VOL. 3, NO. 5, May 2013
ISSN 2225-7217
ARPN Journal of Science and Technology
©2011-2013. All rights reserved.
http://www.ejournalofscience.org
Effect of Sulfate Ion on the -Al2O3 Surface Area Synthesized by
Homogeneous Precipitation
1
1, 2
Adrián Zamorategui, 2 Satoshi Sugita
Department of Chemistry, University of Guanajuato, Guanajuato, Gto. México. Col. Noria
ABSTRACT
Amorphous basic aluminum sulfate (BAS) was synthesized by homogeneous precipitation using an aluminum sulfate
solution and ammonium bisulfate as the precipitating agent. A second neutralization in the solid/liquid phase of the BAS
with ammonia solution eliminates the sulfate ions of the amorphous structure and obtains pseudoboehmite (PB) at pH 10.3
with fibrousnano-sized morphology. The structure of the PB is transformed to the amorphous structure of the gamma
alumina phase (-Al2O3) by thermal dehydroxylation (400°C). Thus, the eliminating water in the form of hydroxides,
maintained the fibrous morphology with a high specific surface area 350 m2/g. The neutralization of the BAS the second
time generates a slight reduction in specific surface area to 338 m2/g. The sample of BAS, PB and -Al2O3 was analyzed
using the techniques TGA-DTA, XRD, FTIR, EDS, TEM and adsorption/desorption of N 2 gas (BET).
Keywords: Homogeneous precipitation, basic aluminum sulfate, pseudoboehmita, -Al2O3.
1. INTRODUCTION
Alumina is a material of significant importance
from the technological point of view, for its application in
electronics, personal protective equipment, refractoriness,
catalysts, catalyst support, oil refining, and control of
pollutant emissions from cars. The fine powder of
nanometer size and high purity is of great interesting the
preparation of ceramics with high hardness and wear
resistance in applications or membranes or catalytic
materials [1,2]. Some methods have been developed to
prepare mono disperse particulate oxides or their
precursors, such assol-gel [20], emulsion, homogeneous
precipitation, etc. Among these techniques, homogeneous
precipitation appears to be industrially advantageous
because it does not require expensive alkoxides, organic
solvents or, surface tents that can raise the cost of
production [3-5].
Aluminum oxy hydroxide, amorphous spherical
particles of Al(OH)3 and boehmite particles of different
shapes, prepared by hydrolysis of aluminum ions in
aqueous solution were prepared. Interestingly, for the
formation of spherical particles of amorphous Al(OH)3, the
sulfate ion was necessary as an intermediate[6]. Dense
particles of basic aluminum sulfate (BAS) of sub micron
size have been prepared by homogeneous precipitation of
aluminum sulfate using urea as the precipitating agent.
The urea rapidly dissolves in water to produce a
uniform solution with pH during decomposition. The pH of
the solution can be controlled via the thermal
decomposition of the urea to ammonia and carbon dioxide
in the temperature range of 70°-100°C.In addition, the
formamide was used to precipitate the BAS to lower pH
than that used with urea to obtain a BAS with the highest
purity [7-10].
The physical properties, crystal structure,
morphology, specific surface area, pore volume and size
must be controlled to increase the catalytic activity and
adsorption characteristics of pseudoboehmite (-AlOOH).
Thus, these properties can be controlled by homogeneous
precipitation, depending on the relationship of H2O/Al2
(SO4)3, precipitating agent, pH, temperature and time of
aging [11,12,13]. The objective of this study is to evaluate
the effect of the remained sulfate ion concentration on the
specific surface area of gamma alumina (-Al2O3)
synthesized by homogeneous precipitation, using
ammonium bisulfate (NH4HSO3) as precipitating agent.
2. EXPERIMENTAL PROCEDURE
Basic aluminum sulfate was synthesized by the
homogeneous precipitation method, mixing solutions of
aluminum sulfate and ammonium bisulfite 0.311M (1.3
g/cm3density). The ratio of ion sulfate to aluminum ([SO42
]/[Al+3]) in the mixture was 2/3.This was heated to 90°C
and maintained at this temperature for 30 minutes. The
heating rate up to 90°C was 2°C/minute and the agitation
was maintained at 350 rpm. The pH during the
precipitation was kept at 3.7 and controlled by the thermal
decomposition of ammonium bisulfate at 75°C, generating
ammonium sulfite, sulfur dioxide and water, as shown in
reaction 1 [6,14].

2 NH4HSO3(ac)→
(NH4)2SO3(ac) + H2O (l) + SO2(g) -- (1)
The Al2(SO4)3 reacts due to the high affinity
between the sulfate and Al3+ ion. Thus, the formation of the
polymeric species of the Al3+ion with the hydroxide ion
(OH-) is strongly inhibited by the sulfate ion in the
precipitation reaction at pH 3.7. Indeed, the precipitation
occurs upon reaching temperature between 75 and
90°C.The reaction (2) describes the precipitation process to
obtain the BAS (Al(OH)x (SO4) (3-x)/2) using the aluminum
salt and ammonium sulfite generated in the first reaction
(1), where the value of x is approximately 2.6.The white
precipitate of the BAS was filtered and washed with
distilled water and subsequently dried for 12 hours in an
oven at 110°C to obtain a fine powder.
Al2 (SO4)3(ac)+ x(NH4)2SO3(ac)+ xH2O (l)→

2 Al (OH)x(SO4)(3-x)/2(s)+ (NH4)2SO4(ac)+ SO2 (g) ---(2)
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ARPN Journal of Science and Technology
©2011-2013. All rights reserved.
http://www.ejournalofscience.org
On the other hand, the optimal time of the BAS
neutralization with an ammonia solution to completely
remove the sulfate ions generated in reaction (2) was
determined. The BAS powder was dispersed in d is tilled
water at a solid/liquid ratio of 1/8 to be neutralized at a
temperature of 75°C and constant agitation of 350 rpm.
Table 1 show the times used in each neutralization test.
The Al(OH)3 solid obtained was filtered and was heated
with hot distilled water, then the solid was dried in an oven
at 110°C for 12 hours to give a white powder
psedoboehmite (PB).Also, the PB was thermally treated at
450°C and was transformed by thermal De hydroxylation
to the -Al2O3 phase. Finally, the effect on the specific
surface area of the sulfate ion remaining in the -Al2O3
structure was evaluated.
The material crystallinity was analyzed by X-ray
diffraction (XRD) using the Siemens D500 diffractometer
in the range of 5° to 80°, with monochromatic copper
radiation (Cu) Kα; spectroscopy Fourier transform infrared
(FTIR) was carried out using the Perkin Elmer (1600
series) in the range of 400cm-1 to 4000 cm-1 using KBr
pellets; thermal analysis (TGA and DTA) was developed in
the equipment model TA Instrument SDT2960
simultaneous TGA-DTA type in air flow. The morphology
of the material was observed by transmission electron
microscopy (TEM) with the field emission microscope
PhilipsTecnai F-20, and textural properties of the powders
were analyzed by adsorption/desorption of N2 gas using the
BET method.
Table 1: Neutralization time
No.
Time (min)
Sample
First
Neutralization
Second
Neutralization
Relation
BAS/H2O
pH
1
2
3
4
5
6
5
6
7
1
10
30
80
120
10
10
10
10
20
30
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
10
10
10
10
10
10
10
10
3. RESULTS AND DISCUSSION
3.1 Titration of the BAS and Al2 (SO4)3
The BAS titration curve (Fig. 1) compared with
the curve of Al2(SO4)3
lacks the segment AB
corresponding to the high consumption of OH- in the
nucleation stage without variation of pH. As is known, the
ion Al3+ predominatesin acidic media and coexists with
basic species (AlOH2+ and Al (OH)2+). In the presence of
ions that exhibit high affinity for the aluminum ion, such as
sulfate, the basic salt precipitation occurs rapidly after
mixing with the counter ion. The affinity of the sulfate ion
for the aluminum ion (Al3+) prevents or at least retards the
polymerization of the Al-OH and its hydrolysis poly
meriting in the crystalline structure of Al(OH)3[9]. Thus,
the point B at pH 3.9 corresponds to the formation of basic
salts of aluminum. The two well-defined regions bc and c
dare very consistent with the regions BC and CD in the two
cases. The region b corresponds to the functional groups
present as weak acid in the system develops by the
hydrolysis of the sulfate surface groups through Kolth off
reaction [10].
pH
The final pH of the neutralization of the BAS was
determined by titration with constant agitation. Thus, a
dispersion of BAS was prepared (0.08 wt. %) and titrated
with 1M NH4OH at an addition rate of 0.02cm3 per minute.
The pH was measured using a potentiometer 210 brand
HANNA pH instrument. The titration curve is shown in
Fig.1.
11
10
9
8
7
6
5
4
3
2
D
d
c
C
----- Al2(SO4)3
SBA
b
B
BAS
A
0
0.5
1
1.5
Vol. NH 4OH (mL)
2
Fig 1: Titration curves of BAS.
The region cd represents the titration of the excess
of base added to the system after the point of equivalence.
This region corresponds to the saturation zone and occurs
at a pH 10.3 corresponding to -AlOOH formation. Since
the sulfate ion is an ion penetrated in the basic aluminum
species, this can be easily exchanged with the hydroxyl ion
favoring the formation of aluminates. This ion later
becomes an aluminum hydroxide by the action of the
addition of an ammonium action (weak acid) at high pH
values[15, 16]. In both cases the end point of neutralization
was at pH 10.3 as shown in Fig. 1.
3.2 Thermal Analysis of BAS and PB
The thermal behavior of the BAS and PB obtained
by homogeneous precipitation was analyzed as shown in
Fig. 2a and b. In the thermal analysis of BAS (Fig. 2a), the
TGA curve shows a weight loss between 50 and
850°C.This corresponds to the loss of structural water and
dehydroxylation temperature of the material. Furthermore,
there is a second weight loss between 850 and 950°C,
resulting from the thermal decomposition of the sulfate ion
in the sample, generating the gas, SO3. Finally, a third
weight loss was observed at approximately 1200°C
attributable to the formation of the -Al2O3 phase.
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a
.00
0.08
0.06
-.05
60
0.04
Temp. Diff ( C/mg)
Weight (%)
80
40
20
0
0
200
400
-.10
0.02
-.15
Deriv. weight (%/ C).
0.10
100
0.00
-0.02
600 800 1000 1200 1400
Temperature (°C)
TG
DTG
DTA
b
Weight (%)
100
100
0.14
0.0
80
0.12
0.10
9060
0.08
0.06
40
-0.1
80
20
0.04
Deriv. Weight (%/ C).
120
0.02
0.00
70 0
0
300
600
900
1200
Temperature (°C)
TG
DTG
DTA
Fig 2: Thermal analysis of: a) BAS and b) PB
The chemical composition in weight percent of
the BAS was determined with the results obtained by
thermal analysis: 47.8% Al2O3, 22% SO3 and 30.2% H2O.
Thus, the empirical formula of BAS agrees with other
reported formulas [17].
Al2O3•0.6SO3•4.4H2O
Al4(OH)10SO4
Fig. 2b shows the thermal analysis of the PB
obtained by neutralizing the BAS for 30 minutes. The
thermal gravimetric analysis curve shows five very
significant weight losses. The first endothermic loss is
between 26 and 132°C attributable to desorption of water.
The second is observed as a slight inflection between 200
and 270°C because of the loss of residual NH3 in the PB
superficially adsorbed during neutralization, the third
corresponding to the temperature range between 300 and
460°C,with a maximum at 390°C,corresponding to the
dehydroxylation and structural transformation of the PB
towards -Al2O3 phase[18]. Likewise, the fourth loss
inflection exists in the range of 800 to 1000°C with a
maximum at 910°C corresponding to the removal of the
residual sulfate ion, adsorbed on the surface of the PB. The
fifth exothermic loss is between 1180 and 1200°C
corresponding to the formation of α-Al2O3 phase.
3.3 XRD analysis
Diffraction patterns were determined to determine
the structure and the degree of structural order of the
samples. The BAS obtained by homogeneous precipitation
is an amorphous material as shown in Fig. 3. This BAS
was neutralized in a solid/liquid reaction in order to obtain
the PB. The diffraction angles correspond to the crystal
planes of the PB (020), (120), (140), (031), (200), and
agree very well with those reported in the JCPDS No. 211307. Thus, the diffraction peaks with little height indicate
that the material has low crystallinity. Furthermore, the
diffraction angles of the -Al2O3 corresponding to the
planes 311, 400 and 440 are consistent with those reported
in the JCPDS No. 10-0425. Additionally, the broad peaks
with low intensity show that the  -Al2O3 powder is very
amorphous [19].
140
120
36.5 45.87
66.65
100
80
u.a
The DTA curve shows a short endothermic band
between 100 and 250°C with a maximum at 170°C, and a
broad band between 250 and 800°C attributable to a
process of structural water loss and progressive thermal
dehydroxylation. A more intense and narrow band is
observed between 850 and 950°C, corresponding to the
decomposition of the sulfate present in the material which
is released as a gas, SO3.Additionally, there is a small
exothermic peak at approximately 1200°C, corresponding
to the formation of the -Al2O3.
-Al2O3
13.30
28.1
38.3
48.9
64.85
60
71.4
PB
40
20
BAS
0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2θ (°)
Fig 3: XRD analysis of the BAS, PB and -Al2O3 (450°C)
3.4 Microanalysis EDS (Energy Dispersive
Spectroscopy).
To remove the remaining sulfate ion in the powder
of the PB observed in the thermo gram (Fig. 2b), various
BA Sneutralizations were performed at different times
(Table 1). Thus, the PB obtained in each test was heat
treated at 450°C to obtain the -Al2O3 phase according to
the thermal analysis of Fig. 2b and the diffractogram of
Fig. 3.
The EDS microanalysis results (Fig. 4) show that
the sulfate ion is not removed completely with only first
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ARPN Journal of Science and Technology
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100%
100.0%
SO3
80%
60%
40%
20%
9.0%
0%
corresponding -Al2O3 sample obtained by double
neutralization: 10 + 10, 10 + 20and10 + 30 minutes
indicates that the absorption band attributable to sulfate
ions does not appear.
610
a
1100
3
3445
Absorbance (u.a.)
neutralization regardless of the time duration thereof.
However a second neutralization with times of 10 minutes
each (10 +10) eliminates 100% of the sulfate ions. The
same results are obtained with 20 +30, 30 +30 minutes
each. To corroborate the results, detection of residual
sulfate ions was performed with FTIR spectroscopy (Fig.
5b).
 O-H
990
1
4
1635
δO-H
3500 3000 2500 2000 1500 1000 500
4.0% 2.6% 2.2% 2.5%
0.0% 0.0% 0.0%
Wave number [1/cm]
SBA
1
10
30
80
120 10+1010+2010+30
Neutralization time (min)
Fig 4: Sulfate ion (SO3) remaining in the -Al2O3 powder
obtained by neutralization of the BAS.
The spectrum of -Al2O3 (Fig. 5b), shows
vibration frequencies below 1000 cm-1which represent
fundamental interactions of the aluminum Al-O,
coordinated in the structure -Al2O3. It has been established
that the octahedral coordination AlO4 and tetrahedral AlO6
are characterized by vibrational frequencies in the range
500-700cm-1 and 700-900cm-1respectively. The spectrum
also exhibits an OH stretching mode associated with water
and the hydroxide species, and a scissor mode H-O-H
associated with the water, found at 3464cm-1 and 1630 cm1
respectively[22]. Thus, the amplitude of the absorption
bands can be due to the amorphous material.
Additionally, the spectra corresponding to 1, 10
and 30 minutes of neutralization shows a band at 1100 cm1
corresponding to the remaining sulfate ion in the Al2O3
powder. The presence of the sulfate ion in the sample is a
result of the sulfate ion affinity for aluminum, being
difficult to remove with onetime neutralization. However,
applying a second neutralization, totally removes any
sulfate ion remaining. The FTIR spectrums (Fig.5b) of the
Absorbance (u.a).
3.5 FTIR of the BAS and -Al2O3 (450°C)
Fig. 5a shows the FTIR spectrum of the BAS.
Five absorption bands can be observed, the first at 3445cm1
corresponding to the vibration modes 1 and 3 of the
water molecules and bond stretching H-O, the second band
located at 1635 cm-1corresponding to the vibration mode 2
of the coordination water, the third, fourth and five
absorptions appear at 1140 cm-1, 990 cm-1 and 610 cm1
corresponding to the vibrations 3, 1 and 4, respectively
of the sulfate ion SO42-[20,21].
b
3464
-Al2O3 10+30
770 598
-Al2O3 10+20
-Al2O3 10+10
-Al2O3 30
-Al2O3 10
-Al2O3 1
1100
1630
0
.
-0.1
1
3500 3000 2500 2000 1500 1000 500
Wave number [1/cm]
Fig 5: FTIR spectra of the BAS and -Al2O3
3.6 Morphology and particle size of the PB and -Al2O3
by TEM
The transmission electron microscope (TEM) was
used to evaluate the shape and particle size of the PB and
the -Al2O3. As shown, the samples are formed by nano
fibers, in arrangements of agglomerates and this is in
agreement with the amorphous material observed by XRD
analysis (Fig. 3). Fig. 6a shows nano fibers of the PB
thinner than those corresponding to the -Al2O3 powder
(Fig. 6b). As is known, the high surface area can be related
to the geometry of the particle. Thus, according to the
specific surface area (338 m2/g) an average length of 75 nm
and a diameter of 3 nm (approx.) was calculated per fiber
for the -Al2O3.
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400
350
SBET (m2 /g)
a
350
100.0%
338
325
300
271
250
234
200
150
100
50
9.0%
4.0%
11
2.6%
0.0%
0.0%
0
SBA
BAS
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
SO3
http://www.ejournalofscience.org
1
10
30 10+10 10+30
Neutralization time (min)
Fig 7: Effect of sulfate ion and neutralization times on the
specific surface area of the -Al2O3
b
b
Fig. 8 shows the adsorption-desorption isotherms
of the BAS and -Al2O3 powder obtained from PB
synthesized with different neutralization times of the BAS.
The isotherms except for the BAS sample presents a type
IV is other according to the IUPAC definition which is
characteristic of a mesoporous material. The irregular
shape isotherms, with the hysteresis loop indicating that the
pores do not have uniform size have an inkwell-type shape.
The BAS is other m is very horizontal due to its low
surface area and negligible porosity.
Fig 6: TEM micrographs of: (a) PB and (b) -Al2O3.
3.7 Adsorption / desorption of N2, BET area analysis
Fig. 7 shows the specific surface area of -Al2O3
and the sulfate ion remaining against the neutralization
time. As can be seen the -Al2O3 powder with higher
surface area (350m2/g) is obtained with a neutralization
time of 30 minutes and the percentage of remaining sulfate
ions in the -Al2O3 powder was 2.63%. Thus, for
neutralizations with time under 30 minutes, the contents of
sulfates present in the solid neutralized (AlOOH)
drastically decreases the surface area of -Al2O3. In fact,
sulfate ion shave not been exchanged for hydroxide ions
during neutralization and they are occupying more space
within the structure of the -Al2O3. Furthermore, with the
double neutralization the sulfate ions are eliminated and
even though slightly affect the specific surface area of the
powder. This can be due to aging of the solid during the
second neutralization. However, the specific surface area is
kept high to 325 m2/g.
Vol. Adsorbed (cm3 /g)
600
500
400
300
10 + 30 min.
30 min.
10 min.
1 min.
0 min.
200
100
0
0.00
0.20
0.40
0.60
0.80
Relative Pressure (P/Po)
1.00
Fig 8: Adsorption-desorption isotherm of -Al2O3 obtained
from different neutralization times
4. CONCLUSIONS
The ammonium bisulfate used as precipitating
agent to synthesize the basic aluminum sulfate (BAS)
maintains a pH of 3.7. The BAS with amorphous structure
obtained by homogeneous precipitation was used as
starting material for the synthesis of -Al2O3 with high
specific surface area (350m2/g). The high surface area of
the -Al2O3 is due to the febrile morphology of the
pseudoboehmite (-AlOOH) which was used as a precursor
material. The removal of sulfate ions from the structure of
BAS by neutralization of the solid is accomplished by
double neutralization each one for a short time (10 + 10
minutes). However, the double neutralization diminishes
the specific surface area (350 m2/g) due to the aging effect.
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