The influence of composition on the functionality of hybrid CuO-ZnO-Al2O3/HZSM-5 for the synthesis of DME from CO2 hydrogenation

Article abstract of DOI:10.1039/c8ra04814b

A series of CuO-ZnO-Al₂O₃/HZSM-5 hybrid catalysts with different Cu/Zn ratios and disparate Al₂O₃ doping were prepared and characterized by XRD, BET, H₂-TPR, NH₃-TPD and XPS techniques. The optimal Cu/Zn ratio is 7 : 3, and the introduction of a suitable amount of Al₂O₃ to form hybrid catalysts increased the BET specific area and micropore volume, facilitated the CuO dispersion, decreased the CuO crystallite size, increased the interaction between CuO and ZnO, enhanced the number of weak acid sites, altered the copper chemical state and improved the catalytic performance consequently. The highest CO₂ conversion, DME selectivity and DME yield of 27.3%, 67.1% and 18.3%, respectively, were observed over the CZA₇H catalyst. The suitable temperature of 260 °C and the appropriate space velocity of 1500 h^-1 for one-step synthesis of dimethyl ether (DME) from carbon dioxide (CO₂) hydrogenation were also investigated. The 50 h stability of the CZA₇H catalyst was also tested.

Full text of DOI:10.1039/c8ra04814b

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The influence of composition on the functionality
of hybrid CuO–ZnO–Al2O3/HZSM-5 for the
synthesis of DME from CO2 hydrogenation
Cite this: RSC Adv., 2018, 8, 30387
*
*
Jie Du, Chunyan Li, Kangjun Wang, Lidong Liu,
Yubing Hu, Yajing Zhang,
Xinrui Yu, Kai Wang and Nan Liu
A series of CuO–ZnO–Al2O3/HZSM-5 hybrid catalysts with different Cu/Zn ratios and disparate Al2O3
doping were prepared and characterized by XRD, BET, H2-TPR, NH3-TPD and XPS techniques. The
optimal Cu/Zn ratio is 7 : 3, and the introduction of a suitable amount of Al2O3 to form hybrid catalysts
increased the BET specific area and micropore volume, facilitated the CuO dispersion, decreased the
CuO crystallite size, increased the interaction between CuO and ZnO, enhanced the number of weak
acid sites, altered the copper chemical state and improved the catalytic performance consequently. The
highest CO2 conversion, DME selectivity and DME yield of 27.3%, 67.1% and 18.3%, respectively, were
observed over the CZA7H catalyst. The suitable temperature of 260 ꢀC and the appropriate space
velocity of 1500 hꢁ1 for one-step synthesis of dimethyl ether (DME) from carbon dioxide (CO2)
hydrogenation were also investigated. The 50 h stability of the CZA7H catalyst was also tested.
Received 6th June 2018
Accepted 18th August 2018
DOI: 10.1039/c8ra04814b
rsc.li/rsc-advances
reaction 2 over a solid acid catalyst. Furthermore, one-step
process (performing the two steps simultaneously over
1. Introduction
a hybrid catalyst) has a better performance as a thermodynam-
ically favorable course because the equilibrium limitation of
reaction 1 can be readily overcame by reaction 2.12,13
CO2 emission is increasing at an unparalleled rate due to fossil
fuel combustion in recent years.1 As a consequence, excessive
CO2 release leads to a series of problems such as global
warming and environmental pollution.2,3 Therefore, CO2 recy-
cling and utilization are very imperative.4,5 CO2 hydrogenation
to dimethyl ether (DME) is an excellent option.6
DME is attracting more and more interest as a green fuel
because it produces less COx, NOx, and SOx particulates when
combusted.7 Therefore, DME is considered as a potential envi-
ronmentally friendly alternative option to petrol diesel for diesel
engines.8 Meanwhile, DME, as a vital chemical intermediate,
can also be used to prepare diversied valuable chemical
products especially esters and lower olens.9 Furthermore, it
can be utilized in several elds, for example, in fuel additives of
diesel, home heating and cooking gas, and pro-environmental
refrigerants.10,11
The hydrogenation of CO2 to DME over a hybrid catalyst for
DME synthesis is principally composed of three independent
reactions, methanol synthesis from CO2 hydrogenation (CO2 +
3H2 4 CH3OH + H2O reaction 1), methanol dehydration to
form DME (2CH3OH 4 CH3OCH3 + H2O reaction 2), and
a reverse water gas shi reaction (RWGS) (CO2 + H2 4 CO + H2O
reaction 3). DME can be produced via a two-step process
including reaction 1 on a copper based catalyst and subsequent
Currently, hybrid catalysts of methanol synthesis component
and methanol dehydration component are applied to the one-
step direct DME synthesis from CO2.14,15 The methanol dehy-
dration components are generally solid acid catalyst such as g-
Al2O3, but the active sites of the g-Al2O3 is Lewis acid sites which
are favored for water adsorption, resulting in a signicant
reduction of the amount of acid sites and then a decrease of
catalytic activity. Another intriguing solid acid catalyst is WOx/
ZrO2, but the catalytic activity of WOx/ZrO2 is lower due to its
weaker acid strength present.16 Besides, SAPO-18 has been used
as acid function owing to its uniform microporous structure
and a high stability, but SAPO-18 possesses a large number of
moderate strength acid sites, which cause catalyst deactivation
as well as undesired products by coke deposition.17 By contrast,
HZSM-5 is the most suitable acidic component due to its more
hydrophobic character and advantage of Brønsted acidity,
which signicantly reduces the impact of water on HZSM-5.18–23
Furthermore, HZSM-5 possesses weak acid sites which are in
favor of methanol dehydration to DME at low temperatures and
exhibits high catalytic activity.24 However, the methanol
synthesis components of the hybrid catalysts are various, and
the Cu–Zn based catalyst is regarded as the most promising
binary catalyst among them. Xiao et al. reported the deposition
of Zn on the surface of copper can increase surface area and can
form a unique site at the Cu–ZnO interface to improve the
College of Chemical Engineering, Shenyang University of Chemical Technology,
Shenyang 110142, PR China. E-mail: yjzhang2009@163.com; angle_79@163.com;
Tel: +86-24-89383902
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adsorption of CO2 and catalyst activity.25,26 To further improve of 2 : 1, for convenience of comparison with previous work.7,31,32
the performance of the Cu–Zn bimetallic compound catalyst, The HZSM-5 with a SiO2/Al2O3 ratio of 38 was purchased from
adding an amphoteric metal oxide species as additive to Cu–Zn Catalyst Plant of Nankai University (China). The slurry was
based catalyst is an efficient method. Al2O3 is a prospective co- heated to 80 ꢀC and was aged for 2 h under stirring. Subse-
catalyst of sufficient attraction due to its mechanical and quently, the slurry was evaporated at the same temperature.
thermal stability, its excellent dispersion feature of copper and Then the obtained product was dried inꢀ muffler furnace at
ꢀ
zinc. Nevertheless, only a few literatures have reported Al2O3 as 120 C for 12 h and calcined in air at 400 C for 4 h. The nal
a single additive of the Cu–Zn based catalyst for one-step direct product was hybrid catalyst named as CxZ10ꢁxH (the x express
DME synthesis from CO2 hydrogenation at present. Lei et al. the molar ratio of CuO : CuO–ZnO) and was pressed, crushed
studied hydrogenation of CO2 to CH3OH over CuO–ZnO–Al2O3 and griddled to get the granules (20–40 mesh).
catalysts prepared via a solvent-free route, they found that the
combustion processes and physicochemical properties of cata-
lysts depend strongly on the type and amount of fuel.27 Allahyari
et al. reported CuO–ZnO–Al2O3/HZSM-5 nanocatalyst for direct
synthesis DME as a green fuel from syngas, the catalysts were
prepared by ultrasound-assisted co-precipitation method at
different irradiation times, and the result revealed the rela-
tionship between irradiation time and catalytic performance.28
Bowker et al. studied the mechanism of methanol synthesis on
copper/zinc oxide/aluminum catalyst, and found that the
energetics of formate hydrogenation/hydrogenolysis on the
copper component of the catalyst was unaffected by the inti-
mate mixing of copper and zinc oxide in the catalyst.29 Liu re-
ported preparation of HZSM-5 membrane packed CuO–ZnO–
Al2O3 nanoparticles for catalysing carbon dioxide hydrogena-
tion to DME, they found zeolite capsule catalysts exhibited an
excellent DME selectivity compared with the conventional
hybrid catalyst because the further dehydration of DME was
restrained.10 Unfortunately, those above mentioned literature of
CO2 hydrogenation over CuO–ZnO–Al2O3 catalyst were mainly
related to the inuence of catalyst preparation methods to the
catalyst activity as well as the mechanism of CO2 hydrogenation.
On the contrary, the effect of the amount of the additive Al2O3 of
CuO–ZnO–Al2O3 catalyst on catalyst activity was rarely investi-
gated. Meanwhile, the Cu/Zn ratio of the CuO–ZnO–Al2O3
catalyst currently still derived from methanol synthesis from
CO2 hydrogenation or from syngas hydrogenation.30
2.2 Preparation of CuO–ZnO–Al2O3/HZSM-5 hybrid catalysts
Thereaer, a series of CuO–ZnO–Al2O3/HZSM-5 hybrid catalysts
of different Al2O3 contents were prepared. Cu(NO3)2$3H2O,
Zn(NO3)2$6H2O were weighed according to the optimal Cu/Zn
mole ratio which was determined in the preliminary experi-
ments. The mixture was dissolved in ethanol followed by the
addition of Al(NO3)2$9H2O. The amount of the Al(NO3)2$9H2O
used depended on the required Al2O3 contents of 0, 1.0, 3.0, 5.0,
7.0, 10.0 wt% in CuO–ZnO–Al2O3. Then, the solution of
precursors was added to a vigorously stirred ethanol solution
containing the oxalic acid and the powdered HZSM-5 in
ꢀ
a constant and slow manner at 60 C. The weight ratio of the
nal oxide of CuO–ZnO–Al2O3 to HZSM-5 is 2 : 1. The slurry was
dealt with the same procedure as the CxZ10ꢁxH and the nal
product was abbreviated as CZAyH, where the y represents
theoretical Al2O3 : CuO–ZnO–Al2O3 wt%.
2.3 Catalyst testing
The hybrid catalyst activity was measured in a continuous-ow
xed-bed reactor made of stainless steel (10 mm inner diam-
eter). 1 mL of the hybrid catalyst was diluted with 0.5 mL of the
quartz sand (both in 20–40 mesh). Prior to the test, the catalyst
ꢀ
was reduced in situ in a stream of 10% H2/N2 at 300 C for 3 h
under atmospheric pressure. Aer the reduction, the reactor
was cooled to room temperature under owing N2 and reactant
gas ow (H2/CO2/N2 ¼ 3/9/1, molar) was introduced, subse-
quently raising the pressure to 3.0 MPa and the reaction
temperature was raised to 260 ꢀC. Analysis of the products were
performed applied a gas chromatograph (SP2100A) equipped
with both a TCD (for N2, CO and CO2, GDX-101 connected with
Porapak T column) and a FID (for CH4, CH3OH and DME,
Porapak Q column). The CO2 conversion as well as the selec-
tivity toward CO, CH3OH and DME are calculated by internal
standard method.32,33 XCO , SP and YDME represent the conver-
Here, we prepared a series of CuO–ZnO–Al2O3/HZSM-5
hybrid catalysts of different Al2O3 contents by the oxalate co-
precipitation method, on the basis of optimizing Cu/Zn ratio
of catalyst for DME synthesis from CO2 hydrogenation. The
effect of Al2O3 content on catalytic properties especially CO2
conversion and DME selectivity of the hybrid catalysts was
investigated. Moreover, the inuence of reaction temperature
and velocity on the catalyst activity of the hybrid catalyst with
optimum Al2O3 content was also investigated.
2
i
sion of CO2, the selectivity of the product (DME, MeOH, CO) and
the yield of DME, respectively. Each experimental data was
corresponds to an average of three independent measurements,
with error of ꢂ2%.
2. Experimental
2.1 Preparation of CuO–ZnO/HZSM-5 catalysts
A series of CuO–ZnO/HZSM-5 catalysts of diverse Cu/Zn molar
ratio were prepared, aiming at conrming optimum composi-
tion. Cu(NO3)2$3H2O, Zn(NO3)2$6H2O (Cu/Zn molar ratio
ranging from 2 : 8 to 8 : 2) were dissolved in ethanol to gain the
metal nitrates solution and coprecipitated by oxalate under
ꢀ
ꢀ
CO2;in N2;in ꢁ CO2;out N2;out
CO2;in N2;in
ꢀ
XCO
¼
(1)
(2)
2
Pi
ꢀ
SP
¼
i
vigorous stirring at 60 C in a solution containing the HZSM-5
1 ꢁ CO2;out
nely dispersed, with a nal CuO–ZnO : HZSM-5 weight ratio
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Table 1 Catalytic performance of the CxZ10ꢁxH Hybrid catalystsa
where Pi stands for the concentration of a specic i product
(DME, MeOH, CO).
Selectivity/%
Conversion
Catalyst of CO2/%
Yield/%
Hydrocarbons DME
YDME ¼ SDMEXCO
(3)
2
DME CH3OH CO
C2Z8H
C3Z7H
C4Z6H
C5Z5H
C6Z4H
C7Z3H
C8Z2H
16.06
16.44
17.71
17.46
18.06
19.20
18.16
25.41 5.34
27.04 8.03
36.71 5.23
37.73 5.22
38.42 6.01
47.80 1.70
39.35 5.15
67.35 1.90
4.08
4.45
6.50
6.59
6.94
9.20
7.15
63.14 1.79
56.16 1.90
55.22 1.83
53.70 1.87
47.80 2.70
53.49 2.01
2.4 Catalyst characterization
X-ray powder diffraction (XRD) patterns of all samples were
performed on a Rigaku D/max 2500 pc X-ray diffractometer with
a
Reaction conditions: T ¼ 260 ꢀC; P ¼ 3.0 MPa; CO2 : H2 : N2 ¼ 3 : 9 : 1;
Cu-Ka radiation (l ¼ 1.54156 A) at a scan rate of 4 minꢁ1 at 50
ꢀ
˚
GHSV ¼ 1500 hꢁ1
.
kV and 250 mA.
The BET surface area of the samples were conducted by N2
adsorption at ꢁ196 ꢀC using a Quantachrome Autosorb 1-C.
Before the absorption–desorption measurements, samples were
degassed under vacuum at 300 ꢀC for 3 h. The specic BET
(SBET) was estimated from the linear part of the Brunauer–
Emmett–Teller (BET) plot.
The X-ray photoelectron spectroscopy (XPS) and X-ray-
induced Auger electron spectroscopy were recorded on an
ESCALA 250 Xi spectrometer, using a standard Al-Ka X-ray
source (1486.6 eV). The binding energy (BE) values were refer-
enced to the adventitious C 1s peak (284.6 eV). Quantication of
the surface atomic concentrations was carried out using the
sensitivity factors supplied for the XPS instrument.
H2-TPR of catalysts were performed on chemisorptions
(ChemBET 3000). Before reduction, 0.02 g of sample was pre-
heated with owing He at 400 ꢀC for 60 min, then cooled down
room temperature. Subsequently, the temperature was raised in
10% H2/Ar (50 mL minꢁ1) at a ramp rate of 10 ꢀC min to 400 ꢀC.
H2 consumption was detected by TCD.
3.2 Catalytic performance of the CZAyH catalysts
The catalytic performances of the CZAyH catalysts with different
Al2O3 contents were listed in Table 2. DME was the major
product and the by products included CO and methanol under
the reaction temperature of 260 ꢀC, pressure of 3.0 MPa and
GHSV of 1500 hꢁ1. Moreover, several hydrocarbons such as CH4
were also detected. The catalytic performances of the Al2O3-
modied hybrid catalysts were enhanced enormously compared
with the Al2O3-free hybrid catalyst. With the Al2O3 contents
increase in the catalysts (range from CZA0H to CZA7H), the CO2
conversion, DME selectivity and the DME yield increased
signicantly. Further increasing the contents of Al2O3 (CZA10H)
decreased the catalysts activity. The CZA7H catalyst exhibited
the highest catalyst performance and a maximum CO2 conver-
sion of 27.3% with a DME selectivity of 67.1% and a yield of
DME of 18.3%, as shown in Table 2. The DME yield and the
DME selectivity of CZA7H catalyst were appreciable in compar-
ison of nearly 13.5% and 63.4%, respectively, obtained over
CuO–ZnO–ZrO2/HZSM-5 catalyst reported by Li et al.34 Mean-
while, the DME selectivity of 67.1% for CZA7H catalyst was
signicantly higher than 49.2% observed over CuO–ZnO–Al2O3/
HZSM-5 catalyst studied by Zhang et al.31 These results indi-
cated suitable Al2O3 additive can improve the catalyst perfor-
mance signicantly.
NH3-TPD of the samples were measured on the same appa-
ratus for H2-TPR measurements. 0.1 g sample was heated to
ꢀ
400 C and maintained for 30 min, then the temperature was
cooled to 100 ꢀC. Aer that, a 6 vol% NH3/Ar mixture stream was
introduced to the sample for 60 min. Then, the examined
sample was ushed with helium stream (30 mL minꢁ1) for
60 min to remove the weak adsorbed NH3. The temperature was
ꢀ
ꢀ
raised from 100 ꢀC to 800 C at a rate of 10 C min.
Fig. 1 shows the effect of reaction temperature on the cata-
lytic performance of CZA7H catalyst. It can be found that
volcanic shape change trends of the CO2 conversion, DME
selectivity and the yield of DME versus the variation of the
3. Results and discussion
3.1 The effect of the Cu/Zn molar ratio on the catalytic
Table 2 Catalytic performance of the CZAyH hybrid catalystsa
performance of CxZ10ꢁxH catalysts
In order to investigate the optimal Cu/Zn molar ratio of the
hybrid catalysts, a series of CxZ10ꢁxH catalysts were prepared.
The catalytic performances of catalysts with different Cu/Zn
molar ratio were summarized in Table 1. The CO2 conversion,
DME selectivity and the yield of DME increased with the Cu/Zn
increased from 2 : 8 to 7 : 3, and decreased if continue raising
the Cu/Zn to 8 : 2. The C7Z3H exhibited the maximum CO2
conversion, DME selectivity and DME yield values of 19.2%,
47.8% and 9.2%, respectively. The result revealed the Cu/Zn
molar ratio can affect the catalytic performance and the
optimal Cu/Zn molar ratio of the hybrid catalysts was 7 : 3.
Selectivity/%
Conversion
Catalyst of CO2/%
Yield/%
DME CH3OH CO Hydrocarbons DME
CZA0H 19.2
CZA1H 23.6
CZA3H 22.9
CZA5H 23.0
CZA7H 27.3
47.8 1.7
60.6 2.0
61.6 2.6
64.3 5.5
67.1 3.9
60.9 3.6
47.8 2.7
34.6 2.8
33.1 2.7
26.5 3.7
25.5 3.5
32.7 2.8
9.2
14.3
14.1
14.8
18.3
13.8
CZA10
H
22.6
a
Reaction conditions: T ¼ 260 ꢀC; P ¼ 3.0 MPa; CO2 : H2 : N2 ¼ 3 : 9 : 1;
GHSV ¼ 1500 hꢁ1
.
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Fig. 1 Effect of temperature on catalytic CO2 hydrogenation to DME
Fig. 3 Effects of the conversion of CO2 and the selectivity of DME with
for CZA7H hybrid catalysts.
CZA7H hybrid catalysts with time on stream.
temperature. With the temperature rising from 220 ꢀC to 260 ꢀC,
the conversion of CO2, the selectivity of the DME and the DME
yield increased obviously 12.7%, 2.7% and 8.9%, respectively,
and then reduced sharply when the reaction temperature
reached 280 ꢀC. Comparing the catalytic performance data at
several reaction temperatures, the optimal catalytic perfor-
contact time between reactants and the catalyst becomes short,
consequently both the CO2 hydrogenation to methanol and
methanol dehydration to DME reactions could not proceed
sufficiently.18 Therefore, the conversion of CO2 and the selec-
tivity of the DME reduced distinctly indicating the efficient
effect the space velocity on the catalytic performance.
ꢀ
mance for the CZA7H catalyst were obtained at 260 C. These
results can be explained by the hydrogenation of CO2 for
synthesis of DME was a reversible exothermic reaction, the
reverse reaction favored with an excessive temperature. Mean-
while, the main side reaction RWGS was an endothermic reac-
tion thus the CO selectivity increased and the selectivity and
yield of DME decreased with the reaction temperature higher
3.3 Stability of the CZA7H catalyst
The stability of CZA7H catalyst was studied for the reaction of
DME synthesis via CO2 hydrogenation at 260 C and 3.0 MPa
ꢀ
with GHSV ¼ 1500 hꢁ1. The stability results were recorded every
2 h, as shown in Fig. 3. It can be found that the CO2 conversion
of and the DME selectivity decrease slightly from 27.3% and
67.1% to 25.3% and 62.1% during the continuous 50 h reaction
process, respectively. The result indicated that the CZA7H
catalyst possessed better catalytic performance and stability.
than 260 C.5 In addition, the CZA7H catalyst activity may also
ꢀ
decrease due to the Cu sintered and crystallized gradually with
the reaction temperature increased.18
Fig. 2 illustrates the inuence of the space velocity of on the
catalytic performance of the CZA7H catalyst. It can be found that
the CO2 conversion, the DME selectivity and the DME yield
decreased continuously from 27.3% to 20.5%, 67.1% to 52.3%
and 18.3% to 10.7%, respectively, as the space velocity increased
from 1500 to 3000 hꢁ1. With the space velocity increasing, the
3.4 The structure of the catalysts
Fig. 4 illustrates the XRD patterns of a series of CZAyH catalysts
with different Al2O3 contents before H2 reduction. For all cata-
lysts, the diffraction peaks at 2q ¼ 35.5ꢀ, 38.7ꢀ, 48.7ꢀ, 53.4ꢀ,
58.2ꢀ, 61.5ꢀ, 66.2ꢀ, 72.3ꢀ and 74.9ꢀ can be ascribed to CuO phase
(JCPDS no. 48-1548), the peaks at 2q ¼ 31.7ꢀ, 34.3ꢀ, 36.1ꢀ, 47.4ꢀ,
56.5ꢀ, 62.8ꢀ and 67.8ꢀ can be attributed to ZnO phase (JCPDS no.
65-3411), and the peaks appear in the 2q range of 21–25ꢀ can be
matched to HZSM-5 (JCPDS no. 44-0003). The peaks corre-
sponding to Al2O3 phase cannot be observed, indicating that the
presence of Al2O3 were amorphous phase or highly dispersed in
the catalyst body phase. Compared with the peaks of catalyst
CZA0H, the peaks of catalysts with Al2O3 become weaker, indi-
cating the CuO–ZnO oxides possess less intensive crystallinity.
The diffraction peaks of CuO and ZnO weakened and broad-
ened gradually with the increasing of Al2O3 contents, even some
diffraction peaks of CuO and ZnO phase disappeared, sug-
gesting the decrease of grain size. The grain sizes of CuO (dCuO
)
and ZnO (dZnO) for CZAxH catalysts calculated by Scherrer's
equation are listed in Table 3. These results indicated obviously
that the additive of Al2O3 can improve the CuO dispersion and
Fig. 2 Effect of space velocity on catalytic CO2 hydrogenation to DME
for CZA7H hybrid catalysts.
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Fig. 4 XRD patterns of the CZAyH hybrid catalysts.
Fig.
5
XRD patterns of the reduced CZA0H and CZA7H hybrid
catalysts.
can decrease the grain size of CuO. For all catalysts, the grain
size of ZnO phase is greater than that of CuO phase, indicating
ZnO phase closely relates to CuO phase. Shimokawabe et al.
considered the grain size of ZnO is larger than CuO due to CuO
can be well dissolved on the ZnO particles.35 As a consequence,
the active interface increased with the improving the dispersion
of CuO.35,36 Fig. 5 shows the XRD patterns of the reduced CZA0H
and CZA7H catalysts. For these two catalysts, the diffraction
peaks at 2q ¼ 43.3ꢀ, 50.4ꢀ and 74.1ꢀ can be ascribed to metallic
copper phase (JCPDS no. 04-0836). No diffraction peaks of CuO
can be detected, indicating all CuO in the catalysts reduced to
Cu. The intensity of diffraction peaks of Cu for reduced CZA7H
catalyst decrease signicantly, compared to the corresponding
peaks for CZA0H catalyst. Furthermore, the grain size of Cu
decreased signicantly for CZA7H catalyst. As shown in Table 3,
and the grain sizes of Cu (dCu) which calculated by Scherrer's
equation for reduced CZA0H and CZA7H are 21.4 nm and
9.7 nm, respectively. These results suggest that the addition of
Al2O3 can effectively inhibit the Cu crystals growth.
The BET Surface areas (SBET) and the calculated average pore
diameter of the catalysts are also listed in Table 3. The SBET of
the catalysts approximately increases from 131.7 m2 gꢁ1 to 161.2
m2 gꢁ1 with the increasing of Al2O3 contents (from CZA0H to
CZA7H), then sharply decreases to 143.5 m2 gꢁ1 when the Al2O3
contents continuing increasing (CZA10H). The variation trend of
the micropore volumes is the same as the SBET while the trend of
average pore diameter of the catalysts is contrary to the SBET
generally. These results illustrate that the pore structure of the
hybrid catalysts can be changed by the introduction of Al2O3.
Meanwhile, a maximum SBET of 161.2 m2 gꢁ1 and a minimum
average pore diameter of 6.8 nm are detected for the CZA7H
catalyst which possesses the smallest grain sizes of Cu (dCu), as
determined from XRD. The result revealed that why the catalyst
of CZA7H exhibited the maximum of the CO2 conversion, the
DME selectivity and the DME yield simultaneously.
3.5 The reducibility of the catalysts
TPR measurements were carried out in order to evaluate the
reduction behavior of all the catalysts. Fig. 6 shows the H2-TPR
proles of the CZAxH catalysts with different Al2O3 contents. It
can be found only a single reduction peak with any satellite
peaks for all the catalysts. The result illustrate the opinion the
copper oxides on the CZAxH catalysts are easy to be reduced.37
Since ZnO and Al2O3 cannot be reduced under the experimental
condition, the single high temperature reduction peak can be
ascribed to the reduction of CuO species with strongly interac-
tion with ZnO and Al2O3.38 With Al2O3 content increasing, the
reduction peak maximum rstly shied towards higher
temperature from CZA0H to CZA7H catalysts, and then shied
towards lower temperature for CZA10H, as shown in Fig. 6. The
CZA7H catalyst possessed the highest reduction temperature at
ꢀ
373 C, which indicated the strongest interaction between the
Table 3 Physicochemical properties of the CZAyH hybrid catalysts
D (Average
pore diameter)/nm
Catalyst
SBET/(m2 gꢁ1
)
V1 (Micropore volume)/(cm3 gꢁ1
)
dCuOa/nm
dCua/nm
dZnOa/nm
CZA0H
CZA1H
CZA3H
CZA5H
CZA7H
131.7
132.1
133.4
134.1
161.2
143.5
0.032
0.024
0.028
0.031
0.037
0.030
8.0
8.7
7.7
7.6
6.8
7.6
20.4
15.4
14.4
11.4
10.9
10.6
21.4
—
—
—
9.7
—
29.1
17.6
16.5
14.1
13.4
12.3
CZA10
H
a
Determined by Scherrer's equation.
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Fig. 6 H2-TPR profiles of the CZAyH hybrid catalysts.
Fig. 7 NH3-TPD profiles of the CZAyH hybrid catalysts.
b exhibited unconspicuous changes with doping different Al2O3
contents, implying that the strength and amount of medium
acid sites cannot be inuenced by Al2O3 additive substantially.
The position of peaks g shied towards higher temperature
gradually, indicating the intensity of strong acid sites increased
with increasing Al2O3 doping, leading to the increase of
hydrocarbons in the products. This might be attributed to the
fact that the HZSM-5 with possessed strong acid sites could be
modied by more oxalic acid with the addition of the more
Al2O3 contents, during the preparation of hybrid catalysts by the
co-precipitation method.11,34 From the above, the amount of
weak acid sites enhanced with optimal Al2O3 modied, which
was in favor of obtaining the highest catalytic performance for
one-step DME synthesis reaction.24
Table 4 The reduction peak areas of the CZAyH hybrid catalysts
Catalyst
CZA0H CZA1H CZA3H CZA5H CZA7H CZA10
H
Peak area 25 065
30 247 26 320 32 048 38 894 15 425
heterogeneous CuO particles with metal oxides. The CZA7H
catalyst exhibited the best catalytic performance which might
be explained that strongly interaction between CuO and oxides
was benecial to CO2 hydrogenation to DME.39 Meanwhile, the
CZA7H catalyst possessed the largest area among the reducing
peaks, which revealed that the hydrogen consumption of the
catalyst was enhanced signicantly, as shown in Table 4. The
result implied the largest amount of easily reducible well-
dispersed copper oxide existed, which might also be ascribed
to the improving dispersion of the CuO particles.40 This may be
one reason why when increase Al2O3 contents in the catalysts
from CZA0H to CZA7H, the conversion and selectivity signi-
cantly increased and a further increase in Al2O3 contents
adversely inuenced conversion and selectivity simultaneously.
3.7 XPS analysis
The XPS spectra of reduced catalysts are shown in Fig. 8. Both of
the spectra contained two peaks at about 1022 and 1045 eV
(Fig. 8a), which were assigned to Zn 2p3/2 and Zn 2p1/2 peaks of
ZnO, respectively, with a spin energy separation of 23 eV.41 This
illustrates that the Zn atoms are in a completely oxidized state
and were not reduced.42 It can be observed a single peak
centered at near 74.2 eV in Fig. 8b, which was attributed to Al
2p3/2 of Al2O3, therefore, the form of Al was Al2O3. Fig. 8c
3.6 Surface acidity of the CuO–ZnO–Al2O3/HZSM-5 catalysts
Fig. 7 shows NH3-TPD proles of the CZAxH catalysts with showed the XPS spectra of the Cu 2p for the reduced CZA0H and
different Al2O3 contents. It can be found three NH3 desorption CZA7H catalysts. Two reduced catalysts displayed Cu 2p3/2 and
peaks which located in the range of 50–200 ꢀC (denoted as peak Cu 2p1/2 main characteristics peaks with binding energy (BE)
a), 200–350 ꢀC (denoted as peak b), and 350–600 ꢀC (denoted as values of approximately 932.6 eV and 952.6 eV, respectively, with
peak g), respectively. The a, b and g peaks were attributed to a Cu 2p core level split spin–orbit components (DCu 2p) of
weak, medium and strong acid sites, respectively, indicating the 20.0 eV. The shake-up satellite peaks between 940 eV and 945 eV
presence of three strength acid sites on the hybrid catalysts.33 were not detected, which illustrates the absence of Cu2+ species
The intensity of the low-temperature peak a of weak acid sites in both CZA0H and CZA7H catalysts can be reduced to Cu+ and/
rstly increased with increasing Al2O3 contents (from CZA0H to or Cu0 species completely.43,44 Meanwhile, the binding energy
CZA7H) and then declined with further increasing Al2O3 addi- (BE) of Cu 2p3/2 and Cu 2p1/2 for reduced CZA7H can be observed
tive, which was agreement with the trend of SBET and H2-TPR. slightly shied towards lower BE values, which revealed an
The CZA7H catalyst of the highest peak intensity possessed the enhancement on the interaction between Cu and ZnO with the
largest amount of weak acid sites which were benecial for increasing Al2O3 contents.45 The result indicated an inuence
methanol dehydration to DME at low temperatures.24 This on Cu chemical combination state due to the increase of the
might be ascribed to the fact Al2O3 owned weak acidic charac- outer-shell electron density of Cu as a consequence of the
ters. In addition, the intensity and the position of the peaks introduction of additive Al2O3. Liu et al. thought that the
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Fig. 8 Spectra of the reduced CZA0H and CZA7H hybrid catalysts: (a) Cu 2p; (b) Zn 2p; (c) Al 2p; (d) Cu (LMM) Auger.
chemical state of Cu was tightly related to the catalysts perfor- properties and catalytic performance of the hybrid catalysts
mance for CO2 hydrogenation to DME.46 This point might be based on the optimizing Cu/Zn ratio. The hybrid catalysts for
one reason answerable for doping Al2O3 affect the activity of the the reaction of CO2 directly hydrogenation to DME improved
hybrid catalysts. To distinguish Cu+ and Cu0 species, the kinetic the CuO dispersion, reduced the CuO crystallite size, decreased
energies (KE) of the Cu LMM X-ray auger electron spectroscopy the grain size of the Cu, enhanced the BET surface areas,
(XAES) spectrum are measured. As shown in Fig. 8d, a broad increased the interaction between the CuO and ZnO, enhanced
and asymmetric peak was divided into two peaks, which the amount of weak acid sites, changed the copper chemical
centered at near 916.5 and 918.7 eV for both reduced CZA0H and state on the catalysts surface and enhanced the catalytic activity
CZA7H catalysts. The two symmetrical peaks are corresponding and stability with the introduction of the Al2O3 contents. In
to the Cu+ and Cu0 species, respectively. The line position in the addition, the inuence of the reaction temperature and the
Cu LMM Auger electron spectrum of the two reduced catalysts space velocity on the catalytic performance of the catalysts
illustrated that the Cu0 was the predominant copper species on modied by Al2O3 was also investigated. The optimal Al2O3
the CZA0H and CZA7H catalysts surface. Furthermore, the Cu0/ contents (Al2O3 : CuO–ZnO–Al2O3 wt%) of the CuO–ZnO–Al2O3/
Cu+ area ratios of the two reduced catalysts were calculated HZSM-5 catalysts was 7.0 wt% under the proper Cu/Zn ratio of
based on the corresponding Cu LMM peaks. The area ratio of 7 : 3. The CZA7H catalyst for CO2 hydrogenation to DME showed
the reduced CZA7H catalyst is a bit higher than that of the the maximum CO2 conversion, DME selectivity and DME yield
CZA0H catalyst, which indicated that Cu0 species but not Cu+ of 27.3%, 67.1% and 18.3%, respectively, under the reaction
species might in charge of the activity of these catalysts.47 The conditions of the optimal reaction temperature 260 ꢀC, the
result might be another reason responsible for the improve- appropriate space velocity of GHSV 1500 hꢁ1 and the conven-
ment in catalytic performance for the Al2O3-modied hybrid tional pressure of 3.0 MPa.
catalysts.48,49
4. Conclusions
Conflicts of interest
The work discussed the effect of the amount of Al2O3 additive of
the CuO–ZnO–Al2O3/HZSM-5 catalysts on the physicochemical There are no conicts to declare.
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20 S. Wang, D. S. Mao, X. M. Guo, G. S. Wu and G. Z. Lu, Catal.
Commun., 2009, 10, 1367–1370.
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Foundation of China (51301114, 21203125, 51602206), Natural
Science Foundation of Liaoning Province (201602598,
2015020649), Science research Foundation of Education
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