Effect of preparation protocol on the surface acidity of molybdenum catalysts supported on titania and zirconia

Article abstract of DOI:10.1016/j.molcata.2016.10.006

Molybdenum catalysts supported on titania and zirconia have been prepared via the impregnation on the corresponding hydroxides and oxides. The solids have been characterized by surface area measurements, XR-Fluorescence analysis, XRD and Raman Spectroscopy. The isopropanol decomposition reaction has been used to probe their acidic properties. These analyses revealed that introduction of molybdenum on the surfaces of the hydroxides and the oxides results in an increase of the specific surface area. Impregnation on the hydroxide leads to a comparatively better molybdenum dispersion and to a greater surface acidity. This indicates that the acid sites on hydroxide derived solids are stronger and more numerous than on their oxide derived counterpart.

Full text of DOI:10.1016/j.molcata.2016.10.006

Accepted Manuscript
Title: EFFECT OF PREPARATION PROTOCOL ON THE
SURFACE ACIDITY OF MOLYBDENUM CATALYSTS
SUPPORTED ON TITANIA AND ZIRCONIA
Author: Nac e´ ra Lamine Amel Benadda Amar Djadoun Akila
Barama Juliette Blanchard
PII:
S1381-1169(16)30427-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.006
MOLCAA 10068
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
30-6-2016
3-10-2016
4-10-2016
Please cite this article as: Nac e´ ra Lamine, Amel Benadda, Amar Djadoun,
Akila Barama, Juliette Blanchard, EFFECT OF PREPARATION PROTOCOL
ON THE SURFACE ACIDITY OF MOLYBDENUM CATALYSTS SUPPORTED
ON TITANIA AND ZIRCONIA, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.10.006
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EFFECT OF PREPARATION PROTOCOL ON THE SURFACE ACIDITY OF
MOLYBDENUM CATALYSTS SUPPORTED ON TITANIA AND ZIRCONIA
1
1,*
2
1
3
Nacéra Lamine , Amel Benadda , Amar Djadoun , Akila Barama and Juliette Blanchard
1
- Laboratoire des Matériaux Catalytiques et Catalyse en Chimie Organique, Faculté de Chimie, USTHB, BP32 El Alia,
Bab Ezzouar, 16111, Algiers, Algeria.
2
3
- Laboratoire de Géophysique, FSTGAT, USTHB, BP 32, El Alia, Bab Ezzouar, 16111, Algiers, Algeria.
- Sorbonne Universités, UPMC Univ Paris 6, UMR CNRS 7197, Laboratoire de Réactivité de Surface, F75005 Paris,
France.
∗
Corresponding author. E-mail addresses: abenadda@usthb.dz ; benaddaamel@yahoo.fr.
(A.Benadda).
1

Graphical abstract
Isopropanol conversion at 120°C
Impregnation of Zr or Ti Oxide
Impregnation of Zr or Ti Hydroxide
Highlights




Mo/TiO
2
and Mo/ZrO were prepared via impregnation of metal hydroxides and oxides.
2
The addition of Mo to the hydroxide or oxide leads to an increase of the surface area.
At close Mo densities, the hydroxide impregnation leads to stronger acidity.
The acid sites strength and density is optimal at Mo density about 6 Mo-At/nm².
Abstract
Molybdenum catalysts supported on titania and zirconia have been prepared via the
impregnation on the corresponding hydroxides and oxides. The solids have been characterized by
surface area measurements, XR-Fluorescence analysis, XRD and Raman Spectroscopy. The
isopropanol decomposition reaction has been used to probe their acidic properties. These analyses
revealed that introduction of molybdenum on the surfaces of the hydroxides and the oxides results in
an increase of the specific surface area. Impregnation on the hydroxide leads to a comparatively
better molybdenum dispersion and to a greater surface acidity. This indicates that the acid sites on
hydroxide derived solids are stronger and more numerous than on their oxide derived counterpart.
2

Keywords: Molybdena; zirconium hydroxide; titanium hydroxide; impregnation; Isopropanol
decomposition
1. Introduction
Molybdenum is an element that has been widely studied as catalyst component; it has been
shown that, due to the electronic and structural properties of molybdenum oxides, they can be used
as the main active phase or as an additive for many catalytic systems [1-3]. Indeed, molybdenum
based catalysts are used in numerous industrial processes such as: olefins and alkynes metathesis,
ammoxidation and oxidative dehydrogenation of light alkanes and hydrodesulphurization [4-11].
Molybdenum oxide is generally supported on metal oxides such as SiO
2
, Al
2
O
3
, ZrO
2
, SnO , MgO,
2
CaO and TiO in order to prevent its sintering. In these supported catalysts, the molybdenum
2
constitutes the active phase and is dispersed on an oxide with a higher surface area. Molybdenum
oxide-support interactions affect the structure and electronic properties of the active phase and
influence therefore the catalytic performances of the solids. These interactions depend on the nature
of the support, the active phase loading and the thermal treatment.
Titania and zirconia have been the object of great interest in the field of heterogeneous
catalysis [12-17]. Both oxides are n-type semi-conductors and zirconia presents specific
characteristics such as high thermal stability, extreme hardness, stability under reducing conditions
and both acid and base functions whereas titania is well known to present SMSI properties as
reported in early studies carried on Pt/TiO
Among supported molybdenum oxide, Mo/TiO
1, 2, 18-21]. Molybdenum sulfide supported on TiO
2
and Ir/TiO
2
catalysts [16, 17].
and Mo/ZrO have been extensively studied
and ZrO showed improved performances as
2
2
[
2
2
hydrotreating catalysts thanks to higher dispersion and reducibility of molybdenum oxide phase
when supported on titania and zirconia than on alumina and silica [2, 21-25]. In fact, the
molybdenum species-support interactions depend on the acid-base properties of the support: the
more basic the support, the stronger the interaction [26] and both TiO2 and ZrO2 are less acidic than
Al2O3. Moreover, Payen et al. have shown that the critical dispersion capacity for molybdenum oxo-
species is higher on zirconia than on alumina [27].
Isopropanol decomposition has been widely used, among other test reactions, to probe acid-
base properties of solid materials. It presents the advantage that dehydration; dehydrogenation and
condensation reactions can occur simultaneously [28]. The analysis of the products distribution gives
information about strength and density of catalysts’ acid-base sites [29-31].
3

The purpose of the present work is to investigate the effect of the preparation method on the
surface acidity of zirconia and titania supported molybdenum catalysts by means of the isopropanol
decomposition test reaction.
2. Experimental
2
.1. Catalyst preparation
The solids Mo/ZrO
2
and Mo/TiO were prepared by the classic Ippatieff impregnation
2
method [32] starting from zirconium/titanium hydroxides and oxides.
2
.1.1 Support preparation
First, the oxyhydroxides ZrO
.8H O (Alfa Aesar 99.9%) and TiCl
A 3M solution of TiCl (pH=0.5) was prepared by adding pure TiCl slowly to distilled water
x
(OH)4-2x and TiO
x
(OH)4-2x were prepared starting from
ZrOCl
2
2
4
(Merck 99%) respectively.
4
4
placed in an ice-water bath because the dissolution is very exothermic and may be explosive [33].
Ammonia solution (6N) was then added to the previous solution under stirring until the pH reached
9
-10. For zirconium oxyhydroxide preparation, ammonia solution was added dropwise to a 0.5 M
ZrO(NO solution (initial pH=2) until the pH reached 9.5. The resulting gels were aged for 24
3 2
)
hours then filtered and washed thoroughly with distilled water until the filtrate reached a neutral pH.
The powders were lastly dried at 100°C overnight.
2
.1.2 Catalyst preparation
For the first series of catalysts, Zr(OH)
solution of ammonium heptamolybdate (AHM) (NH
4
and Ti(OH)
Mo
4
were dry impregnated with an aqueous
O, provider : Strem Chemicals)
4
)
6
7
O
24,4H
2
whose concentration was adjusted in order to obtain different molybdenum loadings. The samples
were dried at 100°C overnight and further calcined in air for 4 hours at 450 and 700°C for the
Mo/ZrO
2
catalysts and at 450°C only for the Mo/TiO systems. The solids of this series were labelled
2
x-MoMOH-T where x represents the molybdenum loading, M the support cation symbol and T the
calcination temperature.
For the second batch of catalysts, Zr(OH)
hours before being used as supports. The obtained oxides were impregnated following the procedure
used for the first series. The solids of this series are noted x-MoMO -T where x represents the
molybdenum loading, M the support cation symbol and T the calcination temperature.
4
and Ti(OH)
4
were first calcined at 450°C for 4
2
2
.2. Catalyst characterization
All samples were characterized using the following physico-chemical methods:
4

Surface area measurements: Specific surface areas of samples (after outgassing during 1 h at
23 K under vacuum) were determined by nitrogen adsorption at 77 K on a Micromeritics ASAP
060 system.
5
2
Powder X-ray diffraction (XRD): X-ray diffraction patterns were collected with a Philips
diffractometer (type PW 1710) using CuKα radiation (λ =1,5418Å).
X-ray fluorescence analysis (XRF) : the molybdenum content was determined by X-ray
fluorescence analyses conducted under He flow with an energy dispersive spectrometer (XEPOS
with Turboquant powder software) equipped with a 50-Watt end-window X-ray tube to excite the
samples.
Laser Raman spectroscopy: The Raman spectra were obtained using a BRUKER RAM II
Raman spectrometer equipped with a Ge detector and using the 1064 nm excitation line of Nd-YAG
laser. The data were collected by keeping the power at 30 mW and by recording 2000 scans per
-1
spectrum with a spectral resolution of 4 cm .
2
.3. Catalytic test
The isopropanol decomposition reaction was carried at 393 K in a fixed-bed continuous flow
reactor under atmospheric pressure (see Supplementary information file) and in the absence of
oxygen. Prior to catalytic tests, 0.05g of the solid was heated under flowing nitrogen at 250°C for 1
-1
hour. The isopropanol was fed by flowing nitrogen (40 mL.min ) through a saturator kept at 0°C to
produce a pressure of 9 mmHg. The reaction was followed using on-line gas chromatography.
Conversion and selectivities were calculated at steady state which was typically reached after less
than 1 hour of reaction.
3. Results and discussion
3
.1. X-Ray diffraction
3
.1.1. Mo-Zr catalysts
Figure 1 depicts the XRD diffractograms of the solids supported on zirconia. Calcination of
zirconium hydroxide at 450 and 700°C results in a mixture of monoclinic (m-ZrO2) and tetragonal
t-ZrO2), with a predominance of the monoclinic phase especially at 700°C (see table 1). This is
(
consistent with previous studies which have shown that the metastable tetragonal phase was formed
at low calcination temperatures, that its proportion was maximum 400°C and that it gradually
declined after calcination at higher temperatures (up to 1200°C) due to its transformation into
monoclinic zirconia [34, 35].
5

For the samples prepared by impregnation of Zr(OH)4 with AHM, the tetragonal phase is the
main form of zirconia whatever the calcination temperature. This clearly confirms that the
molybdenum stabilizes the metastable tetragonal phase as reported in several studies [36, 37]. The
tetragonal-monoclinic transformation at low temperatures has been proposed to occur via oxygen
adsorption on specific sites, which seem to be also the anchoring sites for oxoanions such as sulfates
and molybdates. Hence, the incorporation of oxoanions results in the inhibition of the phase
transformation [38].
Considering the solids prepared by impregnation on ZrO2 followed by calcination at 450°C,
the diffractogram of the solid 5-MoZrO2-450 is similar to the one of Zr(OH)4-450 with a
predominance of the monoclinic zirconia phase, whereas, for the higher loading (10%), the
proportion of tetragonal phase is higher than in the calcined support. At 700°C, the proportion of the
tetragonal phase is more important in the solids containing molybdenum than in the support calcined
at the same temperature.
For the solids supported on ZrO2, the calcination of the catalyst implies, for the zirconia
phase, an additional thermal treatment at 450 or 700°C after the initial calcination step of Zr(OH)4 at
4
50°C. This second thermal treatment results in a further crystallization of zirconia; the presence of
molybdenum during this thermal treatment favors the formation of t-ZrO2 at the expense of m-ZrO2,
leading to an increase of the proportion of tetragonal phase in the Mo/ZrO2 solids compared to the
support alone.
Weak XRD lines corresponding to MoO
with the highest Mo loading (10% Mo), prepared by impregnation on ZrO
Zr(MoO mixed oxide phase lines were observed on the diffractograms of the solids
3
were only observed on the pattern of the catalyst
2
and calcined at 450°C.
)
4 2
calcined at 700°C and with relatively high molybdenum loading, which is consistent with reported
data in the literature [3,36].
3
.1.2. Mo-Ti catalysts
The XRD patterns of the calcined catalysts prepared by using either Ti(OH)
4
or TiO as
2
support (Figure 2) indicate, for all Mo loading and independently of preparation protocol, a high
dispersion of molybdenum on the support as no peaks except those of the anatase phase were
observed.
6

3
.2. Surface area measurements and elementary analysis
The molybdenum contents, BET surface areas and molybdenum surface densities of the Mo-
Zr and Mo-Ti catalysts are shown in Table 1. The N adsorption-desorption isotherms and pore size
2
distributions of selected samples are given in the Supplementary information file.
Whatever the preparation protocol and the calcination temperature, the introduction of
molybdenum on zirconium hydroxide or oxide resulted in a greater surface area of the catalyst
compared to the support. Moreover, within one preparation protocol, the greater the molybdenum
loading is, the higher the surface area of the supported solid. Additionally, the areas of the solids
prepared by impregnation on Zr(OH)
4
are superior to those of the solids prepared by impregnation on
ZrO
2
.
The larger surface area of molybdenum supported on Zr(OH)
4
compared to Zr(OH) alone
4
after calcination at the same temperature has been widely commented in the literature [39, 40]. It has
been proposed that promoters (cations or anions) decrease the sintering and grain-growth rates of
zirconia, which results in a greater surface area and a hindrance of the tetragonal-monoclinic
transformation [40]. Conversely, the increase of the surface area of an oxide after its impregnation by
another component is not common. Indeed, it is generally admitted that impregnation results in a
decrease of the surface, because (i) the active phase contributes to the overall weight of the catalyst
and hence decreases the surface area by the mere fact that the proportion of high surface area support
is lower in the catalyst than in the staring support; (ii) the active phase may partially block the
support pores. We believe that as molybdenum exists as large oxo-species in the impregnation
solution and as the zirconia has been prepared in the laboratory at mild conditions (calcination of
Zr(OH) at 450°C for 4 hours), the interaction between the oxygen atoms of the oxo-species and the
4
zirconia surface defects may result in the de-agglomeration of zirconia particles leading to an
increase of the surface area.
Similarly to the Mo-ZrOH solids, the surface areas of the Mo-TiOH systems are larger than
that of the pure TiO obtained by calcination of the titanium hydroxide at the same temperature.
2
Hence, the presence of molybdenum also reduces the titania grain growth during the thermal
treatment of the titanium hydroxide.
For the samples supported on TiO , a decrease in the specific surface area of the sample is
2
first noted upon the impregnation of the support with 5 wt% of molybdenum, then as the
molybdenum content increases to 10 wt%, the surface area increases and becomes greater than the
initial surface of the support. This finding is different from the results observed for the ZrO
2
supported solids, where the surface of the supported solids are, whatever the molybdenum loading,
superior to the support surface area. This difference may be attributed to the difference in
7

crystallinity between ZrO
2
and TiO
2
. Indeed, ZrO
2
seems, based on the broadness of the lines of
. This implies the existence of a larger
ZrO diffractogram, to be slightly less crystallized than TiO
2
2
number of surface defects which are responsible for the interaction of molybdenum oxo-species with
the support and, therefore for the enlargement of the surface area of the supported solids compared to
the supports. It is noteworthy that a non-monotonic variation of the surface area of titania supported
solids with increasing transition metal loading has already been reported by Tsilomelekis and
Halkides [41, 42].
3
.3. Laser Raman spectroscopy analysis
Supported molybdenum oxide has been thoroughly studied by Raman spectroscopy in the
literature; assignment of the Raman bands is based on a careful examination of these previous
reports.
3
.1.1. Mo-Zr solids calcined at 450°C
Figure 3.a presents the Raman spectra of the Mo-Zr systems calcined at 450°C. The bands
-1
present on all spectra (Figures 3a and 3b) at low wave number (612-645 cm ) are characteristic of
-1
the oxide (the one appearing at ca. 613 cm corresponds to m-ZrO and the second one at ca. 640
2
-1
cm corresponds to t-ZrO
2
).
-1
The bands appearing in the 950 and 996 cm range have been attributed to the stretching of
-1
terminal Mo=O bonds [43–47]; when this band appears at high wavenumber (ca. 990-997 cm ) and
-1
that is accompanied by a band at 819-820 cm [43,44], it is a strong indication of the formation of
MoO crystallites. These bands can even be observed at Mo concentration for which the
crystallization of MoO is not detectable by XRD due to their small size. Based on this, 10-
MoZrOH-450, 15-MoZrOH-450 and 10-MoZrO2-450 contain MoO crystallites.
The band corresponding to (Mo=O) modes in MoO octahedra within polymolybdates is
3
3
3
6
-1
located at lower wavenumbers (ca. 974-978 cm ) and its exact position is very sensitive to the size
of the polymolybdate cluster and to the degree of hydration [36,43,44,46]. It is associated with a
-1
band at ca. 870-885 cm that is due to Mo-O-Mo vibrations in two-dimensional polymolybdates
47–49]. From these assignments, the solids 5-Mo-ZrO2-450, 10-MoZrO2-450, 10-MoZrOH-450
[
and 15-MoZrOH-450 contain polymolybdates clusters.
The band corresponding to (Mo=O) modes in isolated molybdates is located at even lower
-1
wavenumbers, ca. 956 cm [43,48]. Based on this, the solid 5-MoZrOH-450 contains isolated
-1
molybdates. The second band, observed at 821 cm on the spectrum of this solid may correspond to
the antisymmetric vibration mode of Mo-O-Zr [43,50].
8

3
.1.2 The solids Mo-Zr calcined at 700°C
Figure 3.b presents the Raman spectra of the Mo-Zr systems calcined at 700°C. For all these
solids, the two bands characteristic of the formation of crystalline MoO are absent. Instead, new
bands at 940 and 740 cm are clearly visible. They have been assigned, together with a (Mo=O)
3
-1
-1
stretching band at 996 cm , to the formation of the mixed oxide Zr(MoO
4
2
) [43,44]. These three
bands are clearly visible on the spectra of 10-MoZrOH, 15-MoZrOH and 10-MoZrO2, i.e. for the
solids having the highest molybdenum densities. Moreover the Raman spectra of these solids exhibit
-1
bands at 976-980, 950-966, 850-884 and 807-810 cm . The first two bands were attributed to the
stretching vibration mode of terminal Mo=O bonds within the polymolybdates and isolated
molybdates respectively, the bands at 850-884 cm-1 and 807-810 cm-1 correspond to the
antisymmetric vibration modes (Mo-O-Mo) and (Mo-O-Zr) respectively.
The spectra of the solids with the lowest molybdenum loading (5%) exhibit a broad band in
the Mo=O stretching region. The broadness of this band indicates that it may be constituted by two
bands corresponding to the Mo=O stretching bond in isolated molybdates and polymolybdates. The
presence of the bands corresponding to the Mo-O-Zr and Mo-O-Mo stretching confirms the
coexistence of both types of molybdates.
From the analysis of the Raman spectra, it can be concluded that molybdenum on zirconia
exists as isolated molybdate species only at low molybdenum density. At 450°C, for intermediate to
high Mo densities, polymolybates are observed, and also, at densities higher than 4.46 Mo-at/nm²,
crystalline MoO
Zr(MoO are observed at high loadings (molybdenum density  5.9 Mo-at/nm²) whereas bands
characteristic of polymolybdates are observed at all loadings and bands of isolated molybdates are
observed at low and intermediate loadings. 700°C is well above the Tammann temperature of MoO
hence, a thermal spreading of MoO is expected to occur during thermal treatment at 700°C [51],
3
is formed. At 700°C, the MoO
3
bands are absent and bands characteristic of
)
4 2
3
;
3
which could explain why the samples calcined at 700°C contain, at the same Mo loading, a larger
proportion of (poly)molybdate in interaction with the support (antisymmetric Mo-O-Zr vibration at
-
1
8
07-810 cm ) than those calcined at 450°C.
3
.1.3. The solids Mo-Ti
The Raman spectra of the Mo-Ti solids are represented on Figure 4. They all have been
-1
normalized with respect to the strong band at 634 cm assigned to TiO (Anatase).
2
-
1
All the spectra displayed a band at 953-976 cm ; as the molybdenum density increases, this
band is blue-shifted and its intensity increases. This band has been attributed to the stretching of
terminal Mo=O bonds of surface molybdate species. For 5-MoTiOH-450 (i.e. the solid with the
9

-1
lowest molybdenum density), this band is located at 953 cm and could be assigned to the Mo=O
stretching mode of isolated molybdates with a tetrahedral configuration [41,48].
As the molybdenum density increases, the (Mo=O) band shifts to higher wavenumbers (ca.
-1
9
68-976 cm ). This can be explained by a growth of polymolybdates species with an octahedral
-1
-1
coordination [48]. Additionally, new features appear at ~815-820 cm and 878 cm . The band at 878
-1
cm is attributed to Mo-O-Mo linkages within polymolybdates species [48], whereas the band at
8
15-820 cm-1 is assigned to the antisymmetric stretching of Mo-O-Mo bonds within micro-
-1
crystalline MoO
3
.Another band characteristic of MoO is expected around 995 cm . It is observed on
3
-1
the spectrum of 10-MoTiO2 spectra (at ca. 991 cm ). It is however not clearly visible on the spectra
-1
of 5-MoTiO2 and 15-MoTiOH but may be masked by the intense band at 976 cm band for these
two solids.
3
.4. Isopropanol decomposition tests
The isopropanol (IPA) decomposition reaction has been widely used to probe acid-base
and/or redox properties of catalysts depending on the reaction atmosphere[52]. Indeed, under inert
atmosphere, the IPA undergoes dehydration leading to propene and diisopropylether over acid and/or
basic sites and is dehydrogenated to acetone over basic sites according to the following reactions:
Intramolecular dehydration
Intermolecular dehydration
Dehydrogenation
(CH
2 (CH
(CH
3
)
2
CHOH  CH
CHOH  (CH
CHOH  CH COCH
2
CHCH
CHOCH(CH
+ H
3
+ H
2
O
3
)
2
3
)
2
3
)
2
+ H O
2
3
)
2
3
3
2
According to the literature, solid oxides with acidic character lead mainly to the formation of
propene and diisopropylether; dehydrogenation reaction requires strong basic sites and occurs at
temperatures above 573K[53]; propene formation necessitates strong acid sites [54,55] whereas the
formation of diisopropylether involves weak acid sites [56].The conversions and the selectivities for
isopropanol decomposition reaction are reported in Table 2.
Bare supports, prepared by calcination of Zr(OH)
at the reaction temperature chosen for this study. ZrO
4
and Ti(OH)
4
at 450 or 700°C, are inactive
2
and TiO possess mainly Lewis acid sites
2
4
+
4+
associated with Zr and Ti ions [57,58]. Yet, these Lewis acid sites seem to be inactive towards the
isopropanol decomposition at 120°C. This is likely due to the fact that, at moderate temperatures, the
Lewis acid sites of oxides such as alumina, titania and zirconia are hydroxylated and hence their
Lewis acidity is masked (replaced by the weak Brønsted acidity of the surface hydroxyls) and a
thermal treatment above 300°C is required to reveal it [59–61]. Moreover, considering that the
1
0

isopropanol decomposition reaction produces water, even if a few Lewis sites were present after the
pretreatment at 250°C, they would deactivate immediately by reaction with water molecules
produced by the reaction. This finding is consistent with the results reported by Baertsch [62] who
found that aluminium oxide is inactive in butanol dehydration reaction at 130°C despite the presence
of Lewis acid sites.
The inactivity of the bare supports and the development of a catalytic activity upon the
introduction of molybdenum on the support surfaces may indicate that the acid activity of the solids
is mainly due to Brønsted acid sites in agreement with the conclusions of Samaranch et al. [63].
Besides the supports, all the supported catalysts, except 5-MoTiOH-450 and 5-MoZrOH-450,
present an appreciable catalytic activity. Propene is the major product and the proportions of ether
and acetone are very small on the most active catalysts. Only the samples with the lowest Mo loading
(and hence the lowest activity) have significant selectivities towards ether and acetone.
Assuming that the formation of propene is directly related to the abundance of Brønsted acid
sites, a correlation between the propene yield and the molybdenum density has been attempted
Figure 5). The correlation is quite good but not perfect as the propene yield happened to vary quite
(
irregularly with the molybdenum density. A more careful look to Figure 5 reveals that the correlation
between propene yield and molybdenum density is very good when considering the two preparation
protocols separately (Figures 6.a & 6.b).
For the two preparation methods the maximum propene yield is obtained for the solids
presenting a molybdenum density of about 6 Mo-At/nm². This indicates that these solids present the
highest proportion of strong acid sites as the rate of dehydration to propene increases with the
strength of the acid sites [60]. The solids with densities below 5 Mo-At/nm² present the lowest
propene yields but exhibit at the same time the highest ether selectivities (Table 2). This clearly
indicates that they contain weak acid sites as the dehydration to ether does not depend on the acid
sites strength.
Hence, in addition to the molybdenum density, the preparation protocol affects the number
and the strength of the created acid sites. Indeed, if we compare the propene yield obtained on solids
presenting close molybdenum densities and supported on the same support, but synthesized via
different protocols (Table 3), we observe that impregnation on the hydroxide leads always to a
higher propene yield and hence to a surface with stronger acid sites.
Based on this, we can conclude that the variation of the concentration and strength of acid
sites as a function of the molybdenum density and protocol preparation can be related to the structure
of molybdenum species, as follows:
1
1



At very low molybdenum density (0-3.3 Mo-At/nm², Solids 5-MoZrOH-450, 5-
MoTiOH-450 and 5-MoZrO2-450): the molybdenum exists mainly as isolated
molybdates species and the activity of these solids is very low or null which means
that isolated molybdates do not have Brønsted acid sites.
As molybdenum density increases (3.5-5.5 Mo/nm², Solids 10-MoTiOH-450, 10-
MoZrOH-450, 5-MoZrOH-700, 5-MoZrO2-700): the molybdenum exists as
polymolydates and isolated molybdates. For these solids the isopropanol conversion is
within 12-27% and the propene selectivity is between 63 and 83%, which indicates
the formation of acid sites with different strength.


At molybdenum density close to 6 Mo-At/nm² (Solids 10-MoZrOH-700 and 10-
MoTiO2-450): Raman spectroscopy and XRD have shown that these solids contain
polymolybdates species with small amounts of microcrystalline MoO
3
for 10MoTiO2-
4
50 and Zr(MoO for 10-MoZrOH-700. These solids present the largest proportion
4 2
)
of polymolybdates and led to the highest propene yield and hence have the most acid
surfaces (in terms of number and strength of acid sites).
At High molybdenum surface densities (8-17 Mo/nm²): the solids contain either
crystalline MoO
MoZrOH-450) or isolated molybdates, polymolybdates and large particles of mixed
oxide Zr(MoO (10-MoZrO2-700 and 15-MoZrOH-700). Involvement of part of
3
and polymolybdates (5-MoTiO2-450, 15-MoTiOH-450, 15-
)
4 2
the molybdenum in crystalline structures results in the diminution of the proportion of
surface molybdate species and leads to a lower propene yield.
From the above discussion, we can conclude that the polymolybdates clusters are the
molybdenum species responsible for the presence of strong Brønsted acid sites able to decompose
isopropanol to propene. The optimum proportion of these polymolybdates is reached at a
molybdenum density around 6 Mo-At/nm².
4
. Conclusion
TiO and ZrO
ammonium heptamolybdate on the corresponding oxyhydroxide MO
2
2
supported molybdenum oxide solids have been prepared by impregnation of
(OH)4-2x and oxide MO
x
2
.
An increase of the surface area has been observed upon the addition of molybdenum for most
of the prepared solids. Moreover, the solids prepared by impregnation on the hydroxides have higher
surface areas than the solids prepared by impregnation on the oxide. This has been assigned to an
inhabitation of the grain-growth of the supports by the introduction of molybdenum; this grain-
growth inhibition also hinders the tetragonal to monoclinic transformation for zirconia upon thermal
1
2

treatment. Raman spectroscopy allowed determining the speciation of molybdenum (isolated
molybdate, polymolybdate, MoO and Zr(MoO ) and was shown to better detect the formation
crystalline MoO than XRD. Raman spectroscopy especially showed that the polymerization of
3
)
4 2
3
molybdenum species on the surface of the support is higher for the catalysts prepared with the oxides
than with the oxyhydroxides.
The study of isopropanol decomposition led to the conclusion that, at comparable
molybdenum densities, impregnation on the oxyhydroxide leads always to a surface with stronger
acid sites. The strength and the density of acid sites are optimal at a molybdenum density of ca. 6
Mo-At/nm². The correlation between the structure of molybdenum species and the activity related to
strong Brønsted acid sites (conversion of isopropanol to propene) allowed us concluding that the
polymolybdates clusters are the molybdenum species responsible for the strong Brønsted acid sites
and that the optimum proportion of these polymolybdates is reached at a molybdenum density of ca.
6
Mo-At/nm².
Funding
This work was supported by the Algerian Ministry of Higher Education and Scientific
Research “MESRS” and USTHB Chemistry Faculty (PhD scholarship and Travel Grant for N.
Lamine).
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2
1
1
7

T
T
T: t-ZrO M: m-ZrO +: MoO
3
T T: t-ZrO2 M: m-ZrO2 #:ZrMo2O8
2
2
(a)
(b)
T
1
5-MoZrOH-450
#
15-MoZrOH-700
T
T
T
T
T
T
M MM
#
#
1
0-MoZrOH-450
10-MoZrOH-700
T
T
T
T
T
T
T
T
T
#
M M
5
-MoZrOH-450
T
5-MoZrOH-700
T
T
T
M
T
M
1
0-MoZrO2-450
10-MoZrO2-700
M
M
T
#
M
TM
M
M
M
T
T
M
M M
+
M
#
+M+
T
M
M
5
-Mo-ZrO2-700
M
M
M
M
5-MoZrO2-450
M
T
M
M
M
M
M
T
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
T
M
M
Zr(OH) -450
4
M
M
M
Zr(OH) -700
4
T
M
2
0
30
40
50

60
70
80
20
30
40
50
2
60
70
80
2
Figure 1. X-Ray diffraction patterns of pure ZrO
2
and Mo-Zr samples calcined at (a) 450°C and (b)
7
00°C.
1
8

*:
TiO (anatase)
2
*
1
0-MoTiO2-450
*
*
*
*
*
*
*
*
*
5
-MoTiO2-450
1
5-MoTiOH-450
1
0-MoTiOH-450
5
-MoTiOH-450
Ti(OH) -450
4
2
0
30
40
50

60
70
80
2
Figure 2. X-Ray diffraction patterns for pure TiO and Mo-Ti solids calcined at 450°C.
2
1
9

7
43
46
8
85
9
44
(a)
8
19
(b)
884 940
6
46
9
96
9
76
9
90
8
20
6
43
642
6
45
15MoZrOH-700
8
08
1
5-MoZrOH-450
74
980
940
8
70
7
9
9
54
8
77
6
43
9
96
645
998
1
0MoZrOH-700
8
23
810
1
0-MoZrOH-450
56
9
76
9
9
54
8
20
6
13
8
80
6
34
5
MoZrOH-700
7
45
5
-MoZrOH-450
97
807
6
33
8
70
9
40
6
14
976
9
78
9
6
35
6
12
996
6
42
10-MoZrO2-700
67
6
13
8
1
0-MoZrO -450
2
976
9
75
5
-MoZrO -450
2
5-MoZrO2-700
6
00
700
800
900
1000
1100
600
700
800
900
1000
1100
-1
-1
Wavenumber (cm )
Wavenumber (cm
)
Figure 3. Raman spectra of zirconia supported molybdenum catalysts calcined at (a) 450°C and (b)
00°C.
7
2
0

8
15
17
8
78
975
6
34
992
10-MoTiO2-450
8
8
78
9
73
8
20
9
76
5
-MoTiO2-450
1
5-MoTiOH-450
0-MoTiOH-450
-MoTiOH-450
1100 1200
9
68
53
1
9
5
6
00
700
800
900
1000
-
1
Wavenumber (cm )
Figure 4. Raman spectra of Mo-Ti solids calcined at 450°C.
2
1

Figure 5. Propene yield as a function of surface molybdenum density.
2
2

(a)
(b)
Figure 6. Propene yield as a function of molybdenum density; (a): hydroxide impregnation protocol;
b): oxide impregnation protocol.
(
2
3

Table 1. Physicochemical characteristics of zirconia, titania supports and of molybdenum supported
solids.
Solids
Calcination %Moa t-ZrO
2
BET area
(m /g)
surface Densityb
(Mo at/nm )
2
2
temperature
(vol.%)
Mo-Zr catalysts
ZrO
2
-450
450
450
450
450
450
450
700
700
700
700
700
700
-
41
97
100
100
30
73
11
100
96
94
71
99
111
67
80
90
25
49
98
37
47
53
5
1
1
5
1
-MoZrOH-450
0-MoZrOH-450
5-MoZrOH-450
4.1
7.9
12.5
3.9
8.3
-
4.1
9.2
10.2
4.1
8.1
2.6
4.5
11.8
3.0
5.8
-
5.2
5.9
17.2
5.4
9.6
-MoZrO
0-MoZrO
-700
2
-450
2
-450
ZrO
2
5
1
1
5
1
-Mo-ZrOH-700
0-MoZrOH-700
5-MoZrOH-700
-MoZrO
0-MoZrO
2
-700
-700
60
47
2
Mo-Ti catalysts
TiO
2
-450
450
450
450
450
450
450
-
5.0
7
11.8
5.1
64
93
115
93
37
73
-
5
1
1
5
1
-MoTiOH-450
0-MoTiOH-450
5-MoTiOH-450
-MoTiO
0-MoTiO
3.3
3.8
7.9
8.7
6.6
2
-450
-450
2
7.7
a
b
:
Determined from X-Ray Fluorescence analysis
: Calculated from the XRF molybdenum content value.
2
4

Table 2. Conversion and selectivities in the isopropanol decomposition reaction .
Solid
Surface Density
Conversion
(%)
Propene
selectivity
Ether
selectivity
(%)
-
Acetone
selectivity
2
(Mo-at/nm )
(
%)
(%)
-
-
ZrO
ZrO
2
-450
-700
-
-
Inactive
Inactive
Inactive
17.1
19.2
59.6
40.7
39.1
9.5
26.6
37.8
17.3
Inactive
1.4
11.8
56.1
21.3
74.8
-
-
-
2
-
-
5
5
1
1
1
1
5
5
1
1
-MoZrOH-450
-MoZrOH-700
0-MoZrOH-450
0-MoZrOH-700
5-MoZrOH-450
5-MoZrOH-700
-MoZrO2-450
-MoZrO2-700
0-MoZrO2-450
0-MoZrO2-700
2.6
5.2
4.5
5.9
11.8
17.2
3.04
5.4
5.8
9.6
-
3.3
3.8
7.9
8.7
6.6
-
65.8
63.1
96.6
93.8
100
53.1
83
85.2
85.2
-
58.2
74.9
95
75.1
98.4
32.9
20.8
2.8
3.7
0
15.5
10.8
10.1
10.7
-
27.9
20.2
2.2
1.3
11
0.6
2.5
0
31.4
6.2
4.7
4.1
-
TiO
2
-450
5
1
1
5
1
-MoTiOH-450
0-MoTiOH-450
5-MoTiOH-450
-MoTiO2-450
0-MoTiO2-450
10.4
2.7
2.8
9
16
0.9
0.7
2
5

Table 3: Propene and diisopropylether yields for solids with close molybdenum densities and
prepared via different protocols
Iso-density
Mo-At/nm²)
~6
~8
~10
(
Density value
5.75
5.89
7.86
7.9
9.56
11.76
(
Mo-At/nm²)
1
0MoZrO2-
10MoZrOH-
700
5MoTiO2-
450
15MoTiOH-
450
10MoZrO2-
700
15MoZrOH-
450
Solid
4
50
Propene yield
%)
32.2
57.6
16
53.3
14.7
38.2
(
Diisopropylether
yield (%)
3.8
1.7
3.4
1.2
1.9
1.5
2
6