Preparation of nanostructured CuO/ZnO/Al2O3 catalysts for the synthesis of methanol from syngas

Article abstract of DOI:10.1166/jnn.2016.13258

In this work, the effect of surfactants on the catalytic properties of CuO/ZnO/Al₂O₃ (CZA) was studied for methanol synthesis via the hydrogenation of carbon monoxide. CZA catalysts were prepared by a co-precipitation method with various surfactants (tetrapropylammonium hydroxide (TPAOH) or cetyltrimethylammonium bromide (CTAB)) and evaluated for the synthesis of methanol from syngas. CZA catalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis. The results showed that the surfactants have significant influences on the morphology, crystalline structure, and surface area. Their catalytic activities for methanol synthesis from syngas (CO:H₂ = 1:2) were evaluated in a fixedbed reactor at 518 K and 45 bar. The as-synthesized CZA with CTAB exhibited higher reactivity in terms of the conversion and methanol space time yield (STY).

Full text of DOI:10.1166/jnn.2016.13258

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Nanoscience and Nanotechnology
Vol. 16, 10887–10891, 2016
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Preparation of Nanostructured CuO/ZnO/Al2O3
Catalysts for the Synthesis of Methanol from Syngas
Jaeyong Jang1, Gil-Pyo Kim2, Tae Sun Chang3, Beom-Sik Kim3, Sang Eun Shim1,
Seok Hoon Park4ꢀ∗, and Sung-Hyeon Baeck1ꢀ∗
1Department of Chemistry and Chemical Engineering, Center for Design and Applications of Molecular Catalysts,
Inha University, Incheon 402-751, Korea
2Institute of Chemical Processes, School of Chemical and Biological Engineering, College of Engineering,
Seoul National University, Seoul 151-742, Korea
3Greenhouse Gas Resources Research Group, Research Center for Environmental Resources,
Korea Research Institute of Chemical Technology, Daejeon 305-600, Korea
4Department of Environmental Engineering, Anyang University, Anyang, 430-714, Korea
In this work, the effect of surfactants on the catalytic properties of CuO/ZnO/Al2O3 (CZA) was
studied for methanol synthesis via the hydrogenation of carbon monoxide. CZA catalysts were
prepared by a co-precipitation method with various surfactants (tetrapropylammonium hydroxide
(TPAOH) or cetyltrimethylammonium bromide (CTAB)) and evaluated for the synthesis of methanol
from syngas. CZA catalysts were characterized using X-ray diffraction (XRD), scanning electron
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microscopy (SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET)
IP: 79.110.19.37 On: Mon, 07 Nov 2016 09:26:22
Copyright: American Scientific Publishers
analysis. The results showed that the surfactants have significant influences on the morphology,
crystalline structure, and surface area. Their catalytic activities for methanol synthesis from syngas
(CO:H2 = 1:2) were evaluated in a fixedbed reactor at 518 K and 45 bar. The as-synthesized
CZA with CTAB exhibited higher reactivity in terms of the conversion and methanol space time
yield (STY).
Keywords: CZA, Methanol Synthesis, Surfactant, Syngas.
1. INTRODUCTION
its low cost and high selectivity to methanol.9 Reduced
copper enables both CO and CO2 to be activated on
sites during the methanol synthesis reaction.10 Alterna-
tively, ZnO minimizes the sintering of Cu during the
calcination process,11 thereby introducing highly active
sites that are exposed to reactants. In addition, it serves
as an active site for the dissociation of H2,12 which
permits the hydrogenation of CO over Cu surfaces. More-
over, numerous efforts have been made to explore vari-
ous CuO/ZnO-based ternary catalysts.13 Presently, various
catalysts such as Cr,14 Mo,15 Au,16 Ga,17 and Al,18
have been incorporated into CuO/ZnO-based catalysts in
attempts to improve the catalytic activity. Among these,
CuO/ZnO/Al2O3 (CZA) has exhibited excellent catalytic
activity and stability for methanol synthesis. This is the
case because Al2O3 can stabilize the uniformly dispersed
CuO/ZnO structure, prevent the sintering of Cu parti-
cles, and increase the thermal stability.19ꢁ20 Saito et al.17
explored CZA composites as a catalyst for methanol
Methanol is an important raw material for the production
of important chemicals such as DME (dimethyl ether),
DMC (dimethyl carbonate), olefin, formaldehyde, acetic
acid, and a wide variety of other products.1–4 In addition,
it can be used as a hydrogen storage material and a trans-
portation fuel, which may help reduce the dependence on
fossil fuels in the near future.5–7 Therefore, syngas-based
methanol synthesis plays an important role in chemical and
energy industries. Therefore, catalysts with large surface
area, high activity, and good thermostability are needed for
the development of economical processes.
Typically, methanol is synthesized from syngas that con-
tains mostly CO and H2 along with a small amount of
CO2ꢀ8 To achieve enhanced catalytic activity, CuO/ZnO
is considered to be an attractive catalyst because of
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 10
1533-4880/2016/16/10887/005
doi:10.1166/jnn.2016.13258 10887

Preparation of Nanostructured CuO/ZnO/Al2O3 Catalysts for the Synthesis of Methanol from Syngas
Jang et al.
synthesis and demonstrated much higher catalytic activity
compared to that of CuO/ZnO. In this study, CZA cata-
lysts were employed to enhance the catalytic performance
for syngas-based methanol synthesis.
then calcined for 5 h at 623 K in air. The as-prepared cat-
alysts with TPAOH and CTAB are referred to as CZA-T
and CZA-C, respectively. For the sake of comparison, we
synthesized a CZA composite using the same procedure
but without any surfactant.
Since the degree of dispersion of the catalyst is a main
factor for achieving enhanced catalytic activity, attempts
have been made to prepare nanostructured catalysts.21 For
a high-efficiency synthetic process, catalytic materials with
a high surface area and proper pore size distribution are
essential for rapid transfer of reactant gases to the active
sites of catalysts.22 Therefore, the structure and catalyst
properties should be carefully considered and optimized
when attempting to design a more practical methanol syn-
thesis process. In the present work, the CZA catalyst for
syngas-based methanol synthesis was synthesized by a co-
precipitation method. More importantly, tetrapropylammo-
nium hydroxide (TPAOH) and cetyltrimethylammonium
bromide (CTAB) were used as surfactants in order to
develop the nanostructure; the use of surfactants can lead
to higher surface areas for enhanced catalytic activity. This
work mainly focuses on how these two types of surfactants
influence the structural changes and catalytic activities of
CZA catalysts.
2.2. Characterization of the Catalysts
The structures of the as-prepared catalysts were investi-
gated by X-ray diffraction (XRD, Rigaku, MAN 2200 V).
The morphology and microstructure were examined by
scanning electron microscopy (SEM, Hitachi, S-4300
SE) and transmission electron microscopy (TEM, Philips
CM200). The BET surface area was determined using an
ASAP 2020 instrument (Micromeritics). The pore diam-
eter was determined using the Barrett-Joyner-Halenda
(BJH) method.
2.3. Catalytic Activity Test for Methanol Synthesis
Activity test of the synthesized catalysts for methanol syn-
thesis were conducted in a continuous stainless steel tubu-
lar flow fixed-bed reactor (inside diameter = 9 mm), as
shown in Figure 1. 300 mg of catalyst was loaded into
the reactor with closely packed quartz wool in both ends.
Before reaction, all of the catalysts were reduced under a
flow of 40 mL/min H2 at 518 K for 4 h. The composition
of the feed gas was CO:H2 = 1:2 (by volume), and the
reaction was performed at 45 bar and 518 K with a space
velocity of 10,000 h−1. The products were analyzed after
2. EXPERIMENTAL DETAILS
2.1. Preparation of CZA Catalysts
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CZA catalysts, with a molar ratio of Cu:Zn:Al =
IP: 79.110.19.37 On: Mon, 07 Nov 2016 09:26:22
6:3:1, were prepared by a co-precipitation method
Copyright: American ScaierenaticfticioPnutibmlisehoefrs50 min via gas chromatography (GC,
using tetrapropylammonium hydroxide (TPAOH) or
Agilent 6890A), and the conversion of reactants and space
time yield (STY) of methanol were determined.
orcetyltrimethylammonium bromide (CTAB) as surfac-
tants. The proper amount of surfactant was dissolved in
a mixed solution of copper nitrate, zinc nitrate, and alu-
minum nitrate under vigorous stirring. A sodium carbonate
aqueous solution (1 M) was added into the resultant solu-
tion to initiate precipitation and stirred for 2 h at 343 K.
The precipitate suspension was then washed with deion-
ized (DI) water to remove any remaining impurities. The
obtained precipitates were dried at 343 K overnight and
3. RESULTS AND DISCUSSION
3.1. Characterization of the Catalysts
The XRD patterns of the as-synthesized catalysts, with-
out or with surfactants, are shown in Figure 2. Diffraction
peaks arising from CuO and ZnO phases appeared in all
samples; the diffraction peaks at 35.5ꢀ, 38.7ꢀ, and 48.7ꢀ
PI
TC
MFC
CO
BPR
GC
Furnace
Vent
PI
TC
Ball Valve
Pressure Indicator
Thermocouple
MFC
Check Valve
Relief Valve
H2
BPR
Back Pressure Regulator
Filter
Figure 1. Schematic diagram of the tubular fixed-bed reactor used for methanol synthesis.
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Jang et al.
Preparation of Nanostructured CuO/ZnO/Al2O3 Catalysts for the Synthesis of Methanol from Syngas
pore sizes of CZA, CZA-T, and CZA-C, determined by the
BJH method, were 37.0, 28.0, and 14.7 nm, respectively.
The CZA-C catalyst exhibited the highest specific surface
area, followed by CZA-T and CZA. The pore sizes showed
the reverse order of the specific surface area. Therefore,
the changes in the surface areas are closely related to the
pore size distribution. It should be noted that the presence
of surfactants strongly affects the surface area and disper-
sion of the active sites.
The surface morphologies of the as-prepared cata-
lysts were investigated by SEM, as shown in Figure 3.
In the case of CZA, the as-synthesized catalysts were
mainly composed of agglomerated spherical nanoparticles
(Fig. 3(a)). Alternatively, the structure of the as-prepared
catalysts prepared with surfactants showed a bundle of
rod-like shapes with an average diameter 33 nm (CZA-T)
Figure 2. XRD patterns of (a) CZA, (b) CZA-T, and (c) CZA-C.
and 28 nm (CZA-C), as shown in Figures 3(b and c),
respectively.
Table I. Specific surface area and average pore size of synthesized cat-
To further elucidate the degree of dispersion of the cata-
lyst in the mixed metal oxides, the as-synthesized catalysts
were characterized by TEM (Fig. 4). In the case of CZA,
spherical particles were anchored to the nanosheet and the
aggregation of small particles was observed (Fig. 4(a)).
In contrast, the structures of CZA-C and CZA-T were
one-dimensionally oriented and rod-like, consisting of very
small nanoparticles (Figs. 4(b and c), respectively). These
alysts, as determined by BET and BJH analysis.
Material
Specific surface area (m2g−1
ꢂ
Average pore size (nm)
CZA
CZA-T
CZA-C
47
62
78
37.0
28.0
14.7
can be assigned to CuO (JCPDS No. 45-0937) and the
ꢀ
ꢀ
ꢀ
results imply that the surfactant strongly affects the crystal
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peaks at 31.8 , 56.6 , and 68.0 can be indexed to ZnO
(JCPDS No. 36-1451). Interestingly, for the catalysts pre-
pared with surfactants, the peak intensities were decreased
and broadened. This suggests that the surfactants adsorbed
onto the crystal surfaces and retarded the metal crystal
growth to produce well-dispersed small particles.23 For the
three calcined catalysts, peaks corresponding to the Al2O3
phase were not observed. This indicates that Al2O3 was
either highly dispersed in the catalysts or existed as an
amorphous structure.24 Based on the above results, it can
be deduced that CuO was well-dispersed in the mixed
metal oxides when surfactants were used.25
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growth and alters the surface properties (e.g., the crys-
Copyright: American Scientific Publishers
tal shape, surface area, and pore size distribution). This
strongly indicates that the surfactants play an important
role in the formation of the catalyst structure.
3.2. Methanol Synthesis From Syngas
The catalytic performance of the as-synthesized cata-
lysts for methanol synthesis from syngas (CO:H2 = 1:2)
was evaluated by using a tubular fixed-bed reactor at
45 bar and 518 K. Figures 5(a and b) show the H2
and CO conversion, respectively, of the three catalysts,
respectively. CZA-C exhibited the highest conversion,
with average H2 and CO conversions of ∼8.6 and 8.4%,
respectively (Figs. 5(a and b)). In addition, the CZA-C
catalyst also showed the most stable space time yield
(STY) of methanol (Fig. 5(c)). The terminal STY after 4 h
was 0.03 gMeOH/(gCat ·h), which is higher than that of CZA
Further investigation of the structure was carried out
using a nitrogen adsorption–desorption technique. The
BET surface area results and average pore size calcula-
tions of the three samples (CZA, CZA-T, and CZA-C) are
shown in Table I. The BET surface areas of CZA, CZA-T,
and CZA-C were 47, 62, and 78 m2 g−1, respectively. The
Figure 3. SEM images of (a) CZA, (b) CZA-T, and (c) CZA-C.
J. Nanosci. Nanotechnol. 16, 10887–10891, 2016
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Preparation of Nanostructured CuO/ZnO/Al2O3 Catalysts for the Synthesis of Methanol from Syngas
Jang et al.
Figure 4. TEM images of (a) CZA, (b) CZA-T, and (c) CZA-C.
Figure 5. The catalytic performance of the synthesized catalysts: (a) H2 conversion, (b) CO conversion, and (c) space time yield (STY) of methanol.
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prepared without a surfactant. This can be explained by
As-prepared CZA samples, with or without surfactants,
were employed as a catalyst for methanol synthesis from
syngas. As compared with CZA and CZA-T catalysts,
CZA-C exhibited higher conversion and a more stable
STY of methanol. The catalytic activity results demon-
strated that effectively eliminating the surfactant was
important for obtaining improved performance. Moreover,
it is suggested that the catalytic properties are strongly
dependent on the surface properties, including the crystal
shape, surface area, and pore size distribution.
Copyright: American Scientific Publishers
the fact that the relatively high specific surface area has a
significant effect on the performance of the methanol syn-
thesis catalyst. In the case of CZA-T, H2 conversion was
drastically decreased after 60 min. Moreover, the CZA-T
catalyst demonstrated much lower average H2 (4.9%) and
CO (6.9%) conversions than CZA-C, and the stability with
time on stream was poor. The reason for these results is
likely related to the fact that TPAOH is not completely
eliminated by the heat treatment due to its high decom-
position temperature;26 this could result in surfactant cov-
ering some of the active sites. From the above results, it
can be concluded that the higher surface area contributes
to the large and uniformly-distributed active sites, which
can produce enhanced catalytic properties. The effective
elimination of the surfactant from the active sites is a key
strategy for obtaining improved performance.
Acknowledgments: This research was performed as
a cooperation project No “Development of greenhouse
reduction and its practical use technology” and supported
by the KOREA RESEARCH INSTITUTE of CHEMI-
CAL TECHNOLOGY (KRICT). This work was supported
by the National Research Foundation of Korea (NRF)
Grant funded by the Korean Government (MSIP) (No.
2015R1A4A1042434).
4. CONCLUSION
In summary, CZA catalysts were synthesized by a
co-precipitation method, and the influence of surfac-
tants (i.e., TPAOH and CTAB) on the catalytic activ-
ity was investigated. With surfactants, nano-sized active
sites were highly dispersed in the mixed metal oxides.
The as-synthesized catalysts with surfactants consisted of
one-dimensionally-oriented structures, indicating that the
presence of the surfactants strongly affects crystal growth.
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Received: 5 August 2015. Accepted: 8 April 2016.
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J. Nanosci. Nanotechnol. 16, 10887–10891, 2016
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