Encapsulation of Homogeneous Catalysts in Mesoporous Materials Using Diffusion-Limited Atomic Layer Deposition

Article abstract of DOI:10.1002/anie.201712010

The heterogenization of homogeneous metal complex catalysts has attracted great attention. The encapsulation of metal complexes into nanochannels of mesoporous materials is achieved by coating metal oxides at/near the pore entrance by diffusion-limited atomic layer deposition (ALD) to produce a hollow plug. The pore size of the hollow plug is precisely controlled on the sub-nanometer scale by the number of ALD cycles to fit various metal complexes with different molecular sizes. Typically, Co or Ti complexes are successfully encapsulated into the nanochannels of SBA-15, SBA-16, and MCM-41. The encapsulated Co and Ti catalysts show excellent catalytic activity and reusability in the hydrolytic kinetic resolution of epoxides and asymmetric cyanosilylation of carbonyl compounds, respectively. This ALD-assisted encapsulation method can be extended to the encapsulation of other homogeneous catalysts into different mesoporous materials for various heterogeneous reactions.

Full text of DOI:10.1002/anie.201712010

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Accepted Article
Title: Encapsulation of Homogeneous Catalysts in Mesoporous
Materials Using Diffusion-Limited Atomic Layer Deposition
Authors: Shufang Zhang, Bin Zhang, Haojie Liang, Yequn Liu, Yan
Qiao, and Yong Qin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712010
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10.1002/anie.201712010
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Encapsulation of Homogeneous Catalysts in Mesoporous
Materials Using Diffusion-Limited Atomic Layer Deposition
Shufang Zhang,[a,b] Bin Zhang,*[a] Haojie Liang,[a,b] Yequn Liu,[a] Yan Qiao,[a] Yong Qin*[a]
Abstract: The heterogenization of homogeneous metal complex
catalysts has attracted great attention. Herein, we report the
encapsulation of metal complexes into nanochannels of mesoporous
materials by coating metal oxides at/near the pore entrance via
diffusion-limited atomic layer deposition (ALD) to produce a “hollow
plug”. The pore size of the hollow plug is precisely controlled on the
sub-nanometer scale by the number of ALD cycles to fit various
metal complexes with different molecular sizes. Typically, Co or Ti
complexes are successfully encapsulated into the nanochannels of
SBA-15, SBA-16 and MCM-41. The encapsulated Co and Ti
catalysts show excellent catalytic activity and reusability in the
hydrolytic kinetic resolution of epoxides and asymmetric
cyanosilylation of carbonyl compounds, respectively. This ALD-
assisted encapsulation method can be extended to the
encapsulation of other homogeneous catalysts into different
mesoporous materials for various heterogeneous reactions.
FDU-12) by reducing the size of the pore entrance to molecular
scale via silylation.[7] Compared to SBA-16 and FDU-12, the
cylinder-like mesoporous materials (such as SBA-15 and MCM-
41) are more superior in mass and heat transfer.[8] However, it is
a great challenge to encapsulate the metal complexes into
cylinder-like mesoporous materials by silylation, and no other
chemical method has been developed because the reagents for
encapsulation can easily diffuse into the mesopore channels in
the solution system, and the size of the silylating agents
matching the large pore entrance is limited.[4a] Therefore, it is
desirable to develop highly efficient methods to precisely modify
the large pore entrance of cylinder-like mesoporous materials on
a sub-nanometer scale to encapsulate metal complexes with
different molecular sizes.
Atomic layer deposition (ALD) is a cyclic process for the
sequential self-limiting chemical reaction between precursor
molecules and a solid surface.[9] The self-limiting nature of ALD
assures its distinctive and unique advantages in precise film
thickness control and excellent coverage, even in 3D porous
materials.[10] Recently, using ALD, our group has synthesized a
series of highly efficient catalysts with distinctive structures,[11]
which demonstrated greatly enhanced activity and stability by
optimizing the metal-oxide interface.
Chemical reactions can be confined to a nano-space by
encapsulating catalysts in nanoreactors.[1] In past decades,
researchers have focused much effort into the development of
nanoreactors for various applications in cell biology, tissue
engineering, medical diagnostics and therapies, and catalysis.[2]
Among a series of nanoreactors, porous materials provide a
uniform and stable nanochannel to immobilize catalysts.[3]
Compared to the immobilization of metal complexes in porous
materials by chemical bonds, the encapsulation of metal
complexes via physical adsorption in the pore channels is
preferred since the properties and freedom of metal complexes
are similar to those of homogeneous catalysts, which leads to
the advantages of both homogeneous and heterogeneous
catalysts.[4]
Herein, we report a highly efficient and general method to
encapsulate metal complexes into straight nanochannels by
reducing their pore sizes only at/near the pore entrance by
diffusion-limited ALD. The limited diffusion depth due to the short
diffusion time of the precursors results in a “hollow plug” at the
pore entrance (Scheme 1). Moreover, the pore size of the hollow
plug could be precisely controlled by the ALD cycle numbers.
We successfully encapsulated [(R,R)-N,N’-Bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediamine]Co(III)
(Co(salen))
The “ship in a bottle” method[5] and enclosing catalysts by
non-covalently bonding[6] are two typical strategies to
encapsulate metal complexes into ordered porous materials. For
the “ship in a bottle” method, the microporous zeolite structure
limits the molecular size and freedom of the catalysts and even
the diffusion of reactants. To solve these problems, Li and
coworkers have developed a method to encapsulate metal
complexes into cage-like mesoporous materials (SBA-16 and
and [(R,R)-N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediamine)Ti(μ-O)]2 ([(salen)Ti(μ-O)]2) complexes into
SBA-15, SBA-16, and MCM-41. The encapsulated catalysts
show excellent catalytic activity and stability in the hydrolytic
kinetic resolution (HKR) of epoxides and the asymmetric
cyanosilylation of carbonyl compounds, respectively.
Scheme 1 illustrates the process of encapsulating metal
complexes into the channels of mesoporous materials, with
SBA-15 as an example. Co(salen) was first introduced into the
nanochannels of SBA-15 with the most probable pore size of 9.2
nm (Figure S1) (denoted as Co(salen)/SBA-15) by the vacuum
impregnation method[7g] (see supporting information, Methods).
Subsequently, a TiO2 film was deposited by ALD at 45 °C into
the pores of SBA-15 using titanium tetraisopropoxide (TIP) and
H2O as precursors. The low temperature was applied to avoid
the deterioration of chirality of Co(salen). The pulse, exposure,
and purge times for TIP were 0.5, 1, 120 s, respectively, and for
H2O were 0.1, 8, 120 s, respectively. The long purge time was
applied to avoid chemical vapor deposition due to the low
deposition temperature.[12] The short total diffusion time (t = 1.5 s,
[a]
[b]
Shufang Zhang, Bin Zhang, Haojie Liang, Yequn Liu, Yan Qiao,
Yong Qin
State Key Laboratory of Coal Conversion, Institute of Coal
Chemistry, Chinese Academy of Sciences
Taiyuan 030001, PR China
E-mail: zhangbin2009@sxicc.ac.cn, qinyong@sxicc.ac.cn
Shufang Zhang, Haojie Liang
University of Chinese Academy of Sciences
Beijing 100039, PR China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201712010
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pulse plus exposure time) for TIP was used to ensure the
deposition of TiO2 only at/near the pore entrance of SBA-15. The
pore size at pore entrance was gradually reduced from the initial
9.2 nm to the molecule size of the metal complexes (~0.8 nm)[13]
by increasing the number of TiO2 ALD cycle (m). Finally, the
samples were washed in tetrahydrofuran (THF) three times to
obtain the encapsulated catalysts, denoted as mTiO2-
t/Co(salen)/SBA-15.
cross-sectional specimen prepared by focused ion beam (FIB)
milling along the axis of the SBA-15 channels. Moreover, the 13
C
cross polarization magic angle spinning nuclear magnetic
resonance spectra (13C CP/MAS NMR) show that the peaks
corresponding to the 200TiO2-1.5/Co(salen)/SBA-15 are
identical to those of Co(salen) (Figure S5). These results reveal
that Co(salen) was successfully encapsulated in SBA-15.
Scheme 1. (a) Schematic illustration of the encapsulating process of metal-
complexes into ordered mesoporous materials. (b) Increase of ALD cycle
number to reduce the pore size. (c) Increase of diffusion time of precursor to
increase the length of the hollow plugs.
Figure 1a and Figure S2 show a scanning electron
microscope (SEM) image of the original SBA-15 samples with a
channel length ranging from 2 to 50 µm. Figures 1b and 1c show
the transmission electron microscope (TEM) images of SBA-15
before and after encapsulation of Co(salen), respectively. The
darker contrast indicates that TiO2 is successfully deposited in
the channels of SBA-15 after 200 ALD cycles (200TiO2-
1.5/Co(salen)/SBA-15). The ordered pore structure of SBA-15
was maintained after TiO2 ALD, which is consistent with the
small-angle X-ray diffraction results (Figure S3). Figure 1d
shows the TEM image projected with the electron beam parallel
to the channels of SBA-15 after TiO2 ALD. A tubular TiO2 film
with darker contrast is clearly visible at the pore entrance, and
the pore size decreased to approximately 0.8 nm, which is
similar to the molecular size of Co(salen). The growth rate of
TiO2 ALD was measured by TEM analysis of the carbon
nanofibers coated with TiO2 under the same conditions (Figure
S4). According to the determined growth rate of 0.025 nm per
cycle, the reduced pore size of 8.4 nm (9.2-0.8 nm) corresponds
well to the expected filled pore size of approximately 10 nm (two
times the thickness of the deposited TiO2 film), considering that
the substrate has slight influence on the growth rate of the ALD
film. From the energy dispersive X-ray spectroscopy (EDX)
mapping (Figure 1e), the deposition depth (length of the plug) of
TiO2 was found to be approximately 1.1 µm. Further, the
deposition depth is revealed by (scanning transmission electron
microscope (STEM) image and EDX mapping (Figure 1f) of a
Figure 1. (a) SEM image and (b) TEM image of SBA-15; (c) and (d) HRTEM
images of encapsulated catalyst 200TiO2-1.5/Co(salen)/SBA-15; (e) STEM
image and EDX mapping
of encapsulated catalyst 200TiO2-
1.5/Co(salen)/SBA-15. (f) STEM image and EDX mapping of a cross-sectional
specimen of 200TiO2-1.5/SBA-15 prepared by focused ion beam milling.
In principle, when a hollow plug forms at the pore entrance of
SBA-15, the straight tubular channels are converted to “ink-
bottle” pores. The N2 adsorption-desorption isotherms (ADI) of
both SBA-15 and Co(salen)/SBA-15 are typical of type IV with
an H1 hysteresis loop, which is indicative of a cylindrical pore
structure (Figure 2a). As expected, the N2-ADI of 200TiO2-
1.5/Co(salen)/SBA-15 are typical of type IV with an H2
hysteresis loop, which is indicative of an ink bottle” pore
structure (Figure S1).[14]
performed by depositing 200 cycles of TiO2 in SBA-15 without
A control experiment was also
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10.1002/anie.201712010
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loading Co(salen) (200TiO2-1.5/SBA-15). The N2-ADI results
also present a similar H2 hysteresis loop as that of 200TiO2-
some Co(salen) molecules during the recycling. In contrast, the
200TiO2-1.5/Co(salen)/SBA-15 (0.59 wt% Co(salen)) exhibited a
1.5/Co(salen)/SBA-15 (Figure S6), confirming that the “ink-bottle” higher yield (>49%) than that of the 150TiO2-1.5/Co(salen)/SBA-
pore structure indeed resulted from the deposition of TiO2 rather
than from the loading of Co(salen).
The size of the pore entrance 200TiO2-1.5/SBA-15 was also
15 and the homogeneous reaction (using Co(salen) only, Table
1). After recycling, the yield and Co content very slightly reduced
to 46% and 0.57 wt%, respectively, revealing an excellent
stability. However, only a 26% diol yield was obtained for
220TiO2-1.5/Co(salen)/SBA-15 in spite of the stable yield after
recycling (24 wt%). According to the growth rate, 20 cycles of
TiO2 ALD (reducing 1 nm pore size) will completely close the
pores of the hollow plugs (0.8 nm). Thus, the remaining activity
should be ascribed to the original pores that had a size larger
than the most probable pore size of 9.2 nm due to the pore size
distribution of SBA-15 (Figure S1). Finally, when the cycle
number was further increased to 250, the 250TiO2-
1.5/Co(salen)/SBA-15 shows no activity, revealing that all the
pores were completely blocked (Figure S7). These results
suggest that the 200TiO2-1.5/Co-(salen)/SBA-15 shows an
optimized performance considering both the activity and stability.
This matches well with the suitable pore size of the hollow plugs
produced with 200 cycles of TiO2 ALD, as revealed by TEM
analysis (Figure 1c and 1d). For these mTiO2-
1.5/Co(salen)/SBA-15 catalysts, the enantioselectivity of all their
diol products (including the recycling reactions) are 96~98%,
similar to that of the homogeneous reaction, implying that the
molecular structure or chirality of Co(salen) is well maintained
after the encapsulation.
estimated using Co(salen) as
a
probe molecule in
a
dichloromethane (DCM) solution (Figure 2b). As expected, after
treating the Co(salen) solution with 200TiO2-1.5/SBA-15, the
content of Co(salen) in the DCM remained nearly unchanged,
indicating that the pore size at the entrance of 200TiO2-1.5/SBA-
15 was small enough to prevent the diffusion of Co(salen) into
the channels. In contrast, the content of Co(salen) in DCM after
treatment with SBA-15 dramatically decreased, suggesting that
the pore entrance of SBA-15 was large enough to allow for the
inward diffusion of Co(salen). This result further confirms that
200TiO2-1.5/SBA-15 can be used as a suitable host material to
entrap Co(salen).
Figure 2d presents the kinetic plots of the hydration of
propylene oxide over 200TiO2-1.5/Co(salen)/SBA-15 to further
confirm the successful encapsulation. The encapsulated catalyst
was filtered off after the reaction was carried out for 1 h. The
filtrate was continuously stirred for another 17 h, but the yield of
the diol was almost the same as that measured at 1 h. In
addition, the 200TiO2-1.5/SBA-15 was evaluated, and it did not
show any activity for the HKR reaction. Therefore, the
encapsulated Co(salen) is indeed responsible for the catalytic
reaction, and there is no apparent leaching of Co(salen). The
influence of the local density of Co(salen) encapsulated in
200TiO2-1.5/Co(salen)/SBA-15 was investigated to further reveal
its advantages. When the Co content increased from 0.13 wt%
to 0.65 wt%, the yield of diol increased sharply from 17% to 49%
(Table 1, Table S4-S6). Correspondingly, the turnover frequency
(TOF) increased from 48 h-1 to 186 h-1, which can be ascribed to
the enhanced synergistic effect between the Co(salen)
complexes.[7b] The enhanced synergistic effect was further
demonstrated by directly comparing the catalytic performance of
200TiO2-1.5/Co(salen)/SBA-15 with the homogeneous reaction
with Co(salen). The TOF of 200TiO2-1.5/Co(salen)/SBA-15 (up
to 186 h-1) was higher than that of homogeneous Co(salen) (131
h-1), because the local density of Co(salen) confined in the
nanochannels of SBA-15 is much higher than that in the
homogeneous system.
Figure 2. a) N2 adsorption-desorption isotherms of SBA-15, Co(salen)/SBA-15
and the encapsulated catalyst 200TiO2-1.5/Co(salen)/SBA-15. b) UV-vis
spectra of Co(salen) dissolved in dichloromethane before and after treatment
with SBA-15 and 200TiO2-1.5/SBA-15. c) Activity and reusability of mTiO2-
1.5/Co(salen)/SBA-15 produced with different TiO2 ALD cycle numbers for the
hydration of propylene oxide. d) Kinetic study of the asymmetric hydration of
propylene oxide over 200TiO2-1.5/Co(salen)/SBA-15.
The hydrolytic kinetic resolution (HKR) of epoxides was used
as a probe reaction for the encapsulated catalysts. The influence
of the cycle number (m) on the catalytic performance was
investigated for TiO2 ALD with a fixed diffusion time (1.5 s) of
TIP for mTiO2-1.5/Co(salen)/SBA-15 catalysts (m = 150, 200,
220 and 250) since the size of pore entrance gradually
decreases as the cycle numbers increase (Figure 2c, Table S1-
S3). For Co(Salen)/SBA-15 without encapsulation, the yield of
diol dropped rapidly from 44% to 6% after 3 reaction runs due to
the leaching of Co(salen), revealing a poor stability. The
150TiO2-1.5/Co(salen)/SBA-15 provided a 43% yield of diol (for
the first run). After 8 reaction runs, the yield reduced to 32%.
This is due to the insufficient ALD cycle number, resulting in a
pore size of the hollow plug larger than 0.8 nm and leaching of
Generally, the ALD process in a nanochannel is governed by
Knudsen diffusion.[9b] According to the equation mentioned by
Gordon et al.,[15] the deposition depth depends on the diffusion
time of the ALD precursors. Thus, the influence of diffusion time
on the performance of the encapsulated catalyst was also
investigated. The exposure times of 0 s and 8 s after 0.5 s pulse
time were also applied for TIP, and the corresponding catalysts
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10.1002/anie.201712010
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values (for example, 4.5 µm for 1.5 s diffusion time at 0.1 torr
TIP partial pressure),[15] This can be ascribed to the specific
deposition conditions including the deposition temperature, TIP
partial pressure, flow rate of carrying gas, reactor structure, and
so on.
Table 1. The HKR of propylene oxide on homogeneous Co(salen) and
encapsulated Co(salen) in mesoporous materials.[a]
Co
content
(wt%)[b]
Diol
enantioselectivity
(%)
Diol yield
(%)
TOF
Catalyst
(h-1)[c]
Co(salen)
—
44
97
--
131
--
200TiO2-1.5/SBA-
15
--
N.D.
0.13
0.37
0.59
0.65
17(14)[d]
39(35)[d]
>49(47)[d]
49(47)[d]
97(96)[d]
96(97)[d]
98(98)[d]
98(97)[d]
48
200TiO2-
1.5/Co(salen)/SBA-
15
125
186
179
Figure 3. Activity and reusability for the hydration of propylene oxide of mTiO2-
0.5/Co(salen)/SBA-15 (a) and mTiO2-8.5/Co(salen)/SBA-15 (b) produced with
different TiO2 ALD cycle numbers.
80TiO2-
1.5/Co(salen)/SBA-
15 (4.2 nm)
This encapsulation strategy can also be applied to other
ordered mesoporous materials and reactions due to the precise
control of the pore size at the entrance. Table 1 and Figure S10
also show the catalytic performance of Co(salen) encapsulated
in different mesoporous materials, including SBA-15 with a
smaller pore size (4.2 nm), MCM-41 (3.2 nm), and SBA-16 (4.3
nm). All the encapsulated catalysts prepared with optimized TiO2
ALD cycle numbers produced an increased yield of diol and
excellent reusability compared to the homogeneous catalysts.
Additionally, this strategy is feasible for the encapsulation of
other metal complexes by ALD with optional encapsulating
materials due to the extensive availability of various materials.
For example, [(salen)Ti(μ-O)]2 is encapsulated into SBA-15 by
depositing Al2O3 via ALD (Table 2 and Figure S11). The
encapsulated catalyst exhibited excellent activity and reusability
for asymmetric cyanosilylation of carbonyl compounds.
0.31
0.29
0.26
49(46)[d]
48(44)[d]
49(45)[d]
96(95)[d]
96(98)[d]
97(98)[d]
127
116
121
60TiO2-
1.5/Co(salen)/MCM-
41 (3.2 nm)
80TiO2-
1.5/Co(salen)/SBA-
16 (4.3 nm)
[a] Reactions were carried out with 0.55 equiv of H2O and 0.2 mol% Co(salen)
catalyst for 18 h. [b] Determined by ICP. [c] TOF is calculated according to the
following equation: TOF = Nconversed propylene oxide/(NCo(salen) × t), where N denotes
molar numbers, and t denotes reaction time (h). [d] The number in the bracket
is the result after eight cycles of reaction.
were denoted as mTiO2-0.5/Co(salen)/SBA-15 and mTiO2-
8.5/Co(salen)/SBA-15, respectively. Similarly, the number of
ALD cycles was optimized in terms of the activity and stability.
For a 0.5 s diffusion time, the optimized number of cycles is 220,
and the produced catalyst 220TiO2-0.5/Co(salen)/SBA-15 also
shows good activity and stability (Figure 3a). It has a similar “ink
bottle” pore structure as shown in the N2-ADI analysis (Figure
S8), which was further confirmed by EDX mapping, revealing a
TiO2 deposition depth of approximately 550 nm (Figure S9a).
The shorter deposition depth is reasonable in terms of the
shorter diffusion time of TIP. However, for an 8.5 s diffusion time,
the optimized number of cycles is 150 (150TiO2-
Table 2. The asymmetric cyanosilylation of carbonyl compounds on
100Al2O3/[(salen)Ti(μ-O)]2/SBA-15 and homogeneous [(salen)Ti(μ-O)]2.[a]
Ti content
(wt%)[b]
Enantioselectivity
(%)
Catalyst
Yield (%)
[(salen)Ti(μ-O)]2
--
>99
82
8.5/Co(salen)/SBA-15). 150TiO2-8.5/Co(salen)/SBA-15 has
a
96(94)[c]
99(95)[c]
>99(96)[c]
81(80)[c]
83(83)[c]
82(83)[c]
very low activity despite its good stability (Figure 3b). The TiO2
deposition depth reaches 5-6 µm (Figure S9b), which is
consistent with the longer diffusion time (Scheme 1c). Obviously,
the low activity resulted from the coverage of a majority of
Co(salen) by TiO2 and the limited diffusion of reactant molecules
in the long channels with a too small pore size of 0.8 nm.
Therefore, it is important to control the deposition depth of TiO2
by choosing an appropriate diffusion time to optimize the stability
and activity of the encapsulated catalyst. Note that the
determined deposition depth are smaller than the simulated
0.11
0.35
0.52
100Al2O3/[(salen)Ti(μ-
O)]2/SBA-15
[a] Reactions were carried out under an argon atmosphere with 0.1 mol% Ti
catalyst for 2 h. [b] Determined by ICP. [c] The number in the bracket is the
results after 6 cycles of reaction.
In conclusion, we have demonstrated for the first time that
ALD can be applied for the encapsulation of homogeneous
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10.1002/anie.201712010
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by depositing metal oxides at/near the pore entrance via
controlling the diffusion time of the precursors to form a hollow
plug. Its pore sizes can be precisely tailored at the sub-
nanometer scale by controlling the ALD cycle number. Typically,
by accommodating Co(salen) and [(salen)Ti(μ-O)]2 in SBA-15,
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the HKR of the epoxides and asymmetric cyanosilylation of
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Acknowledgements
We appreciate the financial support from the National Natural
Science Foundation of China (21403271 and 21673269) and
Youth Innovation Promotion Association CAS (2017204).
Keywords: encapsulation • hollow plugs • atomic layer
deposition • asymmetric catalysis • mesoporous materials
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This article is protected by copyright. All rights reserved.

10.1002/anie.201712010
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
Metal complexes have been
Shufang Zhang, Bin Zhang,* Haojie
successfully encapsulated in
mesoporous materials by atomic layer
deposition of metal oxides at/near the
pore entrance to form a hollow plug
via controlling the diffusion time of the
precursors.
Liang, Yequn Liu, Yan Qiao, Yong Qin*
Page No. – Page No.
Encapsulation of Homogeneous
Catalysts in Mesoporous Materials
Using Diffusion-Limited Atomic Layer
Deposition
This article is protected by copyright. All rights reserved.