Striking effects of a titania support on the low-temperature activities of Ir catalysts for the dehydrogenative synthesis of benzimidazole and indole

Article abstract of DOI:10.1016/j.cattod.2017.08.014

The crystalline structure of titania supports for iridium catalysts markedly affected their low-temperature activity for the dehydrogenative synthesis of N-containing heteroaromatics, namely benzimidazole and indole. While solid iridium catalysts supported on anatase showed moderate to poor activity for the synthesis of 2-phenylbenzimidazole (3) from o-phenylenediamine (1) and benzyl alcohol (2) at 100 °C, the reaction in the presence of rutile-supported catalysts proceeded smoothly to give 3 in high yields of up to 88%. Similar results were observed for the dehydrogenative conversion of 2-(2-aminophenyl)-ethanol (4) to indole (5). The reaction at 100 °C for 18 h in the presence of 1.0 mol% iridium on rutile gave 5 in a yield of 73%, while the use of anatase-supported catalysts resulted in significantly lower yields. TEM analysis showed the formation of small (ca. 2 nm in diameter), homogeneously-dispersed iridium nanoparticles on rutiles, while the inhomogeneous loading of iridium species was observed on anatase supports. CO pulse experiments revealed that there is a strong correlation between CO uptake by iridium nanoparticles and the activities at 100 °C. These results suggest that the predominant formation of small, well-reduced iridium nanoparticles is one major reason for the excellent activities of rutile-supported catalysts at low temperatures.

Full text of DOI:10.1016/j.cattod.2017.08.014

Accepted Manuscript
Title: Striking Effects of a Titania Support on the
Low-Temperature Activities of Ir Catalysts for the
Dehydrogenative Synthesis of Benzimidazole and Indole
Authors: Tatsuhiro Fukutake, Kenji Wada, Gang Chuan Liu,
Saburo Hosokawa, Qi Feng
PII:
S0920-5861(17)30536-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cattod.2017.08.014
CATTOD 10957
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Catalysis Today
Received date:
Revised date:
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31-5-2017
2-8-2017
7-8-2017
Please cite this article as: Tatsuhiro Fukutake, Kenji Wada, Gang Chuan Liu, Saburo
Hosokawa, Qi Feng, Striking Effects of a Titania Support on the Low-Temperature
Activities of Ir Catalysts for the Dehydrogenative Synthesis of Benzimidazole and
Indole, Catalysis Todayhttp://dx.doi.org/10.1016/j.cattod.2017.08.014
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Striking Effects of a Titania Support on the Low-Temperature Activities of Ir
Catalysts for the Dehydrogenative Synthesis of Benzimidazole and Indole
Tatsuhiro Fukutake,a Kenji Wada,b,* Gang Chuan Liu,a Saburo Hosokawa,c,d Qi Fenga
aDepartment of Advanced Materials Science, Faculty of Engineering, Kagawa University, Takamatsu
761-0396, Japan
bDepartment of Chemistry for Medicine, Faculty of Medicine,
Kagawa University, 1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793, Japan Phone
+81-87-891-2249, Fax +81-87-891-2249, e-mail: wadaken@med.kagawa-u.ac.jp
cElements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara,
Nishikyo-ku, Kyoto 615-8245, Japan
dDepartment of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

TOC/Abstract Graphic:
Highlights
> Rutile-supported iridium catalysts showed excellent activity.
> A correlation between CO uptake by iridium and the activities was observed.
> Well-reduced iridium nanoparticles would be responsible for the excellent activities.
ABSTRACT:
The crystalline structure of titania supports for iridium catalysts markedly affected their
low-temperature activity for the dehydrogenative synthesis of N-containing heteroaromatics, namely
benzimidazole and indole. While solid iridium catalysts supported on anatase showed moderate to poor
activity for the synthesis of 2-phenylbenzimidazole (3) from o-phenylenediamine (1) and benzyl alcohol
(2) at 100 °C, the reaction in the presence of rutile-supported catalysts proceeded smoothly to give 3 in
high yields of up to 88%. Similar results were observed for the dehydrogenative conversion of
2-(2-aminophenyl)-ethanol (4) to indole (5). The reaction at 100 °C for 18 h in the presence of 1.0 mol%
iridium on rutile gave 5 in a yield of 73%, while the use of anatase-supported catalysts resulted in

significantly lower yields. TEM analysis showed the formation of small (ca. 2 nm in diameter ),
homogeneously-dispersed iridium nanoparticles on rutiles, while the inhomogeneous loading of iridium
species was observed on anatase supports. CO pulse experiments revealed that there is a strong
correlation between CO uptake by iridium nanoparticles and the activities at 100 °C. These results
suggest that the predominant formation of small, well-reduced iridium nanoparticles is one major reason
for the excellent activities of rutile-supported catalysts at low temperatures.
Keywords: supported catalyst; iridium; titania; rutile; benzimidazole; indole
1. Introduction
The development of environmentally benign organic synthesis is one of the most important
issues in modern chemistry [1]. While homogeneous catalysts often play a significant role in organic
syntheses, the development of heterogeneous catalysts, particularly solid oxide-supported catalysts, that
enable highly efficient organic synthesis has attracted much attention [2,3], since these solid catalysts
can overcome the practical and environmental disadvantages of homogeneous catalysis, such as the
difficulty of separation and recovery of the catalysts from the reaction mixture as well as contamination
of the products by metallic species. In this context, we have developed various supported catalysts,
namely ceria-supported ruthenium catalysts, which are effective for various highly atom-efficient

organic transformations [3,4].
On the other hand, nitrogen-containing aromatics such as benzimidazoles and indoles are
important pharmaceutical and agrochemical raw materials [5,6]. For example, angiotensin II
receptor blockers (ARBs), which are used for the treatment of hypertension, often have
benzimidazole-type core structures, and indole structures are found in many alkaloids, such as
tryptophan and auxin. Dehydrogenative synthesis is one of the most efficient routes to these
nitrogen-containing chemicals [4e,7,8]. We previously reported that ceria-supported ruthenium
catalyst effectively catalyzed the synthesis of indole via dehydrogenative N-heterocyclization [4e].
In the initial report [8a] dehydrogenative synthesis of benzimidazoles from phenylenediamines and
primary alcohols in the presence of ruthenium complex catalysts required temperatures as high as
200 °C, and the following homogeneous catalyses operated at lower temperature required the
presence of strong bases [8b-d]. While the photo-assisted reactions [8f] proceeded at room
temperature, the dehydrogenative reactions without irradiation using TiO2 on organosilicon-supports
o
[8g] or Pt/TiO2 [8h] were operated at relatively high temperatures (above 160 C). Recently, we have
developed iridium catalysts supported on TiO2 (JRC-TIO-4, Degussa P-25 equivalent), which realize
the smooth dehydrogenative synthesis of benzimidazoles under significantly mild conditions, within
18 h at 120 °C [7,9].
However, further investigation revealed that the iridium species were not homogeneously

loaded on the titania support, JRC-TIO-4. The formation of dense iridium nanoparticles was
observed on some specific facets of the titania particles, but not on other facets. These findings
encouraged us to investigate the effects of the crystalline structures of titania supports in detail to
develop more active iridium catalysts. Herein, we describe the marked improvement of the activity of
titania-supported iridium catalysts for the dehydrogenative synthesis of 2-phenylbenzimidazole and
indole at a low reaction temperature through suitable selection of the titania support. Among those
examined, the catalysts supported on rutile showed the highest activity at 100 °C even in the absence of
any additives such as bases or hydrogen acceptors. On the other hand, the catalysts prepared using
anatase titanias resulted in low activity regardless of their surface areas. Factors that govern the catalytic
activity of iridium species are discussed based on a detailed characterization of the catalysts.
2. Experimental
2.1. General Information
All manipulations were performed under an argon atmosphere using standard Schlenk
techniques. Tris(acetylacetonato)iridium(III) (Ir(acac)3, C15H21O6Ir), bis(1,5-cyclooctadiene)diiridium(I)
dichloride ([(COD)IrCl]2), mesitylene, tetrahydrofuran (THF), methanol, biphenyl, o-phenylenediamine,
and benzyl alcohol were obtained from Wako and used as received. 2-(2-Aminophenyl)ethanol (TCI)
was obtained commercially and used without further purification. JRC (Japan Reference Catalyst)

titanias (JRC-TIO-4, 6, 7, 8, 9) were obtained from the Catalysis Society of Japan. Other titanias (F-1,
F-1R, F-2, F-4, F-6) were obtained from Showa Denko Ceramics Co. Ltd. XRD patterns of F-series
titanias are shown in Figure S1 of the Supplementary content.
Physical and Analytical Measurements
The organic products of catalytic runs were analyzed by GCMS (Shimadzu GC-MS Parvum 2;
Zebron ZB-1 capillary column, i.d. 0.25 mm, length 30 m, at 50250 °C) and gas-liquid
chromatography (Shimadzu GC-14A; Zebron ZB-1 capillary column, i.d. 0.25 mm, length 30 m, at
50250 °C).
Solid catalysts were analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction
(XRD), transmittance electron microscopy (TEM), nitrogen gas adsorption, thermogravimetric analysis
(TG-DTA), wavelength-dispersive X-ray spectrometry (WDX), and CO adsorption. XP spectra of the
catalysts were recorded using an ULVAC-PHI 5500MT system equipped with a hemispherical energy
analyzer. Samples were mounted on indium foil and then transferred to an XPS analysis chamber under
N2. Samples just after reduction in a H2/Ar stream (see below), without passivation, were mounted in a
glove box (Korea Kiyon) with an O2 level below 0.5 ppm. The spectra were measured at room
temperature using Mg K radiation (1254 eV) generated by an X-ray tube operating at 15 kV, 400 W.
The electron take-off angle was set at 45 deg. The residual gas pressure during data acquisition was less

than 1 x 10-8 Torr (1 Torr; 133.3 N m-2). Binding energies were referenced to the C 1s level of residual
graphitic carbon [10]. Powder XRD patterns were recorded using Cu Kα radiation (40 kV, 40 mA) and a
carbon monochromator (Shimadzu XRD-6100). TEM photographs were obtained using a JEOL
JEM-3010 microscope at the Faculty of Engineering, Kagawa University or a JEOL JEM-200EX at the
Hanaichi Ultrastructure Research Institute, Okazaki, Japan. Nitrogen adsorption/desorption isotherms of
catalysts were recorded with a computer-controlled automatic gas sorption system (Quantachrome
NOVA 4200e). Samples were degassed at 300 °C for 2 h just before the measurements.TG-DTA
analyses were performed using a RIGAKU TG8120 in a stream (40 cm3min-1) of synthetic air. The
composition of supported iridium catalysts was confirmed by a wavelength dispersive X-ray
fluorescence spectrometer (Panalytical Axios system equipped with a 4 kW rhodium anode X-ray tube)
at the Kagawa Prefectural Industrial Technology Center. The amount of adsorbed carbon monoxide was
measured at 50 °C using a home-made pulse titration analyzer connected to a TC detector on a
Shimadzu 8AIT gas chromatograph. Before measurement, a sample was reduced at 500 °C for 30 min in
a flow of H2 (2vol.%)/Ar at atmospheric pressure.
2.2. Preparation of supported iridium catalysts
Supported catalysts were prepared by an impregnation method as shown in Scheme 1. To a
solution of Ir(acac)3 or [(COD)IrCl]2 (0.10 mmol as Ir) in 10 mL of methanol (THF for [(COD)IrCl]2),

1.0 g of titania support was added in air at room temperature. After impregnation, the sample was dried
overnight in air at 80°C. Unless otherwise noted, the resulting powder was calcined in air at 400 °C for
30 min in a box furnace (ramp rate 10 °C min-1). About 600 mg of the resulting powder was mounted in
a quartz tube (o.d. 12 mm) and reduced in a H2 (2vol.%)/Ar flow (40 cm3 min-1) at 500 °C (ramp rate;
10 °C min-1) for 30 min by the use of a conventional flow reactor. After reduction, the catalysts were
passivated in an O2 (ca. 2%)/N2 flow (40 cm3 min-1) at 50 °C for 5 min. The loading level of iridium
speceies was finally checked by WDX.
2.3. General procedure for the catalytic runs
The procedure was slightly modified from that in our previous study [7]. All of the reactions
were performed using hot stirrers equipped with cooling blocks to reflux the solution. The Ir/titania
catalyst (0.010 mmol as Ir) was mounted in a glass Schlenk tube (20 cm3). As noted above, all the
catalysts were used just after the reduction at 500 oC and following passivation without additional in-situ
reduction treatment, while such treatment might increase the initial activities of the catalysts to some
extent. A typical reaction procedure for the synthesis of 2-phenylbenzimidazole (3) is as follows:
1,2-phenylenediamine (1) (1.0 mmol), benzyl alcohol (2) (2.3 mmol), and mesitylene (1.0 cm3) were
added to the Schlenk tube containing the reduced catalyst. The reaction mixture was stirred at 100 °C for

18 h under an argon atmosphere. The solid catalyst was removed by passing the mixture through a 0.45
m polytetrafluoroethylene (PTFE) filter (Millipore Millex LH), and the products were quantified by
GC using biphenyl (ca. 50 mg) as an internal standard. For the synthesis of indole (5),
2-(2-aminophenyl)ethanol (4, 1.0 mmol) and mesitylene (1.0 cm3) were placed in a glass Schlenk tube
(20 cm3) under an argon atmosphere together with the reduced catalyst (0.010 mmol as Ir), and the
reaction mixture was stirred at 120 or 100 °C for 18 h. Biphenyl (ca. 50 mg) was used as an internal
standard for the GC analysis.
3. Results and discussion
3.1. Effects of titania supports of iridium catalysts for the dehydrogenative synthesis of benzimidazole
The activities of iridium catalysts supported on various titanias for the dehydrogenative
synthesis of 2-phenylbenzimidazole (3) from o-phenylenediamine (1) and benzyl alcohol (2) were
compared. The supported iridium catalysts were prepared by the procedure shown in Scheme 1, and
used after the reduction at 500 °C in a stream of hydrogen. A chlorine-free iridium complex, Ir(acac)3
(acac = acetylacetonate), was used as an iridium source. Note that a composition analysis of the catalyst
using a WDX analyzer revealed that some of the iridium species loaded with the catalyst might be lost
during calcination in the preparation of the catalysts, probably by the sublimation of Ir(acac)3 due to its

relatively high volatility (see Experimental and Table 1). Such a loss of iridium species was highly
significant for catalysts supported on titania with low surface areas such as F-series titanias. Therefore,
in the following study, the activities were compared under the same molar amount of iridium species by
adjusting the weight of the catalysts used.
Table 1 shows the yield of 3 in the presence of 1.0 mol% (as Ir) of supported iridium catalysts
at 120 °C and 100 °C for 18 h. Anatase-rich and rutile-rich titanias are denoted as (A) and (R),
respectively. As reported in our previous study [7], JRC-TIO-4-supported catalyst showed excellent
activity at 120°C. The desired product was obtained in 96% yield after 18 h reaction under the present
conditions. On the other hand, different kinds of titanias had different effects on the yield of 3 in the
reactions at 100 °C. Catalysts supported on rutile titanias, namely JRC-TIO-6 and F-1R, showed
excellent activities (see entries 2 and 7), whereas the yields of 3 in the presence of anatase-supported
catalysts were not satisfactory (see entries 1, 4-6, 8-11). No significant correlation was found between
surface area and the activity of the catalysts, and catalysts supported on titanias with high surface areas,
such as JRC-TIO-7, -8, or -9, did not show significant activities. The yield of JRC-TIO-6-supported
iridium catalyst further improved to 88% when the catalyst was prepared without calcination (entry 3,
see below).
The loss of iridium species during preparation of the catalysts could be avoided by the use of
chlorine-containing iridium complexes such as [(COD)IrCl]2 (COD = 1,5-cyclooctadiene). However, the

resulting catalysts showed lower activities than that prepared using Ir(acac)3 at 100 °C, as shown in
Table S1. XPS analyses of these catalysts revealed the presence of a larger amount of chlorine atoms on
the surface of the catalysts prepared using [(COD)IrCl]2 (see Table S2), which suggests that the presence
of chlorine decreases the catalytic activity at a relatively low temperature (100 °C).

3.2. Ir/TiO2-catalyzed dehydrogenative synthesis of indole
The effects of various titania supports on the synthesis of indole (5) via a dehydrogenative route
were examined. The results are summarized in Table 2. While previously-reported Ru/CeO2 catalysts
(2.0 mol% as Ru) required a relatively high temperature of around 140 °C for the reaction to be
complete within 18 h [4e], iridium catalysts supported on JRC-TIO-6 successfully gave 5 in an excellent
yield (86 %) at lower temperature (120 °C). As in the dehydrogenative synthesis of
2-phenylbenzimidazole, rutile-supported catalysts generally showed activities at lower temperature.
Again, anatase titanias with high surface areas were not good supports for this reaction. At 100 °C, 5
was obtained in the highest yield (52%) in the presence of the F-1R (rutile)-supported catalyst .
3.3. Optimization and characterization of iridium catalysts supported on various titanias
The activities of Ir/TiO2 catalysts greatly depend on the calcination procedure during their
preparation. Note that all the catalysts were reduced under H2 at 500 °C just before the catalytic runs
(see Scheme 1). As shown in Figure 1, the yield of 5 at 100 °C greatly improved when the catalyst was
calcined below 240 °C, and 5 was obtained in the highest yield of 73% in the presence of the catalyst
without calcination. The Ir/JRC-TIO-6 catalyst without calcination was also effective for the synthesis

of 2-phenylbenzimidazole (see Table 1, entry 3). A TG-DTA study showed that surface Ir(acac)3 on
JRC-TIO-6 oxidatively decomposed at around 230 °C (Fig. S2). Since CO uptake by the reduced
catalysts without calcination, followed by the reduction at 500 °C, was greater than that with catalysts
calcined at 400 °C (see below), calcination at a high temperature would promote the formation of
aggregated iridium oxide species. The BET surface area of the catalyst without calcination (63 m2g-1)
was slightly greater than that of the catalyst calcined at 400 °C (53 m2g-1). Our previous study [7]
revealed that the calcination of spent catalysts is required to maintain excellent activities in repeated
uses to completely remove strongly-adsorbed carbonaceous materials. The present results suggest that
calcination should be performed at a temperature lower than the decomposition temperature of the
surface complex, namely 230 °C.

Figure 2 shows TEM images of several Ir/TiO2 catalysts after the reduction at 500 °C. A
photograph of Ir/JRC-TIO-4 (1 wt% as Ir) is shown for comparison ([7] - Reproduced by permission of
The Royal Society of Chemistry). As previously reported [7,11], iridium species of all the catalysts
examined were very small, less than 2.5 nm. While the formation of homogeneously-dispersed iridium
particles was confirmed on JRC-TIO-6, the effects of calcination temperature on the particle size were
ambiguous, as shown in Figure 2 (a) and (b). Slightly larger particles were formed on F-1R(R). Much
smaller particles of iridium species were formed on JRC-TIO-7, whereas the catalytic activity was low
and CO-uptake by this catalyst was limited (see Table 1 and Figure 3). On F-2, inhomogeneous loading
of iridium species was observed, while the particle size was small. Some facets of titania particles were
thickly covered by small iridium nanoparticles, while other facets had no iridium particles. This could be
due to differences in the surface energies between facets of anatase [12]. Such inhomogeneous formation
of transition metallic species has been reported previously [13].

There was a significant relationship between CO uptake and the activities of the reduced
Ir/TiO2 catalysts, as summarized in Figure 3. In general, the yields of both (a) 3 and (b) 5 via
dehydrogenative routes increased with an increase in CO uptake, suggesting that fully-reduced, probably
zero-valence, iridium species are the active sites of the present dehydrogenative reactions. Note that the
catalysts supported on pure rutile such as JRC-TIO-6 or F-1R adsorbed much amount of CO, and the
fraction of rutile phase in titania was generally correlated with CO uptake ability of the catalyst. Iridium
catalysts prepared without calcination showed higher CO uptake than that calcined at 400 °C, implying
that calcination at relatively high temperature might induce aggregation of the resulting iridium species,
whereas this difference was not clear in TEM images (see above). Despite the formation of very fine
nanoparticles, CO uptake of iridium catalysts supported on JRC-TIO-7 was low. Similar results were
observed for JRC-TIO-8 or -9-supported catalysts. It has been reported that noble metals supported on
reducible supports such as titanias often suffer from strong metal-support interaction (SMSI). Due to
SMSI, the CO-adsorption ability of supported metallic catalysts has been reported to drastically decrease
when the catalysts are reduced at high temperatures without aggregation of metallic species [14]. The
lower CO uptake and the resulting lower activity of iridium catalysts supported on anatase titania with
high surface areas could be attributed to SMSI, and the extent of SMSI would significantly depend on
the titania support. As demonstrated in our previous study using Ir/JRC-TIO-4, the extent of the

reduction of iridium species greatly affects the activity [7]. Therefore, the achievement of highly
reduced states of iridium without SMSI could be one reason for the enhanced activity of
JRC-TIO-6-supported catalysts.
The state of surface iridium species on various titanias just after reduction at 500 °C was
investigated by XPS. The Ir 4f spectra are shown in Figure S3. The ratio of Ir0 (60.7 eV) to Ir0 + IrVI
(62.0 eV) on the surface estimated by curve fitting was 55% for F-1R (R), 52% for JRC-TIO-6 (R), 45%
for F-1 (A), 40% for F-2 (A), 43% for F-6 (A), and 41% for JRC-TIO-7 (A), indicating that, as a general
trend, fully reduced iridium species are predominant on rutile titanias. The formation of well-reduced
iridium species could be one reason of the predominant CO uptake by the rutile-supported catalysts
The results presented here clearly indicate that the predominant formation of highly reduced
and highly dispersed iridium nanoparticles on rutile is important for the excellent activity in the
dehydrogenative synthesis of N-containing chemicals. Kido and coworkers reported characteristic
charge transfer between gold nanoclusters and a rutile support [15]. While an electronic charge transfer
from gold nanoclusters to the O-rich TiO2(110) played a crucial role in the enhancement of CO
oxidation activity, the deposition of Au nanoclusters on reduced-TiO2(110) surfaces resulted in a reverse
electronic charge transfer from TiO2. Therefore, one can expect the formation of more electron-rich

iridium nanoparticles on rutile under a reductive atmosphere by such reverse electronic charge transfer,
although comparative studies on such electronic effects are still required. In addition, further studies on
why electron-rich iridium species promote these dehydrogenative reactions are now in progress.
4. Conclusion
The catalytic activities of titania-supported iridium catalysts for the dehydrogenative synthesis
of benzimidazole and indole at relatively lowtemperature (100 °C) depended on the crystalline structure
of titania. Of those examined, iridium catalysts supported on rutile titanias, such as JRC-TIO-6 and F-1R,
selectively gave the desired products in high yields, while catalysts supported on anatase titanias with
high surface areas, namely JRC-TIO-7, -8, and -9, did not show significant activity. TEM photographs
showed that small nanoparticles (ca. 2 nm in diameter ) were uniformly dispersed on JRC-TIO-6 and
F-1R, while iridium particles are only found on specific facets or particles of several anatase supports.
According to the results of CO pulse experiments and an XPS study, the formation of small,
well-reduced iridium nanoparticles can be considered to be essential for excellent activities in
dehydrogenative reactions. Further application of these catalysts for other dehydrogenative reactions is
underway.
Acknowledgements
Part of this research was supported by a Grant-in-Aid for Scientific Research (No. 17H03458)
from the Ministry of Education, Sports, Culture, Science and Technology, Japan. We deeply
acknowledge Showa Denko Ceramics Co. Ltd. for providing titania samples.

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100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
Calcination temp.(oC)
Fig. 1. Effects of calcination temperature of Ir/JRC-TIO-6-supported catalysts on the synthesis of indole
5 at 100 °C for 18 h. All the catalysts were reduced under H2 at 500 °C before use (see Scheme 1).

(a)
(b)
(c)
10 nm
10 nm
10 nm
80
60
40
20
0
80
80
60
40
20
0
60
40
20
0
1.0 1.5 2.0 2.5 3.0
1.0 1.5 2.0 2.5 3.0
1.0 1.5 2.0 2.5 3.0
Diameter (nm)
Diameter (nm)
Diameter (nm)
(f)
(d)
(e)
10 nm
10 nm
10 nm
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
1.0 1.5 2.0 2.5 3.0
1.0 1.5 2.0 2.5 3.0
1.0 1.5 2.0 2.5 3.0
Diameter (nm)
Diameter (nm)
Diameter (nm)
Fig. 2. TEM photographs and the particle size distribution histograms of iridium catalysts supported on
(a) JRC-TIO-6 (R), (b) JRC-TIO-6 (R, calcined at 240°C), (c) JRC-TIO-7 (A), (d) F-1R (R), (e) F-2 (A),
and (f) JRC-TIO-4 (A) [7]. Loading levels of iridium species are shown in Table 1. Catalysts were
calcined at 400 °C and then reduced at 500 °C for 30 min unless otherwise noted.

(a)
100
80
60
40
20
0
TIO-6*
F-1R
TIO-9
TIO-6
F-6
F-2
F-4
F-1
F-1R**
TIO-4
TIO-7
TIO-8
0
20
40
60
CO uptake X 102 (mol/mol)
(b)
100
80
60
40
20
0
TIO-6*
TIO-4
TIO-6
F-1R**
F-1R
F-2
F-6
TIO-7
TIO-8 TIO-9
0
20
40
60
CO uptake X 102 (mol/mol)
Fig. 3. Relationship between CO uptake of the catalysts and the yields of (a) 3 from 1 and 2 and (b) 5
from 4. Reaction temperature 100 °C, reaction time 18 h. *Prepared without calcination. **Prepared
using [(COD)IrCl]2 as an iridium source.

Ir(acac)3 in MeOH
Impregnation
TiO2
Drying in air at 80 oC overnight
Calcination at 400 oC for 30 min.
Reduction in 2% H2 in Ar at 500 oC for 30 min.
Passivation in 2% O2 in N2 at 50 oC for 5 min.
Ir(2.0 wt% as Ir) /TiO2
Scheme 1. Preparation procedure of titania-supported iridium catalysts.

Table 1. Activities of Ir/TiO2 catalysts for the synthesis of 2-phenylbenzimidazole
Entry TiO2 supporta
BET S.A. of
Ir loading after
Yield of 3 at
Yield of 3 at
catalysts (m2g-1)
calcination (wt%) 120 °C 18 h
(%)
100 °C 18 h
(%)
24
59
88
17
19
27
61
26
24
32
43
1
JRC-TIO-4 (A)b
JRC-TIO-6 (R)
JRC-TIO-6 (R)c
JRC-TIO-7 (A)
JRC-TIO-8 (A)
JRC-TIO-9 (A)
F-1R (R)
51
2.0
2.0
2.0
2.5
2.4
2.0
1.0
1.2
1.4
1.1
2.0
96
96
-
2
53
3
63
4
133
123
129
19
29
55
59
83
87
91
82
94
5
6
7
8
F-1 (A)
21
9
F-2 (A)
30
10
11
F-4 (A)
47
F-6 (A)
85
a (A): Anatase-rich and (R) rutile-rich titanias. b Containing ca. 20% of rutile. c Prepared without
calcination.

Table 2. Activities of Ir/TiO2 catalysts for the synthesis of indole.
Entry TiO2 supporta
Yield of 5 at
Yield of 5 at
120 °C 18 h (%)
100°C 18 h (%)
1
2
3
4
5
6
JRC-TIO-4 (A) 50
JRC-TIO-6 (R) 86
29
45
13
17
16
52
JRC-TIO-7 (A) 15
JRC-TIO-8 (A) 22
JRC-TIO-9 (A) 23
F-1R (R)
80
a (A): Anatase-rich and (R) rutile-rich titanias.