Communications
solutions.[3,17] Pd/TiO2 catalysts were prepared by using Pd acetate
with the following procedure: 50 mg block copolymer PS(x)-b-
P2VP(y) (54300/8800, 81000/14200, 54000/43800, Polymer Source,
Inc., Canada) was dissolved in toluene (10 mL) and stirred for 5 h.
This was followed by the addition of 20 mg Pd acetate (or 16 mg for 3-
nm diameter Pd) into the solution and continuous stirring for 48 h.
The different Pd nanocluster sizes were obtained by changing the
length of the polymer head (PS). The Pd micelle precursors were then
deposited onto anatase TiO2 (1 g, donated by Siant-Gobain) which
was ground into powder and filtered by a sieve (mesh size 60). The
surface area of the support was 34.8 m2 gꢀ1. The solvent was slowly
evaporated in air until the sample was completely dry. The dry sample
(200 mg) was calcined in the mixture of oxygen and argon (20:80 v/v)
at 3008C after increasing the temperature 58C minꢀ1 for 1 hour, after
which the temperature was decreased to 508C. The sample was
reduced at 2508C for 30 min in the hydrogen/argon mixture (10:80
v/v). The sample was cooled to room temperature before starting the
reaction studies. Bulk Pd catalyst was prepared by mixing Pd (2 mg)
with TiO2 (198 mg). The sample was calcined at 10008C for 4 h to
form large Pd and was also reduced with hydrogen at 2508C before
the catalytic reaction.
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formation of water with C H bond activation through an
activated intermediate of the hydrogen/oxygen reaction.
Figure 5 illustrates the proposed reaction cycle. The observa-
tion that benzene formation from propylene is preceded by
A continuous-flow packed-bed reactor (0.6 30 cm) was used for
the activity studies. In a typical run, the volumetric flow ratios of
argon, propylene, hydrogen, and oxygen were 1:1:1:1, controlled by
mass flow controllers. The total flow rate was 480 mLhꢀ1. The
products were analyzed by GC (SRI 8010 A), MS (SRI), and NMR
spectroscopy (Varian UNITY INOVA 400 MHz).
A Porapack
column was used for the separation of products. Both a thermal
conductivity detector and flame ionization detector were used. For
NMR spectroscopic analysis, the products were collected in a CDCl3
solvent cold trap.
Figure 5. Mechanism of propylene coupling, leading to the formation
of benzene. M represents one or more metal sites.
The Pd/TiO2 catalysts were imaged by HRTEM (JEOL 2010FX)
to determine the morphology and size of the Pd nanoclusters. The
binding energies of Pd in the Pd/TiO2 catalyst were monitored by XPS
(Kratos, Axis Ultra) with a monochromated AlKa source.
hydrogenation and oxidation suggests that the Pd surface is
populated by hydroxy and/or peroxide species that result
from the dissociative chemisorption of hydrogen and
oxygen.[10,20] These species abstract a hydrogen atom from
the terminal methyl group of a p-bound propylene molecule
on the surface. This initial hydrogen abstraction produces a
symmetric allyl intermediate[21] which then cyclizes with an
adjacent allyl group to form the cyclic intermediate I and
water. Although it is known that on a bifunctional catalyst,
two allyl groups can couple to form 1,5-hexadiene as a by-
product and benzene by dehydrogenation, 1,5-hexadiene
products were not observed by MS. Thus, in the low-temper-
ature Pd-catalyzed propylene coupling reaction, the cycliza-
tion apparently does not follow the conventional mechanism
via 1,5-hexadiene; instead, it may proceed to form inter-
mediate I (Figure 5). The mechanism of propyne coupling to
form benzene via cyclohexadiene intermediates has been
proposed.[16] The formation of water provides the energetic
drive necessary for the removal of hydrogen. The fact that Pd
nanoclusters are required for efficient low-temperature
coupling suggests that either the very low coordination
(giving rise to the suggested structures analogous to bis-
(allyl)palladium complexes) and/or surface intermediates that
are sufficiently different from the pure metal (or metal–ligand
complexes previously studied) are responsible for this
unusual reactivity.
Received: January 4, 2005
Revised: April 15, 2005
Published online: June 28, 2005
Keywords: benzene · heterogeneous catalysis · nanostructures ·
.
palladium · propylene coupling
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Experimental Section
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Angew. Chem. Int. Ed. 2005, 44, 4735 –4739