Positional immobilization of Pd nanoparticles and enzymes in hierarchical yolk-shell@shell nanoreactors for tandem catalysis

Article abstract of DOI:10.1039/c7cc03177g

A hierarchical yolk-shell@shell nanoreactor that spatially positioned Pd nanoparticles and the CALB enzyme in separated domains is constructed, and served as an efficient bifunctional catalyst for the one-pot dynamic kinetic resolution (DKR) reaction of 1

Full text of DOI:10.1039/c7cc03177g

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Positional Immobilization of Pd Nanoparticles and Enzyme in
Hierarchical Yolk-shell@shell Nanoreactors for Tandem Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Zhang,a‡ Lingyan Jing,a‡ Fangfang Chang,a Shuai Chen,c Hengquan Yang,*a and Qihua
Yang*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
hierarchical yolk-shell@shell nanoreactor that spatially the high density of exposed active sites endowed by the
positioned Pd nanoparticles and CALB enzyme in separated movable core, and the protection effects conferred by the
domains is constructed, and served as an efficient bifunctional shell.12 Moreover, different catalytically active sites could be
catalyst for one-pot dynamic kinetic resolution (DKR) reaction of exquisitely located in the core or on the shell, achieving site
1-phenylethylamine with excellent activity and selectivity.
isolation.13 These intriguing properties make YSNs materials
promising candidates as nanoreactors for cascade reactions.
However, the reports about YSNs as cascade nanoreactors are
still rarely seen in literatures because of the difficulty in
precisely positioning active sites in the core and on the shell.
Dynamic kinetic resolution (DKR) reactions, which
combined the enzyme-catalyzed kinetic resolution of racemic
amine or alcohols mixtures and metal/acid-catalyzed
racemization process together, are regarded as an important
approach to generate enantiomerically pure compounds that
are common building blocks in the pharmaceutical and
agrochemical industries.14,15 In a DKR process, 100% yield of
the desired stereoisomer from a racemic starting mixture
could be got, in contrast to the use of lipases alone, which can
only achieve maximum yields of 50%. Much research efforts
have been extensively devoted to such a cooperative catalysis
processes and several efficient cascade catalysts have been
developed.16-18 For example, Marco and co-workers reported
that novel enzyme-Pd NPs nanobiohybrids could be
synthesized straightforward in aqueous medium and exhibited
excellent yields of target products.16 Hou and co-workers
Cellular biochemistry requires different enzyme-catalyzed
processes performing simultaneously to make the
orchestration of various metabolic pathways, yielding a wide
range of primary and secondary metabolites within the cell.1-3
In an attempt to construct artificial cells, the concept of
“compartmentalization” or “site isolation”, through which
incompatible substrates and enzymes are spatially separated
to avoid undesired interactions, will need to be mimicked to
allow cascade reactions to take place in parallel in an efficient
manner.4,5 Chemists have applied the principle of site isolation
to several catalysis processes, which mainly rely on
immobilization or encapsulation of incompatible reagents
within sol-gel materials, polymers or emulsion droplets.6-9
However, compared with abundant cascade reaction systems,
the site isolated catalysis is still limited, since it is really a
challenge to precisely control the location of different
functionalities.
Over the past decade, nanostructured materials have
undergone explosive growth with the development of efficient
synthetic methodologies.10,11 Among various fascinating
materials, the yolk-shell nanospheres (YSNs) with unique
core@void@shell nanostructure have attracted much research
attentions owing to their superior advantages, including the
easy independent functionalization of the core and the shell,
demonstrated that
a Ru complex catalyst confined in
nanochannels of mesoporous silicas integrated with
a
commercial solid enzyme catalyst Novozym 435 could also be
utilized as fine cascade catalyst.17 Backvall et al. recently
achieved the cooperative cascade catalysis by direct co-
immobilization of enzyme and metal NPs together into the
compartments of mesoporous silica.18 These studies take one
important step towards mimicking complex natural systems.
However, the control over positional assembly of different
active species (enzyme and metal catalyst) in separate
domains is still not achieved so far.
a.School of Chemistry and Chemical Engineering, Shanxi University, Wucheng Road
92, Taiyuan 030006, China. Email: hqyang@sxu.edu.cn
b.State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China.
Email: yangqh@dicp.ac.cn
c. Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, PR China
†Electronic Supplementary Information (ESI) available: Experimental details and
Herein, we here report a yolk-shell@shell structured
mesoporous silica nanoreactor as
a metal NPs-enzyme
additional
DOI: 10.1039/x0xx00000x
These authors attributed equally to this work.
information
about
material
characterization.
See
bifunctional catalyst with Pd NPs loaded in the inner core and
‡
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the enzyme catalyst adsorbed in the outer shell. Such a metal- further coated with a mesoporous silica shell that possessing
core@enzyme-shell structure design not only synchronously large pores through recently reported novel biphasic
a
achieved immobilization of both functionalities into one stratification approach.20 The synthesis process was carried
nanoreactor, but also resulted in excellent spatial separation out in an oil-water bi-phase reaction system, which allowed
of these different catalytic active sites. Moreover, such a the self-assembly of reactants taking place at the oil-water
structure offered a designated diffusion pathway that was interface for one-pot continuous interfacial growth and
favorable for the reaction proceeding. As a result, the cascade resulting dendritic pore structure with large average pore size
nanoreactor showed superb activity and selectivity for the distribution. The resulting Pd/NH2-MSN@BTME@L-mesosilica
one-pot DKR reaction of 1-phenylethylamine, which are much was dispersed into a solution of Candida antarictica lipase B
better than those of the physically mixed counterpart. As far as (CALB) for the purpose of the enzyme immobilization. After
we know, this is the first example of the integration of metal removing the free CALB by centrifugation, the catalysts
catalyst and enzyme through a controlled spatial positioning Pd/NH2-MSN@BTME@enzyme/L-mesosilica
with
yolk-
procedure. Meanwhile, our findings might pave a way to apply shell@shell structure design were obtained, where Pd NPs and
artificial tandem catalytic systems for mimicking organelle or enzyme active sites were spatially isolated.
important chemical transformations.
Scheme 1. The construction process of bi-functional yolk-shell@shell nanoreactors
The construction process of the yolk-shell@shell bi-
functional nanoreactor is shown in Scheme 1. Firstly,
aminopropyl groups modified mesoporous silica nanosphere
(NH2-MSN) as the support for Pd nanoparticles was
synthesized via a common sol-gel process involving TEOS
(tetraethylorthosilicate) in the presence of CTAB
(cetyltrimethyl ammonium bromide) as template reagent,
Fig. 1.TEM images of (a) NH2-MSN, (b) Pd/NH2-MSN, (c) Pd/NH2-MSN@BTME, (d) and
(e) Pd/NH2-MSN@BTME@L-mesosilica with different magnifications, (f) SEM image of
Pd/NH2-MSN@BTME@L-mesosilica, (g) and (h) N2 sorption isotherms and pore size
distribution curves of (I) Pd/NH2-MSN@BTME@L-mesosilica and (II) Pd/NH2-
MSN@BTME@enzyme/L-mesosilica
followed by
a
post-grafting procedure using APTMS
Transmission electron microscopy (TEM) observations
showed that the NH2-MSN cores are relatively uniform
nanospheres with an average particle size of 80-100 nm (Fig.
1a). Highly ordered hexagonal arranged mesopores could also
be clearly observed, which was further demonstrated by X-ray
diffraction characterization (XRD, Fig. S1 in ESI). The
thermogravimetric analysis (TGA) reveals that about 16.6 wt%
of organic aminopropyl groups (2.86 mmol/g -NH2) was
successfully grafted on the surface of MSN (Fig. S2). Due to the
strong coordination ability of –NH2, almost all of the added
Pd2+could be adsorbed by NH2-MSN. And followed by
reduction, Pd/NH2-MSN with NPs loading of 4.83 wt% was got
(0.454 mmol/g). The morphology and distribution of the
supported Pd NPs are shown in Fig 1b and Fig.S3, and almost
no Pd NPs are observed on the outer surface of the support,
which is possibly because the Pd nanoparticles are mostly
loaded into the channels of the mesoporous silicas. It is also
clear to see that ultra-small and uniform Pd NPs with particle
size of 1.0 ± 0.5 nm are homogeneously dispersed along the
surface of mesoporous silica pores (for the particle size
distribution, see in Fig.S3d). The well-dispersed ultra-small Pd
(aminopropyltrimethoxysilane) as precursor (for the
experiment procedures, see ESI). Here, amino groups were
introduced into MSN due to the following considerations.
Firstly, previous reports have demonstrated its superior
activity and selectivity during the racemization process than
other catalysts.18,19 Secondly, with amino groups, ultra-small
dispersed metal nanoparticles could be formed easier.
Pd2+/NH2-MSN complex was prepared by mixing a desired
amount of the metal precursor Li2PdCl4 (5 wt% of Pd relative to
the NH2-MSN support) and NH2-MSN in water. After
centrifugation and washing with water, the Pd2+ was
subsequently reduced to Pd0 with sodium borohydride,
forming core materials Pd/NH2-MSN. In the next step, Pd/NH2-
MSN was protected with a sacrificial silica layer followed by a
facile organosilane-assisted etching process to fabricate the
yolk-shell structured Pd/NH2-MSN@BTME. During this process,
the amount of added TEOS and organosilane BTME (1, 2-
bis(trimethoxysilyl)ethane) was precisely controlled to etch the
preformed outer silica layer without damage of the inner
Pd/NH2-MSN core. Afterward, the yolk-shell nanospheres were
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NPs without aggregation observed might be attributed to the enough to accommodate enzyme catalyst with relative large
stabilization effect of -NH2 groups and the confinement effect nanodimensions (e.g., CALB, 3 nm * 4 nm * 5 nm). It has been
of the ordered mesoporous structure. Through a sol-gel demonstrated that with larger pore size, higher loading
coating and organosilane-assisted etching process under mild content could be achieved. Here, adsorption tests were also
conditions, uniform and nearly monodisperse YSNs with an performed. As shown in Fig. S5, with bimodal mesopores,
average particle size of 200 nm and shell thickness of 20 nm Pd/NH2-MSN@BTME@L-mesosilica could efficiently adsorb
were obtained (Fig. 1c). The inner core diameter is similar to CALB enzyme from its aqueous solution. An adsorption
NH2-MSN, thus suggesting that the framework of Pd/NH2-MSN content as high as 10.5 mg/g could be got within 7 h. However,
was kept without etching during the synthesis process. After with only small pore size, much less enzyme content (5.1
the biphasic shell coating step, as shown in Fig. 1d, a dendritic mg/g) was achieved for Pd/NH2-MSN@BTME within the same
silica shell coating on Pd/NH2-MSN@BTME appeared, and their adsorption time. The enhanced adsorption content might be
average particle size is increased to
∼350-400 nm. In the attributed to the accommodation of large pores to enzyme.
central area of the yolk-shell@shell material, yolk-shell And without large pores, the enzyme catalyst can only be
structure could be clearly observed. The high-magnification adsorbed on the outer surface instead of in the mesochannels,
TEM image (Fig. 1e) shows that their outer shell thickness is yielding a low loading content. It is also noteworthy that the
nearly 80-100 nm, independent of their sizes. At the same time, adsorption rate of Pd/NH2-MSN@BTME@L-mesosilica is faster
no obvious of the metal particle size increasing was observed. than Pd/NH2-MSN@BTME, which is probably due to the
The SEM image of the dendritic Pd/NH2-MSN@BTME@L- unique dendrimer- like pore structure. After enzyme
mesosilica (Fig.1f) shows relatively uniform particle sizes and adsorption, the BET surface area and total pore volume of
irregular craters on the surface of yolk-shell nanospheres. The Pd/NH2-MSN@BTME@enzyme/L-mesosilica
decreased
significant feature of the obtained yolk-shell@shell is the significantly. Only 294 m2/g of BET surface area and 0.592
radially oriented dendrimer-like mesochannels in their shells cm3/g of pore volume were deserved, which is mainly due to
and the pore size seems much larger than the inner the occupation of CALB enzyme in the nanopores. Also, the
nanospheres. It can also be clearly observed that the morphology and structure of the bifunctional catalyst remains
mesochannels aligned along the normal direction of the shell unchanged after enzyme adsorption (see in Fig. S6). On the
surface and perforated the shell, revealing that the other hand, the unique inner shell of yolk-shell@shell
mesochannels of yolk-shell@shell nanoreactors are readily nanoreactors possess small sized nanopores, providing an
accessible, which is critical for the adsorption of enzymes.
efficient barrier for the diffusion of enzyme from outer shell to
The nitrogen sorption isotherms and the corresponding inner core. Therefore, the large sized enzyme component
pore size distribution curve of Pd/NH2-MSN@BTME@L- might exist in the outer large sized shell.
mesosilica are showed in Fig. 1g and Fig. 1h. Typical type-IV
To demonstrate the power of this spatially immobilization
isotherms with sharp capillary condensation steps in the method, the one pot DKR of 1-phenylethylamine was chosen
relative pressure (P/P0) range of 0.40-1.0 and hysteresis loops as a model reaction. In this reaction, the lipase enzyme CALB
in 0.40 < P/P0 < 0.90 are observed, indicating characteristics of acts as a resolving agent that selectively transforms only one
mesoporous materials with a bimodal mesopore. The Barrett- enantiomer of the starting amine into the corresponding
Joyner-Halenda (BJH) pore size distribution curves further amide. The resolution process is coupled to an in situ
confirmed the bimodal mesopores at the mean values of
∼2.3 racemization of the amine performed by Pd NPs for converting
and 7.2 nm, demonstrating the successful fabrication of the unreacted enantiomer into the desired product. The
hierarchical structures. The mesopore size of 2.3 nm is derived utilization of the yolk-shell@shell structured bifunctional
o
from CTAB as template in the inner Pd/NH2-MSN@BTME, catalyst was commenced by performing the reaction at 70 C,
while the large pore size of 7.2 nm is produced by the oil-water as both the kinetic resolution and the racemization previously
biphasic system. For comparison, the N2 sorption tests of MSN, reported high efficiency at this temperature.18,19 The catalytic
NH2-MSN and Pd/NH2-MSN@BTME were also performed. As results were summarized in Table 1. As expected, the Pd/NH2-
shown in Fig. S4, all of them showed mesoporous structures MSN@BTME@enzyme/L-mesosilica could smoothly catalyze
with a narrow size distribution of 2.3 nm. The Brumauer- the cascade reaction with 98% of conversion and 63% of (R)-1-
Emmet-Teller (BET) surface areas and total pore volume of phenylethylamide yield after a reaction time of 8 h, showing
obtained hierarchical yolk-shell nanoreactor are measured to high activity and excellent selectivity of the yolk-shell@shell
be as high as 763 m2/g and 1.04 cm3/g respectively, which structured bifunctional catalyst (entry 1). To further validate
might be beneficial for the adsorption of enzymes. Meanwhile, the utility of the bifunctional nanoreactor, monofunctional
the intermediated materials MSN, NH2-MSN, Pd/NH2-MSN and catalyst
with
similar
yolk-shell
structures,
NH2-
Pd/NH2-MSN@BTME also exhibited high surface areas and MSN@BTME@enzyme/L-mesosilica, was also performed
pore volumes (see in Table S1). For example, before coating (entry 2). And to make these results fully comparable, the
with large pore sized shell, Pd/NH2-MSN@BTME possesses 590 immobilization process of CALB and reaction conditions were
m2/g of BET surface area and 0.398 cm3/g of pore volume.
the same as those employed in the above bifunctional catalyst.
For the following adsorption of enzyme, pore size is a The morphology, structure and textural parameters were
crucial factor. The materials with limited pore size (< 3 nm) are characterized to be similar with the bifunctional catalyst (Fig.
improper for loading enzymes, since the pore size is not large S6, Fig. S7 and Table S1). As shown, only 49% of conversion
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and desired target amide selectivity could be got, because of the bifunctional catalyst although a decrease in efficiency
the absence of racemization catalyst. In addition, a physical could be observed over each cycle. After recycling, the
mixture of NH2-MSN@BTME@enzyme/L-mesosilica and morphology, Pd loading and textural parameters of the reused
Pd/NH2-MSN@BTME@L-mesosilica with similar catalytic sites catalyst were well kept (see in Fig. S6, S7, Table S1). The
concentration was also tested. Both the activity and the deactivation is probably due to the deteriorated enzyme
selectivity of the reaction were significantly lower than those component, of which the structure might be destroyed in
of the bifunctional Pd/NH2-MSN@BTME@enzyme/L-mesosilica organic solvent and high temperature conditions.
catalyst. These results demonstrate that by positional
In summary, we constructed a hierarchical yolk-
integrating Pd nanoparticles and CALB into the hierarchical shell@shell structured bifunctional mesoporous silica
yolk-shell@shell structured nanoreactor, a substantially more nanoreactor with Pd nanoparticles deposited in the inner core
efficient and robust DKR can be achieved.
and CALB enzyme adsorbed in the outer shell. The yolk-
shell@shell structure allowed complete spatial separation of
metal active sites and enzyme sites, leading to excellent
catalytic efficiency in the dynamic kinetic resolution of amine.
The spatial isolation of different active species holds great
promise in other tandem catalysis systems.
Table 1. The catalytic results from the DKR of 1-phenylethylamine catalyzed by the
Pd/NH2-MSN@BTME@enzyme/L-mesosilica and the separate component or
mechanically mixed systems.
O
NH2
OMe
HN
Catalyst, toluene, H2
This work is supported by the Natural Science Foundation
of China (21603128), the Natural Science Foundation for
Young Scientists of Shanxi Province (2016021034), the
Scientific Research Start-up Funds of Shanxi University
(rsc723).
O
OMe
EtO
Time Con. Yield
Ee
Entry
Catalyst
(h)
(%)
(%)
(%)
Pd/NH2-MSN@BTME@enzyme/L-
mesosilica
1
2
8
98
63
>99
>99
NH2-MSN@BTME@enzyme/L-
mesosilica
8
49
69
49
55
Notes and references
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3
12
>99
Pd/NH2-MSN@BTME@enzyme/L-
mesosilica 2nd
4
5
12
12
94
79
51
41
>99
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Pd/NH2-MSN@BTME@enzyme/L-
mesosilica 3rd
Reaction conditions: All of the reactions were carried out in toluene (3 ml) under
1 atm of hydrogen gas, 1-phenylethylamine (0.50 mmol), ethyl methoxy acetate
(1.0 mmol), dry Na2CO3 (40 mg), 70 oC, hexadecane as internal standard.
Conversion, yield and ee values were determined by GC analysis.
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The higher efficiency of the bifunctional solid catalyst can
be attributed to nanoreactor features of the system. The yolk-
shell structure not only spatially separated the two active sites,
but also directed the reaction species to follow the desired
diffusion pathway of the reaction sequence. As shown in Fig.
S8, during the catalytic process, the reactant racemic amines
pass through the outer shell to reach the inner core and are
firstly exposed to the enzyme, leading to a selective acylation
of the (R)-enantiomer of the amine. The (R)-amide exits from
the nanoreactor, whereas the (S)-enantiomer of the amine will
be racemized by Pd NPs inside the nanoreactor. The racemic
amine formed will again undergo an enzymatic resolution
while diffuse out of the reactor, leading to excellent selectivity
toward the target product during the whole catalytic reaction
process. On the other hand, the mesoporous structure was
also beneficial for fast reactions. However, during the
racemization of the amine, side reactions were observed,
leading to the decreased selectivity to final desired product.
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examined by employing it in consecutive cycles. After reaction,
the catalyst was recovered by centrifugation and directly
reused in the next run. Encouragingly, it is possible to recycle
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