A new facile method to synthesize mesoporous γ-Al2O3 of high surface area and catalytic activity

Article abstract of DOI:10.1016/j.catcom.2009.11.011

The present investigation describes a new method to prepare a high surface area mesoporous γ-Al₂O₃ by controlled calcination of Al (III) nitrate with urea. The alumina catalyst thus obtained is characterized by XRD, adsorption-desorption isotherms and acidity measurements. The catalytic activity is evaluated for dehydration of methanol to form dimethyl ether. It showed high conversion and 100% selectivity to DME as compared to the other mesoporous aluminas, as well as zeolite catalysts. The results are discussed in relation to the surface area and the strength of the acid sites.

Full text of DOI:10.1016/j.catcom.2009.11.011

Catalysis Communications 11 (2010) 414–418
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
A new facile method to synthesize mesoporous c-Al O of high surface area
2
3
and catalytic activity
Pranay P. Morajkar, Julio B. Fernandes *
Department of Chemistry, Goa University, Taleigao Plateau, Goa 403 206, India
a r t i c l e i n f o
a b s t r a c t
Article history:
2
The present investigation describes a new method to prepare a high surface area mesoporous c-Al O3 by
Received 3 September 2009
Received in revised form 4 November 2009
Accepted 10 November 2009
Available online 16 November 2009
controlled calcination of Al (III) nitrate with urea. The alumina catalyst thus obtained is characterized by
XRD, adsorption–desorption isotherms and acidity measurements. The catalytic activity is evaluated for
dehydration of methanol to form dimethyl ether. It showed high conversion and 100% selectivity to DME
as compared to the other mesoporous aluminas, as well as zeolite catalysts. The results are discussed in
relation to the surface area and the strength of the acid sites.
Keywords:
Mesoporous c-Al O
2 3
Ó 2009 Elsevier B.V. All rights reserved.
Aluminium nitrate
Urea
Methanol dehydration to DME
1
. Introduction
Zhang et al. [5] reported the synthesis of high surface area mes-
oporous
ture was subjected to hydrothermal treatment at 100 °C for 48 h.
They observed that with only Al(NO /urea mixture, very low sur-
2 3 3 3 3
c-Al O using Al(NO ) /NH /urea/CTAB system. This mix-
The advances in nanotechnology have led to an increasing de-
mand for multifunctional materials. Highly ordered mesoporous
materials function as excellent catalysts due to high surface areas
and surface functionalities. They also function as electrode mate-
rial in solid oxide fuel cells. Thus, synthesis of ordered mesoporous
materials has attracted great deal of research in the recent years.
The novel approach of using structure directing agents for synthe-
sis, has added a new dimension to the synthesis of porous materi-
3 3
)
2
3
face area of 158 m /g with a pore volume of 0.14 cm /g was ob-
2
tained. The surface area then increased to 276 m /g and the pore
3
volume to 0.21 cm /g with the addition of CTAB.
The two above samples were further characterized by XRD.
They report that only one broad peak at 1–2° was observed for
the sample prepared with CTAB, indicating a disordered worm-
like mesostructure and however, no peak appeared in the low-
angle XRD pattern of the sample prepared without CTAB, mani-
festing the vital role of CTAB in production of wormlike
mesopores.
als. However, synthesis of ordered mesoporous
be much more difficult and hence considered as a challenging task
1].
Pinnavaia and coworkers [2] first synthesized mesoporous alu-
2 3
c-Al O is found to
[
mina using aluminium alkoxide via a hydrothermal method. Vau-
dry et al. [3] made use of caproic and lauric acid and synthesized
2 3
Ordered mesoporous c-Al O in particular, with high surface
area and tunable pore structure has attracted much attention due
to its potential application as catalyst support in automotive and
petrochemical industries [6].
2
mesoporous alumina having surface area 710 m /g but the pore
0
size was in the microporous range (19 ÅA ).
Recent research in this area includes the sol gel method of
synthesis using surfactants such as CTAB and non ionic templates
such as Triton X-114 as structure directing agents [4]. The
Dimethyl ether (DME) is an important alternative diesel fuel
due to its low NO emission, near zero smoke and less engine noise
x
compared with those of traditional diesel fuels [7,8]. Hence cata-
lytic dehydration of methanol to DME is one among the many
important researches of the recent years.
2 3
c-Al O obtained is reported to have high surface area but the
use of costly surfactants as template make such a method
uneconomical.
In spite of all the research work done till date, more simpler
methodologies are required to synthesize ordered mesoporous
Al via a cost effective route.
In this paper, we present a simple urea calcination method to
c-
2 3
O
*
Corresponding author. Tel.: +91 9823556759.
E-mail addresses: julio@unigoa.ac.in, juliofernandes01@gmail.com (J.B. Fer-
obtain mesoporous
catalytic activity.
2 3
c-Al O having a high surface area and high
nandes).
1
566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.011

P.P. Morajkar, J.B. Fernandes / Catalysis Communications 11 (2010) 414–418
415
2
. Experimental
less steel reactor. Methanol was introduced to the reaction zone by
bubbling ultra-pure helium gas through a glass saturator (2 L
capacity) filled with ultra-high pure methanol and maintained at
2.1. Materials
2
0 °C. In each run 0.05 g of the catalyst was tested at 260 °C at
ꢁ1
ꢁ1
Aluminium nitrate, DL-malic acid, urea were purchased from
Loba Chemicals, India and used without any further purification.
GHSV of 4000 mL gcat
h . The tail gas out of the reactor was ana-
lyzed by an on-line GC (Agilent 7890 A) equipped with a thermal
conductivity detector and HP-Plot Q capillary column.
2.2. Synthesis of mesoporous
c-Al
O
2 3
3
. Results and discussion
3.1. XRD studies
2.2.1. By sol gel method
About 75.03 g of Al(NO)
3
ꢀ9H
2
O was dissolved in 100 mL of
The wide angle XRD pattern of the alumina showed that they
deionised water. A 2.5% ammonia was added drop wise until the
pH > 3.8. About 350 mL of 2.5% ammonia was then directly added
and then the precipitate formed was washed several times and
then dispersed in 200 mL of deionised water. It was then peptized
with 1 M HNO at 80 °C under vigorous stirring conditions for 4 h
3
to obtain stable boehmite sol of 1 mol/L [9].
are crystalline and belong to the
c phase [5] (JCPDS card 10-0425).
The low-angle XRD pattern (Fig. 1) of AM showed a half peak
centering at 0.6° between 0.5° and 2° which indicates the presence
of randomly distributed pores (interparticle voids) with a wide
2 3
pore size distribution [4]. However, c-Al O prepared using urea,
Desired amount of DL-malic acid was then added to boehmite
sol in 2:1 ratio. The mixture was stirred vigorously at 80 °C for
1
h and aged on water bath at 90 °C for 3 h. The gel was then dried
at 120 °C for 6 h to obtain the dry gel followed by calcination at
00 °C for 4 h to form stable mesoporous -Al [10]. The sample
was labeled as AM = ( -Al prepared using malic acid).
5
c
2 3
O
c
2 3
O
2
.2.2. By calcination with urea
About 75.03 g of Al(NO)
deionised water. Desired amount of urea was then added main-
3
2
ꢀ9H O was dissolved in 100 mL of
3
+
taining Al to urea ratio of 1:2. The mixture was then vigorously
stirred for 1 h and then dried at 120 °C.
The dry solid mixture was heated very slowly at 250 °C for about
3
h till the gas evolution due to decomposition of the nitrates is com-
plete. The nitrate free sample was ground to a fine powder and then
calcined at 500 °C for 3 h to form mesoporous -Al .This sample
was labeled as AU = ( -Al prepared using urea).
c
2 3
O
c
2 3
O
2.3. Catalyst characterization
The catalysts were characterized by X-ray diffraction (Rigaku
Miniflex, Cu K radiation, 30 kV and 15 mA), BET surface area
and adsorption–desorption isotherms (NOVA 1200 Quanta
Fig. 1. The low-angle and wide angle XRD pattern of (1) AM (alumina prepared
using malic acid) and (2) AU (alumina prepared using urea).
a
chrome).
Acidity measurement was carried out using Temperature Pro-
grammed Desorption technique (TPD) using ammonia as a probe
molecule in the temperature range of 40–440 °C. A 0.5 g of the cat-
alyst was activated in air at 300 °C for 1 h in a catalytic reactor. The
catalyst was then cooled to room temperature in nitrogen atmo-
sphere. Anhydrous ammonia was passed over the catalyst for
3
0 min followed by flushing of the catalytic reactor with nitrogen
to remove the unadsorbed ammonia. The sample was then heated
at a rate of 5 °C/min and the amount of ammonia desorbed was
measured as a function of temperature.
Pyridine desorption studies were carried out on the synthesized
samples in the temperature range of 50–400 °C. About 0.5 g of the
sample was activated in air at 300 °C and then gradually cooled in
nitrogen atmosphere to avoid any physisorption of atmospheric
water. A pyridine gas was then passed through the catalyst for
1
h followed by degassing in inert atmosphere to remove any
unadsorbed pyridine. The FTIR spectra of these samples were re-
corded on IR Prestige 21 Shimadzu spectrometer.
2.4. Catalytic dehydration of methanol to dimethyl ether (DME)
The activity of the synthesized mesoporous
2 3
c-Al O catalyst for
Fig. 2. FTIR spectra of (1) AM (alumina prepared using malic acid) and (2) AU
dehydration of methanol to DME was assessed in a fixed-bed stain-
(alumina prepared using urea).

4
16
P.P. Morajkar, J.B. Fernandes / Catalysis Communications 11 (2010) 414–418
2 3
Fig. 3. Nitrogen adsorption–desorption isotherms (1, 3) and corresponding pore size distribution (2, 4) of mesoporous c-Al O prepared using malic acid (1, 2) and urea (3, 4).
displayed the characteristic peak of mesoporous aluminas as evi-
dent at 2h between 0.5° and 2° [11].
AM displayed a wide pore size distribution with average pore
diameter of 3.83 nm and a pore volume of 0.1 cm g .
3
ꢁ1
In contrast, sample AU displayed a narrow pore size distribu-
tion. The nitrogen adsorption isotherms recorded on organized
mesoporous aluminas are characterized by a relatively steep in-
3.2. FTIR studies
The samples were initially activated to 300 °C to remove the
0
crease in the adsorbed amount starting at P/P = 0.4–0.8. The hys-
teresis loop of the type H1 in this region is due to the capillary
condensation within the uniform mesopores.
physisorbed water followed by cooling in nitrogen atmosphere
and then the FTIR spectra were recorded (Fig. 2). All the -alumina
samples displayed a broad absorption peak at around 3500–
c
The sample AU has an average pore diameter of 3. 9 nm and a
ꢁ1
3
ꢁ1
3
700 cm . These are typical absorptions due to O–H stretching
vibrations of chemisorbed water molecules. Water is known to un-
dergo physisorption followed by chemisorption onto the -alu-
pore volume of 0.25 cm g which is more than twice as com-
pared to AM (Table 1). The sample also showed much higher sur-
2
ꢁ1
c
face area of 246 m g , as a result of uniform mesopores. The
absence of the large pores is evident from the fact that the iso-
therm ends with a nearly horizontal plateau [9].
mina surface and forms M–H and M–OH bonds. The presence of
these M–OH bonds is the possible reason for Bronsted acidity.
Absence of peaks corresponding to the organic compounds con-
firms the complete removal of the structure directing agent on cal-
3.4. Catalytic activity studies
ꢁ1
cination at 500 °C. Absorption bands at 473 and 620 cm were
due to the vibrations of Al–O–Al, while the peak at 742 and
Catalytic dehydration of methanol to DME was used to study
and compare the catalytic activities of mesoporous c-Al O The re-
2 .
ꢁ1
1
070 cm can be assigned respectively to the bending vibrations
of Al–O and O–H.
sults of the catalytic dehydration reaction are as shown in Table 1.
From Table 1 it is evident that the percentage conversion of meth-
anol increases in the following order AS < AM < AU with 100% prod-
uct selectivity. However, BET surface area follows the order
AM < AS < AU. Although the highest conversion of 64% was achieved
3.3. Nitrogen sorption studies
Mesoporous aluminas prepared by both the methods, displayed
typical type IV isotherms [12] as evident from Fig. 3. The sample
2
in case of catalyst AU having surface area of 246 m /g, there was no
Table 1
2 3
Catalytic dehydration of methanol over synthesized mesoporous c-Al O catalyst.
Catalyst
Description
BET surface
area (m /g)
Pore diameter (nm)
Pore volume
(cm /g)
% Conversion
of methanol
Selectivity to
DME (%)
2
3
AM
AS
AU
c
c
c
-Al
2
-Al
2
-Al
2
O
O
O
3
3
3
prepared using malic acid
prepared using salicylic acid
prepared using urea
75
106
246
3.83
3.83
3.90
0.10
0.15
0.25
26
20
64
100
100
100

P.P. Morajkar, J.B. Fernandes / Catalysis Communications 11 (2010) 414–418
417
have an important contribution to the observed catalytic conver-
sion in case of -Al as well. Hence acidity measurements were
carried out on the synthesized catalysts.
c
2 3
O
3
3
.5. Acidity measurements
3
.5.1. NH -TPD profiles
It is evident from the NH
3
-TPD profiles (Fig. 4) that both AS and
AM showed a broad peak in the temperature range of 120–250 °C.
However, the number of acid sites in AM is almost twice as com-
pared to those present in AS. This could be one of the reasons for
the higher conversion of AM in spite of having a lower surface area
than AS. Further, sample AU has high surface area and stronger acid
sites than AM and AS as indicated by the broad peak in the temper-
ature range of 200–300 °C which are labeled as MS (moderately
strong acid sites). Both these factors add to the observed high con-
version. It is also clear from the data on HZSM-5 zeolites Z
1
and Z
2
(Table 2) that % conversion is dependent on these moderately
1
strong acid sites. Thus Z having a high Si/Al ratio (225) and hence
lower Bronsted acidity showed lower conversion as compared to Z
HZSM-5, Si/Al = 40) which has a high conversion but mixed selec-
tivity. In Ref. [13] it is reported that Na-H-ZSM-5(30) obtained from
H-ZSM-5(30) by performing ion exchange with aqueous NaNO was
2
(
Fig. 4. NH
3
-TPD profiles of (1) AS (alumina prepared using salicylic acid), (2) AM
(
alumina prepared using malic acid), (3) AU (alumina prepared using urea),
W = weak, S = strong, MS = moderately strong, VS = very strong acid sites.
3
found to have much reduced number of acid sites and, especially,
eliminated the very strong acid sites. Therefore, subsequent reac-
tions of DME over strong acid sites to form light olefins were re-
duced by sodium ion exchange. Thus, Na-H-ZSM-5(30) gave a
higher selectivity for DME than H-ZSM-5. Our results are in good
agreement with the above findings and the presence of moderately
strong Bronsted acid sites in AU explains its higher selectivity as
clear correlation between the surface area and the catalytic activity.
Hence it was necessary to investigate the nature of the acid sites.
As reported in the literature [13], both Bronsted and Lewis acid
sites play a key role in catalytic dehydration of methanol as in case
of zeolites. Hence the type and the strength of the acid sites must
Table 2
Total acidity of the synthesized
c
-Al
2 3
O in relation to the surface area, catalytic activity and selectivity.
*
S. no.
Catalyst
Description
Total acidity
mmol/g)
BET surface area
(m /g)
% Conversion
of methanol
Selectivity
to DME
Others
2
(
(
%)
1
2
AM
AS
c
-Al
acid
-Al
acid
-Al
2
2
2
O
O
O
3
3
3
prepared using malic
prepared using salicylic
prepared using urea
0.074
0.040
75
26
20
100
100
–
–
c
106
3
4
5
AU
c
0.061
0.069
0.240
246
305
358
64
53
75
100
97
80
–
3
20
Z
Z
1
Zeolite HZSM-5 (225)
Zeolite HZSM-5 (40)
2
*
2
Others include ethene, propene, CO/CO .
Fig. 5. FTIR spectra of pyridine desorption on AM (alumina prepared using malic acid) and AU (alumina prepared using urea).

4
18
P.P. Morajkar, J.B. Fernandes / Catalysis Communications 11 (2010) 414–418
compared to the zeolite samples. Clearly, catalyst AU showed a rel-
atively good % conversion as well as excellent selectivity to DME as
compared to rest of the catalysts investigated here.
The high catalytic activity is attributed to a combination of
some favorable factors such as high surface area, uniform pore size
and large presence of relatively strong Bronsted acid sites.
Acknowledgement
3.5.2. Pyridine FTIR studies
Fig. 5 shows temperature dependent FTIR spectra of adsorbed
The authors are grateful to the financial support for this inves-
tigation received vide UGC-SAP, F.540/25/DRS/2007/(SAP-II) 2008.
pyridine. Both AM and AU showed absorptions in the range of
ꢁ
1
ꢁ1
1
500–1570 cm as well as around 1640 cm due to Bronsted acid
ꢁ1
sites. The absorptions at ꢂ1460 cm due to pyridine coordinated
to the Lewis acid sites are less intense and more significant in AM
than AU. A clear indication of the presence of strong Bronsted sites
References
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[
[
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this sample features the strongest Bronsted acid sites of the series.
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lyst AU is due to the large presence of relatively strong Bronsted
acid sites dispersed over large surface area.
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[
[
[
[
4
. Conclusion
2
A new facile method was developed to obtain mesoporous c-
Al
2
O
3
having a large surface area and relatively uniform pore sizes.
The mesoporous -Al prepared using urea showed a high
catalytic activity of ꢂ64% as well as 100% selectivity to DME.
c
2 3
O