High catalytic activity of palladium(II)-exchanged mesoporous sodalite and NaA zeolite for bulky aryl coupling reactions: Reusability under aerobic conditions

Full text of DOI:10.1002/anie.200806334

Angewandte
Chemie
DOI: 10.1002/anie.200806334
Heterogeneous Catalysis
High Catalytic Activity of Palladium(II)-Exchanged Mesoporous
Sodalite and NaA Zeolite for Bulky Aryl Coupling Reactions:
Reusability under Aerobic Conditions**
Minkee Choi, Dong-Hwan Lee, Kyungsu Na, Byung-Woo Yu, and Ryong Ryoo*
Palladium-catalyzed cross-coupling reactions, such as Suzuki,
Heck, and Sonogashira reactions, are very important as a
versatile route to aryl compounds that are highly useful as
pharmaceuticals and fine chemicals.[1] Various Pd complexes
have been used as homogeneous catalysts, but the limited
reusability of expensive Pd has been a serious problem in
industrial applications.[2,3] In this regard, heterogeneous Pd
catalysts are a promising option. Such heterogeneous cata-
lysts can be prepared by grafting ligated Pd complexes onto
polymer resins and metal oxides.[4] Alternatively, ligand-free
catalysts can be prepared by supporting Pd0 nanoparticles or
Pd2+ ions on porous solid materials, such as carbon,[5]
zeolites,[6–9] mesoporous silica,[9,10] and metal oxides.[11–13]
The ligand-free system is preferable in heterogeneous catal-
ysis, owing to economic preparation and high stability in
air.[14–18] However, the use of ligand-free catalysts is often
limited by Pd leaching, and also by changes in the chemical
state of the Pd species.[1,19]
The limited success of ligand-free, heterogeneous Pd
catalysts is attributed to the unique reaction mechanism
involving a Pd0 state. There have been numerous works
indicating that the catalytic active state is a Pd0 species in
coupling reactions.[6,9,14–18] In cases where PdII is used, in situ
reduction to Pd0 is known to occur during the reaction.
De Vries and Reetz provided the evidence that monomeric or
dimeric Pd0 species, being in equilibrium with nanoparticulate
Pd0, is the catalytically active species.[14,15] Similar results were
also reported by Schmidt at around the same time.[17,18]
Notably, it was reported that the reduced Pd0 species were
more easily leached into the reaction media than the PdII
species.[20] The reversible dissolution and redeposition of Pd0
can result in the agglomeration of Pd0 species into large Pd
nanoparticles or Pd black, causing a loss of metal dispersion
and surface area.[14,15] Thus, a heterogeneous catalyst, free of
Pd leaching and agglomeration, is highly desirable for
achieving high activity and catalyst reusability.
Herein, we report that Pd2+-exchanged mesoporous
sodalite and NaA zeolite are suitable as heterogeneous
catalysts, which meet the aforementioned requirements. We
chose the highly mesoporous sodalite and NaA zeolites as a
support for Pd because the mesoporous structure (pore
diameter> 5 nm) can allow much-enhanced diffusion of
bulky aryl substrates as compared with solely microporous
zeolites (< 0.72 nm). Most importantly, the catalysts were
reusable without Pd leaching and agglomeration, as long as
the reactions were carried out in air.
The mesoporous sodalite and NaA zeolite were synthe-
sized by the addition of organosilane surfactant into the
conventional synthesis compositions of sodalite and NaA
zeolite (see the Supporting Information).[21–23] The resultant
sodalite and NaA zeolite exhibited framework compositions
of Si/Al = 1.0 and Na+/Al = 1.0, which are consistent with
those of their conventional counterparts. XRD patterns
(Figure 1a) correspond well to the highly crystalline sodalite
and NaA. N2 adsorption isotherms (Figure 1b) revealed the
presence of highly mesoporous structures with a narrow
mesopore distribution centered at 8 and 10 nm for sodalite
and NaA, respectively. The highly mesoporous morphologies
were also confirmed by scanning electron microscopy (SEM;
see Figure S1 in the Supporting Information). The materials
exhibited high mesopore surface areas (187 m2 gÀ1 for soda-
[*] M. Choi, D.-H. Lee, K. Na, B.-W. Yu, Prof. R. Ryoo
Center for Functional Nanomaterials, Department of Chemistry and
Graduate School of Nanoscience and Nanotechnology (WCU)
Korea Advanced Institute of Science and Technology
Daejeon 305-701 (Korea)
Fax: (+82)42-350-8130
E-mail: rryoo@kaist.ac.kr
[**] This work was supported by National Honor Scientist Program of
the Ministry of Education, Science and Technology in Korea. The
authors are grateful for XAFS measurement facilities at Pohang
Light Source.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806334.
Figure 1. a) XRD patterns and b) N2 adsorption–desorption isotherms
of mesoporous sodalite and NaA zeolite.
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lite, 155 m2 gÀ1 for NaA) and mesopore volumes (0.42 and
Pd2+0.02Na+0.96Si1.0Al1.0O4.0). To investigate the effect of pore
structure on catalytic properties, conventional NaX zeolite,
with a solely microporous structure, was also ion-exchanged
0.38 mLgÀ1). The t-plot analyses revealed solely mesoporous
structures in these materials, with no appreciable micro-
porosities. Notably, even N2 (0.41 nm in size) cannot enter the
narrow pore apertures of sodalite (0.22 nm in diameter) and
NaA (0.40 nm) zeolite.
in
the
same
way
(product
composition:
Pd2+0.02Na+0.96Si1.2Al1.0O4.4). The catalytic properties of zeo-
À
lites in the various C C coupling reactions are summarized in
Owing to the Al-rich compositions (Si/Al = 1.0, Na+/Al =
1.0), the mesoporous sodalite and NaA zeolite provided a
large number of ion-exchange sites. Various cations, including
as alkali (e.g., Li+ and K+), alkaline earth (e.g., Mg2+, Ca2+,
and Sr2+), and transition metal ions (e.g., Pd2+, Ru3+, Pt2+ and
La3+), can penetrate the 6-membered pore aperture (0.22 nm
diameter) of the regular sodalite cage.[24,25] However, Cs+ is
too large to enter the aperture, leading to a negligible
exchange in conventional sodalite (Cs+/Al < 0.01).[25] Con-
versely, the mesoporous sodalite exhibited a considerable ion-
exchange capacity for Cs+ (Cs+/Al = 0.17). The result indi-
cated that as much as 17% of the total ion-exchange sites are
accessible to bulky Cs+ ions at the mesopore walls of the
sodalite. A similar conclusion might be drawn for the
mesoporous NaA zeolite.
Table 1. All of the reactions were carried out in air.
The zeolite catalysts all gave high catalytic conversions in
reactions involving small aryl substrates (Table 1, entries 1–3
and 8). The reagents and products in these reactions are
sufficiently small so that the reactions can take place even
inside the supercages of NaX zeolite (diameterꢀ 0.73 nm).
With bulky substrates (other entries in Table 1), however, the
mesoporous catalysts exhibited much higher catalytic activ-
ities than NaX zeolite. The results indicated that reagents and/
or products in these reactions were larger than the aperture of
the NaX supercage, and hence the reactions could only occur
at the external surface of NaX. Notably, the NaX zeolite
supercage structure has one of the largest pore apertures
among zeolite materials, and hence this zeolite has been most
frequently investigated as a support for Pd catalysts.[6–9] The
present results indicate that the aperture of NaX is still too
small to host coupling reactions that produce such bulky aryl
compounds. As a result of facile molecular diffusion to the
catalytically active sites, the aforementioned mesoporous
catalysts exhibited high catalytic activities even for aryl
bromides and chlorides in the Heck reaction, whereas the
ordinary NaX zeolite only exhibited considerable activity for
couplings of aryl iodides. Furthermore, the mesoporous
zeolites were highly active even without the use of an external
Owing to the highly mesoporous structure and the
significant ion-exchange capacity at the mesopore walls, the
À
zeolites can be modified into highly active catalysts for C C
coupling reactions, including Suzuki, Heck, and Sonogashira
reactions. Pd2+-exchanged mesoporous sodalite and NaA
zeolite were prepared by ion exchange in a 0.01m aqueous
solution of Pd(NO3)2·xH2O, with the Pd2+ loading corre-
sponding to 2% of total Al sites. The Al-rich zeolites
exhibited an extremely high ion-selectivity for Pd2+; more
than 99% of the loaded Pd2+ was exchanged onto the
À
base cocatalyst (Table 1, entries 1 and 3). Conventionally, C
C coupling reactions were carried out in the presence of base
frameworks
(product
composition:
Table 1: Catalytic properties of Pd2+-exchanged zeolites in Suzuki (entries 1–4), Heck (entries 5–7), and Sonogashira (entries 8,9) coupling reactions
in air. Unless stated otherwise, reactions were carried out without external base cocatalysts.
Entry
Reaction[a]
Pd cat. [mol%] Temp. [8C] Time [h]
Conversion [%][b]
Mesoporous sodalite Mesoporous NaA NaX
1[c]
2[c]
3
1.0
0.1
1.0
80
100
80
12
24
12
96
84
95
95
89
91
88
45
84
4
1.0
80
12
83/4[d]
85/6[d]
27/22[d]
5
6
1.0
1.0
100
120
3
90/7[e]
86/6[e]
92/8[e]
89/6[e]
87/8[e]
23/3[e]
12
7
3.0
1.0
1.0
120
80
24
12
15
41/4[e]
94
38/3[e]
91
5/–[e]
83
8[f]
9[f]
80
84
86
16
[a] Reaction conditions, unless otherwise stated: aryl halide (2.0 mmol), another substrate (2.4 mmol), DMF/H2O (3 mL, 1:1 v/v), and Pd catalyst
(approx. 0.1–3.0 mol%), Sonogashira reaction was carried out in Cu–free conditions;[26] [b] Yield was determined by GC based on the amount of aryl
halide, using n-dodecane as an internal standard; [c] with Na2CO3 (1.0 mmol); [d] heterocoupling/homocoupling selectivity (homocoupling product is
biphenyl); [e] trans/cis selectivity; [f] DMF/H2O (3 mL, 4:1, v/v).
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cocatalysts, such as Na2CO3, K2CO3, and K3PO4, to increase
catalytic activity and neutralize acidic byproducts (HCl, HBr
or HI).[1] In the absence of such bases, the homogeneous
Suzuki reaction mediated by Pd(NO3)2·xH2O resulted in a
catalytic conversion no greater than 7%. The marked differ-
ence in the catalytic activities of Pd(NO3)2·xH2O and the
Pd2+-exchanged zeolites indicates that, in the case of the
zeolite catalysts, the catalytic reaction occurred due to the
intrinsic basicity of Al-rich zeolite frameworks.[7,27]
Based on these results, we propose that the catalytic
reaction is carried out by a molecular Pd0 species that is
generated in situ under the reaction conditions. The Pd0
species is immediately re-oxidized into PdII by O2, before
agglomeration to Pd0 nanoparticles can take place
(Scheme 1). According to this mechanism, O2 in the reaction
In the absence of external base, the mesoporous zeolites
exhibited high reusability under the reaction conditions with
an aryl halide/Pd molar ratio of 100:1. The initial activity was
completely maintained during five recycling experiments (see
the Supporting Information, Figure S2), and no leaching of Pd
was detected by inductively coupled plasma (ICP) emission
spectroscopy. With higher aryl halide/Pd molar ratios, how-
ever, excessive generation of acid byproducts (HCl, HBr and
HI) caused the destruction of zeolite frameworks, leading to a
significant loss of catalytic activity and reusability. The
destruction of zeolite frameworks could be prevented by
addition of an external base. For example, the mesoporous
zeolites were highly active even at an aryl halide/Pd molar
ratio of 1000:1, when Na2CO3 was added as an external base
(Table 1, entry 2). The catalyst also completely maintained
catalytic activity during five recycling experiments.
The presence of O2 in the reaction media played a crucial
role in catalyst reusability. When the Suzuki reaction (Table 1,
entry 3) was carried out under an N2 atmosphere instead of
air, catalytic conversion gradually decreased to 52% after the
5th cycle (91% in the first cycle).To investigate the role of O2,
the chemical state of Pd in the mesoporous sodalite was
analyzed by X-ray absorption near-edge structure (XANES)
and extended X-ray absorption fine structure spectroscopy
(EXAFS) studies, before and after the Suzuki coupling
(Table 1, entry 3) in N2 and air. The PdII zeolite showed
exactly the same XANES pattern before and after the Suzuki
reaction in air (see the Supporting Information, Figure S3),
which was similar to that of Pd(NO3)2·xH2O. According to the
EXAFS analysis, no peaks corresponding to the nearest Pd–
Pd distance in the metallic state (approximately 2.8 ꢀ) were
detected in the radial distribution (see the Supporting
Information, Figure S3). This result indicated that the Pd2+
ions remained highly stabilized against metal reduction when
exchanged onto the zeolitic framework, without agglomer-
ation to Pd0 nanoparticles. This observation is remarkable,
whereas Pd0 nanoparticles or Pd black (bigger agglomerates)
were often detected after reactions in both homogeneous and
heterogeneous systems.[6,9,14–18] Contrary to the result in air,
the reaction in N2 produced Pd0 nanoparticles. The XANES
patterns indicated a change from PdII to Pd0 states during the
reaction, and a distinct peak corresponding to the Pd–Pd
distance occurred at 0.28 nm in the radial distribution
function. The results indicated that O2 in air can effectively
suppress the reduction of PdII and subsequent agglomeration
into Pd0 nanoparticles. Bubbling of O2 in the reaction mixture
also stabilized the PdII state during the Suzuki reaction
(Table 1, entry 3), although the catalytic conversion (34%)
decreased compared to the reaction in air (95%).
Scheme 1. Proposed reaction mechanism for Pd2+-exchanged zeolite
catalysts under aerobic atmosphere.
medium prevents the formation of less reactive Pd0 nano-
particles by shifting the equilibrium toward PdII. The equi-
librium seems to be highly shifted toward the PdII state as a
result of the strong electrostatic stabilization of Pd2+ ions by
the negatively charged zeolite surface. However, an exces-
sively high concentration of O2 (or O2 pressure in the gas
phase) can suppress the in situ generation of the molecular
Pd0 species, causing decreased activity.
In this mechanistic consideration, the solubility of the
in situ-generated molecular Pd0 species is a very important
issue. Zeolites have long been used by Djakovitch and
Koehler for aryl-coupling reactions.[6] In many cases, although
Pd is initially supported on the zeolite, it is not clear whether
the Pd was leached to solution so that the reactions were
catalyzed homogeneously by the dissolved Pd species.[6–9]
Traces of soluble Pd were detected in a sufficient concen-
tration to afford catalytic activity. Conversely, the present Pd
catalyst seemed to catalyze the coupling reactions in a truly
heterogeneous manner. This conclusion was based on the
following observations. 1) During the reaction, we removed
the Pd-exchanged zeolite by microfiltration and allowed the
reaction to continue. The conversion stopped increasing
immediately (see the Supporting Information, Figure S4). If
a trace amount of soluble Pd species had been the active
species, the catalytic conversion should have increased
further, even after the removal of the zeolite catalyst. 2) We
tested the coupling reactions in the presence of poly(4-
vinylpyridine),[28] which is well known as a solid poison to trap
homogeneous Pd species in the solution phase through
chelation. However, no significant change in catalytic activity
was detected (see the Supporting Information, Figure S4).
3) Mesoporous zeolites exhibited much higher activity than
the solely microporous NaX zeolite catalyst, especially for
reactions involving bulky molecular species. If the reactions
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Communications
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would be very difficult to explain.
In summary, we have demonstrated that Pd2+-exchanged
mesoporous sodalite and NaA zeolite can be used as a highly
À
efficient heterogeneous catalyst for various C C coupling
reactions of bulky aryl compounds. Notably, Pd2+ ions
exchanged on the zeolite frameworks were highly stabilized
against metal agglomeration, maintaining the high catalytic
activities during a number of recycles under aerobic environ-
ments. It was proposed that the catalytic reactions were
carried out by a Pd0 species, which was generated in situ under
the reaction conditions. This species seemed to be immedi-
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formation of Pd0 nanoparticles. The present strategy, prevent-
ing the formation of large Pd nanoparticles using air or O2,
would provide a versatile and economic way to achieve high
catalytic activity and reusability, even for other heteroge-
neous Pd catalysts.
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Received: December 27, 2008
Revised: March 10, 2009
Published online: April 9, 2009
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Keywords: cross-coupling · heterogeneous catalysis ·
mesoporous materials · palladium · zeolites
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