Direct synthesis of dimethyl ether from CO2 hydrogenation over Cu-ZnO-ZrO2/SO42--ZrO2 hybrid catalysts: Effects of sulfur-to-zirconia ratios

Article abstract of DOI:10.1039/c4cy01568a

Sulfated zirconia catalysts were prepared by a direct sulfation method and were admixed with a CuO-ZnO-ZrO₂ catalyst for the direct synthesis of DME from CO₂ hydrogenation. The effects of sulfur-to-zirconia ratios on the physicochemical properties, activity, selectivity and stability of the catalysts were investigated. The sulfur loading content significantly influenced the structure and surface chemistry of the catalysts. The addition of a small amount of sulfur (5-15 wt%) created numerous mesopores on the catalyst surface, remarkably enhancing the surface area and total pore volume. However, at high sulfur loading (20-30 wt%), the mesopores tended to merge and form a larger pore. The detailed characterization by FT-IR, XANES and NH₃-TPD reveals that the sulfated zirconia with low sulfur content (5-10 wt%) mainly contained weak acid sites and acted as Lewis acids. Increasing the sulfur loading (15-30 wt%) resulted in the formation of Bronsted acid sites, thus increasing the acid strengths. The sulfated zirconia catalyst with 20 wt% sulfur loading achieved a superior DME productivity of 236 g_DME kg_cat^-1 h^-1 at a reaction temperature and pressure of 260 °C and 20 MPa. However, after 75 h of a time-on-stream experiment, the sulfated zirconia catalyst lost approximately 16.9% of its initial activity while a commercial H-ZSM-5 catalyst was more stable as only a 2.85% reduction was observed. This journal is

Full text of DOI:10.1039/c4cy01568a

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Direct synthesis of dimethyl ether from CO hydrogenation over Cu–ZnO–
2
2ꢀ
4
ZrO /SO –ZrO hybrid catalysts: Effects of sulfur to zirconia ratios
2
2
a, b, c, d*
a
a
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Thongthai Witoon
, Tinnavat Permsirivanich , Nawapon Kanjanasoontorn ,
a
a
e
Chalairat Akkaraphataworn , Anusorn Seubsai , Kajornsak Faungnawakij , Chompunuch
b,c
a,b,c,d*
b,c,d
Warakulwit , Metta Chareonpanich
and Jumras Limtrakul
a
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1
Center of Excellence on Petrochemical and Materials Technology, Department of
Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900,
Thailand
b
Center for Advanced Studies in Nanotechnology and Its Applications in Chemical Food
and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand
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c
NANOTECꢀKUꢀCenter of Excellence on Nanoscale Materials Design for Green
Nanotechnology, Kasetsart University, Bangkok 10900, Thailand
d
PTT Group Frontier Research Center, PTT Public Company Limited, 555 Vibhavadi
Rangsit Road, Chatuchak, Bangkok, 10900, Thailand
e
Nanomaterials for Energy and Catalysis Laboratory, National Nanotechnology Center
(NANOTEC), National Science and Technology Development Agency (NSDTA), Klong
Luang, Pathumthani 12120, Thailand
*Corresponding author: Tel.: +66 2579 2083; Fax: +66 2561 4621
Eꢀmail address: fengttwi@ku.ac.th (Thongthai Witoon)
fengmtc@ku.ac.th (Metta Chareonpanich)
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Abstract
Sulfated zirconia catalysts were prepared by a direct sulfation method and were
admixed with CuOꢀZnOꢀZrO catalyst for the direct synthesis of DME from CO
2
2
hydrogenation. The effects of sulfur to zirconia ratios on the physicochemical properties,
activity, selectivity and stability of the catalysts were investigated. The sulfur loading
content significantly influenced on the structure and surface chemistry of the catalysts. The
addition of small amount of sulfur (5–15 wt%) created numerous mesopores on the catalyst
surface, remarkably enhancing the surface area and total pore volume. However, at high
sulfur loading (20–30 wt%), the mesopores tended to merge and form a larger pore. The
1
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1
1
1
1
1
1
1
1
2
2
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2
2
2
2
detailed characterization by FTꢀIR, XANES and NH ꢀTPD reveals that the sulfated
3
zirconia with low sulfur content (5–10 wt%) mainly contained weak acid sites and acted as
Lewis acids. Increasing sulfur loading (15–30 wt%) resulted in the formation of Brønsted
acid sites, increasing the acid strengths. The sulfated zirconia catalyst at 20 wt% sulfur
loading achieved a superior DME productivity of 236 gDME/kgcat h at a reaction temperature
o
and pressure of 260 C and 20 MPa. However, after 75 h of a timeꢀonꢀstream experiment,
the sulfated zirconia catalyst lost approximately 16.9% of its initial activity while a
commercial HꢀZSMꢀ5 catalyst was more stable as only 2.85% reduction was observed.
Keywords: Carbon dioxide; Sulfated zirconia catalysts; Dimethyl ether; Heterogeneous
catalysis.
2

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1. Introduction
Exploring novel alternative fuels is necessary in order to meet the high global
demand for energy and power and reduce greenhouse gases, especially carbon dioxide
(CO ), released into the atmosphere. The combustion of fossil fuels is the main human
2
activity that causes a drastic increase in the concentration of CO in the atmosphere, which
2
is believed to be the major reason for rapid climate change [1]. For these reasons, the
2
production of alternative fuels created from CO is attractive as a prospective solution both
for high energy demand and for global warming concerns [2–4]. Among the alternative
fuels, dimethyl ether (DME) is considered a promising economical transportation fuel
1
1
1
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1
1
1
1
1
2
2
2
2
2
2
2
because it can be used as an efficient H carrier for fuel cell applications, a fuel source in
2
diesel engines, and a replacement for cooking gas (Liquified Petroleum Gas; LPG) [5–7].
In addition, DME is a clean fuel because the combustion of DME with an excess of oxygen
x
produces no particulate matter and low NO emissions.
DME can be produced from CO in a single step by mixing a catalyst for methanol
2
dehydration with a catalyst for methanol synthesis in a close proximity. This process has
received increased attention from researchers because in situ conversion of methanol to
DME would become a driving force to overcome the limitation of thermodynamic
2
equilibrium for CO hydrogenation to methanol [8]. In order to obtain a high yield of
DME, an excellent catalyst that can effectively convert CO to methanol is required. Many
2
kinds of bimetallic compounds, e.g., CuꢀZn [9], PdꢀGa [10], NiꢀGa [11] supported on metal
2
oxides have been investigated for methanol synthesis from CO hydrogenation. Among
them, CuꢀZn bimetallic compound has been found to be the most efficient catalyst in terms
of performance and cost, and therefore they are considered as the methanol conversion
catalyst in this study. To date, several works have focused on the modification of Cuꢀbased
catalysts in order to enhance the catalytic activity and stability. Arena et al. [12] showed
that ZrO
2
modified CuꢀZn oxideꢀbased catalysts were more active for the methanol
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synthesis from CO
2
2 3
hydrogenation than conventional CuꢀZnOꢀAl O catalysts. Wengui et
2 3
al. [13] modified the conventional CuꢀZnOꢀAl O catalyst by adding a small amount of La
and found that La could enhance the dispersion of Cu and reduce the CuO crystallite size.
Zhang et al. [14] reported that by adding 0.5 wt% V O onto CuꢀZnOꢀZrO catalyst CO
2
2
5
2
conversion and DME selectivity increased 12.5% and 6.7% compared to the unmodified
CuꢀZnOꢀZrO catalyst. However, very few studies have considered on the improvement of
acid catalysts [15].
Several solid acid catalysts such as alumina (Al
2
2
3
O ), modified alumina, and Hꢀ
ZSMꢀ5 were extensively studied in the dehydration of methanol. The alumina catalysts
exhibited a good performance in terms of both methanol conversion and DME selectivity.
However, the alumina catalysts rapidly deactivated due to a strong adsorption of water
molecules on Lewis acid sites [16]. HꢀZSMꢀ5 had a strong resistance towards water
adsorption [17]. Nevertheless, HꢀZSMꢀ5 usually contained a narrow pore size, which
limited the diffusion of reactant molecules from the surface to the active sites located in the
pores. This indicated that only the active sites on the external surface of the HꢀZSMꢀ5
could be used effectively. Moreover, the strong acid sites on the surface of HZSMꢀ5 could
lead to coke and hydrocarbon formation as secondary products [17, 18]. The modified
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2
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2
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2
2
2 3
Al O catalyst with 1 wt.% titania was reported to increase the dehydration rate and to
minimize the coke formation. However, the optimum operating temperature over this
o
catalyst was ca. 400 C, which was considerably higher than the optimum temperature
o
(220–280 C) for CO
2
hydrogenation to methanol [12, 19, 20]. Thus, the development of
the acidic catalysts for this process is still desired.
Interestingly, sulfated zirconia catalysts have been reported to be super acid
catalysts because their catalytic activity is relatively high in alkane isomerization [21–24],
esterification [21, 25–29] and alcohol dehydration [21, 30–33]. With the tunable acidꢀbase
properties over sulfur loading content, sulfated zirconia materials could act as bifunctional
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catalysts for the isomerization of glucose to fructose and the dehydration of fructose to 5ꢀ
HMF [34]. Despite the significant number of papers reporting the high activity of sulfated
zirconia catalysts, a few studies have considered the use of sulfated zirconia as catalysts for
methanol dehydration to DME production [30–32], and the performance of a Cu/ZnO/ZrO2
catalyst mixed with various sulfated zirconia catalysts for the direct synthesis of DME from
CO
with different sulfur contents were therefore prepared by a direct sulfation method and used
in a combination with a Cu/ZnO/ZrO catalyst for the direct synthesis of DME from CO
2
hydrogenation has not yet been reported. In this work, the sulfated zirconia catalysts
2
2
hydrogenation. The catalytic performance in terms of activity and stability of the sulfated
zirconia catalysts is discussed on the basis of their structure and surface acidity. Moreover,
a reaction mechanism for methanol dehydration to DME over the sulfated zirconia catalysts
is proposed based on evidence of various characterization techniques.
1
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2
2
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2. Experimental
2.1. Catalyst preparation
A CuOꢀZnOꢀZrO
reverse coꢀprecipitation method. Note that the effect of metal oxide compositions on the
activity and selectivity of methanol production from CO hydrogenation has been primarily
2
catalyst at a Cu:Zn:Zr atomic ratio of ~4:2:4 was prepared by
2
investigated and we found that Cu:Zn:Zr with the molar ratio of ~4:2:4 could provide an
excellent catalytic performance in terms of yield of methanol and stability. As a result, this
particular composition was applied in the present study. A metal salts solution (ca. 100
mL) of Cu(NO
3
)
2
·3H
2
O, Zn(NO
3 2 2 2 2
) ·6H O and ZrOCl ·8H O (Sigma Aldrich) was slowly
added into a 0.1 M NaHCO
3
solution (500 mL) under stirring at room temperature. The pH
of the solution was adjusted to 7.5 by adding 0.1 M NaHCO solution. The mixture was
3
continuously stirred at 400 rpm for 2 h. The precipitate was filtered and washed with 3,000
o
mL deionized water. Subsequently, the obtained product was dried at 100 C for 24 h and
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o
1
2
calcined at 350 C for 2 h. The textural properties of the prepared catalyst are listed in
Table 1.
3
4
2
Table 1 Chemical compositions and textural properties of the CuOꢀZnOꢀZrO catalyst.
Chemical
composition (mol %) surface
BET
Average
pore
diameter
(nm)
Total pore
volume
(cm /g)
a
Catalyst
3
area
2
CuO ZnO ZrO
2
(
m /g)
CuOꢀZnOꢀZrO
2
37.47 21.53 41.00 142
26.4
0.94
a
5
6
7
8
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0
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2
3
4
5
6
7
8
9
0
1
2
3
The chemical compositions of the catalyst were determined by ICP technique.
A series of sulfated zirconia catalysts with different sulfur loadings were
accomplished according to the procedure described by Sun et al. [35]. In a typical
preparation process, ZrOCl ·8H O and (NH ) SO were ground in an agate mortar for 20
2
2
4 2
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
min at room temperature. After standing for 18 h at room temperature in air, the sample
o
was calcined at 550 C for 5 h. The hybrid catalysts were prepared by a physical mixing by
a mortar of methanol synthesis catalyst and acid catalyst with a mass ratio of 1:2.
+
A commercial zeolite catalyst (Na ZSMꢀ5 with Si/Al ratio of 24, SHꢀ55 from
ALSIꢀPENTA Zeolithe GmbH (APZ)) was transformed to HꢀZSMꢀ5 via an ionꢀexchange
method using ammonium nitrate (NH
4
NO
3
) solution. Typically, 1.5 g catalyst was placed
into a bottle. 100 ml of 0.1 M NH NO
4
3
solution was added. The exchange process was
performed at 80°C for 2 h under vigorous stirring. After that, the exchanged sample was
+
washed with 100 mL of deionized water to remove the excess NH
4
ions. The process was
+
repeated for 3 times. Then, the obtained NH ZSMꢀ5 sample was dried overnight at 110°C.
4
Finally, the resulting sample was calcined in air at 550°C for 6 h in order to obtain the Hꢀ
ZSMꢀ5 catalyst.
2.2. Catalyst characterization
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The BET surface area, pore size distribution and pore volume of all catalysts were
determined by N
2
ꢀsorption measurement with a Quantachrome Autosorbꢀ1C instrument at
o
o
–196 C. Prior to measurements, the samples were degassed at 200 C for 24 h. Pore size
distributions of the samples were determined from the adsorption branch of the isotherms
in accordance with the BarrettꢀJoynerꢀHallenda (BJH) method. The specific BET (SBET
was estimated for P/P values to be between 0.05–0.30. The total pore volume was
measured at a relative pressure (P/P ) of 0.995.
)
0
0
The surface morphology and surface chemical compositions of the sulfated zirconia
catalysts were assessed with the application of a scanning electron microscope (SEM; FEI
Quanta 450) equipped with energyꢀdispersive Xꢀray spectroscopy (EDS). The SEM
measurement was taken at 20.0 kV. The samples were sputter–coated with gold prior to
analysis.
1
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1
1
1
1
1
1
1
1
2
2
2
2
2
2
Xꢀray diffraction (XRD) patterns of all catalysts were attained on a diffractometer
(Bruker D8 Advance) with CuꢀKα radiation. The measurements were made at temperatures
o
o
in a range of 15–70 on 2θ with a step size of 0.05 . The diffraction patterns were analyzed
according to the Joint Committee on Powder Diffraction Standards (JCPDS).
The sulfate content in each sample was determined with a DSCꢀTGA 2960 thermal
analyzer (TA Instruments). A 50 mg sample was loaded into an alumina sample pan. In
order to remove preꢀadsorbed H O, the sample was heated from room temperature in a flow
2
o
0
of pure N (100 mL/min) at a rate of 10 C/min until 400 C was achieved, then it was kept
2
at that temperature for 30 min. Subsequently, the temperature was linearly raised to 1100
o
o
C at a rate of 10 C/min. The sulfate content was calculated from the difference in weight
o
o
between the sample at 400 C and 1,100 C.
FTꢀIR spectra of the sulfated zirconia catalysts were obtained with a
−
1
spectrophotometer (Bruker Tensor 27) in the range of 400–4000 cm with a resolution of
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4 cm . The sample preparation included the amalgamation of a fine powder of each
sample with KBr powder.
Sulfur Kꢀedge Xꢀray Absorption Near Edge Structure (XANES) of fresh catalysts
was recorded at room temperature in the fluorescent mode using a silicon (111) doubleꢀ
crystal monochromator at beamline 5, Synchrotron Light Research Institute (SLRI),
Nakhon Ratchasima, Thailand. The collected energy range was from 2372 to 2592 eV, with
4 2 4
0.2 eV steps from 2442 to 2552 and 10 eV steps outside that range. A pure (NH ) SO was
also measured as a standard reference material. The preꢀedge and postꢀedge background
subtraction followed by a normalization procedure of all XANES spectra was achieved
using the ATHENA program.
1
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1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
Acidity measurements were performed by thermogravimetric technique using
ammonia as a probe molecule. A 20 mg sample was pretreated in a flow of He (50
o
0
0
mL/min) at a rate of 10 C/min until 550 C was achieved; the 550 C temperature was
o
o
maintained for 30 min, then the sample was cooled to 100 C. Once the 100 C
temperature was reached and stabilized, 10% v/v NH (He as a balance gas) with a flow
3
rate of 50 mL/min was introduced into the system for 1 h. Subsequently, the 10% v/v NH
flow was disconnected and a He flow was introduced for 1 h to remove physisorbed NH
The NH
3
3
.
3
desorption measurement was conducted in a flow of He (50 mL/min) at a heating
o
o
o
rate of 5 C/min from 100 C to 600 C. The amount of acidity was determined from the
weight loss due to the desorption of NH3.
2.3. Catalytic activity test
2
CO hydrogenation to DME was carried out in a fixed–bed stainless steel reactor
(7.75 mm inner diameter). 0.15 g hybrid catalyst was diluted with 1.35 g inert silica sand
(75–150 ꢁm). The catalyst was reduced in situ under atmospheric pressure with flowing H2
o
o
(60 mL/min) at 300 C and a heating rate of 2 C/min for 4 h. After the reduction, the
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temperature was cooled to 180 C under flowing N
2
; subsequently a flow of CO
2
and H
2
2 2
mixture (CO :H molar ratio of 1:3) was fed through the reactor. The feed flow rate was set
at 60 mL/min. The reactor pressure was slowly raised to 20 bars, and the reactor was
o
heated to a variety of temperatures (240, 260, 270, 280 and 300 C). The effluent gaseous
products were analyzed by using gas chromatography. Analysis of H , CO, CO , and N
2
2
2
was performed using GC–2014 gas chromatography equipped with a thermal conductivity
detector (TCD) and a UnibeadꢀC column. Methanol, DME and other hydrocarbon products
were analyzed by using GC 8A equipped with a flame ionization detector (FID) and a
Chromosorb WAW (20% PEG) column. The activityꢀselectivity data were calculated by
mass balance from an average of three independent measurements. The errors were within
1
1
1
1
1
± 2%. CO
defined as follows:
CO conversion to oxygenated compounds (%) =
2
conversion to oxygenated compounds, yield of DME and yield of CO are
2
(
molesmethanol+ (2 x molesDME)
)
×100
(1)
(2)
(3)
molesCO2,in
1
5
6
Yield of DME (%)
Yield of CO (%)
=
2
x moles DME ×100
moles CO2,in
1
=
moles CO ×100
moles CO2,in
17
18
19
20
21
22
The stability of the catalysts is presented in terms of spaceꢀtime yield of methanol
ꢀ1
ꢀ1
and DME (grams methanol or DME grams catalyst time ) as a function of timeꢀonꢀstream
defined as follows:
Spaceꢀtime yield of methanol
moles methanol
=
(4)
×
VCO2 × MWmethanol
moles CO2,in x total amount of catalyst x MV
23
Spaceꢀtime yield of DME
=
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molesDME
(5)
×
VCO2 × MWDME
molesCO2,in x total amount of catalyst xMV
3
ꢀ1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
where VCO2 is the volumetric flow of CO
2
(cm min ), MWmethanol and MWDME are the
ꢀ1
molecular weight of methanol and DME (grams mol ), and MV is the molar volume of
3
ꢀ1
ideal gas, 22,400 cm mol . Eq. 1ꢀ3 and Eq. 4ꢀ5 are used in Fig. 6 and Fig. 8, respectively.
3. Results and discussion
3.1 Catalyst characterization
Figure 1 shows the N
corresponding pore size distribution (Fig. 1b) of the sulfated zirconia catalysts prepared at
different sulfur loading contents. The isotherm of pure ZrO was a typical type IV with a
H4 hysteresis loop, a characteristic mesopore structure formed by the aggregates of the
plateꢀlike particles giving rise to slitꢀshaped pores. The pore size distribution of ZrO was
broad, ranging from 4 to 30 nm. When 5ꢀ15 wt% sulfur was loaded onto the ZrO , the type
2
adsorptionꢀdesorption isotherm (Fig. 1a) and the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
IV isotherm was observed, but the hysteresis loop was changed to H2, indicating that the
inkꢀbottleꢀlike structure caused by the compacts of the nonꢀuniform size spherical particles
was present. In comparison to the ZrO product, the mesopores ranging from 2 to 10 nm of
2
the 5SꢀZrO
content was increased to 20 wt% (20SꢀZrO
observed, indicating the presence of both mesopores and macropores. At the highest sulfur
content, 30 wt% (30SꢀZrO ), mesopores (2–20 nm) were no longer observed, and instead
only macropores are obtained.
2
, 10SꢀZrO
2
and 15SꢀZrO
2
products becomes more pronounced. When the sulfur
2
), type IV–II composite isotherms were
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Figure 1 N
2
adsorptionꢀdesorption isotherm (a) and pore size distribution (b) of sulfated
o
zirconia catalysts calcined at 550 C.
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Table 2 lists the results regarding the BET surface area, pore volume, and average
pore diameter of the sulfated zirconia catalysts. The BET surface area and the pore volume
2
3
2
of pure ZrO were found to be 26 m /g and 0.08 cm /g, respectively. At 5–15 wt% of the
sulfur addition, the BET surface area and pore volume increased remarkably compared to
those of pure ZrO . The 10SꢀZrO exhibited the highest BET surface area and the largest
2
2
2
3
1
1
1
1
1
1
1
1
1
1
pore volume, 89 m /g and 0.18 cm /g, respectively. The BET surface area decreased
2
2
significantly, to 36 m /g to 7 m /g, when sulfur content increased to 20 and 30 wt%,
respectively. This discrepancy in BET surface area can be explained as follows: at 5–15
2
wt% of the sulfur content, the ammonium sulfate is highly dispersed onto the ZrO surface,
creating the numerous mesopores observed after the calcination. However, at high sulfate
content (20–30 wt%), the sulfate species might fully cover the ZrO surface, resulting in
2
the reduction of BET surface areas and the generation of the larger pores.
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Table 2 Textural, sulfur content and surface acidic properties of the catalysts
Catalysts BET
Pore
Sulfur content Weak
(wt%)
Medium
acid sites acid sites sites
Total acid
surface area volume
2
3
(
m /g)
(cm /g)
(µmol/g)
(µmol/g)
(µmol/g)
ZrO
2
26.1
66.7
89.2
74.9
36.0
6.6
0.08
0.13
0.18
0.17
0.13
0.06
Not detectable 24
ꢀ
20
42
160
261
140
24
95
129
214
306
169
a
b
b
b
5
1
1
2
SꢀZrO
2
1.00 ,0.92
75
87
54
45
a
0SꢀZrO
5SꢀZrO
0SꢀZrO
0SꢀZrO
2
2
2
2
1.20 ,1.56
a
3.20 ,4.24
a
b
7.00 ,12.42
a
b
3
14.00 ,17.78 29
a
3
4
5
The sulfur content determined by the TGA analysis.
The sulfur content measured by the SEMꢀEDS analysis.
b
6
7
8
9
0
1
2
3
4
5
6
7
The thermal stability and the sulfate content of the sulfated zirconia catalysts,
shown in Fig. 2, were analyzed by TG and DTG under nitrogen flow condition. Every
o
sample showed weight loss at temperatures below 100 C, a result ascribed to the
desorption of physically adsorbed water molecules. The samples prepared with low sulfur
o
1
1
1
1
1
1
1
1
contents (5SꢀZrO
2
and 10SꢀZrO
2
) exhibited a small weight loss beginning at about 750 C
o
and ending at 900 C, possibly due to the evolution of SO decomposed from the sulfate ion
3
2
bonded to the zirconia surfaces. With further increase in the sulfur content (15SꢀZrO ,
o
20SꢀZrO
2
and 30SꢀZrO
2
), the DTG curve exhibited an additional peak around 726 C. This
implies that the sulfate that forms on the surface of the zirconia at high coverage is less
stable than that at low coverage. In other words, the number of the coordinated
configuration between sulfate and zirconia of the catalysts with high sulfate coverage is
lower than the catalysts with low sulfate coverage.
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2
o
Figure 2 TG (a) and DTG (b) curves of the sulfated zirconia catalystscalcined at 550 C.
3
4
5
6
7
8
9
0
1
2
3
TG tests determined the catalysts’ sulfur content on the basis of the difference in
o
weights at 200 and 1100 C. The results are shown in Table 2 together with the sulfur
content measured by SEMꢀEDS. The TG and SEMꢀEDS measurement agree for catalysts
prepared with low sulfur contents (5SꢀZrO
2 2
and 10SꢀZrO ). However, for catalysts
prepared at higher sulfur contents (15SꢀZrO , 20SꢀZrO and 30SꢀZrO ), the SEMꢀEDS
2
2
2
measurements were appreciably higher than the TG measurements. This suggests that, at
high sulfur content, sulfate species almost completely cover on the surface of the zirconia,
1
1
1
1
2
preventing the SEMꢀEDS analysis from detecting the ZrO . Note that the catalysts’ sulfur
contents were obviously different from those of the starting composition, which could be
o
due to the fact that the catalysts were preꢀcalcined at 550 C prior to the TG and SEMꢀEDS
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2
3
measurements. Therefore, some sulfate species were decomposed leaving only the stable
sulfate species on the surface of catalysts.
The FTꢀIR spectra of the sulfated zirconia catalysts prepared at different sulfur
ꢀ1
contents are shown in Fig. 3. The pure ZrO showed bands between 700 and 418 cm ,
2
characteristics of crystalline zirconia. The 5SꢀZrO and 10SꢀZrO exhibited bands at 995,
2
2
ꢀ1
1053, 1136 and 1196 cm , characteristic peaks for the SꢀO stretching vibration modes of
2
ꢀ
ꢀ1
the coordinated SO
4
species on the zirconia surface. The band at 1400 cm was assigned
ꢀ1
to the stretching vibrations of S=O bond in the sulfate groups. The band at 1612 cm was
attributed to δOꢀH bending frequency of water molecules associated with the sulfate groups.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
As the sulfur loading was increased up to 15 wt% (15SꢀZrO ), the distinct peaks between
2
ꢀ1
990–1200 cm were no longer observed, and one broad peak appeared instead. With
further increase of the sulfur content from 15 to 30 wt% (20SꢀZrO and 30SꢀZrO ), this
peak, which indicates a complex polyꢀsulfate surface species [36], became more dominant.
It was interesting to see that the spectral splitting between the (S=O) and (S–O) of the
(200 cm ) catalysts was much larger than that between the 15Sꢀ
and 30SꢀZrO catalysts, indicating that tridentate sulfate ions coordinated
2
2
ν
ν
ꢀ1
5SꢀZrO
2 2
and 10SꢀZrO
ZrO , 20SꢀZrO
2
2
2
to the zirconia at the low sulfur contents while a 2ꢀfold coordinated configuration was
formed at high sulfur contents. This FTꢀIR interpretation was clearly consistent with the
TG analysis (Fig. 2), i.e. the catalysts with low sulfur contents had a higher coordination
configuration. This finding was also supported by the theoretical study by Haase and Sauer
ꢀ1
[37]. The band at 1340 cm , assigned to the bending vibration of Zr–OH groups [32, 38],
was observed only for the catalysts with high sulfur contents (15SꢀZrO , 20SꢀZrO and
30SꢀZrO ).
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2
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o
Figure 3 FTIR spectra of the sulfated zirconia catalystscalcined at 550 C.
3
4
5
6
7
8
9
0
1
2
3
4
5
Sulfur Kꢀedge Xꢀray absorption near edge structure (XANES) spectra was used to
investigate the sulfate species. The first derivative analyses on the spectra were performed
in order to detect absorption edges of overlapping species. The sulfur Kꢀedge XANES
spectra and first derivative spectra of (NH ) SO and sulfated zirconia samples are shown
4
2
4
in Figure 4. The (NH
XANES spectra of all sulfated zirconia catalysts were similar to the (NH
indicating that the sulfur species form as a sulfate group. However, the shape of the white
line of the sulfated zirconia catalysts was slightly different than that of the (NH SO
sample. In addition to the white line at 2483.2 eV, an absorption peak at 2475.6 eV was
present for the sulfated zirconia catalysts with high sulfur contents (15SꢀZrO , 20SꢀZrO
and 30SꢀZrO ). This suggests that a portion of the sulfate group was reduced to lower
valence states [39]. The first derivative on these spectra showed a more distinctive
4
2
) SO
4
showed a white line around 2483.2 eV. The sulfur Kꢀedge
4
)
2
SO sample,
4
1
1
1
1
1
1
4
)
2
4
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difference in the white lines of the (NH
derivative of the sulfatedꢀzirconia catalysts had two peaks: one at 2480.2 eV and another at
2483.2 eV. The latter peak was in the same position as that of (NH SO and the former
4 2 4
) SO and the catalysts (Fig. 4b). The firstꢀ
4
)
2
4
peak was interpreted as indicating a protonated sulfate [40–42]. It can be seen that the
former peak was much more developed when the sulfur content was increased and that it
ꢀ1
correlated to the appearance of the FTꢀIR band at 1340 cm (Fig. 3), indicating that the
protonated sulfate occurred at high sulfate surface coverage.
2483.2
(a)
(b)
2
475.6 2480.2 2483.2
2475.6
30SꢀZrO
2
30SꢀZrO
2
2
1
0SꢀZrO
5SꢀZrO
2
2
20SꢀZrO
2
15SꢀZrO2
1
0SꢀZrO
2
2
4
1
0SꢀZrO
2
5
SꢀZrO
2
5SꢀZrO
(
4 2
NH ) SO
(
4 2
NH ) SO
4
2460
2480
2500
Energy (eV)
2520
2540
2460 2465 2470 2475 2480 2485 2490 2495 2500 2505
Energy (eV)
8
9
0
1
4 2 4
Figure 4 Raw data (a) and first derivative of sulfur Kꢀedge XANES spectra (NH ) SO and
1
1
sulfatedꢀzirconia materials.
12
13
14
15
16
17
18
The strength and overall concentration of the acid sites of the sulfated zirconia
catalysts were determined by NH
3
ꢀTPD; the results are shown in Fig. 5. The pure ZrO
2
0
catalyst showed one broad lowꢀtemperature desorption peak ranging from 100 to 200 C,
indicating the presence of weak acid sites. The sulfation treatment at low sulfur content
(5SꢀZrO ) increased the number of the weak acid sites and also generated medium acid
2
0
sites, corresponding to the desorption of NH
3
, in the range of 240–400 C, with the
0
maximum at 266 C. As the sulfur content was increased to 10 wt% (10SꢀZrO
2
), the
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second peak appeared to be broader and of somewhat higher intensity than it did for the 5Sꢀ
ZrO catalyst. With an even higher sulfur content (15SꢀZrO , 20SꢀZrO and 30SZrO ), the
2
2
2
2
second peak shifted towards to a higher desorption temperature, indicating an increase in
the acid strengths corresponding to the presence of the protonated sulfate species as
observed by sulfur Kꢀedge XANES analyses (Fig. 4).
6
7
o
Figure 5 NH ꢀTPD profiles of the sulfated zirconia catalysts calcined at 550 C.
3
8
9
0
1
2
3
4
5
6
7
8
2
The number of acid sites of all samples is shown in Table 2. The pure ZrO had a
1
1
1
1
1
1
1
1
1
low number of total acid sites (24 µmol/g). The addition of sulfur markedly enhanced the
total number of acid sites. The doping 5–20 wt% sulfur provided the total acid contents
higher than the pure ZrO at 3.96–12.75 times. Further increase in sulfur content to 30 wt%
2
caused a significant decrease in the number of the total acid sites, suggesting some sulfate
groups had become embedded in the bulk of the sulfated zirconia catalysts.
3.2 Catalytic activity results
CO hydrogenation was tested over CuOꢀZnOꢀZrO /sulfatedꢀzirconia catalysts in a
2
2
fixedꢀbed reactor at 20 bars. The catalytic activities were determined after 3 h on stream at
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the same GSHV. For all catalysts, CO, methanol and DME were found to be the major
products; each also produced a trace amount of methane. The catalytic performance of the
sulfatedꢀzirconia catalysts prepared at different ratios is shown in Fig. 6 in terms of CO
2
conversion to oxygenated compounds (Fig. 6a), yield of DME (Fig. 6b) and yield of CO
(Fig. 6c). The oxygenated compounds included methanol and DME produced from CO2
hydrogenation and methanol dehydration, respectively.
o
Over the sulfurꢀfree catalyst (ZrO
2
), and at a reaction temperature of 240 C, the
CO
2
ꢀtoꢀoxygenated compounds conversion rate (Fig. 6a) was 2.6%. The CO
2
conversion
decreased monotonically with increasing temperatures. This could be explained by the fact
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
that CO hydrogenation becomes kinetically less favored at higher temperatures [43].
2
2
However, in the presence of sulfatedꢀzirconia catalysts, the CO ꢀtoꢀoxygenated compounds
conversions were considerably higher and followed a volcanoꢀshape trend with a maximum
o
at the reaction temperature of 260 C. This observation is potentially explained by the fact
that the conversion of methanol to DME increases the equilibrium conversion of CO2
hydrogenation to methanol [8].
Fig. 6b shows the performance of the catalysts for converting methanol to DME.
The ZrO
2
catalyst was found to be basically inactive for the methanol dehydration; the
o
maximum DME yield was only 0.05% at a reaction temperature of 280 C. The 5SꢀZrO
2
and 10SꢀZrO catalysts showed a higher yield of DME compared to the ZrO catalyst. The
2
2
maximum DME yield for the 5SꢀZrO2 and 10SꢀZrO2 catalysts 0.5% and 1.0%,
o
respectively, was achieved at 280 C. The catalysts with higher sulfur contents (15SꢀZrO
2
,
2 2
20SꢀZrO and 30SꢀZrO ) provided a greater DME yield (1.5–3.6%), and these maximum
o
conversions were achieved at a lower reaction temperature (260 C). This suggests that the
formation of DME from methanol dehydration over the two groups of the catalysts may
have occurred on different active sites.
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4
3
2
(
a)
ZrO
SꢀZrO
0SꢀZrO
2
5
1
2
1
0
2
1
5SꢀZrO
2
2
0SꢀZrO
2
3
0SꢀZrO
2
ZrO
2
(b)
5SꢀZrO
2
1
0SꢀZrO
2
4
3
2
1
0
15SꢀZrO2
20SꢀZrO
2
3
0SꢀZrO
2
ZrO
2
(c)
5
SꢀZrO
2
1
0SꢀZrO
2
2
5
0
5
0
1
5SꢀZrO
0SꢀZrO
2
2
2
2
1
1
30SꢀZrO
2
5
0
2
20 240 260 280 300 320
o
Reaction temperature ( C)
1
2
2
Figure 6 CO conversion to oxygenated compounds (a), yield of DME (b) and yield of CO
3
4
5
6
7
8
9
0
1
(c) of the sulfated zirconia catalysts prepared with different sulfur loading contents.
To gain further insight on the relationship between the catalytic activity and the
property of the sulfated zirconia catalysts prepared at different sulfur to zirconia ratios, we
then attempted to construct a correlation between DME yield and total number of acid sites
o
of the sulfated zirconia catalysts; the result is shown in Fig. 7. At 240 C, the DME yield
slightly increased from 0.01 to 0.43% when increasing the total number from 24 to 129
µmol/g (i.e. the catalysts with low sulfur content). Increasing the total number of acid site
from 129 to 169 µmol/g, a dramatic increase in DME yield was observed. Similarly, the
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trend was also found for the other reaction temperatures. This implies that the formation of
DME over the sulfated zirconia catalysts with low and high sulfur contents occurs via
different mechanisms. Further increase in the total acid sites, DME yield no longer
changes which is mainly attributed to fast rate of methanol dehydration to DME.
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 7 Correlation of DME yield with total number of acid sites at different reaction
temperatures.
1
1
1
1
1
1
1
1
1
1
2
Various mechanisms for the dehydration of methanol to DME have been proposed
and studied. Bandiera and Naccache [44] proposed that two methanol molecules adsorbed
+
simultaneously on their catalyst surface, forming two surface species ([CH
3
OH
2
] and
ꢀ
[CH O] ), which then condensed to form a DME and a water molecule. Kubelková et al.
3
+
2
[45] predicted that a surface methoxy species via the protonated methanol CH
3
OH
was
initially formed and another methanol molecule reacted with the methoxyl group to
produce the DME product. Said et al. [32] studied the catalytic performance of sulfated
zirconia catalysts and interpreted the mechanism for DME formation in terms of oxidation
– reduction reactions in which one molecule of methanol adsorbed on an acid site and
another methanol molecule adsorbed on a base site, which then condensed to produce the
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DME and water. We agree that dehydration of methanol to DME occurs at dual acidꢀbase
sites but suggest that the details of the mechanism differ depending on the sulfur content of
the sulfated zirconia catalysts.
Scheme 1 proposes the mechanism for the formation of DME from methanol
dehydration over the sulfated zirconia catalysts with low (0ꢀ10S) and high (15ꢀ30S) sulfur
content. The proposed molecular structures and mechanism are based on characterization
analyses using FTꢀIR, TGA, and XANES techniques, performance test results, and
previous findings in the literature [32, 44, 45]. As discussed above in the FTꢀIR and
XANES results, at the low sulfur content the sulfate group preferably forms as
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
O=S(ꢀOꢀZrꢀ) (structure A). Thus, due to the inductive effect of the sulfate group, the
3
zirconium atoms attached to the sulfate group act as a Lewis acid site for DME formation
[21, 32]. As the sulfur content is increased, the sulfate group forms as –SO
4
(structure B)
and seems to cover the surface of the ZrO cluster. It is important to note that FTꢀIR and
2
XANES spectra indicate that ꢀZrꢀOH (i.e. a protonated oxygen) is present with the high
sulfur content catalysts only. This ꢀZrꢀOH plays a vital role in increasing DME production
in the presence of high –SO
group. Thus, when –SO covers the ZrO
become a strong Br nsted acid site, which in turns provides a proton to methanol.
4
coverage since the –SO
4
is a strong electronꢀwithdrawing
4
2
surface it induces the neighboring ꢀZrꢀOH to
Ø
Therefore, the methanolꢀtoꢀDME reaction mechanism at low and high sulfur content
follows two separate pathways, as shown in Scheme 1. At low sulfur content (Pathway I),
one methanol molecule adsorbs on the Lewis acid site and another methanol molecule
adsorbs on a relatively low Lewis base site (i.e. an oxygen atom of ꢀZrꢀOꢀZrꢀ) and then
both undergo the reaction via S
[46, 47]. By contrast, DME molecule creation at high sulfur content (Pathway II) [48–50]
takes place via the strong Br nsted acid site of ꢀZrꢀOH. In the first step, one methanol
molecule adsorbs on ꢀZrꢀOH. Then, it reacts with another methanol molecule from the gas
N
2 transition state mechanism to produce a DME molecule
Ø
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phase via the S
I.
N
2 type mechanism, resulting in a much higher DME yield than via Pathway
3
4
Scheme 1 Proposed mechanism for the formation of DME from methanol dehydration
5
6
7
8
9
0
1
2
3
4
5
6
7
over the sulfated zirconia catalysts at low (pathway I) and high (pathway II) sulfur content.
The behavior of CO formation is shown in Fig. 6c. CO yield of all catalysts was
found to significantly increase with increasing temperatures. However, only small
difference in CO yield of all tested catalysts at each reaction temperature was observed.
This might be the logical consequence of the fact that the reverse waterꢀgas shift reaction
proceeds very fast until equilibrium is achieved over Cuꢀcontaining catalysts [51, 52]. The
equilibrium is shifted towards the formation of CO with ascending temperature. This would
explain why all tested catalysts behave more or less the same assuming that the reaction
rate of the reverse waterꢀgas shift is much faster than the formation rates of methanol and
dimethyl ether.
1
1
1
1
1
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3.3 Stability of catalysts
Stability is a key factor in determining whether the sulfatedꢀzirconia materials can
be used as the acid catalyst for dehydration reactions in industrial production. On the basis
of the activity test and the proposed mechanism (Figs. 6, 7 and Scheme 1), the dehydration
of methanol over the sulfated zirconia catalysts consisting of low and high sulfur contents
provided the significant difference for DME yields. Therefore, the stability of the two
groups of the catalysts was investigated, 10SꢀZrO
2
and 20SꢀZrO
2
catalysts representing low
and high sulfur coverage, respectively, with pure ZrO
2
and HꢀZSMꢀ5 used as the reference.
Before initiating the experiment, a suitable testing condition must be considered. This is
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
because the direct synthesis of DME from CO hydrogenation requires two functional
2
catalysts: the CuOꢀZnOꢀZrO
catalysts can be deactivated over time. Generally, the deactivation of the Cuꢀbased catalyst
for methanol synthesis from CO hydrogenation is caused by the loss of copper surface
2
catalyst and the sulfated zirconia catalyst. Note that these two
2
area due to the low Tammann temperature of copper and the exothermicity of the reaction
[53, 54]. In order to assess the real performance of the sulfated zirconia catalyst without the
2
imposition from the CuOꢀZnOꢀZrO catalyst, the stability test must be performed under a
condition that can maintain a constant yield of methanol.
Fig. 8 shows the STY versus the timeꢀonꢀstream of DME and methanol over four
catalysts: CuOꢀZnOꢀZrO /ZrO , CuOꢀZnOꢀZrO /10SꢀZrO , CuOꢀZnOꢀZrO /20SꢀZrO and
2
2
2
2
2
2
CuOꢀZnOꢀZrO /HꢀZSMꢀ5. The STY of methanol over all catalyst systems was found to be
2
almost constant during the 75 h experiment. This indicated that the methanol catalyst
(CuOꢀZnOꢀZrO
ZrO /10SꢀZrO
75 h timeꢀonꢀstream experiment, while the CuOꢀZnOꢀZrO /20SꢀZrO catalyst exhibited
2
) had an excellent stability. Regarding the STY of DME, the CuOꢀZnOꢀ
2
2
catalyst showed a substantial decrease in DME yield ca. 41.7% during the
2
2
much greater durability, experiencing only a 16.9% reduction under the identical reaction
condition. Saravanan et al. [28] reported two possible reasons for the successive decrease
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in the activity of the sulfated zirconia catalysts: poisoning by water molecules formed
during the reaction, and leaching of sulfate species from the catalyst in the polar alcohol
medium. According to the mechanism we proposed above, for the catalyst with low sulfur
content (10SꢀZrO ), the methanol dehydrated over Lewis acidꢀbase sites. This is in
2
harmony with the direct synthesis of DME from CO hydrogenation, which produces a
2
large amount of water when compared to either the methanol dehydration or the oneꢀstep
synthesis of DME from syngas [31, 32]. This created water is the major cause of the
poisoning at the active sites of the catalysts, since the water molecule competes with the
4
+
methanol molecule for adsorption onto the Lewis acid sites (Zr ). This is a reason the
reduction of STY of DME over time is observed and this catalyst provides the maximum
yield of DME at the higher reaction temperature (Fig. 6b). When the reaction temperature
is increased, the desorption of the water molecules may regenerate the catalyst activity. By
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contrast, the 20SꢀZrO
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catalyst is mainly composed of Br nsted acid sites, which were
Ø
slightly affected by the water [28]. Therefore, the deactivation rate of the 20SꢀZrO catalyst
2
was much lower than that of the 10SꢀZrO catalyst.
2
CuOꢀZnOꢀZrO
2
catalysts are commonly deactivated through sulfur poisoning, but
catalyst. One of the following
we observed no signs of deactivation of our CuOꢀZnOꢀZrO
2
three hypotheses could explain the durability of this particular Cuꢀbased catalyst. (1) Since
the sulfated zirconia – an acid catalyst which was mixed with CuꢀZnOꢀZrO , a methanol
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o
conversion catalyst, to form our catalytic system – was calcined at 550 C for 2 h, and, as a
result, only the stable form of the sulfate remained on the zirconia surface, sulfate ions are
2
not transferred from the sulfated zirconia to the CuꢀZnOꢀZrO and therefore do not
deactivate it. During the experiment, water and methanol are produced, but they cannot
leach out sulfate ions, so that there is no sulfur source to poison the Cuꢀbased catalyst. (2)
The presence of ZnO in the catalyst improves sulfur resistance by forming
thermodynamically stable ZnS [9], so that the few sulfate ions that may have been leached
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out from the sulfated zirconia surface during the reaction are eliminated by the Zn and the
activity of the Cuꢀbased catalyst remains constant. (3) While the leaching process of sulfate
ions is the major cause of the sulfated zirconia deactivation, the Cuꢀbased catalyst exhibits
excellent stability because the sulfate ions are not changed to H S, the most active form for
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poisoning a metal catalyst. [Note: the spent catalyst should be further characterized for a
better understanding of the deactivation phenomena, e.g., coke deposition and leaching of
sulfate ions.]
A benchmark between the new catalyst and a commercial catalyst is required to
allow a straightforward comparison. In the present case, the stability of the 20SꢀZrO2
catalyst is compared to that of the HꢀZSMꢀ5 catalyst, as demonstrated in Fig. 8c. The STY
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of DME over the 20SꢀZrO
catalyst at the beginning of the experiment, indicating that the 20SꢀZrO
active. However, the 20SꢀZrO catalyst exhibited a rapid decrease in the STY of DME
2
catalyst was found to be higher than that of the HꢀZSMꢀ5
2
catalyst was more
2
while the STY of DME of the HꢀZSMꢀ5 slightly decreased at approximately 2.85% of 75 h
on stream, which was significantly lower than that of the 20SꢀZrO catalyst (16.9 %). This
2
indicates that the sulfated zirconia catalysts cannot be considered for the practical catalyst
at this stage since they cannot compete with the commercial HꢀZSMꢀ5 catalyst when
considered the longꢀterm stability. However, given that the stability of the sulfated zirconia
catalyst is improved, it might be considered a good industrial acid catalyst due to its
elevated activity and ease of preparation.
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Figure 8 Spaceꢀtime yields of methanol and DME as a function of timeꢀonꢀstream of CuOꢀ
ZnOꢀZrO /ZrO (a), CuOꢀZnOꢀZrO /10SꢀZrO (b), and CuOꢀZnOꢀZrO /20SꢀZrO and
CuOꢀZnOꢀZrO /HꢀZSMꢀ5 (c).
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Reaction conditions: T = 260 C, P = 2 MPa, Flow rate = 60 mL/min.
4. Conclusions
The physicochemical properties, activity and stability of the sulfated zirconia
catalysts were strongly affected by the sulfur content on the zirconia surface. At low sulfur
content (5SꢀZrO and 10SꢀZrO ), the tridentate sulfate formed on the zirconia surface and
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induced zirconia atoms to act as weak Lewis acid sites. These catalysts were active for
methanol dehydration at relatively high reaction temperatures. At high sulfur contents
2 2 2
(15SꢀZrO , 20SꢀZrO and 30SꢀZrO ), a protonated sulfate species formed and donated a
significant Brønsted acidity, which efficiently catalyzed the methanol dehydration even at
relatively low reaction temperatures. In addition, the catalysts with high sulfur content
showed a much greater stability than those with low sulfur contents because water
molecules preferentially adsorbed on the Lewis acid sites. It was also demonstrated that
Brønsted acidity is essential important for the direct synthesis of DME from CO
2
hydrogenation. This finding may open up a new application for sulfated zirconia catalysts
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(combined with Cuꢀbased catalysts) in the direct synthesis of DME from CO
hydrogenation that could lead to a sustainable fuel in the future.
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Acknowledgements
This work was financially supported by the Thailand Research Fund (Grant No.
TRG5780258), the Center of Excellence on Petrochemical and Materials Technology
(PETROMAT), the National Research University Project of Thailand (NRU), the
Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology,
Thailand through its program of Center of Excellence Network, Rayong Institute of
Science and Technology Foundation, and the Kasetsart University Research and
Development Institute (KURDI). The authors would like to thank the support from the
Synchrotron Light Research Institute (BL5.2: SUTꢀNANOTECꢀSLRI XAS) for XAS
measurement. Financial support for Mr. Tinnavat Permsirivanich from Faculty of
Engineering, Kasetsart University is also appreciated.
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