Stability or flexibility: Metal nanoparticles supported over cross-linked functional polymers as catalytic active sites for hydrogenation and carbonylation

Article abstract of DOI:10.1016/j.apcata.2014.05.001

A novel cross-linked functional polymer was prepared through copolymerization between 1, 3, 4, 6-tetraallylglycoluril and 4-vinyl pyridine. Pt and Pd nanoparticles supported over this polymeric framework (Pt/CFP and Pd/CFP) were detailedly characterized by TEM, EDS, and XPS. Pt nanoparticles were kept in the monodispersed state with the average size of 1.4 nm. Monodispersed Pd nanoparticles were about 4.5 nm. The hydrogenation of nitrobenzenes over Pt/CFP shows high activity and selectivity with the substrate to Pt ratio of 4000 under mild reactions. Pd/CFP was the catalyst for carbonylation of aryl iodides in the presence of secondary amines and acylhydrazines. Double carbonylation with secondary amines produced α-ketoamides with the selectivity of 80%. Diacylhydrazine molecules were synthesized by the direct carbonylation of aryl iodide with acylhydrazine over Pd/CFP. The recyclability and recoverability of Pt/CFP were investigated through a seven-run recycling test of nitrobenzene hydrogenation. The flexibility of Pd/CFP in the carbonylation process was thoroughly explored by a 12-run recycling test. Supported Pt or Pd nanoparticles showed the macroscopic robustness in their catalytic performance in the catalytic cycle. The flexibility of metal nanoparticles and the polymeric supports guaranteed macroscopic catalytic robustness.

Full text of DOI:10.1016/j.apcata.2014.05.001

Accepted Manuscript
Title: Stability or Flexibility: Metal Nanoparticles Supported
over Cross-linked Functional Polymers as Catalytic Active
Sites for Hydrogenation and Carbonylation
Author: Bingfeng Chen Fengbo Li Zhijun Huang Tao Lu
Guoqing Yuan
PII:
S0926-860X(14)00311-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.001
APCATA 14820
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
4-11-2013
13-4-2014
1-5-2014
Please cite this article as: B. Chen, F. Li, Z. Huang, T. Lu, G. Yuan, Stability or
Flexibility: Metal Nanoparticles Supported over Cross-linked Functional Polymers
as Catalytic Active Sites for Hydrogenation and Carbonylation, Applied Catalysis A,
General (2014), http://dx.doi.org/10.1016/j.apcata.2014.05.001
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Stability or Flexibility: Metal Nanoparticles Supported over
Cross-linked Functional Polymers as Catalytic Active Sites
for Hydrogenation and Carbonylation
Bingfeng Chena, Fengbo Li*a, Zhijun Huanga, Tao Lua,b, Guoqing Yuan*a
a Beijing National Laboratory of Molecular Science, Key Laboratory of Green Printing, Institute
of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China; Tel：(86)
-10-62634920; Fax: (86) -10-62559373; E-mail: lifb@iccas.ac.cn; yuangq@iccas.ac.cn
b University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
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Abstract
A novel cross-linked functional polymer was prepared through copolymerization
between 1, 3, 4, 6-tetraallylglycoluril and 4-vinyl pyridine. Pt and Pd nanoparticles
supported over this polymeric framework (Pt/CFP and Pd/CFP) were detailedly
characterized by TEM, EDS, and XPS. Pt nanoparticles were kept in the
monodispersed state with the average size of 1.4 nm. Monodispersed Pd nanoparticles
were about 4.5 nm. The hydrogenation of nitrobenzenes over Pt/CFP shows high
activity and selectivity with the substrate to Pt ratio of 4000 under mild reaction
reactions. Pd/CFP was the catalyst for carbonylation of aryl iodides in the presence of
secondary amines and acylhydrazines. Double carbonylation with secondary amines
produced α-ketoamides with the selectivity of 80%. Diacylhydrazine molecules were
synthesized by the direct carbonylation of aryl iodide with acylhydrazine over Pd/CFP.
The recyclability and recoverability of Pt/CFP were investigated through a seven-run
recycling test of nitrobenzene hydrogenation. The flexibility of Pd/CFP in the
carbonylation process was thoroughly explored by a twelve-run recycling test.
Supported Pt or Pd nanoparticles showed the macroscopic robustness in their catalytic
performance in the catalytic cycle. The flexibility of metal nanoparticles and the
polymeric supports guaranteed macroscopic catalytic robustness.
Keywords: Functional polymer; Metal nanoparticles; Hydrogenation;
Carbonylation; Recoverability and recyclability.
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1. Introduction
Most of the industrial catalysts are made of metal or metal oxide particles of a few
nanometers in size and the reaction proceeds at the molecular scale. Nanocatalysis is
the intrinsic property of the catalyst. Supported metal nanoparticles constitute the
majority of the heterogeneous catalysts and show catalytic properties different from
small atom clusters or the bulk crystal surfaces. Gold is the most famous model
system [1]. The size-dependence effect is the specialty of catalysis over metal
nanoparticles [2, 3]. First, metal nanoparticles have irregular surfaces with abundant
structural defects on the steps and kinks, which are favorable low-coordinated sites
for the breaking and forming of chemical bonds [4, 5]. Second, metal nanoparticles’
surface offers the flexibility of structure recovering in the adsorption and desorption
of the catalytic reaction. The third property, which influences catalysis, is the
electronic structure of metal nanoparticles [6, 7].
Metal nanoparticles are excellent catalysts for chemical synthesis [8-10]. However,
active nanoparticles are not stable and show a high tendency toward agglomeration,
which then leads to deactivation and precipitation. Protecting agents to stabilize metal
nanoparticles include various materials, such as metal oxide [11, 12], carbon materials
[13], porous silica [14] and polymers [15, 16]. The flexibility of metal nanoparticles is
necessary for active catalytic performance. However, it results in their instability
during the catalytic reaction. (One sentence was deleted.)
Metal nanoparticles bridge the gap between homogeneous and heterogeneous
catalysis. Some metal nanoparticles with stabilizing agents are soluble in the reaction
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medium as homogeneous catalysis. On the other hand, metal nanoparticles have an
active surface as heterogeneous catalysis. However, it is a major challenge to separate
the catalyst from the reaction mixture. In our previous researches [17, 18],
cross-linked functional polymers (CFPs) were demonstrated to be excellent supports
for metal nanoparticle catalysts to be employed in the liquid phase. The swelling
behavior of the polymer support facilitates the dispersion of metal nanoparticle
precursors in the polymeric framework, which are further immobilized by the
functional groups. Under liquid-phase conditions, the formation of swollen-gel phase
is favorable for the involvement of all reactants in the catalytic reaction.
CFPs supported metal nanoclusters had been applied in commercial production of
MIBK from acetone and hydrogen in 1965 [19]. EnCat catalyst by Reaxa was another
commercial catalyst based on polyurea supported palladium [20]. Typical CFPs
consist of three components: chemical structure, cross-linked, and functional groups.
Chemical structures are the main polymer chain such as poly-DMA, polyacrylate,
poly-MMA, and PS. The most employed cross-linkers are DVB and MBAA.
Functional groups are used to anchor metal nanoparticles’ precursors. There are two
types of attachment strategies: ion-pair interaction (-SO3-, COO- and –NR3+) and
chelating interaction (Lewis base groups: amino, pyridyl, benzimidazoyl, thiol,
methylsulfide, and nitrile) [21-24]. Supported metal species are mostly noble metals,
such as Pt, Pd, Ru, Rh, and Au [25, 26]. (One sentence was deleted.) There are three
methods to prepare CFPs supported metal nanoparticles: 1) introduction of a suitable
metal precursor into CFPs and generation of metal nanoparticles through reduction or
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decomposition of the precursor (RIMP), 2) incorporation of metal precursors into
CFPs during the polymerization and generation of metal nanoparticles through
reduction or decomposition of the precursor (IMPR), 3) direct introduction of
prepared metal nanoparticles into CFPs (IPMN) [27]. Kobayashi et al. have reported
“polymer-microencapsulated catalysts” and “polymer-incarcerated catalysts”, which
provide new insights into the preparation and catalytic application of
polymer-supported metal nanoparticles [28-30]. Recently, Yamada et al. have found a
novel CFPs-supported metal nanocluster catalyst: self-assembled poly
(imidazole-palladium) exhibits high catalytic activity and excellent resuability in
several C-C formation reactions [31]. The aim of these proposed strategies is to
achieve high catalytic activity with good recoverability and recyclability.
In this work, a new cross-linker based on glycoluril [32, 33] was synthesized and
then copolymerized with 4-vinyl pyridine to produce cross-linked functional
polymeric support for metal nanoparticles. Pt and Pd nanoparticles over this CFP were
prepared and their catalytic properties in hydrogenation and carbonylation were
further investigated. The catalytic behavior and transform of supported metal
nanoparticles during the reaction were explored to interpret the structure-property
relation.
2. Experimental
2.1 Preparation of 1, 3, 4, 6-tetraallylglycoluril
A three-neck 100 ml flask was flushed with argon. Anhydrous DMSO (40 ml) was
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added and glycoluril (1.42 g, 10 mmol) was dissolved. Then potassium tert-butoxide
(5.62 g, 50 mmol) was gradually added and the mixture was further stirred for 1.0 h.
Allyl bromide (7.0 ml, 80 mmol) was injected into the reaction mixture and the
reaction was performed at 25℃ for 20 h. 500 ml aqueous solution of HCl (0.1 M) was
added into the reaction mixture. 100 ml ethyl acetate was used to extract the mixture
and the organic phase was washed by saturated NaCl aqueous solution (100 ml) for
three times. After being dried by MgSO4, the solvent was removed. 1.53 g target
product was obtained after purification over silica column with the eluent of ethyl
acetate/ petroleum ether (60-90℃) (v/v, 3:2). All procedures were performed in argon
atmosphere.
2.2 Preparation of cross-linked functional polymer (CFP)
The copolymerization was performed in a 100-ml two-neck flask under argon flow. 1,
3, 4, 6-tetraallylglycoluril (1.51 g, 5 mmol) and 4-vinyl pyridine (1.07 ml, 10 mmol)
were dissolved in 20 ml anhydrous toluene. Azobisisobutyronitrile (164 mg, 0.05
mol/L) was used as the radical initiator and the polymerization proceeded at 60 ℃ for
21 h. The resulting solid materials were filtered and extrated by petroleum ether for
three times. After being dried under vacuum, yellow solid (0.89 g) was obtained.
2.3 Preparation of Pt/CFP and Pd/CFP
CFP (250 mg) was fully swollen in 30 ml anhydrous ethanol. And then K2PtCl4 (28
mg, 0.0674 mmol) was added. The mixture was further stirred at room temperature
for 20 h. NaBH4 (25.5 mg, 0.674 mmol) in 6.0 ml water was injected and the
reduction proceeded for 3.0 h. Finally, a 100 ml mixture of petroleum ether and
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diethyl ether was added into the mixture and the resulting precipitate was collected.
After being dried under vacuum, brown solid (Pt/CFP, 216 mg) was obtained. Pt/CFP
(50 mg) was calcined in air at 600 ℃ for 3 h. The final ash was dissolved in mixture
HNO3 and HCl aqueous solution (4.0 ml). The solution was diluted to 100 ml with
dionized water. The Pt concentration was determined by ICP-AES. The calculated Pt
load of Pt/CFP was 2.02 wt%.
CFP (400 mg) was fully swollen in 50 ml anhydrous ethanol. And then
PdCl2(CH3CN)2 (70 mg, 0.27 mmol) was added. The mixture was further stirred at
room temperature for 20 h. NaBH4 (70 mg, 1.85 mmol) in 16 ml water was injected
and the reduction proceeded for 3.0 h. Finally, a 150 ml mixture of petroleum ether
and diethyl ether was added into the mixture and the resulting precipitate was
collected. After being dried under vacuum, deep gray solid (Pd/CFP, 392 mg) was
obtained. Pd/CFP (50 mg) was calcined in air at 600 ℃ for 3 h. The final ash was
dissolved in mixture HNO3 and HCl aqueous solution (4.0 ml). The solution was
diluted to 100 ml with dionized water. The Pd concentration was determined by
ICP-AES. The calculated Pd load of Pd/CFP was 1.10 wt%.
2.4 Hydrogenation of nitrobenzenes over Pt/CFP
To a 10-ml Schlenk tube, were added nitrobenzene (1.0 mmol), THF (2.0 ml) and the
catalyst (S/Pt: 4000). The Schlenk tube was flushed by hydrogen flow and then
directly connected to a hydrogen balloon (1.0 atm). After the given reaction time, the
catalyst was separated by simple filtration. The reaction results were determined by
GC. In the seven-run recycling test, the catalyst was separated by filtration and
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washed by ethyl acetate and diehtyl ehter after each testing run. The catalyst was dried
vacuum and then recycled into the next batch.
2.5 Carbonylation of aryl iodide in the presence of secondary amines over Pd/CFP
To a 100-ml autoclave, were added aryl iodide (1.0 mmol), secondary amine (5.0
mmol), the catalyst (Pd: 0.78 mol%), Cs2CO3 (1.14 g, 3.5 mmol), LiI (9.0 mg, 0.0479
mmol) and toluene (5.0 ml). The autoclave was flushed by CO flow and pressurized
to 4.0 MPa. The reaction was performed at 120 ℃ for the given time. The reaction
mixture was extracted with ethyl acetate. Double carbonylation and mono
carbonylation products were obtained by purification over silica column with the
eluent of ethyl acetate/ petroleum ether (60-90 ℃) (v/v, 1:4). In the twelve-run
recycling test, the catalyst was separated by filtration and washed sequently by water,
ethyl acetate and diehtyl ehter after each testing run. The catalyst was dried vacuum
and then recycled into the next batch.
2.6 Carbonylation of aryl iodide in the presence of acylhydrazines over Pd/CFP
To a 100-ml autoclave, were added aryl iodide (1.0 mmol), acylhydrazine (2.0 mmol),
the catalyst (Pd: 0.78 mol%), base (1.06 mmol) and DMSO (5.0 ml). The autoclave
was flushed by CO flow and pressurized to 4.0 MPa. The reaction was performed at
120 ℃ for the given time. The reaction mixture was extracted with ethyl acetate. The
carbonylation product was obtained by purification over silica column with the eluent
of ethyl acetate/ petroleum ether (60-90 ℃) (v/v, 2:1). In the five-run recycling test,
the catalyst was separated by adding 30 ml mixture of petroleum ether and diethyl
ether and washed by ethyl acetate and diehtyl ehter after each testing run. The catalyst
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was dried vacuum and then recycled into the next batch.
2.7 Materials characterization
X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL
electron spectrometer from VG Scientific using 300W Al-Kα radiation. The base
pressure was about 3×10-9 mbar. The binding energies were referenced to the C1s line
at 284.8 eV from adventitious carbon. Eclipse V2.1 data analysis software supplied by
the VG ESCA-Lab200I-XL instrument manufacturer was applied in manipulation of
the acquired spectra. Transmission electron microscopy (TEM) and EDS were
obtained by a JEOL 2010 TEM with an accelerating voltage of 200 kV. FTIR was
performed over Bruker TENSOR-27. Gel Permeation Chromatography (GPC) was
carried out by Water 1515 with the solvent of DMF. Thermal gravimetric analysis was
performed over STA 409PC. ICP-AES was performed by Perkin Elmer Optima 5300
dv. GC was carried out over GC-2014 (SHIMADZU) with high temperature capillary
column (MXT-1, 30m, 0.25mm ID) and FID detector.
Silica gel (200-300 mesh) was used for column chromatography. Melting points
1
were recorded on an uncorrected Melting Point X-4 instrument. H NMR and 13C
NMR spectra were recorded on a 400 MHz spectrometer (Bruker Avance 400 MHz).
Spectra were referenced internally to the residual proton resonance in CDCl3 (δ 7.26
13
ppm). Chemical shifts (δ) were reported as part per million (ppm). C NMR spectra
were referenced to CDCl3 (δ 77.16 ppm, the middle peak). Mass spectra (MS) were
1
obtained on a Thermo Finnigan Surveyor MQS-plus mass spectrometer. H-NMR,
13C-NMR and MS spectra are provided in Supporting Information.
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3. Results and Discussion
3.1 Cross-linked functional polymer (CFP) and immobilized metal nanoparticles
The three-dimensional coplymeric architectures (CFPs) are prepared through the
copolymerization between 1, 3, 4, 6-tetraallylglycoluril and 4-vinyl pyridine. FTIR
spectrum (Fig. S1) reveals that the materials are the copolymeric architectures of
4-vinyl pyridine and 1, 3, 4, 6-tetraallylglycoluril. 1598 cm-1 and 1471 cm-1 are
skeleton vibration absorption peaks of six-member cycle of pyridine. 1700 cm-1 is the
stretching vibration absorption peak attributed to a carbonyl group in glycoluril unit.
2927 cm-1 and 3022 cm-1 are stretching vibration absorption peaks of C-H bond. The
weak peak of 1640 cm-1 is the stretching vibration absorption of unpolymerized
double carbon-carbon bond. CFP begins to decompose at 308 ℃ and ends at 440 ℃
(thermal gravimetric analysis, Fig. S2). The average molecular weight and the
distribution of CFP are determined by Gel Permeation Chromatography (GPC) (Fig.
S3). Number-average molecular weight (Mn) is 13026 and weight-average molecular
(Mw) is 39284. The polydispersity index (Mw/Mn) of CFP is 3.01. All chemical
elements are detected by XPS survey spectrum of the copolymer of 4-vinyl pyridine
and 1, 3, 4, 6-tetraallylglycoluril (Fig. S4).
The Mo/CFP samples are illustrated in Scheme 1. The dispersion of metal
nanoparticles and their chemical states are detailedly characterized by TEM, EDS, and
XPS. The dispersion of Pt nanoparticles is revealed by TEM image (Fig. 1a). All
nanoparticles are kept in the monodispersed state and the average size is about 2.0 nm.
EDS of the selected region shows the presence of Pt species over the polymer. The
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peaks of copper species are from the copper sample net. Fig. 1c shows Pt XPS peaks
of the incipient metalated CFP. Pt species are unreduced and the binding energy of Pt
2-
4f7/2 is 73.14 eV, which is indexed to the precursor (PtCl4 ) [34]. After reduction with
NaBH4, the Pt XPS peaks are complicated. Fitting of the Pt 4f XPS envelope results
in the identification of two chemical states of platinum: one with a binding energy of
72.2 eV with an atomic concentration of 52.4 % and another at 73.2 eV with an
atomic concentration of 47.6 % (Fig. 1d). Pt 4f7/2 at the binding energy of 72.2 eV is
attributed to zero-vanlent Pt nanoparticles. This value is about 1.0 eV higher than the
binding energy of Pt 4f7/2 of bulk Pt metal. The size - effect of nanoparticles causes
this difference. Pt 4f7/2 at the binding energy of 73.2 eV is attributed to unreduced
2-
PtCl4 species. In the reduction process, the surplus amount of NaBH4 and enough
reaction time are applied.
Fig. 2a shows the dispersion of Pd nanoparticles over CFP. The average size of
monodispersed Pd nanoparticles is about 4.5 nm. EDS of the selected region reveals
the presence of Pd species (Fig. 2b). Fitting of the Pd 3d XPS envelope of the reduced
Pd/CFP results in the identification of two chemical states of palladium: one with a
binding energy of 334.9 eV, which is attributed to zero-valent Pd nanoparticles and
another at 336.9 eV of Pd (II). The coexistence of Pd (0) and Pd (II) over the
polymeric support occurs for the same reason as Pt/CFP.
As revealed by the TEM characterization, polymer-supported metal nanoparticles
are mainly embedded in a swollen polymer matrix. Without a swelling agent, metal
nanoparticles are tightly enclosed by the polymer framework and the interaction
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between the reactants and the catalytic sites is impossible. (One sentence was deleted.)
The swollen polymer-supported catalyst contain temporary nanopores and mesopores
than improve the accessibility of the catalytic active sites. The pores of the swollen
CFPs have diameters in the order of nanometer (< 50 nm), which are suitable for
technical application in catalysis. The as-synthesized Pt/CFP is tested in THF for
hydrogenation of nitrobenzens and two types of carbonylation of aryl iodide over
CFP-supported Pd are performed in toluene or DMSO. The reaction solvents are good
swelling agents for CFPs.
3.2 Hydrogenation of nitrobenzenes over Pt /CFP and carbonylation of aryl iodide
over Pd /CFP
Hydrogenation is by far the most intensively investigated catalytic reactions. Platinum
and palladium are generally harnessed as active metals. There are many comparative
researches about metal catalysts supported on CFPs with those on traditional supports
[25, 26]. Hydrogenation of nitrobenzenes is selected as the model reaction to
investigate the catalytic performance of Pt/CFP, as listed in Table 1. (Two sentences
were deleted.) Hydrogenation of nitrobenzene is performed under mild conditions.
Commerical available Pt/C is used as the benchmark catalyst. Pt/CFP is more efficient
than Pt/C when the metal amount of Pt/CFP is half of Pt/C. Total conversions of
several nitrobenzens are achieved with the substrate/Pt ratio of 4000 under low
pressure of hydrogenation at room temperature. Substituent groups show little
influence on the hydorgenation activity. Ortho-substituted substrate has some decrease
in the final yield of hydrogenation product. The subtle relation between
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microstructures of CFP-supported Pt nanoparticles and their catalytic performance
will be investigated in the following section.
Carbonylation is a very important process for large-scale production of industrial
chemicals: the carbonylation of methanol to acetic acid and the hydrocarbonylation of
olefins to aldehydes. However, direct carbonylation with carbon monoxide attracts
little attention in complex organic synthesis. Palladium-catalyzed carbonylation
reactions of aromatic halides are rapidly-emerging carbonylative transformations to
synthesize important intermediates (carboxylic acid derivatives, aldehydes, and
ketones) in the manufacture of dyes, pharmaceuticals, agrochemicals, and other
industrial products [35]. (One sentence was deleted.) The most employed catalysts are
homogeneous palladium complexes. Heterogeneous catalysts are comparatively
seldom considered. Actually, there are three inevitable problems to supported metal
nanoparticle catalysts in carbonylation. First, the formation of metal carbonyl
compound increases the mobility of metal nanoparticles during the reaction. Second,
halide ions may lead to the deactivation of surfaces of metal nanoparticles. Third,
strong base additives may cause the instability of the supporting materials. These
problems are adverse factors of the recoverability and recyclability of supported metal
nanoparticles. However, well-designed support microstructures and attachment
strategy greatly mitigate the deteriorating trend during the catalytic conversion [36,
37].
Pd/CFP is used as the catalyst for carbonylation of iodobenzene in the presence of
secondary amines. Double carbonylation competes with monocarbonylation. The
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main reaction is the double carbonylation process (α-ketoamides). The double
carbonylation of (hetero) aryl, alkenyl, and alkyl halides has been achieved in the
presence of simple palladium complexes [38, 39]. To the best of our knowledge, the
results in Table 2 are the newest discovery of double carbonylation of iodobenzenes
over a heterogeneous palladium catalyst. Iodobenzene is totally converted and the
yield of double carbonylation product nearly reaches 80%.
Iodide salts were proven to promotors for the carbonylation process [40]. Several
iodide salts were attempted to explore their promoting effects on double carbonylation
(Fig. S5a). LiI is the best promotor with a dosage of approximately 5 mol% of
iodobenzene. Increasing the adding amount of LiI causes marked loss of the
selectivity to the double carbonylation (Fig. S5b).
The selectivity to α-ketoamide is related to the secondary amine. The
carbonylation with hexamethylene imine shows the highest selecltivity to
α-ketoamides (Table 2). (One sentence was deleted.) In the carbonylation of
iodobenezene, primary amines are not used as the nucleophiles because of the
formation of imide [41]. For the carbonylation over Pd/CFP, the use of n-butylamine
as the nucleophile results in the α-ketoamide selectivity of 65.8% and no imide
products are detected in the final reaction mixture. The iodobenzenes with
electron-withdrawing groups show lower activity in carbonylation and poor
selectivity to double carbonylation products.
The scope of aminocarbonylation is extended by using acylhydrazines as the
nucleophiles. The products of aminocarbonylation of acylhydrazines are important
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precursors of pharmaceuticals and agrochemical [42]. The general method involves
stoichiometric acyl chloride and leads to a large amount of acid and salt wastes.
Carbonylation of aryl iodide with acylhydrazines is a green and efficient route to
synthesize diacylhydrazines. The reaction conditions, including solvent, base, and
reaction pressure, are detailedly screened (Table S1). Polar solvents are suitable for
the aminocarbonylation of acylhydrazines and DMSO is the best solvent. DMF can be
involved in the carbonylation and has a negative impact on the yield of target product
[43]. When Cs2CO3 is used as the base, there is no carbonylation product. The use of
organic bases, such as trialkylamines and N-heterocycle compounds, leads to the
formation of the target products. Pyrazole and triethylamine are the two best bases
(Table S1). (One sentence was deleted.) Because of the cost and post-treatment,
triethylamine is selected as the base. Various substrates are tested in this reaction
(Table 3). The product yields are in the range from 54.5% to 95.8%. (Two sentences
were deleted.) A unique advantage of aminocarbonylation of acylhydrazines is the
direct formation of diacylhydrazine with complex substituent groups. A selected
iodobenzene molecule can be paired with a certain type of acylhydrazine to produce a
well-designed diacylhydrazine molecule. As listed in Table 3, fourteen
diacylhydrazine molecules are synthesized through the direct carbonylation over
Pd/CFP.
3.3 The recyclability and recoverability of Pt /CFP in catalytic hydrogenation of
nitrobenzene
The recyclability and recoverability of Pt/CFP in hydrogenation of nitrobenzene are
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further investigated through a seven-run recycling test (Scheme 2). After the sixth
recycling run, there is a marked decrease in the catalytic activity. (Three sentences
were deleted.) XPS measurement reveals involvement and transformation of Pt
nanoparticles in the catalytic cycle. After the fifth recycling test, the proportion of
zero-valent Pt species decreases markedly (Fig. 3a, b). Fig. 3c shows the curve of Pt0/
Pt2+ XPS intensity ratio’s changes in each recycling run. The value decreases
gradually after each recycling run. Spontaneous reduction of Pt (II) species is difficult
to proceed under hydrogenation conditions. In our experimental observation, Pt
(II)/CFP have very low activity in catalyzing hydrogenation of nitrobenzene and no Pt
(0) species were detected after the treatment under hydrogenation conditions. The
gradual transformation from Pt (0) to Pt (II) species directly leads to unrecoverable
loss of catalytic activity.
For the fresh catalyst (Fig. 1a), Pt nanoparticles are kept in a uniform dispersion
and their average size is approxomately 1.4 nm. TEM image of the used catalyst
after the fifth run shows some growth in size and their average size is 2.7 nm. The
comparison between the particle density in Region 1 and that in Region 2 reveals the
redispersion of Pt nanoparticles (Fig. S6) during the reaction process. The
redispersion of Pt nanoparticles in CFP matrix is mainly attributed to the mobility of
solutes and self-diffusion of solvents inside swollen CFPs under reaction conditions.
(Two sentences were deleted.) Metal nanoparticles are inevitably involved in the
catalytic cycles and their flexibility during the reaction determines the extent of
activity regeneration after the reaction. Gradual size growth from 1.4 nm to 2.7 nm
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and redispersion is another reason for unrecoverable loss of catalytic activity.
3.4 The flexibility of Pd /CFP during catalytic carbonylation of aryl iodide
As shown by the catalytic cycle of palladiun-catalyzed double carbonylation of
iodobenzenes (Scheme S1), palladium species are intensively involved in several key
steps. Their transfer between the supported catalyst and the reaction solution is
attributed to inevitably leaching of active metal species from palladium nanoparticles.
(Two sentences were deleted.) The recyclability and recoverability of CFP-supported
palladium nanoparticles are thoroughly investigated through a twelve-run recycling
test of double carbonylation of iodobenzene in the presence of hexamethylene imine
(Fig. 4). There are little fluctuations in the catalytic performance during the incipient
six recycling runs. From the seventh to the twelfth recycling run, the conversion of
iodobenzene is approximately 100% and the selectivity to the double carbonylation
product increases gradually from 80.0% to 95.1%. After each test batch, the
concentration of leaching palladium species in the reaction mixture is detected by
ICP-OES. The leaching metal amount is negligible and shows no effect on the
catalytic performance.
For supported palladium catalysts (complexes or nanoparticles), the leaching of
palladium by the reaction flow is a serious problem [44, 45]. However, well-designed
support structures and attachment strategy can efficiently immobilize the active
centers and then greatly mitigate metal leaching during the catalytic conversion [36,
37]. (Two sentences were deleted.) The pyridine groups of CFPs could anchor and
trap palladium carbonyl complexes from the catalytic cycle. The swollen copolymeric
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gel phases of CFPs facilitate completing the entire carbonylation cycle inside the
copolymeric architectures, and greatly mitigate the palladium leaching during mass
transfer between the catalysts and the reaction solution.
The redispersion of palladium nanoparticles during the recycling test is
characterized by TEM (Fig. S7a-d). (Two sentences were deleted.) With the recycling
test proceeding, giant clusters of palladium nanoparticles are formed, but no
agglomeration is observed. (One sentence was deleted.) The formation of giant
clusters of palladium nanoparticles is attributed to swollen gel micro-phases in
cross-linked polymer framework and molecular mobility and diffusion by the reaction
flow [46]. The palladium species transfer among the nanoparticles in a certain form of
palladium carbonyl intermediate, which can be trapped and decompose to generate
new palladium nanoparticles. The palladium chemical states of the used catalyst (after
the ninth recycling test) are detected by XPS (Fig. S7e). (One sentence was deleted.)
The 3d5/2 peak at 335.1 eV is attributed to zero-valent palladium nanoparticles.
Another 3d5/2 peak at 338.1eV is indexed to Pd (II), which are trapped by functional
sites over copolymeric architectures. The Pd0/Pd2+ XPS intensity ratio is calculated as
1.53. This value of the fresh catalyst is 0.78 (Fig. 2c). (Two sentences were deleted.) It
has been demonstrated that Pd (0) species are active catalytic centers for double
carbonylation [47]. The increase in their proportion has directly led to the improved
selectivity to double carbonylation products, as shown in Fig. 4.
The hot filtration test is performed to directly define the metal leaching during the
reaction [48]. Double carbonylation of iodobenzene in the presence of hexamethylene
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imine is carried out for 4.0 h and then the reaction liquid mixture is drained into
another autoclave through a stainless steel pipe with inter filter membrane. The hot
reaction mixture without Pd/CFP is further stirred under typical CO pressure and
temperature with newly added base (Fig. 5a). The amount of carbonylation products
in the mixture is measured each hour in the sequent reaction period. No more products
are formed and the carbonylation ceases. It is demonstrated that there are no leached
palladium species in the reaction mixture, which are active in the carbonylation of
iodobenzene. (Two sentences were deleted.) Double carbonylation of iodobenzene in
the presence of hexamethylene imine is performed with PdCl2[CH3CN]2 and blank
CFP (Fig. 5b). In Region 1, the carbonylation shows a gradual increase in the yield of
carbonylation products. In Region 2, the reaction is terminated and CFP is separated
and cleaned. In Region 3, the separated CFP is used as the catalyst for iodobenzene
carbonylation under the same reaction conditions as in Region 1. The separated CFP
shows catalytic activity in the carbonylation and its TEM image (the insert in Fig. 5b)
reveals the presence palladium nanoparticles over CFP. The blank CFP captures and
traps active palladium carbonyl species, which in situ decompose to generate
palladium nanoparticles.
(One paragraph was deleted.)
A five-run recycling test is carried out over Pd/CFP for aminocarbonylation of
acylhydrazines (Scheme S2). We observe a marked loss of catalytic activity and
leached palladium species in the reaction mixture after each test run (Scheme S3).
TEM image of the used catalyst after the fifth run reveals that palladium nanoparticles
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are transferred from CFP framework to the surface. (One sentence was deleted.)
Pd/CFP is totally dissolved in DMSO under the reaction conditions and separated
from the reaction mixture by adding diehtyl ehter. Products of aminocarbonylation of
acylhydrazines are chelating ligands of palladium species. Their intensive
involvement in the catalytic cycle facilitates leaching of palladium species with the
reaction flow. The leaching of palladium species causes the loss of catalytic activity in
the five-run recycling test.
4. Conclusions
In summary, 1, 3, 4, 6-tetraallylglycoluril was used as the cross-linker for preparing a
cross-linked functional polymer with the monomer of 4-vinyl pyridine. Pt and Pd
nanoparticles over the copolymeric support were introduced by the incipient
metalation of the swollen polymeric framework with the precursors and the sequent
reduction. Pt nanoparticles are kept in the monodispersed state and the average size is
about 1.4 nm. The average size of monodispersed Pd nanoparticles is about 4.5 nm.
Pt/CFP shows excellent catalytic performance in hydrogenation of nitrobenzenes
under mild reaction reactions. Total conversions of several nitrobenzens are achieved
with the substrate / Pt ratio of 4000 under low pressure of hydrogenation at room
temperature. Pd/CFP is used as the catalyst for carbonylation of iodobenzene in the
presence of secondary amines. The main reaction is the double carbonylation process
(α-ketoamides) with the highest selectivity of 80%. A selected iodobenzene molecule
can be paired with a certain type of acylhydrazine to produce a well-designed
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diacylhydrazine molecule through the direct carbonylation over Pd/CFP. Supported Pt
or Pd nanoparticles are intensively involved in the catalytic cycle. (One sentence was
deleted.) The flexibility of metal nanoparticles and CFP supports guarantees
macroscopic catalytic robustness. There are two main mechanisms for confining the
irreversible transfer of metal nanoparticles: anchoring and trapping function of
cross-linked functional copolymeric architectures and the presence of temporary
nanopores and mesopores in the swollen CFP framework.
Acknowledgements
This work was financially supported by the National Natural Sciences Foundation of
China (No. 21101161, 21174148, 21304101) and YMB New Energy Co.
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Scheme 1 The illustration of preparation procedure of cross-linked functional
polymer supported metal nanopartixles.
Scheme 2 Seven-run recycling test of Pt/CFP for hydrogenation of nitrobenzene.
Fig. 1 a) TEM image of CFP supported Pt nanoparticles. b) EDS of the selected
region. c) Pt XPS peaks of the unreduced Pt/CFP sample. d) Pt XPS peaks of the
reduced Pt/CFP sample.
Fig. 2 a) TEM image of CFP supported Pd nanoparticles. b) EDS of the selected
region. c) Comparison of Pd XPS peaks between the reduced Pd/CFP and the
unreduced Pd/CFP.
Fig. 3 a) Pt XPS peaks of the fresh catalyst. b) Pt XPS peaks of the used catalyst after
the fifth recycling test. c) The curve of Pt0 / Pt2+ XPS intensity ratio’s changes in each
recycling run.
Fig. 4 The twelve-run recycling test of double carbonylation of iodobenzene in
precence of hexamethylene imine over Pd/CFP.
Fig. 5 a) The hot filtration test of Pd/CFP for carbonylation of iodobenzene in the
precence of hexamethylene imine. c) Carbonylation of iodobenzene in the precence of
hexamethylene imine performed with PdCl2[CH3CN]2 and blank CFP (the insert is
TEM image of the used CFP).
Table 1 Hydrogenation of nitrobenzenes over Pt/CFP.
Table 2 Carbonylation of aryl iodides in the presence of secondary amines.
Table 3 Carbonylation of aryl iodides in the presence of acylhydrazines.
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Scheme 1 The illustration of preparation procedure of cross-linked functional
polymer supported metal nanopartixles.
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Scheme 2 Seven-run recycling test of Pt/CFP for hydrogenation of nitrobenzene.
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Fig. 1 a) TEM image of CFP supported Pt nanoparticles. b) EDS of the selected
region. c) Pt XPS peaks of the unreduced Pt/CFP sample. d) Pt XPS peaks of the
reduced Pt/CFP sample.
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