Effect of ZrO2 on the catalytic performance of nano γ-Al2O3 in dehydration of methanol to dimethyl ether at relatively low temperature

Article abstract of DOI:10.1007/s11164-015-2101-7

A series of ZrO₂/γ-Al₂O₃ catalysts were prepared by incipient wetness impregnation of zirconyl nitrate hydrate aqueous solutions on nano-γ-Al₂O₃ with ZrO₂ loadings (1-30 % w/w) and calcined at 450 and 550 °C for 3 h in a static air atmosphere. Physicochemical properties of the catalysts were determined using TG, DTA, XRD, FT-IR, Raman spectroscopy, TEM, and N₂ sorption measurements. The surface acidity of the catalysts was investigated by dehydration of isopropanol and adsorption of pyridine, 2,6-dimethyl pyridine and TPD-pyridine. This series of catalysts was used for vapor-phase dehydration of methanol to dimethyl ether in a fixed bed reactor. It was found that all catalysts in this study were active and selective for DME synthesis. According to the experimental results, 1 % ZA catalyst exhibited the highest activity with selectivity of 100 % toward DME.

Full text of DOI:10.1007/s11164-015-2101-7

Res Chem Intermed
DOI 10.1007/s11164-015-2101-7
Effect of ZrO2 on the catalytic performance of nano
c-Al2O3 in dehydration of methanol to dimethyl ether
at relatively low temperature
Abd El-Aziz A. Said1 Mohamed M. M. Abd El-Wahab1
•
•
Mohamed Abd El-Aal1
Received: 12 January 2015 / Accepted: 9 May 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A series of ZrO2/c-Al2O3 catalysts were prepared by incipient wetness
impregnation of zirconyl nitrate hydrate aqueous solutions on nano-c-Al2O3 with
ZrO2 loadings (1–30 % w/w) and calcined at 450 and 550 °C for 3 h in a static air
atmosphere. Physicochemical properties of the catalysts were determined using TG,
DTA, XRD, FT-IR, Raman spectroscopy, TEM, and N2 sorption measurements.
The surface acidity of the catalysts was investigated by dehydration of isopropanol
and adsorption of pyridine, 2,6-dimethyl pyridine and TPD–pyridine. This series of
catalysts was used for vapor-phase dehydration of methanol to dimethyl ether in a
fixed bed reactor. It was found that all catalysts in this study were active and
selective for DME synthesis. According to the experimental results, 1 % ZA cata-
lyst exhibited the highest activity with selectivity of 100 % toward DME.
Keywords ZrO2/c-Al2O3 Á Methanol Á Dehydration Á DME
Introduction
Alumina is an important material in industrial applications and it has a significant
importance from the technological point of view. Alumina nanopowders are utilized
in many areas of modern industry such as electronics, metallurgy, optoelectronics and
fine ceramic composites [1, 2]. Alumina exists in different phases and each phase
possesses some unique properties that make it most suitable for special cases. On the
other hand, zirconia presents interesting catalytic properties because it is the only
metal oxide that demonstrates four chemical properties, on the surface: acidic, basic,
oxidizing and reducing properties [3]. The acid and base bifunctional properties of
&
Abd El-Aziz A. Said
aasaid55@yahoo.com
1
Chemistry Department, Faculty of Science, Assiut University, Assiut 71516, Egypt
123

A. E.-A. A. Said et al.
zirconia and its mixed oxides have a number of catalytic properties [4]. However,
extensive studies have been carried out on ZrO2–Al2O3 mixed oxides in the
heterogeneous catalysis field. They are widely employed as potential catalysts and/or
supports for NOx reduction, Fischer–Tropsch synthesis, acetylation of alcohols and
amines, catalytic oxidation of hydrocarbons, and isomerization and dehydrogenation
reactions, because the mixed system presents higher catalytic behavior than pure ZrO2
or Al2O3 oxides [5]. With the rapid growth of methanol production from coal-derived
syngas, the research and the development of new processes to transform methanol
into valuable products has attracted extensive attention. Through oxidation and/or
dehydration, methanol can be converted to formaldehyde (FA), dimethyl ether
(DME), dimethoxymethane (DMM), methyl formate (MF), and so on. Among them,
DME is one of the most promising types of diesel fuel since it has a thermal efficiency
equivalent to the traditional diesel fuel, lower NOx emission, fewer carbon
particulates, and near-zero smoke and laser engine noise [6, 7]. DME can be
produced by two methods. The first is considered as the indirect method, in which it is
produced by dehydration of methanol over solid acid catalysts [8, 9], and the second
one is the direct synthesis of DME from synthesis gas (STD) over a hybrid catalyst
comprising a methanol-synthesis catalyst and a solid acid in a single reactor [10, 11].
Commercially, DME is produced by the catalytic dehydration of methanol at around
300 °C at 10 atm pressure over solid catalysts such as c-Al2O3 or zeolites, combined
with different additives [12]. The main problems with the catalysts are coke formation
and coating the surface with carbon, which requires their replacement more frequently
than is desirable. This necessitates a search for new catalysts with high activity and
long periods of exploitation. Undoubtedly, alumina is an excellent candidate for the
methanol dehydration process as provided by many reports. However, its unsolved
drawbacks are also obvious, such as low hydrothermal activity, many side reactions,
competitive adsorption of steam resulting from the strong surface hydrophilic ability,
etc. [13]. Recently, numerous researchers have performed a series of effective works
to develop an active, selective and stable alumina-based catalyst. Modification with
another oxide is considered as one of the most powerful approaches to improve the
performance of c-Al2O3 [13]. However, the effect of ZrO2 on the catalytic activity
and selectivity of c-Al2O3 in the dehydration of methanol to DME has to our best
knowledge not yet been investigated. Therefore, the aim of the present work is to
study the effect of ZrO2 contents upon the catalytic performance of nano c-Al2O3
catalyst for methanol dehydration to DME at relatively low temperature (230 °C)
under inert and oxidative atmosphere. In addition, we have investigated the
correlation between the activity and the acid properties of the prepared catalysts.
Experimental
Catalyst preparation
Aluminum nitrate nonahydrate Al(NO3)3Á9H2O (Oxford assay 98.5 %), zirconyl
nitrate hydrate ZrO(NO3)2ÁxH2O (Aldrich), ammonia solution (NH4OH 23 %,),
methyl alcohol, isopropyl alcohol, pyridine and 2,6-dimethyl pyridine were obtained
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Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
as pure reagents and were used without further purification. Calculated amount of
aluminum nitrate nonahydrate with concentration of 0.5 M was dissolved in
distilled water at 60 °C with continuous stirring by a magnetic stirrer then drop-wise
addition of ammonia solution until the pH equalled 7. After that, the precipitated
solution was filtered and washed by distilled water to remove nitrate ions and excess
ammonia. The precipitate was dried in an oven at 100 °C for 24 h and calcined at
500 °C in a static air atmosphere for 3 h. The samples of ZrO2 supported on c-
Al2O3 were prepared by the impregnation method. Calculated amounts of zirconyl
nitrate hydrate were dissolved in small amounts of distilled water. The zirconyl
nitrate hydrate solutions were admixed carefully with calculated amounts of nano-c-
alumina until the formation of homogeneous pastes. The mixtures were dried in an
oven at 100 °C for 24 h, when the contents of ZrO2 were 1–30 wt%. All the
supported samples were calcined at 450 and 550 °C for 3 h in a static air
atmosphere. This series of c-Al2O3 samples with 1–30 wt% ZrO2 loading, were
named (1–30) % ZA.
Catalyst characterization
Thermal analysis
Thermogravimetry (TG) and differential thermal analysis (DTA) were performed on
heating (at 10 °C min-1) test samples to 800 °C in a dynamic atmosphere of air
(30 mL min-1) using a computerized Shimadzu Thermal Analyzer TA6O Appa-
ratus (Japan).
X-ray diffraction (XRD)
XRD analysis of the test samples was performed with a Philips (Netherlands)
diffractometer (Model PW 2103, k = 1.5418 A°, 35 kV and 20 mA) with a source
of CuKa radiation (Ni filtered). An online data acquisition and handling system
facilitated the automatic JCPDS library search and match for phase identification
purposes. Patterns were recorded from 4° to 80° (2h). Particle size was estimated
using the Scherrer equation [14]
Kk
D ¼
bCosh
where D is the mean crystallite diameter (nm), k is the X-ray wavelength, K is the
Scherrer constant (0.89), and b is the observed angular width at half maximum
intensity of the peak.
Fourier transform infrared (FT-IR) spectroscopy
FT-IR spectra of the prepared catalysts calcined at 450 and 550 °C for 3 h were
recorded using a Nicolet spectrophotometer (model 6700), equipped with a data
station in the range of 4000–400 cm-1 with a KBr disc technique.
123

A. E.-A. A. Said et al.
Raman spectroscopy
Raman spectra of the prepared catalysts calcined at 450 °C were obtained using
Raman spectroscopy model SENTERRA Inverted Configuration (Germany) under
ambient conditions, e.g. 25 °C. A 785 nm laser was used as the exciting source on
the sample surface with a power of 50 mW.
Transmission electron microscopy (TEM)
The size and morphology of the investigated catalysts were characterized by
transmission electron microscope (TEM) JEOL Model JSM-5400 LV (Joel, Tokyo,
Japan). The catalyst powder dispersed 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.
Nitrogen gas sorption
Nitrogen gas adsorption–desorption isotherms were measured at -196 °C using a
Nova 3200 instrument (Quantachrom Instrument, USA). Test samples were
thoroughly outgassed for 3 h at 250 °C to a residual pressure of 10-5 Torr, and
the weight of the degassed 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 catalysts was determined from the desorption curves
using Nova enhanced data reduction software (v.2.13).
Acidity determination
The acidity of the catalysts under investigation was determined by studying the
dehydration of isopropyl alcohol (IPA) and the adsorption of pyridine (PY), 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: 500 mg catalyst weight, 2 % IPA in the gas
feed, 50 mL min-1 total flow rate and 180 °C reaction temperature. The measurement
of propene yield (%) 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 acidity populations over
the surface of catalysts, under investigation, were also measured by thermogravimetric
technique (TG) using the adsorption of pyridine as probe molecule. The following
procedure was used: 500 mg of calcined sample was preheated at 250 °C for 1 h in air
before being saturated with pyridine for 7 days after evacuation, then 10–15 mg of
pyridine-saturated sample was subjected to TG and DTA analysis. The analysis was
recorded by heating from room temperature to 400 °C at 10 °C min-1 and
30 mL min-1 flow of N2. The mass loss due to desorption of pyridine from the acidic
sites was determined as a function of total surface acidity [15].
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Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
Catalytic activity measurements
The catalytic activity of the catalysts for the vapor-phase dehydration of methyl
alcohol was carried out at 230 °C in a conventional fixed bed flow type reactor at
atmospheric pressure using air or nitrogen as a carrier gas. The measurement
procedure was similar to that reported previously [16]. Each catalytic run was
conducted using 500 mg of the powdered catalyst and a total flow rate was fixed at
50 mL min-1 and 4 % methanol in the gas feed. The gases after reaction 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 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 conditions).
Results and discussion
Thermal analysis
TG and DTA curves for the prepared c-Al2O3 calcined at 500 °C and 10, 20 and
30 % ZA catalyst original precursors are represented in Fig. 1. The TG and DTA
curves for all samples exhibit two weight loss steps. The first one is in the range of
100
90
80
30% ZA
70
60
100
90
80
20% ZA
70
60
100
90
10% ZA
80
70
100
95
γ-Al2O3
90
85
0
200
400
600
800
1000
Temperature (oC)
Fig. 1 TG and DTA curves for the prepared c-Al2O3 calcined at 500 °C, for 10, 20 and 30 % ZA
original precursors
123

A. E.-A. A. Said et al.
30–200 °C with a weight loss of 10, 11.6, 16.1 and 16.8 %, respectively. The
samples also exhibit endothermic peaks appear on the DTA profiles minimized at
89, 99, 103 and 111 °C for c-Al2O3, for 10, 20 and 30 % ZA catalysts, respectively.
These peaks attributed to the loss of physically adsorbed and crystalline water
molecules. The second step occurs in the range of 200–800 °C with weight loss of
5.1, 6.9, 12.3 and 17.9 for c-Al2O3, for 10, 20 and 30 % ZA catalysts, respectively.
This step was not accompanied by any peak in the DTA curves of c-Al2O3 and 10 %
ZA catalyst, while 20 and 30 % ZA catalysts exhibited exothermic peaks
maximized at 251 and 241 °C for 20 and 30 % ZA, respectively. These peaks
may be attributed to a certain change in the structure of ZrO2 (dehydroxylation
process) [17].
X-ray diffraction (XRD)
XRD patterns determined for the various alumina supported ZrO2 catalysts with 10,
20 and 30 wt% loadings and calcined at 450 °C are illustrated in Fig. 2. Matching
the XRD patterns of pure c-Al2O3 calcined at 500 °C with that identified in the
(JCPDS NO. 290063) results show that the predominant diffraction peaks
correspond to c-Al2O3 are at 2h = 37.4°, 46° and 66.5°. The XRD patterns of
T
M
M
T
T
ZrO2
T
T
M
T
γ
γ
γ
30% ZA
20% ZA
γ
γ
γ
γ
10% ZA
γ
γ
Pure Al2O3
10
20
30
40
50
60
70
80
2
θ
Fig. 2 X-ray diffraction of pure c-Al2O3 calcined at 500 °C, pure ZrO2 and 10, 20 and 30 % ZA
calcined at 450 °C
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
pure zirconia calcined at 450° exhibit both tetragonal (2h = 30.3°, 35.3°, 50.7°,
59.9°, 60.6° and 63.5° [18]) and monoclinic (2h = 24.7°, 28.4°, 31.6° [19]) phases.
However, the XRD patterns of 10, 20 and 30 % ZA calcined at 450° showed
reflections typical of the c-Al2O3 phase. The intensity of the diffraction peaks of the
c-Al2O3 decrease with increasing zirconia contents. The samples also showed a
broadened peak at 2h & 30°, and the intensity of the peak increased with increasing
zirconia contents. The broadened peak was attributed to the tetragonal phase
zirconia [20], but it was also considered as an appearance of amorphous zirconia
[21]. The diffraction peaks at 2h values of 50.7° and 66.46° were selected for
calculating the crystalline size for pure ZrO2 and pure c-Al2O3 phases, respectively.
The crystalline sizes for pure ZrO2 calcined at 450 °C and pure c-Al2O3 calcined at
500 are cited in Table 1. The results obtained reveal that the addition of ZrO2 to c-
Al2O3 leads to increase in the crystallite size of c-Al2O3 phase.
Fourier transform infrared (FTIR) spectroscopy
The FTIR spectra obtained for pure nano c-Al2O3 calcined at 500 °C, pure ZrO2 and
the different loadings of ZrO2 on nano c-Al2O3 calcined at 450 °C are shown in
Fig. 3. The FTIR spectrum of nano-c-Al2O3 calcined at 500 °C shows a strong and
broad band at 3500 cm-1 which is believed to be associated with the stretching
vibrations of hydrogen-bonded surface water molecules and hydroxyl groups.
Additionally, the bands at 1633, 1400 and 1384 cm-1 correspond to the existence of
a large number of residual hydroxyl groups, which implies that the O–H vibration
mode of the traces adsorbed water. The band located at 610–883 cm-1 can be
ascribed to the Al–O vibration of Al2O3. These results are in good agreement with
Table 1 The specific surface areas, pore characteristics and crystalline sizes of the ZrO2–Al2O3 samples
calcined at 450 and 550 °C
Calcination
temp.
% ZrO2
450 °C
550 °C
SBET
m2 g-1
Total pore
volume
Average
pore
DXRD
(nm)a
SBET
m2 g-1
Total pore
volume
Average
pore
(cc g-1
)
diameter
(nm)
(cc g-1
)
diameter
(nm)
Pure Al2O3
222
244
241
229
221
235
230
198
86.2
0.28485
0.31863
0.30794
0.29912
0.28888
0.29697
0.28697
0.24340
0.13486
5.1
5.2
5.1
5.2
5.2
5.0
5.0
4.9
6.2
8.0
–
202
204
203
199
200
203
180
168
49.2
0.29376
0.29747
0.29271
0.29763
0.28726
0.29140
0.24780
0.21734
0.14552
5.8
5.8
5.7
6.0
5.7
5.7
5.5
5.1
11.8
1
3
–
5
–
7
–
10
9.00
10.0
10.5
11.0
20
30
Pure ZrO2
a
Crystalline size: determined by XRD results
123

A. E.-A. A. Said et al.
the results obtained by Zhang et al. [22]. The FTIR spectrum of pure ZrO2 shows
that bands assigned at 490–745 cm-1 correspond to the O–Zr bond [23]. A strong
and broad band assigned at 3600–3000 cm-1 may correspond to physisorbed water,
whereas the band at 1633 cm-1 was assigned to the bending mode (tHOH) of
coordinated water [24]. In addition, the band at 1362 cm-1 was assigned to the
bending vibration of Zr–OH groups [25]. The FTIR spectra of the different loadings
of ZrO2 on nano-c-Al2O3 calcined at 450 °C indicate that the positions of all peaks
showed no apparent change. However, the intensities of the peaks were decreased. It
should be noted that the peaks of Zr–O do not exist. This is possibly due to the
stronger intensity of Al–O vibration than that of Zr–O vibration, thereby the peak of
Zr–O vibration is masked [26], which is due to the dispersion of Zr4? ions within
the alumina lattice, substituting Al3? ions. Finally, there is a small band present at
2170 cm-1, which has been assigned to the Zr–O–Al bond vibration [5, 27]. This
band can also be observed in pure c-Al2O3 sample. Therefore, it should correspond
to the M–O–M0 band vibration, where M and M0 may be the same element or
ZrO2
1362
1633
745
490
3500
2170
30% ZA
2170
1% ZA
2170
γ
-Al2O3
1435
1384
883
610
1633
2170
2200
3500
2300
2100
2000
Wavenumbers (cm-1
)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm-1)
Fig. 3 FTIR spectra of pure c-Al2O3 calcined at 500 °C, pure ZrO2, for 1 and 30 % ZA catalysts
calcined at 450 °C
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
different elements, such as Al–O–Al or Zr–O–Zr. In addition, the FTIR spectra of
the samples calcined at 550 °C (not shown) show the same bands as in the samples
calcined at 450 °C but with a slight decrease in their intensities.
Raman spectroscopy
The Raman spectra of pure c-Al2O3 calcined at 500 °C, for 1 and 30 % ZA catalysts
calcined at 450 °C, are presented in Fig. 4. The laser Raman spectra of c-Al2O3
calcined at 500 °C does not exhibit any Raman peaks [28], but the addition of 1 %
ZrO2 produced a peak at about 970 cm-1. This peak may be attributed to vibrational
modes involving Zr–O–Al linkages. The intensity of this peak increased on
increasing the percentages loading up to 30 % ZA. The results are in a good
agreement with those obtained by Gao et al. [29].
Fig. 4 Raman spectra of pure c-Al2O3 calcined at 500 °C, for 1 and 30 % ZA catalysts calcined at
450 °C
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A. E.-A. A. Said et al.
Transmission electron microscopy (TEM)
To obtain more information about the morphological features of synthesized c-
Al2O3, 1 and 30 % ZA catalysts, TEM studies were undertaken as shown in Fig. 5.
A TEM image of c-Al2O3 calcined at 500 °C shows the presence of small
aggregates with average particle size of about 8.4–14 nm. The average particle size
calculated from the TEM micrographs is consistent with the average crystalline size
obtained from XRD measurement as indicated in Table 1. On the other hand, a
TEM image of 1 and 30 % ZA catalysts calcined at 450 °C shows more fine
aggregates.
Sorption measurements
Adsorption–desorption isotherms of pure Al2O3 calcined at 500 °C, pure ZrO2 and
ZrO2 supported on c-Al2O3 catalysts calcined at 450 °C are represented in Fig. 6a.
All isotherms of pure and mixed calcined samples resulted in type IV of Brunauer’s
classification [30]. The type IV isotherm is characteristic of mesoporous materials,
in which capillary condensation and the multilayer adsorption (for higher pressure)
Fig. 5 TEM images of a c-Al2O3 calcined at 500 °C, b 1 % ZA and 30 % ZA catalysts calcined at
450 °C
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
take place. The hysteresis loop belongs to type H2 of the de Bore classification [31].
The surface areas (SBET), the pore volumes and the average pore diameters of the
catalysts are summarized in Table 1. It is clear from Table 1 that pure c-Al2O3
calcined at 500 and 550 °C has SBET values of 222 and 202 m2 g-1, respectively,
while pure zirconia calcined at 450 and 550 °C has SBET values of 86.2 and
49.2 m2 g-1, respectively. The results also show that, on addition of 1 % ZrO2 into
c-Al2O3, the surface area increased to 244 and 204 m2 g-1 for the samples calcined
(a)
200
γ
-Al2O3
1% ZA
3% ZA
ZrO2
150
100
50
0
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
2
1
0
(b)
ZrO2
6
3
30%ZA
1%ZA
0
12
6
0
10
5
0
γ
-Al2O3
2
4
6
8
10
12
14
16
Pore diameter, (nm)
Fig. 6 a Nitrogen adsorption–desorption isotherms and b the pore size distribution of c-Al2O3 calcined
at 500 °C, pure ZrO2, for 1 and 30 % ZA catalysts calcined at 450 °C
123

A. E.-A. A. Said et al.
at 450 and 550 °C, respectively, which might be due to the dispersion of the
zirconia phase into the alumina phase, leading to structural stabilization [32].
Further addition of ZrO2 leads to a continuous decrease in the SBET values.
However, the 10 % ZA sample provided a higher SBET value than the 5 and 7 %
ZA. Possibly, this was due to the different particle size of ZrO2 in the pores of the
alumina [17]. On the other hand, it is observed that the SBET of the prepared
catalysts decreased with increasing the calcination temperature from 450 to 550 °C.
This behavior may be attributed to the sintering process [33]. The pore size
distribution curves of c-Al2O3, ZrO2 for 1 and 30 % ZA are shown in Fig. 6b. It
shows that the pores of c-Al2O3 have a narrow pore distribution in the range of
4–7.5 nm, while the pores of ZrO2 exhibit a minor contribution for mesopores with
an average pore diameter of 6 nm and a major contribution for narrow pores of
diameter 1.9 nm. In addition, the pore size distribution of 1 % ZA shows a
monomodal with wider pore distribution slightly more than pure c-Al2O3. Further
increasing the contents of ZrO2, the samples exhibit bimodal size distribution as
observed in 30 % ZA.
Determination of the surface acidic sites
Dehydration of isopropyl alcohol (IPA)
The catalytic dehydration of IPA over pure c-Al2O3 calcined at 500 and 550 °C and
the different loadings of ZrO2 on c-Al2O3 catalysts calcined at 450 and 550 °C are
shown in Fig. 7. IPA dehydration has been used by several authors [34], as a test
reaction to determine the acidity of different catalysts and it proceeds quickly on
weak acid sites [35]. Under our working conditions, it was found that IPA reacts on
c-Al2O3 and selectively on the different loadings catalysts to propene. The results
indicate that pure c-Al2O3 calcined at 500 °C exhibits conversion and yield of
propene &70 %. Moreover, in the samples calcined at 450 °C, the addition of
95
90
85
80
75
70
65
450 oC
550 oC
γ
1
3
5
7
10
20
30
-Al2O3
% Loading of ZrO2
Fig. 7 Activity variation of IPA with different loadings of ZrO2 on c-Al2O3 calcined at 450 and 550 °C
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
1 wt% ZrO2 leads an observable increase in the yield of propene by &90.6 %
(maximum yield). Above this ratio, further addition steadily decreases the yield of
propene reaches &81.4 % over 30 % ZA catalyst. The decrease in the activity of
the catalysts in high zirconia contents may be related to the fact that pure zirconia is
an inactive catalyst toward IPA dehydration. The catalysts calcined at 550 °C
exhibit the same behavior as the catalysts calcined at 450 °C but exhibit lower
activity towards propene formation, due to the decrease in the surface area (see
Table 1). The above results show that the catalyst containing 1 wt%. ZrO2 has
sufficient acid sites to proceed the dehydration pathway for IPA better than that of
the c-Al2O3 catalyst. The difference between the performances of 1 % ZA and c-
Al2O3 catalysts is explained on the basis of ZrO2 interacting with c-Al2O3 via
creation of more acidic sites, which is responsible for the dehydration reaction.
Poisoning of acid sites with pyridine and 2,6-dimethyl pyridine over c-Al2O3
and 1 % ZA catalysts during the reaction of IPA
It is known that the chemisorptions of pyridine (PY) and 2,6-dimethyl pyridine
(DMPY) can be used as basic probe molecules to determine the acidity of the
catalyst [36]. It has been reported that PY is selectively adsorbed on both Brønsted
(B) and Lewis (L) acid sites [37]. On the other hand, DMPY is selectively adsorbed
on Brønsted acid sites [38] but not Lewis sites because of the steric hinderance of
two methyl groups. So the difference between PY and DMPY adsorption is a
measure of the Lewis acid sites. The poisoning of the active surface sites of c-Al2O3
and 1 % ZA catalysts in IPA conversion was performed through saturation of the
acid sites with PY or DMPY according to the following procedure. After measuring
the conversion activity of c-Al2O3 and 1 % ZA catalysts at 180 °C, the catalysts
were injected with different volumes of PY or DMPY in the stream of reactants
using N2 as a carrier gas. We selected 1 % ZA catalyst for the poisoning test
because it is the most active and selective catalyst in both IPA and methanol
dehydration reaction. The results obtained are shown in Fig. 8a. It shows the
influence of PY and DMPY additions on the reaction products of IPA over c-Al2O3
calcined at 500 °C. The results indicate that PY and DMPY suppressed IPA
dehydration activity. Thus, the chemisorbed PY and DMPY decreased the yield of
the propene by &45 and 7 %, respectively. It is clear from the above results that the
difference between the amounts adsorbed from PY and DMPY equal 38 %. This
value corresponds to the (L) acid sites present on the surface of the c-Al2O3 catalyst.
Thus, the c-Al2O3 catalyst is an acidic catalyst with major (L) and minor (B) acid
sites. Fig 8a also shows the effect of PY and DMPY additions on the reaction
products of IPA over the 1 % ZA catalyst calcined at 450 °C. The results revealed
that PY and DMPY retard IPA dehydration activity. Thus, the chemisorbed PY and
DMPY decrease the yield of the propene by 56 and 12 %, respectively. The
difference between the amounts adsorbed from PY and DMPY is &44 %. This
value corresponds to the presence of the (L) acid sites. Thus, the 1 % ZA catalyst is
an acidic catalyst with major (L) and minor (B) acid sites. Inspection of the above
results illustrate that the number of acidic sites and its distribution present on the
surface of 1 % ZA catalyst is greater than that on the surface of c-Al2O3 catalyst.
123

A. E.-A. A. Said et al.
Thermal analysis of c-alumina and 1 % ZA catalysts presaturated with pyridine
To identify the strength of acid sites available on the surface of the catalysts under
investigation, thermal analysis of presaturated samples with PY was carried out.
Figure 9a depicts the TG and DTA curves of the pyridine removed from c-Al2O3
catalyst calcined at 500 °C. The TG curve exhibits a total weight loss of about
19.2 % through two steps. The first step lies in the temperature range of 25–120 °C
with a weight loss of about 12.3 %. This step is associated with a strong
endothermic peak on the DTA profile minimized at 71 °C. This peak may be
attributed to the removal of physically adsorbed water and desorption of PY
molecules from weak acid sites. The second step lies in the temperature above
120 °C with a weight loss of about 6.9 %. This step is not accompanied by any peak
in the DTA curve. This behavior may indicate that the removal of PY from
intermediate acid sites occurs without gain or release energy. On the other hand,
Fig. 9b also shows the TG and DTA curves of the desorption of pyridine from 1 %
ZA catalyst calcined at 450 °C. The TG curve exhibits a total weight loss of about
22.7 % within two steps. The first step occurs in the temperature range of
25–120 °C with a weight loss of about 16.4 %. This step is accompanied by a strong
endothermic peak on the DTA curve minimized at 73 °C. This step may also be
attributed to the removal of physically adsorbed water and desorption of PY from
weak acid sites. The second step lies in the temperature above 120 °C with a weight
loss of about 6.3 %. This step is not associated with any peak in the DTA profile.
The above results demonstrate that the c-Al2O3 and 1 % ZA catalysts contain weak
and intermediate acid sites, and the percent of strong acid sites is very small. The
100
Unsaturated
PY
DMPY
80
60
40
20
0
γ
-Al2O3
1% ZA
Fig. 8 Activity variation of IPA with the volume of PY and DMPY over pure c-Al2O3 calcined at
500 °C and 1 % ZA catalyst calcined at 450 °C (500 mg catalyst, 2 % IPA, 50 mL min-1 total flow rate,
180 °C reaction temperature)
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
100
(b)
1% ZA
90
80
100
90
(a)
γ
-Al2O3
80
50
100
150
200
250
300
350
400
Temperature (oC)
Fig. 9 TG and DTA curves of PY desorption from (a) c-Al2O3 and (b) 1 % ZA catalysts
acid site density of c-Al2O3 and 1 % ZA catalysts were calculated and found to be
6.56 9 1018 and 7.08 9 1018 sites m-2, respectively. This indicates that the acid
site density (number of acid sites) of 1 % ZA catalyst is more than that of c-Al2O3.
The above results are in good agreement with the results obtained and presented in
Fig. 8.
Catalytic dehydration of methanol to DME
The catalytic dehydration of methanol over c-Al2O3 (calcined at 500 and 550 °C)
and the different loadings of ZrO2 on c-Al2O3 catalysts calcined at 450 and 550 °C
using air or nitrogen as a carrier gas are presented in Fig. 10. However, we
confirmed previously that using air or nitrogen as a carrier gas gives the same yield
of DME with selectivity to DME 100 % over sulfated zirconia samples [16]. The
results revealed that pure c-Al2O3 calcined at 500 and 550 °C exhibits 81 and 79 %
conversion of methanol, respectively, with selectivity of 100 % toward DME. On
the addition of 1 wt% ZrO2, a noticeable increase in conversion and yield of DME
reaches 87 and 85 % in the catalyst calcined at 450 and 550 °C, respectively. In the
catalysts calcined at 450 °C, with a further increase of the addition of ZrO2, a
continuous decrease in the conversion and the yield of DME reaches 84.7 % over
30 % ZA catalyst. While, in the catalysts calcined at 550 °C, a further increase in
the contents of ZrO2 decreases the conversion and the yield of DME to &84 % over
123

A. E.-A. A. Said et al.
5 % ZA catalyst, then a steady state in the conversion and the yield of DME was
observed with increasing the addition up to 30 wt% ZrO2. The formation of DME as
the only product indicates that the active sites are mainly Lewis acid pair sites,
Al3?–O2- [39]. The methanol interacts with a Lewis acid site, Al3?, through its
oxygen atom, resulting in the dissociative adsorption forming an Al–O–CH3
intermediate and a hydrogen atom adsorbed on a surface oxygen ion. DME then
forms as a result of condensation of two neighboring surface methoxy groups [40].
Furthermore, the degree of conversion of methanol increases as the surface acidity
increases [41]. Therefore, our results indicate an enhancement in the (L) acidity of
1 % ZA catalyst as compared with the pure c-Al2O3. The enhanced acidity of the
surface could be referred to the possible formation of Zr–O–Al bonds. Such
heterolinkage has been suggested in several studies to enhance the acidity in various
metal oxide systems [42]. The lower activity of the catalysts with high ZrO2
contents may be due to the decrease in the acidity of these catalysts as observed in
the acidity test (see Fig. 7). So, the highest catalytic activity of 1 % ZA catalyst may
be attributed to the increased number of acid sites, as concluded from Fig. 8, and a
high surface area.
Consequently, it is important to study the effects of some parameters on the
activity of the most active sample (1 % ZA catalyst) in dehydration of methanol to
optimize the best conditions for DME production.
Effect of catalytic reaction temperature
The effect of reaction temperature on the catalytic dehydration of methanol over
1 % ZA catalyst calcined at 450 °C was carried out using air or nitrogen as a carrier
gas in the temperature range of 100–275 °C. The percentages of conversion, yield
and selectivity are shown in Fig. 11, which shows that the reaction starts at 125 °C
88
450 o
550 o
C
C
86
84
82
80
78
γ
-Al2O3
1
3
5
7
10
20
30
% Loading of ZrO2
Fig. 10 Variation of methanol conversion and DME yield over c-Al2O3 calcined at 500 and 550 °C and
the different loadings of ZrO2 on c-Al2O3 catalysts calcined at 450 and 550 °C using air or nitrogen as a
carrier gas at 230 °C reaction temperature
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
100
80
60
40
20
0
Conversion
S. DME
100
125
150
175
200
225
250
Reaction temperature (oC)
Fig. 11 Variation of methanol conversion and DME yield with the reaction temperature over 1 % ZA
catalyst calcined at 450 °C using air or nitrogen as a carrier gas
100
80
% Conversion
S. DME
60
40
20
0
0
5
10 15 20 25 30 35
Time (min.)
1
2
3
4
5
6
Time (day)
Fig. 12 Variation of methanol conversion and DME yield with the reaction time over 1 % ZA catalyst
calcined at 450 °C (at 230 °C reaction temperature)
and the methanol conversion and the yield of DME increase monotonically with
increasing the reaction temperature up to 230 °C. With increasing the reaction
temperature up to 250 °C, constant values of methanol conversion and yield of
DME were obtained. Unfortunately, when the reaction temperature increased up to
275 °C in the presence of air as a carrier gas, the yield of DME drastically decreased
due to oxidation of methanol to CO and CO2 products. No oxidation of methanol
was observed at 275 °C with a constant value of conversion and yield of DME in the
presence of N2 as a carrier gas. The above results indicate that the optimum reaction
temperature for conversion of methanol to DME with 87 % yield and 100 %
selectivity is at 230 °C under air or nitrogen as a carrier gas. Said et al. [16] studied
123

A. E.-A. A. Said et al.
90
85
80
75
70
65
60
55
1% ZA
γ-Al2O3
2
4
6
8
10
12
14
16
18
20
Reactant (%)
Fig. 13 Variation of methanol conversion and DME yield with % reactant of methanol over c-Al2O3
calcined at 500 °C and 1 % ZA catalyst calcined at 450 °C (at 230 °C reaction temperature)
the catalytic performance of sulfated zirconia in dehydration of methanol to DME
using air as a carrier gas. They stated that using air as a carrier gas prevented the
deactivation of the sulfated zirconia catalyst.
Catalyst stability test
Because dehydration catalysts cannot be regenerated in situ, keeping high stability
during a long-term reaction is very important [43]. The stability of 1 % ZA catalyst
calcined at 450 °C was evaluated under the operating conditions of 230 °C, 1 atm,
500 mg catalyst and 50 ml flow rate for 6 days of time-on-stream using air or
nitrogen as a carrier gas. Figure 12 shows that the dehydration reaction of methanol
is very fast and the conversion of methanol reaches & 82 % after 5 min. The
reaction reaches a steady state of conversion after 30 min. Figure 12 also shows that
the methanol conversion and selectivity to DME remained constant for 6 days,
which indicates that no deactivation of the catalyst was occurring. This result
reflects that the ZrO2-modified c-Al2O3 has good stability for the synthesis of DME
from methanol under inert and oxidative atmospheres.
Effect of % reactant
The effect of % methanol reactant on the activity variation of c-Al2O3 calcined at
500 °C and 1 % ZA catalysts calcined at 450 °C were studied and the results are
represented in Fig. 13, which shows that the higher activity towards DME formation
is achieved at 4 % methanol over c-Al2O3 and 1 % ZA catalysts. The increase in
value of % reactant led to an observable decrease in % conversion of methanol and
consequently in the yield of DME. The figure also shows that, at a relatively high
value of % reactant, the c-Al2O3 and 1 % ZA catalysts lost 26 and 11 % of their
conversion of methanol, respectively. This means that the decrease in the
123

Effect of ZrO2 on the catalytic performance of nano c-Al2O3 in…
conversion of methanol over c-Al2O3 is more than that of 1 % ZA catalyst. These
results again demonstrate that the 1 % ZA catalyst has excellent strength and
stability of the active sites compared with that of pure nano-c-Al2O3 catalyst.
Conclusions
Structural data and catalytic activity measurements of ZrO2–c–Al2O3 systems
provide consistent information about important changes taking place in c-Al2O3
surface properties after the incorporation of ZrO2. Thus, from the results described
above, it can be concluded that:
1. ZrO2–c–Al2O3 systems are active and selective catalysts for the dehydration of
methanol to DME at relatively low temperature (230 °C).
2. The maximum yield of DME (&87 %) was obtained over 1 % ZA catalyst.
This catalyst possessed a greater number of (L) acid sites, which are responsible
for DME formation.
3. The ZrO2-modified c-Al2O3 has good stability for the synthesis of DME from
methanol under inert and oxidative atmospheres.
References
1. S.A. Hassanzadeh-Tabrizi, E. Taheri-Nassaj, J. Am. Ceram. Soc. 91, 3546 (2008)
2. A. Krell, H. Ma, J. Am. Ceram. Soc. 86, 241 (2003)
3. K. Tanabe, Mater. Chem. Phys. 13, 347 (1985)
4. K. Tanabe, T. Yamaguchi, Catal. Today 20, 185 (1994)
´
5. J. Ortiz-Landeros, M.E. Contreras-Garcıa, H. Pfeiffer, J. Porous Mater. 16, 473 (2009)
6. R.H. Williams, E.D. Larson, Energy. Sustain. Dev. 7, 103 (2003)
7. T.A. Semelsberger, R.L. Borup, H.L. Greene, J. Power Sources 156, 497 (2006)
8. G.R. Moradi, F. Yaripour, P. Vale-Sheyda, Fuel Process. Technol. 91, 461 (2010)
9. D. Varisli, K.C. Tokay, A. Ciftci, T. Dogu, G. Dogu, Turk. J. Chem. 33, 355 (2009)
10. J.H. Kim, M.J. Park, S.J. Kim, Appl. Catal. A Gen. 264, 37 (2004)
´
´
11. A. Garcıa-Trenco, A. Martınez, Appl. Catal. A: Gen. 411–412, 170 (2012)
12. K.-W. Jun, H.S. Lee, H.-S. Roh, S.-E. Park, Bull. Korean Chem. Soc. 24, 106 (2003)
13. J. Sun, G. Yang, Y. Yoneyama, N. Tsubaki, ACS Catal. 4, 3346 (2014)
14. B.D. Cullity, Elements of X-ray Diffraction, 3rd edn. (Addison-Wesley, Reading, 1967)
15. A.I. Osman, J.K. Abu-Dahrieh, D.W. Rooney, S.A. Halawy, M.A. Mohamed, A. Abdelkader, Appl.
Catal. B Environ. 127, 307 (2012)
16. A.A. Said, M.M. Abd El-Wahab, M. Abd El-Aal, J. Mol. Catal. A: Chem. 394, 40 (2014)
17. G. Li, W. Li, M. Zhang, K. Tao, Catal. Today 93–95, 595 (2004)
´
18. E.A. Garcıa, E.H. Rueda, A.J. Rouco, Appl. Catal. A: Gen. 210, 363 (2001)
19. T. Yamamoto, T. Tanaka, S. Takenaka, S. Yoshida, T. Onari, Y. Takahashi, T. Kosaka, S. Hasegawa,
M. Kudo, J. Phys. Chem. B 103(13), 2385 (1999)
20. T. Klimova, M.L. Rojas, P. Castillo, R. Contreras, J. Ramırez, Microporous Mesoporous Mater. 20,
293 (1998)
´
21. S. Damyanova, P. Grange, B. Delmon, J. Catal. 168, 421 (1997)
22. L. Zhang, N. Li, P. Fan, X. Chu, S. An, J. Zhang, X. Wang, Hydrometallurgy 127–128, 8 (2012)
23. F. Heshmatpour, R.B. Aghakhanpour, Adv. Powder Technol. 23, 80 (2012)
24. A. Sinhamahapatra, N. Sutradhar, M. Ghosh, H.C. Bajaj, A.B. Panda, Appl. Catal. A Gen. 402, 87
(2011)
25. X. Dou, D. Mohan, U. Charles, J. Pittman, S. Yang, Chem. Eng. J. 198–199, 236 (2012)
123

A. E.-A. A. Said et al.
26. C. Li, Y.-W. Chen, T.-M. Yen, J. Sol-Gel. Sci. Technol. 4, 205 (1995)
`
´ ´
27. M. Moran-Pineda, S. Castillo, T. Lopez, R. Gomez, Cordero-Borboa, O. Navaro, Appl. Catal.
B Environ. 21, 79 (1999)
28. S.S. Chan, I.E. Wachs, L.L. Murrell, N.C. Dispenziere, JR. J. Catal. 92, 1 (1985)
29. X. Gao, J.L. Fierro, I.E. Wachs, Langmuir 15, 3169 (1999)
30. S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Am. Chem. Soc. 62, 1723 (1940)
31. J.H. de Boer, E.P. Everette, F.S. Stone (eds.), The Structure and Properties of Porous Materials
(Butterworths, London, 1958)
32. J.M. Dominguez, J.L. Hernandez, G. Sandoval, Appl. Catal. A Gen. 197, 119 (2000)
33. S. Da Ros, E. Barbosa-Coutinho, M. Schwaab, V. Calsavara, N.R.C. Fernandes-Machado, Mater.
Charact. 80, 50 (2013)
34. J.R. Sohn, H.J. Jang, J. Mol. Catal. 64, 349 (1991)
35. A.A. Said, J. Chem. Technol. Biotechnol. 78, 733 (2003)
36. A.A. Said, M.M.M. Abd El-Wahab, A.M. Alian, J. Chem. Technol. Biotechnol. 82, 513 (2007)
37. M. Ziolek, J. Kujawa, O. Saur, A. Aboulayt, J.C. Lavalley, J. Mol. Catal. A: Chem. 112, 125 (1996)
38. F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, M.C. Moreno, A.A. Romero, Appl.
Catal. A: Gen. 170, 159 (1998)
39. F.S. Ramos, A.M. Duarte de Farias, L.E.P. Borges, J.L. Monteiro, M.A. Fraga, E.F. Sousa-Aguiar,
L.G. Appel, Catal. Today 101, 39 (2005)
40. A. Khaleel, Fuel 90, 2422 (2011)
41. J. Khom-in, P. Praserthdam, J. Panpranot, O. Mekasuwandumrong, Catal. Commun. 9, 1955 (2008)
42. J.B. Miller, E.I. Ko, Catal. Today 35, 269 (1997)
43. B. Sabour, M.H. Peyrovi, T. Hamoule, M. Rashidzadeh, J. Ind. Eng. Chem. 20, 222 (2014)
123