Structural and texture evolution with temperature of layered double hydroxides intercalated with paramolybdate anions

Article abstract of DOI:10.1021/ic0508674

Paramolybdate-LDHs with MgAl or ZnAl cations within the layers have been prepared by the ion-exchange method from hydrotalcites with different interlayer anions (OH^-, NO₃^-, and terephthalate). The samples and the oxides obtained after their calcination were characterized by element chemical analysis, PXRD, FT-Raman spectroscopy, thermal analysis (TG/DTA), N₂ adsorption at -196°C, and SEM. The results show that layered solids with hydrotalcite-type structure were obtained in which the interlayer space is occupied by heptamolybdate with a small amount of MoO ₄^2- units formed through hydrolysis of the polyanion; both oxomolybdenum species undergo a progressive distortion of the octahedral units from 50°C but are roughly stable up to 250°C as a consequence of the interaction between the polyanion and the brucite-like layers. This distortion is responsible for the observed decrease in the height of the gallery for samples heated in the temperature range, 50-250°C, with respect to the original samples. Rehydration of the calcined solids allows recovering of their original structures and the initial values for the gallery heights. Calcination between 300 and 400°C gives rise to a collapse of the layered structure, and amorphous phases are formed, in which molybdenum is both octahedrally and tetrahedrally coordinated. Crystalline magnesium and zinc molybdates (MgMoO ₄ and ZnMoO₄) are formed at 450 and 600°C, respectively. All solids have some microporosity, which decreases with increasing the calcination temperature.

Full text of DOI:10.1021/ic0508674

Inorg. Chem. 2006, 45, 1243−1251
Structural and Texture Evolution with Temperature of Layered Double
Hydroxides Intercalated with Paramolybdate Anions
D. Carriazo,† C. Domingo,‡ C. Mart ´ı n,† and V. Rives*,†
Departamento de Qu ´ı mica Inorg a´ nica, UniVersidad de Salamanca, 37008-Salamanca, Spain, and
Instituto de Estructura de la Materia, CSIC, Serrano 123, 28006-Madrid, Spain
Received May 30, 2005
Paramolybdate-LDHs with MgAl or ZnAl cations within the layers have been prepared by the ion-exchange method
-
-, and terephthalate). The samples and the oxides
from hydrotalcites with different interlayer anions (OH , NO
3
obtained after their calcination were characterized by element chemical analysis, PXRD, FT-Raman spectroscopy,
thermal analysis (TG/DTA), N adsorption at 196 C, and SEM. The results show that layered solids with hydrotalcite-
type structure were obtained in which the interlayer space is occupied by heptamolybdate with a small amount of
2
−
°
2
-
MoO
distortion of the octahedral units from 50
between the polyanion and the brucite-like layers. This distortion is responsible for the observed decrease in the
height of the gallery for samples heated in the temperature range, 50 250 C, with respect to the original samples.
Rehydration of the calcined solids allows recovering of their original structures and the initial values for the gallery
heights. Calcination between 300 and 400 C gives rise to a collapse of the layered structure, and amorphous
phases are formed, in which molybdenum is both octahedrally and tetrahedrally coordinated. Crystalline magnesium
and zinc molybdates (MgMoO and ZnMoO ) are formed at 450 and 600 C, respectively. All solids have some
4
units formed through hydrolysis of the polyanion; both oxomolybdenum species undergo a progressive
°
C but are roughly stable up to 250 C as a consequence of the interaction
°
−
°
°
4
4
°
microporosity, which decreases with increasing the calcination temperature.
-7
8
Introduction
catalysis.5 Their strong Br o¨ nsted type acidity and their
redox properties make them attractive in different catalytic
reactions. However, they are hardly recyclable and their
specific surface area is usually rather low, so giving rise to
a low density of active sites. One way to overcome this
problem is to immobilize them on different solid supports,
Layered double hydroxides (LDHs), also known as anionic
clays or hydrotalcite-type (HT) compounds, form a group
2
+
3+
n-
of minerals with the general formula [M 1-x
M
x
(OH)
2
]Ax/n
‚
O. The M2 and M cations are located in [M(OH)
+
3+
]
mH
2
6
edge-sharing octahedra with the brucite structure, and anions
9-12
such as LDHs, forming hybrid materials.
Pinnavaia et al. intercalated decavanadate and Keggin
ions (XM12O40 ) using the nitrate form of a Zn,Al hydro-
n-
(
A ) are located, together with water molecules, in the
13
1-4
interlayer region.
Many metal cations are able to form
n-
this sort of compound, and consequently, a very large number
of compounds with this structure have been synthesized in
recent years, varying the nature and molar ratio of cations
in the interlayer and the nature of the interlayer anions.
Polyxometalates (POMs) display a large structural diversity
and are largely used in medicine, materials science, and
talcite as a precursor. Afterward, several authors reported
preparation of LDH-POMs systems by reconstruction of the
1
-4
(5) Katsoulis, D. E. Chem. ReV. 1998, 98, 359.
(
(
6) Mizuno, N.; Misono, M. Chem. ReV. 1998, 98, 199.
7) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171.
(8) Furuta, M.; Sakata, K.; Misono, M.; Yoneda, Y. Chem. Lett. 1979,
31.
(
9) Misono, M. Catal. ReV.sSci. Eng. 1987, 29, 269.
*
To whom correspondence should be addressed. E-mail: vrives@usal.es.
Universidad de Salamanca.
Instituto de Estructura de la Materia.
(10) Rozheunikov, I. V.; Sinnema, A.; Jansen, R. J. J.; Pamin, K.; van
Bekkum, H. Catal. Lett. 1995, 30, 241.
(11) Kresge, C. T.; Marler, D. O.; Rav., G. S.; Rose, B. H. U.S. Patent
5,366,945, 1994.
†
‡
(
(
(
(
1) Reichle, W. T. Solid State Ionics 1986, 22, 135.
2) Cavani, F.; Trifir o´ , F.; Vaccari, A. Catal. Today 1991, 11, 1.
3) Rives, V.; Ulibarri, M. A. Coord. Chem. ReV. 1999, 181, 61.
4) Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova
Sci. Pub., Inc.: New York, 2001.
(12) Schwegler, M. A.; van Bekkum, H.; de Munck, N. A. Appl. Catal.
1991, 74, 191.
(13) Kwon, K.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988,
110, 3653.
1
0.1021/ic0508674 CCC: $33.50
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 3, 2006 1243
Published on Web 12/27/2005

Carriazo et al.
layered structure from partially calcined LDHs or by anion
exchange, using LDHs with both organic or inorganic
intercalated anions.2
calcined species. The thermal study for these materials is
important because not only LDH-POMs, but also the mixed
oxides obtained after their calcination, are suitable hetero-
geneous catalysts for oxidation processes.
,3,14-20
21
The structure of the POMs is usually pH-dependent, and
they tend to depolymerize in the basic medium provided by
the LDH suspension; therefore, the experimental conditions
during synthesis (i.e., pH, temperature, contact time between
the solid and the solution, etc.) should be carefully controlled.
In addition, the precise nature of the POM (responsible for
balancing the positive charge of the layers) is also important
through the charge/size ratio in the POM: POMs with a low
charge/size ratio cannot be intercalated in LDHs with high
positive layer charge.
Experimental Section
Samples Preparation. All chemicals were from Fluka (Buchs,
Switzerland) and were used without any further purification.
Five precursor LDHs were prepared. The Mg,Al-OH (meix-
nerite) hydrotalcite was prepared from a Mg,Al-CO
hereafter MgAlC) with a molar Mg/Al ratio of two, which had
been previously prepared by following the method described by
3
sample
(
30
Reichle. A portion (3 g) of sample MgAlC was calcined in N
2
at
500 °C for 3 h, and the resulting solid was suspended in 200 mL
Although many reports have been published on the
intercalation of Keggin and other heteropolyoxometalates,
only a few number of papers have dealt with intercalation
of heptamolybdate.22-25 In all cases, a layered phase with a
of water. The suspension was kept for 2 days at 70 °C and
maintained under nitrogen atmosphere until used for exchange.
The Zn,Al-NO
3
3
and Mg,Al-NO (hereafter ZnAlN and MgAlN,
respectively) hydrotalcites were prepared by coprecipitation fol-
23
lowing the method previously described by Gardner and Pinnavaia
basal spacing close to 12 Å (2θ ) 7.3°) is formed. However,
no definitive agreement exists on the evolution of the
interlayer phase with temperature: In some cases, a fast
reaction between heptamolybdate and layer cations has been
and Arco et al.,31 respectively. The precipitates were centrifuged
and washed with water several times; a portion was dried in a
vacuum and was used for full characterization, while the remaining
2
suspension was kept under N atmosphere for further exchange with
2
3
claimed; in some other cases it seems that heptamolybdate
heptamolybdate.
22
anion keeps its structure up to 400 °C, and depolymeriza-
The Mg,Al and Zn,Al hydrotalcites containing interlayer tere-
phthalate (here after MgAlT and ZnAlT, respectively) were pre-
pared following methods similar to those previously described by
Drezdzon14 and Crespo et al., respectively. In both cases, the
precipitates were treated as above-described for samples MgAlN
and ZnAlN.
tion or even grafting have been also claimed.2
6-29
We here report the intercalation by anion exchange of
heptamolybdate in LDHs containing Mg,Al or Zn,Al in the
layers; the different nature of the layer cations gives rise to
different acid-base properties, and exchange has been made
starting with LDHs containing hydroxide, nitrate, or tere-
phthalate as interlayer anion, to analyze their effect on the
exchange process and the nature and stability of the final
LDH-POM material.
The layered LDH-POM compounds, as well as the
compounds obtained upon thermal decomposition at increas-
ing temperatures (from 80 to 900 °C), have been character-
ized by different physicochemical techniques. We aimed to
analyze the effect of the LDH precursors on the structure
and porosity of the LDH-POM hybrid compound, as well
as to identify the evolution of the Mo-containing species
during calcination, to gain information on the nature of the
32
In all cases the starting solutions were prepared with a M2+/
Al3 molar ratio of 2.0 and decarbonated water was used for
preparing the solutions and for washing the precipitates.
All the heptamolybdate-containing LDHs were prepared by anion
exchange under an inert atmosphere as follows: 50 mL of an
aqueous solution of ammonium heptamolybdate (hereafter AHM)
was added to 100 mL of the precursor suspension (the solid
precursor was not isolated prior to exchange because it is well-
known that in such a case deaggregation of the primary particles
is difficult and thus exchange as well). The amount of AHM was
+
50% larger than the anion exchange capacity of the starting LDHs
(
in other words, 50% larger than that required for balancing the
3+
positive charge due to the presence of Al cations in the brucite-
like layers). Once the addition had been completed, a few drops of
2
3
M HNO were added until a constant pH value of 4.5 was reached,
(
(
(
(
(
(
(
14) Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628.
and then the suspension was stirred at 30 °C for 1 h (samples
containing terephthalate), 24 h at 30 °C (ex-meixnerite sample), or
15) Nijs, H.; de Bock, M.; Vansant, E. F. J. Porous Mater. 1999, 6, 101.
16) Kwon, T.; Pinnavaia, T. J. Chem. Mater. 1989, 1, 381.
17) Kwon, T.; Pinnavaia, T. J. J. Mol. Catal. 1992, 74, 23.
18) Kooli, F.; Jones, W. Inorg. Chem. 1995, 34, 6237.
19) Dimotakis, E. D.; Pinnavaia, T. J. Inorg. Chem. 1990, 29, 2393.
20) Ulibarri, M. A.; Labajos, F. M.; Rives, V.; Trujillano, R.; Kagunya,
W.; Jones, W. Inorg. Chem. 1994, 33, 2592.
24 h at 60 °C (samples containing nitrate); the solid was then
centrifuged, washed, and dried in a vacuum in a desiccator. The
samples will be named as XYMo, where X ) Mg or Zn and Y )
Mx,N,T for those prepared from precursor LDHs containing
hydroxide, nitrate, or terephthalate, respectively, in the interlayer.
Contact time between the suspensions and the solids, as well as
reaction temperature, were chosen after systematic study of optimum
experimental conditions.
Characterization Procedures. All samples prepared, as well
as the solids obtained after thermal decomposition up to 900 °C,
have been characterized using different experimental techniques.
(21) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag:
New York, 1983.
(
(
(
(
22) Twu, J.; Dutta, P. K. Chem. Mater. 1992, 4, 398.
23) Gardner, E.; Pinnavaia, T. J. Appl. Catal., A 1998, 167, 65.
24) Mitchell, P. C. H.; Wass, S. A. Appl. Catal., A 2002, 225, 153.
25) Tatsumi, T.; Yamamoto, K.; Tajina, H.; Tominaga, H. Chem. Lett.
1
992, 895.
26) Wang, J.; Tian, Y.; Wang, R. C.; Clearfield, A. Chem. Mater. 1992,
, 1276.
(
(
(
(
4
27) Narita, E.; Kaviratna, P. D.; Pinnavaia, T. J. J. Chem. Soc., Chem.
Commun. 1993, 60.
(30) Reichle, W. T. J. Catal. 1985, 94, 547.
(31) del Arco, M.; Guti e´ rrez, S.; Mart ´ı n, C.; Rives, V.; Rocha, J. J. Solid
State Chem. 2000, 151, 272.
(32) Crespo, I.; Barriga, C.; Rives, V.; Ulibarri, M. A. Solid State Ionics
1997, 101, 729.
28) Weber, R. S.; Gallezot, P.; Lefebre, F.; Suib, S. L. Microporous Mater.
1993, 1, 223.
29) del Arco, M.; Carriazo, D.; Guti e´ rrez, S.; Mart ´ı n, C.; Rives, V. Inorg.
Chem. 2004, 43, 375.
1244 Inorganic Chemistry, Vol. 45, No. 3, 2006

Layered Double Hydroxides with Paramolybdate Anions
Table 1. Chemical Composition (% Weight) and Formulas of the Samples Prepared
% M2
+
% Al
% C
2.31
% N
% Mo
M2+/Al3+ a
formula
sample
MgAlC
MgAlN
MgAlT
MgMxMo
MgNMo
MgTMo
ZnAlN
ZnAlT
ZnNMo
ZnTMo
16.41
18.47
17.65
9.46
9.16
9.56
31.20
30.80
25.23
24.09
10.45
10.10
10.88
6.78
6.74
7.03
6.43
7.83
5.20
5.25
1.7
2.0
1.8
1.6
1.5
1.5
2.0
1.6
2.0
1.9
[Mg0.63Al0.36(OH)2](CO3)0.18·nH2O
[Mg0.67Al0.33(OH)2](NO3)0.33·nH2O
[Mg0.64Al0.36(OH)2](TA)0.18·nH2O
[Mg0.61Al0.39(OH)2](Mo7O24)0.065·nH2O
[Mg0.60Al0.40(OH)2](Mo7O24)0.07·nH2O
[Mg0.60Al0.40(OH)2](Mo7O24)0.065·nH2O
[Zn0.67Al0.33(OH)2](NO3)0.32·nH2O
[Zn0.62Al0.38(OH)2](TA)0.19·nH2O
[Zn0.66Al0.33(OH)2](Mo7O24)0.06·nH2O
[Zn0.66Al0.34(OH)2](Mo7O24)0.06·nH2O
5.24
19.24
29.23
26.09
26.09
3.2
13.26
24.93
24.09
a
Experimental molar ratio.
Element chemical analyses were carried out at the Servicio
General de An a´ lisis Qu ´ı mico Aplicado (University of Salamanca,
Salamanca, Spain); Mg, Al, Zn, and Mo were determined by atomic
absorption in a Mark-2 EEL 240 apparatus, after dissolving the
samples in nitric acid, while carbon content was determined in a
CHNS 932 Leco analyzer.
during exchange, probably because of the low pH of the
solution during the preparation; the larger basicity of the
2+
Mg,Al than the Zn,Al series favors dissolution of Mg in a
2+
larger extent than Zn .
It is well-known that the nature of the oxomolybdenum
species depends on the pH, tetrahedral at pH > 10 but
octahedral at pH < 6, and the nuclearity of the POM
The powder X-ray diffraction (PXRD) patterns were obtained
by reflection from powder packed in a sample holder, with a
Siemens D-500 diffractometer, using Cu KR radiation (λ ) 1.540 50
Å) with a graphite monochromator and interfaced to a DACO-MP
data acquisition microprocessor. The diagrams were recorded in
21
increases as the pH is decreased. The predominating species
6
-
at pH used for exchange (4.5) is Mo
7
O24 , and this species
has been assumed to exist in these solids, thus giving rise to
-
1
3+
6-
the 5-70° (2θ) range at a scan speed of 0.05° s and a time
constant of 1.5 s. The crystalline phases were identified from
comparison with database software (JCPDS files).33
the formulas included in Table 1. The Al /Mo O
ratio
7
24
is close to 6, thus further evidencing the presence of this
oxomolybdenum species in the interlayer.
The metal contents in the samples calcined at 80-200 °C
is larger than in the original samples due to removal of water
(see TG results below), but the molar ratio between metal
contents is maintained.
Differential thermal analyses (DTA) and thermogravimetric (TG)
analyses were recorded in DTA-7 and TG-7 instruments, respec-
tively, from Perkin-Elmer. The analyses were carried out in flowing
-
1
(
30 mL min ) nitrogen from L’Air Liquide (Valladolid, Spain) at
-
1
a heating rate of 10 °C min
.
FT-Raman spectra were recorded in a Bruker RFS-100 instru-
ment, using the 1064 nm line of a Nd:YAG laser as excitation
source. Dispersive Raman spectra at 785 and 514 nm laser
wavelengths were obtained with a confocal Raman microscope from
Reinshaw, RM-2000, equipped with a Leica microscope, and an
electrically refrigerated CCD camera. The spectra were taken with
Despite for sample ZnAlN a small amount of ZnO seems
to exist (see the PXRD section below), the formula has been
2
+
calculated by assuming all Zn cations are in the brucite-
like layers; the amount of ZnO seems to be rather small,
3+
-
and the molar Al /NO
3
ratio would be unchanged whether
the presence of ZnO is taken into account or not; in any
case, the formula given should be obviously taken as an
approximate one.
50× and 20× objectives (0.75 and 0.40 NA, respectively) with
laser powers ranging from 10 to 30 mW; integration times were in
the 300-500 s range.
Powder X-ray Diffraction. The diagrams for the precur-
sors are included in Figure 1A and are characteristic of the
hydrotalcite-like structure. The first diffraction peak has been
ascribed to planes (003), assuming a R3m symmetry stacking
of the brucite-like layers; it is recorded at 8.8 (sample
MgAlN), 8.9 (ZnAlN), and 14.1 Å (MgAlT and ZnAlT),
corresponding to 2θ values of 10.0, 9.9, and 6.2°, respec-
The nitrogen adsorption-desorption isotherms for specific
surface area and porosity assessment were recorded at -196 °C in
a Gemini instrument from Micromeritics; specific surface areas were
determined by the Brunauer-Emmett-Teller (BET) method.34
Scanning electron micrographs were obtained in a model 940
digital scanning microscope from Zeiss.
Results and Discussion
2
3,31
tively. The first two values are in agreement
with the
Element Chemical Analyses. Results for all samples
interlayer nitrate anions located with their C axis parallel
3
2+
3+
prepared are summarized in Table 1. The M /Al molar
ratio is close, but in many cases lower, than the value in the
starting solution (2.0) for the precursor samples. The value
for the Mo-containing samples is, however, rather small
to the layers. The terephthalate anions are located upward
14,32,35,36
in both ZnAlT and MgAlT samples.
In addition, very
weak diffraction lines at 10.6 (2θ ) 8.2°) and 5.3 Å (2θ )
16.7°) are recorded for sample MgAlT, Figure 1A, which
can be tentatively ascribed to the presence of a minor second
phase where the terephthalate anions adopt a different
orientation in the interlayer, as the value is too high to
2+
(
1.6-1.5) for the samples containing Mg , but 1.9-2.0 for
2+
those prepared with Zn . Such a deviation from the expected
4
value of 2.0 has been reported previously, and it has been
ascribed to a preferential dissolution of the divalent cation
(
35) Vucelic, M.; Moggridge, G. D.; Jones, W. J. Phys. Chem. 1995, 99,
(
33) Joint Committee on Powder Diffraction Standars, International Centre
for Diffraction Data, Swarthmore, PA, 1977.
8328.
(36) Roy, A.; Forano, C.; Besse, J. P. In Layered Double Hydroxides:
Present and Future; Rives, V., Ed.; Nova Sci. Pub., Inc.: New York,
2001; Chapter 1, p 1.
(34) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60,
309.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1245

Carriazo et al.
Figure 1. (A) PXRD patterns of the precursor samples (a) ZnAlN, (b) MgAlN, (c) MgAlT, and (d) ZnAlT. (B) XRD patterns of oriented films of (a)
MgTMo, (b) MgNMo, (c) MgMxMo, (d) ZnNMo, and (e) ZnTMo. Key: (/) terephthalate oriented parallel to brucite-like layers; (ï) ZnO (zincite).
correspond to a Mg,Al-NO
3
hydrotalcite. The PXRD
The diagrams recorded for samples MgNMo and ZnNMo
show a diffraction at 4.9 Å (2θ ) 18°) in addition to the
basal diffractions at 12, 6, and 4 Å (2θ ) 7.3, 14.7, and
22.2°, respectively) and the broader diffraction maximum
at 11-9.8 Å (2θ ) 8.05-9.0°). Its intensity increases with
the synthesis temperature, and its absence in the diagrams
of the ex-terephthalate and ex-meixnerite samples is due to
the lower temperature used to prepare this series of samples.
We also observed that the diffraction lines at 11-9.8 Å
(2θ ) 8.05-9.0°) and 4.9 Å (2θ ) 18°) are recorded and
even become stronger, when any of the solids prepared was
calcined at increasing temperatures, and these are the only
signals recorded when the sample is calcined between 80
and 200 °C, Figure 2.
No clear diffraction maximum is recorded when the
calcination temperature is increased to 300 °C, Figure 2, thus
showing that the layered structure has collapsed, leading to
amorphous solids. The amorphous phase is detected even
when the samples are calcined at 600 (sample MgAl-POM)
or 450 °C (sample ZnAl-POM), and above these temper-
atures new diffraction lines, which intensity increase as the
calcinations temperature is further increased, are again
recorded. So, for sample MgAl-POM calcined at 600 °C
(and even clearer when the calcinations is performed at 700-
800 °C), the diffraction lines at 4.67, 3.37, and 3.28 Å (2θ
diagram for sample ZnAlN shows also some weak diffraction
maxima due to zincite, ZnO (JCPDS file 36-1451), a species
rather usually found coprecipitated with Zn-containing hy-
drotalcites.37
Anion exchange introducing heptamolybdate anions, what-
ever the hydrotalcite precursor used, gives rise to solids also
showing a hydrotalcite-like structure; the corresponding
PXRD diagrams, recorded using the oriented aggregated
method, are shown in Figure 1B. Basal reflections are
recorded at 12, 6, and 4 Å (2θ ) 7.3, 14.7, and 22.2°,
respectively). The first diffraction peak is usually the
strongest one in the PXRD diagrams of these solids; however,
in this case it is less intense than the second peak and even
in some cases it is recorded as a shoulder to a more intense
diffraction maximum. These opposite intensities for the (00l)
maxima has been previously reported by several authors for
different LDHs containing polyoxometalates in the interlayer
space,1
3-29,38
and it has been ascribed to the presence of atoms
38
with large atomic scattering factors in the interlayer. In the
diagrams recorded for unoriented powder samples, the first
diffraction peak is more intense and is recorded as a rather
broad band between 12 and 9.8 Å (2θ ) 7.3-9.0°), probably
due to the presence of a second crystalline phase, responsible
for the peak between 11 and 9.8 Å (2θ ) 8.05-9.0°).
)
19.0, 26.4, and 27.1°, respectively) are characteristic of
The d(003) value of 12 Å (2θ ) 7.3°) was previously
reported by some authors2
2-25
monoclinic MgMoO (JPCDS file 21-0961), where Mo ions
and, after subtracting the
4
14
are octahedrally coordinated by oxide anions. However,
thickness of the brucite-like layers (4.8 Å ), coincides with
39
2 7 2 4
diffraction lines due to MgMo O or MgAl O , which have
the height calculated, by using Crystal Maker software, for
2
2
6-
been reported by other authors, are not recorded, and then
we should conclude that in the Mg-containing samples
aluminum-containing species should be amorphous or very
the [Mo
7 24
O ]
2
anion with its C axis perpendicular to the
brucite-like layers; in this orientation, maximum interaction
between the polyoxometalate and the hydroxyl groups of the
layers is achieved.
well dispersed. Calcination of sample ZnAl-Mo
7
O24 at 450
°
C leads to formation of ZnMoO (JCPDS file 35-0765). A
4
further increase in the calcinations temperature gives rise to
development of diffraction maxima at 3.6, 2.69, and 2.6 Å
(
37) Kooli, F.; Depege, C.; Ennaqadi, A.; de Roy, A.; Besse, J. P. Clay
Clay Miner. 1997, 45, 92.
(
38) Hibino, T.; Tsunashima, A. Chem. Mater. 1997, 9, 2082.
39) Crystal Maker, version 2.1.0.: http//www.crystalmaker.co.uk/
cristal-maker.
(2θ ) 24.7, 33.2, and 34.4°, respectively), probably due to
(
Zn Mo (JCPDS file 21-1483), and diffraction lines due
3
2 9
O
1246 Inorganic Chemistry, Vol. 45, No. 3, 2006

Layered Double Hydroxides with Paramolybdate Anions
Figure 2. PXRD patterns of (A) MgTMo and (B) ZnTMo calcined at different temperatures. Key: (/) Zn3Mo2O9; (ï) ZnAl2O4.
Figure 3. (A) PXRD patterns of MgNMo samples (a) uncalcined, (b) calcined at 120 °C, and (c) after hydration of sample b. (B) PXRD patterns of ZnTMo
samples (a) uncalcined, (b) calcined at 90 °C, and (c) after hydration treatment of sample b.
to normal spinel ZnAl
when the sample is calcined at 800 °C.
2
O
4
(JCPDS file 5-0669) are recorded
hydrotalcites with aqueous solutions of ammonium hepta-
molybdate at pH ) 4.5 and 80 °C. These results confirm
that, despite the low pH used, paramolybdate is unable to
react with these cations giving rise to other solids.
The origin of the diffraction lines close to 11-9.8 Å (2θ
8.05-9.0°) and 4.9 Å (2θ ) 18°) is not clear. Some
)
authors have claimed that the first line, recorded in the PXRD
Development of the grafting process upon heating at 80
°C has been also checked by rehydrating the solid after
calcination at 120 °C; the corresponding PXRD diagram has
been included in Figure 3; it should be remembered that
calcinations between 80 and 200 °C give rise to solids whose
PXRD diagrams only show the diffraction lines at 11-9.8
Å (2θ ) 8.05-9.0°) and 4.9 Å. For these experiments, the
solids calcined at 120 °C were stored for 1 day in a vacuum
desiccator at saturation vapor pressure (water had been
decarbonated previously). The diagrams for the MgAl-POM
hydrotalcite rehydrated showed diffraction lines at 12-9.8,
6, and 4 Å, Figure 3A, similar to those in the original samples
(not calcined). The intensity of the line at 4.9 Å (2θ ) 18°)
decreases upon rehydration being almost unobservable. So,
we can safely conclude that this line (and probably another
obscured by the broad diffraction between 11 and 9.8 Å)
corresponds to another hydrotalcite phase (we will call it
phase II), which is reversibly formed from the first phase
(phase I, characterized by maxima at 12, 6, and 4 Å,
corresponding to 2θ ) 7.3, 14.7, and 22.2°, respectively).
Despite the cautions taken during rehydration, a small portion
of carbonate is intercalated, thus accounting for the diffrac-
diagram of several POM-containing hydrotalcites, is due to
23
ill-crystallized Mg and/or Al polyoxometalates. This should
be the only remaining crystalline phase after heating at 80
°
C. However, other authors have pointed out that this line
is due to polyoxometalate grafting to the brucite-like layers,
upon displacement of some hydroxyl groups from the layers
2
6,29
by oxide anions of the POM moiety.
erization of the polyoxmetalate,
Partial depolym-
26-28
or a mixture of POM
together with small anions in the interlayer,38 has been also
claimed as being the origin of this diffraction line.
For insight into the origin of these lines, we have mixed
an aqueous solution containing Al3+ and Mg (or Zn )
cations with an aqueous solution of ammonium hepta-
molybdate; the mixture was heated at 80 °C and pH ) 4.5
2+
2+
3
(HNO ), to reproduce as close as possible the experimental
conditions during anion exchange. No precipitate was
observed in any case; the diffraction diagram of the solid
residue formed upon evaporation of the solvent does not
show any diffraction line close to 11-9.8 Å (2θ ) 8.05-
9.0°) and 4.9 Å (2θ ) 18°). These diffraction lines are not
recorded after mixing either MgAl-CO or ZnAl-CO
3
3
Inorganic Chemistry, Vol. 45, No. 3, 2006 1247

Carriazo et al.
Figure 4. DTA/TG profiles recorded in N2 atmosphere of samples (A) MgNMo and (B) ZnNMo.
tion line at 7.6 Å (2θ ) 11.6°), corresponding to (003) planes
4
ZnMoO , which presence has been confirmed by other
techniques even for the sample calcined at 450 °C.
solid.1 Only when the Zn-containing
,2
of the Mg,Al-CO
sample had been previously calcined between 80 and 100
C, reversible rehydration occurred, Figure 3B. A small
3
The results from the TG/DTA study are in agreement with
the PXRD study for the samples calcined at increasing
temperatures; calcination at 300 °C leads to amorphous
solids, as collapsing and destruction of the layered structure
starts at ca. 200-250 °C.
°
amount (lower than in the case of the Mg,Al sample) of
carbonate is intercalated during rehydration, probably due
to the lower basicity of the Zn-containing solids.
These results indicate that grafting does not take place and
also that the phase existing in the calcined solids is not a
Raman Spectroscopy. When using exciting lines at 1064,
7
85, and 514 nm, the Raman spectra of the uncalcined Mo-
2+
3+
poorly crystallized salt of the M /M cations, as in such a
case the LDH-heptamolybdate structure would not have been
recovered. Moreover, in the case of the Zn-containing
samples the interaction between heptamolybdate and the
brucite-like layers is stronger than in the case of the Mg-
containing samples, and the interaction becomes more intense
as the temperature is increased. These results are in agree-
ment with the high-temperature transformations observed,
containing LDHs show the same characteristics, implying
the lack of resonance Raman effect at the laser wavelengths
used. The spectra for samples MgNMo and ZnNMo, recorded
with the 785 nm exciting line, are included in Figure 5; the
spectra for the other samples were similar. The baseline of
the spectra for samples calcined at 300 and 400 °C has been
corrected because of the marked fluorescence background
when excited at 514 nm, which considerably decreases at
as ZnMoO
than MgMoO
4
is formed at a lower temperature (450-500 °C)
(600 °C).
7
85 nm and disappears at 1064 nm.
4
Assignment of the bands recorded is summarized in Table
; some of them have been identified by deconvolution of
Thermal Analysis (TG/DTG). The TG and DTG curves
recorded for the heptamolybdate-containing LDHs are very
similar, and so only the curves for one sample within each
series are shown in Figure 4.
2
the experimental spectra in the 270-420 and 720-1020
-
1
cm ranges; the spectra are included in Figure 5.
In both cases, weight is lost in a continuous step from ca.
00 to 400 °C. This weight loss is, however, due to several
The Raman spectra of molybdenum-oxygen isopoly-
4
0-44
1
anions exhibit
five characteristic ranges of vibrational
consecutive overlapped steps, as described by the DTA
curves, also shown in the same figure.
modes at 200, 310-370, 500-650, 700-850, and 900-1000
-
1
cm , which are usually assigned respectively to Mo-O-
Mo deformations, ModO bending vibrations, symmetric
Mo-O-Mo stretching, antisymmetric Mo-O-Mo stretch-
ing, and symmetric and antisymmetric terminal ModO
stretching. All Zn,Al and Mg,Al samples show similar
Raman spectra, with bands basically coincident with those
of the heptamolybdate anion; the intense band recorded close
The Mg-containing samples show two endothermic effects
at 150 and 329 °C, which can be ascribed2,3 to removal of
water molecules adsorbed on the external surface of the
crystallites, removal of interlayer water molecules (unfor-
tunately, evolved gases could not be analyzed), and dehy-
droxylation of the layers, leading to collapsing of the layered
structure. The weight loss associated to these processes is
ca. 17% of the initial sample weight. A progressive weight
loss is recorded for sample Zn,Al-heptamolybdate, Figure
-
1
-1
to 200 cm and the band at 550 cm (ν (Mo-O-Mo))
s
are characteristic of highly polymerized species, although
-
1
the band at 550 cm can be also ascribed (together with
-
1
4
B, up to 500 °C, amounting ca. 15% of the initial sample
weight, and corresponding to four endothermic peaks at 85,
36, 268, and 328; as for the Mg-containing samples, the
another band at 470 or 490 cm ) to Al/Mg-OH or Zn/
1
(
40) Aveston, J.; Anacker, E. W.; Johnson, J. S. Inorg. Chem. 1969, 8, 6.
first two peaks can be due to dehydration, and the next two
peaks, to dehydroxylation. An additional exothermic peak
at 480 °C can be tentatively ascribed to crystallization of
(41) Kepert, D. L. Comp. Inorg. Chem. 1973, 4, 607.
(
(
42) Griffith, W. P.; Lesniak, P. J. B. J. Chem. Soc. A 1969, 1066.
43) Zezlorowski, H.; Kn o¨ zinger, H. J. Phys. Chem. 1979, 83, 1166.
(44) Hardcastle, F. D.; Wachs, I. E. J. Raman Spectrosc. 1990, 21, 683.
1248 Inorganic Chemistry, Vol. 45, No. 3, 2006

Layered Double Hydroxides with Paramolybdate Anions
Figure 5. Raman spectra of the samples (A) MgNMo and (B) ZnNMo, excited with λ of 785 nm. Key: (a) originals samples and calcined at (b) 150, (c)
3
00, (d) 400, and (e) 700 °C.
Table 2. Assignment of the Bands (cm-1) Recorded in the Raman
Spectra of the Samples
but it can be due to an overlapping of the bands due to
-1
4
δ(ModO) of tetrahedral [MoO ] (317 cm ) and octahedral
-
1
bending
[MoO
of MoO
] species (360 cm ), thus accounting for the presence
6
sample
ν(ModOt)s ν(ModO)as (ModO) δ(Mo-O-Mo)
2-
4
species together with heptamolybdate ones.
MgNMo
945
960
936
941
946
951
942
945
943
897
879
879, 803
880
880
880
881, 803
890
856
903
837
331, 357
334, 358
338
216, 246
222
220 (vw)
220 (vw)
215, 245
221
226 (vw)
226 (vw)
219
However, their concentration should be very low, as from
chemical analyses results for the original samples, a coherent
chemical formula is calculated by assuming that all molyb-
denum is as heptamolybdate.
MgNMo/150
MgNMo/300
MgNMo/400
ZnNMo
332
330, 358
326, 358
323
326
362
ZnNMo/150
ZnNMo/300
ZnNMo/400
Upon calcination of the samples above 200 °C, Figure 5,
6
despite that bands characteristic of octahedral [MoO ] species
6
-
Mo7O24
MoO4
-1
are still recorded, the band at 200 cm , due to mode δ(Mo-
O-Mo), is weaker, indicating a lower polymerization degree;
in addition, the δ(ModO) band is recorded between 325 and
2
-
317
Al-OH translation modes of the brucite-like layers.45 The
small differences observed in the positions of the bands, from
those for the free heptamolybdate anions, can be due to the
interactions between the anion and the brucite-like layers.
The spectra for the samples calcined at 150 °C are very
similar to those for the uncalcined samples, both due to
polymolybdate species and to the hydrotalcite-like structure;
the only difference is a slight shift toward larger wave-
-
1
3
40 cm . Consequently, it seems that calcination at 300-
4
00 °C, in addition to destroying the layered structure (the
bands due to Al/Mg-OH and Zn/Al-OH translation modes
are not longer recorded), also decreases the polymerization
degree, increasing the concentration of tetrahedral [MoO
4
]
species, even probably forming dimolybdate species, con-
4
4
taining tetrahedral and octahedral species.
Upon calcination at 500 °C (samples Zn,Al-heptamolyb-
date), 600 °C (samples Mg,Al-heptamolybdate), or above,
important changes are observed in the spectra. Those for the
samples calcined at 700 °C are shown in Figure 5. The
t
numbers of the band due to mode ν(Mo-O) , especially for
-1
the Mg-containing sample (from 945 to 960 cm ). This shift
has been ascribed by other authors to a decrease in the
44
Mo-O bond length; Hardcastle and Wachs have proposed
the equation R(Mo-O) ) 0.48239 ln(32895/ν) to determine
the Mo-O bond length from the position of this band. When
applied to our results, it indicates that the heptamolybdate
species existing in the original samples undergo a slight
shortening of the Mo-O bonds upon calcination at 80-200
-
1
characteristic band of polymolybdates at 200 cm is no
longer recorded, but intense bands at 966, 955, 908, 852,
-
1
2
86, 330, and 365 cm are recorded for the Mg,Al-
heptamolybdate system. These bands are characteristic of
4
6
crystalline monoclinic MgMoO
4
(which presence had been
VI
already detected by PXRD) where Mo is octahedrally
°
C, probably as a consequence of a stronger interaction
coordinated. For the Zn,Al-heptamolybdate sample, Figure
between the ispolyoxometalate unit and the brucite-like
species.
5
B, the bands are recorded between 960 and 930 and 400-
-
1
3
30 cm and would correspond to Zn molybdate and
-
1
The band at 330 cm , recorded for the original and 150
VI
dimolybdate, where Mo ions are tetrahedrally and octa-
hedrally coordinated, in agreement too with the PXRD data.
°
C calcined samples, corresponds to the δ(ModO) mode,
(
45) Kloprogge, J. T.; Hickey, L.; Frost, R. L. J. Raman Spectrosc. 2004,
(46) Cadus, L. E.; G o´ mez, M. F.; Abello, M. C. Catal. Lett. 1997, 43,
35, 967.
229.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1249

Carriazo et al.
Figure 6. N2 adsorption-desorptions isotherms for samples (A) MgTMo and (B) ZnTMo, calcined at the temperatures shown.
Table 3. Specific Surface Area (m g-1) of the Samples Prepared
2
These results show that the interlayer molybdate species
existing in all the original, uncalcined, samples are mainly
heptamolybdate, coexisting with a small amount of tetra-
hedral [MoO ] species; these tetrahedral species, formed via
4
hydrolysis of heptamolybdate, would account for the broad
sample
MgAlC
MgAlN
MgAlT
SBET
St
Sm
sample
SBET
St
Sm
86
26
1
ZnAlN
ZnAlT
28
1
MgMxMo
MgNMo
MgTMo
MgTMo/400
MgTMo/800
34
31
30
31
20
PXRD effect recorded between 11 and 9.8 Å (2θ ) 8.05-
ZnNMo
ZnTMo
ZnTMo/400
ZnTMo/800
31
43
32
8
17
14
10
13
13
10
34
23
8
9
8
0
9.0°). The layered structure and the same Mo-containing
species seem to exist upon calcination at 80-200 °C,
although in such a case the original and calcined samples
would display the same gallery height (from their PXRD
diagrams), unless the orientation of the interlayer anion
would be different in phase II. However, this is not possible,
as all orientations different from that for phase I (hepta-
include them in type II of the IUPAC classification.47 This
sort of isotherm is typical of nonporous or microporous (pore
diameter smaller than 2 nm) solids.
The values for the BET specific surface areas are sum-
marized in Table 3. All Mg,Al-heptamolybdate samples
molybdate units with its C
would give rise to larger gallery heights. Consequently, from
the Raman spectra (shift of the ν(ModO) band to larger
2
axis perpendicular to the layers)
2
-1
show values close to 35 m g , this value decreasing when
the samples are calcined above 600 °C, probably because
t
wavenumbers) we should conclude that heptamolybdate units
in phase II should be slightly distorted, because of its
interaction with the layers. This distortion is responsible for
the diffraction line at 4.9 Å (2θ ) 18.0°) and probably
another close to 10 Å (2θ ) 8.8°) obscured in the broad
band recorded at 11-9.8 Å (2θ ) 8.05-9.0°) in the PXRD
diagrams. The layered structure is destroyed when the
samples are calcined at 300 °C (the characteristic Raman
bands of LDHs are absent) and depolymerization starts; both
tetrahedral and octahedral species coexist in these mostly
amorphous phases, probably as dimolybdate units, which
undergo further transformation when the calcination tem-
perature is further increased, to crystalline Mg or Zn
molybdates.
of the formation of crystalline MgMoO
4
.
The largest specific surface area was measured for sample
2
-1
ZnTMo (43 m g ), while the terephthalate-containing
precursors (MgAlT and ZnAlT) display the lowest values
2
-1
(1 m g ). For the Zn-containing samples, calcination at
00 °C decreases the specific surface area by 20%, due to
5
crystallization of zinc molybdates, this effect being even
more marked when the calcination temperature is increased
up to 800 °C. The decrease in the specific surface area within
the Zn series takes place at lower temperatures than for the
Mg series, as for these samples crystallization of magnesium
molybdates takes place at a higher temperature.
4
8
The t-curves showing positive zero intercepts were
confirming the presence of micropores. These plots were used
to determine the external surfaces areas (from the slopes of
the lines) and the area equivalent to adsorption in micropores
The increased concentration of tetrahedral species via
depolymerization when the samples are calcined above the
temperature required for destroying the layered structure is
probably related to the increase in surface basicity of
hydrotalcites upon dehydration and dehydroxylation, leading
to formation of amorphous solids.
(from the zero intercepts); the values have been summarized
in Table 3. It should be noted that hydrotalcite-like materials
with small interlayer anions (e. g., carbonate, nitrate, chloride,
4
9
hydroxyl, etc.) are not microporous, as the nitrogen
molecules cannot access to the interlayer. In the samples here
Specific Surface Area and Porosity. The nitrogen
adsorption-desorption isotherms at -196 °C on some of
the samples studied (both original and calcined ones) are
included in Figure 6A and are characteristic of type I
isotherms, although the weak inflection recorded would
(
47) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti,
R.; Rouquerol, J.; Sieminiewska, T. Pure Appl. Chem. 1985, 57, 603.
48) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319.
(
(
49) Rives, V. In Layered Double Hydroxides: Present and Future; Rives,
V., Ed.; Nova Sci. Pub., Inc.: New York, 2001; Chapter 8, p 229.
1250 Inorganic Chemistry, Vol. 45, No. 3, 2006

Layered Double Hydroxides with Paramolybdate Anions
Figure 7. SEM micrographs of sample MgNMo (a) original and calcined at (b) 400 and (c) 800 °C.
formation of particles with a shape rather different from the
others and which can be tentatively ascribed to formation
of MoO crystallites.
3
Conclusions
Although the use of LDH precursors with different anions
in the interlayer, such as nitrate, hydroxide, and terephthalate,
allows the intercalation of isopolyoxomolybdates, a faster
anion exchange has been observed when using LDH-
terephthalate as the starting material.
It can be concluded from the Raman spectra that molyb-
denum is present, mainly as heptamolybdate with some
isolated molybdate species, in Zn and Mg systems, both
uncalcined and calcined up to 250 °C. The differences
observed in the PXRD patterns between the uncalcined
samples and those calcined up to 250 °C are due to distortion
of the octahedra, more evident when the calcination tem-
perature is increased, as a consequence of a strong interaction
between isopolyanions and the brucite-like layers. By a
rehydration treatment, calcined samples recover their original
Mo-O distances. When the samples are calcined between
Figure 8. SEM micrographs of sample ZnNMo (a) original and calcined
at (b) 400, (c) 800, and (d) 900 °C.
3
00 and 400 °C, amorphous phases are obtained, in which
studied, however, the microporosity observed can be related
to the incorporation of rather large anions, which have a
swelling effect on the brucite-like layers, thus allowing
insertion of nitrogen molecules in the interlayer space.
Microporosity, however, decreases when the samples are
calcined at increasing temperatures. Pore size distribution
curves indicate that the main pores have a diameter between
the polymerization degree of the Mo-containing species is
lower than in the uncalcined ones, they existing probably as
dimolybdate, as both octahedral and tetrahedral oxo-
molybdenum units are detected. Zn and Mg molybdates are
formed at 450 and 600 °C, respectively. Calcination of zinc-
containing samples above 600 °C gives rise to new crystalline
phases identified as zinc dimolybdate and Zn-Al spinel, and
1
and 2 nm in the original samples, while for those calcined
3
finally, at 900 °C MoO (detected by SEM) is formed.
at 800 °C the dominant pores have a diameter between 3
and 5 nm, probably being interparticle pores.
Type I/II nitrogen adsorption isotherms, showing interlayer
microporosity produced by the polyanion intercalation, are
measured for all the solids prepared. The specific surface
areas are not very large and decrease when the calcination
temperature is increased, as a consequence of the formation
of crystalline phases.
Electron Microscopy. The scanning electron micrographs
for original and some calcined MgAl- and ZnAl-hepta-
molybdate samples are shown in Figures 7 and 8. The
morphologies observed are rather similar. So, the original
samples and those calcined up to 200 °C appear as ag-
glomerates of layered particles with a diameter between 5
and 10 µm. The average size decreases to 2 µm when the
samples are calcined between 200 and 700 °C. The apparent
particle size does not change further when the calcination
temperature is increased for the Mg,Al samples; however,
calcination at 900 °C of the Zn-containing samples leads to
Acknowledgment. Financial support from the MEC
(Grant MAT2003-06605-C02-01) and ERDF is acknowl-
edged; D.C. acknowledges a grant from the Universidad de
Salamanca.
IC0508674
Inorganic Chemistry, Vol. 45, No. 3, 2006 1251