Study of acid-base properties of supported heteropoly acids in the reactions of secondary alcohols dehydration

Article abstract of DOI:10.1016/j.cattod.2013.09.007

The dehydration of secondary alcohols (propan-2-ol and 4-methylpentan-2-ol) was catalyzed by heteropolyacids (HPAs) supported on different solids. Catalysts prepared with 20 wt.% of HPAs were calcined at 400 C and characterized by X-ray diffraction, Raman spectroscopy, XPS and N₂ adsorption measurements. Stability of the Keggin structure of supported HPAs and changes in textural properties of catalysts were analyzed. The catalytic conversion of alcohols to olefins and ethers has been studied over the catalysts prepared. All catalysts presented activity in the reactions, but only molybdophosphoric acid supported on ZrO₂ (MoP-Z) showed selectivity in the formation of acetone and methyl isobutyl-ketone (MIBK). Catalysts with tungstosilicic acid (WSi) and Tungstophosphoric acid (WP) were active in the formation to DIPE. The acid-base properties of the catalysts play a key role in route of the reaction mechanism.

Full text of DOI:10.1016/j.cattod.2013.09.007

Catalysis Today 220–222 (2014) 32–38
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Study of acid–base properties of supported heteropoly acids in the
reactions of secondary alcohols dehydration
J.G. Hernández-Corteza,∗, Ma. Manríquezb, L. Lartundo-Rojasc, E. López-Salinasa
a Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, A.P. 14-805, 07730 Mexico, D.F., Mexico
b ESIQIE, Instituto Politécnico Nacional, Unidad Profesional “Adolfo López Mateos”, Zacatenco, Del. Gustavo A. Madero, 07738 Mexico, D.F., Mexico
c Centro de Nanociencia y Nanotecnología, Instituto Politécnico Nacional, Unidad Profesional “Adolfo López Mateos”, Zacatenco, Del. Gustavo A. Madero,
07738 Mexico, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2013
Received in revised form 4 September 2013
Accepted 6 September 2013
Available online 7 October 2013
The dehydration of secondary alcohols (propan-2-ol and 4-methylpentan-2-ol) was catalyzed by het-
eropolyacids (HPAs) supported on different solids. Catalysts prepared with 20 wt.% of HPAs were calcined
at 400 ◦C and characterized by X-ray diffraction, Raman spectroscopy, XPS and N2 adsorption measure-
ments. Stability of the Keggin structure of supported HPAs and changes in textural properties of catalysts
were analyzed. The catalytic conversion of alcohols to olefins and ethers has been studied over the
catalysts prepared. All catalysts presented activity in the reactions, but only molybdophosphoric acid
supported on ZrO2 (MoP-Z) showed selectivity in the formation of acetone and methyl isobutyl-ketone
(MIBK). Catalysts with tungstosilicic acid (WSi) and Tungstophosphoric acid (WP) were active in the
formation to DIPE. The acid–base properties of the catalysts play a key role in route of the reaction
mechanism.
Keywords:
Heteropoly acids supported
Dehydration secondary alcohols
DIPE
MIBK
© 2013 Elsevier B.V. All rights reserved.
Olefins and ether production
1. Introduction
4-Methylpent-1-ene is the starting material for manufacturing
thermoplastic polymers of interest for their technological proper-
The acidity and basicity of solid catalysts are two influen-
tial factors in their activity and selectivity, not only in typical
acid–base reactions, different authors reported on the impor-
tance of acid–base pair site in many catalytic processes [1,2]. The
acid–base properties of the solids are involved in the catalysis of
the dehydration reaction producing olefins and ethers that can be
accompanied too by alcohol dehydrogenation to the corresponding
ketone. Indications as to an application can be found in the patent
literature: secondary alcohols were reported to give ␣-alkenes over
thorium and cerium oxides supported on alumina [3] and zir-
conium oxide catalysts were found to be very selective toward
alk-1-ene formation [4,5]. Ether synthesis from alcohol is known
to be an acid catalyzed reaction; however, one of the undesirable
products is hydrocarbons. Extensive studies on methanol dehy-
dration, mainly over Al2O3, have been carried out to address the
reaction mechanism. Knözinger and co-workers [6] have proposed
that the ether formation takes place via a surface reaction between
the adsorbed alcohol molecule on an acidic site and an adsorbed
alkoxide anion on a basic site. Other authors have suggested that a
Lewis acid-basic pair is responsible for ether formation [7].
ties. This valuable alkene can be prepared through the dehydration
of 4-methylpentan-2-ol. The acid–base properties of the solid are
evaluated in the dehydration reaction, which can be accompanied
by alcohol dehydrogenation to the corresponding ketone.
Heteropolyacids (HPAs) constitute a alternative for the dehy-
dration reactions and others, as they are characterized by a strong
acidity, fundamentally of the Brönsted type, comparable to that of
fluorhydric and sulfuric acids; some of the more interesting HPAs
show the Keggin structure. The Keggin HPAs comprise heteropoly
anions of the formula [XM12O40] ,
n−, where X is the heteroatom (P5+
Si4+, etc.) and M the addendum atom (Mo6+, W6+, etc.). The struc-
ture of the heteropoly anion is composed of a central tetrahedron
XO4 surrounded by 12 edge- and corner-sharing metal-oxygen
octahedra MO6 [8]. The HPAs are the usual catalyst of choice
because of their acidic strength and relative thermal stability. A
serious problem associated with the use of this type of materials
as heterogeneous catalysts is their low surface area (∼5–8 m2 g−1)
[9]. The use of HPA in supported form is preferable because of its
high surface area compared to the bulk material. Acidic o neutral
solids, which interact weakly with HPAs such as silica, active carbon
and acidic ion-exchange resin, have been reported to be suitable as
HPA supports [10]. Bielánski et al. reported tertiary ether synthesis
using WSi on different supports as catalysts [11]. Recently, we had
shown that zirconia-supported WP acts as an efficient catalyst for
isomerization reaction [12,13].
∗
Corresponding author. Tel.: +52 55 9175 8409.
E-mail address: jhcortez@imp.mx (J.G. Hernández-Cortez).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.09.007

J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
33
Table 1
Tungstophosphoric acid (H3PW12O40), tungstosilicic acid
Surface area, pore volume and pore diameter of the different solids.
(H3SiW12O40) and molibdophosphoric acid (H3PMo12O40) are the
most representative of the family of heteropoly acids (HPAs), and
their structure is Keggin type [14]. These compounds show high cat-
alytic activity as much in acid–base reactions and oxide–reduction.
On the other hand, the HPAs have other special properties, which
are very useful in catalysis, such as high solubility in water and in
some polar organic solvents, a high thermal stability in solid state
and ability to form pseudo-liquid phases. All these properties make
possible their use in homogeneous and heterogeneous catalysis
[15,16].
The objective of the present study is the evaluation of
various solid acid catalysts for alcohols dehydration, mainly
4-methylpentan-2-ol and propan-2-ol. A correlation between cat-
alytic activity and the acid–base properties of the catalysts were
investigated.
Sample
SBET (m2 g−1
)
Pore volume (cm3 g−1
)
Average pore diameter (Å)
Z
Z*
S
ZS
204
132
220
207
229
0.24
0.29
1.05
0.95
0.22
0.16
0.86
0.22
n.e
47
69
191
184
40
83
151
32
n.e
n.e
MoP-Z
MoP-Z* 77
WP-S
WP-Z
WSi-Z
WP-ZS
227
272
n.e.
n.e
n.e
n.e. = not evaluated.
Z* stands for ZrO2 calcined at 400◦C.
2.3. Activity test of catalysts
The catalytic activity of solids was evaluated on the reactions
of dehydration of secondary alcohols (4-methylpentan-2-ol and
propan-2-ol), these reactions were carried out in a fix-bed quartz
tubular reactor. Previous to the reactions, the catalyst was activated
at 350 ◦C with He flow (40 ml/min) during 1 h. On the propan-
2-ol dehydration, 0.1 g of sample was used with 60 ml/min He
flow as carrier gas (molar ration of He/propan-2-ol = 5). Dehy-
dration of 4-methylpentan-2-ol was carried out with 0.05 g of
catalyst and 165 ml/min He flow as carrier gas (molar ration of
He/4-methylpentan-2-ol is 39). The products of the reactions were
analyzed with a gas chromatograph Varian 3600 CX, connected to
the outlet of the reactor and equipped with a Flame Ionization
Detector (FID) and PONA capillary column. The reaction rate and
conversion were calculated assuming a first order reaction. The
following parameters were calculated as follows:
2. Experimental
2.1. Preparation of catalysts
SiO2-Gel (Merk-7734), ZrO2/SiO2 (Grace 18301-14 with 13 wt.%
of ZrO2), ZrO2 (prepared in laboratory by precipitation pH = 10)
[12,17] and ZrO2 thermal treatment at 400 ◦C (Z*) for 4 h is used
as support. These supports are referred like S, ZS, Z and Z*, respec-
tively. The HPAs are supported by the impregnation method by the
following way: First, solutions of HPAs/ethanol are prepared. Later,
the necessary volume of these solutions is added in the different
supports, to obtain 20 wt.% of HPAs, the solvent is evaporated using
a rotavapor. Finally, the catalysts are dried at 120 ◦C and calcined at
400 ◦C for 4 h. The prepared catalysts were denoted as X-Y, where
X is the corresponding HPA (WP, MoP or WSi) and Y is for S, ZS, Z
and Z*, respectively.
ꢀ
ni Yi
Xa (Conversion of secondary alcohol, mol%) =
× 100,
ꢀ
Cout
+
ni Yi
2.2. Characterization of catalysts
Selectivity Si to compound i:
The specific surface area, pore volume and pore size distribu-
tion of the samples were measured in an automatic adsorption
instrument (Quantachrome Autosorb 1C) using low-temperature
N2 adsorption–desorption isotherms. Prior to the measurements,
the samples were evacuated in situ at 300 ◦C for 3 h under vacuum.
The surface area was calculated from these isotherms using the
multi-point Brunauer–Emmett–Teller (BET) method based on the
adsorption data within the partial pressure P/P0 range from 0.05
to 0.35. The pore size distribution was determined by BJH method
from the desorption part of isotherm, the pore volume was deter-
mined from total volume of nitrogen adsorbed at P/P0 = 0.98–0.99.
All the diffraction patters (XRD) of the samples were obtained
with an Siemens D 5005 apparatus equipped with monochroma-
Yi
ni Yi
S (mol %) =
× 100,
ꢀ
i
where Cout is secondary alcohol mole percent in the outlet of reactor
and Yi are yields to the different products.
For a reaction of the type: A → products.
The reaction rate (−ra) was determined using the following
equation:
Fao Xa
−ra
=
m
where −ra = reaction rate [mol g−1 s−1
] Fao = molar flow of A
[mol s−1]; Xa = Conversion of A; and m = catalyst mass [g].
˚
tor of secondary beam for K-radiation ˛ = 1,5406 A (anode of Cu).
Raman spectra were recorded at room temperature on previously
calcined samples in a nearly backscattering geometry using an
ISA Labram micro-Raman apparatus. The excitation line was the
632.8 nm of a He-Ne laser. The laser power on the sample was
kept low (about 1 mW) to avoid thermal effects. The samples were
analyzed by X-ray Photoelectron Spectroscopy (XPS), the spectra
were acquired with a THERMO Scientific K-Alpha spectrometer
equipped with Al K␣ X-ray source (1486.6 eV) and a hemispher-
ical electron analyzer. Experimental peaks were decomposed into
components using mixed Gaussian–Lorentzian functions and a
non-linear squares fitting algorithm. Shirley background subtrac-
tion was applied. An intensity ratio of 2:3 and a splitting of 2.3 eV
were used to fit the Mo 3d peaks. Binding energies were repro-
ducible to within 0.2 eV and the C 1s peak at 284.6 eV was used
as a reference from carbon.
3. Results and discussion
3.1. Physico-chemical characterization
In Table 1, the effects caused by the addition of the HPAs on the
different supports on the specific area, pore volume and pore aver-
age diameter are shown. The ZrO2 calcined at 400 ◦C (Z*), shows
a surface area of 132 m2 g−1. When Z was calcined at 300 ◦C, the
area was 204 m2 g−1. The reduction of area is due to the structural
transformation by the effect of temperature. The presence of the
HPAs produces a stabilization of the area in the supports. The SiO2
(S) showed a high surface area of 220 m2 g−1 and pore volume of
1.05 cm3 g−1, when the support was impregnated with 20 wt % of
WP, the surface area increases to 227 m2 g−1. Similar effect hap-
pened with the samples WP-Z and MoP-Z, with areas of 272 and

34
J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
153, and two small bands to 198 and 218 cm−1 corresponding to
tetragonal phase zirconium [25,26]. The Raman bands assigned
to Mo O vibrations at 234 cm−1, these bands are assigned to
229 cm2 g−1, respectively. The pore volume and average diameter
diminished when HPAs were impregnated on the supports; this is
because Keggin unit has 12 A [18] of diameter and its location in
˚
the symmetric/antisymmetric stretching modes of terminal Mo
O
,
the pores of the supports produces a decrease in the volume and
diameter of the pores. However, in MoP-Z*, the surface area was
very low when MoP was supported on a previously calcined ZrO2
at 400 ◦C (Z*), this also affects the pore volume and pore diame-
ter. The samples WSi-Z and WP-ZS were not evaluated in textural
properties, however, we have reported that the addition of WP or
MoP on ZrO2 stabilizes the surface area of the resulting solid acid in
comparison with that of pure ZrO2 [12,19], so we expect a similar
behavior in the stabilization of the area of the WSi-Z and WP-ZS
samples.
groups. Finally, two bands are also observed at 375 and 335 cm−1
which is typical of the Keggin structure [22,26], indicating that
molybdophosphoric acid was not decomposed upon calcinations
at 400 ◦C. However, when MoP is supported on a hydrated zir-
conia, containing abundant OH groups (MoP-Z), the characteristic
peaks of MoP are not observed, suggesting its high dispersion [12].
Assuming this, a strong interaction between surface Zr OH groups
and MoP framework oxygens may be taking place, particularly with
terminal oxo Mo O bonds.
Raman spectra of WSi-ZS and WSi-S are presented in the same
Fig. 3. The Raman bands can be attributed to Keggin structure, in
accordance with the literature [27–29]. In WSi-ZS sample, Raman
bands from 119 to 205 cm−1 correspond to tetragonal zirconium.
Assignments of the observed bands are as follows, two strong
bands of 1003 and 803 cm−1 attributed to symmetric and asymmet-
ric stretching (W O), respectively. The Raman band at 1062 cm−1
attributed to symmetric stretching of the tungsten-terminal oxy-
gen (W O) within WO6 octahedral. Weak bands are also observed
Kozhevnikov reported that the thermal stability of HPAs has
the following order PW > SiW > MoP [28] and above of 450 ◦C, the
Keggin structure of H3PMo12O40 is completely destroyed [27]. In
the Figs. 1 and 2, XRD patterns of supports, pure HPAs and the
solids with a content of 20 wt.% of HPAs, within 2ꢀ range of 4–70◦
are shown. The XRD of pure HPAs (dried at 120 ◦C) showed large
number of narrow peaks, being the characteristic pattern of a crys-
talline structure type body-centered cubic [20]. The Z (ZrO2) and
ZS showed a broad peak, extending from 15 to 40◦ 2ꢀ range, which
indicated that the supports are present in the amorphous state.
In patterns of the samples, MoP-Z, WP-Z and WSi-Z, recorded
after calcining at 400 ◦C, a portion of the amorphous material
crystallizes into tetragonal ZrO2 showing no indication of any
crystalline phases related to HPAs. The reason of these results may
be that HPAs particles are too small and/or too well dispersed and
therefore undetectable by XRD. However, in samples WP-S and
WSi-S characteristic peaks related to the HPAs were detected. These
results suggest a strong interaction between HPAs particles and
ZrO2 surface, either in the hydrated or stabilized state. In the sample
MoP-Z*, all the amorphous material crystallizes into tetragonal and
monoclinic ZrO2, without detecting any crystalline phase related to
MoP. This good dispersion can be due to the mean pore diameter of
at 784 and 534 cm−1 attributed to (W
O W), see WSi-S sample.
The bands at 235 y 269 cm−1 corresponds to the bridging W
bending modes of the intact Keggin structure.
O W
Fig. 4 shows XPS spectra of the W 4f of the sample WSi-S. The
presence of two shoulders besides the main doublet are observed.
This spectrum was fitted with three different doublets of W 4f with
4f7/2 located at 33.6, 34.2, 34.6 and 38.0 eV. The major contribution
is from the first peak. The samples show only one form of W having
4f7/2 binding energy at 38.0 eV. Therefore, the peaks 33.6, 34.2 and
38.0 eV peaks are related to the presence of water on the sample
surface, the peak a 34.6 eV is assigned to interaction of phospho-
rus species with WO3. The signals at 33.6 Ev correspond to reduced
species of WO2 and W
O W. The signal at 38.0 eV corresponds
˚
to species of WO3. Oxygen (O 1s) presents the following contribu-
tion in the sample WSi-ZS (see Fig. 4). At 531 eV is assigned the
interaction of Oxygen with ZrO2, at 530 eV shows the interaction of
oxygen with WO3 species, already 528 eV is assigned O O species,
it is observed the interaction of oxygen with the W species in its
reduced state as WO2. At 532.27 eV the oxygen interaction with
SiO2 species, and finally to 529 eV are observed OH on the surface
[23,30,31]. The XP spectra of the ZrO2 used as support in the sam-
ple WSi-Z are shown in Fig. 4. The BE of the Zr (3d) photoelectron
peaks at 182.31, 184.31 and 185 cm−1. The peak at 184.31 eV corre-
sponds to interaction HPA with zirconia. The first signal of ZrO2 (Zr
3d5/2) at 182.31 and the third at 185 eV (Zr 3d3/2) are related to the
presence of water on the sample surface of ZrO2 [32,33]. This infor-
mation provides evidence of the presence of one type of ZrO2 with
an oxidation state of +4. The values of peaks for Mo 3d core level
at 228.38–233.08 suggest the presence of Mo (VI) in polymolyb-
dates species [34,35]. The value observed to 233.08 eV suggests the
presence of molybdenum in the oxidation states of MO3 that corre-
spond to higher binding energies of Mo 3d3/2. The peaks at 228.38
and 230.0 eV were assigned to Mo (V) species, accordingly energy
binding value to Mo3d5/2, is due to reduced molybdenum species
of MoO2 [36]. The other peak at 231.41 eV corresponds to MoOx
species [37]. This spectrum shows clearly the presence of reduced
molybdenum.
the supports which is bigger at 30 A (see Table 1) while the diameter
˚
of the HPA is about 12 A [18]. This difference in the pore diameter
can allow a more uniform distribution of HPAs on the supports
surface, minimizing the possibility of agglomerations which could
induce crystallization. Recently, we reported that the collapse of
the WP Keggin structure occurs at temperatures above 500 ◦C,
which yields WO3 and phosphates [17]. A comparison between
the XRD patterns of 20 wt % WP or WSi loaded on Z and S indicates
that the interaction of HPA with S is much weaker than that with Z.
The Raman spectrum of MoP-S (see Fig. 3) under ambient
conditions exhibits the characteristic bands of the Keggin struc-
ture. The relative intensities of the Raman bands due to Mo
O
vibrations at 839, 673, and 234 cm−1, are assigned to the sym-
metric/antisymmetric stretching modes of terminal and bridging
Mo O, and their bending vibrations, respectively. The band at
234 cm−1 corresponds to the bridging Mo
O Mo bending modes
of the intact Keggin. A pronounced shift of the Mo O bands are
observed from the band to 839 and 994 cm−1 of crystalline MoO3
and at 673 cm−1 band result of the three-bridged oxygen stretching
motion. The bands to 821 and 1006 cm−1 correspond of crystalline
MoO [21–23]. PO4 species, present a symmetric stretching mode
to 1071 cm−1 (spectrum Fig. 3) and finally the band at 342 cm−1
indicate the formation of a polyoxoanion with the Keggin structure.
The Raman spectrum of sample MoP-ZS is shown in Fig. 3.
The main bands characteristic of the Keggin heteropolyanion
(MoP) are 1006, 995, 920–896, 800, 620, and 245–214 cm−1
,
3.2. Activity test of catalysts
related, respectively, to stretching modes ꢁs(Mo Ot), ꢁas(Mo Ot),
ꢁs(Mo Ob Mo), ꢁs(Mo Oc Mo) and ꢁs(Mo Oa) [24]. A pro-
nounced shift of the Mo O band from 817 and 992 cm−1 observed
bands of crystalline MoO3 the symmetric stretching mode and
that at 1156 cm−1 of PO4 species. Raman bands at 113, 124,
In order to examine the catalytic activity of the supported HPAs,
the catalysts were evaluated in the decomposition of propan-2-ol
(Table 2). This reaction has been extensively reported as a valuable
catalytic test to analyze the acid–basic properties of heterogeneous

J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
35
WP pure
WSi pure
10
20
30
40
50
60
70
10
20
30
40
50
60
70
MoP pure
MoP-ZS
10
20
30
40
50
60
70
10
20
30
40
50
60
70
Z
ZS
10
20
30
40
50
60
70
10
20
30
40
50
60
70
WP-S
WSi-S
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2 Θ, degree
2 Θ, degree
Fig. 1. XRD patterns of pure HPA’s, supports and HPA-S samples.
catalysts [38]. Pure MoP only showed activity at 200 ◦C, this HPA
presented a conversion of 21.2 mol% and its selectivity was: propyl-
ene (87.2 mol%), acetone (8.8 mol%) and DIPE (4 mol%). The HPAs
supported on ZrO2 presented more catalytic activity than pure
ZrO2. The selectivity is oriented mainly toward the formation of
propylene and as reaction by-product to di-isopropil ether (DIPE).
It is important to notice that the DIPE formation is a potential
additive to increase the octane in reformulated gasoline. On the
other hand, the catalyst with lower activity is MoP-Z. Neverthe-
less, this solid besides dehydrating the alcohol, presented oxidant

36
J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
MoP-Z
MoP-Z*
10
20
30
40
50
60
70
10
20
30
40
50
60
70
WSi-Z
WP-Z
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2
Θ
, degree
Θ
, degree
2
Fig. 2. XRD patterns of HPA-Z samples.
MoP-ZS
MoP-S
100 200 300 400 500 600 700 800 900 1000 1100 1200
100 200 300 400 500 600 700 800 900 1000 1100 1200
-1
-1
Raman Shift, cm
Raman Shift, cm
WSi-ZS
WSi-S
100 200 300 400 500 600 700 800 900 1000 1100 1200
100 200 300 400 500 600 700 800 900 1000 1100 1200
-1
-1
Raman Shift, cm
Raman Shift, cm
Fig. 3. Raman spectra of the different catalysts.

J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
37
WSi-ZS (O 1s)
WSi-S (W 4f)
WO3
W-HP
WO2
OH
WO3
ZrO2
O-O
WO2
SiO2
W-O-W
33
32
34
35
36
37
38
39
528
530
532
534
Binding Energy (eV)
Binding Energy (eV)
Mo2
MoP-ZS
(Mo 3d)
WSi-Z (Zr 3d)
ZrO2
Mo3
Zr-HP
ZrO2
MoOx
Mo
178
180
182
184
186
188
226
228
230
232
234
236
Bindin enegy eV
Binding enegy (eV)
Fig. 4. X-ray photoelectron spectra of: O 1s, Mo 3d, W 4f and Zr 3d.
The results of the catalytic evaluation at 115 ◦C are presented
in Table 3. The supports used in this study do not present activ-
ity in the decomposition of propan-2-ol at this temperature. When
HPAs were deposited on the supports an increment was observed
in activity, which depends on the interaction of the HPAs with the
supports and the generation of acid sites [16]. We reported that
the impregnation of WP on Z yields strong solid acids (Ho ≤ −9.3,
calcined at 400 ◦C), the sites are predominantly Lewis acids when
WP was supported on hydrated ZrO2 and then calcined at 400 ◦C,
but both Bronsted and Lewis acid sites are observed when WP was
supported on a previously calcined zirconia (Z*) or with SiO2 (S)
[12]. The WP-S, WP-Z* and WSi-Z* catalysts showed the major
conversion of propan-2-ol with values of 4.9, 4.8 and 5.0 mol%,
respectively. The high activity of these catalysts is due to the for-
mation or presence of Bronsted acidity. The selectivity is oriented
mainly toward the formation of propylene and DIPE. The other
catalysts presented a lower conversion of propan-2-ol with the
selectivity oriented to the formation of propylene and DIPE. The
selectivity for propylene product (C3=) depended on the acid sites
type generated by HPAs on the supports. In the case of MoP-Z the
formation of 15.5 mol% of acetone was observed. The basic sites
contained in the MoP system reported in the literature [39,40]
have an important role in the selectivity to acetone. The basic sites
present on the catalysis of MoP-Z favor the selectivity to acetone.
In general it is conventionally accepted that acid sites are the
responsible for the dehydration activity yielding propylene. In a
similar way, in the production of acetone (dehydrogenated prod-
uct) acid and basic sites are required. On the other hand, for the
formation of the ether only acid sites of moderated strength are
involved. Considering our previously reported studies [12,16,17],
we can state that dehydratation and formation of DIPE takes place
Table 2
Conversion, reaction rates and selectivity in the reaction of propan-2-ol at 175 ◦C.
Catalyst
Conversiona
(mol%)
Selectivity (mol%)
−ra × 10−4
Propylene
Acetone
DIPE
(mol/g min)
Zb
1.4
21.2
29.9
51.2
22.2
100
87
80
86
95
0
9
0
0
5
0
4
20
14
0
0.5
8.2
11.6
19.8
8.6
MoPb
WP-Z
WSi-Z
MoP-Z
a
Taken after 15 min of reaction.
Reaction at 200 ◦C.
b
Table 3
Conversion, reaction rates and selectivity in the reaction of propan-2-ol at 115 ◦C.
Catalyst
Conversiona
(mol%)
Selectivity (mol%)
−ra × 10−5
Propylene
Acetone
DIPE
(mol/g min)
MoP-Z
WSi-Z
WSi-Z*
WP-Z
WP-Z*
WP-S
2.3
1.6
5.0
1.6
4.8
4.9
42.7
41.8
47.2
38.0
55.7
85.4
15.5
0.0
0.0
0.0
44.3
0.0
41.9
58.2
52.8
62.0
0.0
8.7
6.0
19.4
6.3
14.1
8.1
14.6
Z* stands for ZrO2 calcined at 400◦C.
a
Taken after 15 min of reaction.
properties because of Mo, which not only orients selectively toward
the dehydration of the alcohol, but also toward the formation of
acetone. The reaction rate is influenced by the type of HPA sup-
ported; WSi-Z and WP-Z catalysts had greater reaction rate of 19.8
and 11.6 mol/g min, respectively.

38
J.G. Hernández-Cortez et al. / Catalysis Today 220–222 (2014) 32–38
Table 4
secondary alcohols depends on the type of HPAs used. DIPE produc-
tion is formed only in catalysts that contain WSi and WP (acid sites
of moderated strength are required). The MoP-Z catalyst exhibited
mainly acid and base properties given that acetone and MIBK are
obtained in dehydration of secondary alcohols. These reactions can
be employed to study the acid–base properties of catalysts.
Conversion, reaction rates and selectivity in the reaction of 4-methylpentan-2-ol.
Catalyst
Conversiona
(mol%)
Selectivity (mol%)
−ra × 10−5
A
B
C
D
MIBK
(mol/g min)
WP
5.9
3.1
3.1
3.8
9.0
12.6
15.7
17.6
13.9
16.0
14.1
13.5
57.5
54.7
47.7
74.6
75.8
66.4
60.3
28.0
29.6
34.7
11.5
8.2
1.85
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
9.0
5.5
15.6
8.4
8.4
10.2
22.8
41.4
52.4
WP-S
WP-ZS
WP-Z
WP-Z*
MoP-Z
MoP-Z*
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18.0
22.5
10.5
20.7
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This paper has shown that a combination of Raman spec-
troscopy, X-ray photoelectron spectroscopy and X-ray diffraction
can be used to perform the characterization of supported HPAs.
The interaction between HPAs and supports influences the physi-
cochemical properties of prepared solids. The addition of the HPAs
stabilizes the surface area of the resulting solid in comparison with
the pure supports. The HPAs retained their Keggin structure when
they are supported. The activity of the catalysts in dehydration of