C O MMU N I C A T I O N S
0
.87), showing that the presence of additional alkyl groups at the
R-carbon does not have a significant effect on reactivity. However,
for alumina coated with [PAA/PEI-Pd(0)] PAA (3.5 bilayers of
film, Table 1, column 3), the initial rate for hydrogenation of 1 is
- and 12-fold faster than that for 2 and 3, respectively. To further
selectivity, but they suggest that there are differences in transport
pathways to the commercial catalyst and the embedded nanopar-
ticles.
3
Substrate isomerization is a common but unwanted process in
hydrogenation, and in some cases, this side reaction is even
dominant over hydrogenation.25 Minimization of isomerization is
thus desirable to improve the yield of hydrogenation reactions.
Hydrogenation of 1 with polyelectrolyte-encapsulated Pd nanopar-
ticles produces 60% less acetone than does hydrogenation with the
commercial Pd catalyst at similar yields. Similar decreases in
isomerization were observed with substrate 2.
3
improve selectivity, we capped two bilayers of PAA/PEI-Pd(0)
with five bilayers of PAA/PEI (without any Pd). With the capping
layers, selectivity for 1 over 3 increases to 24 (Table 1, column 5).
The presence of the capping layers decreases the hydrogenation
rate for all of the alcohols, but hydrogenation of 3 is most attenuated.
This likely results from very slow diffusion of 3 through the film
(
see below).
In conclusion, layer-by-layer deposition of PAA and PEI-
Pd(II) on alumina and subsequent reduction of Pd2 is a versatile
method for synthesizing immobilized Pd catalysts. The polyelec-
trolyte matrix stabilizes the particles, introduces selectivity, and
significantly decreases unwanted isomerization. Further exploitation
of the versatility of polyelectrolyte films should increase selectivity
in hydrogenation as well as other reactions.
+
We also carried out the hydrogenation in a 4:1 methanol-water
mixture because hydrogen and the unsaturated alcohols (particularly
2
rate of hydrogenation of 1 is 40% lower in 80% methanol than in
pure water, but selectivities are 60-80% higher (Table 1, column
4
and 3) are more soluble in organic solvents than in water. The
). The increased selectivity and decreased rate probably result from
less swelling of the film in the methanol-water mixture. These
selectivities are, in general, about 2-3-fold higher than those
reported for dendrimer-encapsulated Pd nanoparticles, suggesting
that the polyelectrolyte films provide highly restricted access to
catalytic sites of nanoparticles.22
In practical applications of selective catalysts, such as minimizing
the number of purification steps involved in organic reactions, one
substrate must be hydrogenated in the presence of several impuri-
ties.23 In an equimolar mixture of 1 and 2, use of the commercial
Acknowledgment. We thank the Department of Energy Office
of Basic Energy Sciences and the American Chemical Society
Petroleum Research Fund for financial support of this work.
Supporting Information Available: Procedures for hydrogenation
reactions, synthesis of nanoparticle-containing MPFs, TEM, and
determination of the amount of Pd in different catalysts as well as SEM
images of commercial and nanoparticle-based catalysts (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
5% Pd-on-alumina catalyst results in almost no selectivity between
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Coating of the commercial catalyst with 3.5 bilayers of PAA/
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