Preparation of porous zirconia microspheres by internal gelation method

Article abstract of DOI:10.1016/j.materresbull.2007.12.007

A modified internal gelation process for the preparation of porous zirconia microspheres has been developed. The conventional method has been modified by adding a surfactant in the feed broth. The effects of variation of surfactant concentration, washing techniques and temperature of calcination on the pore volume and the surface area of the microspheres have been studied. The conditions were optimized to obtain porous stable microspheres suitable for various applications. The microspheres were characterized by surface area analysis, pore volume analysis, thermogravimetric analysis and X-ray diffraction. The ion exchange behavior was studied using pH titration.

Full text of DOI:10.1016/j.materresbull.2007.12.007

Materials Research Bulletin 43 (2008) 2937–2945
www.elsevier.com/locate/matresbu
Preparation of porous zirconia microspheres
by internal gelation method
*
Sachin S. Pathak, I.C. Pius , R.D. Bhanushali, T.V. Vittal Rao, S.K. Mukerjee
Fuel Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
Received 7 June 2007; received in revised form 28 November 2007; accepted 14 December 2007
Available online 26 December 2007
Abstract
A modified internal gelation process for the preparation of porous zirconia microspheres has been developed. The conventional
method has been modified by adding a surfactant in the feed broth. The effects of variation of surfactant concentration, washing
techniques and temperature of calcination on the pore volume and the surface area of the microspheres have been studied. The
conditions were optimized to obtain porous stable microspheres suitable for various applications. The microspheres were
characterized by surface area analysis, pore volume analysis, thermogravimetric analysis and X-ray diffraction. The ion exchange
behavior was studied using pH titration.
#
2008 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; A. Oxides; B. Sol–gel chemistry; C. Thermogravimetric analysis; D. Surface properties
1
. Introduction
Inorganic ion exchangers are increasingly being used in nuclear industry, especially for the separation and
immobilization of radioactive wastes [1]. They have certain advantages over their organic counter part such as
radiation and thermal stability, resistance towards oxidants and have stable structure suitable as host materials for the
immobilization of radionuclides. Powder form of inorganic exchangers many times chokes the liquid flow during the
column operations. The problem could be overcome by preparing them in the form of microspheres of required size.
Studies have been reported for the preparation of zirconia microspheres for use as ion exchange material [2].
Zirconia-based matrix is one of the most favored materials for use as inert matrix for transmutation of minor actinides
[
3] and also for the once through, high burn up inert matrix plutonium based nuclear fuels [4].
Several studies have been reported on the preparation of porous inorganic materials by precipitation or gelation [5]
in the presence of artificial opals [6], surfactants [7] and other pore formers [8]. Surfactants form self-assembly in
concentrated solution, and precipitation of inorganic materials in their presence results in composite material retaining
the ordered assembly of the surfactant macromolecules. Careful removal of the surfactant by heat treatment can result
in porous materials [9].
In the present study, attempts were made to prepare stable porous 1–2 mm sized zirconia spheres by internal
gelation method [10]. A cationic surfactant, cetrimide, was added to the feed broth prior to gelation. Feed composition
*
Corresponding author. Tel.: +91 22 25590645; fax: +91 22 25505151.
E-mail address: icpius@lycos.com (I.C. Pius).
0025-5408/$ – see front matter # 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.12.007

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S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
and heat treatment conditions were optimized to obtain porous spheres stable at high temperature, suitable for various
applications. The material was characterized by various physicochemical methods. The ion exchange behavior of the
+
microspheres was studied by using K as exchange species [11].
2
. Experimental
2
.1. Chemicals
Zirconium oxychloride used was of GR-Grade obtained from Loba Chemicals, Mumbai. Extra pure
cetrimide [C H BrN] was procured from Sisco Research Laboratories, Mumbai. All other chemicals used were
1
7 38
of AR-Grade.
2
.2. Preparation of microspheres
The process flow sheet for the preparation of hydrous zirconia microspheres is shown in Fig. 1. Feed solution was
prepared by stirring zirconium oxychloride, water and cetrimide in required proportion at room temperature. The
resulting clear solution was cooled to 5–10 8C and equimolar mixture of 3 M Hexamethylene tetra amine (HMTA) and
3
M Urea was added slowly with stirring to obtain clear feed broth of desired composition. To optimize the conditions
for obtaining porous and physically stable microspheres, different batches were prepared by varying (HMTA, Urea)/Zr
mole ratio in feed solution from 0.35 to 0.56. The concentration of cetrimide was varied from 0 to 18 wt.%. The
assembly [12] used for the preparation of gel microspheres is shown in Fig. 2. The equipment consists of a gelation
column, conveyor belt and wash tank. The feed broth was dispersed as 1–2 mm diameter droplets using drawn weight
burette, into gelation column containing silicone oil (100 cP) maintained at 90 8C to obtain solid gel particles. The gel
particles were separated from oil by collecting them on the conveyor belt. Gelation occurred in 10–15 s, after addition
of droplets into the silicone oil column. The resulting gel microspheres were washed with carbon tetrachloride (CTC)
to remove adhering oil and were further washed with 5% ammonia to remove HMTA, urea, ammonia chloride etc.
Other washing schemes such as washing with excess of 5% ammonia at 80 8C and also with excess of distilled water at
8
0 8C were also tried. The microspheres were first dried at room temperature and then in an oven at 100 8C in air for
6
h.
Fig. 1. Flow sheet for the preparation of zirconia microspheres.

S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
2939
Fig. 2. Gelation assembly used for the preparation of zirconia microspheres.
2
.3. Heat treatment
The dried microspheres were heated at a rate of 28/min and were maintained at 220, 240 and 280 8C for half an hour
each. These were then heated to 500 8C and were maintained for 1 h in air oven. Some samples were heated to 800 or
1
2
2
000 8C and maintained at these temperatures for 1 h.
.4. Characterization
.4.1. Surface analysis
Surface area and pore size distribution studies were carried out using nitrogen adsorption desorption method using
the instrument sorptomatic 99CE from Italy.
2
.4.2. Thermogravimetric analysis
SETARAM TG-DTA 9218 thermal analyzer was used. TG/DTA curves were recorded in air for microspheres dried
at 100 8C.
2
.4.3. X-ray diffraction studies
X-ray analysis was carried out using STOE Powder Diffraction System.
2
.4.4. pH titration of the zirconia microspheres
Ion exchange behavior of the zircoia microspheres was studied by pH titration method. 50 mL of 0.1 M
solution of KCl was stirred with 0.05 g of powdered zirconia microspheres and was titrated with 0.05 M
KCl + 0.036 M KOH solution to pH = 11. After addition of each increment of the solution, stirring was continued
till a constant pH was attained. The reverse reaction and regeneration of the exchange material was performed
similarly by titrating the resulting solution with an aqueous solution of 0.1M HCl + 0.01 M KCl until the pH
returned to original value [11]. The exchange capacity was calculated from the difference in the KOH consumed
between sample and blank, using the weight of ZrO taken for pH titration. The exchange and regeneration steps
2
were repeated to confirm the reversibility of the process and exchange capacity of microspheres was determined
+
for K ions.

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S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
Table 1
Effect of variation of (HMTA, urea)/Zr mole ratio in the preparation of zirconia microspheres
2
Sr. no.
(HMTA, urea)/Zr
Physical appearance at different temperatures
Gelation
time (s)
Sp. surface area (m /g)
2
5 (8C)
280 (8C)
500 (8C)
1
2
3
0.35
0.44
0.56
Opaque, white
Opaque, white
Transparent (partially broken)
Black
Black
Black
Gray
Opaque, white
Yellow (broken)
12
11
7
32.8
145.3
103
[
Zr]: 0.97 meq/g ꢀ 1.22 M, cetrimide: 12 wt.%, time of mixing: 10 min, temperature: 5–10 8C, gelation temperature: 90 8C, washings: CTC and 5%
NH OH at room temperature, heat treatment: 500 8C.
4
3
. Results and discussion
3
.1. Variation of HMTA, urea concentration
Preparation of zirconia microspheres was carried out with varying (HMTA, urea) to metal ratio in the feed broth in
the range from 0.35 to 0.56 at zirconium concentration of 0.97 meq/g and cetrimide concentration of 12% and the
results are given in Table 1.
(
HMTA, urea) to metal ratios higher than 0.56 resulted in premature gelation of the feed broth while ratios lower
than 0.35 resulted in incomplete gelation and ill-formed microspheres. At ratio of 0.56, the gelled microspheres were
transparent which tend to crack during drying and heat treatment. As the (HMTA, urea) to metal ratio varied from 0.35
to 0.56, the gelation time changed from 12 to 7 s. When the (HMTA, urea)/Zr ratio was between 0.35 and 0.44, the
microspheres formed were opaque and this generally resulted in intact microspheres even after heat treatment. The
2
surface area of microspheres heated to 500 8C increased from 32.8 to 145.3 m /g as the (HMTA, urea)/Zr ratio was
varied from 0.35 to 0.44.
3
.2. Variation of cetrimide concentration
The concentration of cetrimide in the feed broth was varied from 0% to 18 wt.%. As the cetrimide concentration
increased, the feed broth became more viscous after addition of (HMTA, urea) mixture and a concentration > 18%
resulted in viscous solution, which was difficult to disperse as droplets. In the absence of cetrimide, transparent
microspheres were obtained and these cracked during the heat treatment. At higher concentrations (12 or 18%), the
gelled microspheres were opaque and the resultant microspheres were physically stable during heat treatment, as given
in Table 2.
2
As the cetrimide concentration increased from 0 to 6%, the surface area increased from 84.6 to 98.0 m /g, while the
pore volume increased from 0.07 to 0.12 cc/g. The adsorption isotherm in Fig. 3 falls into Type C hysteresis curve
produced by mixture of tapered or wedge-shaped pores with open end [13]. The increase in porosity for 6% cetrimide
microspheres is due to its removal during heat treatment and this is reflected in pore size distribution curve shown in
Table 2
Effect of variation of cetrimide during the preparation of zirconia microspheres
Sr. no.
Cetrimide (%)
Physical appearance at different temperatures
Pore volume
cc/g)
Sp. surface
area (m /g)
2
(
2
5 (8C)
100 (8C)
500 (8C)
1
2
0
6
Transparent
Transparent
(partially broken)
Translucent
(partially broken)
Opaque
Yellow (broken)
0.07
0.12
84.6
98.0
(
partially broken)
Transparent
partially broken)
Yellow (broken)
(
3
4
12
18
Opaque
Opaque
Opaque (partially broken)
Opaque (intact)
0.12
0.16
145.3
164.2
Opaque
[
Zr]: 0.97 meq/g ꢀ 1.22 M, (HMTA, urea)/Zr = 0.44, time for mixing: 10 min, temperature: 5–10 8C, gelation temperature: 90 8C, washings: CTC
4
and 5% NH OH at room temperature, heat treatment: 500 8C.

S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
2941
Fig. 3. Adsorption desorption isotherm of zirconia microspheres prepared using 6% cetrimide.
Fig. 4. As cetrimide concentration increased further to 12 and 18%, the surface area values increased to 145.3 and
2
1
64.2 m /g and the pore volume increased to 0.12 and 0.16 cc/g, respectively. In the absence of cetrimide, pores with
radii more than 50 Å are minimal. However, as the cetrimide concentration increased from 6 to 18%, more pores with
radius range 50–150 Å were formed as seen in Fig. 4.
Fig. 4. Pore size distribution curve of zirconia microspheres prepared using different cetrimide content.

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S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
Table 3
Effect of washing technique during the preparation of zirconia microspheres
Sr. no.
Washing liquid
Washing
temperature ( 8C)
Physical appearance after heating at
Pore volume
(cc/g)
Sp. surface
area (m /g)
2
1
00 (8C)
500 (8C)
1
2
3
5% NH
4
OH
OH
25
80
80
Opaque
Opaque
Opaque
Opaque (intact)
Opaque (partially broken)
Opaque (partially broken)
0.091
0.068
0.18
138
108
268
H
2
O
5% NH
4
[
Zr]: 0.97 meq/g ꢀ 1.22 M, (HMTA, urea)/Zr: 0.44, time of mixing: 10 min, temperature: 5–10 8C, gelation temperature: 90 8C, cetrimide: 12 wt.,
heat treatment: 500 8C. The gel microspheres were washed with CCl in all cases.
4
3
.3. Washing of microspheres after gelation
Aqueous washing of the microspheres was carried out to remove the residual chemicals such as HMTA, Urea,
and ammonium chloride etc associated with the microspheres. If these are not removed, then decomposition of
these salts can result in breakage of the microspheres during drying and heat treatment. The microspheres were
washed 3 times using 5% NH OH at room temperature with contact time of 5–10 min Some batches were further
4
washed with 5% ammonia at 80 8C and with distilled water at 80 8C with contact time of 6 h. The washings of the
microspheres at room temperature with 5% NH OH resulted in opaque microspheres, which survived the heat
4
treatment.
The surface area of zirconia microspheres washed with 5% ammonia at room temperature, 5% ammonia at 80 8C
2
2
2
and with distilled water at 80 8C were 138 m /g, 268 m /g and 108 m /g, respectively as shown in Table 3. The values
of pore volume also varied in similar fashion with the type of washing schemes used. Washing with warm 5% NH OH
4
gave maximum porosity but these microspheres were partly cracked. For obtaining intact microspheres, the washing
scheme with 5% NH OH at room temperature was found to be the most effective.
4
3
.4. Heat treatment of microspheres
Effect of heat treatment on zirconia microspheres is given in Table 4. It can be seen that at 250 8C heating, removal
of cetrimide from spheres is not effective and hence the surface area and pore volume values are less. The black color
indicates deposition of carbon due to partial decomposition of cetrimide or residuals chemicals. At 500 8C, the
cetrimide got removed completely, leading to the increase in both surface area and pore volume. At higher
temperatures, i.e. at 800 and1000 8C, there is decrease in surface area as well as pore volume, due to sintering and
densification of the microspheres. The photomicrographs of zirconia microspheres heated at 500 and 1000 8C are
given in Fig. 5A and B, respectively and cross-sectional SEM micrograph of these microspheres shows porous nature
as seen in Fig. 5C and D respectively.
3
.5. Thermo-gravimetric studies
The zirconia microspheres were characterized by TG-DTA in temperature range 30–500 8C in air at a heating rate
of 28/min The microspheres containing 18% cetrimide, dried at 100 8C were used for analysis. In Fig. 6, T1 and H1
Table 4
Effect of heat treatment of zirconia microspheres
2
Sr. no.
Temperature ( 8C)
Physical appearance
Pore volume (cc/g)
Sp. surface area (m /g)
1
2
3
4
250
500
800
Black
White
White
White
0.016
0.162
0.040
0.014
66
164
64
1000
36
[Zr]: 0.97 meq/g, (HMTA, urea)/Zr = 0.44, time of mixing: 10 min, temperature: 5–10 8C, gelation temperature: 90 8C, cetrimide 18 wt.%,
4
washings: CTC and 5% NH OH at room temperature, rate of heating 28/min, soak time: 1 h.

S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
2943
Fig. 5. (A) Photomicrograph of Zirconia microspheres heated up to 500 8C. (B) Photomicrograph of Zirconia microspheres heated up to1000 8C (C)
SEM micrograph of Zirconia microspheres (cross-section) heated up to 500 8C (D) SEM micrograph of Zirconia microspheres (cross-section) heated
up to 1000 8C.
represent the TG and heat flow curves, respectively. It shows a weight loss of 7% up to 150 8C and this may be due to
evaporation of moisture. From 150 to 250 8C, a weight loss of ꢁ40% is observed due to volatilization of unreacted
chemicals such as urea, HMTA, ammonium chloride and part of cetrimide. The sharp exothermic peak seen in the
temperature range 250–280 8C with weight loss of 8% is due to ignition of cetrimide, which resulted in cracked
microspheres. Between 380 and 450 8C, there was 9% weight loss accompanied by broad exotherm due to oxidation of
residual organic carbon. At 500 8C, it attained constant weight with a total weight loss of 67%. In Fig. 6, curves T-2 and
H-2 respectively show the TG and heat flow of a similar sample in which soaks of 30 min duration each were given at
2
50, 270 and 280 8C. The prolonged soak at these temperatures gave enough time for organic material to go out of the
Fig. 6. Thermogravimetric curve of zirconia microspheres prepared using 18% cetrimide.

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S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
Fig. 7. XRD pattern of zirconia microspheres prepared using 18% cetrimide.
microspheres so that there was no sharp exothermic peak and breakage of the microspheres. Therefore, a similar
heating procedure was followed to obtained intact microspheres in further experiments.
3
.6. X-ray diffraction study
The zirconia microspheres containing 18% cetrimide and heated up to 500 8C were analyzed by STOE X-ray
diffractometer using Cu Ka radiation (l = 1.5406 Å). It showed broad hump between 22 and 38, as shown in Fig. 7,
indicating that material is amorphous in nature.
3
.7. Ion exchange behavior
Fig. 8 shows the pH titration curve of powdered zirconia microspheres prepared with 18% cetrimide and heated to
5
00 8C. The results show that the ion exchange property of the material is regenerable. The ion exchange capacity
obtained by titrating against 0.036 M KOH was 1.74 meq/g of zirconia microspheres, indicating good ion exchange
characteristics.
+
Fig. 8. Reversibility study of ion exchange and regeneration curve obtained for K ion exchange.

S.S. Pathak et al. / Materials Research Bulletin 43 (2008) 2937–2945
2945
4
. Conclusion
The present study has shown that porous zirconia microspheres, both physically and chemically stable, could be
prepared by the internal gelation method using the surfactant cetrimide as pore former. The microspheres heated to
+
5
00 8C showed high specific surface area and pore volume and exhibited reasonably high exchange capacity for K ion
exchange. This material has promise as good inorganic exchange material for the sorption and fixation of radioactive
materials and is also suitable as inert host material for transmutation of minor actinides.
Acknowledgements
The authors thank Dr S.K. Aggarwal, Head, Fuel Chemistry Division for his support and encouragement during the
course of investigations as well as for critical reading of manuscript. The authors also express their sincere thanks to
Shri R.V. Kamat for useful discussions and valuable suggestions during the course of the work and Shri Sunil Kumar
from Post Irradiation and Examination Division for providing excellent SEM micrographs of zirconia microspheres.
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