Selective Conversion of CO2 into para-Xylene over a ZnCr2O4-ZSM-5 Catalyst

Article abstract of DOI:10.1002/cssc.202002305

An oxide-zeolite (ZnCr₂O₄-ZSM-5) catalyst for directly converting CO₂ to aromatics was designed and developed. It showed high PX/X (the C-mol ratio of p-xylene to all xylene) and PX/aromatics (the C-mol ratio of p-xylene to aromatics) ratios, which reached 97.3 and 63.9 %, respectively.

Full text of DOI:10.1002/cssc.202002305

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Selective Conversion of CO2 into para-Xylene over a ZnCr2O4-
ZSM-5 Catalyst
Authors: Weizhe Gao, Lisheng Guo, Yu Cui, Guohui Yang, Yingluo He,
Chunyang Zeng, Akira Taguchi, Takayuki Abe, Qingxiang Ma,
Yoshiharu Yoneyama, and Noritatsu Tsubaki
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202002305
Link to VoR: https://doi.org/10.1002/cssc.202002305
0
1/2020

ChemSusChem
10.1002/cssc.202002305
COMMUNICATION
Selective Conversion of CO
ZSM-5 Catalyst
2
into para-Xylene over a ZnCr
2
O -
4
[
a]
[a]
[a]
[a]
[a]
[b]
Weizhe Gao, Lisheng Guo,* Yu Cui, Guohui Yang, Yingluo He, Chunyang Zeng, Akira
[
c]
[c]
[d]
[a]
[a]
Taguchi, Takayuki Abe, Qingxiang Ma, Yoshiharu Yoneyama, Noritatsu Tsubaki*
[
a]
W. Gao, Dr. L. Guo, Y. Cui, Dr. G. Yang, Y. He, Dr. Y. Yoneyama, Prof. N. Tsubaki
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555 (Japan)
E-mail: lsguo07@hotmail.com
tsubaki@eng.u-toyama.ac.jp
[
[
[
b]
c]
d]
Dr. C. Zeng
China Petroleum Chemical Industry Federation, Beijing, 100723 (China)
Dr. A. Taguchi, Prof. T. Abe
Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama 930-8555 (Japan)
Dr. Q. Ma
State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia
University, Yinchuan, 750021 (China).
Abstract: An Oxide-Zeolite (ZnCr
2
O
4
-ZSM-5) catalyst for directly
of CO
direct conversion of CO
oriented catalytic conversion of CO
non-petroleum route, is still a challenge and has received much
attention in recent years, because PX is the main monomer to
polyester and PET bottle.
2
to methanol and methanol to target products, achieving
to aromatics by one-pass. However,
to para-Xylene (PX), as a
converting CO to aromatics was designed and developed. It showed
2
2
high PX/X (the C-mol ratio of para-Xylene to all xylene) and
PX/Aromatics (the C-mol ratio of para-Xylene to aromatics) ratios,
which reached 97.3% and 63.9%, respectively.
2
Direct conversion of CO
2
to high value-added chemicals,
particularly olefins and aromatics, has attracted wide attention in
[
1]
the field of CO
the emission of CO
offers a chemical route to utilize carbon resources. Generally,
the hydrogenation of CO to olefins or aromatics proceeds
through two different routes: direct Fischer-Tropsch synthesis
2
chemistry. It not only relieves, at least partially,
2
by the combustion of fossil fuels but also
[
2]
2
[
3]
(
2
FTS) route and indirect methanol route. For FTS route, CO is
firstly transformed to CO via reverse water-gas shift (RWGS)
reaction with simultaneously catalyzing conversion of CO into
[4]
hydrocarbons by the typical Fischer-Tropsch mechanism.
However, the produced olefins/aromatics obey the traditional
Anderson-Schulz-Flory (ASF) distribution with very low selectivity.
2
Scheme 1. Direct conversion of CO to aromatics with high PX selectivity.
In methanol route, CO
2
is firstly transformed into methanol and
subsequently methanol is converted to olefins/aromatics via the
2
For direct PX synthesis from CO over Oxide-Zeolite system via
[
5]
classical methanol-to-hydrocarbons process.
CO hydrogenation over Oxide-Zeolite catalyst has offered a
new route to directly produce aromatics, in which the oxide
catalyzes CO to methanol, while the zeolite is responsible for
methanol to aromatics. In recent years, different Oxide-Zeolite
methanol route, the step of methanol conversion on zeolite plays
a crucial role. In the traditional methanol to aromatics (MTA)
reaction, ZSM-5 zeolite shows excellent performance for PX
selectivity, owing to the suitable microporous channel size for the
2
2
[
9]
PX product diffusion (shape selectivity). In theory, PX and other
light aromatics (Benzene, Toluene and other Xylenes) are formed
over the internal acid sites of the micropores on ZSM-5 zeolite,
which are subsequently alkylated to heavy aromatics or
isomerized to other xylene, due to the external surface acidity and
[
6]
catalysts have been investigated extensively. For example, Ni et
al. proposed a catalyst composed of ZnAl and ZSM-5 zeolite
directly converting CO to aromatics with 73.9% selectivity. In
this catalysis system, methanol and dimethyl ether (DME) were
firstly synthesized by formate species hydrogenation on ZnAl
surface and then converted into aromatics over ZSM-5 zeolite.
Wang et al. designed a catalyst composed of Cr and ZSM-5
zeolite transforming CO to aromatics with 76.0% selectivity.
x
[
1c]
2
[
10]
x
the thermodynamics effect. In order to enhance PX selectivity,
it is vital to eliminate the external acidity of ZSM-5 zeolite. For
example, Zhang et al. proposed a catalyst with high PX selectivity
2
O
3
[7]
2
2
through coating amorphous SiO layer on the external surface of
[
11]
The fraction of BTX (Benzene, Toluene and Xylenes) was
increased from 13.2% to 43.6% by further modification of ZSM-5
zeolite with a nonacidic silicalite-1 zeolite shell. In these systems
of Oxide-Zeolite catalyst, catalysts were prepared by physically
ZSM-5 zeolite to cover the acid sites. Miyake et al. designed a
ZSM-5@Silicalite-1 core-shell zeolite with silicalite-1 as shell,
[
12]
which effectively inhibited the isomerization of PX. Moreover,
this ZSM-5@Silicalite-1 core-shell zeolite has also been applied
[
8]
mixing metal oxide and acidic zeolite. It coupled both processes
2
in direct conversion of CO to PX reaction. After using the core-
1
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ChemSusChem
10.1002/cssc.202002305
COMMUNICATION
shell zeolite, PX selectivity was increased from 7.6% to 25.3%,
but CO conversion was decreased from 34.5% to 27.6%. Until
2
ZSM-5 (Table S1, Supporting Information, SI). These results
show that the atmospheric MTA process does not exhibit good
aromatics selectivity compared with the Oxide-Zeolite system. As
shown in Table S1, atmospheric MTA reaction performance is
[
7]
now, direct highly selective synthesis of PX from MTA via pristine
ZSM-5 zeolite is still a challenge for scientific research and
industrial production. The key point is regulating the distribution of
Al site to decrease the acid sites density on the external surface
of ZSM-5 zeolite. Wang et al. prepared a ZSM-5 zeolite with very
few acid sites on the external surface of zeolite, demonstrated
extremely high PX selectivity, nearly 100%, in toluene methylation
reaction. However, this catalyst presented very low toluene
rather inferior, if compared with that of CO
Although the selectivity of aromatics and PX, obtained from MTA,
was improved under the same reaction conditions of CO
2
to aromatics process.
2
hydrogenation, it was still worse than that of a composite system
(Figure 1b). Thus, a high PX selectivity is more difficult to obtain
through MTA process. Accordingly, the ZnCr
(physical mixing, ZnCr /ZSM-5 mass ratio as 2:1) exhibited
high PX selectivity of 24.2%, with CO conversion and aromatics
C-mol selectivity as 23.1% and 85.3%, respectively, and the
undesirable CH selectivity was controlled within 1% (Figure 1a
2 4
O -ZSM-5 catalyst
[
13]
conversion. This research implied that high PX selectivity is
realizable over Oxide-Zeolite catalyst with pristine ZSM-5 via
2 4
O
2
methanol route, starting from CO
In this work, we presented an Oxide-Zeolite catalyst for CO
hydrogenation to PX, which composed of ZnCr and tailor-
2
or CO.
2
4
2
O
4
and 1b). PX selectivity is significantly higher than that of one
containing commercial ZSM-5 zeolite, where PX selectivity was
only 5.4%, as shown in Figure 1b. Furthermore, to the best of our
knowledge, the PX selectivity was higher than those of other
reported Oxide-Zeolite catalysts, as compared in Table S2 (SI). It
is worth noting that the reaction gases contain a small amount of
CO (3.94 vol%). The presence of a small amount of CO in the
reaction gases can obviously inhibit the RWGS reaction and
correspondingly reduce the selectivity of CO, promoting the
made ZSM-5 zeolite. It showed significantly high PX selectivity in
total hydrocarbons to 28.6%, which is higher than those over
reported Oxide-Zeolite catalysts under the same reaction
conditions with approximate CO conversion, and even exceeds
2
those on some surface modified ZSM-5 zeolites. Furthermore, by
adjusting the mass ratio of oxide/zeolite to 48:1, the ratios of PX/X
(
the C-mol ratio of para-Xylene to xylene) and PX/Aromatics (the
C-mol ratio of para-Xylene to aromatics) were promoted to 97.3%
and 63.9%, respectively. This study not only realizes direct
2
carbon atom of CO utilization efficiency (Tables S3 and S4, SI).
conversion of CO
but also sheds a sustainable approach for producing PX.
2
to single target product with high selectivity,
Given that RWGS reaction is an endothermic process, more CO
will be formed at high temperature. If CO is also involved in the
total reaction as an intermediate, more intermediates will promote
the formation of aromatics. However, aside from a slight increase
in the selectivity of CO in terms of high reaction temperature, there
is no increase in the selectivity of aromatics (Tables S5 and S6,
SI). These results indicate that CO mainly inhibits the occurrence
of RWGS rather than participates in the generation of aromatics.
These results implied that combining both reactions in tandem
and directly synthesizing aromatics with high PX selectivity from
CO
The effect of reaction temperature on CO
investigated (Figure S1, SI). CO conversion and CO selectivity
2
by one-pass was possible.
2
to PX has been
2
increased from 16.1% to 26.7% and 27.0% to 30.1% when
reaction temperature was increased from 320 to 380 oC,
respectively (Table S7, SI). In this process, the C-mol selectivities
of C2-4 paraffins and C5+ products (non-aromatics) increased
similarly. With the increasing of the reaction temperature,
aromatics selectivity decreased from 86.5% to 71.9% (Table S7,
SI). However, the PX selectivity firstly increased from 21.4% to
Figure 1. (a) Catalytic performance for CO
and CO to aromatics. (b) PX selectivity, CO
methanol and aromatics reaction conditions: 350 C, 4.0 MPa, (23.8vol% CO
2
to methanol, methanol to aromatics,
2
2
and methanol conversion. CO to
2
o
2
,
balance), 10 mL min- and time on stream
1
3.94vol% CO, 4.14vol% Ar and H
2
(
TOS) = 6 h, catalyst weight 0.5 g, oxide/zeolite mass ratio = 2:1. MTA reaction
o
conditions: 350 C, 4.0 MPa, ZSM-5 weight 0.5 g, liquid methanol flow rate
0
flow rate 15 mL min and TOS = 6 h. ZSM-5 sample (SiO /Al O = 300) was
-
1
.008mL min , gas (23.8vol% CO
2
, 3.94vol% CO, 4.14vol% Ar and H
2
balance)
24.2% and then decreased to 19.0% (Table S8, SI). These results
-
1
a
2 2 3
suggested that although high temperature was in favour of CO
2
purchased from Nankai University Catalyst Co. In Figure 1b, for the catalytic
process over single ZSM-5, it stood for methanol conversion.
hydrogenation, the selectivities of CO, C2-4 paraffin and aliphatic
hydrocarbons were simultaneously increased, due to the
accelerated RWGS reaction and side reactions of olefins and
aromatics by acidic sites. Thus, the optimal reaction temperature
In the process of direct conversion of CO
route, ZnCr and ZSM-5 zeolite have been separately
evaluated for CO
2
to PX via methanol
2
O
4
o
2
to methanol and MTA. Prior to reaction, all
is 350 C at 4.0 MPa for direct conversion of CO to PX. Under
2
o
catalysts were pre-treated at 400 C for 2 h under pure H
2
flow
these reaction conditions, 24.2% PX selectivity in hydrocarbons
was achieved.
-1
(
2 4
80 mL min ). As shown in Figure 1a and 1b, the ZnCr O
presented high methanol/DME selectivity of 98.6% with only 1.4%
For ZnCr O , the detailed physical properties were
2
4
o
CH
4
selectivity by 21.8% CO
2
conversion at 350 C and 4.0 MPa.
characterized by XRD, H -TPR, XRF, in situ DRIFT, and XPS.
2
No PX and other aromatics were formed over the single ZnCr
Figure 1b). In comparison, the tailor-made ZSM-5 zeolite was
investigated in MTA under the same reaction conditions with
4.9% aromatics selectivity, including 15.8% PX in all
hydrocarbons (Figure 1a and 1b). Similarly, traditional MTA
experiments at atmospheric pressure have been carried out on
2
O
4
The XRD pattern displayed characteristic peaks associated with
[
14]
(
2 4 2 2 4
ZnCr O spinel oxide (Figure S2, SI). H -TPR profile of ZnCr O
o
showed a single reduction peak in range of 200 to 300 C (Figure
S3, SI), which was ascribed to the reduction of the Zn-Cr spinel.
XRF result also demonstrated that the contents of Zn and Cr in
oxide agree with the theoretical calculating value, as shown in
4
2
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ChemSusChem
10.1002/cssc.202002305
COMMUNICATION
Table S9 (SI). In the process of CO
2
to methanol, the oxygen
species and the peak at 1365 cm-1 was ascribed to the CO 2-*
3
vacancy plays an important role in adsorption and transformation
species, which have been widely recognized as the precursors for
[
15]
[6b,7,17]
of CO
after pretreated by H
2
.
As shown by Figure S4a and S4b and Table S10 (SI),
methanol synthesis.
For ZnCr
2
O
4
-1
-ZSM-5 catalyst, the new
o
-1
2
at 400 C for 2 h, the concentrations of OH
peaks of C-O stretching at 1168 cm and H
appeared, indicating formation of ethers on ZSM-5 zeolite.
The DRIFTS results also confirmed the process of CO
hydrogenation to aromatics through methanol-mediated
pathway. Methanol as the intermediate was obtained from
ZnCr and aromatics were formed on ZSM-5 zeolite. Thus, the
3
CO* at 1101 cm
[
7,17,18]
and Odefect on the surface of ZnCr
2
O
4
were increased from 3% to
14% and 21% to 28%, respectively. These findings implied that
2
H
2
enhanced the concentration of oxygen vacancy on the surface
which was in favour of CO adsorption.
a
of ZnCr
2
O
4
,
2
Simultaneously, the peaks shifted to lower binding energy and the
peak of Cr6+ at binding energy of 578.5 eV disappeared over the
reduced ZnCr O (Figure S4c and S4d, SI). Meanwhile, varied
2 4
2 4
O
oxide/zeolite mass ratio plays a key role in the reaction since it
can control the production and conversion of the intermediate.
oxides combined with the same tailor-made ZSM-5 were also
evaluated, as compared in Tables S11 and S12 (SI). It indicates
that a suitable metal oxide plays a crucial role on CO
2
to aromatics,
and as-prepared ZnCr as a promising catalyst possesses
2
O
4
good catalytic properties. For ZSM-5 zeolite, the typical
characteristic peaks associated with MFI structure (Figure S2, SI).
The N adsorption/desorption isotherm indicated that the new
2
ZSM-5 zeolite had typical microporous structure (Figure S5, SI).
The surface area, pore volume and pore size are shown in Table
S13 (SI). 27Al MAS NMR spectra exhibited a sharp band at 54
ppm (Figure S6, SI), which was associated with tetrahedral
coordinated Al species. At the same time, the overall acidic
amounts and properties of the ZSM-5 zeolite were investigated.
Figure 2. (a) FTIR spectra of adsorbed pyridine and (b) FTIR spectra of
adsorbed di-ter-butyl-pyridine on ZSM-5 zeolite.
3 3
The NH -TPD profile of the ZSM-5 zeolite showed two NH
o
o
desorption peaks at low (173 C) and high temperature (325 C)
in Figure S7 (SI), which were ascribed to the weak acid sites and
[
10]
the sum of the medium and strong acid sites, respectively. In
the FT-IR spectra of adsorbed pyridine (Py-IR), the band at
around 1450 and 1540 cm-1 were assigned to Lewis (L) and
Brønsted (B) acid sites (Figure 2a). It is well-accepted that the
synergistic effect of L and B acid sites is indispensable in the
process of MTA. The acidic properties of the ZSM-5 zeolite are
listed in Table S14 (SI). On the amounts of acid, the results of
3
NH -TPD exhibited more acid sites than Py-IR results, which may
due to the different size of probe molecules and the fact that some
[
16]
acid sites can be approached only by NH
3
.
The FT-IR spectra
of adsorbed di-ter-butyl-pyridine (DTBPy) clarified the amounts of
external Brønsted acid sites (Figure 2b), owing to its larger probe
molecule which cannot enter the internal pores of ZSM-5 zeolite,
2 2 4 2 4
Figure 3. In situ DRIFT spectra of CO hydrogenation on ZnCr O and ZnCr O -
ZSM-5 catalyst.
-1 [8a]
since it was assigned to the band at 1616 cm . It is proved that
appropriate ratio of B/L and few external acid sites are beneficial
Generally, the Oxide-Zeolite catalyst with an oxide/zeolite mass
ratio of 2:1 or 1:1 exhibited the highest aromatics selectivity until
[10]
[6b,7]
for increasing the PX selectivity. On the other hand, employing
butylamine and tetrapropylammoniun bromide as organic
templates during ZSM-5 zeolite synthesis was favorable to
growing of ZSM-5 crystalline with intergrowth structure and
decreasing the external acidic amounts due to the inverse
now.
Based on this consideration, the effect of oxide/zeolite
mass ratio on the catalytic performance was studied systemically.
As compared in Figure S9 (SI), the aromatics selectivity
decreased as the oxide/zeolite mass ratio increased, but PX
selectivity slightly increased to 28.6% at a mass ratio of 12:1. The
aromatics selectivity was decreased significantly when the
oxide/zeolite mass ratio was further increased. It is worth noting
that the PX/X and PX/Aromatics ratios were sharply enhanced
with boosting up the oxide/zeolite mass ratio. High PX/X (97.3%)
and PX/Aromatics (63.9%) ratios were obtained at the
oxide/zeolite mass ratio of 48:1. To our knowledge, these are the
highest PX/X and PX/Aromatics ratios in conversion of CO2 to
aromatics until now, as compared in Table S2 (SI). Increasing the
oxide/zeolite mass ratio can enhance PX selectivity, PX/X and
PX/Aromatics ratios, because PX was readily to be isomerized or
alkylated at the external acidic sited of zeolites without spatial
confinement effect; decreasing total external surface of zeolite will
greatly decrease this possibility. Furthermore, by decreasing
mass of ZSM-5 zeolite, more light hydrocarbons were obtained
[
13]
distribution of Al zoning. In addition, the effect of ZSM-5 particle
size, controlled by rotation speed during hydrothermal synthesis
process on catalytic performance was investigated. Obviously,
the size of ZSM-5 particles can be well regulated by the rotation
speed (Figure S8, SI), in which high speed leads to the formation
of small-size particles. As seen, ZSM-5 of large size is beneficial
for the catalytic activity and the formation of PX product (Tables
S15 and S16, SI).
To further understand the reaction mechanism of CO
2
conversion on the Oxide-Zeolite catalyst, in situ diffuse
reflectance infrared Fourier transform spectroscopy (DRIFTS)
was performed to investigate the surface species on the process
of CO
2
2 2 4
to aromatics (Figure 3). For blank ZnCr O , the peaks at
962, 2865, 2730 and 1598 cm-1 were attributed to the HCOO*
3
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ChemSusChem
10.1002/cssc.202002305
COMMUNICATION
due to the incomplete MTA reaction. On the other hand,
increasing oxide/zeolite mass ratio can reduce the contact area of
PX and external surface of ZSM-5 zeolite during PX diffusion. In
addition, it can still cover more external surface of ZSM-5 zeolite
Acknowledgements
This work was supported by the Japan Science and Technology
Agency (MIRAI-JPMJMI17E2).
(
Figure S10, SI), which effectively suppresses the isomerization
of PX. The results revealed that the appropriate oxide/zeolite
mass ratio was efficient to increase PX selectivity and PX/X ratio.
As discussed above, large-size ZSM-5 controlled by low rotation
speed is more suitable for improving PX selectivity. In reality, on
Keywords: CO
2
conversion • ZSM-5 zeolite • Aromatics • Oxide-
Zeolite catalyst • Green chemistry
the small-size ZSM-5, the amount of Lewis acid sites is increased
from 42.1 to 63.5 μmol g-1 while the amount of Brønsted acid sites
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is unchanged basically (Table S17, SI). It suggests that more
Lewis acid sites on ZSM-5 zeolite are not favorable to increasing
the PX selectivity. When the oxide/zeolite mass ratio is increased,
more methanol intermediate forms while the acidic properties of
the ZSM-5 do not change. For a low oxide/zeolite ratio (1:1 or 1:2),
methanol can be completely converted. As for a high oxide/zeolite
mass ratio catalyst (48:1), more methanol is formed and not
completely converted, then presenting a high methanol and light
olefins selectivity. Based on the discussion above, there is an
optimal equilibrium between the methanol formation rate and the
amount of acid when there is an appropriate B/L ratio (Tables S18
and S19, SI).
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Figure 4. Stability of ZnCr
23.8vol% CO , 3.94vol% CO, 4.14vol% Ar and H
Catalyst weight, 0.5 g, oxide/zeolite mass ratio = 12:1.
(
2
2
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The stability experiment of ZnCr
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O
4
-ZSM-5 (oxide/zeolite mass
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2
in Figure 4, CO conversion and CO selectivity maintained at
around 22% and 28% during 100 h. The aromatics selectivity
decreased from 66.1% to 53.9% at 60 h and then kept stable.
Moreover, the selectivity of C2-4 slightly increased by lowering
aromatics selectivity to little extent in time-on-stream. The PX
selectivity leveled off around 23% after 30 h and PX/X ratio
remained stable at around 89%.
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In conclusion, we prepared an Oxide-Zeolite catalyst composed
of ZnCr
converted CO
zeolite with few external Brønsted acid sites presented excellent
2
O
4
and tailor-made ZSM-5 zeolite, which directly
2
to aromatics with high PX selectivity. The ZSM-5
PX selectivity of 28.6% based on 23.4% CO
2
conversion at 350
o
C and 4.0 MPa (Table S1 and Figure S9, Supporting Information).
Furthermore, PX/X ratio could be promoted to 97.3% by
increasing the oxide/zeolite mass ratio to 48:1. This report
provides a promising strategy to design an efficient Oxide-Zeolite
catalyst for converting CO
2
to highly valuable chemicals,
especially to PX, with high selectivity.
4
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ChemSusChem
10.1002/cssc.202002305
COMMUNICATION
Entry for the Table of Contents
This research achieved the direct conversion of CO
catalyst by one-pass coupling.
2
2 4
into aromatics with high para-Xylene (PX) selectivity over ZnCr O -ZSM-5
5
This article is protected by copyright. All rights reserved.