Enhancement in the catalytic activity of Pd/USY in the Heck reaction induced by H2 bubbling

Article abstract of DOI:10.3390/molecules16010038

Pd was loaded on ultra stable Y (USY) zeolites prepared by steaming NH ₄-Y zeolite under different conditions. Heck reactions were carried out over the prepared Pd/USY. We found that H₂ bubbling was effective in improving not only the catalytic activity of Pd/USY, but also that of other supported Pd catalysts and Pd(OAc)₂. Moreover, the catalytic activity of Pd/USY could be optimized by choosing appropriate steaming conditions for the preparation of the USY zeolites; Pd loaded on USY prepared at 873 K with 100% H₂O gave the highest activity (TOF = 61,000 h^-1), which was higher than that of Pd loaded on other kinds of supports. The prepared Pd/USY catalysts were applicable to the Heck reactions using various kinds of substrates including bromo-and chlorosubstituted aromatic and heteroaromatic compounds. Characterization of the acid properties of the USY zeolites revealed that the strong acid site (OH_strong) generated as a result of steaming had a profound effect on the catalytic activity of Pd.

Full text of DOI:10.3390/molecules16010038

Molecules 2011, 16, 38-51; doi:10.3390/molecules16010038
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Enhancement in the Catalytic Activity of Pd/USY in the Heck
Reaction Induced by H Bubbling
2
Kazu Okumura *, Takuya Tomiyama, Sayaka Moriyama, Ayaka Nakamichi and Miki Niwa
Department of Chemistry and Biotechnology, Tottori University, 4-101, Koyama-cho Minami,
Tottori, Japan
*
Author to whom correspondence should be addressed; E-Mail: okmr@chem.tottori-u.ac.jp.
Received: 17 November 2010; in revised form: 13 December 2010 / Accepted: 24 December 2010 /
Published: 24 December 2010
Abstract: Pd was loaded on ultra stable Y (USY) zeolites prepared by steaming NH
zeolite under different conditions. Heck reactions were carried out over the prepared
Pd/USY. We found that H bubbling was effective in improving not only the catalytic
activity of Pd/USY, but also that of other supported Pd catalysts and Pd(OAc) . Moreover,
4
-Y
2
2
the catalytic activity of Pd/USY could be optimized by choosing appropriate steaming
conditions for the preparation of the USY zeolites; Pd loaded on USY prepared at 873 K
−1
2
with 100% H O gave the highest activity (TOF = 61,000 h ), which was higher than that
of Pd loaded on other kinds of supports. The prepared Pd/USY catalysts were applicable to
the Heck reactions using various kinds of substrates including bromo- and chloro-
substituted aromatic and heteroaromatic compounds. Characterization of the acid
properties of the USY zeolites revealed that the strong acid site (OHstrong) generated as a
result of steaming had a profound effect on the catalytic activity of Pd.
2
Keywords: Heck coupling reaction; USY zeolite; palladium; H bubbling; acid property
1
. Introduction
Heck reactions have been recognized as an important, useful and versatile methodology for the
synthesis of aryl alkenes via arylation or vinylation of olefins. The resulting reaction products are
widely utilized for production of pharmaceuticals, organic electroluminescent devices, and liquid
crystals [1], therefore much effort has been devoted to developing Pd catalysts active in the Heck
reaction. Furthermore, the reaction proceeds under mild conditions to yield products with high

Molecules 2011, 16
39
efficiency. Numerous Pd complexes, including palladacycles [2,3] and N-heterocyclic carbenes [4], have
been developed for use in these reactions. Supported Pd catalysts are other candidates for use in Heck
reactions, which compared to the homogeneous ones, are easily prepared and readily separated from the
products. For this purpose, Pd has been supported on various materials, including activated carbon [5,6],
zeolites [7-10] and polymers such as polyethylene glycol [11,12]. Zeolites are expected to be an
efficient support to accommodate active metal centers because they have large surface area and
uniform micropores. Among the various types on zeolites, faujasite (FAU)-type ones are the most
promising for use as a support for Pd because they have large supercages with diameters of ca. 1.3 nm,
which can be regarded as nano-sized flasks. Indeed, we found that Pd loaded on H-Y type zeolites was
suitable for use in Heck reactions [8]. Furthermore, USY zeolites exhibited excellent catalytic activity
in a Suzuki-Miyaura reaction [13,14]. A remarkable improvement in the catalytic activity in a Suzuki-
Miyaura reaction was achieved by continuously bubbling 6% H through the system during the
2
2
reaction, probably due to the promotion of the reductive elimination step caused by dissolved H . Pd
K-edge extended X-ray absorption fine structure (EXAFS) analysis revealed the formation of atomic
Pd species after H bubbling in o-xylene. In general, USY zeolites are prepared by steam treatment of
2
+
Y-type (FAU structure) zeolites ion-exchanged with NH
4
4
cations (NH -Y). Tuning of the acid
properties of USY is also possible by changing the steam-treatment conditions, i.e., temperature, time,
and H O vapor concentration. A further improvement in activity was achieved after optimization of the
steaming conditions for USY [15]. Here, we tried to apply the Pd/USY catalysts in Heck reactions,
focusing on the effects induced by continuous H bubbling during the reactions as well as the NH -Y
steaming conditions used to prepare the USY zeolite supports.
2
2
4
2
. Results and Discussion
2
.1. Structural Characterizations of USY Zeolites Prepared Under Different Conditions
It has been reported that steam treatment of NH
of mesopores as a result of dealumination of the Y-type zeolite framework [16]. Figure 1(a) shows N2
adsorption isotherms of NH -Y and the USY prepared by steam treatment with NH -Y with 100% H
at different temperatures. Although a gradual decrease in N adsorption capacity was observed up to a
4
-Y under severe conditions results in the formation
4
4
2
O
2
steaming temperature of 1,073 K, mesopore formation was not obvious in every USY sample, except
for the USY prepared at 1,073 K; thus the possibility of mesopores participating in the Pd catalytic
4
reactions described in the following sections may be ruled out. The BET surface areas of NH -Y and
2
−1
the USY prepared at 873 K were calculated to be 710 and 630 m g , respectively.
4
X-ray diffraction (XRD) patterns of NH -Y and USY zeolites prepared by steaming of NH -Y are
4
presented in Figure 1(b). Diffraction patterns characteristic of FAU-type zeolite was seen in every
sample. Although a slight decrease in the intensity of diffraction peaks was observed, it was confirmed
2
that the FAU-type structure was preserved, even after the steaming with 100% H O at 1,073 K.

Molecules 2011, 16
Figure 1. (a) N
40
2
adsorption isotherms. Open symbols: adsorption branches, closed
symbols: desorption branches; (b) XRD patterns of USY zeolites prepared by steaming of
NH
4
-Y zeolites with 100% H
2
O at different temperatures.
(
a)
(b)
5
1
0 cps
1
073 K
3
00
00
00
0
973 K
873 K
823 K
773 K
673 K
573 K
2
1
NH -Y
4
5
7
73 K
73 K
8
9
73 K
73 K
NH -Y
4
1073 K
0
0.2
0.4
0.6
0.8
1
10
20
30
40
50
p/p0
2θ / degree
2
.2. Acid Properties of USY Zeolites Prepared under Different Conditions
Figure 2 shows the difference IR-TPD spectra of H-Y and USY (steam-treatment conditions: 100%
O at 773, 873, 973 K for 1 h) with adsorbed NH , as the temperature was raised from 373 to 773 K.
, and the subsequent TPD, is enlarged.
H
2
3
In these figures, the OH stretching region of the adsorbed NH
3
−1
The OH stretching region consisted of five kinds of OH group: OH groups in the supercage (3,630 cm ),
−
1
−1
−1
extra-framework Al species (3,609 cm ), sodalite cage (3,550 cm ), hexagonal prism (3,520 cm ),
−1
and strong acid sides characteristic of USY (3,598 cm ). In the spectra of USY zeolites, a new
−1
negative band was seen at 3,598 cm (OHstrong), which was not observed in the unmodified H-Y
Figure 2a). The intensity of the OHstrong band increased with increasing temperature, so it is assumed
(
that this band was created as a result of the steam treatment. Table 1 and 2 list the amount and strength
of the each acid site determined based on the IR spectra and MS that was simultaneously measured
during TPD of ammonia, respectively. The detailed data analysis method was described elsewhere [15].
The amount of the OHstrong band was largest when the USY was prepared at 873 K. At this temperature,
the resulting Pd/USY also exhibited the highest activity in Heck reactions, as will be mentioned later.
Excessive steam treatment at 973 K resulted in a decrease in the number of strong acid sites. Therefore,
it is assumed that the interaction with OHstrong band was responsible for the evolution of high activity
of Pd. This tendency was quite different from that of other kinds of acid sites (OHsuper, OHsodalite
,
OHhexagonal); the amounts of these acid sites decreased with increased steaming temperature. It is worth
−1
to mention that the acid strength of OHstrong (153–159 kJmol ) was higher than those of other OH sites
Table 2).
(

Molecules 2011, 16
41
Figure 2. OH region of the difference IR spectra measured on (a) H-Y and (b-d) USY
prepared by steaming with 100% H O for 1 h) with adsorbed ammonia during the
(
2
elevation of temperature from 373 to 773 K. Spectra were taken every 10 K. Bold lines
show the spectra measured at 373, 473, 573 K, 673 K and 773 K. Steaming conditions: (b)
773 K (c) 873 K (d) 973 K.
(
a)
(b)
0
0
.1
.0
0
.0
-
-
-
0.5
1.0
1.5
-
-
-
0.1
0.2
0.3
3800
3700
3600
3500
3400
3800
3700
3600
3500
3400
-
1
-
1
Wavenumber / cm
Wavenumber / cm
(
c)
(d)
0
.1
0.1
OHstrong OHhexagonal
OHsuper OHsodalite
0
.0
0.0
-0.1
-0.2
-0.3
5
73 K
73 K
73 K
4
-
-
-
0.1
0.2
0.3
3
3
800
3700
3600
3500
3400
3800
3700
3600
3500
3400
-
1
-1
Wavenumber / cm
Wavenumber / cm
Table 1. Amount of Brønsted acid sites in USY zeolites determined by NH IRMS-TPD methods.
3
OHsuper
OHstrong
OHsodalite
OH
hexagonal
Sample
−
1
−1
−1
−1
/
mol kg
1.20
/ mol kg
-
/ mol kg
0.38
/ mol kg
0.31
H-Y
USY (773 K)
USY (873 K)
USY (973 K)
0.23
0.17
0.18
0.13
0.08
0.11
0.10
0.05
0.04
0.10
0.04
0.02

Molecules 2011, 16
42
3
IRMS-TPD methods.
Table 2. Strength of Brønsted acid sites in USY zeolites determined by NH
OHsuper
OHstrong
OHsodalite
OH
hexagonal
Sample
−
1
−1
−1
−1
/
kJ mol
112
/ kJ mol
-
/ kJ mol
117
/ kJ mol
107
H-Y
USY (773 K)
USY (873 K)
USY (973 K)
139
159
152
151
141
158
144
143
141
153
142
142
2
.3. Pd K-edge EXAFS of Pd/USY Reduced with Bubbling H
2
in DMAc
In order to obtain some insight into the structure of the active Pd species, Pd K-edge EXAFS data
was collected under in situ conditions. Figure 3 shows Pd K-edge EXAFS spectra of Pd/USY
steaming temperature: 873 K) measured with 6%-H bubbling in dimethylacetamide (DMAc) and Pd
(
2
foil. The Pd–Pd bond characteristic of metallic Pd appeared at 0.22 nm in the Fourier transform (phase
shift uncorrected). Curve fitting analysis using Pd foil as the reference revealed that the coordination
number (CN) of the metallic Pd was 6.5, implying the formation of Pd clusters. The spectrum was close
to that of Pd clusters (CN = 5.5) generated in Pd/H-Y through reduction with 8% H , which were active
13
2
in the Heck reaction between bromobenzene and styrene [8]. Although it was difficult to confirm that the
observed Pd cluster was the active species, taking into account that leached Pd in equilibrium with metal Pd
has been reported to be the active species in many literature reports [17,18], the Pd clusters might be the
precursor for the evolution of catalytic activity.
3
Figure 3. Pd-K edge EXAFS (a) k χ(k) and (b) their Fourier transforms of Pd/USY
reduced with bubbling 6%-H
2
in DMAc (continuous line) and Pd foil (dotted line).
(a) (b)
2
1
0
0
0
30
2
0
0
0
1
-
10
5
10
k / 10 nm
15
0
2
4
6
-
1
Distance / 0.1 nm
2
2
.4. Effect of H Bubbling on the Catalytic Reactions of Pd/USY
Heck coupling reactions were performed over Pd loaded on USY prepared by steam treatment of NH
4
-
−1
2
Y zeolite at 873 K. In order to activate the Pd/USY catalyst, a 6%-H flow at a rate of 30 mL min was
fed into the reactant solution during the catalytic reactions using a glass capillary. It should be
emphasized that a small amount of 0.4 wt % Pd/USY (5 mg) was used with respect to 30 mmol of

Molecules 2011, 16
43
bromobenzene, corresponding to 0.00063 mol % of Pd. We found that Pd/USY worked very efficiently
in Heck reactions when H bubbling was applied during reactions. Figure 4(a) shows a typical change
in the conversion of bromobenzene with time of the reaction with styrene. Without the H -bubbling
2
2
treatment, the activity of Pd/USY was low, with a conversion of 15% being obtained at 4 h. In marked
contrast to this, the conversion of bromobenzene reached 94% in 4 h when 6%-H bubbling was used,
2
2
and the Pd TON reached 150,000. The effect of the 6%-H bubbling on the Heck reaction is therefore
significant. Figure 4(b) shows the TOF plotted as a function of the partial pressure of H . The addition
2
2
of only 1% H to Ar was effective at enhancing the catalytic activity of Pd/USY significantly. TOF
2 2
were slightly depended on the H partial pressure; the highest catalytic activity was attained at an H
pressure of 25%.
Figure 4. (a) Conversion of bromobenzene plotted as a function of the reaction time; (b)
Turnover frequencies plotted as a function of partial pressure of H in the Heck reaction
2
between bromobenzene and styrene over 0.4 wt %-Pd/USY. USY was prepared by
steaming of NH -Y at 873 K.
4
(a)
(b)
100
6
4
2
0
8
6
4
2
0
0
0
0
0
6%-H bubbling
2
N atmosphere
2
0
1
2
3
4
0
10 20 30 40 50
Time / h
Partial Pressure of H / %
2
2
.5. Effect of the Thermal Treatment Temperature of the USY Support
Figure 5 shows the turnover frequency (TOF) plotted as a function of the steam-treatment
temperature for the preparation of USY. TOFs were calculated based on the conversion of bromo-
benzene at 2 h from the beginning of the reaction and the amount of Pd present in Pd/USY measured
by inductive coupled plasma (ICP) analysis. The TOF increased with increasing steam-treatment
temperature, with the highest activity being reached at 873 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. The maximum TOF obtained in Pd loaded on USY
4
−1
prepared at 873 K was 5.3×10 h . The value was higher than those of previous reports such as
−1
−1
Pd/MgO (TOF = 70 h [19]), Pd/SiO
2
(TOF = 110 h [20]) and ferrocenylamine-derived
palladacycles (TOF = 140 h [21]). The optimum condition for Heck reaction (873 K, 100% H O)
was much severer than that for Suzuki reactions where USY was prepared by streaming with 18% H
−1
2
2
O
at 773 K [15]. Nevertheless, we found that that the optimum temperature for preparation of USY
zeolite agreed with that of the maximum amount of OHstrong was obtained as measured by acid
characterizations. The fact suggested that the strong Brønsted acid sites generated as the result of

Molecules 2011, 16
44
steaming of NH -Y and subsequent treatment with ammonium nitrate solution acted as the anchor to
4
keep the dispersed form during reactions [22].
Figure 5. Turnover frequencies plotted as a function of the steaming temperature in the
reaction between bromobenzene and styrene. Catalyst, Pd/USY. 6%-H bubbling was
2
applied during reactions.
6
6
%-H bubbling
2
5
4
3
2
1
0
N atmosphere
2
6
00 700 800 900 1000 1100
Steaming Temp. / K
2
2
.6. Effect of Supports, H Bubbling and Solvents
Heck coupling reaction between bromobenzene and styrene was carried out over Pd/USY (steaming
temperature: 873 K) in various kinds of solvents. The TONs reached at 20 h are compared in Table 3.
A high TON of 630,000 was obtained in DMAc. TONs obtained in dimethylformamide (DMF) and
N-methylpyroridone (NMP) were 390,000 and 100,000, respectively, which were lower than that
obtained in DMAc. The reaction did not proceed in o-xylene as a solvent. The results point to the
importance of choosing DMAc as the solvent.
Table 3. Heck reactions between bromobenzene and styrene catalyzed by Pd/USY in
a
various kinds of solvents.
Br +
,
1
2
b
Conv. / Yield of 1 Yield of 2
Material
balance
99
Entry Condition
TON
%
98
80
18
0
/ %
96
62
16
0
/ %
1.4
0.9
0.2
0
1
2
3
4
DMAc
NMP
630,000
390,000
100,000
0
82
DMF
99
o-xylene
100
a
3
Reaction conditions: Ph-Br (120 mmol), styrene (180 mmol), catalyst (5 mg), CH COONa
−
1
(
144 mmol), solvent (120 mL), 413 K, 20 h. 6%-H bubbling (30 mL min ) was applied during
2
b
reactions; Conversion of bromobenzene.
The TONs of Pd loaded on different types of supports are compared in Table 4. DMAc was
2
employed as a solvent. Pd/USY (steaming temperature: 873 K) activated with 6%-H bubbling

Molecules 2011, 16
45
exhibited the highest activity (entry 3, TON = 630,000). The H
2
bubbling was effective in enhancing
the catalytic activity not only in Pd/USY, but also in Pd loaded on Al
carbon (entry 9, 10). In a similar way, the TON of Pd(OAc) dissolved in DMAc increased from
70,000 (entry 11, without H bubbling) to 420,000 (entry 12) with H bubbling, indicating that H
2 3
O (entry 7, 8) or activated
2
1
2
2
2
bubbling was also effective in a homogeneous catalyst. Hydrogenated or dehalogenated products were
hardly observed in every reaction. It has been reported that the hydrogenation activity decreased as the
size of the Pd cluster decreased [23]. Therefore it is assumed that the hydrogenation of styrene or
products was suppressed by the formation of Pd clusters on USY zeolites. The reaction was also
performed with Pd/USY under an atmosphere of 6% H for a comparison. That is to say, a 6%-H flow
2
2
was introduced at the upper end of the flask to keep the atmosphere in the flask at 6% H . The reaction
2
2
over the 6%-H atmosphere was also effective at enhancing the activity of the Pd/USY (entry 2,
TON = 600,000). In order to obtain insight into the role of hydrogen, hydrogen bubbling was stopped
when the temperature of the solution reached 413 K. The obtained TON after 20 h from the begging of
the reaction was 440,000 (entry 4), which was lower than that obtained by continuous hydrogen
bubbling (entry 3, TON = 630,000). The observation was similar to that of Suzuki reaction. The fact
suggested that the dissolved hydrogen had significant effect not only in the formation of active Pd
species but also in the mechanism of Heck reaction. The role of hydrogen was not clearly understood
at this stage, one hypothesis was that the dissolved hydrogen promoted the reductive elimination of
products through the formation of Pd-H adducts [24].
Table 4. Heck reactions between bromobenzene and styrene catalyzed by Pd supported on
a
different kinds of supports.
Br +
,
1
2
b
Conv. / Yield of 1 Yield of 2 Material
Entry
Catalyst
Pd/USY
Pd/USY
Condition
atmosphere
TON
b
%
/ %
/ %
balance
1
2
N
2
47
37
0.5
90
300,000
600,000
6
%-H2
96
84
1.1
89
atmosphere
3
4
5
6
7
8
9
Pd/USY
Pd/USY
Pd/H-Y
Pd/H-Y
6%-H bubbling
98
70
9
96
60
6
1.4
0.8
0.1
0.2
0.6
0.8
0.1
0.6
0.2
0.7
99
92
98
99
95
90
96
90
93
92
630,000
440,000
55,000
2
c
6%-H bubb., N2
2
N
2
atmosphere
6%-H bubbling
atmosphere
6%-H bubbling
atmosphere
6%-H bubbling
2
11
52
71
17
54
27
67
10
46
61
13
44
20
58
70,000
Pd/Al
Pd/Al
2
O
O
3
N
2
330,000
460,000
110,000
430,000
170,000
420,000
2
3
2
Pd/AC
Pd/AC
N
2
1
1
0
1
2
2
Pd(OAc)
Pd(OAc)
2
2
N
2
atmosphere
1
a
6%-H bubbling
2
b
c
2
Conditions: See the caption of Table 3; Conversion of bromobenzene; 6%-H bubbling was
stopped when the temperature reached 413 K.

Molecules 2011, 16
.7. Heck Reactions Using Various Kinds of Substrates
46
2
Table 5 gives the results of reactions carried out in the presence of Pd/USY using various of
bromobenzene, -pyridine, -thiophene, -quinoline, -naphthalene derivatives under 6%-H bubbling. The
USY support was prepared by steaming of NH -Y at 873 K with 100% H O. High TONs of up to
2
4
2
630,000 were obtained with various bromobenzene and bromonaphthalene derivatives, where the cross-
coupling reaction proceeded almost quantitatively, except for several substrates. We found that the
Pd/USY was also applicable to the heteroaromatic compounds with TON = 1,400–5,900 (entry 13–16).
We tried to repeatedly use the Pd/USY in Heck reactions after separating the catalyst from reaction
mixture by filtration. However, the use of the recycled Pd/USY was difficult; probably due to the
aggregation of Pd. Despite this, it is notable that the Pd/USY is useful in Heck reactions considering that
high TOF and TON values were obtained with different substrates.
a
Table 5. Heck reactions between bromobenzene derivatives and styrene catalyzed by Pd/USY.
Ar
Ar Br
+
,
Ar
3
4
Pd /
mol%
Time Conv. Yield of Yield of 4 Material
Entry
Ar-Br
TON
/ h
20
24
24
27
24
20
20
20
20
20
20
20
2
/ %
98
96
98
91
99
42
99
80
57
55
93
87
80
53
75
99
3 / %
96
91
85
88
97
34
79
76
56
53
88
84
73
26
61
87
/ %
1.4
0.8
0.8
0.8
1.5
0.6
0.8
1.8
0.6
0.6
1.1
1.2
1.3
0.4
0.8
1.4
balance
99
−
4
4
4
4
4
4
4
4
4
4
4
4
2
2
2
2
1
2
3
4
5
6
7
8
9
Bromobenzene
1.6×10
630,000
410,000
420,000
240,000
520,000
190,000
220,000
170,000
140,000
120,000
590,000
550,000
5,900
−
−
−
−
−
−
−
−
−
−
−
−
−
−
−
4-Bromoacetophenone 2.3×10
3-Bromoacetophenone 2.3×10
4-Bromobenzaldehyde 3.7×10
96
88
98
4-Bromotoluene
3-Bromotoluene
2-Bromotoluene
4-bromobenzonitrile
4-bromoanisole
1.9×10
2.3×10
4.6×10
4.7×10
4.7×10
100
92
81
97
99
1
1
1
1
1
1
1
0
1
2
3
4
5
6
4-Bromonitrobenzene 4.7×10
99
1-Bromonaphthalene
2-Bromonaphthalene
3-Bromothiophene
2-Bromothiophene
3-Bromopyridine
1.6×10
1.6×10
1.6×10
1.6×10
2.5×10
6.2×10
97
99
95
2
73
2,100
20
1.5
86
2,400
1-Bromoquinoline
88
1,400
a
Conditions: See the caption of Table 3. DMAc was used as solvent. The scales of all the reagents were
changed, while the catalyst weight was fixed at 5 mg. Reaction was carried out under 6%-H bubbling
2
−
1
(
30 mL min ). The USY zeolite was prepared by steaming of NH -Y at 873 K with 100% H O.
4
2
Table 6 gives the results of reactions using derivatives of bromobenzene and tert-butyl acrylate.
Similarly to the case of the reactions using styrene, high TONs of up to 570,000 were obtained with
various substrates.

Molecules 2011, 16
47
Table 6. Heck reactions between bromobenzene derivatives and tert-butyl acrylate
a
catalyzed by Pd/USY.
O
O
O
Ar Br +
,
t-Bu
t-Bu
t-Bu
Ar
Ar
5
6
Pd
mol%
Time Conv Yield of Yield of 6 Material
Entry
Ar-Br
TON
/
/ h
24
24
30
24
3.5
./%
76
91
88
98
93
5 / %
66
/ %
0.1
0.1
0.1
0.1
0.2
balance
94
b
b
b
b
c
−4
−4
−4
−4
−3
1
2
3
4
4-Bromoacetophenone 1.9×10
4-Bromobenzaldehyde 1.6×10
4-Bromonitrobenzene 1.6×10
4-Bromobenzonitrile 1.6×10
1-Bromonaphthalene 6.2×10
410,000
570,000
530,000
570,000
15,000
87
98
82
95
92
95
5
87
82
b
a
The USY zeolite was prepared by steaming of NH
4 2
-Y at 873 K with 100% H O; Reaction
conditions: Ar-Br (120 mmol), tert-butyl acrylate (180 mmol), catalyst (5 mg), CH COONa
3
c
(
144 mmol), DMAc (120 mL), 413 K; Reaction condition: 1-Bromonaphthalene (3 mmol), tert-
butyl acrylate (4.5 mmol), catalyst (5 mg), CH COONa (3.6 mmol), DMAc (3 mL), 413 K.
3
2
.8. Reactions Using 4-Chloroacetophenone
Finally, Heck coupling reactions between chlorobenzene derivatives and styrene was carried out
over Pd/USY in N-methylpyrrolidone (NMP) as a solvent (Table 7).
a
Table 7. Heck reactions between chlorobenzene derivatives and styrene catalyzed by Pd/USY.
R
R
R
Cl
+
,
7
8
Conv. of
Ar-Cl /%
44
Yield of Yield of 8 Material
Entry
Ar-Cl
Condition
TON
17,000
49,000
c
d
7 / %
/ %
balance
b
1
4-chloroacetophenone
4-chloroacetophenone
N
2
atmosphere
33
0.3
90
6
%-H2
b
2
98
0.5
65
-
97
0.5
48
0.7
0
100
100
83
-
bubbling
%-H2
bubbling
6
3
4-chloroacetophenone
4-chlorobenzonitrile
4-chloronitrobenzene
chlorobenzene
680
35,000
2,600
580
6
%-H2
b
4
0.4
0.1
0
bubbling
6
%-H2
b
5
10
bubbling
6
%-H2
b
6
-
2.1
0.4
-
bubbling
6
%-H2
b
7
4-chloroanisole
-
0
-
100
bubbling
a
Reaction conditions: chlorobenzene derivatives (10 mmol), styrene (15 mmol), catalyst (5 mg),
CH COONa (12 mmol), NMP (10 mL), 413 K, 20 h. The USY zeolite was prepared by steaming of
3
b
c,d
NH -Y at 873 K with 100% H O; TBAB (1 mmol) was added; Conversion and yield of the
4
2
reactions 5–7 were not displayed because much portion of the substrates (Ar-Cl) were evaporated
during reactions.

Molecules 2011, 16
48
The reactions were carried out with or without addition of tert-tetrabutylammonium bromide
(TBAB). Without the addition of TBAB, the activity was almost negligible (entry 3). However,
significant improvement in activity was achieved by the addition of 1 mmol of TBAB in the reaction
using 4-chloroacetophenone as the substrate (entry 2) as reported earlier [25,26]. The positive effect of
H bubbling was confirmed again when the comparison was made between the reactions of entry 1
2
(TON = 17,000) and entry 2 (TON = 49,000). Although high conversions and TONs were obtained with
the use of electron-withdrawing substituents such as 4-chloroacetophenone and -benzonitrile (entry 2, 4),
the activity became lower when chlorobenzene and 4-chloroanisole were used for reactions (entry 6, 7).
3
. Conclusions
We have found that the Heck coupling activity of Pd/USY as well as that of Pd loaded on other
supports was enhanced by the application of H bubbling during reaction. Moreover, preparation
2
conditions of the USY support influenced the catalytic performance of Pd significantly. High TON
values-up to 630,000-were obtained after optimization of the steam-treatment conditions. The Pd/USY
catalysts were applicable in various Heck reactions, including those using bromobenzenes, -
naphthalenes, -heteroaromatic and chlorobenzene derivatives. Characterization of the acid properties of
the USY zeolites revealed that the OHstrong group generated as a result of steam treatment was
important in evolution of high catalytic activity in Pd/USY.
4. Experimental
4
.1. Preparation of USY Zeolites
Na-Y zeolite (320NAA) supplied by the Tosoh Corp. (Tokyo, Japan) was employed as the starting
material for the preparation of USYs. The Na-Y was ion-exchanged three times with a solution of
−
1
NH
with H
placed in a quartz tube and treated with H
4
NO
3
(0.5 mol L ) at 353 K to give NH
4
-Y. USY was prepared from NH
4
-Y zeolites by treatment
-Y (5 g) was
2
O vapor diluted with an N
2
flow. Concentration of H
2
O vapor was 100 vol%. NH
4
2
O vapor for 1 h at 573–1073 K. The flow rate was
−
1
−1
5
0 mL min . The obtained USY was ion-exchanged three times with NH NO (0.5 mol L ) at 353 K
4 3
+
+
to give NH
4
-USY. The completion of ion exchange of H with NH
4
was confirmed by TPD of NH
3
,
+
+
showing that 93% of H had been replaced with NH
N stream to partially remove NH .
4 4
. The NH -USY was then heated to 573 K in an
2
3
4
.2. Loading of Pd on USY Zeolites
Pd was then introduced to the calcined USY by an ion-exchange method using Pd(NH
3
)
4
Cl
solution (3.8 × 10 mol L ; Aldrich, St. Louis, MO, USA) at room temperature (r.t.). That is to say,
USY zeolite (4 g) was added to an aqueous solution of Pd(NH )Cl (400 mL). The suspension was
stirred for 4 h, followed by washing with deionized H O. The obtained solid was dried overnight in an
2
−4
–1
4
2
2
oven at 323 K. Pd was loaded on H-Y with an ion-exchange method using Pd(NH )Cl , in a similar
4
2
way for the preparation of Pd/USY. The H-Y was prepared through calcination of NH
4
-Y at 773 K.
The Pd loading of all samples was 0.4 wt %, as measured by ICP analysis. Pd (0.4 wt %) loaded on

Molecules 2011, 16
49
Al
2
O
3
(JRC-ALO-3, Catalysis Society of Japan) and activated carbon (Wako Chemicals Ltd., Osaka,
Cl solution.
Japan) were prepared by the impregnation method using a Pd(NH
3
)
4
2
4.3. Catalytic Reactions
Under typical conditions, bromobenzene (120 mmol; Tokyo Kasei Chemicals Ltd., Japan), styrene
180 mmol; Tokyo Kasei Chemicals Ltd., Japan), CH COONa (144 mmol; Wako Chemicals Ltd.,
(
3
Osaka, Japan), DMAc (solvent, 120 mL, Wako Chemicals Ltd., Osaka, Japan), and 0.4 wt% Pd/USY
−7
catalyst (5 mg, Pd: 1.9 × 10 mol) were used for the Heck reactions. A 6% H /94% Ar flow at a rate
2
−1
of 30 mL min was introduced into the reactant solution using a glass capillary tube during reactions.
A three-necked flask (300 mL) was placed in an oil bath, preheated to the required temperature, and
subjected to vigorous stirring. The reaction was performed at 413 K. Heck reactions using
chlorobenzene derivatives were performed under similar conditions, except for the use of NMP as the
solvent. After reaction, the reaction mixture was cooled to r.t. and the solution was analyzed using a
Shimadzu 2010 Gas Chromatograph equipped with an InertCap 5 (30 m) capillary column (Shimadzu
Corp., Kyoto, Japan). Tridecane was used as the internal standard.
4
.4. Characterization of the Acid Properties of USY Zeolites
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 was allowed to flow as the carrier
−
1
(
flow rate: 110 mL min ). An IR beam was transmitted to a self-compressed disk (5 mg, and 10 mm
in diameter). After evacuation of the sample at 773 K, IR spectra were recorded before NH adsorption
at 10 K intervals from 373 to 773 K, while the temperature was increased 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
3 kPa, and then gas-phase NH was evacuated for 30 min. IR spectra were again measured at 10 K
3
−1
(
3
1
3
intervals from 373 to 773 K, while the temperature was raised (A(T), recorded). The difference
spectrum, i.e., A(T) − N(T), was calculated at each temperature, and changes in IR absorptions were
+
observed to identify the absorptions of NH
4
and NH
3
adsorbed on the surface. Changes in OH 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. In the present study,
the area of IR absorption was quantified in the absorption range. The IR-TPD was compared with the
3 3
MS-measured TPD of NH (m/e, 16) to identify the nature of the adsorption site for the desorbed NH .
Detailed data analysis was described elsewhere [15].
4.5. Pd K-edge EXAFS Measurements
Pd K-edge EXAFS data were obtained from synchrotron radiation experiments performed at the
BL01B1 station, with the approval of the Japan Synchrotron Radiation Research Institute (JASRI)
Proposal No. 2010A1072). Pd K-edge XAFS data were collected in the quick mode; a Si(111)
monochromator 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 sample position. For the Pd K-edge XAFS measurements, 6% H2
(
(

Molecules 2011, 16
50
was bubbled through a mixture of Pd/USY in DMAc, and the treated Pd/USY was transferred to a plastic
cell at room temperature without contact with air. The X-ray path length of the plastic cell was 2 cm.
Acknowledgements
This research was partially supported by the Ministry of Education, Science, Sports and Culture,
Grant-in-Aid for Scientific Research (C), 21560801, 2009-2011.
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x
materials synthesized by sol-gel coprecipitation as
2
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2010 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/3.0/).
(