Effect of MgO-doping on solid-solid interactions in MoO3/Al2O3 system

Article abstract of DOI:10.1016/s0040-6031(98)00657-1

The effect of MgO-doping (2-10 mol%) on solid-solid interactions and phase transformation process in MoO₃/Al₂O₃ system were studied using thermogravimetry and X-ray diffraction (TG, DTG and XRD) techniques. The proportions of molybdena expressed as weight percent were 12.36, 22.01 and 41.37. The results obtained showed that the MgO-doping promoted the solid-solid interaction taking place at 500°C between Al₂O₃ and MoO₃ to produce Al₂(MoO₄)₃. However, MgO interacts readily with MoO₃ at temperatures starting from 500°C yielding MgMoO₄ which remains stable even when heating at 1000°C. The produced Al₂(MoO₄)₃ decomposed at temperatures starting from 800°C producing α-Al₂O₃ and MoO₃, a portion of which sublimed and the other portion dissolved in alumina matrix forming MoO₃-Al₂O₃ solid solution. MgO-doping decreased slightly the solubility of MoO₃ in Al₂O₃. The promotion of Al₂(MoO₄)₃ formation at 500°C and the decrease in the solubility of MoO₃ in Al₂O₃ by MgO have been attributed to dissolution of a small portion of MgO in the MoO₃ lattice with subsequent increase in the mobility of Mo⁶⁺ ions. The promotion effect of magnesia towards Al₂(MoO₄)₃ formation and dissolution of MoO₃ in Al₂O₃ solid are relatively small when compared to the observed effects reported in the case of Li₂O-doping of MoO₃/Al₂O₃ system. The limited effect of MgO-doping has been attributed to a limited solubility of MgO in MoO₃/Al₂O₃ system due to the formation of MgMoO₄.

Full text of DOI:10.1016/s0040-6031(98)00657-1

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Thermochimica Acta 327 (1999) 39±46
Effect of MgO-doping on solid±solid interactions
in MoO3/Al2O3 system
H.G. El-Shobakya,*, M. Mokhtarb, A.S. Ahmeda
aChemistry Department, Faculty of Science, Cairo University, Cairo, Egypt
bDepartment of Physical Chemistry, National Research Center, Dokki, Cairo, Egypt
Received 21 October 1998; accepted 25 November 1998
Abstract
The effect of MgO-doping (2±10 mol%) on solid±solid interactions and phase transformation process in MoO3/Al2O3
system were studied using thermogravimetry and X-ray diffraction (TG, DTG and XRD) techniques. The proportions of
molybdena expressed as weight percent were 12.36, 22.01 and 41.37. The results obtained showed that the MgO-doping
promoted the solid±solid interaction taking place at 5008C between Al2O3 and MoO3 to produce Al2(MoO4)3. However, MgO
interacts readily with MoO3 at temperatures starting from 5008C yielding MgMoO4 which remains stable even when heating
at 10008C. The produced Al2(MoO4)3 decomposed at temperatures starting from 8008C producing a-Al2O3 and MoO3, a
portion of which sublimed and the other portion dissolved in alumina matrix forming MoO3±Al2O3 solid solution. MgO-
doping decreased slightly the solubility of MoO3 in Al2O3. The promotion of Al2(MoO4)3 formation at 5008C and the decrease
in the solubility of MoO3 in Al2O3 by MgO have been attributed to dissolution of a small portion of MgO in the MoO3 lattice
with subsequent increase in the mobility of Mo6‡ ions. The promotion effect of magnesia towards Al2(MoO4)3 formation and
dissolution of MoO3 in Al2O3 solid are relatively small when compared to the observed effects reported in the case of Li2O-
doping of MoO3/Al2O3 system. The limited effect of MgO-doping has been attributed to a limited solubility of MgO in MoO3/
Al2O3 system due to the formation of MgMoO4. # 1999 Elsevier Science B.V. All rights reserved.
Keywords: Solid±Solid interactions; MgO-doping MoO3/Al2O3 system; TG; XRD
1. Introduction
in the disappearance of all X-ray diffraction lines of
MoO3 due to the formation of two±dimensional
Al2(MoO4)3 ®lm covering the surface of Al2O3 par-
ticles [6±8]. The increase in calcination temperature of
MoO3/Al2O3 to ꢀ5008C enhances the surface and
bulk mobilities of MoO3, leading to well-crystallized
Al2(MoO4)3 phase [9,10].
Molybdenum oxides loaded on an active Al2O3
support are one of the most important solid catalysts
[1±5]. The supported catalysts are usually prepared by
impregnation of an alumina support from an aqueous
solution of ammonium molybdate followed by ther-
mal treatment at suitable temperatures.
The metal±support interactions in the MoO3/Al2O3
system could be in¯uenced by doping with certain
The heating of physical mixture of crystalline
MoO3 and g-Al2O3 at 4008C for about 24 h resulted
‡
foreign cations such as Zn2‡, Ga3‡, Ge4‡, Li and
‡
Na [3,4,11,12]. This in¯uence may result from mod-
i®cation in the mobility of Mo6‡ ions in the MoO3
*Corresponding author.
0040-6031/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved.
P I I : S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 6 5 7 - 1

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40
H.G. El-Shobaky et al. / Thermochimica Acta 327 (1999) 39±46
lattice. It has been reported that doping of MoO3/
Al2O3 mixed solids either with Li2O [11] or Na2O [12]
enhanced metal±support interactions producing
Al2(MoO4)3 phase.
The present investigation reports a study of MgO-
doping on solid±solid interactions in the MoO3±Al2O3
system using TG and X-ray diffraction techniques.
These techniques permitted us to clarify the effect of
MgO-doping in the thermal behaviour of MoO3/Al2O3
mixed solids subjected to thermal treatment at tem-
peratures up to 10008C and to identify the different
crystalline phases produced by heating the mixed
solids at various temperatures.
2.2. Techniques
TG and DTG analyses of various uncalcined mate-
rials were carried out using a Perkin-Elmer (DT-7
Unix system) thermal analyser. A 18 mg solid portion
was taken in each TG measurement. The reference
material was a-Al2O3 (corundum). The rate of heating
1
was kept as 108C min and all the measurements
were carried out in a current of N2 ¯owing at
50 cm3 min
1
.
The X-ray powder diffraction patterns of the various
calcined samples were measured using Nippon denshi
kagaku. The patterns were run with nickel-®ltered Cu
Ê
Kꢀ radiation (ꢁˆ1.5405 A) at 40 kV and 30 mA at a
1
scanning rate of 2ꢂˆ88 min
.
2. Experimental
2.1. Materials
3. Results and discussion
A known mass of Al(OH)3, analytical grade sup-
plied by BDH, was impregnated with ammonium
paramolybdate (BDH) solutions containing three dif-
ferent proportions of (NH4)6Mo7O24Á4H2O. The pro-
portions of ammonium molybdate were calculated so
that the molar compositions of the calcined materials
were 0.1MoO3:Al2O3, 0.2MoO3:Al2O3 and 0.5MoO3:
Al2O3. The impregnated materials were dried at
1208C, then calcined at 5008C, 7008C, 9008C and
10008C. The magnesium oxide doping was effected
by treating the dried impregnated materials with an
aqueous Mg(NO3)2 solution prior to the calcination.
The time of heating of pure and doped materials was
®xed for 5 h, and the amount of magnesium were 2, 4
and 10 mol% MgO, respectively, (with respect to the
sum of Al2O3 and MoO3). The prepared mixed solid
specimens were designated as AlMo-I, AlMoI±2MgO,
AlMo-I±4MgO, AlMO-I±10MgO, AlMo-II, AlMo-
3.1. Thermal behaviour of pure and doped materials
TG and DTG curves of various pure and doped
materials are summarized in Table 1. Representative
TG and DTG curves are given in Figs. 1 and 2 for
AlMo-II and AlMo-III doped with 10 mol% MgO.
The recorded TG curves of the tested materials consist
of seven successive mass loss processes. The ®rst step
extended between room temperature and 1308C and
corresponds to departure of the physiosorbed water,
while the second one extended between 1308C and
1868C indicates the removal of water of crystalliza-
tion. The third and fourth steps were found between
186± 2178C and 217±283.98C, respectively. These
steps characterize the decomposition of Al(OH)3,
(NH4)6Mo7O24 and Mg(NO3)2 producing the corre-
sponding oxides according to
ꢁNH4†6Mo7O24 ‡ AlꢁOH† ‡ MgꢁNO3†
3
2
II±2MgO,
AlMo-II±4MgO,
AlMo-II±10MgO,
! 7MoO3 ‡ 1=2 Al2O3 ‡ MgO ‡ 2NO2
‡6NH3 ‡ 4:5H2O ‡ 1=2 O2 . . .
AlMo-III, AlMo-III±4MgO and Al Mo-III±10MgO.
The nominal molar compositions of the calcined
mixed solids were 0.1MoO3:Al2O3 (I), 0.2MoO3:
Al2O3 (II) and 0.5MoO3:Al2O3 (III) and contained
12.4, 22.01 and 41.37 wt% MoO3, respectively.
Al(OH)3 was used as a starting support material
due to its decomposition during the thermal transfor-
mation of molybdate (into molybdena) which may
provide chances for solid±solid interactions that was
not attainable on using Al2O3 as a support.
(1)
Details of the mechanism of thermal decomposition
of ammonium molybdate yielding MoO3 have been
given elsewhere [9]. The ®fth and sixth steps extended
were between 450±5508C and 550±8008C, respec-
tively. These steps might correspond to the removal
of strongly bound hydroxyl groups and the last traces
of these groups from alumina matrix. The last step

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H.G. El-Shobaky et al. / Thermochimica Acta 327 (1999) 39±46
41
Table 1
TG results of pure and doped materials
Solid
Weight loss (%)
MoO3 (wt%)
Total
100±8008C
100±8008C
(calculated)
800±10008C
Reacted
with MgO
Dissolveda
in Al2O3
AlMo-II
0.0-MgO
4-MgO
33
31.6
33.6
36.25
14.3
9.72
3.88
22.01
21.7
0
7.8
7.7
6.8
34.65
37.26
4.34
10.67
10-MgO
21.35
AlMo-III
0.0-MgO
4-MgO
29.93
30.1
28.8
30.4
32.7
25.29
23.28
18.67
41.37
40.95
40.36
0
16.08
14.40
13.60
3.27
8.09
10-MgO
33.25
aThe data of this column were computed by substracting the sum of the data of columns four and six from those of column five.
Fig. 1. TGA and DTG curves of uncalcined AlMo-II-10 MgO.
Fig. 2. TGA and DTG of uncalcined AlMo-III±10MgO sample.

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42
H.G. El-Shobaky et al. / Thermochimica Acta 327 (1999) 39±46
extended between 800±10008C characterize the sub-
limation of some of the molybdena produced.
The values of mass loss due to heating at the
temperature range 100±8008C for the various samples
were given in column two of Table 1. The theoretical
mass loss accompanying the thermal decomposition of
the tested materials producing the corresponding oxi-
des according to Eq. (1) were calculated and given in
column three of Table 1. The values given in the
columns two and three are close to each other for
the different investigated mixed solid samples which
indicates the thermal stability of MoO3 produced up to
8008C. The total weight loss accompanying the ther-
mal treatment at 800±10008C were calculated for each
sample and the values obtained are given in column
four of Table 1. It is seen from Table 1 (column four)
that the weight loss decreases progressively as a
function of the amount of MgO added. This might
indicate that the MgO present hinders the sublimation
of MoO3 to an extent proportional to its amount
present. This hinderance could be attributed to a
possible decrease in the mobility of the molybdena
species or formation of MgO±MoO3 compounds
which are thermally stable at 10008C. The formation
of such compounds will be identi®ed by XRD inves-
tigation given in the next section of this paper.
Fig. 3. X-ray diffractograms of pure and doped AlMo-III samples
precalcined at 5008C: (1) MoO3, (2) g-Al2O3, (4) MgMoO4, (5)
Al2(MoO4)3.
a-Al2O3 (corundum) phase for the solid sample pre-
heated at 10008C. These results are in agreement with
those published before [13,14]. Table 2 reveals that
the pure and MgO-doped mixed oxide solid samples
calcined at 5008C are composed of g-Al2O3 while that
preheated at 7008C consists of k-Al2O3 and that
calcined at 9008C and 10008C constitute of a-
Al2O3 phase except the doped AlMo-I sample cal-
cined at 9008C. This particular sample consists of a
mixture of a-, g- and q-aluminas. From the above
results it can be concluded that MoO3 catalyses the
crystallization process of Al2O3. In fact, the complete
transformation of pure Al2O3 specimen into the a-
form requires thermal treatment at elevated tempera-
tures >12008C [14±16], or the presence of certain
foreign oxides such as NiO [17], V2O5 [18], Co3O4
[19] and CuO [20]. These oxides enhance the crystal-
lization process of alumina into a-Al2O3 (corundum)
phase. In the present work the role of MoO3 in
catalysing the crystallization of Al2O3 into a-alumina
like the above oxides is clear.
3.2. XRD investigation of various solids
Preliminary experiments showed that the thermal
decomposition of ammonium molybdate in air at 500±
7008C resulted in the formation of well-crystallized
orthorombic MoO3 which sublimes completely by
heating at 8008C.
X-ray differactograms of pure and doped mixed
solids preheated in air at 5008C, 7008C, 9008C and
10008C and those of pure Al(OH)3 calcined at 500±
10008C were determined. Table 2 lists the different
phases present in various calcined solids. Representa-
tive differactograms of pure and doped AlMo-II and
AlMo-III samples are given in Figs. 3 and 4.
It is seen from Table 2 that Al2O3 heated at 500±
7008C is amorphous in nature. While that calcined at
8008C consists of poorly crystalline g-Al2O3. The X-
ray diffractograms of pure Al2O3 preheated at 9008C
and 10008C, not given, include the diffraction lines
characteristic for k-Al2O3 for the solids calcined at
9008C and k-Al2O3 together with a minute amount of
Inspection of the results obtained for pure and
doped mixed oxide solids calcined at 5008C revealed
that: