FULL PAPER
DOI: 10.1002/chem.201101265
Small Pd Nanoparticles Supported in Large Pores of Mesocellular Foam: An
Excellent Catalyst for Racemization of Amines
Mozaffar Shakeri,[a] Cheuk-wai Tai,[b] Emmanuelle Gçthelid,[c] Sven Oscarsson,[c] and
Jan-E. Bꢀckvall*[a]
Abstract: Highly dispersed palladium
nanoparticles (1–2 nm) supported in
large-pore mesocellular foam (MCF;
29 nm) were synthesized. The Pd-nano-
catalyst/MCF system was characterized
by transmission electron microscopy
(TEM), powder X-ray diffraction
(XRD) and X-ray photoelectron spec-
troscopy (XPS). The performance of
the Pd nanocatalyst obtained was ex-
amined for amine racemization. The
Pd nanocatalyst showed higher activity
and selectivity toward racemization of
(S)-1-phenylethyl amine than any other
amine racemization catalyst reported
so far and it could be reused several
times. Our data from TEM and XRD
suggest a restructuring of the Pd nano-
catalyst from amorphous to crystalline
and an increase in Pd nanocatalyst size
during the racemization reaction. This
led to an unexpected increase of activi-
ty after the first use. The Pd nanocata-
lyst obtained can be integrated with
other resolving processes of racemic or-
ganic compounds to increase the yield
of chiral organic products.
Keywords: amines · heterogeneous
catalysis
· mesocellular foams ·
nanoparticles · palladium
Introduction
lent enantioselectivity.[1,2] In most cases metal complexes are
employed as homogeneous racemization catalysts in these
DKR reactions. However, separation, recycling, low-activity,
and low-selectivity of homogeneous racemization catalysts,
and their deactivation by air and humidity are major prob-
lems to overcome for large-scale applications.
Racemization of a chiral compound by a chemical catalyst is
a powerful technique for increasing the yield of a resolution
process. For example, in a resolution process the unwanted
enantiomer can be racemized and the racemate can be recir-
culated for an additional resolution. This increases the effi-
ciency of the resolution and one single recirculation through
racemization will increase the theoretical yield from 50 to
75%. The efficiency can be further increased if the resolu-
tion is combined with an in situ racemization. Such a resolu-
tion process with continuous racemization is termed “dy-
namic kinetic resolution” (DKR) and the yield in such a
process can theoretically reach 100%. Examples of efficient
DKR reactions are the combination of an enzymatic resolu-
tion with a metal-catalyzed racemization of the starting ma-
terial; such systems give products in high yields with excel-
A major challenge is therefore to develop efficient heter-
ogeneous catalysts by immobilizing metal racemization cata-
lysts to various carriers. These catalysts meet the require-
ments for separation and recycling. Metal nanoparticles
have interesting properties as heterogeneous catalysts and
have attracted considerable attention recently. They are
compatible with enzymes, less sensitive to humidity, and
have shown to be more efficient in racemization reactions
than homogeneous catalysts.[3] On the other hand, their ac-
tivity and selectivity is highly dependent on the type of sup-
port, stabilizing ligands, procedure of synthesis, and subse-
quently size and shape of nanoparticles.[4] Among silica-
based mesoporous materials, mesocellular foams (MCFs)
are excellent supports for chemo- and biocatalysts.[5] This
arises from their high surface area, high pore volume, and
adjustable pore size, as well as from the ease with which
their surface might be functionalized with desired ligands.
MCF pores are cage-like structures in which the pores (20–
40 nm) are interconnected by windows of 10–20 nm in diam-
eter.[6,7]
[a] Dr. M. Shakeri, Prof. J.-E. Bꢀckvall
Department of Organic Chemistry
Arrhenius Laboratory, Stockholm University
106 91, Stockholm (Sweden)
[b] Dr. C.-w. Tai
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
106 91 Stockholm (Sweden)
We have recently been involved in the development of
methods for the racemization of amines.[8–10] Several of these
methods have involved the use of a homogeneous rutheni-
um catalyst (Shvo-type catalyst) with racemization at
ꢀ908C,[8,9] and these racemizations were combined with en-
zymatic resolution to give synthetically useful DKR reac-
tions of amines.[8b,c,9] The high temperature required by the
[c] Dr. E. Gçthelid, Prof. S. Oscarsson
Department of Physics and Astronomy
Division of Molecular and Condensed Matter Physics
Uppsala University, Uppsala (Sweden)
Supporting information for this article is available on the WWW
Chem. Eur. J. 2011, 17, 13269 – 13273
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13269