Synthesis and characterization of mesoporous and nano-crystalline phosphate zirconium oxides

Article abstract of DOI:10.1016/j.jallcom.2008.08.094

In this work the preparation and characterization of the materials such as zirconia (ZrO₂) and zirconia promoted with phosphate ion (ZrO₂-PO₄^3-) is presented. Pure zirconium hydroxide [Zr(OH)₄] was synthesized by the sol-gel method using precursors such as zirconium n-butoxide and 1-butanol maintaining a pH 8 during the synthesis. Zr(OH)₄ was impregnated with 15 wt.% of the acid agent. Both were calcined in a dynamic air atmosphere for 3 h at 400, 500 and 600 °C. The supports were characterized by thermal analysis, X-ray diffraction, nitrogen physisorption as well as infrared spectroscopy. The results showed a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)₄ impregnation with the dopping agent (H₃PO₄). Phosphate zirconium oxides remained thermically stable after calcination. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface, inhibiting the particle growth and delaying the syntherization of the material and the apparition of the monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size 1.0-6.5 nm) with high surface areas (210-329 m²/g) and tetragonal structure defined for the calcination temperature of 600 °C.

Full text of DOI:10.1016/j.jallcom.2008.08.094

Journal of Alloys and Compounds 483 (2009) 425–428
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of mesoporous and nano-crystalline
phosphate zirconium oxides
J.M. Hernández Enríqueza,∗, L.A. Cortez Lajasa, R. García Alamillaa,
A. Castillo Maresa, G. Sandoval Roblesa, L.A. García Serranob
a Instituto Tecnológico de Cd. Madero, División de Estudios de Posgrado e Investigación, Juventino Rosas y Jesús Urueta S/N, Col. Los Mangos, 89440 Cd. Madero, Tam., Mexico
b Instituto Politécnico Nacional, Escuela Superior de Ingeniería Textil, Av. Instituto Politécnico Nacional s/n, Edificio #8, Col. Linda Vista,
Delegación Gustavo A. Madero, 07738 México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work the preparation and characterization of the materials such as zirconia (ZrO2) and zirconia
promoted with phosphate ion (ZrO2–PO43−) is presented. Pure zirconium hydroxide [Zr(OH)4] was syn-
thesized by the sol–gel method using precursors such as zirconium n-butoxide and 1-butanol maintaining
a pH 8 during the synthesis. Zr(OH)4 was impregnated with 15 wt.% of the acid agent. Both were calcined
in a dynamic air atmosphere for 3 h at 400, 500 and 600 ◦C. The supports were characterized by thermal
analysis, X-ray diffraction, nitrogen physisorption as well as infrared spectroscopy. The results showed
a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)4 impregnation
with the dopping agent (H3PO4). Phosphate zirconium oxides remained thermically stable after calcina-
tion. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface,
inhibiting the particle growth and delaying the syntherization of the material and the apparition of the
monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size 1.0–6.5 nm) with
high surface areas (210–329 m2/g) and tetragonal structure defined for the calcination temperature of
Received 30 August 2007
Received in revised form 25 July 2008
Accepted 6 August 2008
Available online 11 December 2008
Keywords:
Nanostructured materials
Oxide materials
Sol–gel processes
Crystal growth
Crystal structure
600 ◦C.
© 2008 Published by Elsevier B.V.
2−
1. Introduction
SO4
[9,10]. For that, in this work it is presented the synthesis
of zirconium oxides by the sol–gel method, modified this one with
the presence of the phosphate ion (PO43−) to study the possible
influence of this parameter as a synthesis variable on the thermal,
textural and structural properties.
Zirconia is an important material that has attracted enor-
mous interest in catalytic process such as paraffin isomerization,
hydrogenation of olefins, alcohol dehydrogenation and other tech-
nological uses [1–4]. The use of the zirconia as a support material
or catalyst often requires a high accessible surface area as well
as a large and well-developed porous texture. However, zirconia
oxides generally have surface areas of 50 m2/g or less, which is
rather low compared with conventional supports such as SiO2 or
Al2O3. Higher surface areas are attainable with amorphous zirco-
nia (200–300 m2/g), but this was usually achieved at the expense
of much lower thermal stability [5–8]. On the other hand, zirco-
nia exists in three crystallographic phases: monoclinic, tetragonal
and cubic. Among these phases, the tetragonal phase is preferable
for some acid-catalyzed reactions (paraffin isomerization). Efforts
have been made to partially or fully stabilize the tetragonal phase
with enhanced surface area by some additives such as Y2O3 or
2. Experimental
2.1. Supports preparation
The Zr(OH)4 preparation was carried out by the sol–gel method dissolving zirco-
nium n-butoxide in 1-butanol taking place the hydrolysis and condensation of this
by a slow drop addition of a water/1-butanol solution maintaining a pH 8 during the
synthesis. Later, the gel was aged for 72 h and dried at 120 ◦C for 24 h. The impregna-
tion of zirconium hydroxide (Zr(OH)4) with the acid agent (PO43−) took place using
the wet incipient technique adding the necessary amounts of a phosphoric acid
3−
solution (H3PO4, 2N) that contained the PO4 ion to obtain a 15 wt.% of the acid
agent present in the support. The modified hydroxides were dried at 120 ◦C for 24 h,
calcined afterward in a dynamic air atmosphere at 400, 500 and 600 ◦C for 3 h. For
pure zirconia calcined at 400, 500 and 600 ◦C its nomenclature is: Z400, Z500 and
Z600 and for phosphate zirconia calcined at 400, 500 and 600 ◦C its nomenclature
is: ZP400, ZP500 and ZP600.
2.2. Supports characterization
∗
Corresponding author. Tel.: +52 833 215 8544; fax: +52 833 215 8544.
E-mail address: jmanuelher@hotmail.com (J.M. Hernández Enríquez).
The thermogravimetric and differential thermal analysis took place in a ther-
mogravimetric balance TA Instruments STD 2960 Simultaneous DSC-TGA; samples
0925-8388/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2008.08.094

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J.M. Hernández Enríquez et al. / Journal of Alloys and Compounds 483 (2009) 425–428
3−
Fig. 1. TG–DTG profiles of Zr(OH)4.
Fig. 3. TG–DTG profiles of Zr(OH)4–PO4
.
were analyzed in air flow (10 ml/min) with a heating rate of 10 ◦C/min in the temper-
ature range of 25–900 ◦C. Nitrogen physisorption was used to determine the surface
areas of the materials at the temperature of liquid nitrogen (−196 ◦C) in a Quan-
tachrome Autosorb-1 instrument; previous to the analysis, samples were degasified
for 2 h at 350 ◦C. X-ray diffraction patterns were obtained in a Bruker Advance D800
diffractometer with Cu K␣ radiation (ꢀ = 1.5406 Å) and a graphite secondary beam
monochromator; the intensities of the diffraction lines were obtained in the 2ꢁ
range between 20◦ and 70◦ with a step of 0.02◦ and a measuring time of 2.7 s per
point. Infrared spectroscopy was carried out in a Fourier transforms spectrometer
(PerkinElmer Spectrum One) with transparent pills containing the sample to be ana-
lyzed and KBr as binder (90 wt.%) using a number of scanning of 16 and a resolution
ZrO2 [14]. When the sample was annealed above 500 ◦C, no further
weight loss was observed. In the DTA curve, however, the shift of
the base line shows that the thermal energy was produced during
the calcination, which can be related to transformation from the
tetragonal into the monoclinic phase.
On the other hand, the TG–DTG profiles of the Zr(OH)4–PO4
also presented the same weight loss stages (Fig. 3). The origin of
these stages and peaks are similar to the explanation for the respec-
3−
tivecurves ofthe Zr(OH)4, however, the onlydifferencebetweenthe
of 4 cm−1
.
3−
samples was the minor total weight loss for the Zr(OH)4–PO4
.
This fact could be due to thermal stability of the phosphate ions
to zirconium linkages is much higher than that hydroxyl groups, it
has also been determined into Zr(OH)4–SO4 [14]. Then removal
3. Results and discussion
2−
of the phosphate ions of the zirconia’s surface needs a higher tem-
perature, it is impossible at lower temperature which delays the
formation of a new crystalline phase and the growth crystallite too.
The endothermic peak that appears at 489 ◦C on the DTA curve for
the Zr(OH)4–PO4 is related about that (Fig. 4). This point can be
demonstrated by XRD results of the phosphate zirconia.
3.1. Thermal analysis (TGA–DTA)
Figs. 1 and 2 show the TG–DTG and TG–DTA profiles of the
Zr(OH)4, respectively. Three weight loss stages were observed in the
temperature range of 25–500 ◦C. The largest one located between
25 and 200 ◦C can be attributed to the loss of physically adsorbed
water and the evaporation of 1-butanol, this weight loss stage also
is related with the first endothermic peak in the DTA profile located
at 50 ◦C [11]. The second stage was highly exothermic (200–350 ◦C),
originating from the combustion of the organic compounds in the
sample (alcoxi groups) two peaks located in the DTA profile at 258
and 285 ◦C [12,13]. The last weight loss stage observed between 350
and 450 ◦C, and the third exothermic peak registered in the DTA
curve at 403 ◦C corresponded to the decomposition of Zr(OH)4 into
3
3.2. X-ray diffraction (XRD)
The XRD patterns of the samples ZrO2 and ZrO2–PO43− annealed
at 400, 500 and 600 ◦C are shown in Figs. 5 and 6, respectively.
The results show that there is a difference between pure and acid
zirconia and also a change in the crystallite size. In the first case,
tetragonal and monoclinic phases coexisted [15,16]. Their concen-
3−
Fig. 2. TG–DTA profiles of Zr(OH)4.
Fig. 4. TG–DTA profiles of Zr(OH)4–PO4
.

J.M. Hernández Enríquez et al. / Journal of Alloys and Compounds 483 (2009) 425–428
427
3−
Fig. 7. Surface area versus crystallite size related to ZrO2 and ZrO2–PO4 samples.
Fig. 5. XRD patterns of ZrO2 calcined at 400, 500 and 600 ◦C.
temperature increases, the diffraction peaks tend to sharpen, and
this suggests a further enlargement of the crystallite size (Table 1).
However, the calcination at higher temperatures (600 ◦C) did not
change the XRD pattern phase of the zirconia except for the peaks
becoming sharper and stronger.
3.3. Surface area measurement (BET)
Table 1 and Fig. 7 report the surfaces areas and the crystallite size
3−
of the ZrO2 and ZrO2–PO4 as a function of the calcination tem-
perature. The first striking difference appears from the comparison
between the areas of the two series of supports. The area pertain-
ing to the ZrO2–PO4 calcined at 600 ◦C is about 11 times larger
3−
than the one of the ZrO2 calcined at the same temperature. Further,
also the trend with the temperature of the two series of supports is
different. Pure zirconia shows a sharp and continuous decrease in
the surface area with the temperature. Phosphate zirconia, instead,
exhibits an almost invariant pattern and the calcination tempera-
ture only provokes a slight decrease in area. The present larger areas
of the ZrO2–PO43− are to be related, in the first instance, to the pres-
ence of the PO43− ion that improves the thermal stability into ZrO2
and these results also suggest that the interaction of the acid agent
with the zirconia surface inhibits the strong syntherization on the
material. This fact can be demonstrated by the TG–DTA analysis. In
general, the samples of smaller grain size showed a higher surface
area.
3−
Fig. 6. XRD patterns of ZrO2–PO4 calcined at 400, 500 and 600 ◦C.
trationchangedwiththeannealingtemperature. At400 ◦Cthemain
phase is tetragonal, but when the temperature was increased, the
tetragonal phase concentration decreased with an increase in con-
centration on monoclinic phase and also a fast crystallite growth
was observed (Table 1). At 600 ◦C the monoclinic phase concen-
tration was 100 wt.%. In the second case, the transformation of
tetragonal into monoclinic phase is delayed due to doping agent
3.4. Infrared spectroscopy (FT-IR)
(PO4 ion). Before at 600 ◦C, the broad diffraction patterns were
observed, indicating a characteristic of tetragonal zirconia nanopar-
ticles with a low crystallinity [11,13,17]. When the calcination
FT-IR spectra for the zirconia and phosphate zirconia are shown
in Figs. 8 and 9, respectively. In the different FT-IR spectra of the
unmodified zirconia it is possible to observe in the region between
3−
Table 1
3−
Synthesis variables and textural properties of ZrO2 and ZrO2–PO4
.
Material
Calcination temperature (◦C)
Acid agent
Surface area (m2/g)
Crystallite size (nm)
Z400
Z500
Z600
ZP400
ZP500
ZP600
400
500
600
400
500
600
–
–
–
144
73
18
329
256
210
4.3
6.9
16.2
1.0
1.1
6.5
H3PO4
H3PO4
H3PO4
Crystallite size determined by X-ray diffraction.

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J.M. Hernández Enríquez et al. / Journal of Alloys and Compounds 483 (2009) 425–428
organic compound bands tend to disappear and the Zr–O signals
increasing their intensity.
On the other hand, the FT-IR spectra of modified zirconia
(ZrO2–PO43−) exhibit peaks in the region of 1300–850 cm−1, where
pure ZrO2 does not show any bands. The spectra only show a
broad-sharp band with peak maxima at 1050 cm−1 characteristic
phosphorous–oxygen (P–O) stretching modes in the phosphate ion
[21]. But it is not possible to determine the exact structure of this
species from this data. However, by analogy with the structures
determined for ZrO2–SO42−, the formation of similar structure for
3−
ZrO2–PO4 is expected.
4. Conclusions
The results showed a positive effect on the physicochemical
properties of the catalytic supports after Zr(OH)4 impregnation
with the dopping agent (H3PO4). Phosphate zirconium oxides
remained thermically stable after calcination. It was observed that
the dopping agent remained firmly attached to the zirconium
oxide surface, inhibiting the particle growth and delaying the syn-
therization of the material and the apparition of the monoclinic
phase, obtaining mesoporous and nano-crystalline materials (crys-
tallite size, 1.0–6.5 nm) with high surface areas (210–329 m2/g)
and tetragonal structure defined for the calcination temperature
of 600 ◦C. These materials are considered as promising supports
for potential catalytic applications.
Fig. 8. FT-IR spectra of ZrO2 calcined at 400, 500 and 600 ◦C.
Acknowledgements
The authors acknowledge Instituto Tecnológico de Cd. Madero
for the facilities used in carrying out this work and CONACYT for
its financial support through scholarship 181668. The authors also
acknowledge the financial assistance via project COSNET-479.04P.
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