Heck coupling in zeolitic microcapsular reactor: A test for encaged quasi-homogeneous catalysis

Article abstract of DOI:10.1016/j.jcat.2006.11.028

A new strategy of heterogenizing homogeneous catalysts by encaging the quasi-homogeneous catalytic reaction into a zeolitic microcapsular reactor has been proposed. As an example toward achieving such goal, a highly stable, nanopalladium-entrapped zeoliti

Full text of DOI:10.1016/j.jcat.2006.11.028

Journal of Catalysis 246 (2007) 215–222
www.elsevier.com/locate/jcat
Heck coupling in zeolitic microcapsular reactor:
A test for encaged quasi-homogeneous catalysis
∗
Nan Ren, You-Hao Yang, Ya-Hong Zhang, Quan-Rui Wang, Yi Tang
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433,
People’s Republic of China
Received 25 August 2006; revised 23 November 2006; accepted 26 November 2006
Abstract
A new strategy of heterogenizing homogeneous catalysts by encaging the quasi-homogeneous catalytic reaction into a zeolitic microcapsular
reactor has been proposed. As an example toward achieving such goal, a highly stable, nanopalladium-entrapped zeolitic microcapsular reactor
(Pd@S1) was successfully fabricated and evaluated for Heck coupling reaction. The Pd@S1 not only exhibits excellent reactivity at low Pd
adoption (Pd/PhI = 0.0025) during the reaction, but also can be reused for more than 10 runs with negligible loss of activity, in contrast to rapid
decay of activity of conventional Pd/C catalysts. Such reusability can be explained by the encapsulation of the “quasi-homogeneous” reaction
mechanism of Heck coupling within the micrometer-sized hollow cavities of Pd@S1, which successfully avoids the Pd leaching during the
reaction. The strategy may be helpful in the design of reusable, highly efficient catalysts with advantages deriving from both homogeneous and
heterogeneous catalysts.
©
2006 Elsevier Inc. All rights reserved.
Keywords: Zeolitic microcapsular reactor; Heck coupling; Catalysis; Palladium; Leaching; Recycle
1
. Introduction
cles [12], and palladium complexes liganded with phosphine
13] or carbene [14]. Meanwhile, many groups are exploring
[
Homogeneous and heterogeneous catalysis are two main
various heterogeneous catalysts, such as ligand-protected palla-
dium nanoparticles [15,16], palladium on active carbon (Pd/C)
[17–19] or carbon nanotubes [20], palladium on porous inor-
ganic supports [21–26] or functionalized polymers [27,28], and
zeolite-immobilized palladium species [29–34]. Although some
have claimed that their catalysts can be recycled to some ex-
tent for the redeposition of Pd, the leaching of the palladium
species from the catalyst [6,8,9,28] is still considered the main
problem during its heterogenized process because of the quasi-
homogeneous mechanism of Heck coupling via a soluble inter-
mediate of the solvated Pd clusters or even palladium complex
types of industrial catalytic processes. The former has a high
catalytic efficiency but also has difficulty separating the used
catalyst from the product after the reaction. Adopting a het-
erogeneous catalyst could solve the problem of catalyst re-
covery and reuse, but in most cases, the catalyst’s low utility
and the complexity of the reaction mechanism become new
problems in its application. Consequently, achieving a combi-
nation of the advantages of both the homogeneous and hetero-
geneous processes is always the focus in catalysis [1,2]. The
Heck coupling reaction is considered important in such explo-
rations, not only for its significance in C–C coupling reaction
between aryl halides and olefins [3–5], but also for its charac-
teristic “quasi-homogeneous” reaction mechanism [6–10]. Up
to now, many types of homogeneous catalysts have been used
for this reaction, including palladium acetate [11], palladacy-
[Pd(II)(ArX)(solvent)(base)] [6–10], especially in the systems
using highly active aryl iodides as substrates and highly effi-
cient aprotic polar reagents (e.g., N-methylpyrrolidone [NMP]
or N,N-dimethylacetamide) as solvents. Recently, de Vries and
co-workers [9,35–38] developed a new strategy to enhance the
efficiency of Heck coupling reaction by adopting the homeo-
pathic palladium as the catalyst. Because very little amount of
Pd (0.01–0.1 mol% of the amount of the substrate) is used in
*
Corresponding author. Fax: +86 21 65641740.
E-mail address: yitang@fudan.edu.cn (Y. Tang).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.11.028

216
N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
the reaction, further recycling of the catalyst is not necessary.
However, a longer reaction time seems to be needed to reach
the high product yield.
ed mesoporous silica spheres (MSSs), as described in our pre-
vious reports [52–55]. The MSS template was synthesized ac-
cording to the procedure reported by Unger et al. [56]. After
In another important route to solving the problem, some
deliberately designed reaction conditions and/or catalysts to
enhance the catalyst’s reusability, such as introducing an addi-
tional inorganic base [7], modifying the support with functional
groups (e.g., amino-, thiol-) [28,39–41], or performing the re-
action in an unusual solvent (e.g., molten tetralkylammonium
salts [42], polyethyleneglycol [43]), have been adopted. One
of the most interesting examples in this field is the commer-
cial catalyst [PdEnCat], in which the active palladium species
are protected by a matrix of cross-linked porous polyurea con-
taining heteroatom in its backbone [44–46]. However, the lit-
erature and our recent work found that its organic moiety
bears the poor thermal stability and tends to be dissolved in
the aprotic polar solvents [47,48]. Further studies on the re-
action progress of such catalyst have revealed that the palla-
dium actually leaches into the reaction solution to participate
the reaction and redeposits back onto the support after com-
pletion of the reaction [49–51]. In this work, a new strategy
was proposed to encage the entire quasi-homogeneous cat-
alytic process into an inorganic microcapsular reactor with a
zeolitic shell to realize the heterogenized homogeneous reac-
tion. The nanopalladium-entrapped zeolitic microcapsular reac-
tor (Pd@S1) was fabricated as the first successful achievement
of such strategy and was further evaluated as the catalyst for
Heck coupling reaction. Because of the uniform microporosity
and high thermal/chemical stability of the zeolitic shell, the in
situ formed active species [Pd(II)(ArI)(solvent)(base)] or sol-
vated Pd-clusters during the reaction can be trapped within the
micrometer-sized hollow core of zeolitic capsules to form an
unique encaged microenvironment for the homogeneous catal-
ysis. On the other hand, the whole catalyst apparently behaves
just like a heterogeneous catalyst, which can be recovered and
reused facilely.
surface NH modification with APS at room temperature [57]
to enhance the immobilization capability toward Pd species, the
2
MSSs were impregnated in 0.04 M ethanolic solution of palla-
dium(II) acetylacetonate under stirring at ambient temperature
for 30 min. Then the Pd(II) species loaded in the MSSs were
further transformed into Pd nanoparticles via in situ reduction
4
in a 0.01 M KBH aqueous solution and calcinations in air at
◦
600 C for 3 h.
To obtain the target Pd-trapped zeolitic microcapsular reac-
tor, the external surfaces of the foregoing Pd preloaded MSSs
were seeded with 60 nm silicalite-1 nanocrystals [58] via a
layer-by-layer procedure described previously [52–55] and then
treated through a two-step hydrothermal approach. Briefly,
0.1 g of nanozeolite seeded sample was hydrothermally treated
◦
in 15 mL of 1.5 wt% TPAOH aqueous solution at 100 C for
1
.0 h, and then continuously treated in 15 mL aqueous solution
with a molar composition of TPAOH:TEOS:H2O = 3:10:2000
◦
at 100 C for 8 h. The solid product was separated by centrifu-
gation and washed with distilled water three times. All of the
organic ingredients in the samples were removed by calcination
◦
in air at 550 C for 6 h. Finally, a reduction process was adopted
◦
in a continuous hydrogen flow at 100 C to obtain the metallic
Pd-encapsulated Pd@S1 catalyst. The blank zeolite microcap-
sules without the Pd encapsulation were fabricated according as
described previously [55] and were used as the reference sam-
ple.
2
.3. Catalyst characterization
The SEM and TEM images of the catalysts were obtained on
Philips XL30 and JEOL JEM-2010 instruments with accelerat-
ing voltages of 20 and 200 kV, respectively. The XRD patterns
were obtained on a Rigaku D/MAX-IIA diffractometer over a
◦ ◦
2
θ range of 5 –70 with CuKα radiation. The UV–vis spectra
2
. Experimental
were recorded on a Shimadzu UV-2450 UV–vis spectropho-
tometer. The XPS analysis was carried out with a Perkin-Elmer
PHI 5000C ESCA system using AlKα radiation (1486.6 eV)
at a power of 250 W. The pass energy was set at 93.9 eV, and
the binding energies were calibrated by using contaminant car-
bon at a BE of 284.6 eV. All samples were heavily grinded to
expose the Pd species before XPS measurement. The elemental
analyses were performed by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES).
2
.1. Materials
3
-amino-propyltriethoxysilane (APS), palladium(II) acetyl-
acetonate, dodecane, 4-iodotoluene, 2-iodothiophene, 5-iodo-
m-xylene, and benzothiazole (98%) were purchased from
Aldrich. The 5 wt% Pd/C was supplied from Shanghai July
Chemical Company. N-methyl-2-pyrrolidone (NMP), cyclo-
hexane, potassium borohydride (KBH4, 94 wt%), tetrapropyl-
ammonium hydroxide (TPAOH, 25 wt% in water), tetraethoxy-
orthosilane (TEOS), ethanol (EtOH), styrene, iodobenzene
and methyl acrylate were obtained from Shanghai Chemical
Reagent Company. All chemicals were used without further
purification.
2
.4. Adsorption test of benzothiazole
The adsorption of benzothiazole was conducted to prove the
protective effect of a zeolitic shell on Pd@S1. To do this, a 30-
mg sample of Pd@S1 was added to 30 mL of benzothiazole
−
5
2
.2. Catalyst preparation
solution with a concentration of 3.0 × 10 M in cyclohexane.
For comparison, the blank zeolite microcapsule without Pd en-
capsulation and a shell-crushed Pd@S1 (prepared by heavily
grinding the sample to expose the encapsulated Pd toward the
The Pd@S1 catalyst was synthesized according to the liquid-
phase transformation of nanozeolite-seeded, Pd-preincorporat-

N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
217
Fig. 1. SEM (a) and TEM (b) images of the prepared Pd@S1 catalysts. The inset in (b) is the TEM image at high magnification to depict the encapsulated Pd
nanoparticles.
adsorbate) were tested under the same conditions. The varia-
tion of benzothiazole concentration was detected by UV–vis
absorption at each time interval. The adsorption percentage was
calculated by (C − C )/C , where C and C refer to the con-
0
t
0
0
t
centration of the initial solution and that after adsorption for
t min, respectively.
2
.5. Typical procedure for the Heck reaction
Aryl halide (5 mmol), olefin (8 mmol), Et N (5 mmol, an-
3
hydrous), Pd catalyst (0.0125 or 0.025 mmol), and dodecane
1 mmol, as the internal standard) were sequentially introduced
(
into a 50 mL flask fitted with a reflux condenser and a septum.
Then the solvent (NMP, 30 mL) was added. The mixture was
◦
stirred at 120 C for 3–8 h. After the reaction was completed,
the used catalyst was recovered by filtration and washed with
NMP for reuse, and the filtrate was collected and analyzed by
FID-GC (using an HP5890-II gas chromatograph with a HP-5
capillary column), using dodecane as the internal standard, to
determine the product yields.
The kinetic data were obtained by sampling a small quan-
tity of the reaction mixture at 1, 1.5, 2, 2.5, and 3 h during the
first, second, and tenth reaction runs. After centrifugation, the
supernatant solution of the reaction mixture was collected and
analyzed by FID-GC as described above.
Fig. 2. XRD patterns of MSS template (red) and Pd@S1 (black) at low 2θ
range. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
capsulated palladium nanoparticles (ca. 11 nm). The total Pd
content in catalyst was measured as ca. 7.8 wt% by ICP-AES
analysis.
3
. Results
The transformation of amorphous mesoporous MSS tem-
plate into zeolitic shells can be proven by their XRD patterns
in the low diffraction range (Fig. 2). As expected, the character-
3
.1. Catalyst characterization
◦
istic mesoscopic diffraction peak at ca. 2 of MSS disappeared,
Fig. 1 presents the typical SEM and TEM images of the
illustrating the complete consumption of its silica resource dur-
ing the hydrothermal process. Fig. 3a shows the XRD pattern
of the Pd@S1 catalyst in the wide diffraction range, in which
the composition of the obtained material can be further verified.
The sets of the diffraction peaks attributed to metallic palladium
and silicalite-1 are both clearly shown.
Furthermore, the average size of the Pd nanoparticles in the
catalyst was also calculated from the width of Pd(111) peak
using Scherrer’s formula. The average size of the Pd particles
Pd@S1 samples prepared through the hydrothermal transfor-
mation of nanozeolite-seeded, guest-preincorporated MSSs.
The SEM images in Fig. 1a show the uniform spherical mor-
phology of about 1–1.2 µm and their dense shell composed of
closely packed nanozeolite, whereas the TEM image in Fig. 1b
clearly shows their hollow interior of about 1 µm surrounded
by a thin zeolitic shell of ca. 200 nm. The inset of Fig. 1b is a
TEM image with high magnification, clearly showing the en-

218
N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
Fig. 3. XRD patterns of the fresh Pd@S1 catalyst (a) and that after ten runs
of Heck coupling reaction (b). The characteristic peaks attributed to zeolite
Fig. 5. The Heck coupling activity and reusability of Pd@S1 (") and Pd/C (2).
Reaction conditions: 5 mmol of aryl halide, 8 mmol of olefin, 5 mmol of an-
(silicalite-1) are indicated by ∗.
hydrous triethylamine (Et N), 0.0125 mmol of Pd and 30 mL of NMP under
3
◦
stirring at 120 C for 3 h. The green dashed plot was obtained by doubling the
amount of Pd@S1 catalyst (i.e., 0.025 mmol Pd), which shows that a higher
yield of nearly 100% could be reached with higher catalyst amount. (For inter-
pretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
sule without incorporation of Pd (Fig. 4). This finding indicates
that the most of the Pd nanoparticles in our Pd@S1 samples
were effectively entrapped inside the intact zeolitic shell.
3
.2. Catalytic test
The Heck coupling reaction of iodobenzene with methyl
acrylate was chosen as the model reaction for testing the ef-
ficiency of our Pd@S1 catalyst. The reaction was carried out
in 30 mL of NMP at 120 C using triethylamine as a base.
The amounts of iodobenzene and methyl acrylate were 5 and
8 mmol, respectively, and the catalyst amount corresponded to
Fig. 4. The adsorption plot of benzothiazole on Pd@S1 (black, —),
shell-crushed Pd@S1 (red, - - -) and zeolite microcapsules without Pd encap-
sulation (green, ···). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
◦
0
.0125 mmol of Pd (i.e., Pd/PhI = 0.0025). For comparison, an
industrial Pd/C catalyst was tested under the same reaction con-
ditions. The results, shown in Fig. 5, clearly demonstrate that
both catalysts were effective in the first run of reaction. The re-
action was completed in 3 h and achieved a high yield (>90%)
of trans-methyl cinnamate.
in Pd@S1 is 10.4 nm, which is in accordance with the TEM
observations.
The encapsulation of palladium in the micrometer-sized hol-
low core of Pd@S1 and the intactness of the zeolitic shell can
be proven by a benzothiazole adsorption experiment. Because
of its large molecular size (preventing diffusion through the ze-
olitic shell) and selective interaction with Pd (via its nitrogen
or sulfur atoms), benzothiazole is a good probe for detecting
the Pd nanoparticles possibly left out of the zeolitic shell. To
clearly determine the protective effect of the zeolitic shell, a
Compared with conventional Pd/C catalyst, Pd@S1 demon-
strates superior reusability and stability. The Pd/C deactivates
quickly in two or three cycles, whereas the Pd@S1 can retain
its high yields of the target product even after up to 10 recy-
clings (Fig. 5). This property obviously can be attributed to the
protective effect of the zeolitic shell. Furthermore, according to
the TEM images shown in Fig. 6, the used Pd@S1 particles can
retain their dense zeolitic shell even after 10 recyclings, indi-
cating a promising stability of Pd@S1 in liquid reaction.
To further prove the antileaching effect of the zeolitic shell,
the leaching percentage of the Pd from the catalysts were de-
rived from the ICP-AES analysis of Pd concentration in the
“
shell-crushed Pd@S1” sample with exposed Pd nanoparticles
(
see Section 2) was also tested under the same conditions. It was
found that nearly 70% of the benzothiazole in the solution was
adsorbed by the “shell-crushed” sample in 1 h (Fig. 4, red dash
line), with <3% adsorption on Pd@S1 with an intact silicalite-1
shell even after deducting that on the blank zeolitic microcap-

N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
219
Fig. 6. TEM images of fresh Pd@S1 catalyst (a) and that after ten cycles of Heck coupling reaction (b).
reaction solutions after the catalysts were filtered off. It was
found that about 30% of the Pd in fresh Pd/C was leached
out after the first catalytic reaction, compared with only 2% in
Pd@S1 catalyst (probably due to the small amount of exposed
Pd in a few broken microcapsules). The leaching in Pd@S1 was
negligible in the subsequent reaction recycles, considering the
detection limit of the ICP-AES instrument. The low Pd leach-
ing in Pd@S1 not only reduces the cost of the catalytic process
by recycling the catalyst, but also guarantees a low amount of
PD residue in the product, which is a crucial aspect in improv-
ing the product quality in practical applications, especially in
pharmaceutical production.
Table 1 characterizes the applicability of Pd@S1 reactor in
Heck coupling reactions of various aryl iodides and olefins.
It is shown that all three aryl iodides can be coupled with
methyl acrylate or styrene to provide the respective product.
Even 4-iodotoluene bearing a deactivating electron-donating
group in the para-position of the phenyl ring can be used. The
electron-rich heterocycle 2-iodothiophene works equally well
Scheme 1. Schematic illustration for the encaged quasi-homogeneous Heck-
coupling in Pd@S1 catalyst.
4
. Discussion
(Table 1, entries 6 and 7), indicating that this nanopalladium-
microcapsulated catalyst displays as high a catalytic efficiency
as their homogeneous analogues. When methyl acrylate was
chosen as the olefin, the high yield (>90%) could be obtained
in relatively shorter periods (e.g., 3 h) (Table 1, entries 1, 3, 6).
However, when styrene is used as the substrate, prolonging the
reaction time and increasing the catalyst amount seem to be
necessary to achieve a yield of 70–80% (Table 1, entries 2, 4,
and 7). This can be attributed either to the relatively low reac-
tivity of styrene [3,59] or to the possible diffusion resistance of
the relative large reactants/products in the zeolitic shell. More-
over, when an aryl halide with the diameter larger than the
pore size in silicalite-1 shell (e.g., 5-iodo-m-xylene) is used as
the reactant (Table 1, entry 5), our catalysts have a very low
product yield (4.2%), because of the shape-selectivity of the
zeolitic shell. However, after the zeolitic shell in our catalyst
was destroyed by heavy grinding, the shell-crushed Pd@S1 dis-
played a very high product yield of 92%, close to that of Pd/C
with exposed Pd species (97%). These results further demon-
strate the completeness of the zeolitic shell in our Pd@S1 cata-
lyst.
The most interesting features of Pd@S1 that can be de-
rived from the foregoing results are its promising stability and
reusability due to the encapsulation of active Pd species in-
side the zeolitic shell. It has been reported that the metallic
Pd-catalyzed Heck coupling reaction is essentially a quasi-
homogeneous catalytic process occurring via either a palla-
dium complex intermediate [7] or solvated palladium clusters
[
6,9] in the system of aryl iodides and aprotic polar solvents.
Heck coupling on the Pd@S1 catalyst may follow a similar
mechanism but within the micrometer-sized hollow cavities,
as illustrated in Scheme 1. Adding the aryl halide to the reac-
tion solution triggers an oxidative addition to palladium atoms
in the Pd nanoparticles with the aid of NMP and Et3N, thus
forming a soluble active Pd(II) complex intermediate [7]. Then
the active intermediate reacts with olefin to complete the cat-
alytic cycle. Once all of the aryl halides are consumed, the
palladium complex finally returns to its metallic state and re-
deposits onto the surface of the support. Notably, unlike that
in the classical supported palladium catalyst, the formed ac-
tive [Pd(II)(ArI)(NMP)(Et N)] intermediate in our catalyst is
3

220
N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
Table 1
a
Heck coupling for various substrates
b
Entry
Aryl halide
Olefin
Time (h)
3
Product
Yield (%)
c
1
93 (99 )
c
2
3
4
7
4
8
70 (80 )
94
c
57 (75 )
d
5
6
4
4.2 (92 )
3
7
97
7
a
70
◦
Reaction conditions: aryl halide (5 mmol), olefin (8 mmol), anhydrous triethylamine (5 mmol), Pd (0.0125 mmol) and 30 mL of NMP under atmosphere, 120 C.
Yields were determined by GC analysis using dodecane as the internal standard and were obtained by the ratio of the product amount/the theoretical value
b
supposing the complete consumption of the aryl halides.
c
Yields obtained by doubling the Pd@S1 amount.
Yield obtained on shell-crushed Pd@S1 under the same condition.
d
encaged in the micrometer-sized hollow cavities of the zeolitic
microcapsular reactor, because its molecular diameter is rela-
tively larger than the pore size in the zeolitic shell (silicalite-1).
With this protective effect of the surrounding zeolitic shell,
a stable catalyst for Heck coupling could be constructed by
encaging the quasi-homogeneous process into an unique mi-
croenvironment in the hollow interior of Pd@S1.
To prove the existence of the encaged quasi-homogeneous
mechanism in Pd@S1, we conducted a two-step test originally
proposed by Biffis et al. [6]. All of the reaction conditions were
the same as those for the Heck coupling test, except omitting the
addition of olefins in the first step [60]. After 5 h, the catalyst
was filtered off and the second step in the reaction was contin-
ued by adding methyl acrylate to the filtrate. As expected, only
trace amounts of the product could be detected even after the
filtrate was reacted for another 5 h, indicating an absence of the
leached palladium species in the solution.
Fig. 7. XPS Pd 3d spectra of fresh Pd@S1 (black, —) and filtered Pd@S1 after
the first step reaction on Biffis test (red, - - -). All the samples were washed
with acetone, dried and heavily grinded to expose the Pd species before XPS
characterization. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
In another approach, an XPS experiment was carried out on
the filtered catalyst after the first step of the reaction (Fig. 7).
In contrast to the fresh catalyst, in which Pd is mainly 0 va-
lent (centered at 335 eV for Pd 3d5/2, in Fig. 7), the Pd in
the filtered catalyst is mainly +2 valent (centered at 336 eV
for Pd 3d5/2), indicating that a palladium(II) complex indeed
forms in the interior of the Pd@S1 during the reaction. The
dissolving-redepositing process of the encapsulated palladium
nanoparticles also can be verified by the Ostwald ripening of
some Pd nanoparticles after reaction [7]. As shown in Figs. 3b
and 6b, the 10-times-recycled Pd@S1 catalyst exhibits sharp-
ened Pd diffraction peaks and obvious growth of Pd nanoparti-
cles. The effect of the activity derived from the growth of the
encapsulated Pd particles is indicated by the kinetic behavior of
the Pd@S1 catalyst in the first, second, and tenth reaction runs
(Fig. 8). It can be clearly seen that the steady activity of the cat-
alyst changes little even after the tenth run (also see Fig. 5), and
that the kinetic curves of the second and tenth runs exhibit the
same trend, demonstrating that growth of such Pd species has a
slight affect on activity, probably due to the quasi-homogeneous

N. Ren et al. / Journal of Catalysis 246 (2007) 215–222
221
As a general conclusion, we should point out that the het-
erogenization of the homogeneous catalyst is an eternal theme
due to the combination of the efficiency of the homogeneous
catalyst and the durability of the heterogeneous catalyst. The
approach of encapsulating a homogeneous catalytic microen-
vironment in a porous inorganic shell could provide a novel
and beneficial route for the rational design of catalysts in this
domain, which may find new opportunity to achieve complex
industrial and fine-chemical synthesis through sequentially in-
tegrating the diverse active blocks.
Acknowledgments
Financial support was provided by the Natural Science
Foundation of China (grants 20233030, 20325313, 20303003,
2
0421303, and 20473022), the Science and Technology Com-
mission of Shanghai Municipality (grants 05QMX1403,
5XD14002, and 06DJ14006), and the Major State Basic Re-
0
Fig. 8. Kinetic results of Pd@S1 catalyst in the 1st, 2nd and 10th runs.
search Development Program (grant 2003CB615807).
mechanism. A similar phenomenon has been reported in the lit-
erature [7]. Moreover, a relatively low increase in activity is
evident in the initial reaction period during the first run, possi-
bly attributed to the induction process of Pd species [33,50] for
the fresh catalyst.
The encaged Heck coupling reaction in our catalyst can be
further proved by the different behaviors of para- and ortho-
substituted aryl halides (Table 1). When the catalysts with the
exposed Pd species, such as shell-crushed Pd@S1 and Pd/C, are
applied, both 4-iodo-toluene and 5-iodo-m-xylene demonstrate
high reactivity (yield >90%). However, for our Pd@S1, which
has a protective zeolitic shell, only the para-substitute reactant
with relative small size can permeate into the zeolitic shell and
be successfully converted on the encapsulated Pd species (yield
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[