A metal nanoparticle-based supramolecular approach for aqueous biphasic reactions

Article abstract of DOI:10.1039/b502181b

β-cyclodextrin immobilized on Pd nanoparticles was successfully employed as an efficient phase-transfer catalyst in aqueous biphasic hydrogenation reactions. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502181b

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A metal nanoparticle-based supramolecular approach for aqueous
biphasic reactions{
Shilpa C. Mhadgut,a Kumaranand Palaniappan,a Muralidhara Thimmaiah,a Stephen A. Hackney,b
Be´la To¨ro¨k*a and Jian Liu*a
Received (in Columbia, MO, USA) 11th February 2005, Accepted 3rd May 2005
First published as an Advance Article on the web 24th May 2005
DOI: 10.1039/b502181b
b-cyclodextrin immobilized on Pd nanoparticles was success-
fully employed as an efficient phase-transfer catalyst in aqueous
biphasic hydrogenation reactions.
Aqueous/organic biphasic catalytic hydrogenations1 have received
much attention due to increased economic and environmental
concerns on chemical production. A major problem encountered
in these systems is the lack of phase-transfer efficiency of
hydrophobic substrates to the catalytic species in aqueous media.
In addition, the catalysts are currently limited to the water-soluble
organometallic species. In recent years, dispersed metal nanopar-
ticles were successfully used as a new type of catalyst2 for various
Scheme 1 Catalytic inverse phase transfer/hydrogenation of hydropho-
reactions in mono- or biphasic systems.3 However, aqueous
bic substrate in aqueous biphasic system
biphasic reactions catalyzed by metal nanoparticles have not been
fully explored. Here, we report a new concept of using artificial
These nanoparticles were prepared by the attachment of PSH-
b-CD8 (Chart 1) to the surface of Pd particles during the particle
host immobilized on metal nanoparticle as an efficient phase
transfer catalyst for aqueous biphasic reactions. In particular, this
formation (ESI{). The surface coverage of b-CD on these particles
(3 nm in diameter) is estimated as 50% (elemental analysis).
proof-of-concept model system was demonstrated by water-
soluble, b-cyclodextrin-modified Pd nanoparticles (b-CD/Pd) in
3,3,5-Trimethyl-2-cyclohexenone (isophorone) was used as the
biphasic hydrogenation reactions. We expect that this study may
first substrate for the hydrogenation of the carbon–carbon bond in
serve as a showcase in the development of new generation of
aqueous media (Scheme 2) since we investigated this reaction
multifunctional metal catalysts by the combination of the best
recently in the presence of different catalysts in organic media.9
features of supramolecular chemistry (molecular recognition) and
For comparison, Pd-black (ESI{) was used as a reference catalyst
nanoparticles (high surface-to-volume ratio).
in this study. Hydrogenation of isophorone was completed
It has already been shown that metal nanoparticles4 can act not
(Table 1) within two hours in the presence of b-CD/Pd in water
only as functional components in various nanomaterials, but also
(ESI{). In contrast, only 2% yield was achieved if 10 mg of Pd
as scaffolds for the construction of molecular structures on their
black was used as catalyst under identical conditions. Such
surfaces. Self-assembly of molecular receptors on particle surfaces
difference was due to the aqueous medium and not the poor
leads to supramolecular structures capable of engaging in
quality of Pd black. In fact, 100% yield was achieved for the
molecular recognition at the interface between the solution and
hydrogenation of isophorone with same amount of Pd black in
particles.5 Cyclodextrins (CDs)6 are well-known molecular hosts
ethanol under the same reaction conditions, which was consistent
capable of including small hydrophobic molecules inside their
cavities in aqueous media, which leads to many applications in the
food and pharmaceutical industries. It was observed that self-
assembly of CDs on nanoparticles would bring these nanoscale
entities the ability to hold hydrophobic molecules on their
surfaces.7 In this sense, inverse phase transfer of hydrophobic
molecules from organic phase onto the surface of nanoparticles in
aqueous media would be possible. Consequently, aqueous biphasic
reactions with water-insoluble substrates catalyzed by CD-
Chart 1 Structure of molecular hosts in this work.
modified metal nanoparticles would be facilitated (Scheme 1).
We selected b-CD/Pd as a model catalyst to test our hypothesis.
{ Electronic supplementary information (ESI) available: details of b-CD/
Pd preparation and characterization. Reaction conditions and character-
ization of hydrogenation reaction products. See http://www.rsc.org/
suppdata/cc/b5/b502181b/
*btorok@mtu.edu and jianliu@mtu.edu (Be´la To¨ro¨k)
Scheme 2 CLC double bond hydrogenation of isophorone in an
aqueous biphasic system
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equivalent amount to the b-CD attached to Pd particles), to the
Table 1 Catalytic CLC double bond hydrogenation of isophorone in
the presence of b-CD/Pd or Pd black catalysts in aqueous and organic
mediaa
reaction mixture of b-CD/Pd and isophorone in water did not
change the TOF of the hydrogenation (Table 1, entry 6; Fig. 1,
entry D). Introduction of 10 mg free b-CD only reduced the
original TOF by 10% (Table 1, entry 7; Fig. 1, entry E). In a
control experiment, the combination of free b-CD with Pd black in
the same hydrogenation reaction did not provide the same
enhancement in the TOF as observed in b-CD/Pd.{ We suggest
that the process of complexation and hydrogenation of substrate
on b-CD/Pd particle surfaces was so efficient that the alternative
path through the complexation of substrate with free b-CD and
then diffusion to the surface of Pd nanoparticles was negligible.
Direct complexation of substrate next to the catalytic site
on particles eliminates the kinetic barrier of diffusing free
b-CD/isophorone complex to the surface of Pd particles.
Consequently, a synergistic effect between b-CD (phase transfer
catalyst) and Pd nanoparticle (hydrogenation catalyst) on the
overall catalytic process was achieved. As pointed out in the
introduction, the major motivation to use aqueous biphasic
systems for hydrogenation is to reduce the use of organic solvents.
The catalyst reuse and/or regeneration are, however, also
important questions in developing new applications. As such we
have studied the recyclable character of our catalyst. The activity
of our model catalyst has been tested during several cycles using
the same test reaction (hydrogenation of isophorone, Scheme 1). It
was found that after five consecutive reactions with the same
catalyst, its activity remained the same.
with the results reported earlier.9 The 25% yield provided by
b-CD/Pd in ethanol was attributed to the partial block of active
surface with surface-anchored b-CD. Thus a 250-fold increase
(Fig. 1, entry A and B) in the reaction turnover frequency
(TOF, h21) by b-CD/Pd was achieved in this aqueous biphasic
system, as compared to Pd black. We propose that this enhance-
ment originates mainly from an efficient transfer of hydrophobic
organic substrate onto the surface of Pd nanoparticles by including
these molecules inside the surface-attached b-CDs. Consequently;
a more effective hydrogenation process is achieved.
We then selected adamantane (denoted as AD), an efficient
guest molecule forming complex with b-CD in water, to test this
hypothesis. Introduction of 1 mmol AD, (same amount as
substrate), in the reaction mixture, including b-CD/Pd and
isophorone, dramatically decreased the TOF and yield of
hydrogenation under the same reaction condition (Table 1, entry
5 and Fig. 1, entry C). The adamantane molecules effectively
competed with the substrate to the available CD binding sites on
Pd particles resulting in a decrease in the hydrogenation rate. This
experiment verified that the host–guest complexation of hydro-
phobic substrate with b-CD on Pd particles was the major driving
force for the enhancement of catalytic efficiency. Such driving
force was lost in an organic solvent as the reaction medium leading
to a lower catalytic activity of b-CD/Pd, which was observed in the
ethanol case.
After the mechanistic study hydrogenation of two basic
substrate groups were investigated such as selective CLC double
bond hydrogenation of unsaturated carbonyl compounds and
CLN double bond hydrogenation of imines. For the first part of
the study several aliphatic and cyclic unsaturated carbonyl
compounds have been selected. The hydrogenations have been
carried out under the same experimental conditions used in
Table 1. Table 2 summarizes the experimental results. Most
substrates under this survey could be hydrogenated in excellent to
good yields by b-CD/Pd within two hours in aqueous medium
under mild reaction conditions. These results clearly indicate the
generally applicable nature of our catalyst.
We believe that the efficiency of hydrogenation was improved
not only by the presence of b-CD, but also by the unique
geometric arrangement of b-CD on Pd nanoparticle surfaces. The
proximity of binding site (the cavity of b-CD) to the catalytic site
(the surface of Pd nanoparticle) further enhanced the overall
catalytic efficiency, which resembles the case of catalysis by natural
enzymes. Addition of 1 mg free b-CD (Chart 1), (an approximately
The results of hydrogenation of imines are tabulated in Table 3.
As shown most amines readily underwent selective hydrogenation
to form the expected secondary amines. The results indicate that
the presence of strongly electron-withdrawing substituents lower
the yields slightly. A comparison of the present results to another
recent study on imine hydrogenation with supported Pd catalysts
also highlights the activity of the b-CD/Pd catalyst.10 As a
limitation of the catalyst we mention that b-CD/Pd cannot catalyze
CLO double bond hydrogenations. As these reactions normally do
not occur on Pd catalysts our observation is in agreement with the
literature.
In summary, water-soluble, b-cyclodextrin(b-CD)-modified Pd
nanoparticles (b-CD/Pd) were successfully used as a multifuna-
tional nanocatalyst for the hydrogenation of various hydrophobic
substrates in an aqueous biphasic system. A unique feature of this
catalytic system is that the aqueous phase serves as the only
reaction medium. Thus, the water-soluble nanoparticle catalyst is
not contaminated by organic substrates after the reaction and can
be recycled for the next catalytic process. In addition, organic
solvents are completely eliminated from the reaction mixture. The
Fig. 1 Relative turnover frequency (TOF, h21) as compared with Pd
black (A) for the hydrogenation of isophorone (1 mmol) in the presence of
10 mg of catalyst in aqueous biphasic media. (A) Pd black, 0.1 h21; (B)
b-CD/Pd, 26 h21; (C) b-CD/Pd + 1 mmol of AD, 9 h21; (D) b-CD/Pd +
1 mg of free b-CD, 25 h21and (E) b-CD/Pd + 10 mg of free b-CD, 23 h21
.
3208 | Chem. Commun., 2005, 3207–3209
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efficient and sustainable catalytic processes. Most importantly, the
concept of integration of molecular phase-transfer catalyst with
metal nanoparticle catalyst into a dual-functional heterosupramo-
lecular nanocatalyst represents a new approach in the development
of nanoparticle-based catalysts for future applications.
Table 2 Catalytic CLC double bond hydrogenation of unsaturated
carbonyl compounds in the presence of b-CD/Pd catalyst in aqueous
mediuma
Financial support from NSF (to J. L.) and PRF (to B. T.) is
acknowledged.
Dedicated to Prof. Mihaly Bartok on the occasion of his
receiving the Szechenyi Prize.
Shilpa C. Mhadgut,a Kumaranand Palaniappan,a
Muralidhara Thimmaiah,a Stephen A. Hackney,b Be´la To¨ro¨k*a and
Jian Liu*a
aDepartment of Chemistry, Michigan Technological University, 1400
Townsend Drive, Michigan 49931, USA. E-mail: btorok@mtu.edu and
jianliu@mtu.edu; Fax: 906-487-2061
bDepartment of Materials Science and Engineering, Michigan
Technological University., 1400 Townsend Drive, Michigan 49931, USA
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second phase is actually composed of the organic substrate. Taken
together, the elimination of the organic solvents from the reaction
media and the no contamination of aqueous phase, this new
approach is in line with the fundamental principles of green
chemistry. It represents a new route for the development of
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3207–3209 | 3209