Controllable synthesis of ultrasmall Pd nanocatalysts templated by supramolecular coordination cages for highly efficient reductive dehalogenation

Article abstract of DOI:10.1039/d0ta02725a

Supported ultrasmall noble metal nanocatalysts (UNMNs) are one of the most important classes of solid materials for heterogeneous catalysis. Herein, a simple and efficient supramolecular coordination cage (SCC) template-strategy has been developed to synthesize UNMNs with controllable size and size distribution. A series of SCCs, including M2L4, M4L2, M6L4 and M12L24, with well-defined sizes and shapes as well as different numbers of Pd ions were designed and synthesized as templates. Subsequently, the corresponding Pd nanocatalysts M2&at;CMC, M4&at;CMC, M6&at;CMC and M12&at;CMC were prepared by an impregnation-reduction method on carboxymethylcellulose (CMC) hydrogel supports. It was found that the employment of SCCs as templates could not only significantly reduce the aggregation tendency of Pd nanoparticles but also play an important role in regulating their size and size distribution. For example, the analysis of the catalyst size distribution indicated that the greater the number of Pd ions the cage possesses, the bigger the size of the catalyst. Moreover, with the decrease of the concentration of the template, the size of the Pd nanocatalyst also decreased obviously. Particularly, the resultant catalyst with a nano-Pd loading as high as 12.63percent could still maintain a narrow size distribution. Furthermore, the as-prepared Pd nanocatalyst could serve as a highly efficient polychlorinated biphenyl (PCB) degrader both in a stirred vessel and a continuous flow reactor because of its excellent catalytic efficiency in the reductive dehalogenation reaction under mild conditions. In a word, the SCC template-strategy employed in this study provides new guidelines for the preparation of size-controllable UNMNs on a variety of supports, along with high noble metal loading and catalytic activity. This journal is

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DOI: 10.1039/D0TA02725A
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Controllable Synthesis of Ultrasmall Pd Nanocatalyst Templated by
Supramolecular Coordination Cage for Highly Efficient Reductive
Dehalogenation
Received 00th January 20xx,
Accepted 00th January 20xx
Wei-Ling Jiang,a Ji-Chuang Shen,a Zhiyong Peng,a Gui-Yuan Wu,a Guang-Qiang Yin,b Xueliang Shi,*a
and Hai-Bo Yanga
DOI: 10.1039/x0xx00000x
Supported ultrasmall noble metal nanocatalysts (UNMNs) are one of the most important classes of solid materials for
heterogeneous catalysis. Herein, a simple and efficient supramolecular coordination cages (SCCs) template-strategy has
been developed to synthesize UNMNs with controllable size and size distribution. A series of SCCs, including M2L4, M4L2,
M6L4 and M12L24, with well-defined sizes and shapes as well as different numbers of Pd ions were designed and synthesized
as templates. Subsequently, the corresponding Pd nanocatalysts of M2@CMC, M4@CMC, M6@CMC and M12@CMC were
prepared by an impregnation–reduction method on the support of carboxymethylcellulose (CMC) hydrogels. It was found
that the employment of SCCs as templates could not only significantly reduce the aggregation tendency of Pd nanoparticle
but also play an important role in regulating their size and size distribution. For example, the analysis of the catalysts size
distribution indicated that the greater number of Pd ions the cage possesses, the bigger size of the catalyst can be obtained.
Moreover, with the decrease of the concentration of the template, the size of the Pd nanocatalyst also decreased obviously.
Particularly, the resultant catalyst with nano-Pd loading as high as 12.63% could still maintain a narrow size distribution.
Furthermore, the as-prepared Pd nanocatalyst could serve as highly efficient polychlorinated biphenyls (PCBs) degrader both
in the stirred vessel and continuous flow reactor because of its excellent catalytic efficiency in reductive dehalogenation
reaction under the mild conditions. In a word, the SCCs template-strategy employed in this study provides a new guideline
for the preparation of size-controllable UNMNs on a variety of supports, and meanwhile with high noble metal loading and
catalytic activity.
nanocatalysts (UNMNs) with maximum metal atom-utilization
efficiency.6-14 Such efforts have led to some new concepts of the
supported metal nanoparticle catalyst,15,16 single-site
heterogeneous catalyst (SSHC)17 and even single-atom catalyst
(SAC),18,19 which have been emerging as new research frontier
in catalysis community.
Introduction
Supported noble metal catalysts, as an important type of
heterogenous catalysts, have demonstrated several unique
advantages over their homogeneous counterparts such as the
easier separation from reaction systems, the applicability to
continuous flow processes, good recyclability and much better
stability, which are of great importance in large-scale industrial
production.1-3 However, these heterogenous catalysts
sometimes display the intrinsic poor catalytic performance
relative to their homogeneous counterparts, expressing
particularly with regard to the activity and selectivity.4 In fact,
previous studies have disclosed that, in a particular supported
noble metal catalyst, only the surface atoms are used to
catalyze reactions, thereby revealing the size dependent
catalytic activities.5 To address these issues, tremendous efforts
a. Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University 3663 N.
Zhongshan Road, Shanghai 200062, P. R. China.
Figure 1. An illustrated comparison of the impregnation–reduction method
between employing traditional metal salt precursors and discrete supramolecular
coordination cages for preparing the UNMNs.
E-mail: xlshi@chem.ecnu.edu.cn
b. Department of Chemistry, University of South Florida, Tampa, Florida 33620,
United States.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Generally, the prevailing strategy to design UNMNs is
primarily based on the surface organometallic chemistry,20 i.e.,
have been devoted to developing ultrasmall noble metal
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impregnation and reduction of the metal precursors in a well- mesoporous silica nanoparticle support.43 The gVoieowdArctiaclteaOlynltinice
designed high-surface-area support such as oxides, graphene, performance for reduction of D4O-In: i1t0r.o10p3h9e/Dn0oTlA027a2n5dA
MOFs and so on (Figure 1).21-27 However, this method is likely to Suzuki−Miyaura coupling reaction of the resultant UNMN was
gives rise to a broad size distribution of noble metal particles, also realized.43 Nevertheless, in these two reports, very low
especially under high metal precursors loading, which inevitably concentration of the SCC precursor was able to be applied, thus
enhances the metallic aggregation tendency. Instead, some leading to the relatively low noble metal loading efficiency
reports have demonstrated that the pretreatment of the metal (typical less than 5%) in the prepared UNMNs, which thereby
precursors with some surface capping agents can greatly cannot well embody the distinctive advantages of SCCs in the
facilitate the accurate positioning metals on the supports, thus design of UNMNs with high catalyst loading. Therefore, the
reducing the metallic aggregation tendency and resulting in the development of new UNMNs with high catalyst loading
fully dispersed nanoparticles with narrow size distributions. For templated by SCCs is still in great need. Moreover, the
example, metal precursors either pre-treated with some relationship between the structures of SCCs and the properties
coordinating ligands (e.g., phosphine, amino and thiol), or of UNMNs still remains unknown due to the lack of successful
encapsulated within dendrimers or micelles have been UNMNs systems using different SCCs as templates. In this
successfully applied to the synthesis of UNMNs, thus context, aiming to fill the research gap of this area, this Article
demonstrating distinctly unique advantages over the simple will present the controllable synthesis of ultrasmall Pd
inorganic complexes’ precursors.28-31 Nevertheless, such nanocatalysts employing different SCCs as templates and
surface capping agents inevitably block the catalytic sites, and explore their structure-property relationship.
decrease the activity and selectivity of the resultant UNMNs.
Herein, a series of SCCs, including M2L4, M4L2, M6L4 and
Moreover, the removal of the surface capping agents in harsh M12L24, with well-defined sizes and shapes as well as different
reaction conditions sometimes leads to serious aggregation or numbers of Pd ions have been designed and synthesized. In
sintering of the metallic nanoparticles/nanoclusters on the particular, different from the previous reports, the
support. Besides, the achievement of scalable UNMNs with high corresponding Pd nanocatalysts have been successfully
metal contents on solid supports for practical catalytic prepared by an impregnation-reduction method in the support
applications still remains a synthetic challenge.28 Therefore, the of carboxymethylcellulose (CMC) hydrogels. The results
choice of suitable metal precursors would be essential to reduce revealed a clear structure-property relationship that the
the aggregation tendency of metallic nanoparticles, thus employment of SCCs as templates could significantly reduce the
resulting in the UNMNs with controllable metal size and aggregation tendency of Pd nanoparticles. Therefore, all the
content.
Our group has a long-term interest in construction of well- ranging from 0.8 ± 0.3 nm to 2.6 ± 0.3 nm. Interestingly, the
defined metallosupramolecular architectures through analysis of the catalysts size distribution indicated that the
prepared Pd nanocatalysts exhibited narrow size distributions
coordination-driven self-assembly.32-37 We propose herein that greater number of Pd ions the SCCs have, the bigger size of the
supramolecular coordination cages (SCCs) would be good nanocatalyst can be obtained, thus highlighting the template
candidate of metal precursors for the templated synthesis of effect of the SCCs. Moreover, with the decrease of the
the supported UNMNs (Figure 1). The rationales of such design concentration of cage template, the size of the resultant Pd
strategy are as follows: (a) SCCs have well-defined, robust and nanocatalyst also decreases obviously. Notably, the as-
three-dimensional (3-D) structural geometry with precise type prepared catalyst with nano-Pd loading as high as 12.63% could
and number of metals, which is of high importance to fine-tune still maintain a narrow size distribution, embodying the cage-
the number and size distribution of metal centers in UNMNs template superiority as well. As a representative, the catalytic
when they function as templates; (b) the diverse bridging performance of M12@CMC for reductive dehalogenation has
ligands and solid morphology of SCCs enable them to well been investigated. The catalysis result showed that M12@CMC
hybrid with the support through either physical blending or exhibited an excellent catalytic activity and selectivity for
reactive blending; (c) the coordination environment of the efficiently reductive dehalogenation of various aromatic halides
metals in SCCs is highly dynamical reversible, which makes it under the mild conditions. On this foundation, a continuous
feasible to convert SCCs into metal nanoparticles via a mild flow reactor based on M12@CMC for reductive dechlorination
reduction condition in the support.38-41 Very surprisingly, of polychlorinated biphenyls (PCBs) has been realized,
although numerous well-defined SCCs have been successfully indicating the potential application in the field of PCB
developed, up to date, very few examples on the synthesis of degradation. To the best of our knowledge, this study for the
the supported UNMNs templated by SCCs have been first time demonstrated that employing well-defined
reported.42,43 In 2012, Fujita and co-workers first reported the supramolecular coordination cages as templates could
preparation of hollow silica nanoparticles templated by a effectively regulate the size and distribution of UNMNs and
Pd12L24 spherical complex. They claimed that the incarcerated consequently improve their catalytic properties and
(PdO)n and Pdn clusters were strictly controlled with n = ca. performances. We believe that the SCCs-templated synthesis
12.42 Recently, Brinker and co-workers developed a new strategy developed herein could be also applied to other high-
strategy to prepare an UNMN through in situ reduction of surface-area supports, which thus provides new guidance to the
Pd24L48 cage confined within monosized, thiol-modified design and construction of UNMNs.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0TA02725A
Scheme 1. An illustrated comparison of the impregnation–reduction method between employing traditional metal salt precursors and discrete supramolecular
coordination cages for preparing the UNMNs.
composites of all aerogels have foamlike appearance. Upon
Results and Discussion
mixing the cage@CMC aerogels and NaBH4, their color
immediately turned from grey to gray-black, indicating the
formation of Pd nanoparticles.
Synthesis
An impregnation–reduction method was employed to prepare
the UNMNs. Firstly, four SCCs of M2L4, M4L2, M6L4 and M12L24 as
templates were constructed through coordination-driven self-
assembly. Then, an impregnation process involving a physical
blending of SCCs with CMC for the gelation of cage@CMC was
carried out. Finally, the SCCs were converted into Pd
nanoparticles upon a mild in-situ reduction (Scheme 1). Herein,
CMC was utilized as the support because of its inexpensive,
biodegradable, and easily functionalized features. Moreover,
CMC possesses abundant carboxylate (−COO−) and −OH groups,
which are conducive to coordinate and stabilize Pd particles.44
It should be noted that the four SCCs of M2L4, M4L2, M6L4 and
M12L24 were successfully prepared from the corresponding
ligand and Pd complex via the well-developed coordination-
driven self-assembly approach in nearly quantitative yield,45-48
and all the SCCs were well-characterized by 1H nuclear magnetic
resonance (1H NMR) and mass spectrometry (Scheme S1 and
Figures S1-3). The obtained SCCs were then dispersed in an
aqueous solution with sonication for 30 min, and then CMC was
added to the mixture with a rapid agitation by mechanical
agitation for 5 h. After gelation, the SCCs were successfully
impregnated into the CMC support, making for four cage@CMC
aerogels of M2L4@CMC, M4L2@CMC, M6L4@CMC and
M12L24@CMC with a good cage dispersion (vide infra).
Subsequently, the obtained cage@CMC aerogels were treated
with sodium borohydride (NaBH4), resulting in the CMC-
supported Pd nanoparticles of M2@CMC, M4@CMC, M6@CMC
and M12@CMC. As shown in Figure 2a and Figure S4, the
In order to systematically investigate the cage template
effect on the size and size distribution of the designed
nanoparticles, the Pd loading of M2@CMC, M4@CMC,
M6@CMC and M12@CMC were well controlled and a series of
Pd nanocatalysts were synthesized (Table 1 and Table S1). From
the inductively coupled plasma-atomic emission spectrometry
(ICP-AES) analysis, the determined Pd concentrations
(experimental value) remained almost consistent with the
corresponding theoretical Pd concentrations, indicating the
impregnation and reduction process are highly efficient with
the neglectable cage or Pd losses. Notably, the Pd loading
efficiency could reach as high as 12.63% without significant
sintering of the Pd metal (Table 1), basically benefiting from the
distinct effect of such SCCs on reducing the aggregation
tendency of Pd nanoparticles (vide infra). For comparison, the
conventional metal source of Pd(BF4)2(MeCN)2 was also utilized
to prepare the nanoparticles. However, the mixture of
Pd(BF4)2(MeCN)2 and CMC resulted in
a
precipitate,
consequently the solid composites of Pd(BF4)2@CMC and
M1@CMC rather than the aerogels were obtained (Figures S5a
and S5b). Such incompatible phenomenon between
Pd(BF4)2(MeCN)2 and CMC is likely a result of the coordination
between carboxylic acid group in CMC and Pd(BF4)2(MeCN)2,
thus further emphasizing the importance of SCCs in the aerogel
formation.
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all characteristic bands of CMC remained unchanged in
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Table 1. Palladium loadings of the prepared nanocatalysts as determined by ICP-AES.
M12L24@CMC, implying that no coorDdOinI:a10ti.o10n39/iDn0tTeAr0a2c7t2io5nA
occurred between Pd(II) and CMC in such physical impregnation
process. After reduction, the FTIR spectra of Pd nanocatalysts
all showed the absence of characteristic peaks of SCCs. By
comparison, both the O–H stretching band and COO-
asymmetric stretching band of CMC experienced a shift in Pd
nanocatalysts, confirming the existence of the obvious
coordination interaction between Pd(0) and CMC (Figure 2c,
green line).44,51 The demonstrated coordination interaction
between Pd(0) and O–H and COO- functional groups is supposed
to contribute to the formation and protection of Pd
nanoparticles, and then helps to control their size distributions.
Thus, the FTIR results further confirmed that the SCCs have
been successfully impregnated into the CMC supports and then
been fully reduced to Pd nanoparticles, which was consistent
with the NMR analysis.
Cage(mg) CMC Theoretic Pd Experimental Pd
Entry
(mg)
375
375
375
375
750
375
375
375
(wt %)
3.93
3.62
3.59
3.60
0.09
1.05
5.03
14.48
(wt %)
3.58
3.50
3.28
3.53
0.09
1.07
4.73
12.63
3.5%-M2@CMC
3.5%-M4@CMC
03.5%-M6@CMC
3.5%-M12@CMC
0.1%-M12@CMC
1.0%-M12@CMC
5.0%-M12@CMC
12.0%-M12@CMC
130
66
110
106
5
30
150
480
Sample treatment details: the aerogel sample was ground into a powder and then
dried in a vacuum oven at 100 oC for 24 h. The above powder was digested with
nitric acid and sulfuric acid, and then configured into an aqueous solution for
testing.
Characterization
X-ray photoelectron spectroscopy (XPS) measurements
were then performed to gain the deeper insight into the change
in elemental compositions and valence states of the Pd catalyst
(Figure 2d and Figures S8-10). The XPS spectrum of the
M12L24@CMC clearly revealed the presence of N, F and Pd
elements, which originated from the corresponding cages
(Figure 2d, black line). After reduction, the peaks of N or F
disappeared and the spectrum only featured the characteristic
peaks of O, C, Na and Pd (Figure 2d, red line). Basically, the
binding energies of Pd 3d5/2 in the Pd catalyst were completely
different from those of Pd 3d5/2 in the cage@CMC because Pd
in the nanocatalyst and cage@CMC presented different valence
states. Taking M12L24@CMC and M12@CMC as examples, the
binding energies of Pd 3d5/2 in M12L24@CMC are 343.7 and
338.4 eV, while the binding energies of Pd 3d5/2 in M12@CMC
are 340.5 and 335.3 eV (Figure 2d). The decrease of binding
energy (∼3 eV) further elucidated that Pd(II) in cage@CMC has
been fully reduced to Pd(0) and anchors on CMC,44,51 which is in
well agreement with the above discussions. In addition, in order
to facilitate the subsequent application of the material,
thermogravimetric analysis (TGA) and robust measurement of
M12@CMC have been carried out to test its thermostability and
robustness. Impressively, the results showed that the material
has good thermal stability (Figure S11) and also has an
extraordinary mechanical stability and robustness supported by
weight pressing and tape test (Figure S12).
To further investigate the details of the impregnation and
reduction process, solid-state 13C nuclear magnetic resonance
(13C NMR) was first employed to characterize each composite
(Figure 2b and Figure S6). Taking the case of M12L24 as an
example, CMC displayed a characteristic 13C NMR spectrum
with broad 13C resonances at 173, 101, and 71 ppm (Figure 2b,
black line), which can be well assigned according to the
literature.49,50 After gelation, the M12L24@CMC aerogel showed
the characteristic peaks of both CMC and cage M12L24 (Figure
2b, blue line), indicating the successful impregnation of SCCs
into CMC. Besides, no obvious chemical shifts were observed for
both CMC and M12L24, illustrating the fact that the chemical
structures of CMC and SCC remained unchanged during the
impregnation process. After reduction, the 13C resonances of
each cage were totally disappeared (Figure 2b and Figure S6,
green line), implying the high efficiency of the reduction
process. Similarly, Fourier-transform infrared spectroscopy
(FTIR) was also applied to elucidate the identity of the above as-
prepared composites (Figure 2c and Figure S7). The FTIR
spectrum of the neat CMC depicted the characteristic bands at
3437 cm-1, 1601 cm-1 and 1421 cm-1, resulting from O–H
stretching, COO- asymmetric and symmetric stretching
vibration, respectively. After simple physical impregnation,
taking M12L24 as an example again, the FTIR spectrum of
M12L24@CMC aerogel displayed all characteristic bands of both
CMC and the corresponding cage (Figure 2c, blue line). Notably,
Figure 2. (a) The photos of aerogel of M12L24@CMC (i) and M12@CMC (ii). (b) Stacked solid-state 13C NMR spectra of CMC (black), cage M12L24 (red), M12L24@CMC (blue)
and M12@CMC (green). (c) Stacked FT-IR spectra of CMC (black), cage M12L24 (red), M12L24@CMC (blue) and M12@CMC (green). (d) XPS spectra of M12L24@CMC and
M12@CMC: (i) survey spectra, (ii) high-resolution signals of Pd 3d.
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nanocatalysts with ~ 3.5% Pd loading on the CMC, the mean
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DOI: 10.1039/D0TA02725A
particle diameters of M12@CMC, M6@CMC, M4@CMC and
M2@CMC were estimated to be 2.4 ± 0.3, 1.8 ± 0.2, 1.5 ± 0.2
and 0.8 ± 0.3 nm, respectively (Figures 4a-d). The obtained
results were likely to confirm the close relationship between the
particle size and the type of the coordination cage, i.e., the
greater number of Pd ions the cage possesses, the bigger size of
the metal nanocatalyst can be obtained, thus highlighting the
template effect of the SCCs. As for M12L24@CMC with different
Pd loadings on CMC, the mean particle diameters of 12.63%,
4.73%, 1.07% and 0.09% Pd loaded catalyst were estimated to
be 2.6 ± 0.3, 2.3 ± 0.3, 2.0 ± 0.3 and 0.9 ± 0.4 nm, respectively
(Figures 4e-h). This indicated that with the decrease of Pd
loading in CMC, the size of the metal nanocatalysts became
smaller. Notably, the resulted metal nanocatalysts with a Pd
loading up to 12.63% could still maintain small size with a
narrow size distribution, indicating a distinct advantage of such
SCC-template-strategy toward high Pd loading nanocatalyst
design. The above results reveal that the size and distribution of
the resultant Pd nanocatalysts are closely related to the types
and concentrations of the employed cage templates. The SCCs
template can not only reduce the aggregation tendency of Pd
nanocatalyst but also effectively regulate their size and size
distribution, which is of high importance to realize their
excellent catalytic properties and performances.
Figure 3. TEM images and histograms of size distributions of aerogels of (a)
M12L24@CMC, (b) M6L4@CMC, (c) M4L2@CMC and (d) M2L4@CMC. The Pd
concentrations of all the aerogels of cage@CMC were controlled at ~ 3.5%.
The structure and morphology of cage@CMC aerogels and
Pd nanocatalysts were then investigated by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM).
The SEM figures demonstrated that the prepared aerogel
composites before and after reduction were all microporous
and highly cross linked, suggesting the fact that the reduction
condition didn’t destroy the CMC framework (Figure S13). The
TEM images of cage@CMC aerogels revealed that the SCCs
were uniformly located within the CMC support (Figure 3). The
observed sizes of M12L24@CMC, M6L4@CMC, M4L2@CMC and
M2L4@CMC were estimated to be 2.9 ± 0.4, 2.0 ± 0.3, 2.4 ± 0.3
and 1.2 ± 0.2 nm, respectively (Figure 3). The results indicated
that the mean diameter of the cage@CMC presented in TEM
correlated well with the calculated size of each cage (Figure
S14), illustrating the highly dispersed cage in CMC. After
reduction, the TEM images of the resultant CMC-supported Pd
nanoparticles all showed a highly homogeneous dispersion of
Pd nanocatalysts on the CMC support (Figure 4), implying that
the template effectively reduced the tendency of Pd
nanoparticles to undergo aggregation. In the case of the
Figure 4. TEM images and histograms of size distributions of aerogels of the Pd
nanocatalysts of (a) M12@CMC, (b) M6@CMC, (c) M4@CMC, (d) M2@CMC, and (e-
h) M12@CMC with different Pd loadings. The Pd concentrations of nanocatalysts
in a-d were determined at ~ 3.5%.
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reaction is independent of the halogen position.VNiewotAartbiclley,Ontlhinee
DOI: 10.1039/D0TA02725A
dechlorination of p-chloroaniline with M12@CMC and methanol
Table 2. 12 wt%-M12@CMC catalyzed dechlorination reaction.
but without ammonium formate was not successful since no
dechlorinated product of aniline was detected even after long
duration (Figure S15). This confirms that methanol serves only
as solvent and not as hydrogen donor involved in the reaction
pathways. With regard to the reactivity of the halogen atom,
the organic halides were reduced in the expected order of I > Br
> Cl (Table S2, Figure S16), which is related to the dissociation
energy of carbon-halogen bonds.61 The reaction scope was then
explored with a range of substituted aryl chlorides. As
monitored by gas chromatography–mass spectrometry (GC-
MS) analysis, all substrates were converted into the
corresponding dechlorinated products in good to nearly
quantitative conversions (Table S2, Table S3). Surprisingly, the
substrates bearing carbonyl and nitro groups can even be
further reduced to the corresponding alcohol and amine (Table
2, entry 8-10), revealing the highly catalytic activity of
M12@CMC. As to the multiple chlorinated substrates, mixed
solvent systems of methanol/N, N-dimethyl formamide,
methanol/tetrahydrofuran, and methanol/ethyl acetate were
chosen to evaluate their dechlorination efficiency (Table S4,
Figure S17), since the substrates were insoluble in pure
methanol, made it difficult to (in-situ) detect the reactions and
calculate their conversion through GC-MS analysis. The
preliminary results indicated that the mixed solvent of
methanol and N, N-dimethyl formamide gave a higher reaction
T
(h)
Conversion
(%)
Entry
Substrate
Product
Cl
Cl
NH2
NH2
1
2
6
99
NH2
NH2
6
97
NH2
Cl
3
6
93
NH2
Cl
Cl
4
5
5
1
99
99
Cl
Cl
Cl
Cl
Cl
CH2OH
CH2OH
COOCH3
NH2
6
6
98
COOCH3
NO2
7
6
91
8
5
99
CHO
CH2OH
CHO
9
3
92/8
90/10
COCH3
CHOHCH3
COCH
10
22
Cl
Cl
11
12
10
10
99
99
Cl
Cl
Cl
Cl
Cl
Cl
13
15
99
Cl
Reaction conditions: chlorinated substrate (0.05 mmol), catalyst (calculating
according to 5 mol% of Pd), ammonium formate (1 mmol) and methanol (4 ml) at conversion ratio (Table S4). Remarkably, the multiple
room temperature. A mixed solvent of methanol and N, N-dimethyl formamide
chlorinated substrates could be completely dechlorinated
(V/V, 1:1) was used in entry 11-13 because of the solubility concern. aConversion
without the formation of by-products (Table 2, entry 11-13),
indicating the decent catalytic activity and selectivity in the
methanol/N, N-dimethyl formamide solvent system. Moreover,
the catalytic performance of M12@CMC was also compared
with the common commercial catalysts such as Pd/C (Table S5,
Figure S18). The comparison results showed that Pd/C catalyst
exhibited inferior catalytic performances including lower
reaction conversion and formation of by-product, particularly in
the reductive dechlorination of the multiple chlorinated
substrates. Additionally, the M12@CMC catalyst exhibited good
reusability and could be easily recollected by simple filtration
and washing. As illustrated in Figure S19, the catalyst can still
maintain a high activity, and the reaction conversion could
remain as much as 80% after five cycles of reaction. Moreover,
no obvious sintering was observed since both the XPS survey
and TEM survey of M12@CMC after five cycles of reaction
remained the same information with those of the freshly
prepared M12@CMC (Figures S20 and S21).
determined by GC.
Catalytic performance
Reductive dehalogenation reaction is of paramount importance
no matter from a synthetic point of view or as a destruction
methodology of hazardous materials, especially in application
of practical organic synthesis and degradation of organic
halogenated materials.52 Indeed, chemists have dedicated
many efforts to this research area, and some effectively
reductive dehalogenation methods including photochemical,
electrochemical, microbial and radiolytic reductive methods,
and so on have been well developed.53-56 Among them, the
supported noble metal catalysts have been demonstrated to be
very effective and dominate this field, which is typically
represented by the most widely used catalyst of Palladium on
carbon (Pd/C).57,58 Thus, the good catalytic performance for
reductive dehalogenation of our prepared Pd nanoparticles is
highly expected. Herein, M12@CMC was applied as
a
representative catalyst to study the reduction dehalogenation
reaction. In this work, the reductive dehalogenation is mainly
based on ammonium formate-catalytic transfer hydrogenation
(AF-CTH) techniques, wherein ammonium formate functions as
hydrogen donor dissolving in methanol solution.59,60 Firstly, a
reduction of chloroaniline was carried out to examine the
catalytic activity of M12@CMC. It was found that the
conversions were almost quantitative for the o, m, p-
chloroaniline derivatives (Table 2, entry 1-3), indicating the
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been successfully realized, which was then employed for the
View Article Online
DOI: 10.1039/D0TA02725A
efficiently reductive dehalogenation. The preliminary results
revealed that the designed four supramolecular coordination
cages were capable of being used as template in the efficient
synthesis of cage@CMC aerogels and the corresponding Pd
nanocatalysts through
a simple impregnation–reduction
approach. As a consequence, the coordination cages exhibited
significant template effect on regulating the size and
distribution of the Pd nanocatalysts, i.e., the greater number of
Pd ions the cage possesses, the bigger size of the catalyst can be
obtained. Moreover, with the increase of the concentration of
template, the size of the Pd nanocatalyst also increased
obviously, while still maintaining a narrow size distribution even
with a high metal loading. As a representative, the catalyst of
M12@CMC exhibited excellent catalytic reactivity for reductive
dehalogenation of various aromatic halides. As a demo, a
continuous flow reactor was further designed to demonstrate
its application in the removal of PCBs. This study systematically
investigated the template effect of the supramolecular
coordination cage on Pd nanocatalysts preparation, and clearly
revealed their structure-property relationship. We believe that
the SCCs-templated synthesis strategy developed herein could
be also applied to other high-surface-area supports, and helpful
to the design of high metal loading single-site heterogeneous
catalyst and even single-atom catalyst. Therefore, this strategy
is expected to address the low catalyst loading issue in the
preparation of large-scale size-controllable UNMNs for real
industrial application.
Figure 5. (a) The cartoon of the continuous-flow setup for PCB 77 degradation. (b)
Degradation plot of PCB 77.
Polychlorinated biphenyls (PCBs) are a class of chlorinated
organic chemicals containing 209 isomers that have been
extensively used for a variety of industrial and commercial
purposes for several decades. PCBs have been banned since
1976 because of the serious concerns about their degradation,
bioaccumulation, chronic toxicity, which have seriously
threatened humans and the environment.62-64 Thus, the
degradation of PCBs is one of the most enduring major issues in
the field of environmental sciences. Encouraged by the results
described above, we envision that our designed Pd nanocatalyst
could serve as highly efficient PCBs degrader. Herein, 3,3',4,4'-
tetrachlorobiphenyl (PCB 77) was selected for the degradation
study. The PCB 77 degradation process by M12@CMC and Pd/C
was investigated first in a traditional stirred vessel. The
conversion versus time curve revealed that the degradation was
almost fully complete within 10 hours, and PCB 77 could be
totally dechlorinated to biphenyl after 20 hours with the
catalyst of M12@CMC (Figure S22a). In contrast, the
degradation of PCB 77 was relatively slower with the catalyst of
Pd/C (Figure S22b), illustrating the superior activity of
M12@CMC as well. Based on these promising results, a
continuous flow reactor based on the M12@CMC catalyst was
further designed as a demo to demonstrate its application in the
removal of PCBs. The cylindric aerogel of M12@CMC was well
filled into a short Teflon column (for details of the continuous
flow reactor setup see Figure S23), and the cartoon of the setup
and process is illustrated in Figure 5a. As expected, PCB 77 was
degraded completely within 2 hours and further dechlorinated
to biphenyl within 10 hours under this condition (Figure 5b,
Figure S24), which manifested the high activity and unique
advantages of such heterogenous catalyst. Overall, the
ultrasmall Pd nanocatalyst developed in this work may
represent a versatile and efficient catalyst for the reductive
dehalogenation reaction and degradation of PCB, because all
reactions were carried out at room temperature in good
conversion, and more importantly, with a recyclable catalyst.
Experimental
Synthesis of M4L2 and M6L4
M4L2 and M6L4 were prepared according to the literatures
reported from the corresponding ligand L2 and (BPy)Pd(NO3)2,
L3 and (en)Pd(NO3)2.46,47
Synthesis of M2L4
A mixture of Pd(CH3CN)2(BF4)2 (44.4 mg, 0.1 mmol) and L1 (62.0
o
mg, 0.2 mmol) was stirred in DMSO (4 ml) at 80 C overnight.
The mixture was filtered and the crude product was
precipitated by adding ether. The product was washed by
CH2Cl2 several times and then dried under vacuum for 12 h.
Synthesis of M12L24
A mixture of Pd(CH3CN)2(BF4)2 (44.4 mg, 0.1 mmol) and L4 (52.5
o
mg, 0.2 mmol) was stirred in DMSO (4 ml) at 80 C overnight.
The mixture was filtered and the crude product was
precipitated by adding ether. The product was washed by
CH2Cl2 several times and then dried under vacuum for 12 h.
General procedure for preparation of cage@CMC and Pd
nanocatalysts
Impregnation procedure: Dried powder of cage was dispersed
in aqueous solution with sonication for 30 min in a 50 mL round-
bottom flask. CMC was then added to the mixture with rapid
agitation by mechanical agitation (1,000 rpm) for 5 h. After that,
the mixture was slowly transformed to sol, and then was poured
into a cylindrical mold (3 cm in diameter and 3 cm in depth)
made from PTFE (poly tetra fluoroethylene). Then the sol was
soaked in acetonitrile solution until the sol was completely
Conclusions
In summary, controllable synthesis of ultrasmall Pd
nanocatalysts by using well-defined supramolecular
coordination cages on a carboxymethylcellulose support has
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transformed into a gel. The obtained gel was then pre-frozen at ammonium formate (630 mg, 10 mmol) in meVtihewanArotilc/leNO,nlNine-
-19 oC overnight and freeze-dried under vacuum for 48 h, dimethyl formamide (V/V, 1:1) was continually pumped
DOI: 10.1039/D0TA02725A
resulting in the cylinder-shape aerogel.
through the reaction column using a peristaltic pump with a
Reduction procedure: Pd nanocatalyst was obtained by soaking flow rate of 1 mL/min. The reaction system was periodically
the corresponding cylinder-shape cage@CMC aerogel into the detected by in situ GC-MS analysis.
freshly prepared methanol solution of NaBH4. The mixture was
stirring for 30 minutes, then washed by methanol for three
times. The prepared Pd nanocatalyst was then dried under
Conflicts of interest
vacuum for 12 h.
General procedure for dechlorination reaction
There are no conflicts to declare.
The chlorinated substrate (0.05 mmol), ammonium formate (63
mg, 1.0 mmol) and 12 wt%-M12@CMC (calculating according to
5 mol% of Pd) were added to a vessel with the solvent of
methanol (4 mL) or a mixed solvent of methanol and N, N-
dimethyl formamide (4 mL, V/V, 1:1). Then the mixture was
stirred at room temperature for several hours. The above
reaction liquid was subsequently taken in the GC-MS test.
Continuous flow reactor for degradation of PCB 77
The reactor was designed based on a short Teflon column filled
with the aerogel of M12@CMC, and the figure of the setup and
process was shown in Figure S17. In the continuous flow
experiment, the solution of PCB 77 (150 mg, 0.5 mmol) and
Acknowledgements
H.-B. Y. thanks Innovation Program of Shanghai Municipal
Education Commission (No. 2019-01-07-00-05-E00012),
Program for Changjiang Scholars and Innovative Research Team
in University for financial support. X. S. acknowledges the
financial supports sponsored by Shanghai Sailing Program
(19YF1412900) and the Fundamental Research Funds for the
Central Universities. W.-L. J acknowledges the ECNU Academic
Innovation Promotion Program for Excellent Doctoral Students
(YBNLTS2019-009).
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DOI: 10.1039/D0TA02725A
A simple and efficient supramolecular coordination cages (SCCs) template-strategy is
developed to synthesize Pd nanocatalysts with controllable size and size distribution for
highly efficient reductive dehalogenation.