Heck Reaction Catalyzed by Pd-Modified Zeolites
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 5991
The choice of a zeolite as support was motivated by observations
that complexes immobilized in zeolite supercages have almost
the same activity as the free complexes in solution11 and because
in this manner a very high palladium dispersion can be achieved.
It has been reported that molecules or small metal particles
immobilized in zeolite cavities are stable.11,12 In addition, the
zeolite microstructure could help to overcome the problems of
leaching present with heterogeneous catalysts in solution.
Zeolites are capable of stabilizing intermediate active species
in their cavities and of controlling the reaction pathway via
shape-selectivity.12 Corresponding effects could be expected
from the similar size of the zeolite pores and the molecules
involved (catalyst precursors, adducts and products). For practi-
cal applications, the separation of Pd/zeolites by filtration is
expected to be easier and faster than, for example, high surface
area metal oxides due to the larger zeolite grain size.
The present paper demonstrates that this new approach
allowed us to prepare the most active heterogeneous palladium
catalysts for the Heck reaction of aryl bromides with olefins.
The best catalysts also induced reaction with activated aryl
chlorides with significant yields. The paper also addresses a
number of important and interesting aspects of the catalytic
reaction such as the influence of preparation and treatment of
the catalysts, activity and selectivity for a variety of substituents
at the aryl halide and alkene, kinetic studies, leaching of Pd
species, and recovery and reuse of the catalysts.
Results and Discussions
Catalysts Preparation. To obtain highly active heteroge-
neous Heck catalysts, in particular of different Pd dispersion,
we prepared several “Pd”-exchanged zeolites by immobilization
of different Pd species: Pd particles ([Pd(0)]), ionic species
([Pd(II)] and [Pd(NH3)4]2+), and neutral complexes ([Pd(OAc)2]
and [Pd(C3H5)Cl]2). The catalysts loaded with [Pd(0)], [Pd(II)],
and [Pd(NH3)4]2+ were prepared according to the literature13
by ion exchange of NaY zeolite, using a 0.1 M aqueous solution
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of [Pd(NH3)4]2+and 2Cl-. After a period of 24 h, [Pd(NH3)4]2+
-
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NaY was obtained. Calcination at 500 °C under pure O2 (180
mL/min) of the exchanged [Pd(NH3)4]2+ zeolites gave the
[Pd(II)]-NaY zeolite, and subsequent treatment at 350 °C with
pure H2 (70 mL/min) gave [Pd(0)]-NaY. The absolute pal-
ladium content for these catalysts was determined by AAS to
be 1 ( 0.2 wt % of Pd. As known from the literature, calcination
and reduction of the Pd species at such high temperatures leads
to agglomeration and the formation of palladium (oxide)
particles. These correspondingly pretreated catalysts can be
regarded as the models of lower Pd dispersion.
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Analogous to the ion-exchange process used for the above
preparations, we prepared the following Pd complexes im-
mobilized in zeolites: [Pd(OAc)2] and [Pd(C3H5)Cl]2 in the
zeolite supercage.
Because the diameter of the zeolite channels is limited (for
the Y zeolite: φ ) 7.4 Å),14 we performed molecular modeling
calculations in order to check whether the complexes can pass
through the zeolite window. The results obtained for [Pd-
(NH3)4]2+-NaY, [Pd(OAc)2]-NaY, and [Pd(C3H5)Cl]2-NaY
using the MM2 augmented parameters15 provided with the
CAChe software from Oxford Molecular LTD16 are reported
in Figure 1.
Concerning [Pd(NH3)4]2+, we did not find any limitation due
to its small size and its pseudo-spherical geometry which allows
it to pass through the zeolite pore in all directions and for all
orientations.
Limitations are expected for the[Pd(OAc)2] and [Pd(C3H5)-
Cl]2 complexes. We found that a perpendicular orientation or
an orientation with a minimum angle of 56° for the [Pd(OAc)2]
complex and 42° for the [Pd(C3H5)Cl]2 complex, versus the
plane defined by the pore, is required for the complex to pass
through the pore.
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These calculations were supported by the time necessary for
sufficient ion exchange. To obtain reasonable amounts of
palladium in the zeolite when using the [Pd(OAc)2] and
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(16) CAChe version 4.1.1 for PowerMacintosh from Oxford Molecular
Ltd. was used for the molecular modeling using the parameters provided
by Dr. E. Chamot to calculate the zeolite structure.
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