A double-shell capsule catalyst with core-shell-like structure for one-step exactly controlled synthesis of dimethyl ether from CO2 containing syngas

Article abstract of DOI:10.1016/j.cattod.2011.02.021

A zeolite capsule catalyst Cr/ZnO-S-Z was prepared successfully by a novel dual-layer method, in which one layer of neutral Silicalite-1 zeolite shell was first synthesized on the bimetallic Cr/ZnO core catalyst, acting as the intermediate layer for the following H-type H-ZSM-5 zeolite shell direct synthesis on its surface. By using this dual-layer method, it can be easily realized that the formation of H-ZSM-5 zeolite membrane on the millimeter-sized bimetallic substrates. Direct synthesis of dimethyl ether (DME) from syngas at higher reaction temperature was the application of this zeolite capsule catalyst to test its catalytic performance. In reaction, this zeolite capsule catalyst could realize the exactly controlled synthesis of DME, suppressing the formation of other alkane/alkene by-products simultaneously, demonstrating its exclusive ability for the selective synthesis of target product compared with the conventional hybrid catalyst.

Full text of DOI:10.1016/j.cattod.2011.02.021

Catalysis Today 171 (2011) 229–235
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A double-shell capsule catalyst with core–shell-like structure for one-step
exactly controlled synthesis of dimethyl ether from CO containing syngas
2
Guohui Yang , Montree Thongkamb,c, Tharapong Vitidsantb,∗∗, Yoshiharu Yoneyamaa,
a
d
a,∗
Yisheng Tan , Noritatsu Tsubaki
a
Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand
Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A zeolite capsule catalyst Cr/ZnO–S–Z was prepared successfully by a novel dual-layer method, in which
one layer of neutral Silicalite-1 zeolite shell was first synthesized on the bimetallic Cr/ZnO core catalyst,
acting as the intermediate layer for the following H-type H-ZSM-5 zeolite shell direct synthesis on its
surface. By using this dual-layer method, it can be easily realized that the formation of H-ZSM-5 zeolite
membrane on the millimeter-sized bimetallic substrates. Direct synthesis of dimethyl ether (DME) from
syngas at higher reaction temperature was the application of this zeolite capsule catalyst to test its cat-
alytic performance. In reaction, this zeolite capsule catalyst could realize the exactly controlled synthesis
of DME, suppressing the formation of other alkane/alkene by-products simultaneously, demonstrating
its exclusive ability for the selective synthesis of target product compared with the conventional hybrid
catalyst.
Received 31 October 2010
Received in revised form 16 February 2011
Accepted 17 February 2011
Available online 21 March 2011
Keywords:
Zeolite
Cr/ZnO catalyst
Capsule catalyst
Dimethyl ether synthesis
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
a positive approach also explored a new way for the large-scale uti-
lization of CO . Essentially, the STD reaction consists of a series of
2
Dimethyl ether (DME), the simplest ether, with the simple
sequential reactions: syngas to methanol, methanol dehydration
to form DME, other hydrocarbons formation using DME as feed-
stock, and so on. The conventional hybrid catalysts for STD reaction
were usually made up of methanol synthesis catalysts (such as Pd,
CuO, ZnO, Al O and/or Cr O ) and methanol dehydration catalysts
chemical formula of CH –O–CH3 was a potential alternative of
3
the fossil fuels for the future [1]. It was a well-known propellant,
coolant and an alternative fuel for diesel engines due to its good
properties, such as its high cetane index (>55) and the low emission
of CO, NOx and particulates in its combustion [2,3]. It can be also
used as substitute for liquefied petroleum gas (LPG) that was widely
used as fuel in industry and household now. Usually, DME can be
produced by methanol dehydration over a solid acid catalyst. Here,
the methanol used for DME synthesis was produced from syngas.
Another way for DME synthesis was the direct conversion of syngas
over a physical mixing catalyst [4–8]. The latter way, from syngas to
dimethyl ether (STD), was a potential process and more favorable
in views of thermodynamics and economy [9–11], which made it
get more considerable attentions. Furthermore, the incorporation
of CO2 in the syngas can make the conversion from methanol to
DME thermodynamically more favorable than without it, which as
2
3
2
3
(such as ␥-Al O catalyst, H-Y, H-ZSM-5 or SAPOs zeolite).
2
3
For the methanol synthesis catalyst, the copper based cata-
lyst has been widely investigated because of its high activity and
moderate reaction conditions [12]. But the syngas for methanol
synthesis over copper based catalyst required very low level of sul-
fur, chlorine and so on. Moreover, the existence of CO2 and water
in syngas can deactivate the activity of copper based catalyst more
easily [13], especially under higher reaction temperature. Usually,
the reaction temperature for the conversion from methanol to DME
was higher than 573 K. As we know, the chromium based methanol
synthesis catalyst showed more stability compared with the copper
based catalyst. For methanol dehydration catalyst, zeolite catalyst
such as H-Y, H-ZSM-5 zeolite catalyst having higher activity, sta-
bility occupied prominent positions in the preparation of hybrid
catalyst than ␥-Al O3 catalyst. Until now, studies on the hybrid
2
∗
Corresponding author. Tel.: +81 076 4456846; fax: +81 076 4456846.
Corresponding author.
E-mail addresses: tharapong.v@chula.ac.th (T. Vitidsant),
catalyst preparation only aimed at the simple mixing method, and
a great number of physical mixing catalysts, only various in catalyst
composition or reaction conditions, have been extensively investi-
∗
∗
tsubaki@eng.u-toyama.ac.jp (N. Tsubaki).
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.02.021

2
30
G. Yang et al. / Catalysis Today 171 (2011) 229–235
gated [5,14–17]. However, the catalyst used for this STD reaction
can work more efficiently if it has a well designed core–shell struc-
ture, as proved by the present authors [18]. Furthermore, the acidic
properties of solid acid catalyst may be changed when they were
mixed with the metallic component.
0.48TPAOH:2TEOS:8EtOH:120H O:0.25Al O . Aluminum nitrate
2 2 3
nonahydrate (Al(NO ) ·9H O, 99.9%, Wako) was selected as the
3
3
2
aluminum resource. Other reagents and hydrothermal synthesis
conditions were the same to Silicalite-1 zeolite intermediate layer
preparation mentioned above, but without HNO3 in synthesis
solution. The samples after this second hydrothermal synthesis
were separated from mother liquid, washed by deionized water,
dried at 393 K for 12 h, and then calcined at 773 K for 5 h removing
the organic template settled in zeolite pores. The final zeolite
capsule catalysts were named as Cr/ZnO–S–Z, where the “Z” means
the H-ZSM-5 zeolite shell. The weight increment of this capsule
catalyst after this zeolite shell preparation process was about
9.7 wt%.
H-ZSM-5 zeolite, an excellent catalyst due to its acidic prop-
erties [16], was often used as dehydration catalyst for methanol
dehydration to form DME. The zeolite membranes constructed by
zeolite crystals have more advantages than zeolite powder [19].
However, to our knowledge, it is very difficult to directly synthesize
a defect-free H-type H-ZSM-5 zeolite membrane on the bimetal-
lic substrates [20], especially for the millimeter-sized bimetallic
Cr/ZnO catalyst as support. In this report, a new capsule catalyst
Cr/ZnO–S–Z, for the first time, was prepared successfully using a
novel dual-layer method. One layer of the neutral Silicalite-1 zeo-
lite membrane, as the intermediate layer, was first synthesized on
the bimetallic Cr/ZnO core catalyst, and then the second H-ZSM-5
zeolite membrane grew on its surface to fabricate the dual-layer
final acidic zeolite shell. The illustration for the zeolite capsule cat-
alyst Cr/ZnO–S–Z preparation process by the dual-layer method is
presented in Scheme 1. The obtained zeolite capsule catalyst was
used for the DME direct synthesis from syngas, and the effect of
various reaction temperatures on reaction results, such as catalyst
activity, product distribution, was also investigated and discussed
in detail.
2.3. Physically mixed catalyst of Cr/ZnO with H-ZSM-5
The H-ZSM-5 zeolite catalyst used for hybrid catalyst prepa-
ration was prepared with the same recipe and hydrothermal
conditions to that of zeolite capsule catalyst preparation. The
Cr/ZnO catalyst was physically mixed well with a self-made H-ZSM-
5 (Si/Al = 50) zeolite powder, granulated into the pellet size range
of 0.85–1.70 mm. The new catalyst pellet (Cr/ZnO:H-ZSM-5 = 10:1
in weight) was named as Cr/ZnO–Z–M, where “M” stands for the
physical mixing of Cr/ZnO with H-ZSM-5 zeolite.
2
. Experimental
2.4. Catalyst characterization
2.1. Bimetallic catalyst preparation
The specific surface area, pore volume and average pore diam-
eter of catalysts were determined by nitrogen adsorption in an
automatic gas adsorption system (Quantachrome Autosorb-1).
Before analysis, the samples were first degassed at 573 K and 3.0 Pa
for 3 h. An X-ray diffractometer (RINT 2400, Rigaku Co.) equipped
with Cu K␣ radiation was used to collect the XRD patterns of
catalysts. Operations were performed at 40 kV and 40 mA. The
physical morphology of the naked Cr/ZnO core catalyst, Cr/ZnO–S
and Cr/ZnO–S–Z zeolite capsule catalyst was characterized by a
scanning electron microscopy (SEM) equipped with an energy-
diffusive X-ray spectroscopy (EDS) attachment (JEOL, JSM-6360LV)
which could simultaneously provide the catalyst surface elemental
composition information. The catalysts used for this analysis were
first pretreated by an auto fine coater (JEOL, JFC-1600) to coat a
platinum layer on their surface.
The bimetallic catalyst, chromium and zinc oxide catalyst
Cr/ZnO, Cr:Zn = 1:2 in molar), used as capsule catalyst core
for methanol synthesis was prepared by the conventional co-
precipitation method. The appropriate amounts of metal nitrate
solution and sodium carbonate solution were added dropwise over
(
1
h to a 300 ml of deionized water, with constant pH 9.0 at 338 K
under continuous stirring, followed by 30 min ageing time to obtain
the precipitates. The formed precipitates were filtered out and
repeatedly washed by hot water (343 K), eliminating the effect of
residual sodium ion on catalyst activity. And then it was treated
by drying at 393 K for 12 h, calcination in air at 773 K for 1 h, and
consequently granulating into the size of 0.85–1.70 mm. The final
sample, named as Cr/ZnO catalyst, was a catalyst usually used for
methanol synthesis under higher reaction temperature.
2.2. Tailor-made capsule catalyst by dual-layer method
2.5. Catalyst activity test
Silicalite-1 zeolite membrane as intermediate layer was first
A pressurized flow-type reaction apparatus with a fixed-bed
stainless steel reactor (ID 8 mm) was adopted for catalyst activ-
ity test. The catalysts were loaded at the middle of reactor, dried at
523 K in a flow of nitrogen for 2 h, and then reduced in situ at 673 K
in a flow of hydrogen for 3 h [21]. All the effused products from the
reactor were sampled in gaseous state and first analyzed online
by a gas chromatograph (Shimadzu, TCD, GC-8A) for CH4, CO and
CO2, and then the trail gases were analyzed online using another
gas chromatograph (Shimadzu, FID, GC-8A) for DME, methanol and
other hydrocarbons.
synthesized on the surface of the bare Cr/ZnO core catalyst. The
synthesis solution for its formation was of the molar composi-
tion as 0.48TPAOH:2TEOS:8EtOH:120H O:0.24HNO . The ethanol
2
3
(
EtOH) and deionized water were mixed with tetrapropylammo-
nium hydroxide (TPAOH) in a teflon container under stirring at
room temperature. And then tetraethylorthosilicate (TEOS) and
HNO3 were added to the mixture respectively. After a continu-
ous stirring at room temperature for 6 h, the obtained precursor
solution and Cr/ZnO core catalyst were sealed in the teflon con-
tainer by a stainless steel autoclave, loaded inside the hydrothermal
synthesis instrument (DRM-420DA, Hiro Company, Japan). This
hydrothermal synthesis was performed at 453 K for 24 h with a
rotational speed of 2 rpm. The final samples, named as Cr/ZnO–S
where “S” here indicates Silicalite-1, would be used for the follow-
ing H-type ZSM-5 zeolite membrane growth on its surface.
The total conversion shown in Table 2 was calculated as follow:
a × CO conv. + b × CO conv.
2
total conv. =
a + b
where “a” and “b” were the contents of CO and CO2 in the syngas
respectively.
The successive synthesis solution for H-ZSM-5 zeolite
shell growth on Cr/ZnO–S support had the molar ratio of

G. Yang et al. / Catalysis Today 171 (2011) 229–235
231
Scheme 1. Illustration of zeolite capsule catalyst preparation by using the dual-layer method.
Table 1
Physical property of catalysts.
of Silicalite-1 shell as intermediate layer between core catalyst
and acidic H-ZSM-5 zeolite shell. Furthermore, the zeolite capsule
catalyst Cr/ZnO–S–Z had lower pore volume and pore diameter,
comparing with the naked Cr/ZnO catalyst and physical mixing
Cr/ZnO–Z–M catalyst. For this phenomenon, the hydrothermal syn-
thesis for this zeolite capsule catalyst preparation should be the
main reason. During the hydrothermal synthesis process, few of
zeolite synthesis solution entered the pores of Cr/ZnO core cata-
lyst, and then formed some aluminosilicate that might fill a part of
the pore volume and decrease the pore diameter of zeolite capsule
catalyst.
Catalyst
Specific surface
Pore volume
Average pore
diameter (nm)
area (m2
g
−1
)
(cm
3
g
−1
)
Cr/ZnO
H-ZSM-5
Cr/ZnO–Z–M
Cr/ZnO–S–Z
90.4
595.6
148.2
187.8
0.306
0.308
0.297
0.222
13.5
3.4
10.5
5.9
3
. Results and discussion
3.1. X-ray diffraction
3.3. SEM and EDS analysis of catalysts
X-ray diffraction is helpful to determine whether zeolite crystal
Fig. 2 presents the surface SEM and EDS analysis result of the
was formed on the core catalyst, and what was the zeolite type. The
XRD patterns of the naked Cr/ZnO catalyst, zeolite capsule catalyst
Cr/ZnO–S–Z, physical mixing catalyst Cr/ZnO–Z–M and pure H-
ZSM-5 zeolite are compared in Fig. 1. Comparing the XRD patterns
of Cr/ZnO–S–Z with Cr/ZnO catalyst and pure H-ZSM-5 zeolite,
the typical peaks belonged to pure MFI zeolite in the ranges of
naked Cr/ZnO core catalyst. Only Cr K␣ and Zn K␣ signals, 32.73%
and Zn 67.27% respectively in molar base, can be found on the
Cr/ZnO core catalyst surface, and they are close to the original bulk
◦
◦
2
ꢀ = 5–10 and 21–25 appeared, indicating that H-ZSM-5 zeolite
layer had been successfully coated on the Cr/ZnO core catalyst.
3.2. BET analysis
Table 1 presents the physical properties of the bare Cr/ZnO,
pure H-ZSM-5 zeolite, physical mixing catalyst Cr/ZnO–Z–M and
zeolite capsule catalyst Cr/ZnO–S–Z. For zeolite capsule catalyst
2
−1
was larger
Cr/ZnO–S–Z, its specific surface area of 187.8 m g
than that of bare Cr/ZnO catalyst since the existence of zeolite
shell. It was also slightly higher than that of physical mixing cata-
lyst Cr/ZnO–Z–M, although both had the similar acidic H-ZSM-5
zeolite increment, which should be attributed to the existence
H-ZSM-5
Cr/ZnO-Z-M
Cr/ZnO-S-Z
Cr/ZnO
0
20
40
60
80
2
Theta
Fig. 1. XRD patterns of Cr/ZnO, Cr/ZnO–S–Z, Cr/ZnO–Z–M and pure H-ZSM-5 zeolite
Fig. 2. Surface SEM image of the Cr/ZnO pellet and its EDS surface analysis result.
powder.
Molar ratio: Cr 32.73%, Zn 67.27%, Al 0.00%, Si 0.00%.

2
32
G. Yang et al. / Catalysis Today 171 (2011) 229–235
Fig. 3. Surface SEM image of the Cr/ZnO–S pellet.
ratio of catalyst preparation recipe. After the coating of Silicalite-
zeolite shell onto the naked Cr/ZnO core catalyst, the formed
1
Cr/ZnO–S pellet surface is covered by an integrated, uniform zeolite
membrane shown in Fig. 3. Here, the formed intermediate zeolite
layer of Silicalite-1 of Cr/ZnO–S can induce the growth of the fol-
lowed H-ZSM-5 zeolite shell on its surface effectively, and at the
same time avoid the further damage from hydrothermal synthesis
as much as possible, because pH value of H-ZSM-5 zeolite synthesis
mother liquid is remarkably higher than that of Silicalite-1.
The Cr/ZnO–S–Z zeolite capsule catalyst was prepared by using
the second hydrothermal synthesis, constructing H-ZSM-5 zeolite
shell on the surface of Cr/ZnO–S catalyst. The surface SEM and EDS
analysis results of Cr/ZnO–S–Z are shown in Fig. 4. Slight differences
between this H-ZSM-5 shell and the intermediate layer of Silicalite-
1
zeolite shell (Fig. 3) with respect to their zeolite crystal’s size and
shape can be found clearly, which should be due to the dissimi-
lar pH values of these two zeolite membranes synthesis solution
Fig. 4. Surface SEM image of the Cr/ZnO–S–Z pellet and its EDS surface analysis
result. Molar ratio: Cr 0.00%, Zn 0.00%, Al 2.03%, Si 97.97%.
[
18], as well as the existence or not, of aluminum resource in their
synthesis recipes [22,23]. The surface EDS analysis of Cr/ZnO–S–Z
determined its surface elemental composition as in Fig. 4, where the
zero intensity of Cr K␣ and Zn K␣ signals indicated that the zeolite
shell was integrity and defect-free, suggesting that this dual-layer
zeolite capsule catalyst preparation on the bimetallic core catalyst
was successful. Moreover, the surface Si/Al ratio, Si/Al = 48, of this
H-ZSM-5 zeolite shell can be also obtained according to this EDS
analysis result.
3
.4. Catalytic performance of catalysts in STD reaction
The catalytic performances of the bare Cr/ZnO core catalyst,
physical mixing Cr/ZnO–Z–M catalyst and zeolite capsule catalyst
Cr/ZnO–S–Z prepared by the dual-layer method are compared in
Table 2. The time on stream of different catalysts as a function of
reaction temperature had also been presented in Fig. 6. All the cat-
−1
alysts were tested with the WCr/ZnO/FSyngas = 13.6 g h mol , system
Fig. 5 exhibits the cross-section SEM image and EDS line analy-
sis of Cr/ZnO–S–Z zeolite capsule catalyst. From the SEM image, a
compact and defect-free zeolite shell enwrapping the Cr/ZnO core
catalyst can be observed clearly, which also proves that the zeolite
shell synthesis on the bimetallic Cr/ZnO catalyst by this dual-layers
method is successful. The EDS analysis was performed along the line
in the SEM image, as shown in Fig. 5. The analysis result exhibited
the change of Al K␣, Si K␣, Cr K␣ and Zn K␣ signals in inten-
sity from the core catalyst to zeolite shell. At the interface region
between core catalyst and zeolite shell, the radial distribution of Si
K␣ increased sharply while that of the Cr K␣ and Zn K␣ changed
into zero, which demonstrated the phase change from core cata-
lyst to zeolite shell. It should also be noted that the Al K␣ signal
was not zero at the zone of core catalyst which was near the zeolite
shell. It is clear that some synthesis solution soaked into the outer
surface of core catalyst during the hydrothermal synthesis process,
and the soaked synthesis solution containing aluminum resource
should contribute to this gradient. The zeolite shell thickness of
Cr/ZnO–S–Z was about 9.4 ␮m that can be also identified according
to these EDS radial distribution profiles, as displayed in Fig. 5.
pressure 5.0 MPa and at varied reaction temperatures (573 K, 598 K,
6
3
23 K). The syngas used as feedstock had the composition of Ar
.02%, CO 32.6%, CO 5.16%, H 59.22% in molar base.
2
2
For the naked Cr/ZnO catalyst, the CO conversion exhibited a
linear increment with the increase of reaction temperature shown
in Table 2. At 573 K, the CO conversion was only 5.43%, and then it
reached 7.68% when the reaction temperature up to 623 K. A simi-
lar change also happened on Cr/ZnO–Z–M and Cr/ZnO–S–Z catalyst.
Especially for the hybrid catalyst Cr/ZnO–Z–M, its CO conversion of
4
5.16% at higher reaction temperature of 623 K was five times more
than the 9.30% at lower reaction temperature of 573 K. The higher
reaction temperature, however, adversely promoted the CO con-
2
version, indicating that higher reaction temperature accelerated
the Water-Gas-Shift (WGS) reaction to generate CO [16].
2
The zeolite capsule catalysts Cr/ZnO–S–Z presented lower cat-
alyst activities than the naked Cr/ZnO catalyst or hybrid catalyst
Cr/ZnO–Z–M. For this zeolite capsule catalyst, few zeolite crys-
tals formed during hydrothermal synthesis process covered part
of the active sites of the Cr/ZnO core catalyst, which resulted in the
lower catalyst activity of zeolite capsule catalyst compared with

G. Yang et al. / Catalysis Today 171 (2011) 229–235
233
Table 2
Reaction performance of catalysts at various reaction temperatures.a
Catalyst
Reaction
temp. (K)
Conversion (%)
Selectivity (%)
CO
5.43
7.26
7.68
CO2
3.38
0.48
−5.43
−9.42
−64.96
−122.21
6.97
Total
CH4
2.42
7.79
14.22
4.41
3.96
7.65
7.78
4.83
C2H4
C2H6
C3H6
C3H8
MeOH
DME
C4H10
Others
5
598
73
5.15
6.33
5.89
6.74
12.93
22.29
2.91
5.21
6.22
1.60
1.06
4.57
42.01
23.77
7.34
8.07
7.26
7.87
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.56
1.72
9.62
6.42
2.69
2.39
7.10
6.76
8.54
8.91
12.47
11.15
4.74
4.58
0.72
0.88
1.99
87.44
81.04
63.74
16.06
12.75
12.12
35.62
34.20
21.23
0.00
0.64
3.40
11.12
0.81
0.47
45.42
45.74
50.84
0.00
0.00
0.00
0.00
17.40
27.34
0.00
0.00
0.00
0.00
0.00
0.00
5.63
30.15
37.81
0.00
0.00
0.00
Cr/ZnO
6
5
23
73
9.30
Cr/ZnO–Z–Mb
598
25.25
45.16
2.27
6.03
9.53
6
5
23
73
Cr/ZnO–S–Zc
598
23
0.04
−14.70
6
11.31
a
−1
Reaction conditions: WCr/ZnO/FSyngas = 13.6 g h mol , 5.0 MPa, 5 h; Syngas composition: Ar 3.02%, CO 32.6%, CO2 5.16%, H2 59.22%.
“
“
b
c
Z” stands for the H-ZSM-5 zeolite (Si/Al = 50), and the “M” means the physical mixing of Cr/ZnO with H-ZSM-5 zeolite.
S” means that the Silicalite-1 zeolite intermediate layer and “Z” the H-ZSM-5 shell.
the Cr/ZnO and Cr/ZnO–Z–M catalyst. Another factor, the “speed-
down” for syngas diffusing through zeolite shell, should also be
considered. The bare catalyst was enwrapped by dual-layer zeolite
shells. In order to react on the zeolite capsule catalyst core, syngas
must first pass through the zeolite shells, which may lead to the
diffusion restrictions, comparing with that of the traditional cata-
lyst Cr/ZnO and hybrid Cr/ZnO–Z–M catalyst. However, increasing
the metal content of the core catalyst or extending the contact time
of the reactant with zeolite capsule catalyst can overcome it easily.
It should also be mentioned that, at 623 K, the catalyst activity
of this zeolite capsule catalyst was better than that of the naked
Cr/ZnO catalyst. Here, the promoted catalyst activity should be
attributed to the shifted reaction equilibrium. It is well known that,
different from the single methanol synthesis reaction happened
on the naked Cr/ZnO core catalyst, on the single zeolite capsule
catalyst there were a series of sequential reactions. The reaction
speed of methanol synthesis from syngas occurred on the core
catalyst was significantly affected by the followed DME formation
from methanol dehydration on the zeolite shell, especially at higher
reaction temperature. Removal of the formed methanol to DME
enhanced the CO conversion for the capsule catalyst.
The product selectivity on naked Cr/ZnO, hybrid catalyst
Cr/ZnO–Z–M and zeolite capsule catalyst Cr/ZnO–S–Z are also
presented in Table 2. For the naked Cr/ZnO catalyst, the major com-
ponent of its products was methanol. Moreover, the changes of the
products distribution were obvious with the increase of reaction
temperature, whereas DME always emerged as a tiny component
and increased slightly along with other hydrocarbon by-products
(C1–C3).
For the physical mixing catalyst Cr/ZnO–Z–M, it was made of the
traditional methanol synthesis catalyst Cr/ZnO and the methanol
dehydration catalyst H-ZSM-5 zeolite. Different from the naked
Cr/ZnO catalyst, this hybrid catalyst had a different products dis-
tribution. The methanol selectivity on this hybrid catalyst was
obviously lower than that of the naked Cr/ZnO at each reaction
temperature, as in Table 2. However, DME was still the tiny prod-
ucts, and the major components on this hybrid catalyst were
alkanes and alkenes. Considering the structure composition of
this hybrid catalyst, in STD reaction, the syngas reacted first on
the Cr/ZnO active sites of hybrid catalyst to form methanol, and
then a part of the formed methanol might be randomly con-
verted into DME on the zeolite active sites near the Cr/ZnO catalyst
part in hybrid catalyst. The formed DME from methanol dehydra-
tion could be further converted into hydrocarbons on the zeolite
active sites of one hybrid catalyst particles or another neighboring
one, since it diffused freely without any spatial restriction, espe-
cially at higher reaction temperatures. The remaining methanol
would continue its route to pass through the catalyst bed layer,
might be not converted to DME, without contact with zeolite.
This hybrid catalyst structure, with only a simple mixing, decided
that only a random not ordered sequential reaction process took
place. There were no an effective restriction to stop DME diffus-
ing back into the hybrid catalyst again. As a result, alkanes and
alkenes became the major products when this hybrid catalyst was
used.
Fig. 5. Cross-sectional SEM image and the EDS line analysis of zeolite capsule cata-
lyst Cr/ZnO–S–Z.

2
34
G. Yang et al. / Catalysis Today 171 (2011) 229–235
CO Conv. (573 K)
CO2 Conv. (573 K)
CO Conv. (598 K)
CO2 Conv. (598 K)
CO Conv. (623 K)
CO2Conv. (623 K)
and steps restrictions between the methanol synthesis, methanol
a) Cr/ZnO
dehydration and DME dehydration to hydrocarbon reactions. The
constructionmannerof hybrid catalyst, simply mixing, decidedthat
the sequential reactions will only happen randomly on the two
active sites of hybrid catalyst. The formed DME left the hybrid cata-
lyst by diffusion, but it can also diffuse freely without any limitation
back into the same or another hybrid catalyst to undergo further
reaction transforming into other hydrocarbons. As a result, a series
of sequential reactions happened on this hybrid catalyst without
any restriction or order. They cannot be controlled well, stopped in
hand, ended at the DME formation. The further conversion of DME
to form hydrocarbons on this hybrid catalyst is very easier at least
under the used reaction conditions.
1
5.00%
1
0.00%
5
.00%
.00%
0
0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-
5. 00%
However, it is entirely different for zeolite capsule catalyst
Cr/ZnO–S–Z. This zeolite capsule catalyst has a special core–shell
structure, in which the catalyst shell enwrapped core catalyst
tightly. In STD reaction, each reaction of the sequential reactions
will be restricted in the designated location. Methanol was syn-
thesized from syngas on the core catalyst and then it would be
transformed into DME on zeolite shell, followed by DME escaping
from capsule catalyst. For DME dehydration, the zeolite shell with
the fixed position and homogeneous property was an equal oppor-
tunity provider to each methanol molecular diffused from the core
catalyst, converting them into DME concertedly, while lessening
the probability of the further dehydration of DME generating alka-
nes/akenes. The effused DME from capsule catalyst will join the
flowing syngas. The diffusion of DME back to capsule catalyst, if
DME want to do that, can be effectively suppressed by zeolite shell
-
10. 00%
Time on Stream (h)
CO Conv. (573 K)
CO2 Conv. (573 K)
CO Conv. (598 K)
CO2 Conv. (598 K)
CO Conv. (623 K)
CO2 Conv. (623 K)
b) Cr/ZnO-Z-M
1
00.00%
5
0.00%
0
.00%
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
-50. 00%
especially in the case of competing with syngas (CO + H + CO ),
2
2
which will obviously decrease the possibility of DME further con-
version, leading to the higher DME selectivity in the final products.
Benefitting from the well-confined core–shell structure, this
zeolite capsule catalyst can concertedly settle each reaction of the
sequential reactions in the designated location and stop them at the
desired step according to the catalyst designer’s wishes. All these
results indicated that the concept of this zeolite capsule catalyst
was very effective and available for the controlled synthesis of tar-
get products with high selectivity, especially in the catalytic process
containing a series of sequential reactions.
-
-
100. 00%
150. 00%
Time on Stream (h)
CO Conv. (573 K)
CO2 Conv. (573 K)
CO Conv. (598 K)
CO2 Conv. (598 K)
CO Conv. (623 K)
CO2Conv. (623 K)
c) Cr/ZnO-S-Z
2
1
1
0.00%
5.00%
0.00%
4. Conclusions
5.00%
The acidic H-ZSM-5 shell was successfully coated on the sur-
0.00%
0
.5
1
1.5
2
2.5
3
3.5
4
4.5
5
face of bimetallic Cr/ZnO catalysts by using an improved dual-layer
method, where one layer of neutral Silicalite-1 membrane was
first synthesized on the core catalyst as the intermediate layer for
the followed H-ZSM-5 shell growth on its surface. Characteriza-
tion on the zeolite capsule catalyst prepared by this dual-layer
method indicated that the zeolite shell was compact, integrated
and defect-free, suggesting that this dual-layer method was reli-
able for H-ZSM-5 zeolite shell synthesis on the bimetallic core
catalyst. The direct synthesis of DME from syngas was the applica-
tion of this zeolite capsule catalyst Cr/ZnO–S–Z, where it exhibited
better performance for DME exactly controlled synthesis than the
naked Cr/ZnO catalyst and physical mixing catalyst Cr/ZnO–Z–M.
The dual-layer zeolite shell preparation method and zeolite cap-
sule catalyst application, reported here, should have more potential
applications not only to the zeolite capsule catalyst design and
the sequential reaction processes, but also for the study of zeolite
membrane.
-
5. 00%
-
-
-
10. 00%
15. 00%
20. 00%
Time on Stream (h)
Fig. 6. Time on stream of STD reaction over different catalysts as a function of
reaction temperature: (a) Cr/ZnO, (b) Cr/ZnO–Z–M and (c) Cr/ZnO–S–Z.
Different from the naked Cr/ZnO catalyst and hybrid catalyst
Cr/ZnO–Z–M, zeolite capsule catalyst Cr/ZnO–S–Z gave a very dis-
similar product distribution in Table 2. DME accounted for the
largest proportion in the final products with alkanes and alkenes as
the tiny by-products. The selectivity of DME increased from 45.42%
to 50.84% as the increase of reaction temperature from 573 K to
6
23 K, while the selectivity of methanol decreased correspondingly.
Here, the exactly controlled DME synthesis from syngas, with-
out the formation of C4+ hydrocarbons, should be attributed to
the unique core–shell structure of the zeolite capsule catalyst. For
hybrid catalyst Cr/ZnO–Z–M, there were no spatially confined effect
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