Hydrophobic core-hydrophilic shell-structured catalysts: A general strategy for improving the reaction rate in water

Article abstract of DOI:10.1039/c2cc36273b

A novel solid catalyst featuring a hydrophobic core-hydrophilic shell structure was synthesized for aqueous-phase reactions, which showed significant reaction rate enhancement effects.

Full text of DOI:10.1039/c2cc36273b

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COMMUNICATION
Hydrophobic core–hydrophilic shell-structured catalysts: a general
strategy for improving the reaction rate in waterw
Hengquan Yang,* Xuan Jiao and Shuru Li
Received 29th August 2012, Accepted 29th September 2012
DOI: 10.1039/c2cc36273b
A novel solid catalyst featuring a hydrophobic core–hydrophilic
shell structure was synthesized for aqueous-phase reactions,
which showed significant reaction rate enhancement effects.
core would drive the adsorption of organic reactants from
water. The kinetic barriers caused by the access of organic
substrates to catalytically active sites would be thus alleviated.
6
Our synthesis is inspired by the delayed condensation strategy.
1
Long-standing interest exists in using water as reaction medium.
This strategy allows for preferential locations of the molecular
functionalities in the inner core or on the outer shell through a
However, most catalytic reactions of organic compounds in water
proceed sluggishly because of their low solubility in water, which
leads to incompatibility of a reaction system and severe kinetic
barrier related to the access of the reactant molecules to solid
3 2 2 3
stepwise addition of siliceous precursors. (MeO) SiCH CH CF in
combination with tetramethyl orthosilicate (TMOS) was used to
construct a hydrophobic core in the presence of cetyltrimethyl-
2
catalysts. Although addition of co-solvents can alleviate the
ammonium chloride (CTAC). (MeO)
together with TMOS was used to grow a hydrophilic shell
around the core because the resultant –NH and Si–OH were
3 2 2 2 2
SiCH CH CH NH
kinetic barrier, introduction of auxiliary additives violates green
chemistry principles. To address these obstacles, researchers
attempted to design ‘‘sophisticated’’ catalysts to promote reaction
in pure water. For example, Uozumi’s group and other groups
prepared amphiphilic polymer catalysts that exhibited high activity
2
hydrophilic in nature. After the hydrolysis and condensation, the
resultant materials were extracted with hot alcohol to remove one
portion of CTAC. The other portion of templates still resided in the
nanopores, which was proven to be helpful for stabilizing metal
nanoparticles, to positively affect adsorption, and to improve the
3
in pure water. Crossley and co-workers developed Janus-type
catalysts that exhibited high activity in an aqueous biphasic
4
system. Although these encouraging advances have been made,
7
stability. The resultant material was denoted MF@MN. Pd
the general strategy to design efficient solid catalysts for the
reactions in pure water is still highly desired in view of the fact
that only a few organic reactions in industry are carried out in pure
nanoparticles were loaded onto MF@MN, leading to a catalyst
indexed as Pd/MF@MN. For comparison, we also synthesized a
set of counterpart catalysts: reversed core–shell-structured catalyst
Pd/MN@MF (amino-functionalized core–fluoro-functionalized
shell), Pd/MF/MN (amino–fluoro-bifunctionalized mesoporous
nanospheres synthesized through one step of condensation, see
ESIw), and Pd/MN (amino-monofunctionalized mesoporous silica
nanospheres). The Pd loadings on these catalysts were kept at
1.70 wt%. In parallel, we loaded Pt nanoparticles onto these
materials, and achieved Pt-supported catalysts: Pt/MF@MN,
Pt/MN@MF, Pt/MF/MN and Pt/MN (Pt loadings were kept at
2.46 wt%).
1d
water due to the lack of powerful catalysts.
Micelles can be used as an efficient catalytic system in water
because the outer periphery of the surfactant assembly is
hydrophilic, ensuring good ‘‘dispersion’’ in water and high
reaction interface, and the inner core is hydrophobic, favorably
5
‘trapping’’ the organic reactants from water. The increased
‘
reaction interfaces and enriching substrate effects lead to
improvement in the reaction rate.
We herein propose a general model for designing ‘‘micelle-like’’
solid catalysts toward the aqueous-phase reactions. As shown in
Fig. 1, the catalyst consists of monodispersed mesoporous silica
nanospheres. Each nanosphere contains uniformly distributed
catalytically active centers, featuring a hydrophobic core and a
hydrophilic shell. The core and shell domains are well connected
through nanopores. The hydrophilic shell would ensure good
dispersion of the solid catalyst in water, while the hydrophobic
MF@MN consists of uniform microspheres with diameters
of about 200–300 nm (Fig. 2a). The radically arrayed pores
School of Chemistry and Chemical Engineering, Shanxi University,
Wucheng Road 92, Taiyuan 030006, China.
E-mail: hqyang@sxu.edu.cn
w Electronic supplementary information (ESI) available: Experimental
section; elemental analysis; XRD patterns; solid NMR spectra; TG; XPS
spectra; kinetics over Pd/MFx@MN. See DOI: 10.1039/c2cc36273b
Fig. 1 Evolution of a hydrophobic core–hydrophilic shell-structured
solid catalyst from a micelle. The red spheres represent metal nanoparticles.
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Chem. Commun., 2012, 48, 11217–11219 11217

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coordination interactions. Moreover, the F, N and Pt distributions
on the surface layer of the Pt-supported catalysts are similar to
those of the Pd-supported catalysts (Fig. S8 and Table S3, ESIw).
More interestingly, these catalysts showed different dispersion
behaviours in water (Fig. 3). Pd/MN and Pd/MF@MN can be
well dispersed in water, whereas Pd/MF/MN and Pd/MN@MF
float on the surface of water. The distinct differences originate
from the different surface compositions. The surface of Pd/MN is
relatively hydrophilic because of the presence of hydrophilic –NH
2
and Si–OH on the surface. The hydrophobicity of Pd/MF/MN is
increased due to the introduction of fluoro groups. Pd/MN@MF
is more hydrophobic because the introduced fluoro-terminated
groups are distributed on the shell. Pd/MF@MN is relatively
hydrophilic in that the hydrophobic fluoro groups are
predominantly positioned in the inner core. Such a hydrophobic
core–hydrophilic shell structure makes Pd/MF@MN to be highly
dispersable in water, like a micelle.
Next, we chose hydrogenation of 4-tert-butoxystyrene in pure
water as a model reaction to evaluate the catalytic performances
since it is a typical three-phase reaction (hydrophobic organic
8
mainly dissolved in water). The
substrate, solid catalyst and H
2
hydrogenations were conducted stringently under the same reaction
conditions. We monitored the reaction by recording the H pressure
2
drop with time (Fig. 4). The slope of the pressure change (k) with
time is directly correlated with the reaction proceeding. Pd/MF/
MN exhibits a much faster reaction rate than Pd/MN, suggesting
that the hydrophobic surface can promote the catalytic reaction.
More interestingly, the reaction over Pd/MF@MN proceeded
faster than that over Pd/MF/MN, and much faster than that
over Pd/MN@MF. These results can preliminarily verify our
initial hypothesis that the hydrophobic core–hydrophilic shell
structure can efficiently promote the reaction in pure water.
In order to further verify the role of the hydrophobic core, we
assessed another set of catalysts, where the amount of amino groups
on the shell was kept constant but the amount of fluoro-terminated
Fig. 2 TEM images of (a) MF@MN, low magnified, the bar is
00 nm; (b) MF@MN, more magnified, the bar is 50 nm; (c) Pd/
2
MF@MN, the bar is 20 nm; (d) Pt/MF@MN, the bar is 20 nm.
throughout the materials (ca. 2 nm in diameter) are clearly observed
(Fig. 2b). The small angle XRD patterns of the synthesized
materials exhibit a diffraction peak at 2y = 2.51 (Fig. S1, ESIw).
Solid state NMR spectra reveal that the functional groups are
covalently incorporated on the solid materials (Fig. S2 and S3, and
FT-IR spectra in Fig. S4, ESIw). Elemental analysis and TG
analysis disclose that ca. 16–18% weight of the materials is
contributed by templates (Table S1 and Fig. S5, ESIw). Pd and
Pt nanoparticles with sizes of 2–5 nm are homogeneously
distributed on the supports. A portion of metal particles
(rod-like) was positioned inside the nanopores (Fig. 2c and d). In
the wide angle XRD patterns, the characteristic diffraction peaks of
metals are not found, also indicating the metals with a high
dispersion (Fig. S6, ESIw). The high dispersion is attributable to
the presence of templates in the nanopores (for more explanations
7a
Fig. 3 Photographs for the dispersion of the solid catalysts in water.
of the template role see Table S1, ESIw).
We employed X-ray photoelectron spectroscopy (XPS) to
investigate the elemental composition on the surface layer. It was
found that the surface F concentration of Pd/MF@MN (using Si
as a reference) was lower than that of Pd/MF/MN, and much
lower than that of Pd/MN@MF (Fig. S7 and Table S2, ESIw).
Meanwhile, the surface N concentrations are in the contrary
order: Pd/MF@MN > Pd/MF/MN > Pd/MN@MF. These
results confirm that the core of Pd/MF@MN is actually rich in
fluoro-terminated groups and its shell is rich in amino groups.
More interestingly, the Pd concentration on the surface layer of
Pd/MF@MN is slightly higher than that on Pd/MF/MN, much
higher than that on Pd/MN@MF, which is in line with the change
Fig. 4 The reaction kinetics of hydrogenation of 4-tert-butoxystyrene
over the solid catalysts in pure water. Reaction conditions: 3.2 mmol of
substrate, 8 mL of water, 0.15 MPa, 0.2 mol% Pd, fixed stirring rate.
2
tendency of the surface –NH concentration. The dependence of
the Pd distribution on the –NH distribution is probably due to
2
1
1218 Chem. Commun., 2012, 48, 11217–11219
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a
Table 1 Results of hydrogenations over Pd-supported catalysts
kept after being recycled 5 times; the mesoporous structure was
broadly maintained (Fig. S10, ESIw)), although the elemental
analysis revealed that most template molecules leached from
catalyst even beginning with the first cycle (Table S6, ESIw).
The hydrophobic core–hydrophilic shell-structured model
could be extended to other metal-supported catalysts, for example,
Pt-supported catalysts (Table S7, ESIw). The conversions over
Pt/MF@MN are much higher than those over its counterparts,
and ca. 2–4 times higher than those over Pt/MN. These
findings further highlight the versatility of the hydrophobic
core–hydrophilic shell-structured catalyst.
Conversions over different catalyst (%)
Pd/ Pd/MF/ Pd/ Pd/
Entry Substrate Product MN MN
MF@MN MN@MF
a
1
47
73
90
63
b
c
2
3
22
33
58
75
84
77
67
44
In summary, we have synthesized a novel solid catalyst featuring
a structure of hydrophobic core–hydrophilic shell, through a facile
protocol. It shows significant enhancements of the reaction rate
in pure water. The applications of this concept in preparation
of different metal-supported catalysts and various reactions
sufficiently demonstrate the versatility of this novel solid catalyst.
Our findings may lead to a general model accessible to efficient
solid catalysts for the organic reactions in pure water.
d
e
4
5
a
70
70
73
75
84
80
66
65
2
mmol, 0.1 mol% Pd (with respect to substrate), 4.5 mL of water, 25 1C,
.15 MPa, 1.5 h. 1 mmol, 0.2 mol% Pd, 2 mL of water, 40 1C, ambient
H , 1.5 h. 1mmol, 0.2 mol% Pd, 4.5 mL of water, 50 1C, 0.3 MPa, 1 h.
2
d e
.6 mmol, 0.1 mol% Pd, 4.5 mL of water, 50 1C, 0.3 MPa, 1 h. 1 mmol,
We acknowledge the Natural Science Foundation of China
b
0
(
20903064, 221173137) and Program for the Top Young
Academic Leaders of Higher Learning Institutions of Shanxi
2011002).
c
1
(
0.2 mol% Pd, 10 mL of water, 50 1C, 0.3 MPa, 1 h.
groups in the core gradually increased, namely, Pd/M@MN (pure
siliceous core and amino-functionalized shell, its synthesis is given in
ESIw), Pd/MF2.5@MN (the molar fraction of fluorosilane in the
total siliceous precursors is 2.5%), Pd/MF@MN (the molar
fraction of fluorosilane is 7.5%, as aforementioned) and
Pd/MF12.5@MN (the molar fraction of fluorosilane is 12.5%).
It is evident that the reaction rate gradually increases with the
increase in the amount of fluorosilane (Fig. S9, ESIw). These results
further confirm that the origin of the reaction rate enhancement is
the hydrophobic effects of the solid catalyst core.
Notes and references
1
2
(a) P. G. Jessop, Green Chem., 2011, 13, 1391; (b) C. J. Li and
L. Chen, Chem. Soc. Rev., 2006, 35, 68; (c) S. F. Liu and J. L. Xiao,
J. Mol. Catal. A: Chem., 2007, 270, 1; (d) P. Pollet, R. J. Hart,
C. A. Eckert and C. L. Liotta, Acc. Chem. Res., 2010, 43, 1237;
(
e) C. J. Li, Chem. Rev., 2005, 105, 3095.
(a) S. Minakata and M. Komatsu, Chem. Rev., 2009, 109, 711;
b) R. Abu-Reziq, J. Blum and D. Avnir, Chem.–Eur. J., 2004,
(
10, 958; (c) R. J. Hooley, S. M. Biros and J. Rebek Jr., Angew.
Chem., Int. Ed., 2006, 45, 3517; (d) R. A. Sheldon, Green Chem.,
2005, 7, 267; (e) R. Abu-Reziq, D. Avnir and J. Blum, Angew.
Chem., Int. Ed., 2002, 41, 4132.
We further evaluated the activity of Pd/MF@MN in a
comparative fashion. The results are summarized in Table 1.
In the hydrogenation of 4-tert-butoxystyrene, Pd/MN, Pd/MF/
MN, Pd/MF@MN and Pd/MN@MF afforded conversions of
3
(a) G. Hamasaka, T. Muto and Y. Uozumi, Angew. Chem., Int. Ed.,
2011, 50, 4876; (b) Y. Lan, M. C. Zhang, W. Q. Zhang and L. Yang,
Chem.–Eur. J., 2009, 15, 3670; (c) R. Nakao, H. Rhee and
Y. Uozumi, Org. Lett., 2005, 7, 163; (d) M. Bortenschlager,
N. Sch c¸ llhorn, A. Wittmann and R. Weberskirch, Chem.–Eur. J.,
4
3%, 73%, 90% and 63% within 1.5 h, respectively. These results
2007, 13, 520; (e) F. X. Zhu, W. Wang and H. X. Li, J. Am. Chem.
Soc., 2011, 133, 11632.
(a) S. Crossley, J. Faria, M. Shen and D. E. Resasco, Science, 2010,
are consistent with the kinetic observation that Pd/MF@MN is the
most active. In the cases of cinnamyl alcohol and 4-chloroanisole,
the conversions of these four catalysts showed the same change
tendency as that of 4-tert-butoxystyrene. The conversions
over Pd/MF@MN are 2–4 times higher than those over the
conventional Pd/MN. For somewhat water-soluble substrates,
such as 4-chlorophenol and 2,4-dichlorophenol, the results
also demonstrated the superiority of Pd/MF@MN over its
counterparts. Notably, for all the investigated substrates, the
conversions over Pd/MF@MN were all much higher than those
over Pd/M@MN (pure siliceous core and amino-functionalized
shell, Table S4, ESIw). These sufficient comparisons with the set of
counterpart catalysts can confirm that the high activity of Pd/
MF@MN benefits from the hydrophobic core–hydrophilic shell-
structured architecture. Moreover, Pd/MF@MN was more active
than a commercial Pd/C (Table S4, ESIw), demonstrating the
importance of this novel ‘‘micelle-like’’ solid catalyst. Additionally,
the activity of Pd/MF@MN was kept without significant loss
during reaction cycles (Table S5 ESIw; 91% of initial metal was
4
5
3
(a) H. Groger, O. May, H. Husken, S. Georgeon, K. Drauz and
27, 68; (b) D. J. Cole-Hamilton, Science, 2010, 327, 41.
¨
¨
K. Landfester, Angew. Chem., Int. Ed., 2006, 45, 1645; (b) J. Li,
Y. M. Zhang, D. F. Han, G. Q. Jia, J. B. Gao, L. Zhong and C. Li,
Green Chem., 2008, 10, 608; (c) K. Holmberg, Curr. Opin. Colloid
Interface Sci., 2003, 8, 187.
6
¨
(a) V. Cauda, A. Schlossbauer, J. Kecht, A. Zurner and T. Bein,
J. Am. Chem. Soc., 2009, 131, 11361; (b) V. Cauda, C. Argyo and
T. Bein, J. Mater. Chem., 2010, 20, 8693; (c) A. J. Paula,
L. A. Montoro, A. G. Souza Filho and O. L. Alves, Chem.
Commun., 2012, 48, 591; (d) T. M. Suzuki, T. Nakamura,
E. Sudo, Y. Akimoto and K. Yano, J. Catal., 2008, 258, 265.
7 (a) H. Wang, J. G. Wang, Z. R. Shen, Y. P. Liu, D. T. Ding and
T. H. Chen, J. Catal., 2010, 275, 140; (b) A. Yamaguchi,
M. M. Mekawy, Y. Chen, S. Suzuki, K. Morita and N. Teramae,
J. Phys. Chem. B, 2008, 112, 2024; (c) P. Wang, Q. H. Shi, Y. F. Shi,
K. K. Clark, G. D. Stucky and A. A. Keller, J. Am. Chem. Soc.,
2
009, 131, 182; (d) W. Otani, K. Kinbara, Q. Zhang, K. Ariga and
T. Aida, Chem.–Eur. J., 2007, 13, 1731; (e) Y. Miyake, M. Hanaeda
and M. Asada, Ind. Eng. Chem. Res., 2007, 46, 8152.
(a) K. Nuithitikula and J. M. Winterbottom, Chem. Eng. Sci., 2006,
8
61, 5944; (b) F. Joo, Acc. Chem. Res., 2002, 35, 738.
´
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11217–11219 11219