Catalytic oxidation of aromatic alcohols and alkylarenes with molecular oxygen over Ir/TiO2

Article abstract of DOI:10.1016/j.cattod.2010.10.093

A TiO₂ supported Ir catalyst prepared by the conventional impregnation method exhibited efficient catalytic activity toward the oxidation of organic compounds with molecular oxygen. Aromatic alcohols and alkylarenes selectivity afforded the corresponding carbonyl compounds in high yield under mild conditions. The reaction was quenched by removing the catalyst with filtration suggesting that the catalytic reaction proceeded heterogeneously. The highly dispersed Ir metal particles on TiO₂ surface was considered to be responsible for the catalysis.

Full text of DOI:10.1016/j.cattod.2010.10.093

Catalysis Today 164 (2011) 332–335
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Catalytic oxidation of aromatic alcohols and alkylarenes with molecular oxygen
over Ir/TiO2
Akihiro Yoshida, Youichi Takahashi, Tsuyoshi Ikeda, Kazuki Azemoto, Shuichi Naito∗
Department of Material and Life Chemistry, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2010
Received in revised form 25 October 2010
Accepted 26 October 2010
Available online 4 December 2010
A TiO2 supported Ir catalyst prepared by the conventional impregnation method exhibited efficient cat-
alytic activity toward the oxidation of organic compounds with molecular oxygen. Aromatic alcohols
and alkylarenes selectivity afforded the corresponding carbonyl compounds in high yield under mild
conditions. The reaction was quenched by removing the catalyst with filtration suggesting that the cat-
alytic reaction proceeded heterogeneously. The highly dispersed Ir metal particles on TiO2 surface was
considered to be responsible for the catalysis.
Keywords:
© 2010 Elsevier B.V. All rights reserved.
Alcohol oxidation
Alkylarene oxidation
Supported Ir catalyst
1. Introduction
hols [7] as well as alkylarenes [8] and amines [9]. The key step in
the oxidation reactions by these catalysts is hydride elimination
Carbonyl compounds are one of the important classes of the
compounds not only in chemical industries but also in laborato-
ries. The most common way to produce carbonyl compounds is the
oxidation of the corresponding alcohols and alkanes. In spite of the
formation of toxic by-products and large amount of heavy metal
wastes, these oxidation reactions have been performed mainly
in non-catalytic systems with stoichiometric amount of oxidants
[1]. However, from environmental and economic points of view,
formation of above mentioned by-products and wastes is unde-
sirable. Therefore, the catalytic systems which can utilize greener
and cheaper oxidants, namely molecular oxygen, have attracted
much attention [2]. To date, various kinds of catalytic systems have
been reported in the catalytic oxidation of alcohols utilizing transi-
tion metal complexes [3–5], nitroxy radicals [1], and heterogeneous
supported metal catalysts [6–9]. However, degradation of organic
moieties as well as deposition of inactive metal particles was the
inevitable problems in some homogeneous systems [5].
from substrates to form M–H species (M: Pd, Ru).
Iridium has been attracted much attention as alternative mate-
rials for platinum especially in the field of electro catalysts [10].
However, only few works were reported in the field of aero-
bic oxidation catalysts, while there have been various reports of
both homogeneous and heterogeneous catalytic oxidation utiliz-
ing other group 8–10 metal elements such as Pd, Pt, Ru, Os and Au
[3–9,11]. Ison et al. reported the aerobic oxidation of alcohols to
the corresponding aldehydes and ketones catalyzed by [Cp*IrCl2]2
[12]. Ishii et al. reported the aerobic oxidation of primary alcohols
to esters catalyzed by [IrCl(cod)2]2 [13]. Those reported systems
were utilizing the Ir complexes in homogeneous reaction condi-
tions. Therefore, the applicability of supported Ir catalysts to the
oxidation reaction of alcohols or alkylarenes have still been an
unexplored area. In this manuscript, we firstly report the oxida-
tion of aromatic alcohols and alkylarenes with molecular oxygen
catalyzed by supported Ir catalysts.
Recently, nicely working systems have been developed for alco-
hol oxidation utilizing supported Pd and Ru catalysts, which is
robust enough to achieve high TON and also to provide the recycla-
bility. For example, the nano-sized palladium clusters supported on
hydroxyapatite was reported by Kaneda et al. [6] Mizuno et al. also
reported the nice heterogeneous catalyst of Ru(OH)x/Al2O3 which
exhibited high catalytic activity to the aerobic oxidation of alco-
2. Experimental
2.1. Instruments
GC analyses were performed on a Shimadzu GC-2010 gas chro-
matograph with a FID detector equipped with a TC-WAX capillary
column. Mass spectra were recorded on a Shimadzu GCMS2010
spectrometer equipped with a TC-WAX capillary column at an ion-
ization voltage of 70 eV. Liquid-state NMR spectra were recorded
on a JEOL JNM-ECA-600 spectrometer. 1H and 13C NMR spectra
∗
Corresponding author.
E-mail address: naitos01@kanagawa-u.ac.jp (S. Naito).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.093

A. Yoshida et al. / Catalysis Today 164 (2011) 332–335
333
were measured at 600.17 and 150.92 MHz, respectively. A trans-
mission electron microscope (JEM2010, JEOL) with an acceleration
voltage of 200 kV and LaB6 cathode was applied for the observa-
tion of the images of supported catalysts. Samples were prepared
by suspending the catalyst powder ultrasonically in 2-propanol
and depositing a drop of the suspension on a standard copper
grid covered with carbon monolayer films. The X-ray photoelec-
tron spectra were recorded on a JEOL JPS-9010 spectrometer with
Mg K␣ X-ray source (10 kV, 10 mA). Prior to the measurement, the
sample was reduced by H2 at 573 K in the preparation chamber
and transferred to the analysis chamber without exposure to air.
The adsorption amount of hydrogen was measured by a static volu-
metric adsorption apparatus (Omnisorp 100CX, Beckmann Coulter)
at 298 K. Before measurement, the catalysts were dried at room
temperature and reduced at 573 K for 3 h in a flow of atmospheric
pressure of hydrogen. The dispersion (D(%); percentage of metal
atoms exposed to the surface) of supported metal particles was
evaluated from the amount of chemisorbed carbon monoxide with
assuming the adsorption stoichiometry of CO to an surface metal
atom is unity.
Fig. 1. TEM image of 5 wt% Ir/TiO2.
2.2. Materials and catalyst preparation
be assignable to metallic Ir species. The binding energy of the lat-
ter one at 61.7 eV is close to the value for IrO2 (62.0 eV) [14]. These
results indicate that most of the Ir atoms on Ir/TiO2 are almost zero
valent after hydrogen reduction.
The TiO2 support (P25) was purchased from Nippon Aerosil Co.
Ltd. The solvents used for the catalytic reaction were dried with
activated molecular sieves 4A. For other material, the commercially
available reagents of the highest grade were used without further
purification.
3.2. Oxidation of benzyl alcohol with Ir/TiO2
A TiO2 supported Ir catalyst was prepared from an aqueous
solution of H2IrCl6 by the conventional impregnation method. The
loading amount of Ir was adjusted to 5 wt%. The impregnated sam-
ple was dried at 373 K and then reduced in H2 flow at 573 K for 3 h.
The Ir catalysts supported on other oxide materials were prepared
by the same procedure.
The catalytic activity of Ir/TiO2 toward the oxidation of benzyl
alcohol with molecular oxygen was examined. After 7 h at 353 K,
more than 99% of benzyl alcohol was converted to benzaldehyde
with 99% selectivity without base addition. The detection of only
a trace amount of benzoic acid indicates that the oxidation pro-
ceeded selectively. Low valent or metallic Ir should be the active
species because the catalyst without hydrogen reduction showed
almost no activity. The reaction was quenched by removing the
catalyst with filtration, suggesting that the catalytic reaction pro-
ceeded heterogeneously. In addition, Ir/TiO2 was recyclable at least
for three times as follows; after the separation of the spent Ir/TiO2
catalyst by filtration, and the reduction by hydrogen stream, the
recovered catalyst showed almost the same catalytic activity as a
fresh one even after the three times recovering (the yields of the
2.3. Procedures for catalytic oxidation
The oxidation reaction was carried out as follows: 75 mg of the
reduced 5 wt% Ir/TiO2 (Ir: 20 ␮mol, 1.25 mol% to substrates), the
solvent (1.5 mL), and the substrate (1.5 mmol) were charged into
a glass vial (volume: 17 mL). The reaction mixture was stirred at
353 K, 393 K or 423 K under atmospheric pressure of molecular
oxygen. The products were identified by the comparison of mass
and NMR spectra with those of authentic samples. The yields of
the products were determined by GC analyses with internal stan-
dard technique. TON values were calculated by dividing the number
of the mole of the products by the amount of surface Ir atoms
estimated from the CO chemisorption results. The inter-molecular
competitive oxidation of the primary and secondary alcohols was
examined by using the mixture of benzyl alcohol (1.5 mmol) and
1-phenylethanol (1.5 mmol) as substrates under the same reaction
conditions described above.
3. Results and discussion
3.1. Characterization of Ir supported on TiO2
After hydrogen reduction, highly dispersed Ir particles were
formed on TiO2 support. From TEM observation, the mean diameter
of the particles was 1.7 nm (Fig. 1), which corresponded well to that
estimated from the amount of chemisorbed hydrogen (1.8 nm). In a
X-ray photoelectron spectrum of Ir4f7/2 transition, the larger peak
at 60.4 eV and the smaller one at 61.7 eV were observed (Fig. 2).
The binding energy of the former peak is slightly lower than that
of the reported value for Ir metal (60.8 eV) [14], therefore it should
Fig. 2. X-ray photoelectron spectrum of Ir4f transition of Ir/TiO2.

334
A. Yoshida et al. / Catalysis Today 164 (2011) 332–335
alcohol by Ir/TiO2 was 33 h−1 at 353 K for 5 h (TON: 166), while
that for [Cp*IrCl2]2 at the same reaction temperature was 3.2 h−1
for 12 h (TON: 38) [12]. In the oxidation of benzyl alcohol, the TOF
value of Ru(OH)x/Al2O3, which is known to exhibit high activity to
the oxidation of alcohols, was 40 h−1 (TON: 40) under the almost
same reaction conditions of ours (at 356 K, 1 mmol of benzyl alco-
hol in 1.5 mL of PhCF3) [7], while that for Ir/TiO2 was 24 h−1 (TON:
166). Although the catalytic activity of Ir/TiO2 was a little lower than
Ru(OH)x/Al2O3, Ir/TiO2 showed much better performance than the
previously reported homogeneous Ir complex.
Table 1
Oxidation of benzyl alcohol with molecular oxygen catalyzed by various supported
Ir catalysts.
Support
Conv./%
Yield/%
76
26
18
18
10
6
Selec./%
98
79
79
78
69
54
3
Disp./%a
48
60
53
19
80
23
–
TON
128
44
35
97
14
42
–
TiO2
ZrO2
AI2O3
SiO2
77
33
23
23
14
12
73
CeO2
MgO
The primary cinnamyl alcohol and the secondary 1-phenyl
ethanol also afforded the corresponding aldehyde and ketone,
respectively, although, the secondary 1-phenyl ethanol required
longer reaction period than the primary alcohols for achieving
full conversion. To investigate the lower reactivity of the sec-
ondary alcohol, the competitive reaction of benzyl alcohol and
1-phenylethanol was carried out as shown in Fig. 3. The reaction
rate of 1-phenylethanol was remarkably decreased under the com-
petitive condition, while that of benzyl alcohol was almost the
same. The diminishment of the reaction rates of secondary alco-
hols in the presence of the primary alcohols was also observed in
the Ru(OH)x/Al2O3 catalyzed system, where the formation of metal
alcoholate species were expected [7]. Since the formation of metal
alcoholate intermediate is well known for the selective oxidations
of primary hydroxyl groups [15,16], the formation of alcoholate
species from alcohol with M–OH moiety (M: Ir or Ti) should be
taken part in the present Ir/TiO2 catalyzed reaction and this is
probably a reason for the slower reaction rate of 1-phenylethanol.
In the presence of 2-octanol with a catalytic amount of Ir/TiO2,
acetophenone was converted to 1-phenylethanol under an Ar
atmosphere and the comparable amount of 2-octanone was formed
at the same time (Fig. 4). This result suggests that ␤-hydride elim-
ination of alcohol may take place on Ir/TiO2 with the formation
of Ir–H, which has also been confirmed in the alcohol oxidation
reaction catalyzed by Ir complexes [12,17]. On the basis of these
results, same as the Ru(OH)x/Al2O3 and Pd/hydroxyapatite systems
[6,7], the formation of alcoholate and Ir–H species are possibly
concerned to the present Ir/TiO2 catalyzed aerobic oxidation of
alcohols.
b
TiO2
2
a
Determined by the amount of chemisorbed carbon monoxide.
The catalyst without hydrogen reduction was used.
b
benzaldehyde were 94% (the fresh catalyst), 93% (2nd run), 92% (3rd
run), and 92% (4th run) after 4 h at 353 K).
The Ir catalysts supported on other oxide materials, such as
Al2O3, SiO2, MgO, ZrO2, and CeO2 also exhibited the catalytic
activity, but were less active than Ir/TiO2 (Table 1). Among these
catalysts, the dependency was not found between the catalytic
activity and the dispersion of Ir particles or the acid/base properties
of the supports. The intrinsic role of TiO2 support in this reaction is
under investigation.
3.3. Oxidation of various alcohols with Ir/TiO2
The results for the oxidation of aromatic alcohols with Ir/TiO2
are summarized in Table 2. The p-substituted benzyl alcohols
afforded the corresponding benzaldehyde derivatives in excel-
lent yield, while p-chloro and p-nitro substituted ones required
more severe reaction conditions for achieving complete conversion.
Furthermore, Ir/TiO2 also catalyzed the oxidation of 2-pyridine
methanol, which contains the nitrogen atom being able to coor-
dinate to the metal center, to the corresponding aldehyde. All the
reaction above mentioned proceeded without base addition. Com-
paring to the Ir complex, the catalytic activity of Ir/TiO2 was ten
times higher. The TOF value for the oxidation of 4-methoxybenzyl
Table 2
Oxidation of aromatic alcohols with molecular oxygen catalyzed by 5 wt% Ir/TiO2.
Substrate
Product
Time/h
Conv./%
Yield/%
Selec./%
TON
7»
6a
>99
>99
>99
166
166
>99
>99
99
>99
>99
85
>99
>99
86
5a
166
143
3.5b
80b
98
97
95
95
97
97
161
161
48a
20a
7b
>99
99
86
94
86
95
143
158
a
Reaction temperature (solvent): 353 K (toluene).
Reaction temperature (solvent): 393 K (o-xylene).
b

A. Yoshida et al. / Catalysis Today 164 (2011) 332–335
335
Table 3
Oxidation of alkylarenes with molecular oxygen catalyzed by 5 wt% Ir/TiO2.
Substrate
Product
Time/h
Conv./%
>99
Yield/%
94a
Selec./%
94
TON
12
156
163
12
50
>99
>99
98
62
98
62
103
Reaction temperature (solvent): 423 K (mesitylene).
a
5% of anthrone was yielded.
oxidation of initially formed hydroxylated products of xanthydrol
and 9-fluorenol. On the other hand, the dehydrogenated prod-
ucts were suggested to be formed by the successive dehydration
of hydroxylated products of 9-hydroxy-9,10-dihydroanthracene,
which may be promoted by the Al2O3 support. In the present
alkylarene oxidation catalyzed by Ir/TiO2, the oxygenation and
dehydrogenation are expected to proceed in the similar manner
to the Ru/Al2O3 catalyzed reactions.
4. Conclusion
The TiO2 supported Ir catalyst prepared by the conventional
impregnation method exhibited excellent activity toward the oxi-
dation of aromatic alcohols and alkylarenes. Various kinds of
alcohols including the benzyl alcohol derivatives, the secondary 1-
phenylethanol and the nitrogen containing 2-pyridine methanol
selectively afforded the corresponding aldehydes and ketones
without addition of base or acid. Ir/TiO2 showed the catalytic
activity to the hydrogen transfer reaction from 2-octanol to ace-
tophenone suggesting the oxidation reaction of alcohols proceeded
through the formation of Ir–H species. In addition, Ir/TiO2 also
catalyzed the oxidation of alkylarenes with molecular oxygen
affording xanthone and 9-fluorenone from xanthene and fluorene,
respectively. While 9,10-dihydroanthracene afforded the dehydro-
genated product of anthracene.
Fig. 3. Time courses of the oxidation of benzyl alcohol (solid circles), 1-
phenylethanol (solid diamonds), and their competitive reaction (open marks).
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tion of alkylarenes. Only few examples have been reported for
the liquid-phase oxidation of alkylarenes with molecular oxy-
gen as a solo oxydant [8,18]. Xanthene and fluorene afforded
the oxygenated products of xanthone and 9-fluorenone, respec-
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product of anthracene (Table 3). The oxygenation of xanthone
and fluorene, and the dehydrogenation of 9,10-dihydroanthracene
were also reported by Mizuno et al. in the oxidation of alkylarenes
catalyzed by Ru(OH)x/Al2O3 with molecular oxygen [8]. They sug-
gested that the oxygenated products were formed by the successive
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