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Nowadays the importance of nano-particles and their uses in different industries have attracted many researches. The materials in nano-scale show different characteristics in comparison with their bulk state. Nano-materials have potential applications in optoelectronics, catalysis, and membranes. In this paper Nano-size porous γ-alumina was successfully synthesized by precipitation method under ultrasonic vibration mixing. Sonochemistry help the nano particles to synthesis regular form. The synthesized catalyst was characterized by SEM, XRD, BET, and TPD techniques. The effect of two most important operating conditions (i.e. Temperature and WHSV) on performance of this catalyst was investigated for dehydration of methanol to dimethyl ether (DME). The optimum operating condition was at temperature of 320 º C and WHSV of 15 h-1. Index Terms—Dimethyl ether (DME), γ-alumina catalyst, Sonochemistry, Nano sized catalyst, operating conditions.
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AbstractNowadays the importance of nano-particles and
their uses in different industries have attracted many
researches. The materials in nano-scale show different
characteristics in comparison with their bulk state.
Nano-materials have potential applications in optoelectronics,
catalysis, and membranes. In this paper Nano-size porous
γ-alumina was successfully synthesized by precipitation method
under ultrasonic vibration mixing. Sonochemistry help the
nano particles to synthesis regular form. The synthesized
catalyst was characterized by SEM, XRD, BET, and TPD
techniques. The effect of two most important operating
conditions (i.e. Temperature and WHSV) on performance of
this catalyst was investigated for dehydration of methanol to
dimethyl ether (DME). The optimum operating condition was at
temperature of 320 ºC and WHSV of 15 h-1.
Index Terms—Dimethyl ether (DME), γ-alumina catalyst,
Sonochemistry, Nano sized catalyst, operating conditions.
I. INTRODUCTION
Dimethyl ether (DME) is an important chemical for the
production of gasoline, ethylene, aromatics and other
chemicals [1]-[5]. Its applications as a fuel or a fuel additive
for vehicles and family cooking have been studied [6], [7]. In
view of the environmental protection, the substitution of
DME for Freon as an aerosol spray and a refrigerant is being
considered [8]-[10]. From the literatures, it can be concluded,
almost all commercial DME were produced by the
dehydration of methanol using different solid-acid catalysts
such as zeolites, sillicaalumina, alumina, Al2O3B2O3, etc.
by following reaction (Equation (1)): [11].
-1
3 3 3 2
2 C H O H C H O C H + H O (-22 .6 k j m o l )
(1)
In recent years, increasing attention has been focused on
the development of nano-sized alumina powders for
advanced engineering materials [12], [13]. Conventional
processes for synthesizing ceramic nano powder involve
mechanical synthesis, vapor phase reaction, precipitation,
and combustion and solgel methods [14]-16]. Catalytic
dehydration of methanol over solid-acid catalysts offers a
potential process for dimethyl ether synthesis. Several
catalysts having activity and selectivity for the catalytic
conversion of methanol to DME are known, the so called
acidic dehydration catalysts [17]-[19]. Commercially γ-Al2O3
is used to a large extent for this reaction at a temperature
range of 270 to 380 ºC.
Manuscript received January 25, 2012; revised March 23, 2012.
The authors are with the Petroleum University of Technology, Ahwaz,
Iran (e-mail: orahmanpour@gmail.com; Shariati@put.ac.ir;
mr.khosravi@put.ac.ir).
The sonochemical synthesis of nanophase materials has
the advantage that various classes of materials can be
generated simply by changing the reaction medium. So it is
worthwhile to overview the different applications, where
cavitations can be used efficient. Preparation of metal and
metal oxide nanoparticles immobilized on various materials
is one of the key researches in nanoscience and
nanotechnology, because an excellent synergy and
Bifunctional effect would be expected [20]. It is well
known that the alternative method for generating stabilized
metal nanoparticles involves synthesizing them in or on
nanoporous supports, which help define particle size and
serve to immobilize the resulting particles [21].
The aim of the present study is to find effect of
sonochemistry in particles structure and the optimum
operating condition of new nano size synthesized γ-Al2O3.
Dehydration of methanol to dimethyl ether was carried out in
the fixed bed micro reactor and the structure of solid-acid
catalysts was also studied by SEM, XRD, BET, and TPD
techniques.
II. EXPERIMENTAL
A. Catalyst Synthesis
Aluminum nitrate {Al(NO3)3-9H2O, 99.5%}(Merck),
Ammonia {NH4OH, 32%} (Merck) and deionized water
were used as starting chemicals. A transparent gel-like
precursor containing Al cations is precipitated at pH ~7.58.5
when ammonia (%3.2) and Al nitrate salt solutions (0.26 M)
are mixed together in 400 ml deionized water. The solution
was mixing under ultrasonic vibration and maintained at a
temperature 70 ºC for 2 h. The following chemical reactions
occurred during preparation:
3
3 3 4 4 3
Al(NO ) +3NH OH Al(OH) +3NH NO
(2)
32
Al(OH) AlOOH +H O
(3)
2 3 2
AlOOH Al O +H O
(4)
The precipitate obtained by reaction (2) was aged at a
temperature ~70 ºC helped to homogenize the gel due. The
ageing step is essential to convert Al (OH)3 to crystalline
boehmite precursor by reaction (3). The precipitate is further
processed by washing in deionized water. Then was added
300 ml ethanol to the filtered Al(OH)3 under ultrasonic
vibration for 1 h then dried in oven at ~70 ºC for 18 h.
γ -Al2O3 is produced by calcinations of dried boehmite
(AlOOH) at 550 ºC for 4 h by reaction (4).
New Method for Synthesis Nano Size γ-Al2O3 Catalyst for
Dehydration of Methanol to Dimethyl Ether
Omid Rahmanpour, Ahmad Shariati, and Mohammad Reza Khosravi Nikou
125
International
Journal of Chemical Engineering and Applications, Vol. 3, No. 2, April 2012
B. Characterization Tests of Catalyst
The crystallity of catalyst, measured by PW1840 (40 Kv,
30 Ma) X-Ray diffract meter using Cu radiation source
(λ=1.54056 A°) through the range of 2θ=5°C to 90°. BET
surface area, total pore volume and average pore diameter
were determined by N2 adsorption-desorption isotherm at 77
K using NOVA 2000instrument (Quantachrome, USA). The
pore volumes were determined at a relative pressure (P/P0) of
0.99. Prior to the adsorption-desorption measurements, the
sample was degassed at 200° C in N2 flow for 3h to remove
the physically adsorbed water immediately before analysis.
The pore size distribution of the catalyst was verified by a
BJH (Barett-Joyner- Helenda) model from the adsorption
branch of the nitrogen isotherms. The acidity of the sample
was measured by temperature programmed desorption of
ammonia using BEL- CAT (type A, Japan) instrument with a
conventional flow apparatus. A 0.1 g sample was initially
degassed at 500 °C under The flow rate of 50 ml/min for 60
min at a heating rate of 10°C/min then; the sample was
cooled to 100 °C and saturated with 5% NH3/He for 30 min.
The sample was then purged with the flow for 15 min to
remove weakly and physically adsorbed NH3 on the surface
of the catalyst. After that, the sample was heated at of 10
°C/min under the flow of the carrier gas (30 ml/min) from
100 °C to 700 °C and the amount of ammonia in effluent was
measured via thermal conductivity detector (TCD).
Fig. 1. Schematic view of experimental setup
C. Methanol Dehydration Process
A schematic representation of the experimental set-up is
shown in figure 1.Vapor phase dehydration of methanol to
dimethyl ether was carried out in the fixed bed micro reactor
(stainless steel, O.D=0.75 in, length=10 in). Liquid methanol
(grade AA, 99.9% purity) was injected to the pre heater by
means of HPLC (metering) pump (working range of 0.01 to
9.99 ml/min), then evaporated in the pre heater that kept at
the constant temperature of 300 ºC. After pre-heater, the
vaporized methanol is conducted to the fix bed flow reactor.
Reactor consists of two heating zones. First zone is to raise
the feed temperature to the desirable level and the other one
to maintain the reactor surrounding at the proper temperature
to minimize heat losses and simulate an adiabatic reactor.
The down line effluent was constantly kept at temperatures
above 160°
С, to prevent condensation of the reactant and
products. The reaction temperature is changed between 270º
C
to 380º
C and for each test 1 gr of the catalyst was loaded to
the reactor. Methanol was pumped to the pre-heater before
entering the reactor. The effluent of the reactor was analyzed
with a gas chromatograph (ACME 6100, Younglin
Instrument Korea) which is equipped with nonpolar capillary
column TRB-5(95% dimethyl-5% diphenyl polsiloxane) and
a flame ionization detector (FID).
III. RESULT AND DISCUSSION
A. Phase Analysis
The XRD patterns of the new nano size synthesized
γ-alumina are shown in Figure 2 that the three peaks at
=37.8, 2Ө=45.7 and 2Ө=66.9 are assigned to (311), (4
0 0) and (4 4 0) reflections of γ-Al2O3. The crystallite sizes
were also calculated using the Scherrer (Eq. (5)):
cos
K
D

(5)
where, K is a constant generally taken as ~ 0.9, λ is the
wavelength of the incident radiation, β is the full width of
diffraction peak at half maximum intensity (FWHM) and θ is
the diffraction angle. The calculated crystallite sizes were
found to be in the range of 1-2 nm for nano size synthesized
γ-Alumina. Crystal structures were very close in synthesized
γ-Alumina catalyst that can be good effect in reaction. These
close crystal layers was made under ultrasonic waves.
Fig. 2. XRD patterns of synthesized γ-Al2O3
Fig. 3. Scan electron microscopy images of γ-Al2O3 catalyst
B. SEM
Fig. 3 shows the SEM image of the synthesized γ-Al2O3
sample after calcination. The crystals of the sample reached a
porous and spongy form, and the morphology of the crystals
were regular pores and present enough volume for reaction
and increase surface area. SEM of synthesized catalyst shows
low bulk density and high bulk pores which these structures
c)
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International Journal of Chemical Engineering and Applications, Vol. 3, No. 2, April 2012
made from evaporation of ethanol under ultrasonic vibration
mixing.
C. Acidity Measurements
The TPD spectra Fig. 4 of the synthesized γ-alumina
catalyst contain an intense peak in 160°C and 320 this peak
reduced slightly in the temperature range of 200500 ºC that
corresponds weak to medium acid sites which is attributable
to the removal of adsorbed NH3 on the catalyst surface with
low strengths and the temperature of the desorption maxima
and the acid content of the catalysts are summarized in Table
I. TCD signal shows the synthesized catalyst have medium
acid site and this peak was presented in 320 °C and strong
acid site was not seen in this signal. TCD signal shows the
synthesized catalyst have medium acid site and this peak was
presented in 320 °C and strong acid site was not seen in this
signal.
TABLE I: SUMMARY OF ACIDIC SITE MEASURED BY NH3-TPD OF
SYNTHESIZED Γ-AL2O3 CATALYST
Weak
acid sites (mmol/g)
Total
Nano size γ-Al2O3
0.883
1.07
Temperature (ºC)
170
Fig. 4. NH3-TPD profile of γ-alumina
Fig. 5. N2 adsorption and desorption isotherms of γ-Al2O3 catalyst
D. N2 Adsorption-Desorption (BET)
The N2 adsorption and desorption isotherm remains type
IV, as is seen from Fig. 5. According to IUPAC classification
[22], the hysterics loop (type H1) occurs at a relative pressure
range of p/p0 = 0.60.95, indicating a broad pore size
distribution with uniform size and shape. As it can be seen,
synthesized nano γ-Al2O3 catalyst show a mesoporous
structure with different pore size distributions. The surface
area, pore size distribution and pore volume data obtained for
nano size synthesized γ-Al2O3 catalyst using ammonia agent
obtained by its calcination at 550 °C for 4 h in air are
tabulated in Table II. The BJH pore size distribution curves
are reproduced in Fig. 6. The pore size distribution is wide
and pore size lies between 1 and 20 nm.
Fig. 6. BJH pore size distribution of γ-Al2O3 catalyst
TABLE II: SUMMARY OF SURFACE AREA AND PORE VOLUME OF
SYNTHESIZED Γ-AL2O3 CATALYST
Surface area
(m2 g-1)
Average pore
diameter (nm)
Pore volume
(cm3 g-1)
Nano size
γ-Al2O3
216
9.646
0.5212
E. Catalytic Activity
The reaction performance results, including methanol
conversion calculated according to (Eq. (6)) [23].
initial MeOH final MeOH final DME
MeOH
initial MeOH
n - (n +2n )
X %= 100
n
(6)
The conversion variation trend with temperature was
investigated for different weight hourly space velocities at
range of 15 to 45 hr-1 at a constant pressure of 100 kPa gauge.
As depicted in figure 7 the trend is nearly similar for different
WHSVs but the amount of methanol conversion is reduced
with increasing space velocity in the temperature range of
270 to 380 °C for synthesized γ-Alumina.
Fig. 7. Catalyst bed temperature profile over different weight hourly space
velocity
It is clear from the figure that the system reached to the
equilibrium conversion at temperature range of 310 to 340 °C
under WHSV of 15 hr-1 and by increasing WHSV, the
reaction could not meet the equilibrium conditions. The
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International Journal of Chemical Engineering and Applications, Vol. 3, No. 2, April 2012
highest conversion was resulted in temperature range of
310-340 °C. Equilibrium conversion shows acceptable result
for this nano sized catalyst.
IV. CONCLUSION
Nano-sized porous γ-Al2O3 was successfully synthesized
via a series of synthetic pathways. The calculated crystallite
sizes were 1-2 nm with surface area ~216 m2/g, average pore
diameter of 9.646 nm, and pore volume ~ 0.5212cm3/g.
According to the hysterics loop (type H1) the synthesized
nano γ-Al2O3 catalyst show a mesoporous structure.
The results showed the maximum conversion of reaction
was observed at a temperatures range of 310 to 340 °C and
WHSV of 15 hr-1 with methanol conversion of 84% for
synthesized. All of results showed that sonochemistry is a
new method for synthesized catalysts and have competitive
potential with commercial catalysts. Sonochemistry can play
major role in formation of particles and crystals structure.
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Omid Rahmanpour was born in Marand on March
14th 1985. He received her bachelor degree in
Chemical Engineering from Babol Noshirvani
University of Technology, Mazandaran, Iran, in 2009.
Now, he is studying for MS degree in Gas Engineering
Department, Petroleum University of Technology,
Ahwaz, Iran. Currently his main efforts are taken to
synthesis of γ-Al2O3/CNTs catalyst for dehydration of
methanol and other acidic catalysts.
128
International Journal of Chemical Engineering and Applications, Vol. 3, No. 2, April 2012
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