Immobilization of palladium in mesoporous silica matrix: Preparation, characterization, and its catalytic efficacy in carbon-carbon coupling reactions

Article abstract of DOI:10.1021/ic8004294

Palladium(0) has been immobilized into the silica-based mesoporous material to develop catalyst Pd(0)-MCM-41, which is found to be highly active in carbon-carbon coupling reactions. [Pd(NH₃)₄]²⁺ ions have been incorporated into the mesoporous material during synthesis of MCM-41 and subsequently upon treatments with hydrazine hydrate Pd²⁺ ions present in mesoporous silica matrix were reduced to Pd(0) almost instantaneously. The catalyst has been characterized by small-angle X-ray diffraction, N₂ sorption, and transmission electron microscopy (TEM). TEM and surface area measurements clearly demonstrate that the immobilization of Pd(0) into the mesoporous silica has a significant effect on pore structure of the catalyst. Nevertheless, after immobilization of palladium the meso-porosity of the material is retained, as evidenced in the nitrogen sorption measurement. The TEM micrograph shows that both MCM-41 and Pd(0)-MCM-41 have similar types of external surface morphology; however, Pd(0)-MCM-41 was less ordered. Pd(0)-MCM-41 showed high catalytic activity toward carbon-carbon bond formation reactions like Heck and Sonogashira coupling, as evidenced in high turn-over numbers. In contrast to many other Pd-based catalysts reported so far, Pd(0)-MCM-41 acts as a truly heterogeneous catalyst in C-C coupling reactions. Notably, the new heterogeneous catalyst is found to be efficient in the activation of arylchloride to give impressive conversion in cross coupling (15-45% for Heck and 30% for Sonogashira) reactions under mild conditions.

Full text of DOI:10.1021/ic8004294

Inorg. Chem. 2008, 47, 5512-5520
Immobilization of Palladium in Mesoporous Silica Matrix: Preparation,
Characterization, and Its Catalytic Efficacy in Carbon-Carbon Coupling
Reactions
Sreyashi Jana, Buddhadeb Dutta, Rajesh Bera, and Subratanath Koner*
Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India
Received March 8, 2008
Palladium(0) has been immobilized into the silica-based mesoporous material to develop catalyst Pd(0)-MCM-41,
3 4
)
]2+ ions have been incorporated
which is found to be highly active in carbon-carbon coupling reactions. [Pd(NH
into the mesoporous material during synthesis of MCM-41 and subsequently upon treatments with hydrazine hydrate
Pd2 ions present in mesoporous silica matrix were reduced to Pd(0) almost instantaneously. The catalyst has
+
2
been characterized by small-angle X-ray diffraction, N sorption, and transmission electron microscopy (TEM).
TEM and surface area measurements clearly demonstrate that the immobilization of Pd(0) into the mesoporous
silica has a significant effect on pore structure of the catalyst. Nevertheless, after immobilization of palladium the
meso-porosity of the material is retained, as evidenced in the nitrogen sorption measurement. The TEM micrograph
shows that both MCM-41 and Pd(0)-MCM-41 have similar types of external surface morphology; however, Pd(0)-
MCM-41 was less ordered. Pd(0)-MCM-41 showed high catalytic activity toward carbon-carbon bond formation
reactions like Heck and Sonogashira coupling, as evidenced in high turn-over numbers. In contrast to many other
Pd-based catalysts reported so far, Pd(0)-MCM-41 acts as a truly heterogeneous catalyst in C-C coupling reactions.
Notably, the new heterogeneous catalyst is found to be efficient in the activation of arylchloride to give impressive
conversion in cross coupling (15-45% for Heck and 30% for Sonogashira) reactions under mild conditions.
Introduction
environmental importance in the chemical and pharmaceuti-
cal industry, especially when expensive or toxic heavy metal
complexes are employed. In this context, heterogeneous
Carbon-carbon coupling reactions, such as Heck and
Sonogashira reactions, have profound importance for ap-
plication in the industry. Both the reactions have been studied
extensively in homogeneous condition using a variety of
palladium-containing complexes as catalyst. The mechanistic
pathways of palladium-catalyzed carbon-carbon coupling
5
catalysis reactions are far fetching in comparison to the ho-
mogeneous ones because the former process allows the
production and ready separation of a large quantity of the
desired products with the use of a small amount of catalyst.
In addition, such catalysts can also be used in continuous-
1
,2
reactions in homogeneous medium are well established.
6
7
flow systems or in flow-injection microreactors. Conse-
quently, the search of new efficient and recyclable hetero-
Homogeneous catalysts, however, suffer from the problems
of separation of catalyst from reaction mixture, recycling of
the catalyst and deactivation via the aggregation of metallic
Pd particles formed in situ during the Heck reaction.
Recycling of catalysts is a task of great economic and
8
,9
geneous catalysts has received much attention. Different
approaches like encapsulation or immobilization of catalyti-
3
,4
(3) Zhao, F.; Bhanage, M. B.; Shirai, M.; Arai, M. J. Mol. Catal. A: Chem.
1
999, 142, 383.
*
To whom correspondence should be addressed. E-mail: snkoner@
chemistry.jdvu.ac.in. Tel: +91 33 2414 6666, ext. 2505. Fax: +91 33 2414
223.
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(
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(
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1
5512 Inorganic Chemistry, Vol. 47, No. 12, 2008
10.1021/ic8004294 CCC: $40.75  2008 American Chemical Society
Published on Web 05/06/2008

Palladium in Mesoporous Silica Matrix
cally active metal complex in solid supports such as
mechanism is also described by Arai et al. for Pd/C
catalysts. Jacobs et al. have claimed that truly heteroge-
1
0,11
21
zeolites
and covalent grafting of such active complex
12
onto reactive polymer surfaces or inorganic porous matri-
neous Heck catalyst can be obtained by in situ incorporation
of metallic Pd particles into silica and zeolite (ZSM-5, Y
1
3
ces have been used to develop the efficient heterogeneous
catalyst. Ordered mesoporous silica, for example, MCM-
2
2
and mordenite) matrices. Pd(0) nanoparticles have been
immobilized onto silica or aluminosilicate matrices in the
1
4
4
1, with high surface area and attractive pore structure are
the natural choice to use as matrix.
last 4-5 years to prepare heterogeneous catalyst for C-C
In regard to the preparation and characterization of porous
silica-based heterogeneous Heck catalysts, few studies
deserve particular mention. Mehnert et al. have used vapor
deposition technique for grafting of Pd-allyl complex into
23,24
coupling reactions.
Bedford et al. investigated carbon-
carbon coupling reactions using Pd nanoparticle catalyst
24
immobilized onto the modified silica surface. An enhanced
catalytic activity was noticed in hydrogenation and Heck
15
Nb-MCM-41 pretreated with trimethylchloro-silane. Good
to moderate activity was reported in a variety of Heck
coupling reactions of arylbromide with styrene and n-butyl
acrylate. Although no leaching of palladium was observed
during catalytic reactions, recycling of the catalyst was not
successful because of agglomeration of Pd(0) species, partial
structural damage, and coke deposition. Pd(II) ions have been
immobilized onto the dicyano-functionalized MCM-41 through
25
reactions using Pd nanoparticles encapsulated in dendrimer.
In addition, Pd nanoparticle catalysts have also been used
2
3g,26
27
in hydrogenation,
Pauson-Khand reaction, and oxida-
2
8
tion of alcohols. Immobilization of molecular species of
palladium into the zeolite cage was found to be capable of
activating aryl chlorides, which usually remain inactive in
Heck reactions. Initially it was thought that the reactions
occurred in zeolite cavity which effectively prevents the
agglomeration of inactivate Pd particles, later it was ascer-
tained that palladium species leached from the zeolite into
bulk solution, catalyzed the reactions, and diffused back into
16
complex formation for development of Heck catalyst. Clark
et al. have investigated the Heck reaction using Pd(II) Schiff-
base complex catalyst-supported on porous silica and
observed a high degree of recyclability in respect of both
29
1
7
the pores. PdO particles have been generated on the surface
of modified MCM-41, which catalyzes the hydrogenation
activity and selectivity. Pd-bipyridyl complex has been
immobilized into the nanosized channels of MCM-41 to
1
8
30
catalyze Heck coupling reactions. Djakovitch et al. have
succeeded to prepare highly active Heck catalysts by
entrapping Pd(0) and Pd(II) compounds in the micropores
reaction of cyclic olefins. To our knowledge, no attempts
have been made so far to immobilize the Pd(0) particles into
the silica-based mesoporous materials, MCM-41.
1
9,20
of a variety of zeolites (ZSM-5, Y and mordenite).
It
In this paper we report the immobilization of Pd(0) into
the mesoporous silica matrix to prepare a new catalyst, Pd(0)-
MCM-41 and study its catalytic efficacy in carbon-carbon
cross coupling reactions. Interestingly the prepared catalyst
is capable of activating arylchloride both in Heck and
Sonogashira coupling reactions under truly heterogeneous
and mild conditions to afford an impressive conversion.
While some of the Pd-containing catalysts showed only poor
has been proposed that dissolved active species of Pd(0)
catalyze the Heck reaction and Pd species have been retained
inside the zeolite pores after the reaction through dissolution-
readsorption equilibria of Pd(0) and Pd(II). A similar
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Inorganic Chemistry, Vol. 47, No. 12, 2008 5513

Jana et al.
Scheme 1. (a) Organic Modification of Si-MCM-41: APTES/CHCl3 and (b) Anchoring of p-Bromoacetophenone onto MCM-41 (in Methanol)
activity toward C-C coupling reactions, other Pd systems
have failed to activate the aryl chlorides.
continued for another 2 h. The resulting product was filtered, washed
with copious amount of methanol and, finally, with dichlo-
romethane, and dried under vacuum to give a gray product, Pd(0)-
MCM-41. The Pd content of the Pd(II)-MCM-41 as well as catalyst
was 0.008 wt %.
1
5–18,24,25
Experimental Section
Materials. The precursor PdCl (>99.9%), tetraethyl-orthosili-
2
Preparation of SH-MCM-41. To introduce thiol group into
mesoporous matrix, organic modification of MCM-41 was achieved
by using (3-mercaptopropyl)-triethoxysilane as reported by Rich-
cate (TEOS, 98%), 3-mercaptopropyl-triethoxysilane, the cationic
surfactant cetyl-trimethylammonium bromide (CTAB, 98%), 3-mer-
captopropyl-functionalized silica (SH-SiO
2
) [sulfur content of SH-
33
ardson and Jones. Prepared materials have been characterized by
SiO ) 3.95% (wt)], and all other reagents were purchased either
2
IR spectra and elemental analysis. The elemental analysis showed
that the loading of sulfur was 0.5-3.3% (wt.) in SH-MCM-41.
Preparation of PBA-MCM-41. Anchoring of (3-aminopropyl)-
triethoxysilane (3-APTES) into MCM-41 has been achieved by
stirring 0.1 g of MCM-41 with 0.18 g (0.81 mmol) of 3-APTES in
from Sigma-Aldrich/Fluka or Alfa-Aesar and were used as received
without further purification. The solvents were purchased from
Merck (India) and were distilled and dried before use. [Pd(NH
3 4 2
) ]Cl
complex has been prepared from PdCl by following the reported
2
3
1
procedure.
Preparation of the MCM-41. Pure mesoporous silica MCM-
dry chloroform at room temperature for 12 h under N
2
atmosphere.
The white solid MCM-41-(SiCH CH CH NH thus produced was
2
2
2
2 x
)
3
2
4
1 was prepared as described by Gr u¨ n et al. The white solid
filtered and washed with chloroform and dichloromethane. This
obtained was then calcined at 813K in N atmosphere (heating rate
2
solid was then refluxed with 10 g p-bromoacetophenone (50 mmol)
of 2 K min-1) for 2 h and in air for 5 h to remove the template.
Preparation of the Catalyst, Pd(0)-MCM-41. Pd(0)-MCM-41
was prepared via simultaneous incorporation of palladium and self-
assembling of mesoporous silica by following a method as described
below. In the present procedure, Pd(II) ions were first incorporated
into the mesoporous silica matrix followed by reduction to obtain
3
in methanol (10 cm ) for 3 h at 60 °C. The resulting yellowish
solid PBA-MCM-41 was then collected by filtration and was dried
in a desiccator (Scheme 1).
Catalyst Characterization. The powder X-ray diffraction (XRD)
patterns of the samples were recorded with a Scintag XDS-2000
diffractometer using Cu KR radiation. Cell parameter of the
mesoporous materials were calculated from XRD pattern using d100
values. The Pd content of the Pd(0)-MCM-41 samples was
determined by PerkinElmer A-Analyst 200 atomic absorption
Pd(0)-MCM-41. In a stoppered 250 mL conical flask, 0.025 g (0.1
3
mmol) of [Pd(NH
3
)
4
]Cl
2
complex was suspended in 60 cm of
distilled water, and then 5 mL of aqueous ammonia (0.07 mol)
was added. After complete dissolution of the [Pd(NH ]Cl
complex, 1.2 g (0.003 mol) CTAB was added to the mixture. Then
3
)
4
2
2
spectrometer (sensitivity: up to parts per billion level). N sorption
measurements were performed on a Quantachrome Autosorb
Automated Gas Sorption system. Prior to sorption experiments,
samples were outgassed at 150° under vacuum until a final pressure
5
1
.5 mL of TEOS (0.03 mol) was added dropwise over a period of
0 min. The resulting gel had the molar composition TEOS/CTAB/
3 2 3 4 2
NH /H O/[Pd(NH ) ]Cl ) 1.0:0.11:2.3:111.11:0.003. The mixture
-2
of 10 Torr was reached. BET (Brunauer-Emmett-Teller) surface
was stirred for 1 h, and then the off-white precipitate was filtered
and washed with distilled water until the filtrate gets completely
chloride free. After it was dried at room temperature, the prepared
areas were determined over a relative pressure range of 0.005 to
0
.20. Mesopore size distributions were calculated with BJH
(Barrett-Joyner-Halenda) method of the adsorption branch of the
solid was calcined at 813 K in N atmosphere (heating rate ) 2 K
2
isotherms. Transmission electron microscopic (TEM) images were
recorded with a JEOL JEM 2100 transmission electron microscope
-1
min ) for 2 h and then in air for 5 h to remove the organic template.
After calcination a light orange colored product Pd(II)-MCM-41
was obtained. To reduce the Pd(II) ions to Pd(0), 20 mL of 5%
aqueous solution of hydrazine hydrate was added at room temper-
ature with constant stirring in 15 min, and the stirring was then
equipped with lanthanum hexaboride (LaB
accelerating voltage of 200 kV and with an objective aperture of
0 mm. The samples were ground and sonicated in ethanol and
dispersed over carbon-coated copper grids. Other instruments used
6
) gun, operating at an
6
1
0c,13b
in this study were the same as reported earlier.
(
31) Livingstone, S. E. The Chemistry of Ruthenium, Rhodium, Palladium,
Osmium, Iridium and Platinum; Pergamon Texts in Inorganic Chem-
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32) Gr u¨ n, M.; Unger, K. K.; Matsumoto, A.; Tsutsumi, K. Microporous
Mesoporous Mater. 1999, 27, 207.
Catalytic Reactions. The reactions were carried out in glass
batch reactors. To study the progress of the catalytic reactions the
(
(33) Richardson, J. M.; Jones, C. W. J. Catal. 2007, 251, 80.
5514 Inorganic Chemistry, Vol. 47, No. 12, 2008

Palladium in Mesoporous Silica Matrix
products were collected at different time intervals and identified
and quantified by a Varian CP-3800 Gas Chromatograph using a
CP-Sil 8 CB capillary column. The products of the catalytic
reactions have also been isolated and purified by column chroma-
1
1
tography and identified by H NMR measurement. H NMR spectra
were measured on a Bruker Avance DPX 300 NMR (300 MHz)
spectrometer using TMS as the internal standard. The detail
experimental procedures that were undertaken to study the indi-
vidual reactions are given below.
Heck Coupling Reactions. One millimole of arylhalide and
0
8
.123 g (1.5 mmol) of sodium acetate (NaOAc) were mixed with
mL of N,N-dimethylformamide (DMF) and placed in a batch
reactor. To this 0.05 g of Pd(0)-MCM-41 was added, and the
mixture was heated to the desired temperature in an oil bath. Finally
1.5 mmol of styrene/n-butyl acrylate was added to start the coupling
reaction. The products were collected at different time intervals
and identified and quantified by GC. For isolation of products at
the end of catalytic reaction, the catalyst was first separated out by
filtration, and then the filtrate was treated with distilled water to
remove the NaOAc. The organic layer was collected and dried over
Na SO , and finally the remaining solvent was removed under
2 4
vacuum at 25 °C. The purified desired products are then isolated
by column chromatography.
Sonogashira Coupling Reactions. One millimole of arylhalide,
0.45 g (1.5 mmol) of tetra-n-butylammonium acetate (TBAA), and
0
.001 g (0.05 mmol) of CuI were mixed with 6 mL of DMF and
placed in a batch reactor. To this solution 0.05 g of Pd(0)-MCM-
1 was added. The mixture was then heated to the desired
4
temperature in an oil bath. The coupling reaction started once 1.5
mmol of phenylacetylene was added to the mixture. The products
were collected at different time intervals and identified and
quantified by GC. For isolation of products at the end of the catalytic
reaction, the catalyst was first separated out by filtration. Twenty
milliliters of distilled water was then mixed with the filtrate and
extracted with hexanes (3 × 15 mL). The organic layers thus
Figure 1. Small angle XRD pattern of (a) MCM-41, (b) Pd(II)-MCM-41,
and (c) Pd(0)-MCM-41.
of X-ray powder diffraction patterns of MCM-41, Pd(II)-
MCM-41, and Pd(0)-MCM-41 shows that the typical three
peak pattern of MCM-41 has been retained after the
incorporation of Pd(II) and subsequent immobilization of
Pd(0) into the mesoporous silica. However, all the diffraction
lines shifted to the higher angles. The d100 spacing of the
materials decreased from MCM-41 to Pd(II)-MCM-41 to
Pd(0)-MCM-41. A similar type of behavior was observed
4
collected were combined and dried over anhydrous MgSO . Solid
compounds were obtained on evaporation of the organic solvent
under vacuum at 25 °C. The compounds were recrystallized from
hot ethanol.
Results and Discussion
X-ray Diffraction Studies. The small-angle X-ray dif-
fraction patterns of MCM-41, Pd(II)-MCM-41, and the
catalyst Pd(0)-MCM-41 are shown in Figure 1. In the low-
angle region (2θ ≈ 1.5-5°), three characteristics Bragg
reflections were observed in all the three materials, namely,
MCM-41, Pd(II)-MCM-41, and Pd(0)-MCM-41. These dif-
fraction lines can be indexed by assuming a hexagonal
3
4
by Brukett et al. in phenyl modified mesoporous sieves
and by Lim and Stein for directly synthesized thiol-MCM-
35
4
1. In our case, upon immobilization of Pd(0), a broadening
of the diffraction lines was also noticed. This result can be
attributed to the lowering of local order, such as variations
in the wall thickness, or might be the result of the reduction
of scattering contrast between the channel wall of silicate
framework and palladium present in the pores, as was
1
4,32
symmetry of the materials.
The pronounced d100 reflec-
tions at 2θ ≈ 2.56-2.18° and the less intense d110 and d200
peaks at 2θ ≈ 4.42-3.76° and 5.06-4.32° correspond well
to the hexagonally arranged pore structure of the MCM-41
framework, which confirms the long-range structural ordering
of these materials. MCM-41 and Pd(II)-MCM-41 show
additional peaks at 2θ ≈ 5.74 and 6.26°, respectively, that
can be assigned to d210 reflection, while in the case of Pd(0)-
MCM-41, the weakest reflection for d210 is buried in the
background noise. All the peaks are well-resolved, indicative
of good quality of the material. The values of XRD unit cell
parameter for MCM-41, Pd(II)-MCM-41, and Pd(0)-MCM-
3
5
previously mentioned by Lim and Stein. Therefore, the
changes of the diffraction lines upon incorporation of Pd(II)
into MCM-41 and its subsequent reduction to form Pd(0)
are not inconsistent. Diffraction lines for palladium metal,
which appear at 2θ ≈ 40.1°(111) and 46.7°(200), were not
observed in the XRD pattern of Pd(0)-MCM-41, indicating
that the palladium sites are highly dispersed in the matrix.
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(
1
b) Fowler, C. E.; Burkett, S. L.; Mann, S. Chem. Commun. 1997,
769.
4
1 were 4.05, 3.65 and 3.45 nm, respectively. Comparison
(35) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5515

Jana et al.
Figure 2. N2 adsorption/desorption isotherms of MCM-41 and Pd(0)-MCM-
1. Adsorption points are marked by filled circles and desorption ones are
by open square.
4
2
N Sorption Studies. The nitrogen sorption experiments
showed that the MCM-41 has the BET surface area (AsBET
)
2
-1
of 1136 m g and the primary mesopore volume (V
p
) of
3
-1
0
.61 cm g (Figure 2). The average pore diameter is
calculated to be 22.6 Å for MCM-41 using the BJH method.
All the calculated values are in agreement with those reported
36
for good quality mesoporous MCM-41. The N
isotherm, as well as pore size distribution of the Pd(0)-MCM-
1, was considerably different from that of MCM-41. Pd(0)-
2
adsorption
4
MCM-41 displays a considerably lower BET surface area
in comparison to the MCM-41 (Figure 2). The BET surface
2
-1
Figure 3. Transmission electron micrograph of (a) MCM-41 and (b) Pd(0)-
area of Pd(0)-MCM-41 was 676 m g , while the mesopo-
MCM-41.
3
-1
rous volume (V
p
) was ∼0.31 cm g . The BJH pore size
be the immobilization of significant amount of Pd(0) into
the nanosized pores of the mesoporous material. However,
considering the amount of palladium present in Pd(0)-MCM-
distribution illustrated a narrow peak centered at around 20.3
Å for Pd(0)-MCM-41.
The pore wall thickness of MCM-41 and Pd(0)-MCM-41
has been calculated assuming a hexagonal pore geometry
4
1 the pore volume loss is significantly high. This can be
4
2
37–39
caused by partial amorphization of the layered phase of
mesoporous material during palladium incorporation and
calcination of Pd(0)-MCM-41. It is well-known that meso-
porous materials often tend to collapse during calcination
for both of these materials.
The pore walls are considered
to be amorphous, and in several studies, the density of pore
3
5,38,40
walls is taken as the density of amorphous silica,
that
3
-1 41
is, equal to 2.2 cm g . Accordingly, the pore wall
thickness values were calculated to be 0.96 and 1.32 nm for
MCM-41 and Pd(0)-MCM-41, respectively. The decrease in
4
3
of sample because of their low stability.
TEM Studies. Transmission electron micrographs of
MCM-41 and Pd(0)-MCM-41 have been measured. Figures
p
BET surface area and V value and increase of wall thickness
3
A and 3B show the representative TEM micrographs of
of Pd(0)-MCM-41 in comparison to the corresponding values
for MCM-41 clearly demonstrate that the immobilization of
Pd(0) into the mesoporous silica has a significant effect on
pore structure of the catalyst. One of the possible reasons
Pd(0)-MCM-41 catalyst and MCM-41, respectively. Both
Pd(0)-MCM-41 and MCM-41 feature an open-ended lamellar
type arrangement of hexagonal porous tubules. When
electron beam falls on MCM-41 type mesoporous materials
perpendicular to the pore axis, the pores are seen to be
arranged in patches composed of regular rows as has been
for this remarkable difference in the BET surface area, V
p
,
and b values between MCM-41 and Pd(0)-MCM-41 might
d
44,45
(
36) Kruk, M.; Jaroniec, M.; Sakamoto, Y.; Terasaki, O.; Ryoo, R.; Ko,
C. H. J. Phys. Chem. B 2000, 104, 292.
interpreted by Chenite et al.
TEM micrograph of MCM-
4
1 viewed along the pore axis reveals a hexagonal array
(
37) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1997, 101, 583.
(
38) Dabadie, T.; Ayral, A.; Guizard, C.; Cot, L.; Lacan, P. J. Mater. Chem.
1
996, 6, 1789.
39) Kruk, M.; Jaroniec, M.; Ryoo, R.; Kim, J. M. Microporous Mater.
997, 12, 93.
40) Sonwane, C. G.; Bhatia, S. K.; Calos, N. Ind. Eng. Chem. Res. 1998,
7, 2271.
41) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979.
(42) Alfredsson, V.; Keung, M.; Monnier, A.; Stucky, G. D.; Unger, K. K.;
Sch u¨ th, F. J. Chem. Soc., Chem. Commun. 1994, 921.
(
(
(
1
(43) Monnier, A.; Sch u¨ th, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell,
R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.;
Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299.
3
(44) Chenite, A.; Page, Y. Le; Sayari, A. Chem. Mater. 1995, 7, 1015.
5516 Inorganic Chemistry, Vol. 47, No. 12, 2008

Palladium in Mesoporous Silica Matrix
a
having channel dimension of ∼4 nm,which is consistent with
the XRD results. Pd(0)-MCM-41 also exhibits a similar type
of array of regular rows in TEM micrographs. Therefore, it
can be concluded that both MCM-41 and Pd(0)-MCM-41
have a similar type of external morphology. In our case, the
morphology of the tubules in MCM-41, as well as Pd(0)-
MCM-41, are rectilinear, and they are 1200-1600 Å long.
However, from a comparison of TEM micrographs of MCM-
Table 1. Heck Olefination of Aryl Halides Over Pd(0)-MCM-41
Catalyst
4
1 and Pd(0)-MCM-41, it appears that the channels are less
ordered in Pd(0)-MCM-41 than those in MCM-41, which is
indicative of amorphization of MCM-41 upon immobilization
Pd(0). Palladium particles are not observed on the surface
of the catalyst even at the highest resolution of the TEM
micrographs measured for Pd(0)-MCM-41. It seems, there-
fore, that the agglomeration of Pd particles, if any occurs
on the external surface of the catalyst, is negligibly small.
Coupling Reactions. At first the catalytic activity of
Pd(0)-MCM-41 is tested for Heck coupling reaction using
substituted and nonsubstituted aryl halides and styrene or
n-butyl acrylate as the vinylic substrate. The reactions are
conveniently carried out in batch reactors in air at the normal
pressure in the temperature range of 80-100 °C (Table 1).
The yield of the reaction products in the case of substituted
aryl-halides with respect to reaction time shows that n-butyl
acrylate is more reactive substrate than styrene. Among
nonsubstituted aryl-halides, iodobenzene shows the highest
conversion. Although bromo-benzene shows very poor
activity, its para-substituted substrates (R
1
) -NO
2
,
-
COCH ) not only exhibit enhanced activity but also yields
3
of 100% (Table 1). In all the coupling reactions, high trans-
selectivity was observed. It is noteworthy that the catalyst
is capable of activating chlorobenzene to give a significant
yield of the desired products under the reaction conditions,
those that are normally employed in Heck reactions. While
the reaction of n-butyl acrylate with chlorobenzene showed
an impressive 45% conversion, for styrene the conversion
was ∼15%. As far as we know, only a few Pd(0)-based
heterogeneous catalysts are capable of activating chloroben-
a
All the reactions are carried out in air using 1 mmol of aryl halide, 1.5
mmol of terminal alkene, 8 mL of DMF, 0.05g of Pd(0)-MCM-4,1 and 1.5
mmol of NaOAc. Conversion of reactant is determined by gas chromatography.
In all the reactions, complete E selectivity was observed. Side products:
transformation of styrene s the product of R coupling. TOF (turn over
frequency) ) moles converted/mole of active site/time.
b
c
d
Table 2. Heck Reaction of Aryl Chloride over a Variety of
Heterogeneous Pd-Catalysts
1
5,46
zene in the Heck reaction (Table 2).
Catalytic efficacy of Pd(0)-MCM-41 has been tested in
Sonogashira reaction also. Substituted and nonsubstituted aryl
halides are coupled with terminal alkyne, namely, phenyl
acetylene in DMF in the presence of CuI in air at 80-100°
(
Table 3). Tetra-n-butylammonium acetate is found to be
more efficient base than NaOAc and Et N in this case.
3
Iodobenzene showed the highest activity among the non-
substituted aryl halides, followed by bromobenzene, while
chlorobenzene exhibited a significant amount of conversion
a
TOF (turn over frequency) ) moles converted/mole of active site/time.
4
7a
(
∼30%). Chlorobenzene usually remained inactive in So-
which was suggested by Crabtree and Anton. This test
also indicates that the Pd(0)/Pd(II) cycle is operating to
catalyze the cross coupling reactions.
nogashira reaction toward this type of catalyst.
47
47b,48
Hg(0) Poisoning Test. To determine Pd(0) as the active
To perform the
site of the Pd(0)-MCM-41, Hg(0) poisoning test was done
test, 1 mL (13.5 g) of mercury was added to 8 mL of DMF
along with 0.05 g of Pd(0)-MCM-41, and the mixture was
stirred for 12 h at room temperature. To this, 1 mmol of
iodobenzene, 1.5 mmol of styrene, and 1.5 mmol of NaOAc
(
45) Cheng, C.-F.; He, H.; Zhou, W.; Klinowski, J.; Sousa Gonc u¨ alves,
J. A.; Gladden, L. F. J. Phys. Chem. 1996, 100, 390.
(
(
46) Ding, J. H.; Gin, D. L. Chem. Mater. 2000, 12, 22.
47) (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855. (b)
Yu, K.; Sommer, W.; Richardson, J. M.; Weck, M.; Jones, C. W.
AdV. Synth. Catal. 2005, 347, 161. (c) Widegren, J. A.; Finke, R. G.
J. Mol. Catal. A: Chem. 2003, 198, 317.
(48) (a) Klingelh o¨ fer, S.; Heitz, W.; Oestreich, S.; F o¨ rster, S.; Antonietti,
M. J. Am. Chem. Soc. 1997, 119, 10116. (b) Richardson, J. M.; Jones,
C. W. AdV. Synth. Catal. 2006, 348, 1207.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5517

Jana et al.
a
49
Table 3. Sonogashira Coupling of Aryl Halides Over Pd(0)-MCM-41
in heterogeneous conditions. To address these issues, we
have undertaken a series of tests as described below.
Catalyst
5
0
Hot Filtration Test. To test if palladium is leaching
from the solid catalyst during reaction, the liquid phase of
the reaction mixture is collected by filtration at the reaction
temperature after 30% of the coupling reaction is completed.
Atomic absorption spectrometric analysis of the liquid phase
of the reaction mixtures thus collected by filtration confirms
that Pd is absent in the reaction mixture. It is noticed that
after filtration of the Pd(0)-MCM-41 catalyst from batch
reactor at the reaction temperature, coupling reactions do not
proceed further. Pd is also not detected in the liquid phase
of the reaction mixture after the completion of the reaction.
It is noteworthy that the DMF remains completely colorless
on addition of Pd(0)-MCM-41. These results suggest that
the Pd is not being leached out from the catalyst during cross
coupling reactions.
51
Three-Phase Test. To ascertain if the reactions are truly
heterogeneous we have performed the three-phase test for
Heck and Sonogashira reactions using a designed aryl halide
in usual condition. In this test, one of the reactants (aryl
halide, namely, p-bromo-acetophenone in this case) is
anchored on the modified surface of Si-MCM-41. MCM-
41-anchored p-bromo-acetophenone, PBA-MCM-41, has
been prepared according to the method given in Scheme 1.
A solution of 1 mmol of p-nitrobromobenzene, 1.5 mmol of
styrene, and 1.5 mmol of NaOAc in 8 mL of DMF was
stirred in the presence of 0.05 g of Pd(0)-MCM-41 and 1 g
of PBA-MCM-41 at 100° for 24 h. The reaction mixture
was collected by filtration and washed with fresh DMF
several times. The collected filtrate was then extracted with
a
All the reactions are carried out in air using 1 mmol of aryl halide, 1.5
mmol of phenylacetylene, 1.5 mmol of TBAA, 0.05mmol of CuI, 6 mL of
DMF, and 0.05g of Pd(0)-MCM-41 catalyst for 8 h. Conversion of reactant
is determined by gas chromatography. Side product: homo-coupling
product (Hay coupling).
mole of active site/time.
b
c
53 d
TOF (turn over frequency) ) moles converted/
1
ethyl acetate and analyzed by GC and H NMR, which
showed about 95% of p-nitrobromobenzene converted to
4
-nitrostilbene as expected for Heck reaction.
The residue obtained from reaction mixture was then
hydrolyzed with 2 N aqueous HCl solution (1.7 mL HCl in
.5 mL H O) for 3 h under refluxing condition. The resulting
8
2
solution was neutralized, extracted with ethyl acetate,
1
concentrated, and analyzed by GC and H NMR. The only
compound obtained in this process was p-bromoacetophe-
none. The expected Heck product for p-bromoacetophenone,
that is, 4-acetylstilbene was not detected in extracted the
liquid. This clearly shows that p-bromoacetophenone has not
participated in the coupling reaction while anchored on the
MCM-41.
The same test was also performed for Sonogashira reaction
using 1.5 mmol of TBAA, 1.5 mmol of phenylacetylene,
1.0 mmol of p-nitrobromobenzene, and 0.05 mmol of CuI
in presence of 0.05 g Pd(0)-MCM-41 and 1 g of PBA-MCM-
41 at 80° C for 10 h. In this case, analysis of the filtrate also
showed >99% conversion of p-nitrobromobenzene, while
the compound recovered from residue after hydrolysis was
solely p-bromoacetophenone. Should there be any leaching
of palladium species from Pd(0)-MCM-41, the anchored
Figure 4. Plot of conversion of iodobenzene versus time for Heck catalysis
with Pd(0)-MCM-41 under normal conditions (b) and in presence of excess
Hg(0) (b).
were added. The mixture was then heated up to 100 °C in
an oil bath. The progress of reaction is studied as per the
protocol described in the Experimental Section. The catalyst
is found to be totally inactive toward the coupling reaction
in this condition (Figure 4). This shows that the metallic
mercury amalgamates with Pd(0), thereby quenching the
activity of the palladium(0) particles that catalyze the
coupling reactions.
(
(
(
49) Pr o¨ ckl, S. S.; Kleist, W.; K o¨ hler, K. Inorg. Chem. 2007, 46, 1876.
50) Karimi, B.; Enders, D. Org. Lett. 2006, 8, 1237.
Heterogeneity Test. Recovery and reuse of the catalyst
and leaching of Pd metal in solution are the important issues
in coupling reactions, especially when the reactions undergo
51) (a) Baleizao, C.; Corma, A.; Garcia, H.; Leyva, A. J. Org. Chem.
2004, 69, 439. (b) Das, D. D.; Sayari, A. J. Catal. 2007, 246, 60.
5518 Inorganic Chemistry, Vol. 47, No. 12, 2008

Palladium in Mesoporous Silica Matrix
Table 4. Performance of the Pd(0)-MCM-41 Catalyst in Solid-Phase Poisoning Test
a
Reactions are carried out in air using 1 mmol of aryl halide, 1.5 mmol of terminal alkene, 8 mL DMF, 0.05g of Pd(0)-MCM-41, and 1.5 mmol of
b
NaOAc. Reactions are carried out in air using 1 mmol of aryl halide, 1.5 mmol of phenylacetylene, 1.5 mmol of TBAA, 0.05mmol of CuI, 6 mL of DMF,
and 0.05g of Pd(0)-MCM-41 catalyst for 8 h. Conversion of reactant is determined by gas chromatography. TOF (turn over frequency) ) moles converted/
mole of active site/time.
c
d
p-bromoacetophenone would have also participated in the
above-described Heck and Sonogashira reactions. However,
this is not the case for the present catalyst.
Solid-Phase Poisoning Test. Richardson and Jones
suggested that the solid-phase poisoning test is the most
definitive test available to date to ascertain the true hetero-
geneity of the Pd-based catalyst in C-C coupling reactions.
In this test, metal-free mercaptopropyl-functionalized silica
added to the reaction mixture. Results obtained in a typical
test with sulfur to palladium ratio of 500:1, were collated in
Table 4, showing the effect of mercaptopropyl-functionalized
MCM-41 on catalytic action of Pd(0)-MCM-41. Clearly in
both the coupling reactions catalyst remain active even in
presence of large excess of SH-MCM-41.
In addition to the above solid-phase poisoning test with
SH-MCM-41, we have also performed this test using
commercially available (purchased from Sigma-Aldrich)
3
3
(
2
in our case SH-MCM-41 and SH-SiO ) was used as an
effective palladium scavenger that selectively coordinates and
3-mercaptopropyl-functionalized silica (SH-SiO ). In a typi-
cal Heck reaction, 1 mmol (0.2 g) iodobenzene, 1.5 mmol
2
33,52
deactivates the leached out palladium.
Thereby, cessation
of the reaction is expected if catalysis of the coupling rection
catalyzed by leached palladium species from the solid
support. Richardson and Jones also observed that the
poisoning is effective when the S/Pd (mole ratio) is greater
than 5.3:1. We have performed solid-phase poisoning tests
varying the S/Pd ratio from 5:1 to 1000:1 for Heck and
Sonogashira coupling reactions using SH-MCM-41 and SH-
(
0.2 g) n-butyl acrylate, and 1.5 mmol (0.123 g) NaOAc were
mixed in 15 mL DMF and placed in a batch reactor. To this,
g of Pd(0)-MCM-41 was added and 0.0043 g of 3-mer-
captopropyl-functionalized silica gel (SH-SiO ) was added
1
2
as a solid-phase poison (S/Pd molar ratio ) 7:1). In this case,
also we did not observe any appreciable change of the
catalytic efficacy of Pd(0)-MCM-41 in presence of the
2
SiO as poisoning agent.
poisoning agent SH-SiO
Heck reactions in presence of the poisoning agent SH-MCM-
1 and SH-SiO are shown in Figure 5. Notably there was
2
. The kinetic profiles of the typical
For Heck reactions, appropriate amounts of Pd(0)-MCM-
1 and SH-MCM-41 were taken in the solution of different
4
4
2
arylhalides (1 mmol), styrene/n-butyl acrylate (1.5 mmol),
and NaOAc (1.5 mmol) in 15 mL of DMF. The mixture was
stirred continuously maintaining the desired temperature. The
progress of the reactions was monitored by the method
described above. The poisoning test was performed for
Sonogashira reaction also using 1 mmol of arylhalide, 1.5
mmol of phenylacetylene, 1.5 mmol of TBAA, and 0.05
mmol of CuI at desired temperature in presence of required
amount of Pd(0)-MCM-41 and SH-MCM-41, maintaining
the above-mentioned S/Pd ratio.
no induction period in these reactions in presence or in
absence of the poisoning agent.
Therefore, all the above tests convincingly demonstrate
that there were no leaching of Pd species occurring in the
Pd(0)-MCM-41 catalyzed C-C coupling reactions.
Recycling of the Catalyst. For recycling study, Heck
reaction was performed with styrene and iodobenzene
maintaining the same reaction conditions as described above
using the recovered catalyst. Each time, after the completion
of reaction the catalyst was recovered by centrifugation and
then washed thoroughly with DMF followed by a copious
amount of water to remove the base present in the used
catalyst and finally by dichloromethane. The recovered
catalyst was dried under vacuum at 110 °C overnight. The
performance of the recycled catalyst in C-C coupling
reaction up to four successive runs is summarized in Table
A comparison of percentage of conversion in C-C
coupling reactions clearly shows that the catalytic efficacy
of Pd(0)-MCM-41 is not affected when SH-MCM-41 is
(
52) Webb, J. D.; MacQuarrie, S.; McEleney, K.; Crudden, C. M. J. Catal.
007, 252, 97.
53) Hay, A. S. J. Org. Chem. 1962, 27, 3320.
2
(
Inorganic Chemistry, Vol. 47, No. 12, 2008 5519

Jana et al.
catalyst, Pd(0)-MCM-41, by simultaneous incorporation
of palladium and self-assembly of mesoporous silica
matrix via a simple and hassle free synthesis route. The
prepared catalyst has shown high activity toward Heck
and Sonogashira carbon-carbon coupling reactions. The
catalyst can be reused several times without any loss of
activity. In addition, the Pd(0)-MCM-41 acts as a true
heterogeneous catalyst in cross coupling reactions. There
are only a few catalysts that can catalyze C-C coupling
2
2,48
reactions in truly heterogeneous catalytic conditions.
Notably, Pd(0)-MCM-41 is capable of activating aryl-
chloride in both the Heck and Sonogashira reactions under
mild conditions. Arylchlorides normally remain inactive
toward this type of catalyst especially in heterogeneous
condition. Because the reaction medium is heterogeneous,
the prepared catalyst Pd(0)-MCM-41 might be useful for
industrial applications.
Figure 5. Kinetic profiles for the Heck coupling reaction of iodobenzene
with n-butyl-acrylate in the absence of solid-phase poison (9), in the
presence of SH-SiO2 as the solid-phase poison (O), and in the presence of
SH-MCM-41 as the solid-phase poison (×).
Acknowledgment. Financial support from the Department
of Science and Technology (DST), Government of India,
by a grant (SR/S1/IC-23/2003) (to S.K.) is gratefully
acknowledged. We are grateful to CIF, IIT Kharagpur, for
allowing us to use the TEM facility (on a chargeable basis).
Table 5. Catalytic Efficacy of the Recovered Pd(0)-MCM-41 in
a
Successive Runs
b
c
cycles
conversion (wt %) selectivity (wt %) TOF (h-1)
8
8
90
95
90
95
90
4831
4831
4798
4850
4798
2
Authors wish to thank Dr. A. Bhaumik for N sorption
first reuse
second reuse
third reuse
fourth reuse
a
88
87
89
87
measurement (on a chargeable basis), funded by the Nano-
Science and Technology Initiative of DST, New Delhi.
Authors wish to thank the reviewers for extending useful
suggestions to improve the manuscript.
Reaction conditions: 1 mmol of iodobenzene, 1.5 mmol of styrene, 8
mL DMF, 0.05 g of Pd(0)-MCM-41, and 1.5 mmol of NaOAc. Conversion
of reactant is determined by gas chromatography. TOF (turn over
b
c
frequency) ) moles converted/mole of active site/time.
Supporting Information Available: Pore distribution plots of
5
. The catalytic efficacy of recovered Pd(0)-MCM-41
Pd(0)-MCM-41 and MCM-41 obtained from N sorption measure-
2
remains almost the same for Heck coupling reaction of
styrene and iodobenzene in every run.
ment, IR spectrum of SH-MCM-41, purification procedures of the
products obtained in C-C coupling reactions, and H NMR data
1
of the isolated products. This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusions
In summary, we have succeeded in the preparation of
an efficient Pd(0) immobilized mesoporous heterogeneous
IC8004294
5520 Inorganic Chemistry, Vol. 47, No. 12, 2008