Carbon dioxide hydrogenation to light olefins over ZnO-Y2O3 and SAPO-34 bifunctional catalysts

Article abstract of DOI:10.1016/j.catcom.2019.105711

Conversion of CO₂ using renewable hydrogen to produce valuable chemicals has recently become highly attractive. Light olefins are important basic monomers for the production of various commodities. This work developed a bifunctional catalyst composed of ZnO-Y₂O₃ oxide and SAPO-34 zeolite, which catalyzes the selective hydrogenation of CO₂ to light olefins with a selectivity in hydrocarbons reaching 83.9% at a conversion of 27.6% at 390 °C. The obtained results demonstrated that CO₂ conversion and product distribution are strongly dependent on the oxide composition and structure. This new bimetallic oxide catalyst appears promising for further development of CO₂ conversion to other valuable chemicals.

Full text of DOI:10.1016/j.catcom.2019.105711

Accepted Manuscript
Carbon dioxide hydrogenation to light olefins over ZnO-Y2O3
and SAPO-34 bifunctional catalysts
Jian Li, Tie Yu, Dengyun Miao, Xiulian Pan, Xinhe Bao
PII:
S1566-7367(19)30173-6
https://doi.org/10.1016/j.catcom.2019.105711
105711
DOI:
Article Number:
Reference:
CATCOM 105711
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
23 December 2018
21 May 2019
23 May 2019
Please cite this article as: J. Li, T. Yu, D. Miao, et al., Carbon dioxide hydrogenation
to light olefins over ZnO-Y2O3 and SAPO-34 bifunctional catalysts, Catalysis
Communications, https://doi.org/10.1016/j.catcom.2019.105711
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ACCEPTED MANUSCRIPT
Carbon dioxide hydrogenation to light olefins over
ZnO-Y O and SAPO-34 bifunctional catalysts
2
3
a,b
a
a
a,*
Jian Li , Tie Yu , Dengyun Miao , Xiulian Pan panxl@dicp.ac.cn and Xinhe Bao
a,c,*
xhbao@dicp.ac.cn
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, Liaoning, China
b
University of Chinese Academy of Sciences, Beijing 100049, China
c
Department of Chemical Physics, University of Science and Technology of China,
Hefei 230026, Anhui, China
*
Corresponding authors.
Abstract
Conversion of CO2 using renewable hydrogen to produce valuable chemicals has
recently become highly attractive. Light olefins are important basic monomers for the
production of various commodities. This work developed a bifunctional catalyst
composed of ZnO-Y2O3 oxide and SAPO-34 zeolite, which catalyzes the selective
hydrogenation of CO2 to light olefins with a selectivity in hydrocarbons reaching 83.9%
at a conversion of 27.6% at 390 °C. The obtained results demonstrated that CO2
conversion and product distribution are strongly dependent on the oxide composition
and structure. This new bimetallic oxide catalyst appears promising for further
development of practical CO2 conversion to other valuable chemicals.
Keywords:
carbon dioxide, ZnO-Y2O3, bifunctional catalysts, light olefins.
1
. Introduction
Alternative energy sources preferably renewable and environment protection are
two major issues for a future sustainable society. Excessive carbon dioxide in the
atmosphere is considered a major cause of ocean acidification and climate change [1].

ACCEPTED MANUSCRIPT
It is widely agreed to control carbon dioxide emissions and more scientific discoveries
have been inspired for effective valorization of CO2, which is an alternative way for
its emissions reduction. In the meanwhile, utilization of CO2 as a carbon source is also
proposed, e.g., via electro-catalysis [2,3], photo-catalysis [4,5] and thermal catalysis
[6,7]. Although there are still debates on the energy and carbon efficiency, as well as
economic viability of heterogeneous catalytic hydrogenation of CO2, hydrogenation
using renewable hydrogen might be a solution for the conversion of large-scale
industrial CO2.
Light olefins, including ethylene, propylene and butylene, are important
feedstocks for the production of a variety of chemicals, such as polymers and drugs.
CO2 can be hydrogenated to light olefins via a modified Fischer-Tropsch synthesis
[8,9]. However, the distribution of hydrocarbon products over conventional
Fischer-Tropsch synthesis catalysts is limited by the surface polymerization
mechanism and it follows the Anderson-Schulz-Flory (ASF) distribution model. The
selectivity of C2-C4 hydrocarbons (including olefins and paraffins) is typically less
than 58% [10-12]. For example, the sodium-containing Fe3O4 catalysts were used for
carbon dioxide hydrogenation, which showed a selectivity of 46.6% for C2-C4 light
olefins among hydrocarbons at a CO2 conversion of 40.5% [13].
We recently proposed a catalyst design concept of a bifunctional oxide-zeolite
(OX-ZEO) system, which separates CO activation and C-C coupling onto two
different types of active sites [12]. It was demonstrated to be an effective strategy to
surpass the ASF distribution limit in CO hydrogenation to light olefins, which yields a
selectivity of C2-C4 hydrocarbons up to 94% [12]. By changing the zeolite to
mordenite or HZSM, this OX-ZEO catalytic system allows syngas conversion to
ethylene with a selectivity up to 73% and to aromatics 73.9%, respectively [14,15].
It was recently shown that this concept is also applicable for CO2 hydrogenation.
For example, by combining ZnO-ZrO2 [16], ZnGa2O4 [17] and In2O3-ZrO2 [18] with
SAPO-34 zeolite as composite catalysts, CO2 can be hydrogenated to light olefins
with a selectivity in hydrocarbons reaching over 80%. Although the direct CO2
conversion through bifunctional catalysts has gained wide attention, the catalytic

ACCEPTED MANUSCRIPT
performance still needs to be improved and a better understanding of the reaction
mechanism is also needed for further optimization of catalysts.
Herein, we report that ZnO-Y2O3 in combination with SAPO-34 as a composite
catalyst gives a high selectivity towards light olefins (83.9%) in hydrocarbons with
only 1.8% methane at a CO2 conversion of 27.6%. The roles of ZnO and Y2O3 in the
mixed oxide were studied, and results showed that this mixed oxide composition
affects CO2 conversion and product selectivity significantly.
2
. Experimental
2
.1. Catalyst preparation
The oxide catalysts were prepared by the co-precipitation method. A 6.7 wt%
ZnO-Y2O3 catalyst was taken as an example to describe the synthesis procedures: 1.79
g Zn(NO3)2 · 6H2O and 22.99 g Y(NO3)3 · 6H2O were dissolved in 100 mL deionized
water in a beaker A. 15.6 g aqueous ammonia (25 wt% solution) was added to a
beaker B containing 100 mL water. The solution in beaker A and B were
simultaneously added to a three-necked flask with 100 mL deionized water (at a flow
-
1
rate of 5 mL min ) under vigorous stirring at 70 °C to form a precipitate. After aging
for 2 h at 70 °C, the suspension was cooled to room temperature, filtered, and then
washed five times with deionized water. The filter cake was dried at 110 °C for 12 h
and then calcined at 500 °C in static air for 8 h. Other samples with different
compositions x wt% ZnO-Y2O3 were synthesized by the same method, and x
represents the ZnO content.
SAPO-34 was synthesized hydrothermally using trithylamine (TEA) as the
template. The molar composition was 1SiO2 : 11Al2O3 : 18H3PO4 : 33TEA : 667H2O.
The resulting gel mass was stirred for 2 h at room temperature, then transferred into
2
00 mL autoclave and heated for 24 h at 180 °C. The solid product was finally
obtained through centrifugation, washed several times with deionized water till the pH
of the supernate was 7.0-7.5, followed by drying overnight at 120 °C. Finally, the
template was removed by calcination at 550 °C for 3 h.

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2
.2. Catalytic activity studies
The carbon dioxide hydrogenation reaction was performed in a tubular quartz
fixed-bed reactor. Typically, 400 mg composite catalyst (20-40 mesh) with x wt%
ZnO-Y2O3/SAPO-34 = 1/1 (mass ratio) was used, unless otherwise stated. The
-
1
-1
catalyst was treated at 400 °C with H2 (900 mL h gcat ) for 2 h at 1 bar pressure prior
-
1
to reaction, where the temperature was increased at a ramp of 1.5 °C min . The
temperature was then decreased to 390 °C and the feed gas flow was switched to a
pre-mixed stream of CO2, H2 and Ar (19 vol% CO2 + 76 vol% H2 + 5 vol% Ar).
Reaction was carried out using a H2/CO2 molar ratio of 4, P=4.0 MPa, T=390 °C and
-
1
-1
a GHSV of 1800 mL h gcat . The composition of the effluent stream was analyzed by
an online GC (Agilent 7890B), which was equipped with a thermal conductivity
detector (TCD) and a flame ionization detector (FID). Hayesep Q and 5A molecular
sieves packed columns were connected to TCD, while HP-FFAP and HP-AL/S
capillary columns were connected to FID. Oxygen-containing compounds and
hydrocarbons up to C5+ were analyzed by the FID, while CO2, CO, CH4, C2H4 and
C2H6 were analyzed by the TCD. CH4 and C2H4 were taken as a reference bridge
between FID and TCD. CO2 conversion and product selectivities were calculated on a
carbon atom basis according to the following Eqs. (1)-(3):
F(CO2 inlet)−F(CO2 outlet)
CO2 Conversion =
× 100%,
(1)
F(CO2 inlet)
where, F(CO2 inlet) and F(CO2 outlet) represent the molar flow rates of CO2 at the
inlet and outlet, respectively. The CO selectivity was calculated according to:
F(CO outlet)
CO Selectivity =
× 100%,
(2)
F(CO2 inlet)−F(CO2outlet)
where F(CO outlet) denotes the molar flow rate of CO at the outlet of reactor.
The selectivity of the individual hydrocarbon, CnHm among hydrocarbons was
obtained according to:
n F(CnHm outlet)
CnHm Selectivity =
(3)
F(CO2 inlet)−F(CO2outlet)−F(CO outlet )
The carbon balance fell in a range of 98%-102%.
2
.3. Catalyst characterization

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Powder X-ray diffraction (XRD) was carried out on an Empyrean-100
X-ray diffractometer with a Cu Kα (λ = 1.541 Å), operated at 40 mA and 40
kV. XRD patterns were recorded in the range of 2θ = 5-80°.
Brunauer-Emmett-Teller (BET) surface area and the textural
characteristics of x wt% ZnO-Y2O3 catalysts were characterized by nitrogen
adsorption/desorption isotherms at 77 K using a Quantachrome autosorb iQ2
gas adsorption analyzer. The oxide samples were degassed at 300 °C for 2 h
before measurements.
The temperature-programmed desorption of CO2 and CO (CO2-TPD and
CO-TPD) of x% ZnO-Y2O3 catalysts were conducted on an AutoChem 2910
adsorption/desorption system. In a typical CO2-TPD run, a 100-mg sample was
-
1
treated in situ in a H2 gas stream (30 mL min ) at 400 °C for 2 h, flushed with
-
1
He gas (30 mL min ) at 400 °C for 30 min, and then cooled to 50 °C. The inlet
gas flow was then switched to CO2 to allow adsorption for 60 min. Afterwards,
the sample was flushed with He until a stable baseline was obtained. Then the
-
1
sample was heated from 50 to 600 °C at the heating ramp of 10 °C min , while
the composition of effluent was monitored by a TCD and an OmniStar 300
mass spectrometer. The adsorption amounts of CO2 and CO were estimated
based on the peak area of CO2 and CO calibration pulses.
Hydrogen temperature-programmed reduction (H2-TPR) was carried out in
a tubular quartz reactor. A 100- mg of each sample was placed in the reactor and
treated with pure He at 120 °C for 1 h, then cooled to 50 °C in He flow. The
-
1
sample was then heated in 1 vol% H2/Ar at the ramp of 10 °C min from 50 to
00 °C, while the composition of effluent was monitored by the on-line mass
spectrometer.
High-resolution transmission electron microscopy studies (HRTEM) were
conducted on a JEM 2100 microscope. Scanning electron microscopy studies
SEM) were conducted on a ZEISS Orion nanoFab.
8
(
3
. Results and discussion

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Table 1 lists the catalytic performance of various ZnO-Y2O3 combined with
SAPO-34 zeolite bifunctional catalysts towards CO2 hydrogenation, where the mass
ratio of ZnO/(ZnO+Y2O3) was varied. Y2O3+SAPO-34 catalyst gives a CO2
conversion of 6.6%, producing only CO and no light olefins. By the addition of ZnO,
=
=
light olefins C2 -C4 products start to appear. In the meanwhile, the catalytic activity
is gradually improved with the increasing amount of ZnO, and CO2 conversion
reaches a maximum value (ca. 27.6%) when the oxide contains 6.7 wt% ZnO.
However, by further increasing the ZnO content results in a declining CO2 conversion.
Since the same SAPO-34 zeolite has been used, the different performance between
these bifunctional catalysts should be attributed to the ZnO-Y2O3 mixed oxides phase.
In order to understand the role of these oxides in the catalytic system, a series of
characterizations were applied.
X-ray diffraction (XRD) patterns (Fig. 1) show crystal phases of the ZnO-Y2O3
oxides. As shown in Fig. 1, diffraction peaks at 2θ = 31.7, 34.4 and 36.3° are assigned
to the (100), (002) and (101) plane facets of the hexagonal ZnO crystal (JCPDS
3
6-1451), respectively. In the single component Y2O3 sample, the diffraction peaks at
2
θ = 20.5 and 29.2° are assigned to the (211) and (222) plane facets of Y2O3 crystal
(JCPDS 41-1105), respectively. Only pure ZnO and Y2O3 diffraction peaks are
detected for the single components of ZnO and Y2O3 samples. For the bi-component
ZnO-Y2O3 samples, there is no new crystal phase observed. However, the relative
peak intensity for each oxide varies with their contents as expected. In addition, the
ZnO crystal phase is not detected when ZnO content is less than 6.7 wt%, indicating
that ZnO is dispersed well (< 4.0 nm primary crystal particles). However, the crystal
plane (101) of hexagonal ZnO crystal is still discernible in the transmission electron
microscopy (TEM) images recorded for the 6.7 wt% ZnO-Y2O3, as shown in Fig. S1
(see ESI). This also indicates that ZnO still retains the hexagonal crystal phase in this
mixed oxide system.
Scanning electronic microscopy (SEM) images of ZnO-Y2O3 oxides shown in
Fig. S2 (see ESI) exhibit different morphologies. The morphology of bi-component
oxides appears fluffy and more finely dispersed. BET measurement indicates that

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these samples exhibit higher specific surface areas (SSA) than either of the parent
single metal oxide (see Table S1, ESI). It indicates that the addition of ZnO increases
the pore volume and surface area, which then it is reasonably expected to offer a
larger number of surface catalytic sites, thus enhancing catalytic activity.
H2-TPR was used to elucidate the reducibility of the solid samples. As displayed
in Fig. 2, there is a weak and broad reduction peak for ZnO centered at  405 °C. In
comparison, Y2O3 is much more difficult to reduce than ZnO, as its reduction starts
only above 400 °C showing a peak maximum at 497 °C. Therefore, it appears that
under reaction conditions, Y2O3 is not expected to be found in a partially reduced
form. With the addition of ZnO into Y2O3, the reduction gradually becomes more
facile.
The CO2-TPD profiles shown in Fig. 3a exhibit three CO2 desorption peaks for
Y2O3, 0.18 wt% ZnO-Y2O3 and 6.7 wt %ZnO-Y2O3 solids. However, there is almost
no CO2 adsorption over ZnO. The total amount of CO2 adsorption estimated on ZnO
is two orders of magnitude lower than that estimated over the other three oxides (see
Table S2, ESI). Surprisingly, the total amount of CO2 adsorption gradually increases
with increasing ZnO content, implying that ZnO can promote the adsorption of CO2
over Y2O3. Therefore, CO2 likely adsorbs on Y2O3, and the presence of ZnO likely
promotes its adsorption on Y2O3. The three CO2-TPD peaks observed over Y2O3, 0.18
wt% ZnO-Y2O3 and 6.7 wt% ZnO-Y2O3 fall in a similar temperature range. The peak
at  94 °C is assigned to weakly adsorbed CO2, while the other two peaks at  203 ºC
and 390 °C belong to strongly adsorbed CO2.
Although single component Y2O3 metal oxide exhibits the highest capacity of CO2
adsorption, its CO2 hydrogenation activity appears to be the lowest (CO2 conversion
only 6.6%) among these mixed-metal oxides when combined with SAPO-34 (see
Table 1). Furthermore, CO is the only product over Y2O3 combined with SAPO-34. In
contrast, even though ZnO exhibits a very poor CO2 adsorption capacity it gives a 16%
CO2 conversion when combined with SAPO-34. This might be related with the H2
activation ability of the metal oxide phase, as this is considered as a critical step for
hydrogenation.

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In fact, ZnO is known to be active towards activation of molecular H2 [19]. Liu and
co-workers [20] observed the formation of hydroxyl (OH) species on the ZnO (10 1̅ 0)
surface at room temperature when the solid was exposed to 0.3 mbar H2. This result
indicated the dissociative adsorption of H2. Griffin and co-workers [21] reported the
dissociation of H2 on commercial ZnO, as evidenced by the bond-stretching
-
1
frequencies recorded at 1710 and 3490 cm in their IR studies. It was shown that H2
adsorbed rapidly and reversibly to form active hydrogen species at room temperature
[22, 23], which could be reactive for the hydrogenation of ethylene to ethane [24]. On
the other hand, there is lack of reports on the activation of H2 for Y2O3 to the best of
our knowledge.
The above results and discussion show that Y2O3 is active for CO2 adsorption,
while H2 adsorption and activation are likely taking place at the ZnO surface. This
may explain the activity behaviour of ZnO-Y2O3 mixed oxides for the CO2
hydrogenation. However, there is no linear relationship between CO2 conversion and
the content (wt%) of ZnO. Furthermore, the co-precipitated ZnO-Y2O3 solid exhibits
a much higher activity and light olefin selectivity than its physical mixture counterpart
(ZnO+Y2O3) (see Table 2). For example, the co-precipitated 6.7 wt% ZnO-Y2O3
catalyst gives a 27.6% CO2 conversion, while the physical mixture counterpart gives
only a 17.4% CO2 conversion. The co-precipitated oxide is also more active than the
impregnated (6.7 wt% ZnO/Y2O3) one with same chemical composition. It is
suggested that this activity behaviour is likely attributed to the interface developed
between ZnO and Y O during the co-precipitation process of synthesis followed by
2
3
the calcination step, which seems to play an important role for the simultaneous CO2
and H2 activation, hence the hydrogenation reaction. As a result, the 6.7 wt%
ZnO-Y2O3+SAPO-34 catalyst exhibits the best CO2 hydrogenation activity.
CO-TPD studies were also conducted in order to gain some good understanding
about the high selectivity of CO in the present CO2 hydrogenation reaction. It can be
seen from Fig. 3b that ZnO gives almost no CO desorption up to 500 °C. This is
surprising because it has been demonstrated in our previous study that ZnO is active
in CO hydrogenation for light olefin synthesis when combined with SAPO-34 [25].

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The amount of CO desorption increases stepwise with reducing content (wt%) of ZnO.
The Y2O3, 0.18 wt% ZnO-Y2O3 and 6.7 wt% ZnO-Y2O3 metal oxides all exhibit two
CO desorption peaks. The first one appears at  85 ºC is attributed to some weakly
chemisorbed adsorbed CO, and the second one in the 260-290 ºC range corresponds
to a more strongly chemisorbed CO. Note that the total amount of CO2 desorbed is
dozens of times higher than that of CO over the mixed oxides containing Y2O3 (see
Table S2, ESI), and that the CO desorption temperature overlaps with that of CO2
desorption. Therefore, there is likely competitive adsorption between CO2 and CO
over these metal oxides, thus further adsorption and conversion of CO might be
suppressed. This might be one reason for a high selectivity of CO in the CO2
hydrogenation reaction in this work.
4
. Conclusions
In summary, we reported that a bifunctional ZnO-Y2O3+SAPO-34 catalyst
largely promotes the CO2 hydrogenation into C2-C4 olefins with an outstanding
selectivity. The activity and selectivity varied with composition and structure of the
metal oxides, which is likely attributed to their different functions, i.e. CO2 adsorption
and activation over Y2O3 and H2 over ZnO. Hence, the co-precipitated composite 6.7
wt% ZnO-Y2O3 catalyst gave the highest CO2 conversion and light olefin selectivity
when combined with SAPO-34, ca. selectivity to light olefins in hydrocarbons 83.6%
at a single pass CO2 conversion of 27.6%.
Acknowledgments
This work was supported by the Ministry of Science and Technology of China
(No.2017YFB0602201), the Chinese Academy of Sciences (XDA21020400) and the
National Natural Science Foundation of China (Grant Nos. 21621063, 21425312).
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9

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Table 1. Hydrocarbons distribution and CO
2
conversion over different ZnO-Y
2
3
O samples in
a
combination with SAPO-34 as bifunctional catalysts for the hydrogenation of CO
2
.
Hydrocarbon distribution [%]
CO
2
Conv.
CO
Oxide sample
=
2
=
-
-
[
%]
CH4
C -C
C -C
C5+
Sel. [%]
4
2
4
Y
2
O
3
6.6
0
0
0
0
100
97.4
85.0
89.3
75.8
0
.18 wt% ZnO-Y O
17.7
27.6
17.4
16.0
5.1
1.8
1.4
1.4
81.4
83.9
82.8
88.6
8.6
4.9
1.4
2.3
1.6
2
3
6
.7 wt% ZnO-Y
9.4 wt% ZnO-Y
ZnO
Reaction conditions: CO
2
O
3
12.9
13.5
8.4
1
2
O
3
a
-1 -1
2
/H = 1/4, 390 °C, 4.0 MPa, GHSV = 1800 mL g h .
2

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Table. 2. Hydrocarbons distribution and CO
2
conversion over different preparation methods of
a
ZnO-Y
2
O
3
oxides combined with SAPO-34 as bifunctional catalysts of CO hydrogenation .
2
Hydrocarbons distribution [%]
CO
2
CO
Oxide sample
=
=
-
-
Conv. [%] CH4 C -C
C -C
C5+ Sel. [%]
2
4
2
4
b
6
.7 wt% ZnO-Y
2
O
3
-CP
27.6
19.3
17.4
1.8
2.9
4.9
83.9
12.9
16.9
26.7
1.4
2.1
3.8
85.0
84.3
91.4
c
6.7 wt% ZnO/Y O -I
78.1
64.6
2
3
d
6
.7 wt% ZnO+93.3 wt% Y
2
O
3
-M
a
b
c
d
-1 -1
Reaction conditions: CO
.7 wt% ZnO-Y
.7 wt% ZnO/Y
.7 wt% ZnO+93.3% wt% Y
with a mass ratio of 6.7/93.3.
2
/H
2
= 1/4, 390 °C, 4.0 MPa, GHSV = 1800 mL g h .
6
2
O
3
-CP was prepared by co-precipitation.
-I was prepared by impregnation of ZnO to Y
-M was prepared by grinding the ZnO and Y
6
2
O
3
2
3
O .
6
2
O
3
2
O
3
mixture

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Figure captions
Fig. 1. XRD patterns of ZnO-Y2O3 oxides.
Fig. 2. H2-TPR profiles of ZnO-Y2O3 oxides.
Fig. 3. TPD profiles of ZnO-Y2O3 oxides (a) CO2-TPD; (b) CO-TPD.

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Fig 1.

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Fig 2.

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Fig 3.

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Graphical abstract:
Highlights



CO
2
hydrogenation to light olefins over bifunctional catalyst (ZnO-Y
2
O + SAPO-34).
3
A high CO
2
conversion (27.6%) was achieved through interaction between ZnO and Y
2
3
O .
=
=
A selectivity for C
2
–C was up to 83.9% in hydrocarbons with 1.8% CH4.
4