons by MOR but non-selectively even though formation
of methanol from syngas is detected over metal oxides (Figure S5)
whereas ketene can be highly selectively converted to ethylene
over the 8MR sites. When both the 8MR and 12MR sites are
accessible (MOR#3), syngas is also converted preferably to C2
hydrocarbons with a similar distribution profile (Figure 4G) as
ketene conversion but a lower C2 selectivity (Figure 4H), whereas
methanol conversion (Figure 4I) resembles that by the 12MR sites
with C2 selectivity lower than 20% (Figure 4F). Furthermore,
hydrocarbon distribution is also very wide when feeding dimethyl
ether (DME) together with syngas to these three MOR zeolites,
which is dominated by C3+ and the highest C2 selectivity among
hydrocarbons is only 38% (Figure S10 and Table S2). These
results support that highly selective syngas-to-ethylene
conversion proceeds more likely via ketene as an intermediate
over the MOR 8MR sites rather than methanol or DME over the
8MR or 12MR sites.
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The above results demonstrate that by combining partially
reduced ZnCrOx with the confined active sites of the MOR 8MR
side pockets, syngas conversion is steered along a new reaction
pathway via ketene as an intermediate leading to an extraordinary
ethylene selectivity as high as 73% at a CO conversion 26%. This
opens up a new avenue for development of syngas-to-ethylene
technology. Furthermore, it demonstrates the versatility of the
bifunctional OX-ZEO catalyst concept, which turns syngas
conversion into a tandem catalysis. Consequently, the product
selectivity can be controlled by shape selective zeolites. This
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Acknowledgements
[12]
This work was supported by the Ministry of Science and
Technology of China (No. 2016YFA0202803), the National
Natural Science Foundation of China (Grant No. 91645204,
21425312 and 21621063).
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: heterogeneous catalysis • syngas conversion •
zeolites • ethylene • shape selective
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Ethylene blessed by the 8MR side
pockets of MOR: Syngas conversion
can be effectively steered toward
ethylene via ketene intermediates by
the 8MR side pockets of MOR
Feng Jiao,‡ Xiulian Pan,‡* Ke
Gong, Yuxiang Chen, Gen Li,
Xinhe Bao*
Page No. – Page No.
employing the ZnCrOx-MOR composite.
The selectivity of ethylene alone
reaches 73% at CO conversion of 26%.
In comparison, the 12MR sites of MOR
may favor methanol as intermediates
but lead to a remarkably different
product distribution dominated by
longer chain hydrocarbons.
Shape-Selective Zeolites
Promote Ethylene Formation
from Syngas via a Ketene
Intermediate
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