Dioxygen activation in photooxidation of diphenylmethane by a dioxomolybdenum(VI) complex anchored covalently onto mesoporous titania

Article abstract of DOI:10.1007/s11243-012-9668-2

A dioxomolybdenum(VI) complex has been covalently anchored onto mesoporous titania by a silicon-assisted transesterification route. The grafting of the complex to the mesoporous structure was confirmed by diffuse reflectance infrared Fourier transform, Raman and UV-Vis spectroscopy and by nitrogen sorption experiments. The ability of the grafted complex to activate molecular oxygen (O₂) has been evaluated in the photooxidation of diphenylmethane to produce benzophenone. The photooxidation of diphenylmethane was monitored continuously by in situ dispersive Raman spectroscopy. A scheme for the activation of molecular oxygen under very mild conditions is proposed. A comparison with the same complex anchored onto commercial titanium P-25 and silica gel revealed both the beneficial effect of the mesoporous structure and the existence of a synergistic effect between MoO/TiO₂/O ₂/light entities, which promotes the photooxidation process under green chemistry conditions. Finally, the heterogeneous catalyst is sustainable; it can be recycled and reused without significant loss in activity or selectivity.

Full text of DOI:10.1007/s11243-012-9668-2

Transition Met Chem (2013) 38:119–127
DOI 10.1007/s11243-012-9668-2
Dioxygen activation in photooxidation of diphenylmethane
by a dioxomolybdenum(VI) complex anchored covalently
onto mesoporous titania
•
Nelson J. Castellanos Fernando Martınez
•
´
•
•
•
´ ´
Frederic Lynen Shyam Biswas Pascal Van Der Voort
Henri Arzoumanian
Received: 30 September 2012 / Accepted: 29 October 2012 / Published online: 19 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A dioxomolybdenum(VI) complex has been
covalently anchored onto mesoporous titania by a silicon-
assisted transesterification route. The grafting of the com-
plex to the mesoporous structure was confirmed by diffuse
reflectance infrared Fourier transform, Raman and UV–Vis
spectroscopy and by nitrogen sorption experiments. The
ability of the grafted complex to activate molecular oxygen
(O2) has been evaluated in the photooxidation of diph-
enylmethane to produce benzophenone. The photooxida-
tion of diphenylmethane was monitored continuously by
in situ dispersive Raman spectroscopy. A scheme for the
activation of molecular oxygen under very mild conditions
is proposed. A comparison with the same complex
anchored onto commercial titanium P-25 and silica gel
revealed both the beneficial effect of the mesoporous
structure and the existence of a synergistic effect between
MoO/TiO2/O2/light entities, which promotes the photoox-
idation process under green chemistry conditions. Finally,
the heterogeneous catalyst is sustainable; it can be recycled
and reused without significant loss in activity or selectivity.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-012-9668-2) contains supplementary
material, which is available to authorized users.
N. J. Castellanos
´
Facultad de Quımica Ambiental, Grupo de Investigaciones
Ambientales para el desarrollo Sostenible-GIADS, Universidad
Santo Tomas, Campus Universitario Floridablanca, Santander,
Colombia
Introduction
The endeavor to develop selective oxidation processes for
organic molecules has been the aim of innumerable
chemists both in academia and industry for several decades
[1, 2]. Many workshops, agencies or councils have
repeatedly placed this objective as critical for the devel-
opment of future technologies [3, 4]. Nowadays, selectivity
is not the unique goal; a process must also be ‘‘green.’’
Indeed, many liquid phase oxidations were performed in
fine chemical and pharmaceutical industries with such
oxidants as hydrogen peroxide, potassium permanganate,
chlorinated molecules or various hydroperoxides, which
are today economically uncompetitive and environmentally
unacceptable [5–8]. Current research in this area is focused
on the use of molecular oxygen O2 as the direct oxidant, in
the presence of heterogeneous catalysts at room tempera-
ture and pressure, opening new perspectives for the
development of green photocatalytic processes for phar-
maceutical, food and cosmetic industries [9]. Specifically,
´
N. J. Castellanos ꢀ F. Martınez
´
Escuela de Quımica, Centro de Investigacion en
Catalisis-CICAT, Universidad Industrial de Santander,
´
Km 2 vıa El Refugio, Piedecuesta, Santander, Colombia
´
´
N. J. Castellanos (&) ꢀ S. Biswas ꢀ P. Van Der Voort
Department of Inorganic and Physical Chemistry, Centre for
Ordered Materials, Organometallics and Catalysis-COMOC,
University of Ghent, Krijgslaan 281 (S-3), 9000 Ghent, Belgium
e-mail: njc.castellanos@gmail.com;
nelson.castellanos@mail.ustabuca.edu.co
F. Lynen
Laboratory of Separation Sciences, Department of Organic
Chemistry, University of Ghent, Krijgslaan 281 (S-4),
9000 Ghent, Belgium
H. Arzoumanian (&)
Chirosciences UMR CNRS 7313 Institut des Sciences
´
Moleculaires de Marseille (iSm2), Aix-Marseille Universite,
Campus St Jerome, 13397 Marseille, France
´
´ ˆ
e-mail: henri.arzoumanian@univ-cezanne.fr
123

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Transition Met Chem (2013) 38:119–127
photooxidation reactions of organic compounds catalyzed
by TiO2 in acetonitrile have been studied, and different
modifications of the support have been tested in search of a
more efficient photocatalyst [10–14]. In this context, and
inspired by the performance of molybdenum enzymes [15],
dioxomolybdenum(VI) complexes have been chemically
supported on the surface of commercial TiO2 P-25 and
yielded interesting results in catalytic oxidation reactions,
reaching catalytic levels under ambient conditions of
pressure and temperature, using O2 as the primary oxidant
and illumination with UV–Vis light [16–18].
tellurium) detector operating in vacuum. Nitrogen adsorp-
tion–desorption measurements were carried out at -196 °C
on a Micromeretics Tristar 3,000 apparatus to determine
the Brunauer–Emmett–Teller (BET) surface area and to
estimate the pore size distribution using the Barrett–Joy-
ner–Halenda (BJH) method. Before measurement, samples
were evacuated for 12 h at 120 °C and 1 ltorr. The pore
volume was taken from the adsorption branch of the iso-
therm at Pi/P0 = 0.95 assuming complete pore saturation.
Diffuse reflectance UV–Vis spectra of solid samples were
recorded on a Hitachi U-3000 UV–Vis spectrophotometer.
Raman analysis of both solid and liquid samples was per-
formed on a RXN1 Raman spectrometer (Kaiser Optical
Systems) fitted with a 532 nm laser operating at 40 mW
and using a 5 m optical probe. The CHN elemental analysis
was performed on a Thermo Scientific Flash 2000 CHNS/O
analyzer equipped with a TCD detector. Thermogravi-
metric analysis (TGA) was performed with a Netzsch STA-
409CD thermal analyzer in a temperature range of
Recently, interest in TiO2 mesoporous materials has
grown significantly due to their large surface area and many
improved properties involved in the photocatalytic process
[19–21]. In the present work, we have prepared a dioxo-
molybdenum(VI) complex anchored covalently onto mes-
oporous titania, according to a reported synthetic route [18].
Novelty of present work resides mainly in the significant
increase in activity by the use of mesoporous titania com-
pared with commercial P25 Degussa titania. The surface
modification of the support materials was confirmed by
comparing the diffuse reflectance infrared Fourier trans-
form (DRIFT), Raman and UV–Vis spectra of the modified
supports with those of the respective nonmodified supports.
Furthermore, the modification of the surface support was
verified by nitrogen sorption analysis. The loading of
dioxomolybdenum(VI) complex grafted was studied by
elemental analysis and corroborated by atomic absorption
spectroscopy (AAS) of molybdenum. The ability of the
mesoporous titania-supported complex to participate in
photooxidation reactions in the presence of molecular O2
has been evaluated in the conversion of diphenylmethane to
benzophenone. A comparison with the same complex
anchored onto commercial titanium P-25 and silica gel
revealed both the beneficial effect of the mesoporous
structure and the existence of a synergistic effect between
MoO/TiO2/O2/light entities, which promotes the photooxi-
dation process under green chemistry conditions. An anal-
ysis of the reuse of the catalyst was performed, showing
both stable grafting of the complex on the surface and
preservation of catalytic activity and selectivity.
25–800 °C under air at a heating rate of 5 °C min-1
.
Molybdenum elemental analysis was carried out with an
atomic absorption spectrophotometer Thermo S4. The
samples were analyzed after acid digestion with previous
calcination in a muffle furnace at 500 °C for 5 h. The
detection limit of the AAS method is 0.230 mg/L. The
irradiation intensity of the halogen lamp used in photo-
chemical experiments was measured by a Handheld Opti-
cal Power Meter Model 840-C and the photon flux
emission corresponded to 85 mW cm-2 at 365 nm. Band
gap energy (Eg) of the photocatalysts was calculated from
their diffuse reflectance UV–Vis spectra according to the
Kubelka–Munk theory [22]. The band gap was obtained
from a plot of [F(R?)ꢀE]1/2 versus energy of the exciting
light (E) assuming that both pure and modified mesoporous
TiO2 are indirect crystalline semiconductors [23].
Synthesis and characterization
Synthesis of mesoporous TiO2 (1)
Mesoporous TiO2 was synthesized following a previously
published procedure [24], using cetyltrimethylammonium
bromide (CTAB) as surfactant in acidic medium. Ti(OiPr)4
was added to an ethanolic 1.25 M HCl solution. The
resulting titanium precursor solution was added to the
ethanolic CTAB solution. After controlled addition of
water, the solution was stirred for 15 min. The resulting
mixture was transferred into an open Petri dish and heated
at 60 °C for 5 days to evaporate the solvent. The molar
ratio was Ti:CTAB:HCl:H2O:EtOH = 1:0.16:1.4:17:30.
Afterwards, the yellow solid was refluxed with 0.11 M
NaOH during 48 h [25]. Finally, the stabilized TiO2 was
calcined at 450 °C for 2 h with a heating rate of 120 °C/h.
Experimental
All preparations and manipulations were performed using
standard Schlenk techniques under oxygen- and water-free
argon atmosphere. Commercial grade solvents were dried
and deoxygenated by refluxing at least 12 h over appro-
priate drying agents under argon atmosphere and were
freshly distilled prior to use. DRIFT spectra were measured
on a Thermo 6700 FLEX FTIR/FT-Raman instrument with
a liquid nitrogen-cooled MCT-A (mercury–cadmium–
123

Transition Met Chem (2013) 38:119–127
121
Synthesis of dioxomolybdenum(VI)-dichloro-(2,20-
StableBond C18 column (length: 250 mm, inner diameter:
4.5 mm, particle size: 5 lm). The whole system was con-
trolled by the Waters MassLynx software. The mobile
phase was a mixture of 70 % acetonitrile and 30 % water
(v/v). The eluent was delivered at a rate of 1.0 mL/min,
and the wavelength of detection was 275 nm. Additionally,
the main product (benzophenone) was followed in situ by
dispersive Raman spectroscopy using a special probe
adapted to the reactor, monitoring a peak at 1,660 cm-1
that corresponds to the carbonyl group of benzophenone.
bipyridine-4,40-dicarboxylate)/TiO2-mesoporous (2)
Dioxomolybdenum(VI)-dichloro(4,40-trimethylsilylcarboxy-
2,20-bipyridyl) was prepared according to a previously
reported route [18]. A benzene solution (50 mL) containing
the dioxomolybdenum(VI) complex (294 mg, 0.5 mmol)
was added to thoroughly dried mesoporous TiO2 (1 g,
estimated 0.987 mmol of –OH/g). The suspension was
stirred at 80 °C for 24 h, filtered, and the resulting light
purple solid was washed thoroughly with acetonitrile
(CH3CN, 2 9 40 mL) and benzene (C6H6, 2 9 30 mL)
and dried under vacuum. DRIFTS (KBr, cm-1): 1728
(C=O), 908, 942 (Mo=O). Raman (cm-1): 954 (Mo=O).
Loading: 0.073 mmol of Mo/g of TiO2 determined by
AAS.
Results and discussion
Preparation of the catalyst
The dioxomolybdenum(VI) complex has been anchored
covalently onto the surface of mesoporous TiO2 using a
methodology recently reported [18]. The grafting method
used is based on a transesterification reaction between the
trimethylsilyl carboxylic ester group of the dioxomolyb-
denum(VI) complex and the titanol functionality present on
the surface of mesoporous titanium oxide (Scheme 1). To
ensure the stability of the grafted complex, the light purple
solid obtained was washed with an excess of polar
(CH3CN) and nonpolar (C6H6) solvent after the transeste-
rification reaction.
Synthesis of dioxomolybdenum(VI)-dichloro-
(2,20-bipyridine-4,40-dicarboxylate)/SiO2 gel (3)
This material was synthesized using the same methodology
reported for mesoporous TiO2 (2), but using commercial
silica gel. DRIFTS (KBr, cm-1): 1725 (C=O), 896, 935
(Mo=O). Raman (cm-1): 952 (Mo=O). Loading:
0.104 mmol of Mo/g of SiO2 determined by AAS.
Synthesis of dioxomolybdenum(VI)-dichloro-
(2,20-bipyridine-4,40-dicarboxylate)/TiO2 P-25 (4)
Characterization of the catalyst
This material was synthesized using the same methodology
reported for mesoporous TiO2 (2), but using commercial
TiO2 P-25. DRIFTS (KBr, cm-1): 1718 (C=O), 896, 931
(Mo=O). Raman (cm-1): 955 (Mo=O). Loading:
0.283 mmol of Mo/g of TiO2 P-25 determined by AAS.
DRIFTS analysis: Evidence for the covalent anchoring
onto the surface of mesoporous TiO2 is provided by the
DRIFT spectra shown in Fig. 1. The grafting of the com-
plex on the surface of TiO2 can be evidenced by an analysis
of the vibration bands between 700 and 1,900 cm-1
,
Photocatalytic oxidation of diphenylmethane
comparing the DRIFT spectra of mesoporous TiO2 with
those of mesoporous TiO2 modified with the dioxomolyb-
denum(VI) complex. The signal at 1,728 cm-1 is assigned
to the vibration of the carbonyl (C=O) group [26]. The
asymmetric and symmetric vibrations of the –COO group
are observed at 1,401 and 1,229 cm-1, respectively [27].
The C=C and C=N stretching vibrations of the pyridyl
The photocatalytic oxidation of diphenylmethane was
carried out using a microscale photochemical reaction
assembly (model 7880, Pyrex, ACE Glass) equipped with
an immersion lamp (Phenix, 220 V, 150 W). In a typical
experiment, a 0.01 M solution of diphenylmethane in
acetonitrile was bubbled constantly with oxygen (or
nitrogen), and 20 mg of mesoporous TiO2 powder was
added and illuminated with the halogen lamp for 8 h. In all
cases, the mixture was kept in the dark and under nitrogen
for 1 h before reaction to assure adsorption/desorption
equilibrium. The oxidation products were followed by an
Alliance high-performance liquid chromatography (HPLC)
system from Waters (Milford, MA, USA), and their con-
centrations were corroborated by previous calibration using
commercial standards. The HPLC system was equipped
with a dual absorbance UV detector and an Agilent Zorbax
O
O
O
O
Cl Mo Cl
N
TiO2-Mesop
C6H6
Cl Mo Cl
N
N
N
HO-Si(CH3)3
O
O
O
O
O
O
O
O
Si(CH3)3
(H3C)3Si
2
MoO2Cl2-bipyridine-COO/TiO2-Mesop ( )
Scheme 1 Synthetic route for the preparation of the dioxomolybde-
num(VI) complex anchored onto mesoporous TiO2 surface (2)
123

122
Transition Met Chem (2013) 38:119–127
rings are observed at 1,617 and 1,560 cm-1, respectively
[28]. The vibration band at 1,025 cm-1 corresponds to the
C–H out-of-plane bending mode of the pyridyl rings.
Finally, the asymmetric and symmetric stretching vibra-
tions of the cis—MoO2 group are observed at 942 and
908 cm-1, respectively [29–31].
Fig. 2). Pure mesoporous TiO2 does not show any
absorption in the visible range. Additionally, the determi-
nation of band gap energies (Eg) of the photocatalysts
revealed no significant change in the electronic structure of
mesoporous TiO2 after modification with the dioxomo-
lybdenum(VI) complex (2).
Diffuse reflectance UV–Vis Analysis: Diffuse reflec-
tance UV–Vis spectra (Fig. 2) also provided additional
evidence for the successful immobilization. Pure meso-
porous TiO2 shows typical band gap absorption of TiO2 in
the UV region, resulting from the electron transitions from
the valence band (VB) to conduction band (CB)
(O2p ? Ti3d) [32]. When comparing the TiO2 matrix
before and after modification with the dioxomolybde-
num(VI) complex, a new absorption band is observed at ca.
560 nm, corresponding to n ? p* transition of the Mo=O
group [30, 33–35], as well as a slight shoulder ca. 450 nm,
which could be attributed to the bipyridyl moiety (inset of
Raman spectroscopic analysis: Raman spectroscopy is a
powerful tool to characterize the crystalline quality of
TiO2. All Raman spectra measured confirm the presence of
anatase phase in this mesoporous TiO2 powder. Figure 3
shows a comparison of the Raman spectra of pure meso-
porous TiO2 (1) and MoO2Cl2-bipyridine-COO/TiO2-
mesoporous (2). In case of pure mesoporous TiO2, char-
acteristic bands at 146, 397, 516 and 637 cm-1 correspond
to the Eg(1), B1g(1), A1g ? B1g(2) and Eg(2) modes of ana-
tase, respectively [36–38]. For the system MoO2Cl2-
bipyridine-COO/TiO2-mesoporous (2), all the Raman
bands for anatase can be found. In addition, the Raman
spectra contain vibration bands for the symmetric stretch-
ing of the bipyridyl ligand at 1,638, 1,569, 1,509, 1,438 and
1,307 cm-1 and the ring breathing at 1,025 cm-1, respec-
tively [39, 40]. The Mo=O symmetric vibration is observed
at 954 cm-1 [41, 42].
908, 942
1025
1560
Nitrogen sorption analysis: The porosity of pure meso-
porous TiO2 (1) and MoO2Cl2-bipyridine-COO/TiO2-
mesoporous (2) were investigated using nitrogen adsorp-
tion–desorption measurements. The results are summarized
in Table 1. Grafting of the dioxomolybdenum(VI) complex
onto mesoporous TiO2 (2) resulted in a change of the shape
of the nitrogen adsorption isotherm (Figure S1, Supporting
Information) in the pure mesoporous TiO2 (1). A typical
H1 and H2 hysteresis phenomenon was observed for the
solids parent (1) and modified (2), respectively, that is
usually attributed to different size of pore mouth and pore
body [24, 57]. In our case, this change of hysteresis is
attributable to the presence of the complex, where its
1401
1229
1728
1617
700
900
1100
1300
1500
1700
1900
Wavenumber (cm-1)
Fig. 1 DRIFT spectra of mesoporous TiO2 (1) (dashed line) and
mesoporous TiO2 modified with dioxomolybdenum(VI) complex (2)
(solid line)
Eg= 3.19 eV
Eg= 3.20 eV
450
560
1569
1438
954
1509
1025
Eg(1)
A1g + B1g(2)
B1g(1)
1638
Eg(2)
1307
400
500
600
700
850 1000 1150 1300 1450 1600 1750
Raman Shift (cm-1
Wavelength (nm)
)
280
380
480
580
680
780
100 250 400 550 700 850 1000 1150 1300 1450 1600 1750 1900
Wavelength (nm)
Raman Shift (cm-1)
Fig. 2 UV–Vis diffuse reflectance spectra of pure mesoporous TiO2
(1) (dashed line) and dioxomolybdenum(VI)-dichloro-(2,20-bipyri-
dine-4,40-dicarboxylate)/TiO2-mesoporous (2) (solid line). The mag-
nified view of the spectra in the region 400-750 nm is shown in the
inset
Fig. 3 Raman spectra of pure mesoporous TiO2 (1) (dashed line) and
dioxomolybdenum(VI)-dichloro-(2,20-bipyridine-4,40-dicarboxylate)/
TiO2-mesoporous (2) (solid line). The magnified view of the spectra
in the region 850–1,800 cm-1 is displayed in the inset
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Transition Met Chem (2013) 38:119–127
123
Table 1 Results of nitrogen sorption, elemental analysis and atomic absorption spectroscopy for pure mesoporous TiO2 (1) and dioxomolyb-
denum(VI) complex grafted onto mesoporous TiO2 (2)
Catalyst
BET surface Pore volume Pore sizea Elemental analysis
Mo loadingb
(g of Mo/g of catalyst)
area (m2/g)
(cm3/g)
(nm)
%C
%H
%N
TiO2-mesoporous (1)
203
0.26
0.11
5.2
0.12
5.49
1.0
1.0
–
–
MoO2Cl2-bipyridine-COO/TiO2-mesoporous (2) 103
4.38
1.2
7.02 9 10-3
a
The pore size was calculated using the BJH method
b
Determined by atomic absorption spectroscopy (AAS)
presence modifies the pore size and affects the adsorption–
desorption phenomenon. Likewise, the values of specific
BET surface areas (SBET) and specific pore volumes (VP)
of the starting mesoporous TiO2 solid decrease signifi-
cantly upon modification. It is well known that the intro-
duction of metal complexes onto supports leads to a
decrease in the specific surface area of the supporting
material [43, 44].
24000
a
0.01 M
0.008 M
0.005 M
0.003 M
0.001 M
19000
14000
9000
1660 cm-1
4000
Determination of loading of dioxomolybdenum(VI)
complex onto mesoporous TiO2: An elemental analysis
was initially performed before and after grafting treatment
to determine the amount of dioxomolybdenum(VI) com-
plex covalently grafted onto the surface. The surface-bound
complex was estimated to be present at a loading of
0.43 mmol/g of TiO2, using the percentage of nitrogen
obtained (1.2 %), as the total percentage of nitrogen pro-
vided by the complex grafted using 1 gr as reference, and
taking into account the theoretical percentage of nitrogen
obtained for the dioxomolybdenum(VI)-dichloro-(2,20-
bipyridine-4,40-dicarboxylate (7.2 %). Although elemental
analysis is technically well suitable for the determination of
loading of this complex, it is very susceptible to errors due
to the presence of solvent molecules trapped inside the
pores during the synthesis process. These guest solvent
molecules are difficult to remove by applying simple vac-
uum or high temperature, which may affect the stability of
the supported material.
-1000
1550
1570
1590
1610
1630
1650
1670
1690
Raman Shift (cm-1)
b
12000
10000
8000
6000
4000
2000
0
0
100
200
300
400
500
Time (min)
Fig. 4 In situ monitoring of benzophenone at different concentra-
tions by Raman spectroscopy: a evolution of benzophenone at
1,660 cm-1 and b variation of intensity of the band at 1,660 cm-1 in
the photooxidation of diphenylmethane by dioxomolybdenum(VI)-
dichloro-(2,20-bipyridine-4,40-dicarboxylate)/TiO2-mesoporous (2)
Therefore, we also performed an elemental analysis of
molybdenum using AAS to obtain an accurate value of the
number of active centers covalently anchored on meso-
porous surface; 0.073 mmol of molybdenum complex per
gram of catalyst was obtained and used as reference in the
photocatalytic oxidation reactions.
mixture. A signal at 1,660 cm-1 attributed to the carbonyl
group of the main product (benzophenone) was used for the
respective calibration (Fig. 4). A cross-validation by HPLC
was carried out to determine the concentration of the
reactants and products at each point.
Photocatalytic oxidation of diphenylmethane
O2 activation by mesoporous TiO2 modified with
dioxomolybdenum(VI) complex: Under optimized condi-
tions, the catalytic activities of mesoporous TiO2 (1) and
mesoporous TiO2 modified with dioxomolybdenum(VI)
complex (2) were explored under both N2 and O2 (O-
donor) atmosphere. As shown in Fig. 5, when the illumi-
nation of diphenylmethane solution was carried out in the
In situ monitoring by Raman spectroscopy: In situ moni-
toring of the photooxidation of diphenylmethane was car-
ried out using a new methodology based on the different
Raman vibration bands (Figure S2, Supporting Informa-
tion) obtained for each species present in the reaction
123

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Transition Met Chem (2013) 38:119–127
presence of mesoporous TiO2 (1) bubbled with oxygen,
benzophenone was obtained. However, when the irradia-
tion was carried out in the presence of N2 (i.e., absence of
O2), no reaction was observed.
enzymes, the apparent low activity observed must take into
consideration the very mild reaction conditions as well as
the nature of the substrate used in the present study. A
significant improvement is nevertheless observed when a
relevant comparison is allowed.
The photooxidation reaction of diphenylmethane to
benzophenone using O2 as a primary oxidant catalyzed by
TiO2 in CH3CN was first reported some years ago by Liang
et al. [45]. The photooxidation products are generally
obtained via a photogenerated charge-transfer process
between the photogenerated hole (h?VB) and the substrate.
In our case, a significant increase in the production of
ketone was obtained in the presence of mesoporous support
modified with dioxomolybdenum(VI) complex (2), as
shown in Fig. 5. After 8 h of irradiation, one observes an
increase ranging from 105 to 320 lmoles of ketone per
gram of solid catalyst, reflected in an increase in both
conversion of diphenylmethane from 3.4 to 12.9 %, and
selectivity toward the ketone from 75 to 85 % (Table 2).
When compared with other catalytic oxidation reactions,
either in the presence or in the absence of TiO2, or with
oxidants such as ROOH, RCOOOH, H2O2 or even P450
Previousstudieshavereportedthetransferofoxygenatoms
to arylalkanes, and alcohols by complexes of dioxo-Mo in
solution, using DMSO as an oxidant [46, 47]. It has been
determined that the oxidation in all cases is on the benzylic
carbon, yielding the corresponding alcohol or ketone.
Although the mechanism of oxo-transfer from the Mo=O unit
to the arylalkanes is not fully defined, ithas been hypothesized
that the carbonyl products are obtained most probably via
oxidation of the alcohol initially formed [48, 49].
Although the transfer of oxygen atoms to different a-
rylalkanes using O2 as an oxidant has recently been shown
for the MoO2 entity in both homogeneous and heteroge-
neous phases [17, 18, 48–51], we have performed the
reaction also under inert atmosphere (N2) to show the real
effect of the MoO2 complex/TiO2 mesoporous system in
the selective oxidation of diphenylmethane in the presence
of molecular O2 and UV–Vis light under very mild con-
ditions. As shown in Fig. 6, the catalytic reaction quickly
stops in the absence of any oxidant. The loss of catalytic
activity of the system under N2 atmosphere is rationalized
by a stoichiometric oxygen transfer from the MoO2(VI)
unit to diphenylmethane, obtaining the reduced species
MoO(IV) (Scheme 2).
350.0
MoO2Cl2-Bipyrdine-COO/TiO2-Mesop in O2
300.0
TiO2 Mesoporous in O2
250.0
TiO2 Mesoporous in N2
200.0
150.0
100.0
50.0
Evidence of formation of the reduced MoO(IV) species
was observed when comparing the DRIFTS spectrum of the
complex before and after the reaction, as shown in Fig. 8a.
Unlike the starting material, which has two bands in the
range 900-950 cm-1 corresponding to symmetric and
asymmetric vibrations of the MoO2 unit [29–31], the IR
spectra of the used solid catalyst exhibit a single strong
sharp band at 920 cm-1 characteristic of the Mo=O(IV)
0.0
0
100
200
300
400
500
Time (min)
Fig. 5 Evolution of [lmol of ketone]/[g of TiO2] as a function of
irradiation time for mesoporous TiO2 (1) in the presence or absence of
O2 and mesoporous TiO2 modified with dioxomolybdenum(VI)
complex (2)
500
450
400
350
300
250
200
150
100
50
MoO2Cl2-Bipyrdine-COO/TiO2-Mesop in O2
MoO2Cl2-Bipyrdine-COO/TiO2-Mesop in N2
Table 2 Conversion and selectivity obtained after 8 h in the photo-
oxidation of diphenylmethane for mesoporous TiO2 (1) and dioxo-
molybdenum(VI)-dichloro-(2,20-bipyridine-4,40-dicarboxylato)/TiO2-
mesoporous (2)
Catalyst
Conversion (%)a Selectivity (%)b
Alcohol Ketone
0
0
100
200
300
400
500
TiO2-mesoporous (1)
3.4
24.7
14.6
75.3
85.4
Time (min)
MoO2Cl2-bipyridine-
COO/TiO2-mesoporous (2)
12.9
Fig. 6 Evolution of [mol of ketone]/[mol of MoO2] as a function of
irradiation time for mesoporous TiO2 modified with dioxomolybde-
num(VI) complex (2) in the absence or presence of O2 as O-donor.
*Turnover number (TON) at 8 h = [mol of product]/[mol of Mo]
a
b
% Conversion: (mol of product/mol of reactant) 9 100;
selectivity: (mol of product/total mol of reactants and products) 9 100
%
123

Transition Met Chem (2013) 38:119–127
125
a
MoO2Cl2-Bipyridine-COO/TiO2-Mesop (2) (as-made)
MoO2Cl2-Bipyridine-COO/TiO2-Mesop (2) (used)
Scheme 2 Stoichiometric oxygen transfer from MoO2(VI) unit to
diphenylmethane, obtaining the reduced species MoO(IV)
700
900
1100
1300
1500
1700
1900
Wavenumber (cm-1)
b
100
MoO2Cl2-Bipyridine-COO/TiO2-Mesop (2) ( as-made)
MoO2Cl2-Bipyridine-COO/TiO2-Mesop (2) (used)
98
96
94
92
90
88
86
84
82
80
0
100
200
300
400
500
600
700
800
Temperature ( C)
Fig. 8 a DRIFTS and b TG analysis for as-made and used
mesoporous TiO2 modified with dioxomolybdenum(VI) complex (2)
Scheme 3 Proposed mechanism for the activation of molecular
oxygen by dioxomolybdenum(VI)-dichloro-(2,20-bipyridine-4,40-
dicarboxylate)/TiO2-mesoporous (2) under very mild conditions
500
MoO2Cl2-Bipyridine-COO/TiO2-Mesop in O2
450
400
350
300
250
200
150
100
50
500
MoO2Cl2-Bipyridine-COO/TiO2-Mesop in O2 (Used)
MoO2Cl2-Bipyridine-COO/TiO2-Mesop in O2
MoO2Cl2-Bipyridine-COO/TiO2-P-25 in O2
MoO2Cl2-Bipyridine-COO/ Silica gel in O2
450
400
350
300
250
200
150
100
50
0
0
100
200
300
400
500
0
Time (min)
0
100
200
300
400
500
Time (min)
Fig. 9 Evolution of the [mol of ketone]/[mol of MoO2] as a function
of irradiation time for as-made and used mesoporous TiO2 modified
with dioxomolybdenum(VI) complex (2)
Fig. 7 Evolution of [mol of ketone]/[mol of MoO2] as a function of
irradiation time for mesoporous TiO2 modified with dioxomolybde-
num(VI) complex (2), silica gel modified with dioxomolybdenum(VI)
complex (3) and commercial TiO2 P-25 modified with dioxomolyb-
denum(VI) complex (4) in the presence of O2 as O-donor. *Turnover
number (TON) at 8 h = [mol of product]/[mol of Mo]
possibility for the MoO(IV) unit to activate the molecular
O2 and reoxidize to give a Mo(VI) oxo-peroxo entity,
which allows the catalytic cycle to continue in a photo-
oxidative process (Scheme 3). Therefore, the transfer of
oxygen from MoO2(VI) to diphenylmethane under O2 is
significantly higher than the stoichiometric amount
obtained when an N2 atmosphere is used, resulting in a
photocatalytic process with a turnover number (TON)
moiety in the mononuclear oxomolybdenum(IV) complexes
[51–55].
In contrast, in the presence of O2 as O-donor, although
one cannot exclude the existence of any product via a
photogenerated charge-transfer process by TiO2, there is a
123

126
Transition Met Chem (2013) 38:119–127
Table 3 Results of elemental analysis, atomic absorption spectroscopic and TOF activity for as-made and used mesoporous TiO2 modified with
dioxomolybdenum(VI) complex (2)
Catalyst
Elemental analysis
Mo loadinga
TOFb 9 10-3 (h-1
)
%C
%H
%N
(g of Mo/g of catalyst)
MoO2Cl2-bipyridine-COO/TiO2-mesoporous (2) (as-made)
MoO2Cl2-bipyridine-COO/TiO2-mesoporous (2) (used)
5.45
5.17
1.02
0.79
1.2
7.02 9 10-3
6.90 9 10-3
655
613
1.16
a
b
Determined by atomic absorption spectroscopy. Turnover frequency (TOF) at 2 h = [mol of product]/[mol of Mo] 9 time (h-1
)
around 4.8 after 8 h. (Figure 6). It is important to note that
in the absence of light, the O2 activation effect by the
reduced species MoO(IV)/TiO2 has been observed and
discussed recently [16–18, 50, 51].
In order to assess the reusability of the heterogeneous
catalyst during the photooxidation reaction, the mesopor-
ous TiO2 modified with dioxomolybdenum(VI) complex
(2) was used in a second reaction cycle. Interestingly, as
shown in Fig. 9, the catalytic activity is preserved, with
only a slight reduction in the yield of benzophenone for the
second catalytic cycle.
Additionally, to corroborate the influence of the semi-
conductor mesoporous matrix in the photocatalytic process,
a comparison was made under the same conditions
(O2 ? light) with the same complex anchored onto com-
mercial titanium P-25 and silica gel as shown in Fig. 7. The
characterizations of these catalysts are presented in the
Supporting Information (Figure S3-S8 and Table S1-S2).
When compared with the commercial TiO2 P-25 matrix,
the study revealed that although the oxygen atom transfer
process also was catalytic (TON around 2.5 after 8 h), a
significant increase of about twice was obtained by the use
of mesoporous matrix. This can be understood by the
greater surface area of the mesoporous support, which
encourages and promotes the absorption–desorption pro-
cesses of the substrate and product [56].
Conclusion
A dioxomolybdenum(VI) complex has been covalently
anchored onto a mesoporous TiO2 matrix. The presence of
the complex in the mesoporous structure has been con-
firmed by using DRIFT, Raman and UV–Vis spectroscopy.
The complex is able to catalyze the photooxidation of
diphenylmethane to benzophenone in the presence of
molecular oxygen. The proposed mechanism includes the
ability of the complex to transfer oxygen atoms to diph-
enylmethane as well as activate molecular oxygen during
the photooxidative cycle. A significant increase in the
production of ketone is observed in the presence of the
complex grafted onto mesoporous titania, compared to pure
mesoporous titania. Further comparison with the same
complex anchored onto commercial titanium P-25 and
silica gel revealed both the beneficial effect of the meso-
porous structure and the presence of a synergistic effect
between MoO/TiO2/O2/light entities, which promotes the
photooxidation process. Finally, an analysis of the stability
and reactivity of the used catalyst revealed that under green
chemistry conditions (O2 ? light), the complex remains
bound to the surface without loss of activity.
On the other hand, while noting the ability of the complex
grafted onto silica gel to transfer oxygen atoms to diph-
enylmethane, the moles of ketone obtained per mole of active
site uniquely correspond to a stoichiometric reaction
(TON % 0.93). No reoxidation of the active site is observed
for silica as compared to titania used as support for the com-
plex, suggesting a synergistic effect between MoO/TiO2/hv/
O2 for the complex anchored onto the latter support material.
Stabilityand reusabilityofmesoporousTiO2 modifiedwith
dioxomolybdenum(VI) complex: In order to evaluate the
stability of the Mo-species, the solid catalyst was separated
from the reaction medium by filtration after a catalytic run for
480 min, consequently washed with acetonitrile and finally
dried in vacuum. This solid was characterized by DRIFT and
AAS, atomic elemental and TG analysis. The IR spectra of the
used solid in the range 700–1,900 cm-1 (Fig. 8a) exhibit
bands due to stretching vibrations of the complex that are
identical to that of the precursor MoO2Cl2-bipyridine-COO/
TiO2-mesoporous (2) before catalysis. Both thermogravi-
metric and elemental analysis of the used catalyst (Fig. 9b and
Table 3) corroborated the presence of dioxomolybdenum(VI)
complex grafted onto mesoporous titania. The data in Table 3
showonlyaslightdecreaseinthevaluesofelementalanalysis,
loading of complex and turnover frequency (TOF) activity for
the used catalyst.
Acknowledgments Part of this work was performed under the
auspices of a COOPEN program funded by the European Community.
N.J.C. is grateful to COLCIENCIAS ‘‘Fondo de Apoyo a los Doc-
torados Nacionales.’’ The Raman instrument is funded by the Ghent
University, GOA grant Nr. 01G00710.
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