Origin of the excellent catalytic activity of Pd loaded on ultra-stable y zeolites in Suzuki-Miyaura reactions

Article abstract of DOI:10.1016/j.jcat.2010.05.009

Suzuki-Miyaura reactions were performed over Pd loaded on (ultra-stable Y) USY zeolites prepared by steam treatment of NH₄-Y. We found that the catalytic activity of Pd increased significantly with steam treatment of NH ₄-Y when 6% H₂ was applied before and during the reactions. For instance, a TON of 13,000,000 was obtained in the reaction between bromobenzene and phenylboronic acid in 1.5 h. Pd K-edge and Pd L ₃-edge X-ray absorption fine structure analyses revealed the formation of atomic Pd with a cationic character. The catalytic activity of Pd/USY prepared under different steam-treatment conditions was in good correlation with the strong Bronsted acid sites induced by the extra-framework Al. Based on the catalytic performance data, the structure of Pd, and acidic analysis of the support, atomic Pd anchored to the strong Bronsted acid sites of the USY zeolite was proposed to be the active species.

Full text of DOI:10.1016/j.jcat.2010.05.009

Journal of Catalysis 273 (2010) 156–166
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Origin of the excellent catalytic activity of Pd loaded on ultra-stable Y zeolites in
Suzuki–Miyaura reactions
*
Kazu Okumura , Takuya Tomiyama, Shizuyo Okuda, Hiroyuki Yoshida, Miki Niwa
Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Suzuki–Miyaura reactions were performed over Pd loaded on (ultra-stable Y) USY zeolites prepared by
Received 17 April 2010
Revised 18 May 2010
Accepted 19 May 2010
Available online 26 June 2010
steam treatment of NH
treatment of NH –Y when 6% H
3,000,000 was obtained in the reaction between bromobenzene and phenylboronic acid in 1.5 h. Pd
4
–Y. We found that the catalytic activity of Pd increased significantly with steam
4
2
was applied before and during the reactions. For instance, a TON of
1
3
K-edge and Pd L -edge X-ray absorption fine structure analyses revealed the formation of atomic Pd with
a cationic character. The catalytic activity of Pd/USY prepared under different steam-treatment condi-
tions was in good correlation with the strong Brønsted acid sites induced by the extra-framework Al.
Based on the catalytic performance data, the structure of Pd, and acidic analysis of the support, atomic
Pd anchored to the strong Brønsted acid sites of the USY zeolite was proposed to be the active species.
Ó 2010 Elsevier Inc. All rights reserved.
Keywords:
Suzuki–Miyaura reaction
Palladium
Ultra-stable Y zeolite
X-ray absorption fine structure
Acid property
1
. Introduction
Cross-coupling reactions, such as Suzuki–Miyaura and Mizor-
ratio [23]. Using zeolites as supports for Pd is therefore expected to
lead to high catalytic activity. Among the various zeolites, faujasite
(FAU)-type zeolites are the most promising for use as a support for
Pd because they have large supercage with diameters of ca. 1.3 nm.
Indeed, we found that Pd loaded on ultra-stable Y (USY) zeolites
exhibited excellent catalytic activity in a Suzuki–Miyaura reaction
oki–Heck reactions, are important in the production of pharmaceu-
ticals, organic electroluminescent devices, and liquid crystals [1–
4
]. Numerous Pd complexes, such as palladacycles [5] and N-het-
erocyclic carbine [6,7], have been developed for use in these reac-
tions [8]. However, these Pd(0) or Pd(II) complexes cause
difficulties in the synthesis and purification of the final product.
Another class of catalysts for these reactions, namely heteroge-
neous catalysts, is easy to prepare and readily separated from the
products. For this purpose, Pd has been supported on various mate-
rials, including active carbon [9], zeolites [10–13], double-layer
hydroxides [14], modified silica [15–17], hydroxyapatite [18],
and polymers such as dendrimers [19,20] and polyethylene glycol
2
when H was bubbled through the system prior to the reaction
[24]. The reaction only took place when o-xylene and Pd ammine
complexes were used as the solvent and the Pd precursor, respec-
tively. The catalytic activity was improved significantly by contin-
uously bubbling H
K-edge extended X-ray absorption fine structure (EXAFS) analysis
revealed the formation of atomic Pd species after H bubbling in
2
through the system during the reaction [22]. Pd
2
o-xylene. In addition to studying the active sites, it is important
to obtain insights into the role of the support, taking into account
that the catalytic performance of Pd varies significantly, depending
on the type of support. Of particular interest is the USY zeolite, on
which Pd exhibited extremely high activity in Suzuki–Miyaura
reactions. However, little is known about the reason for the devel-
opment of such a high activity in the Pd/USY catalyst. In general,
[
21]. Among these supported catalysts, there are several reports on
supported Pd catalysts that are nearly as active as the best homo-
geneous ones [13,14,22]. These highly active heterogeneous cata-
lysts have been realized when well-dispersed active sites with a
homogeneous structure are fabricated on supports. Moreover,
well-dispersed metals having surface atoms with a low coordina-
tion number (CN) are anticipated to exhibit high activity; this
behavior is different from that of a bulk-type catalyst. Zeolites have
a large surface area and uniform micropores, which can accommo-
date dispersed metal clusters, leading to a high surface-to-volume
USY zeolites are prepared by steam treatment of Y-type (FAU
þ
structure) zeolites ion exchanged with NH4 cations (NH
4
–Y) at
temperatures of around 823 K [25]. The NH
4
–Y steam treatment
causes the formation of mesopores and the evolution of strong acid
sites as a result of dealumination from the framework structure
[
26]. Tuning of the acid properties of USY is also possible by chang-
ing the steam-treatment conditions, i.e., temperature, time, and
2
H O vapor concentration.
*
Corresponding author. Fax: +81 857 31 5684.
E-mail address: okmr@chem.tottori-u.ac.jp (K. Okumura).
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.05.009

K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
157
Here, we focused on the strong acid sites present in USY, with
the aim of obtaining insight into the genesis of active species in
Pd/USY which showed excellent catalytic performance in Suzuki–
Miyaura reactions that was not realized on other kinds of supports
(Shimadzu Corp., Kyoto, Japan). Tridecane was used as the internal
standard.
2.3. Pd K-edge and L -edge XAFS measurements and data analysis
3
2 3
including H–Y, ZSM-5, Mordenite, Al O , and active carbon [22].
This is because acid sites of zeolites played an important role in
the determination of the dispersion of Pd, as proved primarily by
in situ X-ray absorption fine structure (XAFS) studies [27–29].
The IR spectroscopy/MS spectrometry-temperature programmed
Pd K-edge and Pd L -edge XAFS data were obtained from syn-
chrotron radiation experiments performed at the BL01B1 station,
3
with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2009B1107), and the BL10 station
at the Ritsumeikan University SR Center, respectively. Pd K-edge
XAFS data were collected in the quick mode; a Si(1 1 1) monochro-
mator was continuously moved from 4.75° to 4.40° for 5 min. The
beam size was 5 mm (horizontal) ꢁ 0.8 mm (vertical) at the sam-
ple position. For the Pd K-edge XAFS measurements, 6% H2 was
bubbled through a mixture of Pd/USY in o-xylene, and the treated
Pd/USY was transferred to a plastic cell at r.t. without contact with
air. The X-ray path length of the plastic cell was 2 cm. In the EXAFS
analysis, oscillations were extracted by a spline smoothing meth-
3
desorption (IRMS-TPD) method of adsorbed NH was used to study
the acid properties of USY zeolites prepared under different steam-
treatment conditions in detail. In this method, IR and MS work to-
gether to follow the thermal behavior of adsorbed and desorbed
3
NH , respectively [30]. Using this method, information on the
number and strength of acid sites is obtained simultaneously.
The obtained data were correlated with catalytic performance cou-
pled with XAFS data. Such a combined study enabled us to reveal
the origin of the extremely high activity of Pd/USY catalysts.
3
od. The Fourier transformation of the k -weighted EXAFS oscilla-
3
tions and k
v(k) from the k space to the r space was carried out
ꢀ1
2
. Experimental methods
over the range 25–130 nm . The inversely Fourier-filtered data
were analyzed in the k range between 25 nm
ꢀ1
ꢀ1
and 130 nm
2.1. Catalyst preparation
using a curve-fitting method. The inversely Fourier-filtered data
were analyzed by the usual curve-fitting method, based on Eq. (1)
Na–Y zeolite (320NAA) supplied by the Tosoh Corp., Tokyo, Ja-
pan was employed as the starting material for the preparation of
USY. The Na–Y was ion exchanged three times with a solution of
X
vðkÞ
¼
N
F
ðkÞ expðꢀ2r j2 k Þ sinð2kr
2
j
þ / ðkÞÞ=kr
2
j
j
j
j
j
ð1Þ
2
2
1=2
k
j
¼ ðk ꢀ 2m
D
E0j= ꢀh Þ
ꢀ
1
NH
from NH
an N flow. Typical concentration of H
an exception of steaming with 40 vol.%-H
placed in a quartz tube and treated with H
73–873 K. The total flow rate was 50 mL min . The obtained
4
NO
3
(0.5 mol L ) at 353 K to give NH
–Y zeolites by treatment with H
O vapor was 18 vol.% with
O. NH –Y (5 g) was
O vapor for 1–10 h at
4
–Y. USY was prepared
where N
Debye–Waller factor, and the threshold energy difference between
the reference and the sample, respectively. F (k) and u (k) represent
j j j
, r , r , and DE0j represent the CN, the bond distance, the
4
2
O vapor diluted with
2
2
j
j
2
4
amplitude and phase shift functions, respectively. In the curve-fit-
ting analysis, the empirical phase shift and the amplitude functions
of Pd–O and Pd–Pd were extracted from the data obtained for PdO
and Pd foils, respectively. CdS was used as the reference for Pd–Si/
Al, as described in the literature [32].
For the collection of Pd L
by bubbling 6% H in o-xylene was transferred to a polyethylene
bag (thickness = 10 m) at r.t. Pd L -edge X-ray absorption near
edge structure (XANES) data were recorded under He at atmo-
spheric pressure in a fluorescence mode. A Ge(1 1 1) monochroma-
tor was moved stepwise from 37.3° to 35.0° for 30 min. The XAFS
data were analyzed using the REX2000 (ver. 2.5) program devel-
oped by Rigaku Ltd., Tokyo, Japan. Error bars for each parameter
in the EXAFS curve fitting were estimated by stepping each param-
eter, while optimizing the other parameter, with R factor becomes
two times as its minimum value.
2
ꢀ1
6
ꢀ
1
USY was ion exchanged three times with NH
at 353 K to give NH
with NH4 was confirmed by TPD of NH
4
NO
3
(0.5 mol L
)
+
+
4
–USY. The completion of ion exchange of H
þ
3
, showing that 93% of H
þ
3
-edge XAFS data, the Pd/USY treated
had been replaced with NH . The NH
4
–USY was then heated to
. Pd was then introduced to
the calcined USY by an ion-exchange method using Pd(NH Cl
4
5
73 K in air to partially remove NH
3
2
l
3
3
)
4
2
ꢀ4
ꢀ3
solution (3.8 ꢁ 10 mol dm ; Aldrich, St. Louis, MO, USA) at
room temperature (r.t.) [31]. The suspension was washed with
2
deionized H O and dried in an oven at 323 K overnight. The Pd
loading of all samples was 0.4 wt.%, as measured by inductive cou-
pled plasma (ICP) analysis.
2
.2. Catalytic reactions
Under typical conditions, bromobenzene (0.2 mol; Tokyo Kasei
2.4. IRMS-TPD of NH
3
Chemicals Ltd., Japan), phenylboronic acid (0.32 mol; Tokyo Kasei
Chemicals Ltd., Japan), K CO (0.4 mol; Wako Chemicals Ltd., Osa-
ka, Japan), o-xylene (solvent, 560 mL, Wako Chemicals Ltd., Osaka,
2
3
A Fourier-transform IR (FT-IR) spectrometer (Perkin-Elmer
Spectrum-One; Perkin-Elmer, Waltham, MA, USA) and a mass
spectrometer (Pfeiffer QME200; Pfeiffer, Asslar, Germany) were
connected with a vacuum line kept at 3.3 kPa through which He
ꢀ8
Japan), and 0.4 wt.% Pd/USY catalyst (0.5 mg, Pd: 1.9 ꢁ 10 mol),
weighed using an electronic microbalance, were used for the Suzu-
ꢀ1
ꢀ1
ki–Miyaura reactions. A 6% H
2
/94% Ar flow at a rate of 30 mL min
was allowed to flow as the carrier (flow rate, 110 mL min ). An
was introduced into the reactant solution using a glass capillary
tube before and during the reaction, for 1 h at r.t. A three-necked
flask (1 L) was placed in an oil bath, preheated to the required tem-
perature, and subjected to vigorous stirring. The reaction was per-
formed at 383 K. Suzuki–Miyaura reactions using chlorobenzene
derivatives were performed under similar conditions, except for
IR beam was transmitted to a self-compressed disk (5 mg and
10 mm in diameter). Details of the experimental apparatus have
been described elsewhere [33]. After evacuation of the sample at
773 K, IR spectra were recorded before NH
3
adsorption at 10 K
intervals from 373 to 773 K, while the temperature was increased
ꢀ
1
at a rate of 10 K min (N(T), recorded). The bed temperature was
then lowered to 373 K, at which point NH was adsorbed at
13 kPa, and then gas-phase NH was evacuated for 30 min. IR spec-
the use of DMF + H
2
O (1.5 vol.%) and Cs
2
CO
3
as the solvent and
3
base, respectively. The reactions were performed in an Ar atmo-
sphere. After reaction, the reaction mixture was cooled to r.t.,
and the solution was analyzed using a Shimadzu 2010 Gas Chro-
matograph equipped with an InertCap 5 (30 m) capillary column
3
tra were again measured at 10 K intervals from 373 to 773 K, while
the temperature was raised (A(T), recorded). The difference spec-
trum, i.e., A(T) ꢀ N(T), was calculated at each temperature, and

1
58
K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
100
3. Results
3
.1. Enhancement of the catalytic activity of Pd by steam treatment of
8
6
4
2
0
0
0
0
0
4
NH –Y
1
5
0
0
6
%-H bubbling
2
3.1.1. Catalytic performance of Pd/USY in bromobenzene and
bromonaphthalene derivatives
Suzuki–Miyaura coupling reactions of bromobenzene and bro-
monaphthalene derivatives were carried out over Pd loaded on
USY prepared by steam treatment of NH
activate the Pd/USY catalyst, a 6%-H flow at a rate of 30 mL min
was fed into the reactant solution using a glass capillary before and
during the catalytic reactions. It should be emphasized that a small
amount of 0.4 wt.% Pd/USY (0.5 mg) was used with respect to
4
–Y zeolite. In order to
ꢀ1
2
Without H bubbling
2
ꢀ5
0
.2 mol of bromobenzene, which corresponds to 0.7 ꢁ 10 mol%
0
1
2
Pd. We found that Pd/USY worked very efficiently in Suzuki–Miya-
ura reactions. Fig. 1 shows a typical change in the conversion of
bromobenzene with time of the reaction with phenylboronic acid.
Without the H -bubbling treatment, the activity of Pd/USY was al-
2
most negligible, with a turnover number (TON) of 30,000 being ob-
tained. In marked contrast to this, the conversion of bromobenzene
Time / h
Fig. 1. Time course change in the conversion of bromobenzene over 0.4 wt.%-Pd/
USY with or without 6%-H bubbling. USY was prepared by steaming of NH –Y at
23 K for 10 h with 18% H O.
2
4
8
2
reached 88% in 30 min when H
was completed in 1.5 h, and the Pd TON reached 13,000,000. The
effect of the H bubbling on the Suzuki–Miyaura reaction is there-
fore significant.
Table 1 lists representative data for reactions carried out in the
presence of Pd/USY using various combinations of bromobenzene
or bromonaphthalene derivatives and boronic acids carried out un-
2
bubbling was used. The reaction
changes in IR absorptions were observed to identify the absorp-
þ
tions of NH4 and NH
3
adsorbed on the surface. Changes in OH
2
bands were also detected in the difference spectra as negative
peaks. The difference spectra with respect to temperature, i.e.,
ꢀ
d(A(T) ꢀ N(T))/dT, were calculated at selected band positions (re-
ferred to here as IR-TPD). In the present study, the area of IR
absorption was quantified in the absorption range. The IR-TPD
2
der 6%-H bubbling. Very high TONs, higher than 2,400,000, were
was compared with the MS-measured TPD of NH
identify the nature of the adsorption site for the desorbed NH
i.e., a Lewis or a Brønsted acid site.
3
(m/e, 16) to
obtained with various bromobenzene derivatives in which the
cross-coupling reaction proceeded almost quantitatively (entries
1– 5). The Pd/USY catalyst was also applicable to naphthalene
3
,
Table 1
Suzuki–Miyaura coupling reactions of aryl bromides catalyzed by 0.4 wt.%-Pd/USY activated with H
2
under in situ conditions.a
ArABr þ Ar ABðOHÞ ! ArAAr0
0
2
0
Pdb (mol%)
Entry
1
ArABr
Ar AB(OH)
2
Time (h)
1.5
Yield (%)
99
TON
7.7 ꢁ 10ꢀ
6
13,000,000
9.2 ꢁ 10ꢀ
9.5 ꢁ 10ꢀ
1.3 ꢁ 10ꢀ
5.0 ꢁ 10ꢀ
1.3 ꢁ 10ꢀ
6
1.5
3
99
96
89
83
99
11,000,000
11,000,000
8,900,000
2,400,000
760,000
2
3
4
5
6
6
5
6
5
18
1
4
2.2 ꢁ 10ꢀ
4.3 ꢁ 10ꢀ
9.2 ꢁ 10ꢀ
6.4 ꢁ 10ꢀ
3
1
1
1
1
75
99
84
78
60,000
230,000
130,000
20,000
7
8
9
4
4
3
1
0
a
Typical reaction conditions (Entry 1): see Section 2.2. The scales of all reagents were changed, while the catalyst weight was fixed at 0.5–1.0 mg.
Pd mol% with respect to the bromobenzene or bromonaphthalene derivatives used for reactions.
b

K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
159
100
8
6
4
2
0
80
60
40
20
0
1
5
0
000
00
700
800
900
300 400 500 600 700 800
Steaming Temp. / K
Calcination Temp. / K
Fig. 2. TOFs plotted as a function of the steaming temperature for the preparation
of USY in the coupling reaction between bromobenzene and phenylboronic acid.
Steaming time for the preparation of USY was 1 h.
Fig. 3. Yield and TONs plotted as a function of the temperature for the calcination of
NH –USY in the coupling reaction between 4-chloroacetophenone and phenylbo-
ronic acid. The TON was obtained at 10 min from the beginning of the reaction.
4
0
derivatives (entries 6–10), including reactions to give 1,1 -binaph-
thyl (entry 9).
4
phenyl plotted as a function of the NH –USY calcination tempera-
ture in the reaction between 4-chloroacetophenone and
phenylboronic acid. It can be seen from the figure that the yield
is highly dependent on the NH –USY calcination temperature;
4
the maximum yield was attained at 600 K. Similar data were ob-
tained for the reaction using bromobenzene, as already reported
Fig. 2 shows the turnover frequency (TOF) plotted as a function
of the steam-treatment temperature for the preparation of USY in
the reaction between bromobenzene and phenylboronic acid. TOFs
were calculated based on the conversion of bromobenzene at
3
0 min from the beginning of the reaction and the amount of Pd
[
4
22]. The NH –Y steam-treatment temperature in the preparation
present in Pd/USY. The TOF increased with increasing the steam-
treatment temperature, with the highest activity being reached
at 823 K. The activity declined on further increasing the steaming
temperature. These data indicated the remarkable effect of
steam-treatment conditions on the catalytic performance of Pd.
of USY also affected the catalytic performance of Pd/USY. Fig. 4
shows the yield plotted as a function of steam-treatment temper-
ature. The optimum temperature for the preparation of USY was
8
73 K, at which a yield of 94% and TON = 1300 was obtained.
3
.2. Pd K-edge and Pd L -edge XAFS analysis of Pd/USY in o-xylene
3
3.1.2. Catalytic performance of Pd/USY in chlorobenzene derivatives
Suzuki–Miyaura coupling reactions of chlorobenzene deriva-
tives were carried out over Pd/USY in an Ar atmosphere. After
screening of the solvents, we found that the addition of small
Fig. 5 shows Pd K-edge k3
v
(k) EXAFS spectra and their Fourier
transforms for Pd(NH
3
)
4
Cl
2
/USY, measured with 6%-H bubbling
2
in o-xylene, in which USY was prepared by steam treatment at dif-
ferent temperatures. The color of the sample changed from white
to black at 383 K. The EXAFS data of Pd/USY immersed in o-xylene
were collected under in situ condition at 300 K after reduction with
2
amounts of H O (ca. 1.5 vol.%) to N,N-dimethylformamide (DMF)
led to high Pd/USY catalyst activity. As shown in Table 2, we could
obtain TON = 2000 and 1200 for the reactions between 4-chloroac-
etophenone or 4-chloronitrobenzene and phenylboronic acid,
respectively, in 10 min. Unlike Pd/USY, Pd/NaX (NaX: Zeorum F-
6
2
%-H at 383 K using a plastic cell. The data for the curve-fitting
analysis are summarized in Table 3. The Pd–Pd bond characteristic
9
, supplied by the Tosoh Corp., Tokyo, Japan) showed negligible
activity in an Ar atmosphere (entry 3). A similar performance
was observed in the reaction carried out in air.
100
4
The influence of the thermal treatment of NH –USY before Pd
loading was then examined. Fig. 3 shows the yield of 4-acetylbi-
80
60
40
20
0
1
000
Table 2
Suzuki–Miyaura coupling reactions of chlorobenzene derivatives catalyzed by
0.4 wt.%-Pd/USY.
ArACl þ PhABðOHÞ2 ! ArAPh
Entry
1a
Catalyst
USY
Ar–Cl
Pdc (mol%)
Yield (%)
81
TON
500
0
4.1 ꢁ 10ꢀ2
2000
2b
USY
NaX
9.1 ꢁ 10ꢀ2
92
1200
4
3b
9.1 ꢁ 10ꢀ2
0.3
600
700
800
900
1000
Steaming Temp. / K
a
ArCl (5 mmol), PhB(OH)
2
(8 mmol), Cs
2
CO
3
(10 mmol), DMF + H
2
O (14 mL, H
2
O:
O:
1
1
.5 vol.%). Temperature: 373 K. Time: 10 min, Ar atmosphere.
Fig. 4. Yield and TONs plotted as a function of the steaming temperature for the
preparation of USY in the coupling reaction between 4-chloroacetophenone and
phenylboronic acid. The TONs were obtained at 10 min from the beginning of the
reaction.
b
ArCl (2.5 mmol), PhB(OH)
2
(4 mmol), Cs
2
CO
3
(5 mmol), DMF + H
2
O (7 mL, H
2
.5 vol.%). Temperature: 373 K. Time: 10 min, Ar atmosphere.
c
Pd mol% with respect to the chlorobenzene derivatives used for reactions.

1
60
K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
(a)
(b)
Pd foil
0.2
Pd-Pd
3
0
0
40
30
20
10
0
Pd foil
0.2
8
8
73 K
23 K
8
8
73 K
2
(
k
)
Pd-O Pd-Al
f
f
23 K
773 K
3
k
7
7
7
73 K
48 K
7
7
48 K
23 K
10
0
23 K
5
4
6
8
10
12
0
1
2
3
4
6
k / 10 nm-1
Distance / 0.1 nm
Fig. 5. Pd K-edge EXAFS: (a) k3
v(k) and (b) their Fourier transforms of Pd loaded on USY prepared by steaming of NH
4
2
–Y at different temperatures. Steaming time: 1 h, H O
concentration: 18%.
Table 3
5
Curve-fitting analysis of Pd K-edge EXAFS data measured at r.t. for 0.4 wt.%-Pd loaded
USY after the treatments with bubbling H in o-xylene.
2
Temp.a Scatter CNb Rc (nm)
D
E
d
DWe
(nm)
R
f
f
0
4
3
2
1
0
(
K)
(eV)
(%)
7
7
8
23
73
23
Pd
Pd
O
Al(Si)
Pd
O
4.3 ± 1.0 0.275 ± 0.001
3.5 ± 0.7 0.276 ± 0.001
5
5
0.0085
0.0087
0.0074
0.0061
1.3
1.7
1.7
t
1.1 ± 0.2 0.218 ± 0.001 ꢀ4
1.0 ± 0.2 0.256 ± 0.001
2
Pd foilg
PdOg
12
4
0.274
0.202
K PdCl6
2
a
b
c
Steaming temperature of NH
Coordination number.
Bond distance.
4
–Y for the preparation of USY zeolites.
Pd(NH ) Cl
2
3
4
d
Difference in the origin of photoelectron energy between the reference and the
Pd/USY
Pd foil
sample.
e
Debye–Waller factor.
Residual factor.
Data of X-ray crystallography.
f
g
3170
3180
3190
Photon Energy / eV
of metallic Pd appeared in the samples prepared at 723–773 K with
CNs of 3.5–4.3. The intensity of the Pd–Pd peak gradually de-
creased with increasing steam-treatment temperature, with the
peak eventually vanishing at 823 K. In the spectrum of USY pre-
pared at 823 K, two small peaks can be seen at 0.17 nm and
Fig. 6. Pd-L
and reference samples.
3
edge XANES of Pd/USY treated with bubbling-H
2
in o-xylene at 383 K
4
3
00
00
0
.22 nm in the Fourier transforms. These bonds were assignable
to Pd–O and to Pd–Al(Si) from the USY-zeolite framework, respec-
tively, as reported earlier [22]. The Pd–O bond distance (0.218 nm)
was longer than that of the covalent Pd–O bond (0.202 nm) in PdO,
as shown in Table 3. This indicates the formation of mono-atomic
Pd, because only the atoms present in the USY-zeolite framework
were observed by EXAFS. On further increasing the temperature
to 873 K, the Pd–O and Pd–Pd bonds appeared at 0.15 and
4
NH -Y
200
USY
0
.25 nm, respectively, implying that the Pd structure became
inhomogeneous.
1
00
0
Information on the Pd valence state can be obtained in the Pd
L
3
-edge XANES region because electronic state of absorbing atom
reflects on the spectra [34]. The Pd L -edge XANES data were re-
corded in an He flow (under atmospheric pressure), with the sam-
ple placed in a polyethylene bag 10 m thick. Fig. 6 gives a
comparison of the XANES region of Pd/USY, measured after bub-
bling with 6% H in o-xylene, together with reference samples of
Pd foil, Pd(NH Cl , and K PdCl , which have oxidation states of
, 2, and 4, respectively. The catalyst USY was prepared by the
3
0
0.2
0.4
0.6
0.8
1
l
p/p0
2
Fig. 7. N
23 K for 10 h with 18% H
desorption branches.
2
adsorption isotherms of NH
4
–Y and USY samples prepared by steaming at
3
)
4
2
2
6
8
2
O. Open symbols: adsorption branches; Closed symbols:
0

K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
161
and positions of the white lines represent the valences and the
energies of the core electrons, respectively [35,36]. The relative
electron deficiency and ionic valence can be determined on the ba-
sis of the white line intensity [37,38]. It can be seen from the figure
reference
3
73 K
4
73 K
5
3 4 2
that the peak position and shape of the XANES for Pd(NH ) Cl /USY
73 K
are close to those of the Pd foil but shifted to a higher energy by
about 0.6 eV. Judging from the intensity of the white line, the va-
lence of Pd grafted on USY could be estimated to be +0.3. This
means that the atomic Pd grafted on the USY support had a slight
673 K
0
positive charge compared with Pd .
3
2 4
.3. N adsorption isotherms of NH –Y and USY supports
Fig. 8. Difference IR spectra measured on USY (steaming conditions: 18% H
8
3
2
O at
23 K for 10 h) with adsorbed ammonia during the elevation of temperature from
73 to 773 K. Spectra were taken every 10 K. Bold lines show the spectra measured
It is well known that steam treatment of NH
formation of mesopores as a result of dealumination of the frame-
work of Y-type zeolites [39,40]. Fig. 7 shows N adsorption iso-
therms of NH –Y and the USY prepared by steam treatment with
4
–Y results in the
at 373, 473, 573, 673, and 773 K.
2
4
steam treatment at 823 K. It can be seen in the spectra of the ref-
erence samples that the intensities of the absorption peaks (white
lines) increased, and their positions shifted to higher energies
when the oxidation state of Pd increased. These white lines are
attributed to the transition of the core electrons in 2p3/2 to the
unoccupied 4d states above the Fermi level, and the intensities
NH –Y (steam-treatment conditions: temperature 823 K; time
4
10 h; 18% H O). Although a slight change was observed after steam
2
treatment with NH –Y compared with untreated NH –Y, no clear
4
4
difference was found in USY. Moreover, mesopore formation was
not seen in every USY sample; thus the possibility of mesopores
participating in the Pd catalytic reaction is not plausible. The BET
(a)
(b)
(c)
(d)
Fig. 9. Difference IR spectra measured on: (a) H–Y and (b–d) USY (prepared by steaming with 18% H
3
8
2
O) with adsorbed ammonia during the elevation of temperature from
73 to 773 K. Spectra were taken every 10 K. Bold lines show the spectra measured at 373, 473, 573, 673, and 773 K. Steaming conditions: (a) untreated, (b) 773 K, 1 h, (c)
23 K, 10 h, and (d) 873 K, 1 h.

1
62
K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
(
a)
(b)
4
2
0
0
0
1
5
0
0
MS
0.8
0.4
2
0
.0
.0
MS
NH4+
NH4+
C
C
g
g
T
T
1326 cm-1
0.0
T
T
N
IHexagonal
ISuper
N
T
A
-20
T
A
-
-
0.4
0.8
IUnknown IStrong
-5
-
-
2.0
4.0
ISodalite
IHexagonal
-40
ISodalite
-10
ISuper
-1.2
4
00
500
600
700
400
500
600
700
Temperature / K
Temperature / K
(
c)
(d)
0
0
0
0
.6
.4
.2
.0
0.6
0.4
0.2
0.0
MS
5
0
MS
5
NH4+
NH4+
C
C
g
g
T
T
1
326 cm-1
1326 cm-1
T
T
IUnknown
IStrong
N
0
N
-
-
-
0.2
0.4
0.6
T
A
T
A
IStrong
IHexagonal
-
0.2
IUnknown
IHexagonal
ISodalite
ISuper
ISodalite
-
5
-0.4
ISuper
-
5
400
500
600
700
400
500
600
700
Temperature / K
Temperature / K
þ
ꢀ1
ꢀ1
Fig. 10. IR-TPD of NH and hydroxyl bands in the supercage, sodalite cage, strong acid site, hexagonal prism, and the unknown peak appeared at 3630 cm , 3550 cm
3
,
4
ꢀ
1
ꢀ1
ꢀ1
598 cm , 3520 cm , and 3609 cm , respectively, together with MS-TPD as a comparison. Steaming conditions: (a) untreated, (b) 773 K, 1 h, 18% H
O, and (d) 873 K, 1 h, 18% H O.
2
O, (c) 823 K, 10 h, 18%
H
2
2
surface areas of NH
4
–Y and USY were calculated to be 710 and
not observed in the unmodified H–Y (Fig. 9a). The intensity of
the OHstrong band increased with increasing temperature and
2
ꢀ1
6
90 m g , respectively.
NH
4
Y steam-treatment time (Fig. 9b and c), so it is assumed that
3.4. IRMS-TPD studies of acid sites in USY zeolites
deconvolution
Fig. 8 shows the difference IR-TPD spectra of USY (steam-treat-
ment conditions: 18% H
2 3
O at 823 K for 10 h) with adsorbed NH , as
simulation
original
the temperature was raised from 373 to 773 K. A sharp absorption
ꢀ1
þ
at 1432 cm , ascribable to the NH4 bending vibration, was clearly
observed. Furthermore, a small absorption was observed at
ꢀ1
1
665 cm , and this band was identified as weakly adsorbed
NH . NH stretching vibrations also appeared in the broad region
400–2500 cm . A background spectrum measured at 373 K
3
ꢀ1
3
ꢀ1
showed two OH bands at 3623 and 3550 cm , which were identi-
fied as OH groups in the supercage and sodalite cage of the Y zeo-
abs.=0.1
lite, respectively [41]. On NH
disappeared. The OH stretching region of the adsorbed NH
3
adsorption, these OH bands almost
, and
3
the subsequent TPD, is enlarged in Fig. 9. The OH stretching region
consisted of five kinds of OH groups: OH groups in the supercage
3
700
3600
3500
-1
ꢀ
1
ꢀ1
Wavenumber / cm
(
OHsuper, 3630 cm ), unknown species (OHunknown, 3609 cm ),
ꢀ1
sodalite cage (OHsodalite, 3550 cm ), hexagonal prism (OHhexagonal
3
3
,
ꢀ
ꢀ
1
Fig. 11. Deconvolution of OH bands measured at 373 K into six components. Sum of
them (simulation) was fitted well to the original drawn OH band in the difference
spectrum. USY was prepared by steaming at 823 K, 10 h, with 18%-water vapor,
corresponding to Figs. 8, 9c and 10c.
520 cm ), and strong acid sides characteristic of USY (OHstrong
,
1
598 cm ) [42]. At high temperatures, a new negative band was
ꢀ
1
seen at 3598 cm (indicated by an arrow in Fig. 9c), which was

K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
163
(a) 3
(b) 1
OHtotal
NH +
4
NH +
4
0.8
0.6
0.4
0.2
0
OHunknown (3609)
OHtotal
2
OHSuper
OHStrong (3598)
OHSuper
g
OHSodalite
g
1
0
C
C
OHSodalite
OHHexagonal
OHHexagonal
400
500
600
700
400
500
600
700
Temperature / K
Temperature / K
(c) 8
(d) 8
6
4
6
4
+
OHunknown (3609)
OHStrong (3598)
OHunknown (3609)
OHStrong (3598)
NH4
OHtotal
OHtotal
NH +
OHSodalite
4
OHSuper
OHSodalite
g
g
C
C
OHSuper
2
2
OHHexagonal
OHHexagonal
0
0
400
500
600
700
400
500
600
700
Temperature / K
Temperature / K
Fig. 12. Corrected IR-TPD of five kinds of OH groups, sum of them (I
0
), and MS-TPD of ammonia to measure the amounts of Brønsted OH in the USY. Steaming conditions: (a)
O.
untreated, (b) 773 K, 1 h, 18% H O, (c) 823 K, 10 h, 18% H O, and (d) 873 K, 1 h, 18% H
2
2
2
this band was created as a result of the steam treatment. Excessive
steam treatment at 873 K resulted in a decrease in the number of
strong acid sites (Fig. 9d).
Here, I
at supercage, sodalite cage, the strong acid site, hexagonal prism,
and unknown species, respectively, in which I should agree with
1 2 3 4 5
, I , I , I , and I mean the intensity of IR-TPD of the OH bands
0
IR-TPD profiles of USY prepared under different steaming condi-
tions were then calculated for OH bands and adsorbed ammonia
species. Fig. 10 shows the IR-TPD spectra of H–Y and the USY pre-
pared by steaming under different conditions. MS-measured TPD is
MS-TPD in size and shape. IR-TPD and MS-TPD have different units,
but these are convertible because of the coincidence between IR-
þ
TPD of NH4 and MS-TPD of NH
3
shown in Fig. 10. The number of to-
tal Brønsted acid sites was calculated from the desorbed amount of
NH . The parameter a is a negative value and corresponds to the re-
also shown for a comparison in which desorption of NH
3
was found
3
at ca. 500 K. Sum of the amounts of desorbed ammonia species
shown by the IR-TPD curves must be equal to the MS-TPD of
ammonia. In the calculation of IR-TPD, the absorbance area due
to NH4 was used for the quantification of adsorbed ammonia. On
the other hand, the deconvolution of OH bands was made, as
shown in Fig. 11, and the area of the OH band was quantified.
ciprocal of an extinction coefficient of the OH band. In other words,
a is a parameter to cancel the difference in extinction coefficients in
IR bands. By taking parameters as a
1
= ꢀ3.4, a
2
= ꢀ9.2, a = ꢀ9.2,
3
þ
a
4
= ꢀ1.3, and a = ꢀ1.3, I could be compared with MS-TPD, as
5
0
shown in Fig. 12. The amounts of acid sites determined by curve-fit-
ting analysis are summarized in Table 4. The number of acid sites
originating from supercage, sodalite cage, and hexagonal prism
tended to decrease with increasing steam-treatment temperature
and time. The maximum number of strong acid sites was reached
at a steam-treatment temperature of 823 K (steaming time: 10 h).
This trend was in good agreement with the catalytic performance
in the reaction between bromobenzene and phenylboronic acid, in
which maximum activity was attained at 823 K.
The Brønsted OH bands should show a mirror-image correlation
þ
with that of NH . As a matter of fact, ammonia was desorbed at dif-
4
ferent temperatures from the OH species at ca. 470, 500, 530, 550,
and 580 K arising from unknown species, OH groups in the super-
cage, sodalite cage, hexagonal prism, and newly created sites,
respectively.
Then, the numbers of these OH groups were individually deter-
1 5
mined as follows: parameters, a –a , were determined by compar-
ison of the IR-TPD of OH and MS-measured TPD of ammonia
according to Eq. (2):
3.5. Correlation of acidic properties of USY and Pd catalytic activity
We then tried to correlate the number of acid sites for USY pre-
pared under different steam-treatment conditions (temperature,
I
0
¼ a
1
I
1
þ a
2
I
2
þ a
3
I
3
þ a
4
I
4
þ a
5
I
5
ð2Þ

1
64
K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
Table 4
3
Amount of acid sites in USY determined by NH IRMS-TPD methods.
a
OHsuperb
(mol kg
c
OHstrongd
(mol kg
e
f
Steaming temp. Steaming time
H
(%)
2
O conc.
OHtotal
OHunknown
(mol kg
OHsodalite
(mol kg
OHhexagonal
(mol kgꢀ1)
ꢀ1
)
ꢀ1
)
ꢀ1
)
ꢀ1
)
(mol kg
ꢀ1
)
(
K)
(h)
7
8
8
8
8
8
73
23
23
23
23
73
1
0.2
1
5
10
1
18
18
40
18
18
18
0.86
0.85
0.82
0.59
0.56
0.34
0.37
0.37
0.23
0.21
0.19
0.11
0.11
0.14
0.03
0.06
0.03
0.04
0.14
0.12
0.13
0.16
0.21
0.10
0.12
0.12
0.08
0.08
0.07
0.05
0.12
0.10
0.10
0.08
0.06
0.04
a
b
c
d
e
f
Total acids.
Supercage.
Unknown species.
Strong acid sites.
Sodalite cage.
Hexagonal prism.
2
time, and H O concentrations) with the TOF in the reaction be-
4. Discussion
tween bromobenzene and phenylboronic acid (Fig. 13). No obvious
correlation was observed for the OH groups from supercage, soda-
lite cage, and hexagonal prism (Fig. 13b–d). In contrast, a linear
relationship was obtained between TOF and the number of strong
We found that Pd/USY catalysts exhibited excellent activity in
Suzuki–Miyaura reactions. This was achieved by choosing appro-
priate steam-treatment conditions for preparation of the USY sup-
port. Pd/USY could be used with various substrates, including
bromobenzene, bromonaphthalene, and chlorobenzene deriva-
tives. Bulky products, such as 1,1-binaphthyl, may be larger than
ꢀ
1
acid sites, i.e., OH groups appearing at 3598 cm in the IR spectra
Fig. 13a), clearly indicating that the strong acid sites are related to
the catalytic activity of Pd.
(
(
b)
(
a) 20
20
10
0
(
823 K,10 h,18%)
(823 K,10 h,18%)
(
823 K,5 h,18%)
(823 K,5 h,18%)
(773 K,1 h,18%)
10
(
773 K,1 h,18%)
(
823 K,1 h,40%)
873 K,1 h,18%)
0.2
(823 K,1 h,40%)
(
823 K,0.2 h,18%)
(823 K,0.2 h,18%)
(
(
873 K,1 h,18%)
0
0
0.1
0
0.1
0.2 0.3
0.4
-1
-1
Amount of Strong Acid Sites / mol kg
Amount of Acid Sites in Supercage / mol kg
(c)
(d) 20
2
1
0
(
823 K,10 h,18%)
(823 K,10 h,18%)
(
823 K,5 h,18%)
(823 K,5 h,18%)
0
10
(
773 K,1 h,18%)
(773 K,1 h,18%)
(823 K,1 h,40%)
(823 K,0.2 h,18%)
873 K,1 h,18%)
(
823 K,1 h,40%)
(
823 K,0.2 h,18%)
(
873 K,1 h,18%)
(
0
0
0
0.05
0.1
0.15
0
0.05
0.1
0.15
Amount of Acid Sites in Sodalite Cage / mol kg-1
-1
Amount of Acid Sites in Hexagonal Prism / mol kg
Fig. 13. Turnover frequencies plotted as a function of acid amount in: (a) strong acid sites, (b) supercages, (c) sodalite cages, and (d) hexagonal prisms. Numbers in
parenthesis indicate the temperature, time and H O concentration for steaming.
2

K. Okumura et al. / Journal of Catalysis 273 (2010) 156–166
165
the supercage in USY zeolites. Reactions involving bulky molecules
5. Conclusions
are therefore thought to take place at the Pd located at the external
USY surface. This is probably possible because USY has a relatively
high external surface area (20 m g ), as measured using the ben-
zene-filled pore method [43].
Two possibilities are considered for the outstanding catalytic
activity in Suzuki–Miyaura reactions of Pd loaded on USY. The first
is the formation of mesopores in the USY support, which would as-
sist transportation of reactants and products. This hypothesis may
Suzuki–Miyaura reactions were carried out over Pd/USY cata-
lysts. We found that the preparation conditions of the USY support
influenced the catalytic performance of Pd. Very high TON values—
higher than 10,000,000—were obtained after optimization of the
2
ꢀ1
þ
steam-treatment conditions and the amount of NH4 cations pres-
ent in the USY. The Pd/USY catalysts could be used in various Suzu-
ki–Miyaura reactions, including those using naphthalene and
chlorobenzene derivatives. On the basis of the catalytic activity,
2
be ruled out because N adsorption isotherms show that mesopore
generation does not occur. Another possible mechanism is the sta-
bilization of atomic Pd by the interaction with strong acid sites in
the USY support. It is well known that USY zeolites have strong
acid sites formed from extra-framework Al on steam treatment
Pd K-edge and Pd L -edge XAFS analyses, and IRMS-TPD studies,
the active species was proposed to be the atomic Pd with partially
cationic character, which was anchored to the strong acid sites in-
duced by extra-framework Al species.
3
4
of NH –Y or Na–Y zeolites. The characteristic OH stretching band
ꢀ1
appeared at 3598 cm in the IR spectra, accompanied by dealumi-
nation, which is ascribed to generation of strong acid sites on USY.
The electron-withdrawing effect of the AlOH2 unit gives rise to
formation of strong acid sites. In agreement with this assumption,
Pd catalysis is sensitive to the steam-treatment temperature used
Acknowledgments
+
This research was partially supported by the Ministry of Educa-
tion, Science, Sports and Culture, Grant-in-Aid for Scientific Re-
search (C), 21560801, 2009-2011. The authors acknowledge Dr.
Koji Nakanishi (Ritsumeikan University) for his technical support
4
in the preparation of USY from NH –Y; the highest activity is at-
tained at 823 K. As can be seen from Fig. 13a, a linear relationship
is obtained between the number of strong acid sites and the TOF.
This means that the strong acid sites play an important role in
the development of extremely high Pd catalytic activity. Atomic
Pd is probably anchored on the USY support by interaction with
strong acid sites, thus agglomeration of Pd is suppressed. In agree-
ment with our studies, Xu et al. reported strong interactions be-
tween Pd and protons in zeolites [44]. Another possible role for
the strong acid sites is the tuning of the electronic properties of
Pd. In general, the Suzuki–Miyaura reaction is supposed to proceed
through three steps: oxidative addition, transmetallation, and the
reductive elimination [45]. It has been reported that the reductive
elimination of products (biphenyl derivatives) is promoted by the
electron-withdrawing effect of Pd [46]. The electronic effect of
strong acid sites promotes the reductive elimination of products
from the active center; the reaction is therefore greatly acceler-
ated. Thus, we observe an interesting role for zeolite acidity, i.e.,
the interaction of Pd with the strong acid sites in zeolites induces
extremely high catalytic activity in organic reactions.
3
in the Pd-L XANES measurements.
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(
low concentrations under specific conditions. In addition, Prockl
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that Pd/USY provided the precursor for active species in the reac-
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is the true active species, and it is possible that o-xylene plays a
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