Gelification process to prepare phosphate modified alumina: Study on structure and surface properties

Article abstract of DOI:10.1016/j.jallcom.2006.09.102

P-modified alumina was prepared through gelification with phosphate compounds introduced during the formation of alumina gel. Common characterizations such as XRD, BET and IR were employed to investigate the effect of phosphate dopant on the structure and

Full text of DOI:10.1016/j.jallcom.2006.09.102

Journal of Alloys and Compounds 441 (2007) 311–316
Gelification process to prepare phosphate modified alumina:
Study on structure and surface properties
Weiwei Gua, Meiqing Shena, Xuefeng Changa, Yanhong Wangb, Jun Wangb,∗
a Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology,
Tianjin University, Tianjin 300072, PR China
b School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China
Received 21 June 2006; received in revised form 4 September 2006; accepted 26 September 2006
Available online 16 November 2006
Abstract
P-modified alumina was prepared through gelification with phosphate compounds introduced during the formation of alumina gel. Common
characterizations such as XRD, BET and IR were employed to investigate the effect of phosphate dopant on the structure and surface properties
of alumina. The results showed that the addition of phosphate could effectively improve thermal stability and textural properties of alumina.
For G5 sample, the ␣-phase formation temperature is enhanced up to 1573 K and the surface area retains 85 m2 g−1 as well as the pore volume
0.356 cm3 g−1 after calcination at 1473 K for 3 h. Moreover, the surface area and pore volume both have increasing trends with the increasing
phosphorous contents in alumina after calcinations at the same temperature. There is also an increasing trend of lattice constants of P-modified
aluminas with the increase of phosphorous content in alumina. Considering the defective spinel structure of ␥-alumina, it can thus be deduced that
some of the P5+ ions might occupy the vacancy of cationic site and interrupted the track of Al3+ ions transfer from tetrahedral to octahedral in the
processes of sintering and transformation.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Phosphate; Alumina; Gelification; Phase transformation; Thermal stability
1. Introduction
metal (La, Ce, Pr) [3–10], alkaline earth metal (Ba, Ca) [11–14],
silicon [15–17] and other oxides.
Active alumina as one of the most common catalyst support
has a good performance in the field of catalysis applications due
to its physical and chemical properties [1]. As catalyst support,
large surface area of ␥-alumina improves the dispersion degree
of these “active” elements and enhances the effect of the cat-
alyst. However, in some systems, for example the automotive
emission control system, high temperature, vapor surround-
ing and long-time operation always induce an undesired phase
transformation from ␥-alumina to the thermodynamically stable
corundum phase, which decreases its high surface area and leads
to a deactivation of the whole catalytic activity [2].
Phosphates have also been claimed to act as a support stabi-
lizer [18–25]. Gishti et al. [18] evidenced a stable effect of phos-
phate by studying the phase transitions of alumina in the range
673–1323 K. Abbattista et al. [19] found phosphate ions hin-
dered the transformation of amorphous alumina to crystallized
transition alumina as well as the transformation of ␥-alumina to
␣-alumina. But the surface area decreased with the increasing
content of phosphate in alumina, even less than the blank sample
aftercalcinatedat1073 K. Morterraetal. [20]alsoprovednosur-
face area stabilization effect from phosphates in the case of the
transition phase (spinel) aluminas. The positive effect appeared
only at high temperature.
The procedure for the synthesis of phosphate-modified
alumina mainly contained impregnation [21,22] and sol–gel
method [23,24]. Reductive decomposition of aluminium nitrate
in the presence of phosphoric acid could also obtain high-
surface-area P-modified alumina samples [19]. Mekhemer et al.
[22] prepared phosphate modified alumina with larger surface
It is found that addition of some elements can improve the
thermal stability of alumina, hold the surface area and improve
the surface acidity [2–17]. These elements contain rare earth
∗
Corresponding author. Tel.: +86 22 27892301; fax: +86 22 27892301.
E-mail address: wangjun@tju.edu.cn (J. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.09.102

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W. Gu et al. / Journal of Alloys and Compounds 441 (2007) 311–316
area by impregnation, and it was attributed to the small particles
after doping with phosphate ions. Quartararo et al. [23] used
aluminium tri(sec-butoxide) and orthophosphoric acid, H3PO4,
as the Al and P sources, respectively, 2-butanol as the solvent
and 1,3-butanediol as the chelating agent to prepare P-alumina
catalytic supports.
treated under vacuum (10−2 Pa) at 673 K for 1 h. After cooling down to room
temperature, the samples were exposed to pyridine vapor for 1 h. Then, IR spec-
tra were recorded after evacuation (10−2 Pa) for 0.5 h at different temperature
[27].
3. Results and discussion
Modification effect of phosphate on the surface acidity of
alumina had been studied a lot [19–22,26]. Phosphates play a
key role in stabilizing the amounts of the strong Lewis acid site
[20]. Some researchers [19,22] found existence of Brønsted acid
sites on phosphate-modified alumina when phosphate ions were
added to an alumina gel.
In this work, a new process to synthesis P-modified alumina
was introduced. The effect of phosphate on corresponding alu-
mina was discussed from the point of view of phase stability,
texture properties and surface chemistry.
3.1. Phase stability
XRD results of G0, G1, G3 and G5 after calcined at var-
ious temperatures are displayed in Fig. 1a–d. Fig. 1a shows
that G0 is an imperfect crystal of ␥-phase at 773 K, and then
transformed into ␥-phase with some ␪-phase at 1273 K. After
calcinated at 1373 K for 3 h, G0 had already turned into per-
fect ␣-phase, which is a common phenomenon for pure alumina
[28]. Comparing with blank sample (G0), phosphate modified
alumina (G1, G3, and G5) all have similar profiles like G0 at
773 K (shown in Fig. 1b–d), which show very amorphous com-
pounds. It is reported that short time digestion may decrease
the crystallinity of alumina [29]. However, there are obvious
differences for P-modified alumina at high temperature which
showed that the addition of phosphate had a positive effect to
inhibit the phase transformation of alumina. G1 still contains
lots of ␥-, ␪-phase and a few of ␣-phase at 1373 K and the phase
transformation into perfect ␣-phase was retarded up to 1473 K
(Fig. 1b). While at this temperature, G3 and G5 retain compa-
rable content of ␥-phase (Fig. 1c and d). It is noted that the
more phosphate we added during the preparation, the higher
phase stability we obtained in the final products. Even the tem-
perature increases to 1573 K, G5 still holds ␪-phase in certain
content while other samples have already turned into corundum
phase.
2. Experimental
2.1. Preparation
The phosphate modified alumina in the present work was obtained
through gelification method. First, some aluminum nitrate nonahydrate
(Al(NO3)3·9H2O) (98%, Tianjin Reagent Company) was dissolved in deionized
water at room temperature to obtain 0.5 M aluminum nitrate solution (100 ml).
Different amounts of (NH4)2HPO4 (98%, Tianjin Reagent Company) were
added into 100 ml ammonia (1:3). Secondly, the mixture was dripped into the
0.5 M aluminum nitrate solution (40 drip/min) with continuous stirring, adjusted
pH with ammonia (1:3) until 9, kept stirring for 1 h to gain the gel. Then, the
gel was placed in air for 24 h and digested in a water bath at 333 K overnight.
Finally, the samples were dried at 353 K until the weight kept unchanged, and
then calcined at 773 K, 1273 K, 1373 K, 1473 K, and 1573 K for 3 h, respec-
tively. In this way, the final products were obtained with three different mass
percentage of phosphor in alumina (P/Al2O3 = 1%, 3% and 5%).
A small peak at 2θ = 25◦ was investigated for phosphate
modified alumina which was confirmed as crystallized AlPO4
(JCPDS 72-1161) when the temperature increased to 1373 K.
This peak increases with the temperature and the content of
phosphorous. The previous work showed that the existence of
AlPO4 in ␥-Al2O3 was related to the inhibition of phase trans-
formation [19–21].
Blank samples were prepared without addition of (NH4)2HPO4 during the
first step, while other steps were the same as described above. All the phosphate
modified samples were marked as GTM, where M represented the mass percent-
age of phosphor in Al2O3 and T represented the calcination temperature. (For
example, G7173 was the sample contained 1 wt% of P in Al2O3 after calcinated
at 773K).
2.2. Characterization
In order to have a further understanding of the effect of phos-
phate addition to the crystal structure of alumina, we calculate
the lattice constants of G0, G1, G3 and G5 samples with the calci-
nation at 1273 K (all the phosphate modified alumina contained
most of the amorphous phase at 773 K and the sample G0 had
already transformed into ␣-phase at 1373 K) using following
formula:
X-ray diffraction patterns were taken in a Philips X’PERT MPD diffrac-
˚
tometer (The Netherlands) using the Co K␣ (λ = 1.7890 A, tube voltage:
40 kV, tube current: 40 mA) at room temperature. XRD pattern scanned within
the range 2θ = 10–90◦. Identification of the different crystalline phases was
performed by comparison with the corresponding JCPDS diffraction data
cards.
BET surface area of sample was measured with the NOVA-2000 instru-
ment of surface area and pore size distribution (Quanta chrome, USA) at
liquid-N2 temperature. Physical nitrogen adsorption at −196 ◦C occurred at
a relative pressure (P/P0) between 0.05 and 0.998. Samples were subjected
to control heating at 200 ◦C under vacuum until degasification was achieved.
Nitrogen was used as the adsorbent. The surface area was determined by the
BET method and the pore size distribution by the Barret–Joyner–Halenda (BJH)
method.
FTIR analysis of the catalysts was carried out over the frequency range
4000–400 cm−1, using a NICOLET model 560 spectrometer. Samples for the
IR measurements were pressed into self-supporting wafers with a diameter of
2 cm in an IR-cell for investigation. The spectra were recorded from thin, lightly
loaded KBr-supported discs of the test samples at room temperature. In situ IR
spectra of adsorbed pyridine (Py) was derived from a spectra taken from sam-
ples pressed into self-supporting discs, which were placed in an IR cell, and
1
d2
h2
a2
k2
b2
l2
c2
=
+
+
(1)
where d is the crystal plane spacing, a, b and c the lattice con-
stants, and h, k and l are the miller indices. As a = b = c for
␥-alumina (cubic system), the formula can be simplified and
transformed into:
ꢀ
a = d h2 + k2 + l2
(2)
The results are simulated in Fig. 2 and appear a gradu-
ally enhancement with the increasing contents of phosphorus.

W. Gu et al. / Journal of Alloys and Compounds 441 (2007) 311–316
313
Fig. 1. XRD patterns of G0 (a), G1 (b), G3 (c) and G5 (d) at various temperatures.
It is supposed that part of the phosphorous ions might insert
into the bulk of alumina to extend the lattice of alumina. The
more phosphate added, the better effect of phase stability is
obtained.
␥-Alumina was proven to have a defective spinel structure
[30]. To satisfy the Al2O3 stoichiometry of aluminum oxide,
however, in ␥-alumina 22/3 of the 24 cation sites need to be
vacant. The formula for ␥-alumina is, therefore, typically given
as ꢀ8/3Al64/3O32, where ꢀ stands for a vacancy [31]. NMR
investigation proved most phosphorous atoms were close to alu-
minum atoms in P-modified alumina [25], Meanwhile, the P5+
ion occupies the tetrahedral (Td) sites in phosphate and P5+
ionic radius (0.017 nm) is less than Al3+ ionic radius (0.039 nm)
greatly, which made it possible to occupy the vacancies in the
cationic sites of alumina. During the phase transformation, alu-
minum ions migrate from tetrahedral to octahedral in order to
increase the degree of crystal order [32]. The P5+ ions in the
tetrahedral (Td) sites may interrupt the track of Al3+ transfor-
mation and make it more difficult for the migration of aluminum
ions than that of pure alumina [33]. In this way, the phase trans-
formation can be delayed to a higher temperature. With the
temperature increasing, the P5+ ions were compelled to leave
tetrahedral site to surface and form AlPO4. Therefore, there were
signals of AlPO4 observed in XRD spectra after calcinations
above 1373 K.
Fig. 2. Lattice constants of GM vs. phosphorous content followed by calcination
at 1273 K.

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W. Gu et al. / Journal of Alloys and Compounds 441 (2007) 311–316
Table 1
Besides the largest surface area, G5 also kept the largest pore
The surface area (m2 g−1) of all samples (GMT
)
volume (Table 2), which is two times more than that of G0 at
lowtemperature(773 K). Othertwosamples(G1, G3)havelarger
pore volume than G0 in some sort. With the temperature increas-
ing, all samples lost their pore volume. However, there is a trend
that the pore volume increases with phosphate content irrespec-
tive of the calcination temperature. To our unexpected, G5 still
held 0.356 cm3 g−1 pore volume at 1473 K.
T (K)
M (wt%)
0
1
3
5
773
1273
1373
1473
1573
217.1
85.1
13.0
8.5
285.5
114.7
62.6
38.7
4.5
309.4
135.5
106.6
69.0
329.5
198.0
143.5
85.3
It is reported that phosphorous introduction step determines
the structure of phosphate-modified aluminas after calcinations
[23]. The impregnation of (NH4)2HPO4 solution on alumina or
alumina gel had an opposite effect on textural properties [22].
In this work, the phosphate was introduced during the forma-
tion of alumina gel. At the step of dripwise and continuous
stirring, aluminum nitrate, ammonia and phosphate adequately
mixed. When the temperature increased, nitrate and ammo-
nium decomposed and left the micropore, which resulted in the
enhancement of surface area and the pore volume. The phos-
phate may be adequately dispersed in the system and no signal
of aluminum phosphate appeared in XRD after the first calcina-
tion at 773 K. When calcined at higher temperature, phosphate
acts as a structure stabilizer and gradually inhibits the sintering
and phase transformation. Therefore, G5 still hold large surface
area (85 m2 g−1) and pore volume (0.356 cm3 g−1) at 1473 K.
Table 3 lists the average pore diameter of all samples with
the increasing temperature. G5 has a larger pore diameter about
10 nm at 773 K, whereas the other three samples are around 5 nm
in this case. The pore diameter of G0 increased at 1273 K, and
suddenly decreased at higher temperature. For G1 and G3, there
are increasing trends until 1473 K. G5 always keeps the aver-
age pore diameter around 15 nm in the range of 1273–1473 K.
Fig. 2a–e shows the pore size distribution of G0, G1, G3 and G5
after the calcinations temperatures at 773 K, 1273 K, 1373 K,
1473 K and 1573 K, respectively. At 773 K, G0, G1, and G3 have
similar pore distributions with most of their pores concentrating
around 5 nm. G5 concentrates around 15 nm. This is consistent
with the average pore diameter results. When the temperature
increased to 1373 K, G0 had already destroyed its pore struc-
ture (Fig. 3c). All the P-modified aluminas have good control
over pore structure until 1473 K. They encountered completely
damage of their pore structure when temperature increased up
to 1573 K (Fig. 3e). Associated with the surface area, pore
volume and average pore diameter analysis, these evidences
prove the positive effect of phosphate on the pore structure of
alumina.
5.0
3.4
10.7
Table 2
Pore volume (cm3 g−1) of all the samples (GMT
)
T (K)
M (wt%)
0
1
3
5
773
1273
1373
1473
1573
0.318
0.251
0.029
0.013
0.008
0.384
0.315
0.239
0.211
0.007
0.445
0.339
0.294
0.251
0.005
0.854
0.749
0.620
0.356
0.022
3.2. Textural properties
The textural properties of P-alumina samples were studied by
BET analysis and pore structure analysis. The results are listed
in Tables 1–3 and Fig. 3a–e.
Table 1 shows that large surface area is obtained for blank
sample prepared by gelification after calcinated at 773 K for 3 h.
This surface area is larger than Morterra’s result [20] at same
calcinations temperature, which may be due to the preparation
procedure. G0 seems to have the poor thermal stability in that
more than 90% of primary surface areas loss happened after cal-
cinated at 1373 K, which consists with XRD result. P-modified
aluminas have larger surface areas than G0 after calcination at
773 K. Even at 1373 K, they all retain surface area in a large
number. It is also found that the increase of surface area by
the raising phosphorous content is accomplished for all sam-
ples at each calcination temperature except for 1573 K. This
behavior is just opposite to previous works [19] that reported
a decrease of surface area by increasing phosphate content in
aluminas (treated at the same calcinations temperature). After
calcination at 1573 K, associated with the XRD results, the sur-
face structure of G1 and G3 samples were damaged completely
and the phase had already transformed into corundum phase.
Only G5 sample has a positive performance.
3.3. Surface chemistry
Table 3
Average pore diameter (nm) of all the samples (GMT
)
The FTIR spectra (KBr disk) of the samples G0, G1, G3 and
G5 calcined at 773 K are compared in Fig. 4. In all cases a strong
band is found centred in the region 500–750 cm−1, due to Al–O
stretchings [34]. In addition, the phosphate modified alumina
also contains a broad adsorption nearly at 1094 cm−1, due to
P–O stretchings [19]. The –OH region (3900–3200 cm−1) in P-
modified aluminas were reported including the vibration of Al
(IV, VI)-OH and P–OH bands [22], where contained a complex
of four to seven closely overlapped bands that attributed to the
T (K)
M (wt%)
0
1
3
5
773
1273
1373
1473
1573
5.86
11.81
8.89
6.29
6.25
5.371
5.75
10.37
15.14
17.27
16.68
8.27
10.98
15.26
21.77
5.80
10.01
11.05
14.56
6.05

W. Gu et al. / Journal of Alloys and Compounds 441 (2007) 311–316
315
Fig. 3. Pore diameter distribution of GM at various temperature: (a) 773 K, (b) 1273 K, (c) 1373 K, (d) 1473 K, (e) 1573 K with amplifications in (d) and (e).
free or quasi-free surface OH groups for transition aluminas
[34]. The band at 1644 cm−1 was generated by the stretching
vibration of the OH− ions of residual water [35].
tively the most sensitive 8a mode of vibration of two different
Py adsorbed species. The higher frequency mode was due to Py
species coordinated to Lewis sites of medium acidity, involving
both octahedral and tetrahedral sites. Another mode was due to
Py coordinated to the unsaturated nearly octahedral Al3+ ions,
which was weak acidity site. The other three bands were also due
to Py species coordinated to Lewis acid sites. Moreover, there
appears an obvious enhancement at 1444 cm−1 and 1593 cm−1
for G5 which may be related to its higher surface area and not to a
Fig. 5 compares the in situ IR spectra of G7073 and G5773
sample with the pyridine adsorption. There are five peaks at
1444 cm−1, 1490 cm−1, 1577 cm−1, 1593 cm−1, 1612 cm−1
,
all of which are attributed to the hydrogen bonded pyridine and
pyridine coordinating to Lewis acid sites. According to Morterra
and Magnacca [34], the 1612 cm−1 and 1593 cm−1 were rela-

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W. Gu et al. / Journal of Alloys and Compounds 441 (2007) 311–316
Acknowledgements
This paper was supported by the Programme of Introduc-
ing Talents of Discipline to Universities, NO. B06006 and
the Program for New Century Excellent Talents in Univer-
sity. The authors are also grateful to the Science and Technol-
ogy Association of Tianjin of China for its financial support
(05YFGPGX08000).
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4. Conclusions
1. Inthiswork, P-modifiedaluminaisobtainedbyanewprocess
that adding the phosphate compounds during the formation
of alumina gel. The addition of phosphate could effectively
improve thermal stability and textural properties of alumina.
For G5 sample, the ␣-phase formation temperature of alu-
mina is enhanced up to 1573 K and the surface area retains
85 m2 g−1 as well as the pore volume 0.356 cm3 g−1 after
calcination at 1473 K for 3 h. Moreover, the surface area and
pore volume both have increasing trends with the increasing
phosphorous contents in alumina. The steps of introducing
phosphate compounds during the formation of alumina gel
are the key for the phosphate to act as a good phase stabilizer
and structure modifier.
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2. The surface of ␥-alumina prepared by gelification mainly
forms Lewis acidic site, which can be not affected by the
addition of phosphate ions to alumina.