Toward Homogenization of Heterogeneous Metal Nanoparticle Catalysts with Enhanced Catalytic Performance: Soluble Porous Organic Cage as a Stabilizer and Homogenizer

Article abstract of DOI:10.1021/jacs.5b04029

Organic molecular cage (CC3-R) with intrinsically porous skeleton is used as a support for immobilizing Rh nanoparticles (NPs) in an ultrasmall size of ～1.1 nm for the first time. The CC3-R with the unique characteristic of high solubility can be utilized

Full text of DOI:10.1021/jacs.5b04029

Communication
pubs.acs.org/JACS
Toward Homogenization of Heterogeneous Metal Nanoparticle
Catalysts with Enhanced Catalytic Performance: Soluble Porous
Organic Cage as a Stabilizer and Homogenizer
Jian-Ke Sun, Wen-Wen Zhan, Tomoki Akita, and Qiang Xu*
National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan
S
* Supporting Information
contact between the MNPs and reactants, resulting in a negative
effect on the catalytic activity.6 One of the major goals in catalysis
ABSTRACT: Organic molecular cage (CC3-R) with
intrinsically porous skeleton is used as a support for
immobilizing Rh nanoparticles (NPs) in an ultrasmall size
of ∼1.1 nm for the first time. The CC3-R with the unique
characteristic of high solubility can be utilized to
homogenize the heterogeneous catalyst in solution. The
obtained homogenized heterogeneous catalyst Rh/CC3-R-
homo exhibits significantly enhanced catalytic performance
toward various liquid-phase catalytic reactions, as
compared with the heterogeneous counterpart Rh/CC3-
R-hetero. Moreover, Rh/CC3-R-homo shows excellent
durability and recyclability. The advantage of combining
homogeneous and heterogeneous catalysts is likely to be
beneficial for many applications.
is to combine the advantages of molecular catalysts and
heterogeneous processes, ideally maintaining or even improving
the activity of the resultant catalysts, while facilitating the
recovery and recycling.7 However, catalysts that allow selective
control of both modes are extremely rare.8
Herein, for the first time, we demonstrate a facile method of
enhancement of liquid-phase catalytic performance by homog-
enizing the heterogeneous catalysis on the basis of a porous
support-immobilized MNP catalyst. As a proof of concept, we
employ a porous organic molecule, CC3-R,9 a class of [4 + 6]
cycloimine cage with an intrinsically porous structure, as a novel
support for immobilizing MNPs. The main feature of CC3-R is
its good solubility in certain solvents with well-kept prefabricated
open skeleton,9b which provides an ideal platform for
homogenizing the catalysis in solution. To demonstrate the
feasibility of this method, the CC3-R-supported Rh NPs are
prepared in a solution process and used as catalysts for catalytic
reactions. The resultant homogenized heterogeneous catalyst
Rh/CC3-R-homo gives a solution without any precipitation,
which resembles a homogeneous catalytic system upon mixing
with reactants. Such “soluble” nanocatalyst has several
remarkable features including (1) high stability and dispersibility
in solution, (2) effective control of MNPs in an ultrasmall size of
∼1.1 nm, (3) significant enhancement of catalytic activity toward
liquid-phase reactions as compared with the heterogeneous
counterpart as in a form of solid, and (4) excellent durability and
reusability, which are likely to be beneficial for many applications.
The CC3-R is synthesized according to a previous literature.9
The channel diameter in CC3-R varies between 0.58 nm at the
narrowest point within the triangular cage window to 0.72 nm
within the cage itself. The phase purity of solid CC3-R is verified
by powder X-ray diffraction (PXRD) measurements, which is
consistent with a calculation from the single crystal data (Figure
S1). The N2 sorption measurement shows that the specific
surface area of CC3-R is 452 m2 g−1, demonstrating the
intrinsically porous characteristic (Figure S2). The solid CC3-R
is insoluble in strong polar solvents like alcohol and water but can
completely dissolve in dichloromethane or a dichloromethane−
alcohol mixture under certain volume ratio with maintaining the
prefabricated skeleton.
he newly emerging open frameworks, such as metal−
T
organic frameworks (MOFs) and porous organic frame-
works (POFs), have been targeted as particularly attractive
supports for heterogeneous metal nanoparticle (MNP) catalysts
over the past several years.1 The highly tunable pore structures
and high porosities of these kinds of materials open a new avenue
for the synthesis of heterogeneous catalysts with high catalytic
performance in various catalytic reactions.2 Generally, control of
MNP size, shape, and dispersity is the key to enhance the activity
of these kinds of catalysts.3 Despite the great progresses that have
been achieved over the past few years, it is still a great challenge to
deliberately control these factors and optimize the synthesis
condition for the performance of the catalysts. For example, the
contradiction of decreased size of MNPs will lead to high surface
energy, which is prone to agglomerate or change shape during
catalytic reactions, especially, for the weak interaction between
the porous support and MNPs, resulting in a dramatic decrease
of their activity. Therefore, the development of facile and general
method to improve the catalytic performance of porous support-
immobilized MNP catalysts is highly desirable.
It is well-known that the homogeneous catalysts are generally
more active than their heterogeneous counterparts because of
the well-dispersed catalytic sites in solution, which are highly
accessible and favorable for the kinetic performance of the
catalysis.4 As for heterogeneous catalysts such as porous support-
immobilized MNPs, the effective contact between the MNPs and
reactants often suffers from the diffusion problem, which leads to
a slow kinetic performance of catalysts.5 Although the surfactant-
protected MNPs have a good dispersibility in solution, the long
alkyl chains grafted on the MNPs generally block the direct
Received: April 19, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b04029
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Journal of the American Chemical Society
Communication
The Rh/CC3-R-homo catalyst could be easily prepared by a
solution process (Scheme 1). Briefly, a solution (12 mL) of
Scheme 1. Schematic Illustration of Preparation of
Homogenized Heterogeneous Catalyst and Its
Heterogeneous Counterpart
Figure 1. (a) TEM and (b,c) HAADF-STEM images of Rh/CC3-R-
homo catalyst. (d) Size histograms of Rh NPs based on 300 NPs.
no signs of aggregation (Figure S3). It should be noted that the
redispersion of MNPs after drying to solution has been often
difficult because of the weak binding interaction between the
supports and MNPs, easily leading to NP aggregation in
solution.11 Presumably, the CC3-R here provides good solubility
as well as effectively anchors the Rh NPs’ surface, thus preventing
them from aggregation.
It is well-known that ammonia borane (NH3BH3, AB) is a
promising material for chemical hydrogen storage, from which
hydrogen can be released through hydrolysis or methanoly-
sis.12,13 In this work, methanolysis of AB is employed as a test
reaction for evaluating the catalytic activity of Rh/CC3-R-homo
catalyst. The reaction is initiated by introducing AB into the
reaction flask containing the as-synthesized Rh/CC3-R-homo
catalyst with vigorous shaking at room temperature (298 K). H2
generated from the methanolysis of AB is collected in the buret,
with which the H2 volume is monitored. Figure 2 shows the H2
dichloromethane and methanol mixture (volume ratio = 2:1) was
added to a flask containing dried CC3-R (80 mg) to give a
colorless solution (Figure S3). The color of solution became
yellow upon adding the rhodium acetate. After aging for 20 min,
the mixture was subsequently reduced with a methanol solution
of sodium borohydride (9 mg, rt). The solution immediately
changed color from yellow to dark brown without any
precipitates, indicating efficient Rh(II) reduction and further
stabilization of the resulting Rh NPs by CC3-R. The conversion
of Rh(II) to Rh(0) can be followed in the UV−vis absorption
spectra taken during the reaction (Figure S4). The disappearance
of adsorption band of Rh(II) at 361 nm upon addition of NaBH4
into the solution indicates that the metal ions are reduced
completely in the present system.10
The Rh NP diameter and size distribution were analyzed using
transmission electron microscopy (TEM). The sample was
prepared using a solution of Rh/CC3-R-homo catalyst with 10-
fold dilution. The solution was drop-cast onto carbon-coated
copper grids and allowed to dry before the measurements. The
TEM image of Rh/CC3-R-homo shows the formation of well-
dispersed Rh NPs (Figure 1a). The morphology of Rh NPs was
further revealed by high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) images
(Figure 1b,c). The average diameter of Rh NPs is 1.1 0.2
nm, and about ∼9% of Rh NPs have particle sizes smaller than
the estimated cage internal cavity size (0.72 nm), indicating that
most of the particles are anchored on the surface of CC3-R.
Energy-dispersive X-ray spectroscopy (EDS) elemental mapping
of Rh/CC3-R-homo shows the well-dispersed Rh NPs stabilized
by CC3-R (Figure S5). X-ray photoelectron spectroscopy (XPS)
analysis also identifies the formation of Rh NPs with binding
energies at 307.5 and 312.2 eV (Figure S6), corresponding to Rh
3d5/2 and Rh 3d3/2 of metallic Rh, respectively.
Figure 2. Time course plots of H2 generation for the methanolysis of AB
over the (a) Rh/CC3-R-homo and (b) Rh/CC3-R-hetero catalysts at 298
K (Rh/AB = 0.0199). Inset: the corresponding TOF values of the
catalysts.
generation from AB in the presence of Rh/CC3-R-homo catalyst.
The hydrogen release rate is highly dependent on the content of
Rh (Figure S7). Under our evaluation conditions, the Rh/CC3-
R-homo catalyst with Rh loading of 2.4 wt % shows the highest
catalytic activity, with which the reaction can be completed (H2/
AB = 3.0) within 0.7 min (Rh/AB = 0.0199) at 298 K (Figure 2a),
giving a TOF value as high as 215.3 mol of H2 per mol of Rh per
Despite the ultrasmall size, Rh/CC3-R-homo catalyst shows
excellent stability. It is stable in solution, with no evidence of
agglomeration, and without noticeable color change, over
periods of days. More importantly, it can be dried under N2
flow atmosphere and then redissolved in a solution of
dichloromethane−methanol mixture (volume ratio = 2:1) with
B
DOI: 10.1021/jacs.5b04029
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Journal of the American Chemical Society
Communication
min, which, notably, is the highest value among the catalysts ever
reported for methanolysis of AB reaction.13 Such extremely
excellent catalytic performance demonstrates that the present
homogenized heterogeneous catalyst is effective for liquid-phase
catalytic reaction.
solution by NaBH4 to produce 4-aminophenol (4-AMPh) as
another model reaction. The conversion of 4-NPh can be readily
monitored by UV−vis spectra, recording the change of
characteristic absorbance at λ = 400 nm in alkaline solution. As
shown in Figure 3a, the Rh/CC3-R-homo catalyst gives complete
It is reasonable that such unexpected high catalytic activity is
attributed to the ultrasmall Rh NPs with highly accessible
catalytic sites in solution, which is favorable for the direct contact
between the reactants and Rh NPs. To further demonstrate our
hypothesis, the corresponding heterogeneous catalyst Rh/CC3-
R-hetero in a form of solid toward AB methanolysis is compared.
The powder sample of Rh/CC3-R-hetero is obtained by direct
drying freshly prepared Rh/CC3-R-homo catalyst in N2 flow
atmosphere, and then the resultant solid is mixed with methanol
solution for the use of AB methanolysis reaction. It is notable that
the Rh/CC3-R-hetero catalyst is insoluble in methanol, which
provides the heterogeneous condition for AB methanolysis
reaction. As shown in Figure 2b, the catalytic reaction by the Rh/
CC3-R-hetero catalyst is completed (H2/AB = 3.0) within 2.3
min (Rh/AB = 0.0199) at room temperature (298 K), giving a
TOF value of 65.5 mol of H2 per mol of Rh per min (Figure 2b).
Such a kinetic rate is much lower than that of Rh/CC3-R-homo
catalyst. It is notable that the catalyst obtained by redissolving the
Rh/CC3-R-hetero in pure dichloromethane solution is inactive
toward H2 release reaction (Figure S8). These experiments
unequivocally demonstrate the role of soluble cage in promoting
the kinetics of liquid-phase catalytic reaction. To further
demonstrate the importance of the CC3-R stabilizer/homoge-
nizer in liquid-phase catalysis, the control experiment by using
support-free catalyst (Rh-SP-Free) for AB methanolysis under
the condition similar to that of Rh/CC3-R-homo catalyst is
performed. Figure S9 shows that the completion of reaction
needs as long as 35 min (Rh/AB = 0.0199), with a TOF of 4.3
mol of H2 per mol of Rh per min, which is only 1/50 of Rh/CC3-
R-homo catalyst. Actually, the obtained Rh-SP-Free catalyst
shows serious aggregations through reduction of metal precursor
by NaBH4 without CC3-R (Figure S10). We have also performed
the catalytic reaction based on the conventional surfactant
polyvinylpyrrolidone (PVP) protected Rh NP catalyst (Rh-
PVP), which also gives a poor catalytic kinetic with a TOF of 15.1
mol of H2 per mol of Rh per min (Figure S11). These control
experiments indicate that the soluble cage can successfully serve
as not only stabilizers but also efficient dispersing agents for the
synthesis of MNPs in solution. Moreover, the catalytic activity of
soluble catalyst is much higher than its heterogeneous counter-
part toward the same liquid-phase catalytic reaction, which
highlights the advantage of using such strategy in improving the
catalytic performance. This is understandable because the cage
features intrinsic characteristics of high solubility and dispersi-
bility in certain solvents, which make them act as stabilizing/
homogenizing agents for the MNPs to prevent them from
aggregation. In addition, the functional group (CN) can
efficiently anchor MNPs and thus control their size and
distribution on the cage during the synthetic process. Moreover,
the absence of long alkyl chains on the cages provides the MNPs
with more available active sites. Especially, the open skeleton of
cage provides much higher accessibility than the conventional
surfactant for reactants to the active sites. Such characteristics in
principle could boost the kinetics of the catalytic process.
The present strategy can be extended to other liquid-phase
catalytic reactions with enhanced catalytic performance. Here,
the catalytic performance of catalyst is investigated by employing
the catalytic reaction reduction of 4-nitrophenol (4-NPh) in the
Figure 3. UV−vis spectra showing gradual reduction of 4-NPh over (a)
Rh/CC3-R-homo and (b) Rh/CC3-R-hetero catalysts at room temper-
ature. Inset: the corresponding slope of the plot ln(At/A0) vs time
(min).
conversion in 3.5 min (Rh/4-NPh/NaBH4 (mol/mol/mol) = 1/
21/16596). A linear correlation of ln(At/A0) versus time (At and
A0 represent the absorbance at the intervals and the initial stage
of 4-nitrophenolate ion, respectively) is obtained (Figure 3a
inset), and the pseudo-first-order rate constant (k) is estimated
to be 2.5 × 10−2 s−1. Such catalytic activity is superior to that of
Rh-based catalysts under ambient conditions.14 In contrast, the
corresponding heterogeneous catalyst Rh/CC3-R-hetero gives a
lower catalytic activity under similar condition as inferred from
Figure 3b. The estimated rate constant k value is 0.75 × 10−2 s−1.
Such enhanced catalytic activity further demonstrates that the
present strategy could be generalized to other liquid-phase
reactions with enhanced the catalytic activities.
Durability and recyclability of MNPs are important parameters
for potential applications of the catalysts. Typically, MNP-based
catalysts suffer from both low separation efficiency from reaction
system, owing to their overall small size, and reduced catalytic
activity caused by coagulation of MNPs during reactions. The
present Rh/CC3-R-homo catalyst can overcome these chal-
lenges. We have chosen AB methanolysis reaction to check the
durability and recyclability of Rh/CC3-R-homo catalyst. The
durability of Rh/CC3-R-homo catalyst is examined by
successively adding AB into the reactor after completion of the
previous run, and no significant loss in activity and selectivity is
observed over 5 cycles (Figure S15). After catalytic experiments,
we analyze Rh/CC3-R-homo catalyst by STEM-HAADF
measurements, which clearly reveal that the size and morphology
of Rh NPs remain the same even after catalytic reaction (Figure
S16). In fact, after completion of the reaction, the catalyst could
be recovered by drying in a N2 flow atmosphere and washing with
water and methanol (Figure S3). The PXRD measurement
reveals that the crystalline characteristic of CC3-R is preserved,
and no noticeable peaks from Rh NPs are detected, indicating the
high stability of catalysts after catalytic reaction (Figure S1).
Moreover, the dried sample can redissolve in the dichloro-
methane−methanol mixture without precipitation (Figure S3),
which can be further used for catalytic reaction (Figure S17).
Therefore, the present Rh/CC3-R-homo catalyst possesses the
advantages of both homogeneous and heterogeneous catalysts
with high dispersibility and stability and excellent recyclability,
which are essential for its superior catalytic performance.
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Communication
(4) (a) Coper
́
et, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J. M.
In conclusion, for the first time, a facile method to homogenize
heterogeneous Rh NP catalyst has been developed based on the
immobilization to soluble porous organic cage CC3-R. The
soluble cage molecule with an intrinsic open channel provides
unique properties as a versatile platform to immobilize MNPs
and exhibits an unexpected talent for controlling of MNPs in an
ultrasmall size, preventing them from aggregation and providing
high stability and dispersibility in solution. Illustrating the
application of this method, Rh/CC3-R-homo catalyst shows
superior catalytic performances toward the AB methanolysis and
4-NPh reduction reactions. The activities are much higher than
those of the heterogeneous counterpart. Moreover, the Rh/CC3-
R-homo catalyst exhibits excellent durability and reusability.
Considering the facile process of catalyst preparation, the present
strategy could be easily expanded to prepare other kinds of
MNPs for various liquid-phase catalytic applications by using a
soluble cage stabilizer/homogenizer. Moreover, the present work
may bring new inspiration to the catalysis field in terms of the
exploration of advanced catalysts that combine the merits of both
homo- and heterogeneous catalysts.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, sorption isotherms, PXRD patterns, XPS,
UV−vis spectra, catalytic reaction data, and TEM images for Rh
NPs. The Supporting Information is available free of charge on
the ACS Publications website at DOI: 10.1021/jacs.5b04029.
(12) (a) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan,
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Blacquiere, J. M.; Baker, R. T. J. Am. Chem. Soc. 2007, 129, 1844.
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AUTHOR INFORMATION
Corresponding Author
*q.xu@aist.go.jp
Notes
■
The authors declare no competing financial interest.
(13) (a) Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46,
ACKNOWLEDGMENTS
The authors thank AIST for financial support. J.K.S. thanks JSPS
for a postdoctoral fellowship.
■
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DOI: 10.1021/jacs.5b04029
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