Ethanol as a Binder to Fabricate a Highly-Efficient Capsule-Structured CuO?ZnO?Al2O3@HZSM-5 Catalyst for Direct Production of Dimethyl Ether from Syngas

Article abstract of DOI:10.1002/cctc.201901938

This work reports a highly efficient capsule-structured CuO?ZnO?Al₂O₃@HZSM-5 (CZA@HZSM-5-EtOH) core-shell catalyst for the direct conversion of syngas to dimethyl ether by a facile physical coating method with ethanol as a binder through coating micrometer-sized HZSM-5 shell on the prior-shaped millimeter-sized CZA core, it shows 2.9 times higher CO conversion with the 2.7 times higher turnover frequency and 9.2 times higher dimethyl ether space-time yield of the CZA@HZSM-5-SS catalyst prepared by a similar process but with silica sol as a binder (315.5 vs 34.3 g_DME kg_cat ^?1 h^?1). The relationship between the structure and performance was explored by a variety of characterization techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), ammonia temperature programmed desorption (NH₃-TPD), nitrogen adsorption-desorption, H₂-temperature-programmed reduction (H₂-TPR) and H₂-TPR after oxidation of the samples by N₂O. CZA@HZSM-5-EtOH can be considered as a highly efficient and practical catalyst for dimethyl ether synthesis from syngas. This work presents a new avenue to design other bifunctional catalysts for the cascade reactions in which the raw materials can be converted into an intermediate over the core and then the as-formed intermediate over the core can be subsequently converted into the final product.

Full text of DOI:10.1002/cctc.201901938

Accepted Article
Title: Ethanol as a Binder to Fabricate a Highly-Efficient Capsule-
Structured CuO-ZnO-Al2O3@HZSM-5 Catalyst for Direct
Production of Dimethyl Ether from Syngas
Authors: Yongle Guo and Zhongkui Zhao
This manuscript has been accepted after peer review and appears as an
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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
published online in Early View as soon as possible and may be different
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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: ChemCatChem 10.1002/cctc.201901938
Link to VoR: http://dx.doi.org/10.1002/cctc.201901938
A Journal of
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ChemCatChem
10.1002/cctc.201901938
COMMUNICATION
Ethanol as a Binder to Fabricate a Highly-Efficient Capsule-
Structured CuO-ZnO-Al O @HZSM-5 Catalyst for Direct
2
3
Production of Dimethyl Ether from Syngas
Yongle Guo and Zhongkui Zhao*
[
7-12]
[13-15]
Abstract: This work reports a highly efficient capsule-structured
CuO-ZnO-Al @HZSM-5 (CZA@HZSM-5-EtOH) core-shell
HZSM-22,
heteropolyacid,
HY,
and
H-SAPO),
γ-alumina,
[
16,17]
[18-20]
2
O
3
and mixed metal oxides
in the second
catalyst for the direct conversion of syngas to dimethyl ether by a
facile physical coating method with ethanol as a binder through
coating micrometer-sized HZSM-5 shell on the prior-shaped
millimeter-sized CZA core, it shows 2.9 times higher CO conversion
with the 2.7 times higher turnover frequency and 9.2 times higher
dimethyl ether space-time yield of the CZA@HZSM-5-SS catalyst
prepared by a similar process but with silica sol as a binder (315.5
step. However, methanol, as a relatively high-cost chemical,
increases the cost for producing DME, besides the two-step
[
2]
DME synthesis is limited by the availability of methanol. On the
contrary, syngas is cheap raw material and can be available
from coal gasification, biomass gasification and natural gas
partial oxidation or reforming. The direct syngas-to-DME (STD)
via a single process has an economic strength. Moreover, the
STD process is more thermodynamically favorable, because the
thermodynamic equilibrium limitation of syngas-to-methanol
process can be broken by the subsequent the methanol
dehydration process. Therefore, the STD route has drawn great
-
1
-1
vs 34.3 gDME kgcat h ). The relationship between the structure and
performance was explored by variety of characterization
a
techniques including X-ray diffraction (XRD), scanning electron
microscope (SEM), energy dispersive spectrometer (EDS), X-ray
diffraction (XRD), ammonia temperature programmed desorption
[
2-4,21-30]
attention of worldwide researchers from 1990’s.
The hybrid catalysts prepared by physically mixing
methanol synthesis catalyst (Cu-ZnO-Al and methanol
dehydration catalyst (solid acid) were attempted to catalyze the
(
NH
programmed reduction (H
samples by N O. CZA@HZSM-5-EtOH can be considered as a
3
-TPD),
nitrogen
adsorption-desorption,
2
H -temperature-
2
-TPR) and H -TPR after oxidation of the
2
2 3
O )
2
[
31-40]
highly efficient and practical catalyst for dimethyl ether synthesis
from syngas. This work presents a new avenue to design other
bifunctional catalysts for the cascade reactions in which the raw
materials can be converted into an intermediate over the core and
then the as-formed intermediate over the core can be subsequently
converted into the final product.
STD process.
The methanol synthesis unit in the hybrid
[21-
catalysts have been focused on tuning catalysts composition,
25]
[26,27]
synthetic method,
preparation parameters of Cu-based
[
28,29]
[30]
catalysts
and alternatives to Cu-based.
The methanol
dehydration unit in the hybrid catalysts has been focused on the
[
31]
[32-34]
modulation of particle size , structure
acidic sites by metal
and modification of
[
35-38]
[27-29,39,40]
of zeolite and γ-Al
O
2 3
.
Especially, HZSM-5 has been considered as an excellent
[
31-38]
candidate for methanol dehydration.
In addition, reaction
It has become a consensus to seek clean energy to replace
[41-48]
[49]
conditions,
catalyst regeneration and interaction between
[
1]
fossil fuel because the latter is liable to pollute the environment.
[50]
different functional components have also been investigated.
However, some deficiencies still exist in STD process over the
hybrid catalyst. For example, the far distance between active
sites of syngas-to-methanol and solid acidic sites leads to the
low catalytic performance including activity and selectivity.
Apart from the aforementioned hybrid catalysts, the
Dimethyl ether (DME) is expected to replace liquefied petroleum
gas (LPG) for its alike physicochemical nature. In addition, DME
can also be employed as an eco-friendly replacement for diesel
because of its high cetane number and low emission of toxic
compounds in the outlet gas. In addition, DME is an important
chemical intermediate to synthesize useful chemicals (methyl
acetate, methyl carbamate and dimethylsulfate).
Currently, the industrial synthesis of DME is carried out in a
two-step process. Syngas-to-methanol reaction is performed
supported bifunctional catalysts were also frequently used in the
[
2-4]
[2,51-60]
STD reactions.
The supported catalyst can be produced by
the coprecipitation, impregnation, sol−gel, and wet-chemical
[61-70]
ways.
The character and performance of the supported
2 3
over copper-based catalysts (mainly, Cu-ZnO-Al O ) in the first
catalysts in the STD reaction strongly depend on their synthesis
method. In the last few years, new synthetic technologies such
as ultrasound-assisted method and physical sputtering were
also employed to prepare the supported bifunctional
step, which is thermodynamic equilibrium limitation at high
[
4-6]
reaction temperature.
Methanol-to-DME reaction is carried
out over solid acid catalysts such as acidic zeolites (HZSM-5,
[
71,72]
catalysts.
randomly on the surface of solid-acids, the two tandem reactions
methanol synthesis and the subsequent methanol dehydration)
Because the Cu/ZnO-based active sites distribute
[a]
Dr. Yongle Guo, Prof. Zhongkui Zhao
(
State Key Laboratory of Fine Chemicals, Department of Catalysis
Chemistry and Engineering, School of Chemical Engineering, Dalian
University of Technology, Dalian 116024, P. R. China.
E-mail: zkzhao@dlut.edu.cn
are out of order and take place independently, which results in a
low DME selectivity. To address the problem of the open-
structure of bifunctional catalysts for direct synthesis of DME
[
73-83]
from syngas, core-shell capsule catalysts were designed.
The selectivity of capsule-structured catalysts obviously is higher
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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ChemCatChem
10.1002/cctc.201901938
COMMUNICATION
than that of the physically mixed catalysts for DME synthesis
owing to their closed capsule-structure. Especially, the capsule-
structured CuZnAl@HZSM-5 catalyst by in-situ coating H-ZSM-5
shell over the outside surface of CuZnAl core has been
considered as a smart candidate for highly selective synthesis of
DME. However, it can hardly avoid the erosion of Cu during the
hydrothermal synthesis process of HZSM-5, which leads to
samples is confirmed by SEM and EDS analysis. Figure 1
demonstrates the surface SME images and EDS analysis of
[
73]
much lower CO conversion.
previously reported a facile and robust strategy for preparing a
highly efficient CZA-oa@H-ZSM-5 capsule-structured
In our research group, we
bifunctional catalyst by coating an H-ZSM-5 shell on
millimetersized copper–zinc–aluminum oxalate but not on
copper–zinc–aluminum oxide
via the hydrothermal
crystallization process with the subsequent calcination at 500 °C
for 5 h in air. It was found that the direct use of CZA-oa to
replace CZA could efficiently inhibited Cu leaching in the coating
process, besides turning down the necessity of using a rotary
oven for the good coating of the H-ZSM-5 shell on the core
[
76]
owing to high hydrophilic property. In addition, the dual-layer
strategies with the formed silicalite-1 zeolite shell as a protection
layer was employed for preparation of CuZnAl@H-ZSM-5
capsule catalysts. Unfortunately, the CO conversion also needs
[
78-80]
Figure 1. (a) Surface SEM image, (b) surface EDS, (c) cross-section SEM
image and (d) EDS line analysis of CZA@HZSM-5-EtOH catalyst.
to be improved.
In order to avoid the damage to the Cu-
based core in the process of zeolite coating, a physical coating
[
82,83]
method as a simple way was developed,
was used as a binder for coating zeolite shell. However, the
resulting SiO from silica sol inevitably covers the active sites of
CuZnAl methanol synthesis catalyst. Moreover, SiO also can
in which silica sol
CZA@HZSM-5-EtOH. From Figure 1a and b, the surface of the
CZA@HZSM-5-EtOH is mainly composed of Si while the molar
ratio of Cu/Zn/Al/Si is 1.00/0.38/0.19/0.33. This shows the
HZSM-5 shell has been covered on the millimeter-sized CZA
core. The SEM image and EDS line analysis of cross-section of
CZA@HZSM-5-EtOH are shown in Figures 1c and d. The EDS
analysis is collected along the white line in the SEM image. It
displays the alteration of elemental signals from the HZSM-5
shell to the CZA core part. Figure 1d shows that the Si signal
sharply increases in the HZSM-5 shell and then reduces at the
interface between HZSM-5 layer and millimetre-sized CZA core,
displaying the HZSM-5 shell with about 6.1 µm thickness coating
on core. Figures 2a and b exhibit the surface SME images and
EDS analysis of CZA@HZSM-5-SS. The signals of Si and Al on
the surface of CZA@HZSM-5-SS are the overwhelming majority,
and the signal of Cu and Zn on the outer surface of samples are
very low, indicating successful coating of HZSM-5 zeolite by
using silica sol is a binder. The Figures 2c and d show SEM
images and EDS line analysis of cross-section of CZA@HZSM-
2
2
block the microporous channels of zeolites. Therefore, it is
urgent to search a new binder to prepare core-shell capsule
catalyst by the physical coating method.
In this study, capsule-structured CZA@HZSM-5 catalysts
were synthesized by using physical coating method with variety
viscosities of ethanol, water, methanol, and ethylene glycol as
binder to coat micrometer-sized HZSM-5 shell on the prior-
shaped millimeter-sized CZA core. The silica sol was also used
as binder for comparison. Owing to too low viscosity of methanol,
HZSM-5 cannot be attached to the prior shaped CuZnAl
microparticles. However, the too large vescosity of ethylene
glycol leads to the serious abscission of HZSM-5 from the
particles. As a consequence, the HZSM-5 also cannot be
attached on the core. Therefore, the liquid binder with
appropriate viscocity is required. The CZA@HZSM-5-EtOH
catalyst synthesized with ethanol as a binder shows an excellent
catalytic performance in syngas-to-DME. It displays 2.9 times
higher CO conversion with the 2.7 times higher TOF and 9.2
times higher STYDME compared to the CZA@HZSM-5-SS
catalyst prepared by using silica sol as a binder (315.5 vs 34.3
5
-SS. The EDS analysis displays the alteration of element signal
from the HZSM-5 zeolite to the CZA core part. What is worth
noting is that there are two sharp Si signs in Figure 2d. It shows
that the Si signal increased sharply in the HZSM-5 zeolite shell
and then gradually decreased at the interface between HZSM-5
-
1
-1
g
DME kgcat h ). The effects of types of binder on the nature of
capsule-structured CZA@HZSM-5 catalysts and their catalytic
performance in dimethyl ether synthesis from syngas were
investigated. This work developed a highly-efficient and practical
catalyst for syngas-to-dimethyl ether.
The capsule-structured CZA@HZSM-5 catalysts are
composed of a millimetre-sized CAZ core as methanol synthesis
catalyst and a micrometer-sized HZSM-5 shell as methanol
dehydration catalyst. The capsule-structure of the as-prepared
zeolite shell and the formed SiO
existence of zeolite shell with 5.4 µm of thickness and the middle
SiO layer with 4.4 µm of thickness. Moreover, from Figure S1,
2
from silica sol, indicating the
2
the HZSM-5 zeolite shell is also successfully covered on the
millimetre-sized CZA core by using water as a binder. The
thickness of zeolites shell is 3.6 µm, which is thinner than that of
CZA@HZSM-5-EtOH and CZA@HZSM-5-SS. The above results
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ChemCatChem
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COMMUNICATION
H
2
O catalysts are consistent with those of CZA, which shows
that the damage towards CZA core by the physical coating
process.
Nitrogen adsorption is conducted to investigate the textural
properties of catalysts. The adsorption-desorption isotherms and
pore size distribution of CZA, CZA@HZSM-5-EtOH and
CZA@HZSM-5-SS catalysts are shown in Figure 4. From
Figure 4, the pores with a size of 5-25 nm in the catalysts can be
Figure 2. (a) Surface SEM image, (b) surface EDS analysis, (c) cross-section
SEM image and (d) EDS line analysis of CZA@HZSM-5-SS catalyst.
demonstrate that HZSM-5 shell can be successfully coated on
the millimeter-sized CZA core by using ethanol, water or silica
sol as a binder. However, when silica sol is used to binder, the
Figure 4. Nitrogen adsorption-desorption isotherms and pore size distributions
from adsorption branch of, CZA, CZA@HZSM-5-EtOH and CZA@HZSM-5-SS.
2
resulting SiO layer between millimeter-sized CZA core and
HZSM-5 shell cannot be avoided. From Table 1, the decrease in
the exposed Cu active sites of the core-shell catalysts compared
to the CZA core can present a further evidence for the
successfully coating HZSM-5 shell on the CZA core. Moreover,
compared to the CZ@HZSM-5-EtOH catalyst without Al in the
core, the CZA@HZSM-5-EtOHcatalyst show higher exposed Cu
active sites. This suggests the promoting Cu dispersion in the
CZA@HZSM-5-EtOH catalyst by the Al in the core.
assigned to the intercrystalline pores. The coating of zeolite shell
on the millimeter-sized core CZA catalyst leads to a reduced
amount of the intercrystalline pores of CZA, which may be due
to the filling and/or of coverage of pores by HZSM-5.
Furthermore, from Table 1, the mesoporous volumes of
Table 1. Characterization data of the CZA core and the capsule-structured
CZA@HZSM-5-EtOH and CZA@HZSM-5-SS catalysts.
The XRD patterns of core CZA catalyst, CZA@HZSM-5-
EtOH and CZA@HZSM-5-SS capsule catalysts are showed in
Figure 3. The peaks in the ranges of 5~10 and 21~25 of
[
a]
[b]
[c]
[d]
Cu
[e]
acid
[f]
H2
S
BET
2
S
t‐plot
V
meso
n
n
n
o
o
Catalyst
‐1
‐1
‐1
‐1
2
‐1
‐1
[m g ] [m g ] [ml g ] [μmol g ] [μmol g ] [mmol g ]
CZA
64.8
97.6
97.7
‐
0
35.4
37.4
‐
0.22
0.19
0.21
‐
422.1
362.9
389.3
157.4
‐
2.3
1.9
2.0
2.2
CZA@HZSM‐5‐SS
CZA@HZSM‐5‐EtOH
CZ@HZSM‐5‐EtOH
112.9
133.5
63.0
[
a] BET specific surface area from N
plot method. [c] Vmeso=Vtotal-Vt-plot. [d] Amount of surface active Cu, determined
by H -TPR after oxidation of the samples by N O. [e] Measured by NH -TPD.
f] Determined by H -TPR.
2
adsorption-desorption experiments. [b] t-
2
2
3
[
2
-
1
CZA@HZSM-5-EtOH and CZA@HZSM-5-SS are 0.21 ml g
-
1
and 0.19 ml g , respectively, which are lower than that of CZA
-
1
(
0.22 ml g ). Besides, the degree of blocking of CZA@HZSM-5-
SS is more serious than that of CZA@HZSM-5-EtOH. From
Figures 4 and S3, the sharp increase in amount of adsorbed N
Figure 3. XRD patterns of CZA, CZA@HZSM-5-EtOH and CZA@HZSM-5-SS.
2
at the low P/P0 on the isotherms of the three core-shell
CZA@HZSM-5 catalysts, indicating the existence of micropores..
From Table 1 and S1, CZA@HZSM-5-EtOH, CZA@HZSM-5-SS
CZA@HZSM-5-EtOH and CZA@HZSM-5-SS belong to HZSM-5.
As it is shown in the Figure 3 and S2, the diffraction patterns
corresponding to CuO and ZnO on the capsule-structured
CZA@HZSM-5-EtOH, CZA@HZSM-5-SS and CZA@HZSM-5-
2
-1
and CZA@HZSM-5-H
2
O have 37.4, 35.4, and 26.3 m
g
of
microporous surface area, respectively. This is an evidence for
the successfully coating HZSM-5 on the millimetre-sized CZA
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ChemCatChem
10.1002/cctc.201901938
COMMUNICATION
cores by using the different binders in the physical coating
process.
higher than that over CZA@HZSM-5-SS (26.3 %) under the
same reaction conditions. The high DME distributions (based on
H
2
-temperature-programmed reduction was conducted to
the total hydrocarbons and oxygenates formed without CO
2
) of
investigate the impact of binder type on the reduction behavior
of various capsule-structured CZA@HZSM-5 catalysts. From
Figure S4, there exist two kinds of reduction peaks for all
samples, which can be assigned to surface and bulk reducible
Cu species in the core. The changes in reduction peaks of the
coating CZA catalysts compared to the bare CZA might be led
by the coating process. Moreover, the redox behavior of
CZA@HZSM-5-EtOH catalyst and CZ@HZSM-5-EtOH catalyst
without Al in the core was compared. The CZ@HZSM-5-EtOH
catalyst shows higher reduction temperature than CZA@HZSM-
CZA@HZSM-5-EtOH and CZA@HZSM-5-H O are 95.0 % and
2
88.0%, respectively. However, the value of DME selectivity over
the CZA@HZSM-5-SS catalyst is only 25.8%, and the methanol
is the main production. It is worth to note that, from the above
analysis, HZSM-5 is successfully covered on the outer surface
of millimetre-sized core CZA catalyst when using silica sol as
binder. In addition, the amounts of the acidic sites over the
CZA@HZSM-5-SS are similar as that over the CZA@HZSM-5-
H O. However, the DME selectivity over the CZA@HZSM-5-H O
2 2
is further higher than that of CZA@HZSM-5-SS catalyst. The
reason why the selectivity of DME over the CZA@HZSM-5-SS is
[
4]
5-EtOH, implying the promoting effect of Al in the core on the Cu
dispersion. The promoted Cu dispersion of CZA@HZSM-5-EtOH
compared to CZ@HZSM-5-EtOH can also be confirmed by its
more exposed Cu active sites listed in Table 1.
so low is the SiO
and HZSM-5 zeolites. It can be seen from Figure 2c and d that
the SiO layer nearly capped on the outer surface of millimetre-
2
layer between the millimetre-size core CZA
2
Figure S5 shows the NH
3
-TPD profiles of the capsule-
sized CZA core catalyst and was covered by the HZSM-5
zeolites. The schematics of methanol dehydration over the (a)
CZA@HZSM-5-EtOH and CZA@HZSM-5-H O, and (b)
2
structured CZA@HZSM-5 catalysts. It is observed that weak
o
o
acid sites (150 - 300 C) and medium acid sites (300 – 450 C)
exist in these core-shell CZA@HZSM-5 capsule catalysts. Weak
and medium acid sites are identified as the ideal acid active
CZA@HZSM-5-SS catalyst were drawn in Figure 5. When
[
2]
sites for methanol dehydration to dimethyl ether. From Table 1
and S1, CZA@HZSM-5-EtOH shows more the acidic sites
-
1
-1
(
2
133.5 μmol g ) than CZA@HZSM-5-H O (115.3 μmol g ) and
-
1
CZA@HZSM-5-SS (112.9 μmol g ), which can promote the
conversion of the as-formed methanol on the cores into DME.
Moreover, the existence of Al in the core increases the acidic
amounts of CZA@HZSM-5-EtOH catalyst compared to the
CZ@HZSM-5-EtOH catalyst without Al in the core.
The STD reaction is a tandem reaction, which includes the
methanol synthesis process from syngas over the Cu-based
catalysts and methanol dehydration to DME over the solid acid.
The STD process was carried out using the core-shell capsule
catalyst composed of millimeter-sized CZA core and micrometer-
sized HZSM-5 shell, which is thermodynamically beneficial and
more efficient than the conventional DME synthesis by two
Figure 5. Schematic of methanol dehydration to DME over the (a)
CZA@HZSM-5-EtOH and CZA@ZSM-5-H2O, (b) CZA@ZSM-5-SS.
ethanol and water are used as binder, the HZSM-5 nearly clings
on the outer surface of millimetre-sized CZA core, so the
methanol molecules formed over the CZA core can entirely pass
through the micropores of HZSM-5 and dehydrate on the acidic
[
73-83]
separated processes.
CZA@HZSM-5-EtOH, CZA@HZSM-5-H
The catalytic performances of the
O and CZA@HZSM-5-
2
SS are shown in the Table 2 and S2. The TOF was calculated
2
sites. However, owing to the existence of SiO layer in the
Table 2. Catalytic performance of the CZA@HZSM-5-EtOH and
CZA@HZSM-5-SS catalysts for dimethyl ether direct synthesis from syngas.
CZA@HZSM-5-SS catalyst, the formed methanol molecules
over the CZA core can’t directly enter into the micropore of
HZSM-5 to access to the acidic sites. As a result, CZA@HZSM-
[
a]
Product distribution [Carbon
mol %]
CO‐to‐CO
2
CO Con.
TOF
5
-SS shows much lower DME selectivity than the developed
Catalyst
‐
1
[
mol %] [min ]
[mol %]
CZA@HZSM-5-EtOH by using EtOH as a binder. Furthermore,
the compressed methanol dehydration of methanol over the
ZA@HZSM-5-SS catalyst depresses the CO conversion since
MeOH
72.7
4.8
DME
25.8
95.0
89.6
CH
4
CZA@HZSM‐5‐SS
CZA@HZSM‐5‐EtOH
CZ@HZSM‐5‐EtOH
26.3
76.5
9.8
0.3
2.8
17.7
1.9
1.5
0.2
3.1
0.8
0.3
the methanol synthesis from syngas is
a thermodynamic
7.3
equilibrium limited reaction. As a result, CZA@HZSM-5-EtOH
catalyst shows both higher CO conversion and DME selectivity
than the other catalysts. Moreover, owing to the promoting effect
of improved methanol dehydration, CZA@HZSM-5-EtOH
catalyst shows much higher TOF than others. . Furthermore, as
o
[
1
a] Reaction condition: 0.5 g catalyst, P = 3.0 MPa, T = 250 C, H
0/5/5, GHSV = 1800 ml g h , 5 h of TOS.
2
/CO/N
2
=
-1
-1
based on the amount of exposed Cu active sites per gram
catalyst listed in Table 1. It can be seen that CZA@HZSM-5-
[73]
in shown in the previous report, Al migration form CuZnAlO
core to Silicalite-1 membrane is a key factor to get H-ZSM-5
membrane finally. Encouraged by the result, we explored what
EtOH and CZA@HZSM-5-H
2
O show 76.5% and 61.0% of high
CO conversion, respectively, which are 2.9 times and 2.3 times
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ChemCatChem
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COMMUNICATION
1
would happen when CuZnO is the core of catalyst in the
absence of Al (CZ@HZSM-5-EtOH). From Table 2, CZ@HZSM-
) catalyst. From Table S4, in contrast to the previously reported
syngas-to-DME catalysts, the capsule-structured CZA@HZSM-
5-EtOH catalyst indicates much higher DME STYDME under
similar conditions of GHSV. From the above results, we
developed an efficient syngas-to-dimethyl ether core-shell
5
7
-EtOH shows unexpectedly lower CO conversion (9.8% vs.
6.5%) and TOF (0.3 vs. 0.8) than CZA@HZSM-5-EtOH. From
Table 1, the much higher exposed Cu on CZA@HZSM-5-EtOH
catalyst by Al promoted Cu dispersion than that on CZ@HZSM-
bifunctional
CZA@HZSM-5-EtOH
catalyst
with
5
-EtOH leads to its higher CO conversion. For another, the
1.00/0.38/0.19/0.33 of Cu/Zn/Al/Si molar ratio (by XRF). The
CZA@HZSM-5-EtOH catalyst has a similar Cu/Zn ratio to the
CZA core, suggesting the physical coating process of HZSM-5
shell with ethanol as a binder doesn’t have visible influence on
the CZA core. This endows CZA@HZSM-5-EtOH catalyst with
outstanding catalytic performance concerning activity and
selectivity in direct synthesis of DME from syngas. The stability
of CZA@HZSM-5-EtOH for DME synthesis from syngas was
illustrated in Figure 6b. From Figure 6b, the developed
presence of Al in the CZA core of CZA@HZSM-5-EtOH catalyst
can strengthen the CO adsorption,
CO hydrogenation. Moreover, Al in the core of CZA@HZSM-5-
[
85]
which can also promote
[
76]
EtOH catalyst also endows it with some extra acidic sites. As
a result, CZA@HZSM-5-EtOH catalyst shows much more acidic
sites than CZ@HZSM-5-EtOH (Table 1), and then the former
shows lower methanol content than the latter (4.8% vs. 7.3%).
Furthermore, CZA@HZSM-5-EtOH catalyst shows lower
methane (0.2% vs. 3.1%) contents than CZ@HZSM-5-EtOH,
which might be ascribed to the resulted change in the redox
behaviour of CZA@HZSM-5-EtOH by the Al in the core of the
catalyst, compared to the CZ@HZSM-5-EtOH without Al in the
core.
CZA@HZSM-5-EtOH catalyst by
a facile physical coating
method with EtOH as a binder shows excellent catalytic stability.
No visible decrease in either CO conversion or DME selectivity
along with 30 h of time of stream can be observed. In a word, we
develop
a highly-efficient and practical capsule-structured
Moreover, the space time yield of DME over the diverse
capsule-structured catalysts prepared with various binders and
the catalytic stability of the optimized CZ@HZSM-5-EtOH
catalyst were investigated. From Figure 6a, the space time yield
catalyst by a physical coating approach using ethanol as a
binder, which shows excellent catalytic performance for direct
synthesis of dimethyl ether from syngas.
In summary, a capsule-structured CZA@HZSM-5-EtOH
catalyst was prepared by a physical coating method with ethanol
as a binder through coating prior-shaped millimeter-sized CZA
core with micrometer-sized HZSM-5 shell, which shows
outstanding catalytic performance for direct synthesis of
dimethyl ether from syngas. The developed CZA@HZSM-5-
EtOH exhibits 2.9 times higher CO conversion with 2.7 times
higher turnover frequency and 9.2 times higher dimethyl ether
space-time yield than the previously reported CZA@HZSM-5-SS
catalyst with silica sol as a binder. The much superior catalytic
performance of CZA@HZSM-5-EtOH to CZA@HZSM-5-SS can
be ascribed to the elimination of block effect of silica through
replacing silica sol by ethanol on the access of the reactant to
copper active sites and the synthesized methanol to acidic sites
of HZSM-5 shell. This work not only produces an outstanding
CO-to-DME catalyst, but also develops a new method for
designing highly-efficient core-shell catalysts by a physical
coating method.
Experimental Section
Materials: Copper nitrate trihydrate (Tianjin Damao Chemical Reagent
Co. AR), zinc nitrate hexahydrate (Aladdin, AR), aluminum nitrate
nonahydrate (Sinopharm Chemical Reagent Co., Ltd, AR), ethanol
solution (Tianjin Tianda Chemical Reagent Co. AR), oxalate (Tianjin
Tianda Chemical Reagent Co. AR), tetraethyl orthosilicate (TEOS ,
Xilong Scientific Reagent Co. AR), tetrapropylammonium hydroxide
Figure 6. a) The space time yield of DME over CZA@HZSM-5-EtOH,
2
CZA@HZSM-5-H O and CZA@HZSM-5-SS. b) Catalytic stability of the
developed CZA@HZSM-5-EtOH for dimethyl ether direct synthesis from
syngas.
(
TPAOH, 25 wt.% Sinopharm Chemical Reagent Co., Ltd, AR),
aluminium isopropoxide (Aladdin, AR), silica sol (Ludox: 30 wt. %,
Shandong Usolf Chemical Technology Co.).
-
1
-1
of DME over CZA@HZSM-5-EtOH is 315.5 gDME kgcat h ,
Catalyst preparation: The core CnO/ZnO/Al
2 3
O (CZA) catalyst was
which is much higher than those over the CZA@HZSM-5-H
2
-1
O
h
synthesized by oxalate co-precipitation method at room temperature with
subsequent calcination and shaping. Firstly, 45 mmol of copper nitrate
trihydrate, 20 mmol of zinc nitrate hexahydrate and 10 mmol of aluminum
[73]
-
1
-1
-
(234.7 gDME kgcat h ) and CZA@HZSM-5-SS (34.3gDME kgcat
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ChemCatChem
10.1002/cctc.201901938
COMMUNICATION
nitrate nonahydrate were dissolved in 75 ml of ethanol solution. Then 90
mmol of oxalate was added into 90 ml of ethanol solution. Above two
ethanol solutions was mixed and follow by stirring for 4h at room
temperature. The colloid was separated by centrifugal process. The
consumptive H
(NH
1200) instrument with
2
molecules. NH
3
temperature-programmed desorption
3
-TPD) experiment was conducted on a Builder Chemisorption (PCA-
TCD. The 100 mg of sample was firstly
a
o
pretreated in the flowing Ar at 500 C for 60 min, and then cooled to
o
o
o
obtained samples was dried at 105 C and calcined at 370 C for 1h. The
powder of CnO/ZnO/Al was squeezed and sieved into granules with
0-40 mesh. HZSM-5 was synthesized as the follow method: the molar
composition ratio of the synthesis compounds was
TEOS:0.00625Al :0.27TPAOH:57.69H O. After the mixture was
below 100 C. The ammonia adsorption process was performed at
O
2 3
100 °C via pulse injection of ammonia. NH
3
desorption process was
[84]
o
2
conducted by heating the sample from 100 to 500 C with a heating rate
o
-1
of 10 C min in the flowing Ar.
1
O
2 3
o
2
stirred for 2 h at 35 C, the precursor of HSM-5 was treated at 80 °C in
order to remove the ethanol produced during the hydrolysis of TEOS.
Then the water was added to keep a constant volume. Crystallization
Catalytic performance test: Syngas–to-dimethyl ether reaction was
performed in fixed-bed with a stainless steel reactor (8 mm) at 3.0 Mpa
o
and 250 C. Firstly, 0.5 g of sample was reduced at atmospheric
o
process was conducted at 180 C for 72 h, the product was collected by
-
1
-1
o
pressure in the flowing gas (10 ml min H
for 2h. Then STD reactions were carried out under the conditions: 250 C,
2
with 20 ml min
N
2
) at 250 C
o
centrifugation, dried and calcination in static air at 540 °C for 6 h. The
CZA@HZSM-5-EtOH capsule catalyst is synthesized by physical coating
method as given in the following: 0.2 g of HZSM-5 powder was added
into 5 ml of ethanol solution in a 20 ml beaker and stirred by glass bar for
uniform disperse of HZSM-5. Then 1.0 g of CZA granules was added into
the liquid mixture and immersed at 2h. Then, the mixture was turned into
-1
-1
3
2 2
.0 Mpa, GHSV is 1800 ml g h , and feed gas (H /CO/N = 10/5/5
o
molar ratio). The effluent products were heated at 160 C and analyzed
by an online gas chromatograph (FULI 9790 II) that was equipped with
Porapak N and 5A molecular sieve packed columns. The carbon balance
2
of the reaction over the CZA@HZSM-5-EtOH, CZA@HZSM-5-H O and
o
oven and dried at 40 C. In this process, the capsule catalyst was
CZA@HZSM-5-SS catalysts are 101.4%, 99.4% and 99.3%, respectively.
The internal standard method was used to calculate the CO conversion
and organic product distribution by the Eq. 1 and 2, respectively:
vigorously sharked and stirred carefully with a glass bar every 20 min in
order to obtain a uniform HZSM-5 shell on the out of CZA catalyst. When
water as a binder, the preparation of sample is similar with CZA@HZSM-
o
5
-EtOH, apart from the ethanol was replaced by water and dried at 60 C.
It is noted by CZA@HZSM-5-H
indicated by signed CZA@HZSM-5-SS. For comparison, CZA@HZSM-5-
SS was prepared according to the reference. Typically, 0.33 g of silica
sol was diluted with 2 times water in weight. The 1.0 g of CZA granules
was added into the diluted silica sol and was immersed. Then, the
2
O. As silica sol is binder, the sample is
ꢃꢄꢅꢆꢇ
CO conversion (mol %) ꢀ ꢁ1 ꢂ ꢃꢄꢈꢉ ꢊ ꢋ 100ꢌ% ( Eq. 1)
[82]
ꢈ
i
ꢍꢎꢏꢐꢌꢎꢑꢌꢃ ꢋꢒ
C distribution (mol%) ꢀ
ꢋ 100ꢌ%ꢌ(Eq. 2)
ꢉ
ꢈꢋꢒꢌ
ꢍꢎꢏꢐꢌꢎꢑꢌꢃ
∑
ꢈꢓꢔ
2 3
HZSM-5 powder was mixed with the soaked CnO/ZnO/Al O catalyst in a
Where COin and COout stands for the molar fraction of CO at the inlet
break, follow by vigorously shaking and stirring. Then, the sample was
i
o
and outlet, respectively. C represent the organic product of CH OH, CH ,
turned into oven and dried at 60 C. In this process, the capsule catalyst
3
4
DME, and i is for the carbon number in the molecules.
was vigorously sharked and stirred carefully with a glass bar every 20
o
min. Finally, dried samples were treated by calcinations at 500 C for 2 h
o
-1
at heating rate 5 C min .
2 2
The formed CO was assessed by the CO conversion to CO
(
mol %). In addition, the turnover frequency (TOF) of CO was calculated
as the converted CO per min per surfacial Cu atom. The space time yield
of DME (STYDME) was calculated as the gram of as-formed DME per h
per g catalyst. They were calculated employing equations 3 and 4,
respectively:
Catalyst characterization: The surface architecture of the catalysts and
distribution of element of catalysts cross-section were investigated by
scanning electron microscope (SEM) and energy dispersive
spectrometer (EDS) on FEI QUANTA 450 SEM/EDX instrument. N
adsorption–desorption experiments were conducted using a Beishide 3H-
2
o
-1
ꢕꢖꢗꢌꢋꢌꢘꢙꢚꢎꢛꢜ.
2
000PS1 apparatus at -196 C. The specific surface areas and
TOF (min )ꢀ
(Eq.3)
ꢝꢞꢞꢞꢌꢋꢌꢟꢠꢌꢋꢌꢡꢖꢆ
micropores areas of the samples were calculated by BET method and t-
plot method, respectively. The mesoporous volume was defined as total
volume reduced microporous volume. X-ray diffraction (XRD) patterns of
ꢡꢢꢠꢣꢌꢋꢌꢤꢢꢠꢣ
STYDME
ꢀ
(Eq. 4)
ꢥꢦꢧꢇ
o
o
o
the catalysts were collected in the 2θ range of 5 –80 with a speed of 8
-1
Where the FCO is defined as the CO flow rate (ml min ), V
m
is the
-
1
min on a Rigaku D/max-2400 instrument. H
reduction (H
2
-temperature-programmed
-TPR) measurements were carried out on a Builder
-1
molar volume of an ideal gas at 298 K (24.5 L mol ), nCu is the mole of
surface Cu atom of the loaded catalyst for the catalysis test, and the nDME
is defined as the as-formed DME mol per h, MDME is the molecular mass
of DME, and Wcat is the mass of catalyst.
2
Chemisorption (PCA-1200) instrument equipped a thermal conductivity
detector (TCD). The sample (50 mg) was placed in a U-shape quartz
o
tube and treated at 250 C for 1h in the flowing Ar , and then cooled
o
down to 50 C. After that, gas flow was switched to 10 % H
2
/Ar at 50 mL
-
1
o
o
-1
min , the catalyst was reduced at 400 C by a heating rate of 5 C min .
The numbers of consumption were gained by integrating and
transforming the TPR peak area. The quantities of superficial Cu atoms
of samples were investigated by H -TPR after samples oxidized by N
at 50 C. The procedure was as follows: 50 mg of samples were reduced
H
2
Acknowledgements
2
2
O
This work was financially supported by the National Natural
Science Foundation of China (U1610104) and also by the
Chinese Ministry of Education via the Program for New Century
Excellent Talents in Universities (NCET-12-0079).
o
-
1
o
in flowing 10 % H
down to 50 C. After being purified with He for 30 min, the metallic copper
2
/Ar (30 ml min ) at 250 C for 2 h, and then cooled
o
-
1
o
were oxidized by 10 % N
order to remove the residual N
0 min. Finally, the 10 % H /Ar (30 ml min ) was introduced through the
sample and the samples were reduced at 400 C by a heating rate of 10
2
O/He (30 ml min ) at 50 C for 1 h. Next, in
2
O, the samples were purified with Ar for
-
1
3
2
Keywords: capsule-structured catalyst • physical coating •
o
ethanol as a binder • H-ZSM-5 • Cu/ZnO/Al
dimethyl ether
2 3
O • syngas to
o
-1
C min . The quantities of surface Cu molecules are twice the quantity of
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ChemCatChem
10.1002/cctc.201901938
COMMUNICATION
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ChemCatChem
10.1002/cctc.201901938
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Simple but efficient: This work
reports a new and highly efficient
capsule-structured CuO-ZnO-
Yongle Guo and Zhongkui Zhao*
Page No. – Page No.
2 3
Al O @HZSM-5 catalyst for direct
Ethanol as a Binder to Fabricate a
Highly-Efficient Capsule-Structured
CuO-ZnO-Al O @HZSM-5 Catalyst for
2 3
Direct Production of Dimethyl Ether
from Syngas
conversion of syngas to dimethyl ether
by a facile physical coating method
with ethanol as a binder, it shows 2.9
times higher CO conversion with 2.7
times higher TOF and 9.2 times higher
dimethyl ether space-time yield of
CZA@HZSM-5-SS catalyst with silica
sol as a binder (315.5 vs 34.3 g DME
-
1
-1
kg cat h ).
This article is protected by copyright. All rights reserved.