Palladium-grafted mesoporous MCM-41 material as heterogeneous catalyst for Heck reactions

Article abstract of DOI:10.1039/a705104b

Palladium grafted mesoporous MCM-41 materials, prepared by vapor grafting, show remarkable activity in Heck carbon-carbon coupling reactions.

Full text of DOI:10.1039/a705104b

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Palladium-grafted mesoporous MCM-41 material as heterogeneous catalyst for
Heck reactions
Christian P. Mehnert and Jackie Y. Ying*
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Palladium grafted mesoporous MCM-41 materials, pre-
pared by vapor grafting, show remarkable activity in Heck
carbon–carbon coupling reactions.
Current research focuses on the synthesis and application of
modified mesoporous MCM-41 materials1,2 that have an active
species attached to the framework via host–guest interactions,
creating discrete and uniform catalyst sites on the inner walls of
the porous systems. Herein we report the synthesis of
palladium-grafted mesoporous materials, designated Pd-
TMS11, and their application as heterogeneous catalysts in
Heck reactions. Palladium catalyzed carbon–carbon bond
formation, the Heck reaction,3 represents one of the most
versatile tools in modern synthetic chemistry and has great
potential for industrial applications. Although recent advances
in homogeneous4 and heterogeneous5 Heck catalysis have
attracted considerable attention, the properties and activities of
the existing catalysts have, as yet, limited industrial application.
Fig. 1 TEM of Pd-grafted mesoporous catalyst, Pd-TMS11, showing the
hexagonal array of 26 Å pores
Here we report the synthesis of a highly active heterogeneous
palladium catalyst, Pd-TMS11, that exhibits excellent activity
towards activated and non-activated haloarene substrates.
dispersed over the pore surface of the mesoporous support.
Elemental analysis of Pd-TMS11 samples shows palladium
metal contents of 10–30 mass%, depending on the amount of
volatile complex [Pd(h-C5H5)(h -C3H5)] used in the vapor
Palladium-grafted mesoporous material (Pd-TMS11) is syn-
thesized by vapor deposition of a volatile palladium complex
onto the inside walls of the porous framework, followed by
reduction. The success of grafting the inside of a porous
material with a highly dispersed metal film is strongly
dependent on the properties of the volatile complex and the
reaction conditions.† To maintain the uniform structure, high
surface area and accessibility of the molecular sieves, it is
important to minimize the cluster growth inside and outside of
the porous material. The concept of vapor grafting enables a
uniform distribution of a volatile complex and ensures the
discrete deposition of metal fragments throughout the porous
structure of the material without cluster formation, giving
metal-grafted molecular sieves.
3
grafting process. The BET surface area of a typical sample of
Pd-TMS11 with a palladium content of 19.1 mass% is 750
m2 g21. The BJH (Brunauer–Joyner–Halenda) pore size
distribution of Pd-TMS11 illustrates a narrow peak centered at
26 Å for the pore diameter which is, as expected, slightly
smaller than that determined for the MCM-41 starting material
(27.4 Å) owing to the grafted palladium metal. The CO
adsorption studies of Pd-TMS11 show metallic surface areas
between 120 and 140 m2 g21 (Pd metal) with a metal dispersion
of more than 30%. In comparison, commercially available Pd/
g-Al2O3 (Aldrich) shows a metallic surface area of only 60
m2 g21 (Pd metal) with a metal dispersion of 15%.
The palladium grafting of the degassed silicate-based MCM-
41 material‡ was carried out via sublimation of the volatile
3
organometallic complex6 [Pd(h-C5H5)(h -C3H5)] under vac-
The Pd-TMS11 materials catalyze the Heck carbon–carbon
coupling reaction of aryl halides (Scheme 1). The catalytic
activity of these materials was investigated using activated and
non-activated aryl halides and butyl acrylate as the vinylic
substrate (Table 1). The yields for the activated aryl halides,
with respect to reaction time and amount of catalyst, showed
that the Pd-TMS11 catalysts have an excellent activity espe-
cially in view of other palladium-based heterogeneous sys-
tems.** Full conversion and a yield of 99% was obtained after
uum through the porous material. After the palladium complex
is deposited onto the inside walls of the pores, the material is
reduced under a stream of hydrogen gas, giving an air-stable,
black powder.
The prepared catalyst,§ Pd-TMS11, retains its hexagonally-
packed porous structure as shown in X-ray diffraction (XRD)
pattern. Although the diffraction patterns of Pd-TMS11 and the
MCM-41 starting material are almost identical, the peak
intensity for Pd-TMS11 is reduced owing to radiation diffusion
caused by the grafted palladium metal. The XRD pattern of
palladium metal has major diffraction peaks at 2q = 40.1° (111)
and 46.7° (200), which are not found in the XRD pattern of Pd-
TMS11, indicating that the palladium metal is highly dispersed
in the latter. Further evidence is provided by transmission
electron micrograph¶ (TEM) that shows the hexagonally
packed porous structure of Pd-TMS11 (Fig. 1), with no
noticeable palladium clusters. In addition, scanning transmis-
sion electron micrographs (STEM) and energy dispersive
analysis by X-ray (EDAX) studies have been conducted on Pd-
TMS11 particles, indicating that palladium is uniformly
R
X
O
C
Pd-TMS11
+ base
Bun
O
+
O
– HX•base
R
C
Bun
H
E/Z
O
R
X
H
NO2
C(=O)Me
Cl Br Br
Br
Scheme 1
Chem. Commun., 1997
2215

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Table 1 Heck alkenationa of aryl halides with n-butyl acrylate over Pd-
TMS11 catalysts
2196 °C. The small round-bottom flask containing the volatile palladium
complex and the loaded frit were heated to 120 °C (ramp 1 °C min21) using
an oil bath. During the heating process, the white MCM-41 starting material
turned black and the excess volatile organometallic complex was condensed
into the cooled round-bottom flask. The resulting solid was reduced under
a stream of hydrogen (50 ml min21) at 300 °C, giving a black powder
designated Pd-TMS11 [XRD (100) 39.6 Å, (110) 22.4 Å, (200) 19.4 Å].
¶ An average of 50 particles per sample was investigated via TEM. No
palladium clusters were observed on the as-synthesized Pd-TMS11
material, even at high magnification. TEM investigation of used Pd-TMS11
catalyst showed the formation of palladium clusters (ca. 50 Å diameter).
∑ Catalyst deactivation is noted in the form of palladium metal agglomera-
tion, fracturing of the MCM-41 support material and coking of the catalyst
surface. Pd-TMS11 recovered from the reaction of n-butyl acrylate and
1-bromo-4-nitrobenzene showed reduced activity in the second and third
catalysis cycles under identical condition. The total lifetime of the catalyst
is ca. 3000 TON for the latter reaction.
** Control experiments using commercially available Pd/Al2O3, Pd/SiO2
and Pd/carbon (all containing ca. 10 mass% palladium) were conducted
giving TONs which were 60% lower than those obtained for Pd-TMS11
with respect to activated aryl substrates. No activity was observed for non-
activated aryl substrates in commercial supported catalysts.
†† Reaction between bromobenzene and n-butyl acrylate using homoge-
neous catalysts showed a TON of 96. In comparison, a TON of 624 was
obtained for Pd-TMS11 at 170 °C in the same reaction. Unlike the existing
homogeneous catalysts, our Pd-TMS11 is not air-sensitive and can be easily
recovered by filtration after usage.
Amount
of
catalyst
%
Aryl halide
substrateb
Conversionc
(mol%) T/°C Time % (Yield)d E:Z TONe
C6H5Cl
C6H5Br
4-BrC6H4NO2
4-BrC6H4C(O)CH3 0.1
4-BrC6H4C(O)CH3 0.02
0.1
0.1
0.1
170 32 h
170 48 h
120 90 min 100 (99)
120 25 min 100 (99)
120 60 min 100 (99)
16 (40)
67 (92)
99:1
64
99:1 624
99:1 1000
99:1 1000
99:1 5000
a All reactions are carried out in air. b 1.1 equiv. of base NEt3 with respect
to the aryl halide substrate is added to the reaction mixture. c Conversion of
reactant is determined by gas chromatography. d Mol of coupling product (E
+ Z)/mol reactant converted. e TON = mol product/mol catalyst; recycled
catalyst showed reduced activity.∑
only 1 h for the reaction of n-butyl acrylate and 4-bromoaceto-
phenone using as little as 0.02 mol% catalyst. The Pd-TMS11
material therefore provides an extremely simple and efficient
Heck catalyst that even rivals some of the best homogeneous
catalysts.7,†† A moderate conversion of non-activated aryl
halides was observed, and a conversion of 67% for bromo-
benzene has been achieved (see Table 1).
The new catalyst system Pd-TMS11, prepared by vapor
grafting, has successfully been used in carbon–carbon coupling
reactions. Remarkable activity, easy accessibility and excep-
tional stability of the described material provides an excellent
example for a new generation of heterogeneous Heck cata-
lysts.
We thank Dr. T. J. Garrett-Reed and M. Frongillo (M.I.T.
CMSE) for their assistance with transmission electron micro-
scopy, and David Weaver for his assistance in conducting
catalytic characterizations. The financial support of the David
and Lucile Packard Foundation and the National Science
Foundation (CTS-9257223) is gratefully acknowledged.
‡‡ The prepared MCM-41 had a niobium content of 1.5 mass% which
contributed to the larger wall thickness of the material (18 Å). The dopant
had no effect on the catalysis results.
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Footnotes and References
* E-mail: jyying@mit.edu
† MCM-41 material was degassed at 60 °C under reduced pressure to
prevent decomposition of the palladium complex due to air oxidation or
hydrolysis via surface adsorbed water. To achieve a uniformly grafted Pd-
TMS11 it is necessary that the surface of the MCM-41 be water-free and
only consist of surface bound oxo and hydroxo groups.
‡ Modified synthesis of mesoporous MCM-41. Hexadecyltrimethyl-
ammonium bromide (15.9 g, 44.9 mmol) was dissolved in H2O (1.2 l) and
treated with sodium silicate (14% NaOH/27% SiO2) and 1 g (3.14 mmol)
Nb(OEt)5 in H2O (0.4 l) to give a white precipitate.‡‡ The pH value of the
mixture was adjusted to pH = 11.5 using sulfuric acid (30%). The gel was
aged at room temp. for 12 h before it was heated in pressure tubes to 100 °C
for 7 d. The supernatant of the aged gel was decanted off and the resulting
white residue washed with H2O (1 l) and EtOH (1 l). The isolated solid was
dried in air at room temp. for 12 h before the material was calcined at 560 °C
for 6 h giving MCM-41 with a hexagonally-packed cylindrical mesoporous
structure.
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¨
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§ Synthesis of palladium-grafted mesoporous material (Pd-TMS11). MCM-
41 (0.5 g) [XRD (100) 39.9 Å, (110) 22.5 Å, (200) 19.6 Å, (210) 14.8 Å;
BET surface area of 997 m2 g21, BJH adsorption pore size of 27.4 Å] was
degassed at 600 °C under reduced pressure (1022 Torr) for 6 h. The resulting
material was loaded into a short path frit and contained with glass wool
under an inert gas atmosphere. A small round-bottom flask was filled with
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¨
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3
the red complex [Pd(h -C3H5)(h-C5H5)] (0.25 g, 1.2 mmol) and connected
to a prepared frit that was further attached to a condensation bridge with a
round-bottom flask. The apparatus was evacuated, and a constant pressure
(1022 Torr) was maintained by cooling the empty round-bottom flask to
Received in Bloomington, IN, USA; 17th July 1997; 7/05104B
2216
Chem. Commun., 1997