Inorganic micelles as efficient and recyclable micellar catalysts

Article abstract of DOI:10.1021/nl4045372

An "inorganic micelle" structure that has a hydrophilic cavity and hydrophobic surface has been synthesized. The inorganic micelles possess large surface area and controllable hydrophobic/hydrophilic interface. It shows high catalytic efficiency and great recyclability in the bromination of alcohols. This work suggests that inorganic micelles may be suitable for selective organic syntheses as well as industrial applications and demonstrates the value of translating nanostructure design from organic to inorganic.

Full text of DOI:10.1021/nl4045372

Letter
pubs.acs.org/NanoLett
Inorganic Micelles as Efficient and Recyclable Micellar Catalysts
†
,‡
†,§
∥
,†,§
,†,‡
Qiao Zhang, Xing-Zhong Shu, J. Matthew Lucas, F. Dean Toste,* Gabor A. Somorjai,*
,
†,‡
and A. Paul Alivisatos*
†
Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States
‡
§
Materials Sciences Division and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720,
United States
∥
Department of Mechanical Engineering, University of California, Berkeley, California 94720, United States
*
S Supporting Information
ABSTRACT: An “inorganic micelle” structure that has a hydrophilic
cavity and hydrophobic surface has been synthesized. The inorganic
micelles possess large surface area and controllable hydrophobic/
hydrophilic interface. It shows high catalytic efficiency and great
recyclability in the bromination of alcohols. This work suggests that
inorganic micelles may be suitable for selective organic syntheses as well
as industrial applications and demonstrates the value of translating
nanostructure design from organic to inorganic.
KEYWORDS: Inorganic micelle, silica, catalysis, hollow structure
otivated by the exciting possibility of obtaining catalytic
Here we present the preparation and catalytic application of
M
effects close to those observed with natural biocatalysts,
micellar catalysis that utilizes the hydrophilic/hydrophobic
an inorganic micelle structure. The structure is prepared by
utilizing the knowledge from the well-developed silica-based
1
1,12
interface of organic micelles has attracted long-standing and
core−shell nanostructures.
To achieve selective surface
1
−5
continuing attention since the 1960s. However, the practical
application of micellar catalysis has been limited by some of the
problematic features of organic micelles. For example, micelles
are extremely sensitive to the reaction conditions, such as
temperature, pH, and water/oil ratio, resulting in a very narrow
stability window, substantially limiting the range of reactions
that can be investigated. It is thus appealing to translate the
organic structure into an inorganic structure that also has the
hydrophilic/hydrophobic interface and large surface area. As
rigid structures, such solid inorganic micelles should possess
some advantageous features over their organic counterparts,
such as exceptional mechanical and thermal stability, easy
separation, and high recyclability. The difficulty in making such
an inorganic micelle is the creation of a permeable inorganic
shell with selective modification of the inner- and outer-
surfaces. In situations where the reaction between two reagents
is enhanced at the hydrophilic−hydrophobic interface, an
additional synthetic challenge arises from the need to keep the
surface properties of the permeable shell channels compatible
with that of the inner cavity to maximize hydrophilic−
hydrophobic contact area and to facilitate the transportation
of reagents. Recently, the creation of the hydrophilic−
hydrophobic interface has attracted some attention. For
example, Yang and co-workers reported the synthesis of a
solid mesoporous silica spheres consisting of a hydrophobic
modification, CTAB micelles were first explored as a “pore
blocking” agent during the surface modification process and
then removed to “open” the mesoporous channels. In this way,
the surface properties of the mesoporous channels could be
kept the same as that of the inner surface. The accessibility of
the inner nanocavity was confirmed by monitoring the
6
reduction of aqueous 4-nitrophenol using Au@SiO micelles
2
as the catalysts. We further demonstrated that the hydro-
phobicity of inorganic micelles could be conveniently adjusted.
As a result, the mass transfer of hydrophilic species was easily
tuned, which is beneficial for catalytic selectivity and not
possible in organic micelles. We also examined the application
of these inorganic micelles to model catalytic reactions, the
brominations of primary- and secondary-alcohols with HBr
solution, and found that the inorganic micelles showed superior
catalytic properties to their organic counterparts.
7
−9
Two types of inorganic micelles, including pure SiO micelles
2
and core−shell Au@SiO micelles, were successfully prepared.
2
The incorporation of metal nanoparticles within the major
cavity of the inorganic micelle not only assists with the
characterization of the micelle structures but also holds
promising potential for fabricating attractive tandem catalysts
13,14
that utilize both metal nanocatalysts and the micelle itself.
As shown in Figure 1a, the preparation process first required
10
core and a hydrophilic shell through a sol−gel approach.
However, no catalytic application of such structure has been
reported yet.
Received: December 7, 2013
Published: December 8, 2013
©
2013 American Chemical Society
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Letter
Figure 1. Synthetic protocol and TEM characterization. (a) Schematic illustration of the synthesis of inorganic micelles. (b−d) and (e−g) TEM
images shown the stepwise preparation process of silica micelles and Au@SiO micelles, respectively: (b) resin nanoparticles; (c) resin@SiO
2
2
particles; (d) hollow SiO micelles; (e) Au@resin nanoparticles; (f) Au@resin@SiO particles; and (g) Au@ SiO micelles. The inset digital image in
2
2
2
(
f) shows that hydrophilic Au@resin@SiO nanoparticles could be successfully transferred from water to toluene after hydrophobic surface
2
modification.
the generation of spherical resin nanoparticles employing a
the organosilanes through an alcoholysis reaction, resulting in
the formation of a monolayer of hydrophobic alkyl chains on
the silica surface through covalent −Si−O−Si− bonds, which
has been confirmed by FTIR (Supporting Information Figure
S1). The as-obtained particles were well dispersed in most
nonpolar solvents (toluene, chloroform, and hexane) and some
less-polar solvents (acetone). As shown in the inset in Figure 1f,
the core−shell particles were well dispersed in toluene after
treatment, further confirming the successful surface modifica-
tion.
1
5,16
previously reported colloidal synthesis.
Figure 1b shows a
TEM image of a typical resin sphere sample with average
diameter of 40 nm. The Au@resin nanoparticles were prepared
through an in situ polymerization process in which Au3+ ions
were first reduced to form Au nanoparticles, followed by the
17
polymerization of resin spheres around the Au nanoparticles.
Figure 1e shows a TEM image of a sample of Au@ resin
nanoparticles with average diameter of 50 nm.
The mesoporous silica shell was subsequently obtained
through a surfactant-assisted approach, using a cationic
surfactant, CTAB (cetyltrimethylammonium bromide), as the
Because of the formation of the CTAB−silica complex
1
9,22
during the coating process,
it is believed that the existence
1
8,19
structure-directing agent.
CTAB molecules can form
of the CTAB micelle could “block” the mesoporous channel. As
a result, the inner cavity surface and the channel surface were
not modified and remained hydrophilic. This was verified by
the successful extraction of CTAB and removal of the resin
templates using hydrophilic species. For the extraction process,
the as-prepared products were refluxed in a mixture of acetone
cylindrical micelles and attach to the surface of resin spheres
due to electrostatic attractive forces. Silica coating around the
CTAB scaffold was achieved by the hydrolysis of TEOS
(
tetraethyl orthosilicate) in the presence of NH OH. The
4
successful coating of the mesoporous silica shell was confirmed
by TEM characterization, as shown in Figure 1c,f. The particle
size was increased from around 40 nm to around 100 nm,
suggesting a shell thickness of about 30 nm. Additionally, the
23
and HCl for 24 h. The extraction process was repeated twice
to fully remove CTAB and open the mesopores. After removing
the CTAB micelles, the product was well dispersed in ethanol
and partially dispersed in water, suggesting hydrophilic
mesopores had been opened. Figure 1d,g shows the typical
19
mesopores within the shell were radially ordered.
The outer surfaces of the silica shells could be modified to be
hydrophobic by taking advantage of well-developed silane
morphology of pure silica micelles and Au@SiO micelles,
2
2
0,21
chemistry.
Typically, the resin@SiO particles were dried in
respectively. By tuning the size of the original resin or Au@
resin nanoparticles, the size of the final micelles was controlled
in the range of about 50 to 170 nm, as shown in Supporting
Information Figures S2 and S3.
2
air and then transferred to a solution of 1,2-dichlorobenzene
containing n-octadecyltrimethoxy-silane (ODTMS). The sur-
face silanols were attacked by the hydrolyzable alkoxy groups of
3
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Letter
Figure 2. BET characterization and controllable mass transfer test. (a) Nitrogen adsorption/desorption isotherms for hollow silica (red) and Au@
SiO (black), respectively. The inset shows the corresponding pore size distributions; (b) Plot of ln(C /C ) as a function of reaction time t for the
2
t
0
reduction of 4-NP by NaBH in the presence of different catalysts. (From left to right) Au nanoparticles (black); hydrophilic Au@SiO core−shell
4
2
structures obtained by removing CTAB and resin template (red); and inorganic micelles obtained with different ratios of ODTMS/TEOS: 1/1
(
cyan); 2/1 (blue); and 1/0 (green). The amount of Au@SiO micelles used is 2.5 times as many of pure Au and hydrophilic Au@SiO to get results
2
2
within a reasonable time period.
The surface area and porosity of the inorganic micelles were
further investigated by nitrogen adsorption−desorption iso-
therms. As shown in Figure 2a, there is a single and sharp
adsorption step at intermediate relative partial pressure values
minutes). However, upon addition of freshly prepared Au sol, a
dark precipitate was formed, indicating the occurrence of severe
agglomeration. When hydrophilic Au@SiO core−shell struc-
2
tures were used as the catalyst, no agglomeration was observed,
around 0.3 for both hollow silica and Au@SiO , which is in
demonstrating enhanced stability. The reaction constant (k =
2
−
1
−1
good agreement with the typical type IV curves of surfactant-
0.606 min ) and the TOF (1527 h ) for the Au@SiO core−
2
24
assisted mesoporous silica. For the hollow silica micelles, the
shell structures were slightly lower than those of pure Au
−1
−1
Brunauer− Emmett−Teller (BET) surface area and single-
nanoparticles (k = 0.800 min , TOF = 2003 h ), mainly due
to the diffusion through mesoporous shell. Although the
diffusion of hydrophilic species through the inorganic micelle
structure was much slower, as shown by the reaction constant
and TOF (k = 0.108 min , TOF = 108 h ), it is clear that the
inner cavity of the inorganic micelle was accessible by
hydrophilic reactants. The accessibility of the inner cavity was
further demonstrated by a seeded-growth experiment in which
Au@Ag nanoparticles were obtained using the encapsulated Au
nanoparticles as the seeds for the deposition of silver reduced
by ascorbic acid (Supporting Information Figure S4).
2
−1
3
−1
point total pore volume are 1066 m g and 1.1 cm g ,
respectively, suggesting a highly porous structure. The average
Barret−Joyner−Halenda (BJH) pore diameter calculated from
the desorption branch of the isotherm was measured to be 4.7
nm (see inset in Figure 2a). The sharp peak indicates the
−1
−1
uniform pore size distribution. For the Au@SiO core−shell
2
2
−1
structure, the BET surface area is 979 m g . Although the
surface area is slightly decreased, the total pore volume and
pore diameter of the Au@SiO2 yolk−shell structure is
consistent with the pure hollow silica case with the values of
−1
1
.1 cm3
g
and 4.5 nm, respectively, suggesting good
We further demonstrate that the hydrophobicity of the
inorganic micelle structure could be engineered. As a result, the
diffusion rate of molecules through the shells could be
conveniently adjusted, which cannot be easily achieved in
organic micelles. The hydrophobicity control was achieved
through the coalcoholysis of ODTMS and TEOS. After
hydrolysis of TEOS, more −OH groups were introduced
onto the surface and therefore the density of long alkane chains
reproducibility for this synthetic protocol.
The permeability of the inorganic micelle is of great
importance since catalytic reactions of interest can occur only
at the hydrophilic/hydrophobic interface. The successful
preparation of Au@SiO micelles enables us to quantitatively
2
analyze their surface properties. An aqueous phase reduction of
4
-nitrophenol (4-NP) by NaBH was used as a model reaction
4
25,26
27
to characterize the inorganic micelle structure.
This
from earlier ODTMS treatment was lowered. As shown in
reduction can be considered as pseudo-first-order reaction
with regard to 4-NP, because the concentration of NaBH4
greatly exceeded that of 4-NP. In the absence of Au
nanoparticles, the reduction process did not proceed even
Supporting Information Table S1 and Figure 2b, increasing the
concentration of TEOS raised both the rate constant (k) and
−
1
−1
TOF (k = 0.152 min , TOF = 152 h for TEOS/ODTMS =
−1
−1
1:2; and k = 0.181 min , TOF = 181 h for TEOS/ODTMS
= 1:1.), suggesting higher catalytic efficiencies. This observation
is consistent with enhanced diffusion of hydrophilic reactants
through the mesoporous shell.
with a large excess of NaBH . Figure 2b shows the linear
4
relationship between ln(C /C ) and reaction time t for
t
o
reductions under various conditions and by different catalysts.
It is clear that all these plots obey first-order reaction kinetics
very well. The rate constant k was calculated from the rate
equation ln(C /C ) = kt, while the turnover frequency was
The inorganic micelles demonstrated here were also able to
act as highly efficient and recyclable catalysts for some organic
reactions. The bromination of alcohols with HBr solution has
been selected as a model system, as alkyl halides, particularly
bromides, are very useful reagents in organic chemistry,
0
t
defined as the number of moles of reduced reactant per mole
surface Au atoms per hour when the conversion has reached
28−32
90%. The performance of different catalysts is summarized and
chemical biology, and pharmaceuticals.
Most traditional
plotted in Supporting Information Table S1 and Figure 2b,
respectively. Au nanoparticles are excellent catalysts for this
reduction process, as confirmed by the rapid reduction process
using Au nanoparticles (reduction completed within several
protocols require highly toxic reagents and high temper-
33,34
ature.
With the help of inorganic micelles, the bromination
of both primary- and secondary-alcohols was achieved under
mild conditions. Dichloromethane (CH Cl ) was used as the
2
2
3
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Figure 3. Catalytic study. The yields of (a) benzyl bromide and (b) α-methylbenzyl bromide in the bromination of alcohols using HBr solution with
the assistance of different catalysts.
Figure 4. Kinetic and reusability study. (a) Plot of ln(C /C ) as a function of the reaction time for pseudo-first-order bromination of benzyl alcohol
t
0
without micelles (black) or with different amounts of micelles: 1.3 mg/mL (red), 2.7 mg/mL (green), 4.1 mg/mL (blue) and 5.5 mg/mL (cyan).
The inset is the plot of rate constant k as a function of the concentration C of micelles. (b) Conversion of benzyl alcohol in 10 successive cycles of
bromination with 5.5 mg/mL of silica micelles.
solvent (see Supporting Information Table S2 for other
solvents). As shown in Figure 3a, in the absence of any catalyst
the reaction rate for bromination of benzyl alcohol was very
low, as evidenced by the ∼16% yield of benzyl bromide after 6
suggesting a general approach for the bromination of alcohols.
A secondary alcohol, α-methylbenzene alcohol, was used as the
model molecule for this bromination process. In the presence
of inorganic micelles, the bromination process completed with
full conversion (>99%) within 15 min, which is much faster
than the case of primary alcohols. The catalytic effect of
inorganic micelles was verified by the control experiments
(Figure 3b). Although the organic micelles based on AOT and
Igepal CO-520, showed some catalytic effect, it was not very
significant with a yield of 23 and 30%, respectively. The low
yield might be attributed to the relatively slow diffusion of
reagents through the micelle structure, suggesting the mass
transport advantage of inorganic micelles.
h. When the inorganic micelles, either pure silica or Au@SiO ,
2
were used as catalysts, a dramatic increase of the yield of benzyl
bromide was detected. As shown in Figure 3a, the yield
increased to more than 95% for the inorganic micelle catalyzed
process. To investigate the influence of the inorganic micelle
structures, a series of control experiments were conducted. As
displayed in Figure 3a and Supporting Information Figure S5,
the use of SiO nanoparticles, Au nanoparticles, Au@resin
2
nanoparticles, entirely hydrophilic Au@SiO2 core−shell
structures, ODTMS-modified entirely hydrophobic Au@SiO2
core−shell structures, and ODTMS-modified entirely hydro-
To gain further insight into the catalytic effect of inorganic
micelles, the bromination of benzyl alcohol was studied in more
depth. As shown in Figure 4a and Supporting Information
Table S3, in the absence of inorganic micelles, the bromination
reaction is a pseudo-first-order reaction with respect to benzyl
alcohol as the plots fit first-order reaction kinetics. The rate
constant k calculated from the rate equation ln(C /C ) = kt is
phobic Au@resin@meso-SiO as catalysts did not show a
2
significant improvement over the uncatalyzed reaction. These
results are consistent with the requirement that a true micellar
structure is required for catalysis. To compare with organic
micelles, two types of reverse micelles composed of an anionic
surfactant, AOT (dioctyl sodium sulfosuccinate), and a
nonionic surfactant, Igepal CO-520 (polyoxyethylene (5)),
were used as catalysts. In both systems, HBr solution was
encapsulated inside the micelles first, followed by the mixing
with benzyl alcohol. The yields for using AOT micelle and
Igepal-520 micelle were approximately 50% and 65%,
respectively.
0
t
5.4 × 10− min . With the assistance of silica micelles, the
bromination process was significantly accelerated while
remaining pseudo-first-order. When the concentration of silica
micelles in the system was 2.7 mg/mL, the rate constant
4
−1
increased to 5.9 × 10− min , an order of magnitude
3
−1
enhancement. The rate constant was further increased to 1.3
−2
−1
× 10 min when the concentration of silica micelle was
increased to 5.5 mg/mL, which is a factor of 24 greater
compared to the catalyst-free solution. The inset in Figure 4a
The inorganic micelles not only catalyze the bromination of
primary alcohols but are also applicable to secondary alcohols,
3
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Nano Letters
Letter
plotted the rate constant k as a function of the concentration C
of micelles used, which fits a linear relationship very well,
suggesting that the reaction occurred at the hydrophilic/
hydrophobic interface. The reusability of inorganic micelles was
also investigated. As displayed in Figure 4b, no notable
deactivation of the catalyst was observed with up to ten cycles,
indicating excellent recyclability. Compared with organic
micelles that cannot be recovered, the inorganic micelles are
more promising in practical applications.
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ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures and additional data including
TEM images, FTIR, and catalysis data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
*
S
(
1
4, 931.
AUTHOR INFORMATION
Corresponding Authors
(F.D.T.) E-mail: fdtoste@berkeley.edu;
(G.A.S.) E-mail: somorjai@berkeley.edu.
(A.P.A.) E-mail: alivis@berkeley.edu.
(23) Fang, X. L.; Chen, C.; Liu, Z. H.; Liu, P. X.; Zheng, N. F.
■
*
*
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Nanoscale 2011, 3, 1632.
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Author Contributions
(
2
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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The authors declare no competing financial interest.
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Lett. 2005, 15, 3126.
ACKNOWLEDGMENTS
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We thank the financial support from the Dow Chemical
Company through funding for the Core−Shell Catalysis
Project, contract #20120984 to University of California,
Berkeley. J.M.L. is supported as part of the Light-Material
Interactions in Energy Conversion, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under
Contract DE-SC0001293. We are grateful to Dr. Y. Sure-
ndranath and Dr. E. Gross for helpful discussions.
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