New ZnCe catalyst encapsulated in SBA-15 in the production of 1,3-butadiene from ethanol

Article abstract of DOI:10.1016/j.cclet.2019.04.038

ZnO-CeO₂/SBA-15 catalysts were prepared by two kinds of solid-state grinding method and used for the production of 1,3-butadiene (1,3-BD) from ethanol. A mixture of SBA-15 (with or without organic template) and metal precursors were ground in solid-state. The obtained catalysts were characterized by TG, N₂ adsorption-desorption, TEM, XRD, Py-FTIR and NH₃-TPD techniques. Superior dispersion of metal oxides and more exposed acid sites were achieved on the catalyst 10Zn₁Ce₅-AS with the presence of organic template in SBA-15 during the solid-state grinding process. The catalytic performance was evaluated in a fixed-bed reactor and a 1,3-butadiene selectivity of as high as 45% is achieved. This is attributed to the coupling effect of Zn and Ce species in the mesopores of SBA-15, in which Zn promotes ethanol dehydrogenation and Ce enhances aldol-condensation, respectively. Additionally, solvent-free method inspires new catalyst synthesis strategy for the production of 1,3-butadiene from ethanol.

Full text of DOI:10.1016/j.cclet.2019.04.038

Accepted Manuscript
Title: New ZnCe catalyst encapsulated in SBA-15 in the
production of 1,3-butadiene from ethanol
Authors: Yujun Zhao, Sijia Li, Zheng Wang, Shengnian Wang,
Shengping Wang, Xinbin Ma
PII:
DOI:
Reference:
S1001-8417(19)30208-6
https://doi.org/10.1016/j.cclet.2019.04.038
CCLET 4931
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
14 March 2019
11 April 2019
16 April 2019
Please cite this article as: Zhao Y, Li S, Wang Z, Wang S, Wang S, Ma X, New
ZnCe catalyst encapsulated in SBA-15 in the production of 1,3-butadiene from ethanol,
Chinese Chemical Letters (2019), https://doi.org/10.1016/j.cclet.2019.04.038
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Communication
New ZnCe catalyst encapsulated in SBA-15 in the production of 1,3-butadiene from
ethanol
a,
a
a
b,
a
a
Yujun Zhao , Sijia Li , Zheng Wang , Shengnian Wang , Shengping Wang , Xinbin Ma
a
Key Laboratory for Green Chemical Technology of Ministry of Education. Collaborative Innovation Center of Chemical Science and Engineering, School of
Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
Chemical Engineering, Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA
b
ARTICLE INFO
Article history:
Received 14 March 2019
Received in revised form 11 April 2019
Accepted 12 April 2019
Available online

Corresponding authors
E-mail addresseses: yujunzhao@tju.edu.cn (Y. Zhao), swang@latech.edu (S. Wang)
Graphical Abstract
2
We report a highly efficient ZnO-CeO /SBA-15 catalyst prepared by solid-state grinding method with organic template. The limited
space between the template and the silica wall of SBA-15 and the strong interaction between metal ions and template improve the
dispersion of metal oxides, which help obtain more effective acid sites, resulting in a 1,3-butadiene selectivity as high as ~45%.
ABSTRACT
ZnO-CeO
ethanol. A mixture of SBA-15 (with or without organic template) and metal precursors were ground in solid-state. The obtained catalysts were
characterized by TG, N adsorption-desorption, TEM, XRD, Py-FTIR and NH -TPD techniques. Superior dispersion of metal oxides and more
exposed acid sites were achieved on the catalyst 10Zn Ce -AS with the presence of organic template in SBA-15 during the solid-state grinding process.
2
/SBA-15 catalysts were prepared by two kinds of solid-state grinding method and used for the production of 1,3-butadiene (1,3-BD) from
2
3
1
5
The catalytic performance was evaluated in a fixed-bed reactor and a 1,3-butadiene selectivity of as high as 45% is achieved. This is attributed to the
coupling effect of Zn and Ce species in the mesopores of SBA-15, in which Zn promotes ethanol dehydrogenation and Ce enhances aldol-
condensation, respectively. Additionally, solvent-free method inspires new catalyst synthesis strategy for the production of 1,3-butadiene from ethanol.
Keywords:
1
,3-Butadiene
Ethanol
SBA-15
Dehydrogenation
Aldol-condensation
As an important organic feedstock, 1,3-butadiene is widely used in the production of polybutadiene, synthetic rubber, polymer resin,
—
——

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and many other petrochemical products [1, 2]. In current world market, 1,3-butadiene is mainly produced by separating from the C4
fraction following ethylene manufacturing via the steam cracking of naphtha. However, its production is largely tied with the
petroleum resources.
With the advancement of bioethanol technology and abundant ethanol source in recent years, synthesizing 1,3-butadiene from
renewable ethanol draws great interests [3, 4]. The synthesis of 1,3-butadiene from ethanol have two major routes: one-step direct
synthesis of 1,3-butadiene and two-step production process (ethanol is first partially converted to acetaldehyde and acetaldehyde is
further reacted with ethanol to obtain 1,3-butadiene) [1, 5, 6]. The two-step process has advantages of low reaction temperature and
high selectivity to 1,3-butadiene, but the process is more complicated and the equipment investment is higher in comparison with one-
step method. For one-step process, how to fabricate an efficient catalyst with high 1,3-butadiene selectivity is always a big challenge.
The reaction mechanism of this system is complicated and involves five consecutive reaction steps (Scheme 1) [7, 8]: (i) ethanol is
dehydrogenated to acetaldehyde, (ii) two acetaldehyde molecules are converted to acetaldol by aldol-condensation, (iii) acetaldol is
dehydrated to form crotonaldehyde, (iv) Meerwein–Ponndorf–Verley (MPV) reaction between crotonaldehyde and ethanol to obtain
crotyl alcohol, (v) 1,3-butadiene is generated by the dehydration of crotyl alcohol. In this multiple-step reaction system, ethanol
dehydrogenation and aldol-condensation are two critical steps [9], which require two different active sites of the catalyst. For ethanol
dehydrogenation, redox active sites are required, typically provided by copper oxide [1, 5, 8], silver [1, 10] or zinc oxide [1, 11]. The
aldol-condensation reaction requires Lewis acid or base sites. The Lewis acid sites are generally supplied by elements from the IIIB,
IVB and VB groups in the periodic table, such as Y [12], Zr [13], Hf [14] and Ta [15]. Magnesium oxide is the typical candidate for
Lewis base sites [16]. An excellent ethanol-to-1,3-butadiene catalyst requires wise coupling of these two types of active sites.
Baerdemaeker et al. developed a stable and active catalyst by combining Hf(IV) with Zn(II), in which the adding of Zn(II) to the
Hf(IV)-containing catalyst was crucial important to force the Hf(IV) into catalyzing the acetaldehyde condensation rather than the
ethanol dehydration [17]. Dai et al. prepared a bi-component Zn−Y catalyst confined in zeolite cages, and exhibited a high 1,3-
butadiene selectivity of 63% in the one-step process [18].
Scheme 1. Reaction mechanism of ethanol to 1,3-butadiene.
Ordered mesoporous silica (OMS) materials, such as SBA-15 [19], KIT-6 [19], MCF [20] and MCM-41 [21], were explored as
catalyst supports in this reaction system. Their large pore size and specific surface area were believed to improve the accessibility of
active sites for reactants and intermediates, effectively solving the mass transfer and coking formation during 1,3-butadiene production
from ethanol [19]. Chae et al. have reported that Ta/OMS catalysts had better coke tolerance, catalytic longevity and catalytic activity
than conventional silica-based catalysts [19]. For these catalysts, the metal was generally loaded by wet impregnation method, which
was not time and energy effective. However, solvent-free method has not yet reported in the ethanol to 1,3-butadiene system.
2
In this study, we compared two kind of ZnO-CeO /SBA-15 catalysts with different solid-state grinding method (SBA-15 with or
without organic template P123 (PEO-PPO-PEO) during grinding process). The limited space between the template P123 and the silica
wall of SBA-15 prevents the infiltrated metal oxides from forming large aggregates, and achieving a high dispersion of oxides. The
effect of the template P123 in SBA-15 on the formation process and physicochemical properties of the catalysts, as well as the catalytic
performance in ethanol-to-1,3-butadiene were discussed in detail. In addition, as a solvent-free method, this solid-state grinding
strategy is energy effectively and environmentally friendly in comparison with wet impregnation method.
The mesoporous silica SBA-15 support was prepared by a hydrothermal synthesis method reported by Zhao et al. [22] The as-
prepared SBA-15 material (with organic template) was recovered by filtration, washed and dried. Then through calcination, the
1 5 1 5
template-free SBA-15 material was obtained. The catalysts 10Zn Ce -AS/10Zn Ce -CS were prepared by solid-state grinding the as-
prepared/template-free SBA-15 with the metal precursors. 10Zn-AS and 10Ce-AS were only loaded single metal oxide. The detailed
information about synthesis method was given in Supporting information.
In order to explore the decomposition process of catalysts, the as-prepared SBA-15, 10Zn
characterized by TG technique. As shown in Fig. 1a, the decomposition of template (P123) of as-prepared SBA-15 takes place between
50 °C and 280 °C, corresponding to a sharp DTG peak at 180 °C in Fig. 1b. As the temperature further rises, the subsequent weight
loss is attributed to the removal of residual carbonaceous species [23]. The decomposition of 10Zn Ce -CS presents two DTG peaks at
0 °C and 200 °C, which is the result of the removal of water and the thermal decomposition of Zn(NO and Ce(NO , respectively.
For 10Zn Ce -AS, the TG curve shows 50% weight loss due to the decomposition of organic template and metal salt precursors.
1 5 1 5
Ce -AS and 10Zn Ce -CS were
1
1
5
8
3
)
2
3 3
)
1
5
However, the decomposition temperature of P123 is at around 235 °C, much higher than that for SBA-15 support itself (180 °C) [22].
Similar phenomenon has also been observed on CuO/SBA-15 [24] and ZnO/SBA-15 [25]. This is believed as the result of the strong

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interaction between metal ions and the head group of PEO molecules. The creation of helical crown ether-like metal-PEO complexes
inevitably impedes the decomposition of the nonionic surfactant [26]. And the interaction contributes to a high dispersion of oxides on
the mesoporous support.
(a)
10Zn1Ce5-CS
(b)
1
00
0
4
9
8
7
6
5
4
0
0
0
0
0
0
1
0Zn1Ce5-CS
-
-8
12
16
1
0Zn1Ce5-AS
SBA-15
-
-
1
0Zn1Ce5-AS
SBA-15
1
00 200 300 400 500 600 700
100 200 300 400 500 600 700
Temperature (ºC)
Temperature (ºC)
1 5 1 5
Fig. 1. (a) TG curve and (b) DTG curve of the uncalcined SBA-15, 10Zn Ce -AS and 10Zn Ce -CS.
Fig. S2 (Supporting information) shows the nitrogen adsorption-desorption isotherms and the pore size distribution for various
samples. All samples display Langmuir Type IV isotherms with a H4 hysteresis loop (Fig. S2a), indicating the presence of mesoporous
structure with uniform pore size [27]. Compared with the parent SBA-15 sample, the hysteresis of the modified samples shift to lower
P/P values, suggesting that a decrease of pore size after active species loading [28], which is also evidenced by their pore size
0
distribution profiles shown in Fig. S2b. Therefore, it can be inferred that the zinc-cerium species are to some extent dispersed into the
mesopores of the parent SBA-15. According to the physical properties of catalysts listed in Table S1 (Supporting information), the
specific surface area and pore volume of 10Zn Ce -CS are much smaller than 10Zn Ce -AS, which can be explained by partial
1 5 1 5
blockage of pores caused by aggregation of zinc and cerium species [29]. It also suggests that the presence of organic template can help
well distribute the precursors of zinc and cerium in the mesopores of SBA-15 during the solid-state grinding process.
The surface morphology and pore structure of SBA-15 and two SBA-15 supported catalysts (10Zn
further revealed by TEM and X-ray diffraction results. As shown in Fig. S3a (Supporting information), SBA-15 exhibits the highly
ordered pore-arrangement. Such regular pore patterns are well reserved in 10Zn Ce -AS, as shown in Fig. S3b (Supporting
information). Moreover, no nanoparticles can be observed in the TEM images, which elucidates a well dispersed state of the oxides in
the mesopores of SBA-15. But for 10Zn Ce -CS, some large particles are obviously seen on SBA-15 support, implying the
agglomeration of ZnO or CeO particles (Fig. S3c in Supporting information).
1 5 1 5
Ce -AS and 10Zn Ce -CS) were
1
5
1
5
2

CeO2 •SiO
2
(a)

(b)
•


1
0Zn1Ce5-CS
SBA-15
10Zn1Ce5-AS
10Zn1Ce5-AS
SBA-15
10Zn1Ce5-CS
1
2
2
3
4
5
10 20 30 40 50 60 70 80 90
2 Theta (degree)
Theta (degree)
1 5 1 5
Fig. 2. (a) Low-angle and (b) wide-angle XRD patterns of SBA-15, 10Zn Ce -AS and 10Zn Ce -CS.
Low-angle X-ray diffraction measurement was also performed to reveal the possible structural changes of SBA-15 support induced
by oxides loading. As shown in Fig. 2a, the characteristic peaks of SBA-15 are clearly observed on samples of SBA-15 support alone
and 10Zn
1
Ce
5
-AS. This further endorses the TEM results that the ordered mesoporous structure of SBA-15 largely retains when the
Ce -CS catalyst, the
template is not removed during the solid-state grinding process for catalyst preparation. However, for 10Zn
1
5
characteristic peaks of SBA-15 support in the low-angle XRD spectrum (Fig. 2a) become much weaker, implying the destruction of the
ordered mesoporous structure. The aggregation of metal oxide species inside or outside of the mesopores could be the reason for this
change, which can be further evidenced by the wide-angle x-ray diffraction measurement.
As shown in Fig. 2b, the peaks of CeO
samples beside a broad peak at 23° which is assigned to amorphous silica. This suggests that aggregation of cerium species is
inevitable during the solid-state grinding process. Furthermore, these peaks are much stronger from 10Zn Ce -CS than that from
0Zn Ce -AS, implying more serious aggregation of cerium species at the absence of organic template in the SBA-15 support. In either
2 1 5 1 5
at 28.6°, 47.5° and 56.3° appear in the XRD pattern of 10Zn Ce -AS and 10Zn Ce -CS
1
5
1
1
5
case (with or without organic template), no characteristic peaks of ZnO are found, indicating the better dispersion of ZnO in both
samples. The TEM and X-ray diffraction results are in good agreement and suggest that the importance of the organic template on the
dispersion of metal oxides during the solid-grinding process. It can be inferred that the limited space between the organic template and
the silica wall of SBA-15 makes it difficult for oxide particles to aggregate, which helps improve the dispersion of oxides on SBA-15
[
30].
The acidic properties of catalysts were measured by Py-FTIR and NH
sites. As can be seen from Fig. 3a, both 10Zn Ce -AS and 10Zn Ce -CS have only Lewis acid sites (around 1450 cm ), and no
3
-TPD. Py-FTIR was performed to analyze the type of acid
-
1
1
5
1
5

Please donot adjust the margins
-
1
Bronsted acid sites (around 1540 cm ) are found. The FTIR peak intensity also suggests more Lewis acid sites on 10Zn
that on 10Zn Ce -CS. The strength and amount of acid sites were further characterized by NH -TPD. As shown in Fig. 3b, the parent
SBA-15 has almost no acidity. However, after being modified by ZnO and CeO , it appears two desorption peaks at about 180 °C and
90 °C, indicating the existence of two types of acid sites. The desorption peak around 180 °C is attributed to the weak acid sites of the
samples and the strong acid sites are located near 290 °C. Moreover, the total amounts of acid sites on 10Zn Ce -AS are much more
than 10Zn Ce -CS, which is in consistent with the Py-FTIR results. When taking consideration of the better dispersion of CeO on
0Zn Ce -AS than 10Zn Ce -CS, we can conclude that the well dispersed cerium species could be the reason for the increased acid
sites.
1 5
Ce -AS than
1
5
3
2
2
1
5
1
5
2
1
1
5
1
5
(
a)
SBA-15
(b)
1
1
0Zn1Ce5-AS
0Zn1Ce5-CS
1
0Zn1Ce5-CS
1
0Zn1Ce5-AS
SBA-15
1
650160015501500145014001350
100
200
300
400
500
Ce
-1
Wavenumber (cm )
Temperature (°C)
Fig. 3. (a) Py-FTIR spectra and (b) NH
3
-TPD profiles for SBA-15, 10Zn
1
5 1 5
-AS and 10Zn Ce -CS.
The catalytic activity of catalysts was examined in the synthesis of 1,3-butadiene from ethanol at the same reaction conditions (375
-
1
°
C, 1.62 h ). As shown in Table 1, remarkably higher ethanol conversion and 1,3-butadiene selectivity are achieved on 10Zn
1 5
Ce -AS
than 10Zn Ce -CS. As confirmed by the characterization results, the existence of template P123 in the parent SBA-5 can help embed
1
5
the precursors of ceria and zinc into the mesopores during the grinding process, leading to a higher dispersion of metal oxides in the
final catalyst. On the one hand is the strong interaction between metal ions and template, the other is the confined space between the
organic template and the silica wall of SBA-15. The enhanced dispersion of the ceria species can provide more Lewis acid sites, which
is believed to be the key reason for the higher activity and selectivity of catalyst. Similar observations were also found in some other
pioneer work of various catalysts. Ivanova et al. reported that the activity of catalysts correlated well with the amount of Lewis acid
sites of Zr-containing molecular sieves [21]. Pomalaza et al. have also observed a strong correlation between the selectivity of 1,3-
butadiene and the amount of acid sites on various Zn(II)-Ta(V) based-catalyst [31]. For 10Zn
1 5
Ce -CS catalyst, the absence of template
resulted in the agglomeration of CeO particles, leading to less exposure of Lewis acid sites. Therefore, the catalyst presents a lower
2
activity in the conversion of ethanol to 1,3-butadiene. In addition, the catalysts supporting only one metal oxide were also evaluated.
For 10Zn-AS, the main product is acetaldehyde, which may be attributed to the redox site of zinc that promotes only the
dehydrogenation of ethanol. Due to the absence of enough acid sites, the aldol-condensation is inhibited, resulting in a much low
selectivity of 1,3-butadiene. As regard to 10Ce-AS, ethanol is effectively dehydrated to produce ethylene, suggesting that the acid sites
of cerium mainly catalyze the dehydration of ethanol. About bi-component Zn-Ce catalyst, the addition of zinc to Ce-containing
catalyst inhibits the dehydration of ethanol effectively and enhances the dehydrogenation of ethanol, and the aldol-condensation ability
of cerium is promoted as well. This result was in consistent with Zn-Hf catalyst reported by Baerdemaeker et al. [17] The synergy
between the dehydrogenation sites and Lewis acid sites determines the product distribution in the reaction. A matched combination of
the two kinds of active sites could improve the formation of 1,3-butadiene with a higher yield.
-1
Table 1. Catalytic performance of catalysts at 375 °C and WHSV of 1.62 h for 5 h.
Selectivity (%)
Sample
Conversion (%)
1,3-BD yield (%)
Ethylene
22.1
Propylene
3.9
Acetaldehyde
21.6
Butene
5.6
1,3-BD
45.3
15.0
17.9
4.8
Ethyl Ether
1
1
1
1
0Zn
0Zn
1
Ce
5
5
-AS
-CS
78.7
34.9
66.3
17.6
1.6
2.2
1.4
6.5
35.7
5.2
1
Ce
39.0
1.3
41.0
1.6
0Zn-AS
0Ce-AS
22.1
1.9
56.0
0.7
11.9
1.4
82.6
0.7
4.5
0.8

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1
00
Butadiene
Ethylene
Ethyl ether
Propylene
8
6
4
2
0
0
0
0
0
Acetaldehyde
Butene
Ethanol
0
2
4
6
8
10 12 14 16 18 20
TOS (h)
1 5
Fig. 4. The stability of 10Zn Ce -AS in 20 h (reaction conditions: T = 375 °C, WHSV = 1.08 h ).
-1
1 5
A slow declining trend on the conversion of ethanol is shown in Fig. 4, which indicates the slow deactivation of 10Zn Ce -AS. The
selectivity of 1,3-butadiene is sustained in 20 h, while acetaldehyde is increased and ethylene production is slowly decreased as the
reaction continues. It suggests that the deactivation of acid sites which are responsible for aldol-condensation could be the main reason
for the coke formation and catalyst deactivation. But overall, our 10Zn
deactivation of the catalyst.
1 5
Ce -AS exhibits pretty good stability despite the slow
In this study, we report a highly efficient ZnO-CeO
good dispersion of ZnO, the presence of organic template in the as-prepared SBA-15 improves the dispersion of CeO
2
/SBA-15 catalyst prepared by modified solid-state grinding method. Despite
. This is
2
contributed to the limited space between the template and the silica wall of SBA-15, and to the strong interaction between metal ions
and template. The enhanced dispersion of the ceria species provides more Lewis acid sites, therefore a higher butadiene selectivity of
about 45% is achieved on 10Zn Ce -AS, in which the coupling effect of ethanol dehydrogenation and aldol-condensation is realized
1 5
effectively. As a solvent-free method, this strategy could provide an inspiration on the catalyst preparation for the synthesis of 1,3-
butadiene from ethanol.
Acknowledgements
We are grateful to the financial support from the National Natural Science Foundation of China (No. 21878227).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at

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