C?C Bond Formation in Syngas Conversion over Zinc Sites Grafted on ZSM-5 Zeolite

Article abstract of DOI:10.1002/anie.201912869

Despite significant progress achieved in Fischer–Tropsch synthesis (FTS) technology, control of product selectivity remains a challenge in syngas conversion. Herein, we demonstrate that Zn²⁺-ion exchanged ZSM-5 zeolite steers syngas conversion selectively to ethane with its selectivity reaching as high as 86 % among hydrocarbons (excluding CO₂) at 20 % CO conversion. NMR spectroscopy, X-ray absorption spectroscopy, and X-ray fluorescence indicate that this is likely attributed to the highly dispersed Zn sites grafted on ZSM-5. Quasi-in-situ solid-state NMR, obtained by quenching the reaction in liquid N₂, detects C₂ species such as acetyl (-COCH₃) bonding with an oxygen, ethyl (-CH₂CH₃) bonding with a Zn site, and epoxyethane molecules adsorbing on a Zn site and a Br?nsted acid site of the catalyst, respectively. These species could provide insight into C?C bond formation during ethane formation. Interestingly, this selective reaction pathway toward ethane appears to be general because a series of other Zn²⁺-ion exchanged aluminosilicate zeolites with different topologies (for example, SSZ-13, MCM-22, and ZSM-12) all give ethane predominantly. By contrast, a physical mixture of ZnO-ZSM-5 favors formation of hydrocarbons beyond C₃₊. These results provide an important guide for tuning the product selectivity in syngas conversion.

Full text of DOI:10.1002/anie.201912869

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: C-C bond formation in syngas conversion over Zn sites grafted
on ZSM-5
Authors: Yuxiang Chen, Ke Gong, Feng Jiao, Xiulian Pan, Guangjin
Hou, Rui Si, and Xinhe Bao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912869
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10.1002/anie.201912869
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C-C bond formation in syngas conversion over Zn sites grafted
on ZSM-5
Yuxiang Chen+, Ke Gong+, Feng Jiao, Xiulian Pan*, Guangjin Hou*, Rui Si and Xinhe Bao
Abstract: Despite significant progress achieved in Fischer-Tropsch
synthesis (FTS) technology, the selectivity control remains
SAPO-34 enabled highly selective formation of mixed light
=
=
a
olefins (C2 -C4 ) reaching up to 80% among hydrocarbons.[3a]
The product can be further tuned toward ethylene with a
selectivity up to 83% by combining ZnCrOx and mordenite
zeolite.[3b] A mix of ZnCrOx-ZSM-5 led to selective formation of
aromatics up to 74%.[4] We show here that a catalyst of ion-
exchanged Zn/ZSM-5 leads highly selectively to ethane.
challenge in syngas conversion. Herein, we demonstrate that zinc
ion-exchanged ZSM-5 steers syngas conversion to ethane with a
selectivity in hydrocarbons (excluding CO2) reaching as high as 87%
at 20% CO conversion. NMR spectroscopy, X-ray absorption
spectroscopy and X-ray fluorescence indicate that this is likely
attributed to the Zn sites grafted within the ZSM-5 pores. Quasi-in-
situ solid state NMR by quenching the reaction in liquid N2 detects
C2 species of acetyl [-COCH3] bonding with an oxygen, ethyl [-
CH2CH3] bonding with a Zn site, and epoxyethane molecules
adsorbing on a Zn site and a Brønsted acid site of the catalyst,
respectively. These could provide some insights to C-C bond
formation during ethane formation. Interestingly, this selective
reaction pathway toward ethane appears to be general, because a
series of other Zn2+ ion-exchanged aluminosilicate zeolites with
different topologies e.g. SSZ-13, MCM-22, ZSM-12 all give
predominantly ethane. By contrast, a physical mixture of ZnO-ZSM-5
favors formation of hydrocarbons beyond C3+. These results provide
an important guidance for tuning the product selectivity in syngas
conversion.
Zn modified zeolites had been widely studied in
aromatization of light alkanes and methanol-to-hydrocarbon
(MTH) reactions.[5] The presence of Zn was believed to facilitate
dehydrogenation of paraffins and to improve the yield of
aromatics.[5c, 5d] Pinilla-Herrero et al. reported that ZnOH+ over
ZSM-5 facilitated aromatization in the methanol-to-aromatics
(MTA) process.[5b] Almutairi et al. reported that highly dispersed
multinuclear cationic Zn complexes exhibited a much higher
activity toward propane dehydrogenation than isolated zinc sites
over ZSM-5.[6] However, such catalysts were not studied yet in
direct syngas conversion.
Ion-exchange is a widely practiced method to graft metal
species onto the Brønsted acid sites of zeolites in a highly
dispersed state.[6-7] The obtained samples are denoted as
Zna/ZSM-5 with a indicating the Zn molar loading with respect to
the catalyst weight (mmolZn/gcat). The 1H magic angle spinning
(MAS) NMR spectra in Figure S2 demonstrate that the Brønsted
acid sites of ZSM-5 (Si/Al molar ratio 12) are reduced upon ion-
exchange. This is confirmed by NH3-temperature programmed
Fischer-Tropsch synthesis (FTS) is the core technology of coal-
to-liquid (CTL) and gas-to-liquid (GTL) processes.[1] The
extensive studies over the years have led to significant progress
not only in the industrial applications but also in fundamental
understanding. However, to bring the product selectivity beyond
desorption (NH3-TPD) (Figure S3).
A new shoulder peak
appears around 533 K in the profile, which can be attributed to
Zn-Lewis-acid sites.[5a] By measuring the number of Brønsted
acid sites of the zeolites before and after ion-exchange (Figure
S4), one can calculate how many protons are exchanged by Zn.
By plotting the number of exchanged Brønsted acid sites as a
function of the Zn loadings (measured by X-Ray Fluorescence,
XRF), one clearly sees two different linear correlations (Figure
1a). When Zn loading is below 0.19 mmolZn/gcat (equivalent to
1.2 wt% and Zn/Al = 0.15), the slope is around 1.9 implying that
every Zn atom exchanges with about two Brønsted acid sites on
average. When the Zn loading increases up to 0.34 mmolZn/gcat
(equivalent to 2.2 wt% and Zn/Al = 0.28), the slope is around 1.1
suggesting that every Zn atom exchanges with about one
Brønsted acid site on average. The two linear correlations in
Figure 1a are consistent with previous studies on Zn-ion-
exchanged ZSM-5 and BEA.[7a, 7b] Biscardi et al. proposed that
Zn species existed as [O--Zn2+-O-], with one Zn interacting with
two Al sites via oxygen at low loadings (<1.3 wt%) and as [Zn-O-
Zn]2+ with two Zn2+ cations interacting with two Al sites and
connected by a bridging oxygen at higher loadings over ZSM-5
with a similar Si/Al ratio.[7b] Penzien et al. reported a similar trend
for Zn species over Zn/BEA, i.e. Zn exchanging with two
Brønsted acid site below 1.3 wt% and with one Brønsted acid
site above that loading.[7a] [Zn-O-Zn]2+ sites were also suggested
by Song et al. for Zn-ZSM-5/ZSM-11 prepared by ion-exchange
with a loading of 1.5 wt%.[8] Therefore, Zn over Znx/ZSM-5 here
exists mainly as [O--Zn2+-O-] at a low loading and [Zn-O-
Zn]2+increases with the loading.
the Anderson-Schultz-Flory (ASF) distribution[2] remains
challenge. Typical FTS products cover wide range of
a
a
hydrocarbons. For example, the ASF selectivity of C2-C4
distillates is predicted to be less than 58% (Figure S1). An
increasing number of studies demonstrate that the bifunctional
catalyst concept of physically mixed metal oxides and zeolites
(OX-ZEO) provides
a promising approach to tackle the
selectivity challenge.[3] For instance, a composite of ZnCrOx-
[*]
Y. Chen,[+] K. Gong, [+] Dr. F. Jiao, Prof. X. Pan, Prof. G. Hou, Prof.
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 (China)
E-mail: panxl@dicp.ac.cn
ghou@dicp.ac.cn
Y. Chen,[+] K. Gong, [+]
University of Chinese Academy of Sciences
Beijing 100049 (China)
Prof. R. Si
Shanghai Synchrotron Radiation Facility, Shanghai Institute of
Applied Physics, Chinese Academy of Sciences
Shanghai 201204 (China)
[+]
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. The structure of Zn0.34/ZSM-5 in comparison to a physical mixture of ZnO0.35-ZSM-5. (a) Exchanged Brønsted acid sites as a function of the Zn loading;
(b) the Fourier transform of the k3-weighted extended X-ray absorption fine structure (EXAFS) of Zn K-edge signal in the R-space (R) (solid line: experimental
data; dashed line: fitting data); (c) 31P-31P CP POST-C7 double quantum-single quantum (DQ-SQ) MAS NMR spectrum upon TMP adsorption on Zn0.34/ZSM-5.
Trimethylphosphine (TMP) adsorption is sensitive to the acid
sites in zeolites,[9] which was employed to study the acid
properties of Zn0.34/ZSM-5 by 31P cross polarization (CP) MAS
NMR spectroscopy. Figure S5 shows that the pristine ZSM-5
gives one signal at -4.5 ppm, which is attributed to TMP
adsorption on Brønsted acid sites. In comparison, two additional
signals appear at -42 and -47 ppm on Zn0.34/ZSM-5 upon TMP
adsorption indicating the presence of another two types of Lewis
acid sites. Further experiments with 31P{27Al} Rotational Echo
Adiabatic Passage Double Resonance (REAPDOR) technique[10]
demonstrate no correlation of these two TMP signals with Al
(Figure S6) and therefore they should be associated with Zn
sites. However, they are obviously different from those on ZnO
nanoparticles (Figure S5). Since the acidity of [Zn-O-Zn]2+ is
weaker than that of [O--Zn2+-O-], the signal at a higher field i.e. -
47 ppm should be attributed to TMP adsorbing on [Zn-O-Zn]2+
and the other one corresponds to [O--Zn2+-O-]. This is further
validated by the 2D 31P-31P double quantum-single quantum
(DQ-SQ) MAS NMR correlation spectroscopy based on
homonuclear dipolar couplings.[11] The presence of the diagonal
peak at (-47, -94) in Figure 1c demonstrates a spatial proximity
between the co-adsorbed TMP molecules, reflecting the spatially
correlated zinc sites of [Zn-O-Zn]2+. By contrast, the -42 ppm
signal does not show a diagonal peak, corresponding to [O--
Zn2+-O-]. These data validate the coexisting [O--Zn2+-O-] and [Zn-
O-Zn]2+ sites over Zn0.34/ZSM-5.
X-ray diffraction (XRD) does not detect metallic Zn or ZnO
crystal phases on Zn0.34/ZSM-5 (Figure S7). Further X-ray
absorption near edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) of Zn0.34/ZSM-5 again indicate
the absence of ZnO clusters (Figure S8-S10). The EXAFS
spectrum in Figure 1b only shows the first shell Zn-O at 2.05 Å.
There is neither a peak at 2.6 Å corresponding to Zn-Zn nearest
neighbour in Zn foil, nor a peak at 3.23 Å corresponding to Zn-
Zn nearest neighbour in ZnO. The second shell Zn-Zn
contribution of Zn0.34/ZSM-5 is very weak (Figure S10) because
of low coordination and low concentration of [Zn-O-Zn]2+ sites
(Table S1). For comparison, ZnO nanoparticles physically mixed
with the same ZSM-5 with a similar zinc molar loading as
Zn0.34/ZSM-5 (denoted as ZnO0.35-ZSM-5) clearly exhibit the Zn-
O feature at 1.97 Å and Zn-Zn of ZnO at 3.23 Å in the EXAFS
spectrum (Figure 1b and Table S1).
Figure 2a demonstrates that Zn0.34/ZSM-5 catalyzes direct
syngas conversion giving an ethane selectivity up to 87% among
hydrocarbons (excluding CO2) at 20% CO conversion at 673 K,
4.0 MPa, H2/CO = 2.5 and 1,500 ml/(gcat·h). Note that CO2
selectivity is 45% among all carbon products. However,
Zn0.34/ZSM-5 exhibits a rather low water gas shift (WGS) activity
(Figure S11), implying that CO2 should not come from WGS.
CO-TPR and H2-TPSR (Figure S12) indicate that CO2 is more
likely the product of CO dissociation. For simplicity of discussion,
the hydrocarbon distribution is reported excluding CO2 in this
study (Table S2). Figure 2a further shows that there are very
little C3+ hydrocarbons (selectivity < 3%). Note that there is a
very short induction time, i.e. C2H6 selectivity increases in the
first 4 h and then levels off (Figure S13). CO conversion can
increase up to 30% while the ethane selectivity remains around
80% when the temperature, pressure and space velocity vary
(Figure 2b and S14). The C3+ selectivity remains less than 6%
(Table S3). This hydrocarbon distribution is obviously different
from those in MTH processes where ZSM-5 or Zn/ZSM-5 was
also employed as a catalyst. ZSM-5 is a typical 10 member-ring
zeolite and is known to favor formation of hydrocarbons beyond
C3+.[3a, 4, 12] Even a ZSM-5 with a similar Si/Al ratio upon ion-
exchange with different loadings of Zn, was reported to give C3+
hydrocarbons as main products in MTH.[13] Therefore, the
selective formation of ethane here is not just due to the shape
selectivity of ZSM-5.
Figure S15 shows that methane selectivity is over 20% at a
[7a, 7b]
Zn loading below 0.09 mmolZn/gcat
.
Ethane selectivity
increases almost linearly with the increasing Zn loading, and
above 0.19 mmolZn/gcat ethane selectivity levels off around 83%,
as shown in Figure 2c. Correspondingly, ethane formation rate
picks up slowly with the Zn loading increasing to 0.13
mmolZn/gcat and then it increases almost linearly till 0.34
mmolZn/gcat, which could be related to the rapidly increasing
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Figure 2. Syngas conversion over different catalysts. (a) Zn0.34/ZSM-5, ZnO0.35-ZSM-5 and ZnO2.47-ZSM-5; (b) Activity of Zn0.34/ZSM-5 at different temperatures;
(c) Ethane formation rate and ethane selectivity over Zn/ZSM-5 as a function of Zn loading (mmolZn/gcat). Reaction conditions: H2/CO = 2.5, 673 K, 4.0 MPa,
GHSV = 1,500 ml/(gcat·h).
number of [Zn-O-Zn]2+ sites. Interestingly, in the selective
catalytic reduction (SCR) of NOx, the [Cu-O-Cu]2+ species was
also proposed to be the active site of the ion-exchanged
Cu/SSZ-13.[14] A threshold density of Cu species was also
observed.[14a] However, the role of [O--Zn2+-O-] sites here cannot
be excluded completely yet.
To understand the role of Brønsted acid sites on Zn/ZSM-5
catalysts, a control sample was prepared by Na+ ion-exchange
of the pristine ZSM-5, followed by Zn2+ ion-exchange. The
resulting NaZn0.29/ZSM-5 contains 0.29 mmolZn/gcat and the
used Zn0.34/ZSM-5 shows almost no carbon residues (Figure
S18). Therefore, this reaction does not appear to go via the
conventional hydrocarbon pool mechanism in MTH. Figure S17a
shows a series of 13C signals in the regions of 0 - 50 and 170 -
190 ppm. To identify them, 2D Heteronuclear Correlation
(HETCOR) spectroscopy was carried out to characterize the
spatial proximity of heteronuclei of 13C and 1H, and Incredible
Natural Abundance Double Quantum Transfer Experiment
(INADEQUATE) to identify 13C-13C through-bond homonuclear
correlations (see the supporting information for details).[15] The
NMR spectra and assignments are shown in Figure 3a and S17.
The signal pairs at 172.5 ppm (13C) and 8.3 ppm (1H) in the
upper panel of Figure 3a can be attributed to formyl group [-
CHO] bonding to an oxygen of the catalyst.[16] Those at 25 ppm
(13C) and 2.8 ppm (1H) are assigned to methylene group [-CH2-].
The 13C resonance at 5.6 ppm, correlated to 1H 0.8 ppm, is
assigned to ethane, and 13C signals at 45.2 and 34.2 ppm
correspond to epoxyethane (C2H4O) adsorbing on a Zn site and
a Brønsted acid site, respectively. The lower panel of Figure 3a
shows 13C signals at 185.9 and 20.5 ppm corresponding to
acetyl group [-COCH3] bonding to an oxygen of the catalyst
forming acetate (Figure S19).[17] The 13C 15.2 and 16.2 ppm
signals, correlated to 1H at 0.8 and 1.3 ppm, respectively, are
assigned to the -CH3 and -CH2- groups of propane, and 12.8
and 25.4 ppm signals to -CH3 and -CH2- groups of n-butane.[18]
The quasi-in-situ solid-state NMR spectra in Figure 3b
demonstrate the evolution of the carbon species with time on
stream, which were also trapped by quenching. C1 species ([-
CHO] and [-CH2-]) are already observed on the catalyst after
reaction for 5 min. As the reaction proceeds, their intensities
decline while C2 species i.e. acetyl [-COCH3], ethyl [-CH2CH3]
bonding with Zn (13C at 9 and -1 ppm)[19] and epoxyethane
molecules C2H4O become more prominent, in addition to
products ethane C2H6, propane and n-butane. Epoxyethane is
not detected by GC, which is not well understood yet. Note that
methoxy ([-OCH3]) species is almost indiscernible and appears
to be marginal in this reaction. The surface acetyl and/or acetate
species were reported to be in a rapid equilibrium with ketene
number of Brønsted acid sites is reduced to 0.30 mmolH+/gcat
,
but ethane selectivity retains 86% (Figure S16). Furthermore,
ethane formation rate and ethane selectivity as a function of Zn
loading still fall on the same curves as other Zn/ZSM-5 catalysts
(Figure S16b), suggesting again the importance of the Zn sites
in ethane formation. However, it is not clear if further reducing
Brønsted acid sites below 0.30 mmol/gcat would affect ethane
formation.
In comparison, the physically mixed ZnO0.35-ZSM-5 is much
less active than Zn0.34/ZSM-5 because CO conversion is only 2%
(Figure 2a), one order of magnitude lower, under the same
reaction conditions. A comparable CO conversion is only
obtained when the Zn loading is increased by 7 times to 2.47
mmolZn/gcat (denoted as ZnO2.47-ZSM-5). But this comes at the
expense of ethane selectivity and the selectivity of C3+
hydrocarbons increases to 46% including aromatics. This
appears to be in accordance with the shape selectivity of ZSM-5,
which favored formation of hydrocarbons beyond C3+ in MTH
reactions.[3a, 4, 12]
To understand the highly selective reaction pathway toward
ethane, we turned to solid-state NMR by quenching the catalysts
in liquid N2 after 30 min reaction at 673 K and 4.0 MPa. Note
that 13C-enriched CO was used in the feed. It is interesting that
both the 13C single pulse (SP) and CP/MAS spectra (Figure
S17a) show no signal in the region of 120 - 155 ppm, indicating
the absence of (methylated) aromatic and olefinic groups, which
are characteristic species of hydrocarbon pool observed in MTH
processes.[5b, 5c] Furthermore, thermal gravimetric analysis of the
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Figure 3. Quasi-in-situ NMR study over Zn0.34/ZSM-5. (a) The upper panel showing 2D 13C-1H heteronuclear correlation (HETCOR) spectrum characterizing the
correlation between carbon and proton nuclei; the lower one showing 13C-13C refocused Incredible Natural Abundance Double Quantum Transfer Experiment
(INADEQUATE) Double Quantum/Single Quantum (DQ/SQ) spectrum, which characterizes 13C-13C bonding via through-bond spin-spin couplings. (see SI Figure
S17 for the detailed assignment); (b) 13C Cross Polarization (CP) MAS NMR spectra of intermediates and products trapped over Zn0.34/ZSM-5 after the syngas
reaction at various time and reaction conditions.
over zeolites.[20] This is interesting because ketene was
experimentally detected over the mixed ZnCrOx oxides and
impregnated ZnCrAl[23] may also be attributed to the presence of
highly dispersed Zn sites, as reported by Almutairi et al. for Zn
impregnated ZSM-5.[6]
=
=
could be converted to mixed light olefins (C2 -C4 ) by SAPO-
34.[3a] Jiao and coworkers theoretically predicted that ketene was
the intermediate of ethylene and ethane formation over Fe5C2 in
conventional FTS.[21] Therefore, the surface acetyl species
and/or ketene might represent the first C-C bond formation
towards ethane over Zn/ZSM-5 in syngas conversion. Although
acetate species were also reported in MTH, they were generally
accepted to form over Brønsted acid sites upon carbonylation of
surface methoxy species.[17, 22] In comparison, CO activation and
ethane formation here are related with Zn sites.
Interestingly, experiments with aluminosilicate zeolites with
different pore shapes and dimensions such as SSZ-13 (8-
membered ring, CHA), MCM-22 (10-membered ring, MWW) and
ZSM-12 (12-membered ring, MTW) suggest that this highly
selective reaction pathway toward ethane appears to be general
for the ion exchanged Zn sites. These zeolites were subjected to
ion-exchange with Zn2+ following the same procedure as
Zn/ZSM-5. The resulting Zn/SSZ-13, Zn/MCM-22 and Zn/ZSM-
12 can catalyze syngas conversion to hydrocarbons, which are
all dominated by ethane, as displayed in Figure 4. XAFS
analysis of the structure (Figure S20) indicates that the zinc
species are also highly dispersed as those on Zn0.34/ZSM-5. The
correlation between the densities of Brønsted acid sites and the
Zn loadings (Table S4) show that each Zn exchanges with
around 1.4-1.6 Brønsted acid sites, indicating again coexisting
[Zn-O-Zn]2+ and [O--Zn2+-O-] species. These results further
corroborate the more importance of these Zn sites in ethane
formation compared to the zeolites‘ shape selectivity. The
previously observed high ethane selectivity over ZSM-5
Figure 4. Syngas conversion over Zn-ion exchanged aluminosilicate zeolites
with different topologies.
In summary, we report here that Zn ion-exchanged ZSM-5
catalyzes syngas conversion toward ethane with a selectivity in
hydrocarbons reaching 87%. NMR spectroscopy, XAFS and
XRF as well as the activity correlation with the Zn loading
suggest that [Zn-O-Zn]2+ sites likely play an important role in
ethane formation but [O--Zn2+-O-] cannot be excluded completely.
The quasi-in-situ solid state NMR spectroscopy shows the
evolution of surface carbon species from C1 such as formyl [-
CHO] and [-CH2-] to C2 species acetyl [-COCH3] and ethyl [-
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the detailed pathways toward ethane will require further
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zeolites. However, the topologies and distribution of acidic sites
and Zn sites may also play a role by forming a special
microenvironment. These findings also point to future design of
metal oxide-zeolite catalysts for synthesis of more valuable
molecules such as olefins, i.e. avoiding formation of Zn single
site species within the zeolite pores.
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This work was financially supported by the Ministry of
Science and Technology of People's Republic of China (No.
2016YFA0202803) and the National Natural Science Foundation
of China (Grant No. 91645204, 91945302 and 21773230). The
authors thank beamline BL14W1 (Shanghai Synchrotron
Radiation Facility) and BL04B (National synchrotron Radiation
Laboratory) for the beam time and assistant in the experiments.
The authors thank Prof. Fengshou Xiao and Prof. Xiangju Meng
from Zhejiang University for kindly providing SSZ-13.
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10.1002/anie.201912869
Angewandte Chemie International Edition
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COMMUNICATION
Zn sites grafted within the
Yuxiang Chen+, Ke Gong+, Feng Jiao,
Xiulian Pan*, Guangjin Hou*, Rui Si and
Xinhe Bao
aluminosilicate zeolite pores via ion-
exchange steer C-C coupling
selectively toward ethane in direct
syngas conversion.
Page No. – Page No.
C-C bond formation in syngas
conversion over Zn sites grafted on
ZSM-5
Layout 2:
COMMUNICATION
Yuxiang Chen+, Ke Gong+, Feng Jiao,
Xiulian Pan*, Guangjin Hou*, Rui Si and
Xinhe Bao
Page No. – Page No.
C-C bond formation in syngas
conversion over Zn sites grafted on
ZSM-5
This article is protected by copyright. All rights reserved.