Tailoring acid-metal functions in molybdenum oxides: Catalytic and XPS-UPS, ISS characterization study

Article abstract of DOI:10.1016/j.apcata.2014.02.006

Formation of acid, metal and metal-acid (bifunctional) functions in titania-supported molybdena following controlled partial reduction of MoO ₃ using hydrogen at different temperatures are monitored by a combination of surface XPS-UPS, ISS techniques. Addition of controlled amount of sodium or potassium alkali metals to the bifunctional MoO_2-x(OH) _y/TiO₂ (MoTi) enabled to neutralize the Br?nsted acid Mo-OH functions. Sodium or potassium molybdenum bronze metallic function is formed following the addition of the alkali metal to the Mo salt following its calcination at 773 K. The catalytic functions of these MoTi and Na, KMoTi systems were evaluated for the dehydration/hydrogenation, oxidation reactions of isopropanol to acetone as well as the ring shortening of cyclohexane to methylcyclopentane MCP and its oxidative dehydrogenation to benzene.

Full text of DOI:10.1016/j.apcata.2014.02.006

Applied Catalysis A: General 475 (2014) 497–502
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Tailoring acid–metal functions in molybdenum oxides: Catalytic and
XPS-UPS, ISS characterization study
a
b
a
a
a,∗
S. Al-Kandari , H. Al-Kandari , A.M. Mohamed , F. Al-Kharafi , A. Katrib
a
Chemistry Department, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
College of Health Sciences, PAAET, P.O. Box 1428, Faiha 72853, Kuwait
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2013
Received in revised form 5 February 2014
Accepted 5 February 2014
Available online 14 February 2014
Formation of acid, metal and metal–acid (bifunctional) functions in titania-supported molybdena fol-
lowing controlled partial reduction of MoO3 using hydrogen at different temperatures are monitored by
a combination of surface XPS-UPS, ISS techniques. Addition of controlled amount of sodium or potas-
sium alkali metals to the bifunctional MoO2−x(OH)y/TiO2 (MoTi) enabled to neutralize the Brønsted acid
Mo-OH functions. Sodium or potassium molybdenum bronze metallic function is formed following the
addition of the alkali metal to the Mo salt following its calcination at 773 K. The catalytic functions of these
MoTi and Na, KMoTi systems were evaluated for the dehydration/hydrogenation, oxidation reactions of
isopropanol to acetone as well as the ring shortening of cyclohexane to methylcyclopentane MCP and its
oxidative dehydrogenation to benzene.
Keywords:
MoO2−x(OH)y catalyst
XPS-UPS, ISS
Acid, metal functions
2
-Propanol
© 2014 Elsevier B.V. All rights reserved.
Cyclohexane
1
. Introduction
Certain catalytic reactions such as dehydration of alcohols,
Optimization of the MoTi catalytic performances in terms of
acid, metal or both metal–acid functions toward specific catalytic
reactions is achieved by defining the exact experimental con-
ditions of preparation and activation of the catalyst. Moreover,
addition of controlled amount of alkali metals such as sodium,
potassium and cesium in view of enhancing the metallic proper-
ties of MoTi is evaluated for well-known catalytic reactions. This
methodology consists of in situ characterization-catalytic activity
measurements conducted in the same experimental conditions. In
this work, we have added small amounts of sodium or potassium
to the impregnated molybdenum salt on titania. Catalyst activation
and performances were studied for the different catalytic reactions
of 2-propanol and cyclohexane. Several types of catalytic reactions
take place in these systems. Each of them is performed by specific
catalytic function(s) at certain experimental conditions. Associa-
tion of each catalytic reaction in the above systems with the active
catalytic function(s) in MoTi and modified MoTi by Na or K alkali
metals constitutes the primary objective of this work.
hydrogenation of olefins and isomerization of alkanes are per-
formed by specific acid, metal or both, metal–acid (bifunctional)
catalytic functions. Identification of the chemical structure(s) hav-
ing these functions present on the outermost surface layer in
heterogeneous catalysts constitutes a major challenge. Several
spectroscopic and physical techniques as well as catalytic reactions
are employed for such objective. The major difficulty relies on the
fact that catalytic active site(s) are formed in situ following specific
catalyst activation procedure(s) of the initial compounds. In other
words, exposure of the catalyst to air will cancel its effect due to
modification of its chemical structure. Consequently, characteriza-
tion results in this case will be misleading. In previous works [1–3],
we have reported the formation of a metal–acid (bifunctional) sys-
tem following controlled partial reduction by hydrogen of bulk or
supported MoO3 on TiO2 (MoTi). Identification of these catalytic
function(s) to specific molybdenum chemical species is based on,
in situ, characterization using XPS-UPS, ISS and FT-IR techniques in
parallel to its catalytic activity of well-defined catalytic reactions.
In this catalytic MoTi system, titania is employed as a support in
order to increase the surface area and add mechanical strength to
Mo oxide catalyst.
Dehydration of propanol and isomerization of cyclohexane are
typical catalytic reactions performed by Brønsted acid function(s)
[4–6]. On the other hand, dehydrogenation of 2-propanol to ace-
tone, hydrogenation of an olefin to saturated hydrocarbon and
dehydrogenation of cyclohexane to benzene are performed by
metallic function(s).
In order to get more insight about the formation and suppression
of Brønsted acid function, addition of small amount of alkali metals
such as sodium or potassium to the molybdenum oxide is used. It is
expected in this case that Na or K atoms will replace the hydrogen
∗ Corresponding author. Tel.: +965 24985582; fax: +965 24816482.
E-mail address: ali.katrib@ku.edu.kw (A. Katrib).
http://dx.doi.org/10.1016/j.apcata.2014.02.006
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

4
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S. Al-Kandari et al. / Applied Catalysis A: General 475 (2014) 497–502
in MoOH, if any, and lead to the suppression of the acid function(s)
in the catalytic system. Moreover, calcination of the modified sys-
tems is expected to produce sodium and potassium molybdenum
bronzes of certain metallic property [7,8]. The proposed, in situ, sur-
face chemical structure-catalytic activity methodology will enable
to identify the chemical process(es), responsible for the formation
of specific catalytic active site(s). Moreover, very weak electronic
interaction, if any, takes place between the partially reduced MoOx
O
(b)
Ti
Na
Mo
(
x < 3) and the titania surface as evaluated by the XPS of Ti(2p) after
each treatment. This is compared to strong electronic interaction
SMSI) which takes place with the alumina support and results in
the formation of aluminum molybdate complex [9,10].
(
(a)
2
. Experimental
2.1. Catalyst preparation
The equivalent of
5
monolayers of molybdenum triox-
ide were deposited on TiO2 using ammonium heptamolybdate
NH ) Mo7O ·4H O (99.9%) supplied by STREM Chemicals. Tita-
(
4
6
24
2
nium dioxide, TiO , is Degussa P-25 (25% rutile) with a pore volume
250
400
550
700
850
1000
2
3
2
of 0.5 cm /g and a BET surface area of 50 ± 5 m /g. The reported
Binding Energy(eV)
error margin of 10% by the supplier covers widely the experimental
2
value of 52 m /g performed in the laboratory. Supported catalysts
Fig. 1. ISS spectrum after hydrogen reduction at 673 K for MoTi (a), NaMoTi (b).
are prepared by impregnating the appropriate amount of molyb-
denum in ammonium heptamolybdate salt following the method
described by Pines et al. [11]. The exact amount of the molybdenum
Na, K modified MoTi catalytic systems. A continuous H2 flow of
40 cm3/min was allowed through 500 mg of the catalyst which
(
0.3 g of salt per gram of the support) is dissolved in distilled water
followed by the impregnation procedure. The excess water has
been eliminated by evaporation then the catalyst is dried at 383 K
for 8 h and eventually calcined at 773 K for 12 h. It is worth mention-
ing that the loaded ammonium heptamolybdate was quantitatively
decomposed into MoO3 species during the calcination process as
characterized by XPS. 2.5% concentration by mass of sodium or
potassium, in form of NaNO3 and KNO3 with respect to Mo were
added following post-impregnation of the dried heptamolybdate-
impregnated TiO2 pellets prior to the calcination process at 773 K.
contains 65 mg of Mo. The reaction mixture was separated and
analyzed with an on-line gas chromatograph Chemito, India 1000
equipped with a Petrocol-DH column and a flame ionization detec-
tor.
3. Results and discussion
3.1. Catalyst characterization
In situ reduction of the MoO /TiO system before and after Na and
3
2
3
K additions in a flow of 40 cm H /min as a function of temper-
Characterization of the active metal–acid functions in partially
reduced MoO3 deposited on TiO2 using XPS-UPS and ISS techniques
was reported in previous works [3]. Addition of controlled amounts
of Na or K to the MoTi structure and its effect on these functions will
be reported. Moreover, extended reduction of MoO3 to the metal-
lic Mo(0) and its effect on the dehydrogenation of cyclohexane to
benzene will be studied.
2
ature was carried out in different ways in order to elucidate the
surface structure and its stability in relation to catalytic activity.
The hydrogen gas was a 99.9% pure product of KOAC (Kuwait).
2.2. Catalyst characterization
X-ray photoelectron spectroscopy (XPS) was conducted using
a Thermo Scientific ESCALAB-250Xi spectrometer. The radiation
source was monochromatic of AlK␣ operating at the power
of 300 W (15 kV, 20 mA). Ultraviolet Photoelectron Spectroscopy
3
.1.1. ISS spectra
Elemental composition of the outermost surface structure of
MoTi, NaMoTi and KMoTi systems determined by ISS technique,
following in situ exposure of each sample to hydrogen at 673 K for
12 h reveals the presence of the alkali metal, Mo, Ti and oxygen
(
UPS) He(I) resonance 584 A˚ radiation of 21.217 eV was employed
for the Valence Band (VB) energy region measurements. Vacuum
−
9
in the analysis chamber was better than 7 × 10 bar during all
measurements. In situ reduction was carried out in a high-pressure
gas cell housed in the preparation chamber. Binding energies were
based on the carbon contamination C 1s at 284.8 eV within an
experimental error of ± 0.2 eV. Ion scattering spectroscopy (ISS)
measurements were performed on the same test sample using He+
with a kinetic energy of 1 keV.
(Fig. 1 and Table 1). The presence of titanium as part of the surface
can be attributed to the inhomogeneous deposition of Mo on TiO2
substrate. Most probably, the Mo oxide is present in form of aggre-
gate needle like crystallites structure as postulated by Bond et al.
[12].
Table 1
2.3. Catalyst tests
ISS relative intensities of catalysts elements composition following its exposure to
hydrogen at 673 K for 12 h.
Time on stream catalytic reactions under atmospheric hydrogen
Catalyst
% O
% Ti
% Mo
% Na
% K
pressure was studied. The reactant (2-propanol or cyclohexane)
was drawn from the reservoir throw HPLC pump of flow rate
of 0.1 ml/min then it passed through vaporizer and eventually it
passed over a fixed bed quartz reactor containing either MoTi or
MoTi
NaMoTi
KMoTi
49
45
47
38
31
23
13
16
22
–
8
–
–
–
8

S. Al-Kandari et al. / Applied Catalysis A: General 475 (2014) 497–502
499
Table 2
Characteristic BE, relative intensities, surface areas and alkali Na and K metals atomic ratio in different catalysts employed in this research work.
Catalyst
MoO3
Mo2O5
MoO2
Mo(IV)/Mo(VI)/Mo(VI) Surface area
Atomic ratio
2
relative intensities
(m /g)
3d3/2
3d5/2
3d3/2
3d5/2
3d3/2
3d5/2
Na/Mo
K/Mo
MoTi after calcinations
236.28 233.08
0/0/1
33.9
–
47.8
45.5
49.0
MoTi after hydrogen reduction at 573 K
MoTi after hydrogen reduction at 673 K
NaMoTi after hydrogen at 673 K
KMoTi after hydrogen at 673 K
236.38 233.18 234.68 231.58 232.78
236.08 232.88 234.68 231.48 232.88
235.88 232.68 234.98 231.78 232.18
229.59
229.68
229.08
229.42
0.57/0.32/0.11
0.56/0.31/0.13
0.80/0.14/0.06
1/0/0
0.13
–
–
–
–
232.6
0.09
3.1.2. XPS-UPS spectra
to the formation of discrete entities of the modified system such as
Calcination of equivalent 5 monolayers of ammonium hepta-
Na MoO4 in the process of calcination of the system at 773 K. Con-
2
molybdate deposited on TiO2 (MoTi) surface at 773 K enabled to
convert all Mo to MoO3 structure (Fig. 2a). In situ exposure of the
sample to hydrogen at different temperatures up to 673 K results
in gradual reduction of MoO3 as follows: MoO → Mo O5 → MoO
sequently, partial depolymerization of the polymolybdate, present
on the sample surface, will take place in the reduction process by
hydrogen [3].
The UPS of the calcined MoTi sample does not show any den-
sity of state at the Fermi level (Fig. 3a). This is in agreement with
3
2
2
characterized by Mo 3d5/2,3/2 spin–orbit coupling at 233.1, 236.3 eV
(
(
MoO ), 231.7, 234.9 eV (Mo O5) and 229.1, 232.3 eV for MoO
Fig. 2b). A stable structure takes place at 673 K, at which the three
the insulating properties of MoO . Sample exposure to hydrogen at
673 K for 12 h results in the presence of two structures at 0.4 and
3
2
2
3
Mo(VI), Mo(V) and Mo(IV) states co-exist at a certain stoichiomet-
ric ratio regardless of exposure time of the sample to hydrogen
1.4 eV (Fig. 3b), assigned to the ␲ and ␴ spectral lines in MoO struc-
2
ture [13,14]. The specific metallic properties of MoO2 deformed
rutile structure is the result of the delocalized ␲ electrons above the
Mo atoms. Taking into consideration the limited UPS depth analy-
sis of few angstroms (Å) as compared to XPS, it is concluded that
(Table 2). The MoO2 with its metallic properties constitutes the
outermost surface layer (Fig. 1c). Further increase in the reduction
temperature to 873 K results in the reduction of molybdenum to
the metallic Mo(0) state characterized at 227.7 and 230.9 eV of
Mo 3d5/2,3/2 spin–orbit components. Moreover, the surface area
MoO , having metallic properties, is constituent of the outermost
2
surface layer of the partially reduced MoTi sample. On the other
hand, the Brønsted Mo-OH acid function is observed in form of O
1s shoulder at 531 eV as well as FT-IR measurements [3,15].
The two ␲ and ␴ spectral lines are also observed in the case
of modified MoTi by alkali metals (Fig. 3c and d). In this case, the
metallic function of MoO2 remains unaffected. However, Brønsted
Mo-OH is suppressed due to the replacement of the acidic Mo-OH
hydrogen by the metal Na or K atoms [3].
2
2
increased from 47.8 m /g at 673 K to 60.0 m /g at 773 K (Table 2).
It is interesting to note that addition of alkali metal improves the
reduction process of MoO3 to the MoO2 structure at 673 K (Fig. 2d)
as compared to pure MoO /TiO2 system (Fig. 2c). The exact rea-
3
son(s) for such difference in the reduction process can be attributed
(d)
(e)
(c)
(d)
(b)
(c)
(b)
(a)
(a)
226 228 230 232 234 236 238 240
-2
-1
0
1
2
3
4
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. XPS of Mo(3d) energy region of calcined MoTi at 773 K (a), after reduction by
hydrogen of MoTi at 573 K for 2 h (b), after reduction by hydrogen MoTi at 673 K for
Fig. 3. UPS of calcined MoTi at 773 K (a); after reduction by hydrogen for 12 h at
673 K: MoTi (b), NaMoTi (c), KMoTi (d); after reduction by hydrogen for 12 h at 873
MoTi (e).
2
h (b), after reduction of NaMoTi by hydrogen at 673 K for 2 h (d).

5
00
S. Al-Kandari et al. / Applied Catalysis A: General 475 (2014) 497–502
100
Conversion
Conversion
100
Propane
8
6
4
2
0
0
0
0
0
8
6
4
0
0
0
Propene
Propane
Acetone
20
Propene
700
Acetone
500
0
4
00
500
600
4
00
600
700
Reduction Temp. (K)
Reaction Temp. ( K)
Fig. 4. Variation of the 2-propanol conversion and selectivity to main reaction prod-
ucts during the 2-propanol catalytic reaction as a function of calcined MoTi reduction
temperature for 1 h each. The reaction temperature is the same as the reduction
temperature.
Fig. 5. Variation of the 2-propanol conversion and selectivity as a function of reac-
tion temperature to main reaction products during the 2-propanol catalytic reaction
on reduced MoTi by hydrogen at 673 K for 12 h.
reduction process at 673 K, a stable bifunctional (metal–acid) struc-
3.2. Catalytic measurements
ture in form of MoO2 (OH)y/TiO is formed. Thus, catalytic activity
−x
2
of this system is controlled by kinetic and thermodynamic effects.
Apparently, the relative concentration of the Brønsted acid sites
on the surface of this bifunctional system is less than in the cal-
cined MoTi prior to the reduction process. Catalytic performances
of the bifunctional system toward 2-propanol is studied as a func-
tion of reaction temperature. Complete conversion of 2-propanol
takes place beyond 450 K reaction temperatures with continuous
increase in propane concentration to reach 92.5% at 673 K (Fig. 5).
It is worth mentioning that no acetone is produced on MoTi before
and after hydrogen treatments regardless of the reaction tem-
perature. Apparently, dehydrogenation of 2-propanol to acetone
requires stronger metallic function, high DOS structure, than what
The formation of metal and acid functions following the expo-
sure of MoO /TiO to hydrogen at 673 K for 12 h is represented in
3
2
a new MoO2 (OH)y/TiO2 bifunctional structure. The modification
−x
of this system by the addition of 2.5% by mass of alkali Na or K
metals will be evaluated using well-known catalytic reactions of
2
-propanol, cyclohexene and cyclohexane.
3
.2.1. 2-Propanol
The main catalytic reactions of 2-propanol performed by either
acidic, metallic or both functions are dehydration to propene
acidic), followed by its hydrogenation to propane (metallic). A
more difficult reaction is the dehydrogenation to acetone per-
formed by a metal function. In the case of calcined MoTi system at
(
is present in form of ␲ delocalized electrons in MoO2 (OH)y/TiO2.
−x
The catalytic behavior of MoTi following the addition of 2.5%
of Na or K alkali metal is completely different than the unmodi-
fied MoTi system. Different types of metallic characters seem to be
formed on the NaMoTi or KMoTi surfaces depending on the reduc-
tion temperature (Figs. 6 and 7). Acetone formation takes place at
reduction-reaction temperatures between 423 and 523 K with a
maximum of 54.2% in selectivity and 10.7% conversion at 423 K in
NaMoTi. Higher conversion of 38.1% and 50.9% selectivity to ace-
tone was obtained at 473 K in the case of KMoTi (Fig. 7). The acetone
selectivity decreases to 1.0% while the conversion increases to 100%
at 573 K (Figs. 6 and 7). In parallel, a propene selectivity of 38.3%
is observed at 423 K and increases to 99.0% at 573 K. Partial hydro-
genation of the formed propene takes place at higher temperatures
than 573 K with a maximum of 55% at 673 K (Figs. 6 and 7). The
above mentioned catalytic results of Na, KMoTi as a function of
7
73 K and prior to the reduction treatment by hydrogen, complete
conversion of 2-propanol to propene takes place at 423 K (Fig. 4).
This is in agreement with FT-IR measurements related to the pres-
ence of Brønsted type acid functions in MoTi prior to hydrogen
reduction [3]. Gradual increase in the reduction temperature up
to 673 K for 1 h each under hydrogen is carried out. It is worth to
mention that the reaction temperature, in this case, is the same
as the reduction temperature. A conversion of 100% of 2-propaol
is obtained regardless of the reduction (reaction) temperature up
to 673 K. However, the selectivity to propene and propane varies
as a function of reaction temperature. Although all the reacted 2-
propanol molecules were dehydrated to propene at 423 K, part of
the produced propene (11.6%) were hydrogenated to propane at
5
23 K. A maximum of 81.7% in propane selectivity at 100% conver-
sion were obtained at 673 K (Fig. 4) These results are consistent
with the relative increase of metal function concentration on the
surface of the sample expressed in terms of the reduction of MoO3
to MoO2 as observed by XPS (Fig. 2) [15]. Furthermore, the con-
version and selectivity remain unchanged at 673 K as a function of
reaction time. In other words, hydrogenation of propene to propane
is related to the reduction process of MoO to MoO which reaches
a stable state at 673 K as demonstrated by XPS-UPS and catalytic
results [2]. In this context, it is of interest to compare the catalytic
performances of the MoTi system upon its gradual reduction as
discussed above and its direct reduction at 673 K for 12 h. Indeed,
in the direct reduction process at 673 K, interpreted as the for-
100
80
Conversion
6
4
2
0
0
0
0
3
2
Propane
Propene
Acetone
500
mation of the bifunctional MoO2 (OH)y/TiO , the conversion of
2
−x
2
4
00
600
700
-propanol is 13.8% at 423 K reaction temperature (Fig. 5) as com-
Reduction Temp.
pared to 100%, obtained in the gradual process (Fig. 4). Moreover,
the nature of products in both reduction processes is different.
Only propene product is obtained in the gradual reduction pro-
tocol, as compared to 72% propene and 28% propane in the direct
one (Fig. 5). These results are interpreted as follows: In the direct
Fig. 6. Variation of the 2-propanol conversion and selectivity to main reaction
products during the 2-propanol catalytic reaction as a function of calcined NaMoTi
reduction temperature. The reduction process is 1 h for each temperature. The reac-
tion temperature is the same as the reduction temperature.

S. Al-Kandari et al. / Applied Catalysis A: General 475 (2014) 497–502
501
MecycloC5
1
00
Conversion
100
Conversion
C6H6
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Propane
Propene
Acetone
500
4
00
500
600
700
400
600
700
Reaction Temp. (K)
Reduction Temp.
Fig. 8. Variation of cyclohexane conversion and selectivity as a function of reaction
temperature to main methylcyclopentane and benzene reaction products during
cyclohexane catalytic reaction on reduced MoTi by hydrogen at 673 K for 12 h.
Fig. 7. Variation of the 2-propanol conversion and selectivity to main reaction
products during the 2-propanol catalytic reaction as a function of calcined KMoTi
reduction temperature. The reduction process is 1 h for each temperature. The reac-
tion temperature is the same as the reduction temperature.
and reaches a maximum of 59.2% at 673 K. The sequence of cyclo-
hexene reactions performed by specific catalytic functions of the
bifunctional MoO (OH)y/TiO2 system could be summarized as
follows:
reduction temperature could be interpreted as follows: addition of
2−x
the alkali Na or K alkali metal to the MoO /TiO2 system and its cal-
3
cination at 773 K seem to result in bronze formation with specific
metallic character [7,8]. Moreover, Brønsted acid function(s) are
present on the sample surface as revealed by FT-IR measurements
acidic
[
3]. These bronze metallic and Brønsted acid function(s) are respec-
tively responsible for the 2-propanol dehydrogenation to acetone
as well as its dehydration to propene.
metallic
metallic
3.2.2. Cyclohexene and cyclohexane
The different catalytic functions formed on the surface of MoTi
and modified by the addition of alkali metals such as NaMoTi
will be evaluated for the possible catalytic reactions of cyclohex-
ene and cyclohexane. Hydrogenation of cyclohexene, ring opening
and dehydrogenation of cyclohexane are performed by metallic
function(s). On the other hand, isomerization of cyclohexane is
catalyzed by acid function. Dehydrogenation of cyclohexane to ben-
zene represents a safe and practical way of hydrogen storage and
transportation.
Platinum based catalysts are the most common and widely used
systems for dehydrogenation/hydrogenation of hydrocarbon com-
pounds. The active site, responsible for the dehydrogenation of
cyclohexane to benzene is attributed to finely dispersed particles
deposited on the surface of nanomaterials supports [16] or car-
bon black supports covered with microporous silica layers [17]. The
objective of microporous silica layers support is to avoid the cata-
lyst deactivation due to Pt particles sintering. Also, gold, palladium
and gold–palladium catalysts were employed for such objective
It is worth mentioning that ring opening catalytic process activity
is very low to negligible in this case.
The catalytic activity of the MoO2 (OH)y/TiO2 bifunctional
−x
system toward cyclohexane has been studied as a function of reac-
tion temperature. Initial activity of 0.8% of the system in form of
cyclohexane isomerization to methylcyclopentane MCP has been
observed at 473 K. The conversion increases to 59.9% with 97.5% of
MCP at 573 K (Fig. 8). Dehydrogenation of cyclohexane to benzene
starts at 623 K reaction temperature. Benzene product selectiv-
ity of 54.7% and 98% conversion were obtained at 673 K. These
results clearly demonstrate that isomerization of cyclohexane to
MCP, catalyzed by the Brønsted acid function of MoO2 (OH)y/TiO2
−x
is easier than the dehydrogenation process to benzene performed
by the metallic function of this catalytic system. The effect of
metal function particle size such as the delocalized ␲ electrons
along the Mo–Mo atoms in deformed rutile structure of MoO2 in
MoO2 (OH)y/TiO2 in comparison with the relatively large aggre-
−x
[
18]. Molybdenum deposited on various oxide supports were also
gates of Mo atoms in the Mo(0) metallic state is investigated.
Further increase in the reduction temperature of MoTi carried out
under hydrogen up to 873 K for more than 12 h enabled to con-
vert all the Mo atoms to the elemental Mo(0) state as revealed by
the XPS of the Mo(3d) energy region as well as the UP spectrum.
In other words, the relative density of states at the Fermi level is
substantially increased (Fig. 3d). The dehydrogenation activity of
cyclohexane on this Mo(0) system starts also at 623 K with a con-
version of 7.4% and 97.1% selectivity to benzene formation (Fig. 9).
At 733 K reaction temperature, the conversion increases to 81.4%,
while the benzene selectivity remains constant at 97%. Apparently,
623 K reaction temperature is the energy barrier for the dehydro-
genation of cyclohexane on this Mo based catalysts. Isomerization
of cyclohexane to MCP did not take place on the Mo(0) due the
absence of Brønsted acid function(s) formation on the surface of
this system.
employed for the dehydrogenation of cyclohexane to benzene [19].
Mo/Cr–Al–O catalyst was found to be effective in cyclohexane
6+
3+
dehydrogenation. The active phase is attributed to Mo and Cr
species present on the catalyst surface. On the other hand, Plat-
inum deposited on MoO is found to have relatively high selectivity
2
toward methylcyclopentane ring opening [20].
In the case of stable bifunctional MoO2 (OH)y/TiO2 structure,
−x
obtained following exposure of MoO /TiO to hydrogen at 673 K for
3
2
1
2 h, a conversion of 100% of cyclohexene and a selectivity of 97.5%
to cyclohexane were obtained at 423 K reaction temperature. Pro-
duced cyclohexane undergo two possible catalytic reactions, these
are isomerization to methylcyclopentane MCP and dehydrogena-
tion to benzene. Catalytic results show that isomerization activity
to methylcyclopentane is catalyzed first with a selectivity of 15.4%
at 473 K. It increases to 72.1% at 573 K, while the conversion remains
constant at 100%. To note at this point that ring opening of produced
MCP did not take place in this case. Dehydrogenation to benzene
seems to be more difficult. A selectivity of 15.4% is obtained at 623 K
Addition of alkali Na or K alkali metals to MoTi system results
in the suppression of the Brønsted acid function(s) formation, as
discussed above, is confirmed in the case of cyclohexane reactant.

5
02
S. Al-Kandari et al. / Applied Catalysis A: General 475 (2014) 497–502
Conversion
terms of dehydration to propene, performed by the Brønsted acid
100
function, followed by its hydrogenation to propane, were per-
formed by the metallic function. The relative concentration of
propane increases from 23.3% at 473 K to 92.5% at 673 K reac-
tion temperature. In the case of cyclohexene, a conversion of 100%
and 97.5% selectivity to cyclohexane were obtained at 423 K reac-
8
6
4
2
0
0
0
0
0
C6H6
tion temperature on MoO2 (OH)y/TiO2 catalyst. Moreover, 15.4%
−x
of the produced cyclohexane at 473 K is isomerized to methylcy-
clopentane and increases to 72.1% at 573 K. On the other hand,
dehydrogenation of the produced cyclohexane to benzene takes
place at higher reaction temperature with a maximum of 59.2%
MecycloC5
700
5
00
600
800
900
at 673 K. Further reduction of the bifunctional MoO2 (OH)y/TiO2
−x
Reaction Temp. (K)
under hydrogen at 873 K enabled to produce the elemental Mo(0)
structure. A conversion of 81.4% of cyclohexane to mainly benzene
with a selectivity of 97% was obtained at 733 K reaction tempera-
ture.
Fig. 9. Variation of cyclohexane conversion and selectivity as a function of reaction
temperature to main methylcyclopentane and benzene reaction products during
cyclohexane catalytic reaction on reduced MoTi by hydrogen at 873 K for 12 h.
Acknowledgements
No isomerization to MCP is observed. In fact, catalytic activity of
the order of 13% cyclohexane conversion to benzene starts at 673 K
on Na or K modified system. At higher reaction temperatures than
The support by Kuwait University through research grant #
SC03/09 and GFS project # GS02/08 is gratefully acknowledged.
7
50 K, the alkali metal modified MoTi system behaves in similar
way to the unmodified system in terms of dehydrogenation of
cyclohexane to benzene. This is attributed to the reduction of MoO3
to Mo(0) state.
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1
MoO2 (OH)y/TiO2 catalyst takes place at reaction temperatures
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[
between 473 and 673 K. Two consecutive catalytic processes in