The catalytic performance of sulfated zirconia in the dehydration of methanol to dimethyl ether

Article abstract of DOI:10.1016/j.molcata.2014.06.041

Sulfated zirconia catalysts were prepared by wetness impregnation of zirconium hydroxide with an aqueous solution of (NH₄) ₂SO₄ with SO₄^2- loadings (1-30%, w/w) and calcined at 450 °C for 3 h in a static air atmosphere. The catalysts were characterized by FT-IR, XRD, TEM and BET measurements. The surface acidity of the catalysts was investigated by the dehydration of isopropanol and the adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The catalysts were tested for dehydration of methanol in a fixed-bed reactor at 230°C using air as a carrier gas. The results revealed that among different catalysts, 10% SO₄^2- supported onto zirconia showed the highest catalytic activity with 83% conversion and 100% selectivity toward dimethyl ether. A good correlation was found between the acidity of the catalysts and their ability to dehydrate methanol.

Full text of DOI:10.1016/j.molcata.2014.06.041

Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The catalytic performance of sulfated zirconia in the dehydration
of methanol to dimethyl ether
∗
Abd El-Aziz A. Said , Mohamed M. Abd El-Wahab, Mohamed Abd El-Aal
Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2014
Received in revised form 28 June 2014
Accepted 30 June 2014
Available online 8 July 2014
Sulfated zirconia catalysts were prepared by wetness impregnation of zirconium hydroxide with an
2−
◦
aqueous solution of (NH4)2SO4 with SO4
loadings (1–30%, w/w) and calcined at 450 C for 3 h in a
static air atmosphere. The catalysts were characterized by FT-IR, XRD, TEM and BET measurements. The
surface acidity of the catalysts was investigated by the dehydration of isopropanol and the adsorption of
pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The catalysts were tested for dehydration of methanol in
◦
a fixed-bed reactor at 230 C using air as a carrier gas. The results revealed that among different catalysts,
Keywords:
ZrO2
2
−
1
0% SO4 supported onto zirconia showed the highest catalytic activity with 83% conversion and 100%
selectivity toward dimethyl ether. A good correlation was found between the acidity of the catalysts and
their ability to dehydrate methanol.
(
NH4)2SO4
Acidity
Methanol
© 2014 Elsevier B.V. All rights reserved.
Dimethyl ether
1
. Introduction
Dimethyl ether (DME) is a well-known building block in the pro-
because their strong acid sites are responsible for the formation of
significant amounts of undesired by-products (hydrocarbons and
carbon deposits) [7]. On the other hand, ␥-Al O , which possesses
2
3
duction of valuable chemicals such as methyl acetate and dimethyl
sulfate, as well as of petrochemicals (BTX aromatic and light olefins)
and conventional fuels by replacing methanol as raw material.
Moreover, DME is attracting increasing interest as a potential
eco-friendly substitute for petroleum-derived diesel fuel given its
high cetane number (55–60), low auto-ignition temperature, and
reduced emissions of hazardous compounds such as NOx, SOx, and
particulate matter [1,2]. DME may also serve as an efficient hydro-
gen carrier, which could be used to store renewable energy in a
chemical form that is easy to handle, distribute, store and use [3].
DME can be produced by methanol dehydration over a solid-acid
catalyst or direct synthesis from syngas by employing a hybrid
catalyst, comprising a methanol synthesis component and a solid-
acid catalyst [4]. The formation of DME from methanol requires a
catalyst with optimum acid properties. Usually, strong acid sites
favor secondary reactions that lead to undesired hydrocarbons and
coke formation, which eventually causes catalyst deactivation [5]. A
myriad of solid acid catalysts has been explored for the conversion
of methanol into DME, including ␥-Al O , modified alumina with
strong Lewis acid sites, exhibits lower methanol dehydration rates
compared to zeolites, and thus, could be due to the preferential
adsorption of generating H O molecules on the Lewis acid sites
2
under the reaction conditions [5]. It is important to develop new
catalysts to replace the acids commonly used in this reaction and
to overcome the problems take place by using other solid acid
catalysts. Sulfated zirconia is the most promising heterogeneous
catalyst [8,9]. Its acidity is much higher even than that of concen-
trated H SO , with its surface sulfate groups acting as Lewis and
2
4
Brønsted sites [10]. The high catalytic activity of sulfated zirconia
has been attributed to super-acidity. In some cases, super-acids are
significantly more active catalysts for reactions than conventional
solid catalysts. Super Acid catalysts can offer many advantages:
lower loadings, lower reaction temperatures, increased selectiv-
ity and fewer by-products, shorter reaction times. Satoshi et al.
[11] studied the catalytic action of sulfated tin oxide for etherifi-
cation and esterification of methanol in comparison with sulfated
zirconia. Mohamed et al. [12] used methanol dehydration as a test
reaction to compare the acidity of pure and sulfated metal oxides
such as ZrO , TiO -anatase, -rutile and Al O . However, the cat-
2
3
silica, TiO –ZrO , and zeolite and zeotype materials (HZSM-5, HY,
2
2
2
2
2
3
AlPO , SAPOs, etc.) [6]. Zeolite materials tend to deactivate rapidly
alytic dehydration of methyl alcohol to dimethyl ether on sulfated
zirconia systems using air as a carrier gas, to our best knowledge,
has not been reported. The objectives of the present study are
prepared, characterization and evaluation of sulfated zirconia cat-
alysts, for methanol dehydration to DME. Accordingly, the catalytic
4
∗ Corresponding author. Tel.: +20 882412427; fax: +20 882342708.
E-mail address: aasaid55@yahoo.com (A.E.-A.A. Said).
http://dx.doi.org/10.1016/j.molcata.2014.06.041
1
381-1169/© 2014 Elsevier B.V. All rights reserved.

A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
41
dehydration of methanol to DME was performed over a series of sul-
fated zirconia calcined at 450 C for 3 h. Finally, an attempt has been
made to correlate the catalytic activity of these catalysts, during
dehydration of methanol, with their structure and the acidity.
2.3.4. Nitrogen gas adsorption
◦
Nitrogen gas adsorption–desorption isotherms were measured
◦
at −196 C using a Nova 3200 instrument (Quantachrom Instru-
ment Corporation, USA). Test samples were thoroughly outgassed
◦
−5
for 3 h at 250 C to a residual pressure of 10 Torr, and the
weight of the outgassed sample was that used in calculations.
The specific surface area, SBET was calculated by applying the
Brunauer–Emmett–Teller (BET) equation. The porosity of the cat-
alysts was determined from the desorption curves using Nova
enhanced data reduction software (Version 2.13). The theoretical
particle sizes are also calculated from specific surface area, assum-
ing spherical particles according to the following equation:
2
. Experiments
2.1. Materials
Zirconium hydroxide (Aldrich), ammonium sulfate (NH ) SO ,
4
2
4
methyl alcohol, isopropyl alcohol, pyridine and 2,6-dimethyl pyri-
dine were obtained as pure reagents and were used without further
purification.
6
000
DBET =
p · SBET
2.2. Catalyst preparation
where DBET is the average particle size (nm), p is the theoretical
−
3
density of the sample (g cm ) and SBET is the specific surface area
A series of sulfated zirconia samples having different weight
2
−1
(
m g ).
percentages of sulfate were synthesized by wetness impregnation
of zirconium hydroxide with an aqueous solution of (NH ) SO .
4
2
4
2.3.5. Acidity determination
Calculated amounts of ammonium sulfate were dissolved in small
amounts of distilled water. The ammonium sulfate solutions were
admixed carefully with calculated amounts of zirconium hydroxide
till the formation of homogeneous pastes. The samples were dried
The acidity of the catalysts under investigation were deter-
mined by studying the dehydration of isopropyl alcohol (IPA), the
adsorption of pyridine (PY) and 2,6-dimethyl pyridine (DMPY). The
dehydration of IPA was carried out in a conventional fixed-bed flow
Pyrex glass tube reactor, at atmospheric pressure using nitrogen
as a carrier gas. The reaction conditions were: A 500 mg catalyst,
◦
2−
in an oven at 100 C for 24 h. The contents of SO
1
were (1, 3, 5,
0, 20 and 30 wt.%) before being calcined at 450 C for 3 h in static
air atmosphere. Sulfated zirconia catalyst donated by (SZ).
4
◦
−
1
2
2
% reactant of IPA in the gas feed, 50 ml min total flow rate and
00 C reaction temperature. The measurement of propene yield
◦
2.3. Catalyst characterization
(%) was made after 1 h to achieve steady-state reaction conditions
of the IPA. The chemisorptions of PY and DMPY were carried out
by injection of different volumes at steady state conditions. The
exit feed was analyzed by direct sampling of the gaseous products
into a Unicam ProGC gas chromatograph using a flame ionization
detector (FID) with a 10% PEG 400 glass column (2 m). The acid-
ity populations over the surface of catalysts, under investigation,
were measured also by thermogravimetric technique (TG) using
the adsorption of pyridine as probe molecule. The procedure was
2
.3.1. Fourier transform infrared (FT-IR) spectroscopy
FT-IR spectra of the prepared catalysts calcined at 450 C
◦
for 3 h were recorded using a Shimadzu Spectrophotometer,
model (Nicolet 6700), equipped with data station in the range of
4
−1
000–400 cm with a KBr disc technique.
2
.3.2. X-ray diffraction (XRD)
XRD analysis of the test samples was performed with a Philips
◦
500 mg of calcined sample was preheated at 250 C for 1 h in the air
(
The Netherlands) diffractometer (Model PW 2103, ꢀ = 1.5418 A˚ ,
before saturated with pyridine for 7 days after evacuation. About
15 mg of pyridine-saturated sample was subjected to TG analysis.
The TG analysis was recorded heating from room temperature up to
3
5 kV and 20 mA) with a source of CuK˛ radiation (Ni filtered). Pat-
◦
terns were recorded from 4 to 80 (2ꢁ). Particle size was estimated
using Scherrer equation [13]:
◦
◦
−1
−1
400 C using at 10 C min and 30 ml min flow of N , using Com-
2
puterized Shimadzu Thermal Analyzer TA60 Apparatus (Japan). The
mass loss due to desorption of pyridine from the acidic sites, was
determined as a function of total surface acidity as sites (g cat)
Kꢀ
D =
ˇ cos ꢂ
−1
[
15].
where D is the mean crystallite diameter (nm), ꢀ is the X-ray wave-
length, K is Scherer constant (0.89), ˇ is the observed angular width
at half maximum intensity of the peak and was calculated by the
following equation [14]:
2
.3.6. Catalytic activity measurements
The catalytic activity of the catalysts under investigation for
the vapor – phase dehydration of methyl alcohol was carried
out at 230 C in a conventional fixed bed flow type reactor at
ˇ2 = ˇs − ˇo
2
2
◦
atmospheric pressure using air as the carrier gas. The system com-
prised two reactors. One was used without any catalyst and filled
with glass beads (control reactor), to enable a measurement of
the ‘control’ conversion (if any), which was subtracted from that
measured in the flow reactor. 500 mg of catalyst was placed in
the middle of the second reactor with quartz wool. Space in the
reactor pre- and post-heating zone was filled with glass beads
to reduce the effect of auto-oxidation of the substrate and prod-
ucts in the gas phase. A methyl alcohol and air were introduced
into the reactor after air was bubbled through methyl alcohol
where ˇs is the full width of a diffraction peak under consideration
(
radian) in the middle of its height that was considered after com-
puter fit of X-ray data using the Gaussian line shape and ꢂ is the
Bragg’s angle and ˇo is the instrumental broadening, ˇo = 0.16 with
the apparatus used.
2.3.3. Transmission electron microscopy (TEM)
The size and morphology of the investigated catalysts were
characterized by transmittance electron microscope (TEM) JEOL
Model JSM-5400 LV (Joel, Tokyo, Japan). The catalyst powder dis-
persed in ethanol using ultrasonic radiation for 20 min and a drop
of that suspension was placed onto the carbon-coated grids. The
degree of magnification of TEM images was the same for all the
different investigated catalysts.
−1
saturator. The total flow rate was fixed at 50 ml min and used
4
% reactant of methanol in the gas feed. The gases after reac-
tion were chromatographically analyzed by FID with a Unicam
ProGC using a 2 m DNP glass column for analysis of the reaction
products of methyl alcohol on the tested catalysts. Measurements

4
2
A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
3. Results and discussion
3.1. Infrared analysis of zirconia and sulfated zirconia
30%SZ
FT-IR spectra of pure ZrO2 and sulfated with 10, 20 and 30 wt.%
◦
−1
calcined at 450 C were measured in the range 4000–400 cm and
presented in Fig. 1. It shows that, the FT-IR spectrum for pure ZrO ,
2
−
1
the bands assigned at 423–654 cm are corresponded to Zr
O Zr
bond [16]. A strong and broad band with a maximum centered at
20%SZ
−1
3
398 cm which is attributed to physisorbed water, whereas the
−
1
band at 1635 cm , assigned to the bending mode (ꢃHOH) of coor-
−
1
dinated water [17]. The band at 1362 cm
was assigned to the
bending vibration of Zr OH groups [18]. The FT-IR spectra of sul-
−
1
fated zirconia catalysts show bands at 1053, 1120 and 1224 cm
.
These bands are assigned to asymmetric and symmetric stretching
modes of oxygen bound to the sulfur of sulfate [19]. Moreover, the
10%SZ
−
1
band assigned at 1429 cm is due to the presence of S O stretch-
ing vibration of the surface sulfate species [20]. The wave number
of this band increases with the sulfur content, due to a change in
the type of sulfate species from isolated ones (a) to poly nuclear
ones (b) [21].
O
ZrO2
O
O
O
S
S
S
O
O
O
O
O
Zr
O
O
O
Zr
O
Zr
Zr
Zr
Zr
4
000 3500 3000 2500 2000 1500 1000 500
Zr
Zr
Zr
-1
Wavenumber (cm )
(
a)
(b)
◦
Fig. 1. FTIR spectra of non-sulfated and sulfated zirconia calcined at 450 C for 3 h.
The partially ionic nature of the S O bond is responsible for the
Brønsted acid sites in sulfated zirconia samples [22]. Furthermore,
−
1
the band observed at 1636 cm is due to the bending vibration
mode of an O H group of water associated with the sulfate. The
broad band around 3200 cm is attributed to the O H stretching
of the conversion and yield (%) were recorded after 1 h from
the initial introduction of the reactant into the reactor to ensure
the attainment of the reaction equilibrium, (steady state condi-
tions).
−
1
vibration of water associated with ZrO and the broadness indicates
2
the effect of hydrogen bonding [20]. It is worth mentioning here
that the O H stretching, vibration band decreases by the increasing
2−
percentage loading of SO4 . This behavior may be attributed to
−
2−
the successive formation of H SO , HSO and SO4 from water
2
4
4
sensitive SO3 groups [23].
3.2. X-ray diffraction (XRD)
30%SZ
The XRD patterns of sulfated and non-sulfated zirconia samples
◦
calcined at 450 C are presented in Fig. 2. The pure ZrO sample
2
◦
◦
◦
◦
◦
exhibits both tetragonal (2ꢂ = 30.3 , 35.3 , 50.7 , 59.9 , 60.6 and
◦
◦
◦
◦
6
3.5 [24]) and monoclinic (2ꢂ = 24.7 , 28.4 , 31.6 [25]) phases,
2
0%SZ
whereas only tetragonal phase is observed in the sulfated sam-
ple containing 10 wt.%. This behavior suggests that the presence
of sulfate within dried Zr(OH)4 before calculation is responsible
for inhibiting the crystal phase transformation from tetragonal to
monoclinic. The change in the crystal phase is due to the strong
interaction between the OH groups of ZrO2 and the sulfate ions
T
T
T
10%SZ
T
of (NH ) SO to form a chemically stable SO /ZrO2 complex [26].
4
2
4
4
These results are in good agreement with the results reported by
Mishra et al. [27]. On the other hand, the disappearance of the char-
acteristic peaks of tetragonal and monoclinic (amorphous) phases
of the samples with loading above 10% (w/w) reflects the high dis-
T
M
T
M
T
T
ZrO2
T
2−
M
T
persion of SO4
in the porous zirconia. However, the diffraction
◦
peak at 2ꢂ values of 30.1 was selected for calculating the crystal-
10
20
30
40
2
50
60
70
80
lite size for pure ZrO2 and 10% SZ phases. The crystallite sizes of
◦
samples calcined at 450 C are cited in Table 1. The results obtained
◦
demonstrate that crystalline sizes, decreases with the addition of
sulfate with 10 wt.%. These results are confirmed by the results
Fig. 2. X-ray diffractograms of non-sulfated and sulfated zirconia calcined at 450
for 3 h.
C

A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
43
Table 1
◦
Texture data of sulfated and non-sulfated zirconia calcined at 450 C for 3 h.
SO42 (%)
−
SBET (m2
g
−1
)
Total pore volume (ccg
−1
)
Average pore diameter (Å)
DXRD (nm)a
DBET (nm)b
Pure ZrO2
120.6
158.2
184.9
185.8
178.3
107.3
46.70
0.1674
0.1476
0.1397
0.1290
0.1152
0.0688
0.0574
55.5
35.8
30.2
27.8
25.8
25.7
49.2
9.5
–
–
–
4.9
–
8.76
6.67
5.71
5.68
5.90
1
3
5
1
2
3
%
%
%
0%
0%
0%
22.6
51.8
–
a
Crystalline size: determined by XRD results.
Particle size: determined by BET surface area.
b
reported by Mishra et al. [27]. A further increase in the amount
surface areas, theoretical particle sizes, pore volume and average
pore diameter of sulfated and non-sulfated zirconia samples are
2
−
of SO4
(more than 10 wt.%) resulted in the formation of amor-
phous samples which is difficult to calculate the crystalline sizes of
these samples from their XRD patterns.
presented in Table 1. It shows that, pure ZrO exhibits a surface area
2
2
−1
−2
of 120.6 m g . In addition of (1–5) wt.% SO , the surface area
4
2
−1
significantly increases to 185.8 m g and thereafter decreased. All
the samples are essentially mesoporous materials (Average pore
diameter between 25 and 55.5 A˚ ). The pore volume is between
3
.3. Transmission electron microscope (TEM)
−
1
0
.0574 and 0.1674 cc g . The theoretical particle sizes of pure ZrO2
To precise the morphology and size of the nanoparticles deter-
and 10% SZ samples that calculated from surface area are close to
the crystalline sizes that calculated from XRD analysis. So, it is clear
from the texture investigation of prepared catalysts that, sulfating
of ZrO2 causes an enormous increase of the surface area with con-
comitant decrease of pore diameter, pore volume and particle size
with loadings up to 10%.
mination, TEM is more suitable and powerful tool. Fig. 3 shows the
TEM micrographs of pure ZrO2 and 10% SZ calcined at 450 C for
3
spherical nanosized particles and the powders are uniform in size.
It can also notice that the particle sizes of a 10% SZ are smaller than
that of pure ZrO . The mean particle size of both samples observed
◦
h. It can be observed that the presence of agglomerates of semi-
2
from TEM image is larger than the crystalline size estimated from
XRD and surface area analysis, which could be attributed to the
crystalline agglomeration through heat treatment [28].
3
3
.5. Determination of the surface acidic sites
.5.1. Dehydration of isopropyl alcohol
3
.4. Surface area measurements
The catalytic dehydration of isopropyl alcohol (IPA) over pure
◦
ZrO2 and sulfated zirconia calcined at 450 C are carried out and
the results are shown in Fig. 4. The results indicate that the reac-
tion product of IPA was only propene. However, the dehydration
reaction of IPA catalyzed at acidic sites has been used by several
authors [30,31] as a test reaction for determining the acidity. Thus,
it can be observed from Fig. 5 that ZrO2 is an inactive catalyst toward
All isotherms (not shown) for pure zirconia and different load-
◦
ings of SZ calcined at 450 C were found to be of type IV, which
are generally observed for mesoporous solids. However, there is a
large increase in adsorption at a higher relative pressure showing
the presence of larger size mesopores in the samples. The inflec-
o
2−
tion point for pure zirconia at around p/p = 0.4, is not sharp, which
IPA reaction under our conditions. Eventually, the addition of SO4
reflects that the pores are not uniform in size and have broad
distribution. The hysteresis loop belongs to type H2 of de Bore
classification [29] for pure zirconia, 1, 3, 5, 10 and 20% SZ, while
for 30% SZ the hysteresis loop belongs to H3 type. In addition, the
hysteresis loop of pure zieconia with a wider gap, while on increas-
sharply increases the dehydration capacity, reaching a maximum
at 10 wt.%, then gradually decreases up to 30 wt.%. The difference
between the performances of the supported catalysts is explained
on the basis of SO42 interaction with the ZrO2 support via cre-
ation of more acidic sites, which is responsible for the dehydration
reaction.
−
ing the addition of % SO42 the gap becomes narrower. The BET
−
◦
Fig. 3. TEM images of (a) pure ZrO2 and (b) 10% SZ calcined at 450 C for 3 h.

4
4
A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
was injected with different volumes of PY or DMPY in the stream
100
of the reactants using N as a carrier gas. Fig. 5 represents the effect
2
of PY and DMPY additions on the distribution products of the IPA
reaction on 10% SZ catalyst. The results show that the injection of PY
or DMPY led to a continuous decrease in the conversion of IPA and
the yield of propene up to the addition of 6 ␮L then a steady state is
reached. This means that PY and DMPY suppressed IPA dehydration
activity. Thus, the chemisorbed PY and DMPY decrease the yield of
the propene by ≈97 and 92%, respectively on the addition of 6 ␮L. It
is clear from the above results that there is a little difference (≈5%)
between the amount adsorbed from PY and DMPY, which is corre-
sponding to the presence of (L) acid sites. Thus, the catalyst with
80
60
40
20
0
Conversion
Yield
Selectivity
1
0% SZ is an acidic catalyst with major (B) acid sites and minor (L)
acid sites. This finding is in good agreement with the results that
obtained by Kemnitz et al. [35]. In addition the existence of a high
concentration of (B) relative to (L) acid sites was expected as a result
of moisture adsorption; Zhang et al. [36].
0
1
3
5
-
10
20
30
2
SO4 , wt (%)
3
.5.3. Thermal analysis of 10% SZ presaturated with pyridine
DSC, TG and DTA curves of the desorption of PY from presatu-
Fig. 4. Activity variation of IPA with % SO42− loading on ZrO2 calcined at 450 C for
◦
3
h.
rated 10% SZ catalyst are carried out and presented in Fig. 6. The
TG curve exhibits a total weight loss of about 12.3% through two
◦
steps. The first step lies in the temperature range of (29–100 C).
3
.5.2. Poisoning of acid sites with pyridine and 2,6-dimethyl
This step is associated with a strong endothermic peak on the
DTA curve minimized at 62 C. This peak may be attributed to the
removal of adsorbed water and desorption of PY molecules from
weak acid sites. The second step lies in the temperature range of
100–250 C) and is not accompanied by any peak in the DTA curve.
This behavior may indicate that the removal of PY of weak and inter-
mediate acid sites occurs without gain or release energy. The DSC
curve shows only one endothermic peak minimized at 105 C. This
peak covered the temperature range from 50 to 200 C and may be
pyridine over 10% (SZ) catalyst during the reaction of IPA
◦
It is known that the chemisorption of pyridine (PY) and 2,6-
dimethyl pyridine (DMPY) can be used as basic probe molecules to
determine the acidity of a catalyst [32]. It was reported that PY is
selectively adsorbed on both Brønsted (B) and Lewis (L) acid sites
◦
(
[
33]. On the other hand, DMPY is selectively adsorbed on (B) acid
site [34] but not (L) sites because of the steric hindrance of two
methyl groups. So the difference between PY and DMPY adsorption
is a measure of the (L) acid sites. The poisoning of the active surface
sites of 10% SZ catalyst in IPA conversion was performed through
saturation of the acid sites with PY or DMPY according to the fol-
◦
◦
attributed to the removal of PY of weak and intermediate acid sites.
So, it is clear from the above results that, the catalyst containing
1
0% SO42 exhibits an acidic catalyst with weak and intermediate
−
lowing procedures. After measuring the conversion activity of the
1
◦
(major) acidic sites.
0% SZ catalyst toward the dehydration of IPA at 200 C the catalyst
100
80
60
40
20
0
PY
DMPY
0
1
2
3
4
5
6
7
Volume, L
μ
◦
Fig. 5. Activity variation of IPA with the volume of PY and DMPY over 10% SZ catalyst calcined at 450 C for 3 h.

A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
45
100
80
60
40
20
0
Conversion
Yield
Selectivity
100
125
150
175
200
225
250
o
Reaction temperature ( C)
Fig. 6. DSC, TG and DTA curves of desorption of PY from 10% SZ catalyst.
Fig. 8. Effect of reaction temperature on the dehydration of methanol over 10% SZ
catalyst calcined at 450 C using air or nitrogen as a carrier gas.
◦
3
.5.4. Effect of reaction temperature on the dehydration of IPA
over 10% SZ catalyst presaturated with PY and DMPY
IPA see Fig. 5. In conclusion, the results of the acidity determination
formulate that the investigated catalysts have mainly acidic sites of
(B) type, with weak and medium strengths.
It was reported that [37] the activity of solid – acid catalysts
depends on many factors, but the (B) acid site density is usually one
of the most crucial parameters. However, it is known that (B) and (L)
acid sites are capable of retaining pyridine at certain temperatures.
The poisoning of the active acidic sites of 10% SZ catalyst under
investigation was performed through the previous saturation of
the acidic sites with PY and DMPY for 7 days after evacuation. The
unsaturated and saturated catalyst was subjected to the catalytic
reaction of IPA under different reaction temperatures using similar
conditions of the catalytic runs as mentioned above and the results
are shown in Fig. 7. It shows that the saturation of catalyst with PY
or DMPY retards its catalytic activity to higher temperature. This
retardation means that PY and DMPY molecules prevent the (B) acid
sites to react with IPA. According to DSC, TG and DTA results (Fig. 6),
it can be observed that the chemisorbed molecules of PY and DMPY
need energy to be removed from weak and medium strength (B)
3.6. Catalytic activity
It is well known that methanol conversion can carry on a solid-
acid catalyst either as simply dehydration producing only DME and
water or as deep dehydration, yielding hydrocarbons [7]. Previ-
ous researchers have reported that the (B) acid or (L) acid sites are
the active sites for methanol dehydration to DME [38]. It is com-
monly agreed that (B) acid sites of medium strength are required
to obtain high reaction rates without causing relevant deactiva-
tion phenomena [5,39]. The effect of reaction temperature on the
◦
catalytic dehydration of methanol in the range 100– 275 C over
◦
10% SZ catalyst calcined at 450 C was carried out under air and
nitrogen. The percentages of conversion, yield and selectivity were
◦
◦
acid sites in the range of temperature of (50–200 C). So, the results
represented in Fig. 8. It shows that the reaction started at 150 C
presents in Fig. 7 go parallel with that shown in Fig. 6. On increasing
the reaction temperature, the catalyst restores its original catalytic
activity at 275 C. The obtained results indicate that there is a lit-
and the methanol conversion and yield of DME increase monoton-
◦
ically with increasing the reaction temperature up to 230 C. With
◦
◦
increasing the reaction temperature up to 250 C constant values
tle difference between the conversion of the IPA and the yield of
propene over the catalyst saturated with PY or DMPY. This behav-
ior reflects that the catalyst exhibits acidity with almost (B) type.
This is in good agreement with the results that obtained in case of
chemisorption of PY and DMPY during the catalytic dehydration of
of methanol conversion and yield of DME were obtained by using
air or nitrogen as a carrier gas. By raising the reaction temperature
up to 275 C and using air as a carrier gas, the yield of DME drasti-
◦
cally decreased and methanol oxidized to CO and CO products. No
2
◦
oxidation of methanol is observed at 275 C with constant values of
conversion and yield of DME when using nitrogen as a carrier gas.
The above results indicate that the optimum reaction temperature
is 230 C under both air and nitrogen is accompanied with 83% yield
100
◦
and 100% selectivity to DME. However, G. Lauget et al. [40], studied
methanol dehydration into dimethyl ether over ZSM-5 type zeolites
under inert and oxidative atmosphere. They found that using air as
a carrier gas inhibit the methanol-To-Hydrocarbons (MTH) reac-
tion and also prevent the deactivation of MFI zeolite and increase
80
60
40
20
0
Untreated
PY
DMPY
◦
its stability at 275 C.
The effect of catalyst calcination temperature on the catalytic
activity of a 10% SZ toward methanol dehydration was studied on
◦
the catalyst calcined at 300, 350, 400, 450 and 500 C using air as a
carrier gas and the results are cited in Table 2. The experimental
results revealed that a significant increase in methanol conver-
◦
sion as well as DME yield is observed up to calcined at 450 C.
◦
With increasing the calcination temperature to 500 C the cata-
100
125
150
175
200
225
250
275
lyst lost a certain amount of its water. It was proposed earlier
o
Reaction temperature, C
[
41] that for a catalyst to be effective in methanol dehydration,
a minimum amount of water is necessary. In addition, the same
substance within a certain concentration of water limits acted as
Fig. 7. Activity variation of IPA with reaction temperature over 10% SZ catalyst and
saturated with PY and DMPY.

4
6
A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
Table 2
Effect of catalyst calcination temperature on the dehydration of methanol over 10%
SZ catalyst using air as a carrier gas.
100
Calcination
temperature ( C)
Conversion (%)
Selectivity (%)
Yield of
DME (%)
◦
80
Conversion
Yield
3
3
4
4
5
00
50
00
50
00
69
74
80
83
76
100
100
100
100
100
69
74
80
83
76
60
40
20
0
Selectivity
a catalytic promoter. On the other hand, a higher concentration
of water may be poisoned the catalyst. So, the effect of calcina-
tion temperature on the catalysts suggests that the water present
in these catalysts played an important role in their activities in
methanol dehydration. Ki-Won Jun et al. [42] stated that the dehy-
droxylated SiO -Al O becomes rather hydrophobic resulting in
0
10
20
30
50 100 150 200 250 300
2
2
3
Time (min.)
the decrease of sorption capacity of water. They also mentioned
that water blocks the active sites for methanol dehydration through
competitive adsorption with methanol on the catalyst surface. This
is why the catalyst calcined at lower temperature showed lower
activity. Besides, the high calcination temperature may change
the strength of the acidity. Consequently, we can suggest that the
optimum value of water needed to make the catalyst under inves-
tigation exhibits higher activity and selectivity should be calcined
Fig. 10. Effect of reaction time on the catalytic dehydration of methanol over 10%
SZ catalyst calcined at 450 C.
◦
Moreover, such a process enhances the catalyst acidity amount and
retards the particle aggregation, which leading to smaller particle
size and larger surface area as seen in Table 1.
◦
at 450 C.
3.7. Stability of 10% SZ catalyst
The effect of sulfate loadings on dehydration of methanol over
◦
the catalysts calcined at 450 C in presence air as a carrier gas is
To evaluate the stability of the 10% SZ catalyst, the dehydra-
tion of methanol to DME was investigated using air as a carrier gas
and the results are presented in Fig. 10. It shows that the maxi-
mum conversion of methanol (83%) with 100% selectivity to DME
was observed after 15 min from the introduction of the reactant
followed by steady till 300 min. However, for the application of
shown in Fig. 9. The results reveal that pure zirconia has no activity
toward methanol dehydration under our conditions. This behav-
ior may be explained on the basis of the catalyst has no acidity
as observed in the acidity determination see Fig. 4. On the other
hand, sulfated zirconia shows a significant increase in both con-
version and yield toward the formation of DME on increasing the
SO4 reaching a maximum at the addition of 10 wt.%, then grad-
ually decrease up to the addition of 30 wt.%. The maximum yield
of DME (≈83%) is obtained over 10% SZ with selectivity of 100%.
As mentioned earlier [43], sulfating of zirconia induces the struc-
tural stabilization of the tetragonal phase as seen from X-ray results
in Fig. 2, which is known to associate with the catalyst activity.
Srinivasan et al. [44] suggested that oxygen adsorption by oxy-
gen deficient surface sites could be responsible for the transition
of the tetragonal phase into monoclinic phase at low tempera-
tures. They also showed that the presence of sulfate ions would
protect the surface against oxygen adsorption and so the transition
of the tetragonal phase into monoclinic phase would be prevented.
−
2
SO4 /MxOy in various reactions, deactivation is the fatal problem
2
−
[45]. Nowadays, the main mechanism of solid super acid deacti-
−
2
vation can be described as follows: first, the lost of SO4 [46] as
−
2
a result of the presence of water or vapor in contact with SO4
during catalytic reactions, which caused the acid sites number to
decrease and weakened the acid strength, leading the catalyst’s
activity to decline and second, carbon coke [47]. To regenerate the
used catalyst, Li and Gonzalez [48] have determined that calcining
a deactivated conventional sulphated zirconia catalyst in oxygen at
◦
450 C is critical in regaining the original activity. It is of interest to
mention here that under our reaction conditions the sulphated zir-
conia catalysts not undergo any deactivation. This behavior due to
the use of air as a carrier gas which prevent the deactivation of the
catalyst. It was reported that the presence of air allows to maintain
the catalyst surface active, via reducing the deactivation probably
by burning hydrocarbons present in the zeolite micropores [40].
100
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
3.8. Mechanism of methanol dehydration
Conversion
Yield
During the dehydration of alcohol, both intramolecular and
intermolecular dehydration may occur. The relative amounts of
these pathways depend on the reaction conditions as well as the
reactant and catalyst used. The first step of alcohol conversion
on metal oxide catalysts is the adsorption of the reactant on the
surface. The adsorbed alcohols can be converted through a two-
step mechanism involving the intermediate carbocation formation
Selectivity
(E1), a concerted pathway (E2) or via carbanion formation mech-
anism (E1cb) [49]. E1 mechanism needs a strong acidic catalyst to
form carbenium ions by abstraction of OH group. The carbenium
ion is rearranged via isomerization followed by abstraction of a ␤-
hydrogen resulting in the thermodynamically controlled alkenes.
For E2 mechanism, reaction occurs at dual acid-base sites to elimi-
nate a proton and a hydroxyl group. Whereas for E1cb mechanism,
ZrO2
1
3
5
-
10
20
30
2
SO4 ,wt (%)
Fig. 9. Effect of sulfate loadings on dehydration of methanol over catalysts calcined
at 450 C for 3 h using air as a carrier gas.
◦

A.E.-A.A. Said et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 40–47
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3
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7
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