Tailoring the window sizes to control the local concentration and activity of (salen)Co catalysts in plugged nanochannels of SBA-15 materials

Article abstract of DOI:10.1002/anie.201304640

Ship shape! Chiral (salen)Co^III complexes (spheres) inside plugged nanochannels of SBA-15 materials is achieved using a ship-in-a-bottle synthesis technique. The local concentration of the metal complexes and the catalytic activity (such as the hydrolytic kinetic resolution of 1,2-epoxyalkanes; see scheme) showed a strong dependence on the size of the window. Copyright

Full text of DOI:10.1002/anie.201304640

Angewandte
Chemie
DOI: 10.1002/anie.201304640
Heterogeneous Catalysis
Tailoring the Window Sizes to Control the Local Concentration and
Activity of (salen)Co Catalysts in Plugged Nanochannels of SBA-15
Materials**
Mozaffar Shakeri, Robertus J. M. Klein Gebbink, Petra E. de Jongh, and Krijn P. de Jong*
Organometallic complexes with excellent activity and selec-
tivity have found many applications in homogeneous catalysis
for the transformation of organic molecules. The use of chiral
metal complexes in particular has facilitated the economically
viable synthesis of enantiopure organic compounds with
a global market of 5.3 billion US$ in 2011 with an annual
growth rate of 6.5%.[1] Among these, chiral (salen)metal
complexes catalyze many asymmetric transformations of, for
example, the hydrolysis of terminal epoxides through hydro-
lytic kinetic resolution (HKR) into single enantiomers.[2–5]
The reaction kinetics are second order in catalyst concen-
tration through the close proximity of two metal complexes in
the activation of the substrates, which resembles the active
site of an enzyme (Supporting Information, Scheme S1).[6–9]
Increased local concentration of these metal complexes by
confinement would be highly desirable to enhance their
performance. The “ship-in-a-bottle synthesis” of metal com-
pounds in the micropore structure of zeolites by diffusing
small precursors and building up complexes larger than the
window sizes of the pore turned out to be a promising
approach to immobilize these complexes.[10–13] The resulting
catalysts display the beneficial properties of homogeneous
catalysts, namely high selectivity and activity, and of hetero-
geneous catalysts through facile recovery and reuse. How-
ever, the small pore sizes (less than 1 nm) of zeolites limit the
scope of applications of ship-in-a-bottle catalysts.
smaller windows have been used to encapsulate either pre-
synthesized (salen)Co complexes followed by silylation[17,18]
or pre-synthesized ligands.[19] The catalysts obtained using the
former approach were superior to the homogeneous catalyst
in terms of activity and selectivity,[17] while those obtained
through the latter approach showed lower activity compared
to the homogeneous catalyst.[19] A main challenge of host–
guest catalysis is the decrease in activity owing to the limits of
mass transfer relating to the small window sizes (Figure 1).
Therefore, tailoring the window sizes might enhance the
performance of catalysts confined within nanoporous sup-
ports.
Inspired by these results we searched for new nanoporous
supports and related the performance of confined metal
complexes with the nanocavity accessibility. Open-ended
(1D) nanochannel materials, for example SBA-15, have been
used widely in immobilization, but their application to ship-
Silica-based nanoporous (2–20 nm) materials, synthesized
using supramolecular templates, are better alternatives for
catalysis because of their larger pore sizes and various pore
geometries.[14–16] Among others, SBA-16 and FDU-12 with
(3D) cage-like pore structures in which each cage is inter-
connected to 8 or 12 neighboring pores, respectively, by
[*] Dr. M. Shakeri, Dr. P. E. de Jongh, Prof. Dr. K. P. de Jong
Inorganic Chemistry and Catalysis
Debye Institute for Nanomaterials Science, Utrecht University
Universiteitsweg 99, 3584 CG Utrecht (The Netherlands)
E-mail: k.p.dejong@uu.nl
Prof. Dr. R. J. M. Klein Gebbink
Organic Chemistry and Catalysis
Debye Institute for Nanomaterials Science
Utrecht University (The Netherlands)
[**] This work is supported financially by the Dutch National Research
School Combination Catalysis (NRSCC). The TEM study was done
by Hans Meeldijk. Dr. Gonzalo Prieto and Rafael de Lima Olivera
were involved in physisorption. salen=(S,S)-(+)-N,N-bis(3,5-di-
tert-butylsalicylidene)-1,2-cyclohexanediamine.
Figure 1. The high-angle annular dark-field (HAADF) image of the
plugged nanochannels of SBA-15 after Pt staining (top). Scheme of
a plugged nanochannel in which pore and window sizes are controlled
by the time and temperature of the synthesis (bottom).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304640.
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Table 1: Characteristics of chiral (salen)CoIII in (plugged) nanochannels
of SBA-15 materials.
in-a-bottle synthesis is still not possible. Van der Voort and
co-workers, reported the synthesis of a new type of SBA-15,
so-called plugged SBA-15, that combines the features of
conventional SBA-15 and short nanochannels aligned
through two smaller windows[20,21] and synthesis of these
materials was later modified by other research groups.[22,23]
The plug formation in SBA-15 materials has been studied by
X-ray diffraction and gas physisorption. The plugged SBA-15
materials possess low pore volume, but enhanced wall thick-
ness and thermal and mechanical stability compared to
conventional SBA-15. We recently reported a reproducible
procedure for the synthesis of plugged SBA-15 materials
which enables manipulation of the window (2–5 nm) and pore
(4–7 nm) sizes with sub-nanometer precision by carefully
varying the time and/or temperature of the synthesis.[24]
Herein, we report TEM studies and assess the suitability
of (1D) plugged nanochannels of various SBA-15 materials
with well-defined window sizes to host (S,S)-(+)-N,N-bis(3,5-
di-tert-butylsalicylidene)-1,2-cyclohexanediamino cobalt(III)
(chiral (salen)CoIII) complexes through ship-in-a-bottle syn-
thesis from external precursors for the first time. The chiral
(salen)CoIII complex in plugged nanochannels of SBA-15
materials were used in the HKR of terminal epoxides. We
were able to control the (local) catalyst concentration and
activity of the confined metal complexes in plugged nano-
channels of SBA-15 materials by changing the size of the
window. The results showed that the ship-in-a-bottle synthesis
from external precursors with control over the window sizes
was a key factor for maximal activity of the confined metal
complexes in the host materials.
Host material
Catalyst Pore size Window size Loading
N[b]
[nm]
[nm]
[mggÀ1 [a]
]
plugged SBA-15
1
2
3
4
5
4.0
4.4
4.2
4.6
6.3
1.0
1.6
1.8
2.2
6.3
16.0
18.0
15.0
9.0
5.9
5.8
4.8
2.7
0
SBA-15
0.0
[a] Loading indicates the amount of chiral (salen)CoIII in mg per gram of
modified (plugged) SBA-15 material. [b] N is defined as the number of
chiral (salen)CoIII complexes per 100 nm3 of modified (plugged) SBA-15
pore volume.
troscopy, UV/Vis spectroscopy, elemental analysis, and N2
physisorption. The FTIR spectrum of catalyst 4 showed
À1
=
a strong and characteristic C N vibration at 1624 cm
,
indicating the presence of the salen ligands in the plugged
nanochannels of SBA-15 (Figure S9).[4] UV/Vis spectra of
catalysts 3 and 4 suggested the formation of the (salen)Co
complex because of the absence of a peak at 334 nm (n-p*)
and the presence of a strong peak at 410 nm (d-p*)
corresponding to cobalt complexation with salen ligands
(Figure S10). The N2 physisorption of catalysts 1–5 showed
that (plugged) SBA-15 materials maintained their original
features (Figure S4–S8) and were accessible enough to
facilitate mass transfer during catalysis. These results indi-
cated the presence of an intact chiral (salen)CoIII complex
inside plugged nanochannels of SBA-15 materials.
Plugged SBA-15 materials with tailored pore and window
sizes and a sample of conventional SBA-15 were synthesized
by changing the time and/or temperature of the hydrothermal
treatment.[24,25] The structural properties of (plugged) SBA-15
materials obtained from N2 isotherms (Figure S1) are given in
Table S1. The (plugged) SBA-15 materials after staining with
Pt were characterized by transmission electron microscopy
(TEM; Figure 1; Figure S3). The results for the first time
clearly show that plugged SBA-15 consists of aligned short
nanochannels (the length is a few times the diameter)
connected through smaller windows making them suitable
for the ship-in-a-bottle synthesis of metal complexes, while
conventional SBA-15 consists of long open nanochannels.
The inner surfaces of (plugged) SBA-15 materials were
modified by n-propyltrimethoxysilane to cap the silanol
groups and adjust the size of the windows (Figures S4–S8
and Table S2).[24] The structural properties of the modified
(plugged) SBA-15 including the calculated window sizes are
given in Table 1. The chiral (salen)CoIII complexes were built
up using the ship-in-a-bottle synthesis inside the modified
(plugged) nanochannels of SBA-15 from the external pre-
cursors under mild reaction conditions, which was followed by
thorough washing to remove physically adsorbed complexes
(Scheme S2).[11,12] Chiral (salen)CoIII catalysts in four differ-
ent plugged nanochannels of SBA-15 are designated as
catalysts 1–4 and that in conventional SBA-15 as catalyst 5
(Table 1).
Tailoring the window sizes influences the (local) concen-
tration of the metal complexes in the nanocavities by
controlling their diffusion during synthesis and their retaining
power during washing steps. Loadings of 16.0, 18.0, 15.0, and
9.0 mg of the chiral (salen)CoIII per gram of support in
catalyst 1–4 were obtained from the elemental analysis
(Table 1), respectively. For catalysts 2–5, the trend shows
that a smaller window size increases the ability of the cavity to
retain the complex during washing. The low loading of
complex with catalyst 1 might be caused by limitations of
diffusion during the synthesis, as the window size is only
1.0 nm. The local concentration of the active sites (N; number
of chiral (salen)CoIII complexes per 100 nm3) were 5.9, 5.8,
4.8, and 2.7 for the window sizes of 1.0, 1.6, 1.8, and 2.2 nm,
respectively (Table 1). This indicates an average number of
ten chiral (salen)CoIII molecules per nanocavity for catalyst 3.
The obtained catalysts were used in the HKR of 1,2-
epoxyhexane and all of them were active without the need of
pre-activation. In contrast, the catalyst synthesized through
encapsulation of (salen)Co complexes by Li and co-workers
initially was inactive and was reactivated by the HKR of
a batch of propylene oxide.[17] Catalysts 1–4 showed much
higher activity than the homogeneous catalyst for the same
catalyst loading (0.015 mol.%) suggesting the synergy of
bimetallic cooperation inside the plugged nanochannels by
increasing the local concentration of the active sites from 0.8
to 2.7–5.9 (Table S3; entries 1–4 versus entry 6).
The formation of chiral (salen)CoIII inside the modified
plugged SBA-15 materials was investigated by FTIR spec-
The activity of the catalysts changed significantly by
varying the window sizes and N (Figure 2). The activity
2
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HKR with more aliphatic terminal epoxides with longer
and shorter alkyl chains under the same reaction conditions
and recyclability of the catalysts were conducted. The size of
the substrate and hydrophobic interaction of the substrate
with its host influence diffusion inside the cavity and
consequently the reaction rate.[26] Catalyst 3 resolved the
terminal epoxide of 1,2-epoxydecane with a very long alkyl
chain and higher viscosity in the absence of any solvent at
similar reaction rates for 1,2-epoxyhexane and higher than
that of 1,2-epoxypropane (Table S5). Catalyst 3 could be
recycled at least four times, operating for a total of 200 hours
while maintaining its activity and enantioselectivity (Fig-
ure S11). The isolated reaction mixture from the first batch
was supplied with a new batch of substrate and the conversion
was followed over time. No additional conversion was
observed after 48 hours, which together with the stable
performance of the catalyst over the runs confirms the
absence of leaching. Consequently, these results showed that
the obtained catalysts meet the criteria of a stable heteroge-
neous catalyst, while accepting various substrates.
We presented for the first time the use of one-dimensional
plugged nanochannels of SBA-15 materials to host metal
complexes using ship-in-a-bottle synthesis. The metal com-
plex loading in the plugged nanochannels of SBA-15 depends
on the window size which controls the synthesis and protects
against washing. The activity is strongly affected by the size of
the window, increasing with the window size provided that
bimetallic cooperation is still effective. The ship-in-a-bottle
synthesis of chiral (salen)CoIII complexes from external
precursors with tailored window sizes resulted in a large
enhancement of activity compared to the homogeneous
catalyst. Our finding for plugged SBA-15 materials, that
a simple change of hydrothermal treatment conditions allows
tailoring of the window sizes for the nanocavities for
maximum performance of the confined catalysts, might be
applicable in other fields of host–guest chemistry.
Figure 2. Percent conversion of 1,2-epoxyhexane and N (number of
chiral (salen)CoIII complexes per 100 nm3) versus window size by
~
&
^
*
^
catalyst 1 ( ), 2 ( ), 3 ( ), 4 ( ), or 5 ( ). Reactions conditions:
catalyst loading of 0.015 mol% (percent molar ratio of confined chiral
(salen)CoIII relative to 1,2-epoxyhexane), 0.75 equiv H2O (relative to
1,2-epoxyhexane), room temperature, and reaction time of 48 h. The
scheme shows the HKR of terminal epoxides using a confined chiral
(salen)CoIII (spheres) inside the plugged nanochannel of SBA-15. The
maximum theoretical conversion is 50%. For further details see
Supporting Information, Table S3.
initially increased with increased window sizes and reached
a maximum at 1.8 nm when N was still high enough and then
decreased sharply with further increases in the size of the
window because of the much lower N that was achieved
(Figure 2; Table S3). Catalyst 2 with a larger window size than
catalyst 1 (1.6 nm versus 1.0 nm), despite a similar N value,
showed a twofold increase in conversion while both provided
high enantioselectivity (> 99%). Catalyst 3 (only 0.2 nm
larger window size than 2) had a lower N but showed higher
activity, possibly arising from faster mass transfer. Catalyst 4
with a still larger window size (2.2 nm) but N being half that of
catalysts 2 and 3, gave about fourfold lower conversion,
confirming that the reaction rate follows a second-order
dependency on the local concentration of catalyst.[6] Catalyst
5 showed no conversion owing to a low loading of chiral
(salen)CoIII complexes. Consequently, the activity of the
catalysts showed a strong dependence on the size of the
window and increased with the window size, provided that the
bimetallic cooperation effect is satisfied by N ꢀ 5.
Experimental Section
The synthesis of chiral (salen)CoIII complexes in plugged nano-
channels of SBA-15 materials was conducted using ship-in-a-bottle
synthesis. The inner surfaces of (plugged) SBA-15 materials were first
modified with n-propyltrimethoxysilane to cap the silanol groups and
vary the size of the windows. The ship-in-a-bottle synthesis inside the
modified (plugged) SBA-15 materials was conducted by diffusing the
precursors one-by-one under mild reaction conditions, oxidating the
chiral (salen)CoII by acetic acid in toluene (1:9) under air, and
washing the catalysts thoroughly with toluene, methanol, and
dichloromethane then drying under vacuum. Catalysts 1–5 (see
Table 1) then were used in the hydrolytic kinetic resolution of the
terminal epoxides.
We then compared the turnover frequency (TOF) of
chiral (salen)CoIII in plugged SBA-15 and that in SBA-16,
relative to the homogeneous catalyst in the corresponding
study.[17–19] The TOF of chiral (salen)CoIII in SBA-16 obtained
through encapsulation of metal complexes was sevenfold
higher than the homogeneous counterpart (Table S4,
entry 1).[17] However, the TOF of catalyst 3 in this study was
35-fold higher than the homogeneous counterpart (Table S4,
entry 2). Clearly, the ship-in-a-bottle synthesis from external
precursors with tailoring of the window sizes used here is
a more efficient strategy for achieving high activity of the
confined metal complexes.
Received: May 29, 2013
Revised: July 8, 2013
Published online: && &&, &&&&
Keywords: heterogeneous catalysis · heterometallic complexes ·
.
porous materials · ship-in-a-bottle synthesis · window size
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Communications
Heterogeneous Catalysis
M. Shakeri, R. J. M. Klein Gebbink,
P. E. de Jongh,
K. P. de Jong*
&&&& — &&&&
Tailoring the Window Sizes to Control the
Local Concentration and Activity of
(salen)Co Catalysts in Plugged
Ship shape! Chiral (salen)CoIII complexes
(spheres) inside plugged nanochannels
of SBA-15 materials is achieved using
a ship-in-a-bottle synthesis technique.
The local concentration of the metal
complexes and the catalytic activity (such
as the hydrolytic kinetic resolution of 1,2-
epoxyalkanes; see scheme) showed
a strong dependence on the size of the
window.
Nanochannels of SBA-15 Materials
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