Catalytic activity of Mo oxide before and after alkali metal addition for methylcyclohexane and methylcyclopentane compounds

Article abstract of DOI:10.1016/j.molcata.2015.06.036

Abstract Different catalytic reactions of methylcyclohexane MCH are performed depending on the nature of the catalytic active site (s) and experimental conditions. Ring contraction RC catalytic processes, producing dimethylcyclopenanes DMCP's of high octane numbers as compared to MCH are catalysed by acidic function of zeolites systems such as HY. Better activity, selectivity and stability concerning these RC reactions were obtained using Pt/HY catalyst. At higher reaction temperature, dehydrogenation of MCH to toluene and hydrocracking reactions are catalyzed by Pt. Comparable catalytic behavior is obtained using a bifunctional (metal-acid) MoO_2-x(OH)_y/TiO₂ (MoTi) system. Different metallic character strength is observed following the suppression of the Br?nsted acid MoOH function(s) to MoO_2-x(OA)_y/TiO₂ (A = Na, K, Rb) by the addition of small amount of alkali metal A. Rubidium addition seems to be the most performant in the dehydrogenation of MCH to toluene. The metallic functions in MoTi and modified AMoTi are not efficient for RO in MCP. In-situ characterization of the different oxidation states of Mo at different experimental conditions were conducted using in-situ XPS-UPS techniques.

Full text of DOI:10.1016/j.molcata.2015.06.036

Journal of Molecular Catalysis A: Chemical 407 (2015) 189–193
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic activity of Mo oxide before and after alkali metal addition for
methylcyclohexane and methylcyclopentane compounds
H. Al-Kandarib, A.M. Mohameda, F. Al-Kharafia, A. Katriba,∗
a Chemistry Department, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
b College of Health Sciences, PAAET, P.O. Box 1428, Faiha 72853, Kuwait
a r t i c l e i n f o
a b s t r a c t
Article history:
Different catalytic reactions of methylcyclohexane MCH are performed depending on the nature of the
catalytic active site (s) and experimental conditions. Ring contraction RC catalytic processes, producing
dimethylcyclopenanes DMCP’s of high octane numbers as compared to MCH are catalysed by acidic
function of zeolites systems such as HY. Better activity, selectivity and stability concerning these RC
reactions were obtained using Pt/HY catalyst. At higher reaction temperature, dehydrogenation of MCH
to toluene and hydrocracking reactions are catalyzed by Pt. Comparable catalytic behavior is obtained
using a bifunctional (metal–acid) MoO2-x(OH)y/TiO2 (MoTi) system. Different metallic character strength
is observed following the suppression of the Brønsted acid Mo OH function(s) to MoO2-x(OA)y/TiO2
(A = Na, K, Rb) by the addition of small amount of alkali metal A. Rubidium addition seems to be the most
performant in the dehydrogenation of MCH to toluene. The metallic functions in MoTi and modified
AMoTi are not efficient for RO in MCP. In-situ characterization of the different oxidation states of Mo at
different experimental conditions were conducted using in-situ XPS–UPS techniques.
Received 8 January 2015
Received in revised form 29 June 2015
Accepted 30 June 2015
Available online 8 July 2015
Keywords:
MoO2-x(OH)y catalyst
XPS
Acid–metal functions
Methylcyclohexane
Methylcyclopentane
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
such as Pt–Rh supported on chlorinated alumina acidic support for
example, appear to be the most active for RO reactions. This cat-
In general, Catalytic reactions are rationalized in terms of
acidic, metallic or bifunctional catalysts. Methylcyclohexane MCH,
undergo several types of catalytic processes requiring the pres-
ence of either acidic, metallic or both metal–acid (Bifunctional)
functions. The most commonly used catalysts for such objectives
are acid catalysts such as zeolites and platinum based systems at
specific experimental conditions. In this respect, the principal cat-
alytic reactions of MCH are: ring contraction (RC)-isomerization
producing different non aromatic dimethylcyclopentanes (DMCP)
of higher octane number as compared to MCH [1]. The isomeriza-
tion reactions are catalyzed by Brønsted acid function(s) such as
the zeolite HY catalyst. However, beside the instability of the zeo-
lite catalysts, considerable increase in the relative concentration of
the highest octane number 1,1-DMCP is achieved using the bifunc-
tional Pt/HY as compared to HY zeolite alone. It is supposed that
platinum, in this case, activates the hydride transfer step [1–3].
Less desired ring opening (RO) processes leading to heptane and 2,
3- methylhexanes, of much lower octane number, seem to require
stronger metallic function(s) catalytic systems. Bimetallic catalysts
alytic process is rationalized in terms of bifunctional mechanism in
which MCH is isomerized to diverse dimethylcyclopentanes DMCP,
catalyzed by the acidic function as first step, followed by RO per-
formed by relatively strong metal function [4–5]. Dehydrogenation
of MCH to toluene as well as cracking reactions are performed by
metallic function of highly dispersed platinum particles [6]. In the
case of methylcyclopentane MCP, catalyst particle size in terms
of relative high density of active site(s) as well as the nature of
the catalytic active species are important in defining the order and
magnitude of activity and selectivity [7–10].
Platinum is widely used in catalysis due, in part, to its specific
metallic properties. However, platinum is rare and expensive. It is
also difficult to prepare it in finely dispersed particles deposited on
an acidic or carbon supports [11]. Moreover, Pt undergoes sintering
at certain experimental conditions which results in drastic changes
in its catalytic properties. It is also sensitive to poisoning by sulfur
and other heavy trace metals present in crude oil cuts reactant(s).
Possible replacement of Pt becomes a subject of importance from
scientific and economical points of views. In this respect, in-situ
partial reduction by hydrogen at 673 K of molybdenum trioxide
deposited on titanium dioxide enabled to produce a bifunctional
(metal–acid) MoO2-x(OH)y/TiO2 (MoTi) catalyst [12–14]. In this
system, the metallic function consists of delocalized ␲ electrons
∗
Corresponding author. Fax: +965 24816482.
E-mail address: ali.katrib@ku.edu.kw (A. Katrib).
http://dx.doi.org/10.1016/j.molcata.2015.06.036
1381-1169/© 2015 Elsevier B.V. All rights reserved.

190
H. Al-Kandari et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 189–193
above the Mo–Mo atoms placed along the C-axis of the deformed
rutile structure of MoO2. It is important to note that metallic elec-
trons are delocalized on arrays of atoms, an atomic like wire. This
is in comparison to highly dispersed Pt particles deposited on a
support. On the other hand, Brønsted acid Mo OH function(s) are
formed on the sample surface following hydrogen dissociation by
the ␲ metallic function to active hydrogen atoms which are bonded
to surface oxygen Mo OH. The Brønsted acid groups were charac-
terized by in-situ IR spectra of adsorbed pyridine molecules [13,14].
Hydrogen reduction of the sample at 873 K for 12 h results in the
formation of relatively large Mo(0) metallic aggregates. Suppres-
sion of the Brønsted acid function(s) in MoO2-x(OH)y/TiO2 could be
achieved by controlled addition of relatively small amount (2.5%)
of alkali metals such as Na, K [13,14] Rb. In-situ XPS–UPS charac-
terization of the different states of molybdenum trioxide deposited
on TiO2 pellets, were performed following in-situ hydrogen reduc-
tion at different temperatures. The catalytic properties of the Mo
based catalysts before and after modifications by alkali metals,
MoO2-x(OA)y/TiO2 (A = Na, K, Rb) for the well-defined catalytic reac-
tions of MCH and MCP will be presented in comparison with the
reported data using HY zeolite and Pt/HY based catalytic systems.
(d)
(c)
(b)
(a)
2. Experimental
2.1. Catalyst preparation
A wet impregnation method using ammonium heptamolybdate
salt has been used in the preparation of five monolayers MoO3 sup-
ported TiO2 salt following the method described by Pines et al.
[15]. An appropriate amount of this salt was completely dissolved
in excess distilled water. This salt solution was then poured into
the corresponding amount of TiO2 in a rotary evaporating flask
with continuous mixing for 30 min, and then the solution was
completely dried under vacuum at 343 K. After that an appropri-
ate amount of each sodium, potassium and rubidium nitrate salt
was separately dissolved in excess distilled water. The solution was
added with continuous mixing to the dry mixture for 30 min. The
resultant solution was completely dried under vacuum at 343 K.
The prepared catalyst was then placed in electric oven at 383 K
overnight. Finally, the calcination step was carried out using a tube
furnace (F21100/ USA) through the passing air at 773 K for 12 h.
The sample was cooled in argon atmosphere and then was kept dry
over silica gel.
225
228
231
234
237
240
Binding Energy (eV)
Fig. 1. XPS of Mo(3d) energy region after calcination at 773 K of MoTi (a), RbMoTi
(b), after hydrogen reduction of RbMoTi at 473 K (c), after hydrogen reduction of
RbMoTi at 673 K (d).
chromatograph Chemito, India 1000 equipped with a Petrocol-DH
column and a flame ionization detector.
3. Results and discussion
3.1. Catalyst characterization
2.2. Catalyst characterization
Characterization of the active metal-acid functions in partially
reduced MoO3 deposited on TiO2 as well as the modified system
by the addition of small amounts of alkali metals, using XPS–-UPS,
ISS, FT-IR techniques was reported in previous works [13–14]. To
elucidate the effect of changes in the relative density of the metallic
active site(s) in relation with the catalytic activity of the system, the
changes in the XPS of the VB of the uppermost surface layer in paral-
lel with the Mo (3d) spin-orbit components before and after in-situ
hydrogen reduction at different temperatures will be reported. In
this study, we report data related to the addition of rubidium Rb to
the Mo catalyst (RbMoTi).
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). Vacuum in the analysis chamber was better
than 7 × 10−9 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 contamina-
tion C1s at 284.8 eV within an experimental error of 0.2 eV.
2.3. Catalyst tests
3.2. XPS spectra
Time on stream catalytic reactions under atmospheric hydrogen
pressure was studied. The reactant was drawn from the reservoir
throw HPLC pump of flow rate of 0.1 mL/min then it passed though
vaporizer and eventually it passed over a fixed bed quartz reactor
containing either MoTi or Na, K, Rb modified MoTi (AMoTi) cat-
alytic systems. A continuous H2 flow of 40 cm3/min was allowed
through 500 mg of the catalyst which contains 65 mg of Mo. The
reaction mixture was separated and analyzed with an on-line gas
Calcination of equivalent 5 monolayers of ammonium hepta-
molybdate deposited on TiO2 (MoTi) surface at 773 K enabled to
convert all Mo atoms to MoO3 with Mo (3d3/2,5/2) spin-orbit com-
ponents at 235.8 and 233.6 eV (Fig. 1a). The VB energy region does
not show any density of states at the Fermi level (Fig. 2a). This
is in agreement with the insulating properties of MoO3 structure.
Similar results were obtained in the case of the Rb modified MoTi

H. Al-Kandari et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 189–193
191
(d)
(c)
(c)
(b)
(a)
(b)
(a)
455
458
461
464
467
470
Binding energy (eV)
Fig. 4. XPS of Ti(2p) energy region after calcination at 773 K of MoTi (a), RbMoTi (b),
after hydrogen reduction of RbMoTi at 673 K (c).
-2
0
2
4
6
8
10
12
Binding Energy (eV)
Table 1
Fig. 2. XPS of the VB energy region after calcination at 773 K of MoTi (a), RbMoTi
(b), after hydrogen reduction of RbMoTi at 473 K (c), after hydrogen reduction of
RbMoTi at 673 K (d).
Isomerization reactions of MCH and products distribution on 0.5 g MoTi reduced at
673 K for 12 h, hydrogen flow 40 mL/min.
Products/Selectivity
Reaction temperature (K)
573
27.4
9.5
23.1
19.8
23.9
21.5
2.2
623
46.3
5.7
12.9
11.6
13.7
10.0
46.2
0
653
74.7
1.4
3.0
2.8
3.3
2.5
77.8
9.1
673
88.8
0.4
0.9
0.8
1.1
0.8
81.7
14.1
(Fig. 1b). In-situ exposure of the sample to hydrogen at 473 K for 2 h
enabled to partially reduce MoO3 to Mo2O5 state (Fig. 1c). Low con-
centration of MoO2 could be observed in form of DOS at the Fermi
level in the VB energy region (Fig. 2c). Extended exposure of the
sample to hydrogen at 673 K for 12 h results in the conversion of
MoO3 to MoO2 state, characterized at 232.3 and 229.1 eV (Fig. 1d).
The presence of MoO2 could also be identified as DOS structure
in the valence band (Fig. 2d). The presence of the ␲ and ␴ energy
levels, characteristics of the MoO2 state, could also be observed in
the UPS of RbMoTi after hydrogen reduction at 673 K (Fig. 3b). To
note at this point that the reduction process by hydrogen of MoTi
and RbMoTi seems to be different. In the case of the unmodified
MoTi system, only part of MoO3 is converted to MoO2 at 673 K for
12 h, whereas most if not all of MoO3 in RbMoTi are converted to
Conversion%
1,1DMCP
Trans 1,3DMCP
Cis 1,3DMCP
Trans 1,2DMCP
Et CP
Toluene
Others
0
MoO2. Apparently, molybdenum deposition on titania surface in
MoTi takes place in form of aggregate needle like crystallites struc-
ture as postulated by Bond et al. [16]. So, only the upper MoO3
surface layers are converted to MoO2. In the case of alkali metal
addition to MoTi, discrete entities of the modified system such as
Rb2MoO4 could be formed in the calcination process of the system
at 773 K. Consequently, partial depolymerization of the polymolyb-
date, present on the sample surface, will take place in the reduction
process by hydrogen. Titanium dioxide is employed as a support in
order to increase the surface area of the Mo active phase and inherit
mechanical strength to the catalyst without any involvement in the
catalytic activity of the system. The Ti(2p3/2 1/2) binding energies at
,
458.5 and 464.2 eV did not change after the calcination of the Mo
salt or the modified one by the addition of Rb as could be observed
in Fig. 4.
( b )
Table 2
Comparative RC isomerization products distribution of MCH performed by Pt/HY at
533 K (ref.1) and MoTi at 573 K.
( a )
Products/Selectivity
MoTi
Pt/HY
Conversion%
1,1 DMCP
Trans 1,3 DMCP
Cis 1,3 DMCP
Trans 1,2 DMCP
Et CP
27.4
9.5
23.1
19.8
23.9
21.5
29.2
13.3
15.6
16.0
26.0
27.0
-1
-0.5
0
0.5
1
1.5
2
Binding Energy (eV)
Fig. 3. UPS of Rb modified Mo catalyst RbMoTi after calcination at 773 K (a), after
in-situ hydrogen reduction at 673 K for 12 h (b).

192
H. Al-Kandari et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 189–193
Table 3
Isomerization reactions of MCH and products distribution on 0.5 g A MoTi (A = Na, K, Rb) reduced at 673 K for 12 h, hydrogen flow 40 mL/min.
Reaction Temp. (K)
2.5AMoO3/TiO2
573
Mo
623
Mo
653
Mo
673
Mo
Na
0.8
0.0
100
0.0
0.0
K
Rb
Na
5.8
0.0
100
0.0
0.0
K
Rb
21.0
0.0
100
0.0
0.0
Na
13.2
0.0
100
0.0
0.0
K
Rb
Na
20.3
0.0
100
0.0
0.0
K
Rb
Conversion,%
DMCP
Toluene,%
Et CP,%
27.4
76.3
2.2
21.5
0.0
0.7
0.0
0.9
0.0
100
0.0
0.0
46.3
43.8
46.2
10.0
0.0
12.9
0.0
100
74.7
10.5
77.8
2.5
30.3
0.0
100
57.3
0.0
98.6
0.0
88.8
3.2
81.7
0.8
42.5
0.0
97.5
0.0
76.2
0.0
96.3
0.0
100
0.0
0.0
0.0
0.0
0.0
0.0
Cracking, %
9.2
1.4
14.3
2.5
3.7
3.3. Catalytic measurements
Pt/HY at 533 K (Table 2). The contribution of the metallic function
of MoTi at this 573 K reaction is obvious from the presence of low
concentration of toluene.
The catalytic properties of the bifunctional (MoTi) and
MoO2-x(OA)y/TiO2 (A = Na, K, Rb) AMoTi will be evaluated for the
catalytic reactions of MCH and MCP.
Different catalytic behavior of MCH takes place on the mod-
ified MoTi by alkali metals AMoTi (Table 3). There is almost no
catalytic activity at 573 K reaction temperature as compared to
the unmodified MoTi system. Furthermore, the catalytic activity
remains relatively low at reaction temperatures up to 673 K. The
main reaction product is the dehydrogenation process to toluene.
A conversion of 76.2% and selectivity of 96.3% to toluene
were obtained at 673 K using the RbMoTi catalyst. The complete
absence of dimetylcyclopentanes isomerization products in this
case (Table 3), is due to the absence of the Brønsted acid function(s).
Addition of Rb seems to enhance the metallic function strength of
MoTi.
3.3.1. Methylcyclohexane MCH
The main catalytic reactions of MCH could be summarized
as follows: Ring contraction RC leading to different dimethylcy-
clopentanes DMCP isomerization products of relatively high octane
numbers as compared to MCH. These isomerization reactions are
performed by acidic function such as HY or bifunctional, mainly
Pt deposited on zeolite Pt/HY catalysts [1]. Dehydrogenation to
toluene is performed by a metallic function. On the other hand,
ring opening RO, producing 2, 3- methylhexanes and heptane, of
lower octane number than MCH, are performed by metallic func-
tion. Furthermore, RO catalytic reactions seem to be more difficult
and require a catalyst of high hydrogenolysis activity such as Ir/SiO2
or a bimetallic Pt–Rh deposited on acidic support. It is of interest
to compare the bifunctional MoTi and the metallic AMoTi catalytic
properties of the Mo systems concerning RC isomerization, dehy-
drogenation and RO catalytic reactions of MCH, to the commonly
used zeolites and supported noble metals catalysts.
The catalytic reactions of MCH performed by MoTi as a func-
tion of reaction temperature up to 673 K are presented in Table 1.
The conversion increases from 27.4% at 573 K to 88.8% at 673 K.
However, two major types of catalytic reactions take place in this
reaction temperature region. At low temperature of 573 K, isomer-
ization ring contraction RC reactions are dominant, while MCH
dehydrogenation to toluene relative concentration increases as a
function of reaction temperature. Ring contraction dimethyl and
ethylcyclopentanes constitute 97.8% of reaction products, and 2.2%
of toluene at 573 K (Table 1). This to be compared with less than
10% of isomerization products and 81.7% toluene obtained at 673 K.
The RC isomerization selectivity at low reaction temperature is
attributed to the Brønsted acidic groups in agreement to what
is obtained using zeolite HY and Pt/HY catalysts [1]. However,
dehydrogenation of MCH to toluene seems to be more difficult
to dissociate C H chemical bond, performed by the metallic func-
tion of MoTi at relatively high reaction temperature. The addition
of a metallic function such as Pt to the unstable zeolite HY cat-
alyst enhances the stability of the catalyst performances. This is
attributed to a higher rate of hydrogen transfer accelerated by the Pt
metal [1]. This is what expected to take place concerning the metal-
lic function in MoTi. It is worth mentioning that catalyst activity and
selectivity are completely reproducible at any reaction tempera-
ture following ascending or decreasing in reaction temperature up
to 673 K. Most probably, this is due to the atomic wire like metal-
lic ␲ electrons above the Mo–Mo atoms placed along the C-axis
of the deformed rutile structure of MoO2 based system, on which
hydrocarbon poisoning species are not permanently adsorbed. This
is not the case using the zeolite HY catalyst in which the activity
drastically decreases after 6 h time on stream experiment [1].
It is of interest to notice the comparable RC isomerization prod-
ucts distribution trend, obtained by MoTi at 573 K with those using
3.3.2. Methylcyclopentane MCP
Ring opening inactivity of the bifunctional MoTi and metallic
AMoTi, Mo(0)Ti catalysts towards MCH reactant seems to prevail
in the case of MCP. Apparently, strong metallic function(s) in form
of large particle size of noble metals such as Pt and Ir or bi-metallic
systems is required for such catalytic processes. The most selec-
tive products in this case are 2 and 3-methylpentanes and hexane
[7–9,17–19]. Moderate catalytic activity in form of ring opening/
enlargement followed by dehydrogenation to benzene takes place
in the case of the metallic Mo(0)Ti at 873 K. A conversion of 7.8%
and 29.6% selectivity to benzene were obtained at 673 K reaction
temperature. Although the conversion increases as a function of
reaction temperature, benzene selectivity does not exceed 35%.
Low concentration of the order of 10% of MCH is obtained at 773 K.
Much lower activity is observed in the case of the modified AMoTi
system. This could be considered as a demonstration that ring
opening-ring enlargement require the presence of an acid func-
tion as well as relatively strong metallic function such as bimetallic
noble metals deposited on acidic support.
4. Conclusions
In situ XPS–UPS characterization enabled to define the chemical
state of Mo following the calcination at 773 K of ammonium hep-
tamolybdate deposited on TiO2, modification of this system by the
addition of small amount of rubidium and in-situ hydrogen reduc-
tion at different temperatures. It was found that the calcination
process at 773 K of both systems enabled to convert all Mo salt to
MoO3 state. However, in-situ hydrogen reduction results in par-
tial conversion of MoO3 to MoO2 in the case of MoO3/TiO2. This is
attributed to presence of MoO3 in a needle like crystal structure. The
molybdenum dioxide is present in form of bifunctional (metal/acid)
MoO2-x(OH)y/TiO2 (MoTi) structure. On the contrary, the majority
of MoO3 in the Rb modified system is converted to MoO2. This is
due to formation of isolated RbMoO4 structure. In the case of Rb
modified system, the Brønsted Mo OH acid function is suppressed
due to hydrogen replacement by the Rb atom (MoO2-x(ORb)y/TiO2.
The bifunctional catalytic properties of MoTi and the only
metallic properties were evaluated for the catalytic reactions of

H. Al-Kandari et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 189–193
193
methylcyclohexane and methylcyclopentane. It was found that in
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The support by Kuwait University through research grant #
SC01/13 and GFS project # GS02/08 is gratefully acknowledged.