Metallo-supramolecular polymer engineered porous carbon framework encapsulated stable ultra-small nanoparticles: A general approach to construct highly dispersed catalysts

Article abstract of DOI:10.1039/c8ta05369c

The development of a general approach for fabricating stable ultra-small heterogeneous nanocatalysts has been intensively pursued. However, issues related to complex synthesis processes and structural stability have restricted their investigation and application. Here we report a facile organometallic conjunction strategy for the large-scale fabrication of porous carbon framework encapsulated highly dispersed sub-3 nm ultra-small nanoparticles (USMNPs@PCF). This methodology is based on the convenient aldol condensation reaction to manufacture a metallo-supramolecular polymer precursor and then consequent annealing to form the target nanocomposite. This technique was successfully applied to the preparation of varieties of USMNPs@PCF, including Fe, Co, Ni, Mo, Ru, Rh, Pd and Pt. As a representative application, the PCF encapsulated sub-3 nm Pd nanoparticles demonstrated remarkable durability and efficiency for chemoselective hydrogenation of nitroarenes to their corresponding anilines under ambient conditions with low catalyst loading. All hydrogenation reactions can complete in 4 min with >99% conversion and >99% chemoselectivity. The turnover frequency (TOF) was up to 11:400 h^-1 for p-nitrophenol. This work provides a general, scalable and economical route for the manufacture of sub-3 nm and highly dispersed nanocomposites, which can be used in many other important fields, such as electrochemistry, energy science and environmental protection.

Full text of DOI:10.1039/c8ta05369c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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ARTICLE
Metallo-supramolecular polymers engineered porous carbon
frameworks encapsulated stable ultra-small nanoparticles: a
general approach to construct highly dispersed catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Yongjian Ai,a Mengqi He,b Feng Zhang,a Yang Long,a Yunzheng Li,a Qiang Han,a Mingyu Ding,a Hong-
DOI: 10.1039/x0xx00000x
bin Sun,*b and Qionglin Liang*a
www.rsc.org/
The development of a general approach to fabricating stable ultra-small heterogeneous nanocatalysts has been intensively
pursued. However, issues related to complex synthesis processes and structure stability have restricts their investigation
and application. Here we report a facile organometallics conjunction strategy for the large-scale fabrication of porous
carbon frameworks encapsulated highly dispersed sub-3 nm ultra-small nanoparticles (USMNPs@PCF). The methodology
is based on the convenient aldol condensation reaction to manufacture the metallo-supramolecular polymer precursor,
then a consequent annealing to forms the target nanocomposite. This technique was successfully applied to the
preparation of varieties of USMNPs@PCF, including Fe, Co, Ni, Mo, Ru, Rh, Pd and Pt. As a representative application, the
PCF encapsulated sub-3 nm Pd nanoparticles demonstrated remarkably durability and efficiency for chemoselective
hydrogenation of nitroarenes to the corresponding anilines under ambient condition with low catalyst loading. All
hydrogenation reactions can complete in 4 min with >99% conversion and >99% chemoselectivity. The turnover frequency
(TOF) was up to 11400 h-1 for p-nitrophenol. This work provides a general, scalable and economical route to the
manufacture of sub-3 nm and highly dispersed nanocomposites, which can be used in many other important fields, such as
electrochemistry, energy science and environmental protection.
and his colleagues developed the cluster beam method for
synthesizing ultra-small Au clusters.16 Hyeon and co-workers
reported the solution-phase colloidal method for the
1. Introduction
Metal or metal oxides constitute the most significant
heterogeneous catalytic systems for chemical transformation.
fabrication of ultra-small nanoparticles.17 Browning used
aromatic liquids in the electron microscope to fabricated sub-3
nm palladium nanoparticles.18 Recently, Wang described the
wettability-driven technique for the fabrication of ultrafine Pd
nanoparticles.19 However, these techniques are not universal
for more than one specific metal and the individual ultra-small
nanoparticles are structurally unstable.20-21 Intrinsically, as the
decrease of the metal particles to several nano-size, the
specific surface area was enhanced and the surface free
energy of metals increases steeply.22-23 Consequently, during
the application process, these nanocatalysts will aggregate
into bulk particles and finally lose their original catalytic
activity.24-25 Confronting of the above challenge, designing a
universal strategy for the scalable engineering of ultra-small
nanocatalysts with structurally stable active phase is of crucial
relevance for academic research.
1-2
Generally, an outstanding catalyst possesses two significant
metrics at least, that is the robust catalytic activity and reliable
stability.3-5 Considering this, numerous efforts have been
focused on the synthesis of ultra-small (1-3 nm) nanocatalyst
with reproducibility and mono-dispersity.6-10 The mono-
dispersed catalysts consist of less than 500 atoms for each
ultrafine activity site, thus remarkably enhances the ratio of
surface atom. For example, the utilization efficiency of
platinum atoms can be increased from 9.5 to 26% by reducing
the size of Pt nanoparticles from 11.7 to 3.9 nm. Moreover,
the tiny particle’s optical, electrical and chemical properties
are distinctly different from the free molecule or larger-sized
counterpart.11-15 A plenty of pioneering endeavors have been
reported on the synthesis of ultra-small nanoparticles. Kappes
Loading the ultra-small nanoparticles onto appropriate
supporter is a practical avenue for this challenge.26-29 Varieties
of materials, such as silicon, metal-oxides, zeolite and carbon
have been used as carriers to fabricate heterogeneous
catalysts, as they are reusable without causing any dispose
problems.30-33 Since the beginning of 21st century, the
exploration of carbonaceous heterogeneous catalysts is a
a.Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry
of Education), Beijing Key Lab of Microanalytical Methods & Instrumentation,
Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China. *E-
mail: liangql@tsinghua.edu.cn
b.Department of Chemistry, Northeastern University, Shenyang 110819, P.R. China.
*E-mail: sunhb@mail.neu.edu.cn
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Scheme 1. The general methodology for the engineering of M@PCF (M= Fe, Co, Ni, Mo, Ru, Rh, Pd and Pt)
highly prior research direction for satisfying the sustainable selected as the probe reaction to examine the catalytic activity.
development and green chemistry requirement.34 Metallo- In our test, all of the prepared catalysts were active. Especially,
supramolecular polymers fabricated carbonaceous catalysts PCF encapsulated sub-3 nm Pd nanoparticles (Pd@PCF)
have attracted considerable attention, not only for their exhibited high durability and inimitable efficiency for
fundamental scientific interest, but also for many chemoselective hydrogenation of nitroarenes to the
technological applications.35 The metallo-polymers were corresponding anilines under room temperature. With only 0.2%
synthesized through the metal-ion ligand coordination catalyst loading, all the reactions can complete in 4 min (TOF>
interactions and the metal centers were incorporated into the 11400 h-1 for p-nitrophenol). The conversion and
backbone of polymers substrate. Thus, the derived chemoselectivity were up to 99%. What’s more, the catalyst
carbonaceous catalysts own many unique properties, such as system can be used for more than 10 times without any
large surface areas, outasite activity center scattering, catalyst performance weakening.
sumptuous chemical and mechanical stability.36-38 With those
properties,
carbonaceous
catalysts
have
exhibited
2. Experimental
2.1. Typical procedure for the preparation of metallo-
supramolecular polymeric precursor
unimaginable and unlimited potential in heterogeneous
catalysis.
In the past decades, lots of highly active carbocatalysts have
been reported.39-40 They have been widely used in various
chemical and electrochemical protocols, including oxidation -
reduction reaction (ORR), oxygen evolution reaction (OER),
hydrogen evolution reaction (HER), catalytic coupling reaction
and hydrogenation.41-45 In heterogeneous catalysis, metal-
carbon nanostructures have been studied as the most
universal platform for hydrogenation. The development of
heterogeneous catalysts with high activity, extreme stability
and good regioselectivity is highly meaningful.46 Up to now,
numerous nano noble or non-noble metal catalysts with
various block and nanostructures have been developed.
However, most of the catalysts preparation methodologies are
The methodology for the engineering of Pd@PCF was
described as the following steps. Firstly, 100 mg Pd(acac)2 was
completely dissolved in 50 mL Hacac under ultrasound, then
the mixture solution was added 16 g NaOH under vigorous stir.
After consequently stirred for 40 min, the mixture was turned
into homogeneous pasty. Finally, the pasty was deposed at
homoeothermy without any special treatment. Two weeks
later, the blend transformed into metallo-supramolecular
polymeric precursor.
2.2. Typical procedure for the preparation of Pd@PCF
nanocatalyst
The solid polymeride precursors were first ground into
powders and placed in the porcelain boat. After drying the
powders at 80 °C overnight, this precursor was carbonized at
400 °C for 2 h in high purity N2 atmosphere with a heating rate
of 5 °C min−1. After cooling to room temperature, the products
were ground and washed with ethanol and deionized water to
remove NaOH and other impurities. Finally, the
nanocomposites were filtrated under reduced pressure and
dried at 100 °C overnight to obtain the ultimate Pd@PCF
nanocatalyst.
Other metals of Fe, Co, Ni, Mo, Ru, Rh, Pt were treated with
the same method for preparing the corresponding PCF
encapsulated ultra-small metal nanocomposites.
2.3. The general process for the reduction of nitroarene by
NaBH4
non-universal and unscalable. The designing of
a
straightforward tactic to the scalable fabrication of stable
heterogeneous catalyst has attracted chemists and materials
scientists’ great attention.
Herein, we present a general strategy for the scalable
fabrication of stable porous carbon frameworks supported
sub-3 nm ultra-small metals (oxides) nanoparticles (Scheme 1).
This methodology was based on the cross-linking of Metal-
(acetylacetonate)x ((M(acac)x) with acetylacetonate (Hacac)
under strong alkali condition at room temperature, then the
conjuncted metallo-supramolecular polymer precursors were
carbonized at 400 °C for 2 h in N2, with a heating rate of 5 °C
min−1. This route was successfully applied to prepare eight
mostly used ultra-small metals (oxides) nanoparticles. For
noble metal such as Ru, Rh, Pd and Pt, the metal exists as zero
value, but for non-noble metal Fe, Co, Ni and Mo, they are
oxides. The hydrogenation of functionalized nitroarenes was
The typical procedure was described as the following: 0.1
mmol p-nitrophenol, 7 mg (2 wt‰) Pd@PCF was added in to 2
mL deionized water, then the mixture solution was sonicated
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to dissolve the nitro compound. After that, 50 mg NaBH4 was
added into the reaction solution under room temperature. The
reaction was extremely intense, and 4 min later the yellow
blend converted to the colourless one, which suggested that p-
nitrophenol was entirely reduced to p-aminophenol. The
detailed reaction procedure was further detected by the UV-
visible spectrophotometer (UV-6100 double-beam spectroph
otometer). This reaction system was also applied to the
hydrogenation of varieties of nitroaromatics with different
functional groups. All reactions were monitored by TLC
(petroleum ether/ethyl acetate), HPLC (Waters, Kromasil 5µm
C18 column) and detected by GC-MS (Bruker 450GC-320MS).
View Article Online
DOI: 10.1039/C8TA05369C
(a1)
(a2)
(b1)
(b2)
(c1)
(c2)
Results and discussion
3.1. Mechanism for the fabrication of metallo-supramolecular
polymer
Figure 2. HRTEM and STEM images of Pd@PCF with different
dosage of Pd(acac)2/Hacac (mg/mL): (a1-a2) 200/50, (b1-b2)
50/50, (c1-c2) 10/50.
The synthesis of palladium constructed metal-porous carbon
composite was elected as the probe process. The morphology
and structure of the metallo-supramolecular polymer
precursor were firstly characterized by scanning electron
microscopy (SEM). As shown in Figure 1(a-b), it is obvious that
the structure of the metallo-supramolecular hybrids was
sheet-like and stacked layer by layer. We further proposed the
mechanism for the synthesis of metallo-supramolecular
polymer. The carbon sheet skeletons were formed by the
homogeneous aldol condensation reaction, which connected
the Pd(acac)2 and Hacac molecules (Figure S1 in the ESM).
During the process, Hacac was deprotonated to generate the
carbon anion intermediates, which acted as a nucleophile to
attack the carbonyl carbon atom of another Hacac molecule or
M(acac)x. Finally, as the crossing-link and dehydration
continued, the layer by layer sheet-like metallo-
supramolecular polymer precursor was finally fabricated.
In order to verify the mechanism, the FT-IR (Figure 1(c)) and
the solid-state NMR ((Figure 1(d)) characterization were
conducted. From the FT-IR spectrum, the absorption bands at
3436, 2960, 1710, 1660, 1450 and 1380 cm-1 are attributed to
the corresponding C-OH, CH2/CH3, C=O, C=C, C-O and CH2
group, respectively. The solid-state NMR shows that the
chemical shifts δ are 28.81, 38.85, 108.28, 119.17, 187.73 and
198.72 ppm which belongs to the carbons of CH2, CH2/CH3,
C=C, C=C-OH, C=C-CO and C=O. The characterization results
illustrated that the metallo-supramolecular polymer
precursors were fabricated by the aldol condensation as we
assumed. Due to the high unsaturation degree of the
precursor, the metallo-supramolecular polymer could be
carbonized under relative low temperature as the TGA
characterization (Figure S2 in the ESM) indicated, and this was
agreed with Raman test (Figure S10).
(c)
(b)
(a)
CH2/CH3
C-OH
C=O
C=C
C-O
CH2
1500
4000
3500
3000
2500
2000
1000
Wavenumber (cm-1
)
(d)
3.2. Fabrication of ultra-small Pd@PCF nanoparticles
After understanding the mechanism of the manufacture of the
metallo-supramolecular polymer precursors, we predicted that
this precursor was an excellent candidate to create ultra-small
metal nanocomposites that were encapsulated by porous
carbon frameworks. We conducted a series of investigations to
fabricate ultra-small Pd@PCF nanoparticles by this synthetic
methodology. Originally, the carbonization temperature was
optimized. As shown in Figure S3 in the ESM and Figure 2, as
the temperature decreased from 800 to 600 °C and 400 °C, the
Pd NPs was turned from almost 25 nm into 10 nm and sub 3
nm, respectively. In addition, the influence of Pd(acac)2 loading
in the metallo-supramolecular polymer precursor was also
inspected. The HR-TEM and STEM were used to characterize
the obtained products from adding 200, 50, and 10 mg
Pd(acac)2 to 50 mL Hacac. From Figure 2, the size of the Pd
nanoparticles in the 200 Pd@PCF, 50 Pd@PCF and 10 Pd@PCF
were all sub 3 nm and Pd nanoparticles was evenly dispersed
on the PCF. With the decrease of the Pd(acac)2 dosage, the
Figure 1. SEM images of metallo-supramolecular polymer
precursor (a) Top view, (b) Side view; (c) FT-IR spectrum of the
metallo-supramolecular polymer precursor; (d) The solid-state
13C NMR of metallo-supramolecular polymer precursor.
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(a)
(b)
The HR-TEM images (Figure 3(b)) of the Pd@PCF demonstrates
that the Pd nanoparticles are uniformly encapsulated by the
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PCF and the particle size of the Pd is about 2-3 nm, the Pd
nanoparticle distribution histogram shows that the average
diameter is about 2.0 nm. From the high magnification image
(Figure 3(c)), the d-spacing of adjacent fringes is 0.223 nm and
it is attributed to the (111) crystal plane of metallic Pd. The
STEM image (Figure 3(d)) of the Pd@PCF also indicated that
the Pd NPs are homogeneously distributed on the PCF with the
average size of 1.9 nm. The energy-dispersive X-ray
spectroscopy (EDS, Figure 3(e-g), Figure S4 in the ESM)
mapping obviously shows that the Pd nanoparticles is ultra-
small and the containing of Pd was 0.2 wt%. The solid-state
NMR characterization of the Pd@PCF (Figure S5 in the ESM)
indicated that the peaks at 142 and 152 ppm was
corresponding the Pd-C. The atomic force microscope (AFM,
Figure S6 in the ESM) shows that the ultra-small Pd
nanoparticles was encapsulated by the carbon frameworks
and the thickness of the carbon was about 1.1 nm.
As to investigate the crystalline phase structure of the catalysts
that prepared by annealing at different temperature, the
nanocomposites were characterized by X-ray diffraction (XRD)
(Figure S7 in the ESM). The peak of 2θ= 23° was corresponding
to the (002) facets of the PCF. Furthermore, the intense
diffraction peaks of 2θ= 40, 46.6, and 68° were corresponded
to the (111), (200) and (220) lattice facets of the Pd
(d)
(c)
0.223 nm
(111)
(e)
(f)
(g)
50 nm
50 nm
50 nm
Figure 3. (a) SEM, (b) HRTEM, (c) high magnification (d) STEM nanoparticles respectively. It was obvious that as the
images of Pd@PCF; (e-g) EDS mapping of Pd@PCF.
treatment temperature decreased, the peaks become weaken.
This was attributed to that the nanoparticles were
particles size of the Pd NPs was not demonstrated a significant encapsulated by the PCF.
change. However, the distance between the neighboring Pd Figure S8 in the ESM displays the X-ray photoelectron
nanoparticles was longer. This means that the Pd spectroscopy (XPS) analysis that reveals the valence states of
nanoparticles were formed through collecting Pd2+ in a certain Pd nanoparticles in the catalysts. The surveyed spectrum
territory and the sizes of the nanoparticles were highly indicates that the average atomic concentrations of C, O and
sensitive to carbonization temperature.
Pd on the surface of the catalysts are about 86.33% 13.64%
The sample that containing 100 mg Pd(acac)2 was further and 0.03%, respectively. The spectrum of Pd 3d region
analyzed by more characterization. After the metallo- demonstrated that the peaks of 340.2 eV and 334.8 eV which
supramolecular polymer precursor was annealed in a tube assigned to the Pd 3d3/2 and Pd 3d5/2 of Pd nanoparticles. The
furnace at 400 °C, the morphology has transformed into the inductively coupled plasma mass spectrometry (ICP-MS)
stretched sheet-like carbonaceous materials (Figure 3(a)).
analysis indicated that the accurate weight of Pd is 0.3%, the
Figure 4. HRTEM images of M@PCF: (a) Fe, (b) Co, (c) Ni, (d) Mo, (e) Ru, (f) Rh, (g) Pd, (h) Pt.
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Fe 2p3/2
Mo 3d3/2 Mo 3d5/2
Fe
Co
Ni
(c)
Ni 2p3/2
Co 2p3/2
(d)
(b)
MVieow Article Online
Fe 2p1/2
DOI: 10.1039/C8TA05369C
Ni 2p1/2
Co 2p1/2
240
845
238
236
234
232
230
228
226
740
735
730
725
720
715
710
705
700
810 805 800 795 790 785 780 775 770 765
Binding energy (eV)
885
880
875
870
865
860
855
850
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Pt 4f7/2
Pt 4f5/2
Ru
Pd 3d5/2
Ru 3d
(f)
Rh
(g)
(e)
Pd
Rh 3d3/2
Rh 3d5/2
(h)
Pt
Pd 3d3/2
292 290 288 286 284 282 280 278 276 274320 318 316 314 312 310 308 306 304 302
346
344
342
340
338
336
334
332
330
80 79 78 77 76 75 74 73 72 71 70 69 68 67 66 65
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Bindign energy (eV)
Figure 5. XPS spectroscopy of M@PCF: (a) Fe, (b) Co, (c) Ni, (d) Mo, (e) Ru, (f) Rh, (g) Pd, (h) Pt.
result indicates that the metal was encapsulated by carbon protected in the N2 before characterization) and the Ru, Rh, Pd
framework. The N2 adsorption/desorption measurement and Pt were existed as metal respectively.
(Figure S9 in the ESM) shows that the adsorption/desorption 3.4. Catalysis test
isotherm belongs to type II and the hysteresis loop at p/p0 We conducted the catalytic activity test through the reduction
0.45–1.0 indicates the presence of large mesopores. The of nitroarenes, because we have concentrated in this field for
Brunauer–Emmett–Teller (BET) specific surface area of the several years,47-49 and this transformation is a typical catalytic
sample was 60 m2g-1. The pore size distribution shows that the hydrogenation whose products are considerable organic
Pd@PCF has a wide pore size distribution from 2 to 100 nm. intermediates in terms of pharmaceuticals, agrochemicals,
The Raman spectroscopy analysis (Figure S10 in the ESM) antioxidants, dyestuffs and pigments.50-51 It was messaged that
shows that the Pd@PCF possessed two predominant bands the requirement of anilines and their derivatives is more than
that located at approximately 1350 and 1600 cm-1. they 4,000,000 tons every year around the world. The output value
corresponding to the D and G bands of the carbon matrix. The of these important chemical feedstocks will increase to £ 10.17
ID/IG ratio of the Pd@PCF catalyst was 0.91, this indicates that billion by 2020.52-54 To improve the chemical synthesis protocol
the 3DPCF is highly graphitized to the carbon frameworks at is highly vital.55-58 The hydrogenation of 4-nitrophenol (4-NP)
o
400 C in the N2. The TGA-DSC (Figure S11-S12 in the ESM) was acted as the probe reaction. The reaction was conducted
measurement shows that the catalyst was stable below 300 °C in a general procedure as follows: 13.9 mg 4-NP, 7 mg catalyst
in air condition, as the temperature increased to 400 °C, most and 2 mL solvent was firstly added into a glass bottle, then the
of the catalyst was devastated.
3.3. Application scope of this strategy
mixture solution was ultrasonicated for 10 min to dissolve the
substrates and disperse the catalyst. After sonication
In order to investigate the application scope of this strategy, treatment, 50 mg reductant was added at room temperature.
another seven metals, namely, Fe(acac)3, Co(acac)2, Ni(acac)2, The optimized results were summarized in table 1. Obviously,
Mo(acac)2, Ru(acac)3, Rh(acac)3, and Pt(acac)2 were explored this reaction can’t occur in the absence of catalyst. When we
to fabricate the M@PCF ultra-small nanoparticles. The used the PCF as the catalyst, the conversion of 4-NP was 0%
reaction condition was unanimous with the fabrication of (entries 1-2). All of the USMNPs@PCF nanocatalysts were
ultra-small Pd nanoparticles. The HR-TEM (Figure 4) and STEM applied to the catalytic reduction of 4-NP. For the non-noble
(Figure S13 in the ESM) images indicated that this strategy has USNPs@PCF nanocatalysts, the conversions are not very high,
been successfully used for the fabrication of ultra-small PCF but the selectivities are up to 99% (entries 3-6). For the noble
anchored metals nanoparticles and the average size of metals USNPs@PCF nanocatalysts, all of them demonstrated excellent
(oxide) nanoparticles were all sub 3 nm. The average size of Fe, activity for this reaction (entries 7-10). In particular, the
Co, Ni, Mo, Ru, Rh, Pd and Pt was 2.5 nm, 1.8 nm, 1.7 nm, 2.8 palladium showed the best results. Then, the palladium
nm, 2.0 nm, 2.2 nm, 1.9 nm, 3 nm respectively. Furthermore, catalyst was further optimized. A series of catalysts that were
in order to confirm the valence states of those obtained ultra- made by adjusting calcination temperatures or mass ratios
small nanoparticles, the XPS analysis was also conducted. As were tested. As the mass ratio of Pd(acac)2/acetylacetonate is
shows in Figure 5 and Figure S14 in the ESM, as the loading of 1/500, the annealing temperature shown significant influences
the metal was a litter and encapsulated by the PCF, the for the catalytic activity. In our experiment, over the Pd@PCF
intensity of the XPS spectroscopy was not so satisfactory. catalyst, this reaction can be completed in 4 min (entry 10).
However, the binding energy of Fe2p, Co2p, Ni2p, Mo3d, However, as the carbonization temperature increased, the
Ru3d, Rh3d, Pd3d and Pt4f demonstrated that the Fe, Co, Ni, catalytic activity was getting worse (entries 11-12). The
Mo were existed as ultra-small metal oxides (samples were
reaction results were supported by the HR-TEM images (Figure
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(a)1.4
Table 1. Catalyst and reaction parameters optimization for the
hydrogenation of p-nitrophenol.[a]
(b)
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DOI: 10.1039/C8TA05369C
0 min
1 min
2 min
3 min
4 min
0.05
4-NP
1.2
0.04
0.03
0.02
0.01
0.00
1.0
0.8
0.6
0.4
0.2
0.0
Catalyst, Solvent,
4-NP
4-AP
Time, Hydrogen source
4-AP
300
Mass
ratio[b]
/
Hydrogen
source
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
NaBH4
/
T
Conv.
(%)[d]
0
Sel.
(%)[e]
0
Entry
Catalyst
Solvent
(min)
180
180
180
180
180
180
20
0
1
2
3
4
250
350
400
450
500
550
Time (min)
Wavelength (nm)
1
/
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
EtOH
MeOH
H2O
H2O
2
PCF
/
0
0
Conversion
Selectivity
(c)
(d)
3
Fe@PCF
Co@PCF
Ni@PCF
Mo@PCF
Ru@PCF
Rh@PCF
Pt@PCF
Pd@PCF
Pd@PCF-600
Pd@PCF-800
Pd@PCF
Pd@PCF
Pd@PCF
Pd@PCF
Pd@PCF
Pd@PCF
Pd@PCF
1/500
1/500
1/500
1/500
1/500
1/500
1/500
1/500
1/500
1/500
2/500
1/1000
1/5000
1/500
1/500
1/500
1/500
5
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
0
4
10
100
80
ty (%)
5
12
60
40
20
0
6
15
7
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
0
C
8
14
Conversion &Selectivi
B
10
9
8
9
10
7
6
Cycles
5
4
3
2
10
11
12
13
14
15
16
17
18
19
4
1
40
Figure 6. (a) The UV-Vis absorption spectra for the catalytic
reduction of 4-NP in the presence of a high activity Pd@PCF
nanocatalyst; (b) Concentrations as a function of time over Pd@PCF
catalyst; (c) Typical optical images of reaction mixtures at 0 and 4
min (d) Recyclability tests of Pd@PCF nanocatalyst for the hydro-
genation of p-nitrophenol.
60
1
30
120
120
120
180
180
previous literatures reported, as the present of NaBH4, the
absorption peak of 4-NP is about 400 nm and 4-AP is about
300 nm.10 As shown in Figure 6 (a-c), the absorption peak of 4-
NP gradually turned weaker, while the peak of 4-AP gradually
increased. This phenomenon indicated that the 4-NP was
successfully transformed to the 4-AP over the Pd @PCF
nanocatalyst in 4 min. The concentrations of 4-AP and 4-NP
versus reaction time indicated that the selectivity towards 4-
AP was close to 100%. From Table S1 in the ESM, compared
with previous reported noble metal-based catalysts (Pd, Pt, Rh,
Ru, Au and Ag) to catalytic transfer hydrogenation of
nitroarenes, the Pd @PCF was one of the best catalyst
systems.
The stability and recyclability of the Pd@PCF catalyst was also
tested. As shown in Figure 6 (d), the conversion and selectivity
did not exhibit obvious decline in ten cycles. This indicated the
catalyst was stable in this reaction system. Compared with the
ultra-small Pd nanoparticles which Prasad reported58 (Figure
S15 in the ESM), the excellent catalytic activity and stability of
the Pd@PCF nanocatalyst could be attributed to the following
reasons according to previous literatures reported59-60: (1) the
highly effective ultra-small Pd nanoparticles were
homogeneously dispersed and intensively encapsulated by the
3D PCF, which offers the sinter-resistance to prevent the
migration and agglomeration of catalyst center during the
catalytic reactions; (2) the pores carbon substrate provides a
good mass transport arena for the reaction and the
appropriately oxygen-functionalized carbon framework helps
to adsorb the hydrophobic nitro-compounds; (3) just as the
“chainmail for catalyst”, the electron of the Pd particles
transferred to the surface PCF and leading to a decreased of
.
N2H4 H2O
0
0
Isopropyl
alcohol
H2
20
Pd@PCF
1/500
1/500
H2O
180
0
0
21[c]
Pd@PCF
H2O
180
0
0
[a] Reaction condition: 4-NP (0.1 mmol), solvent (2 mL), catalyst (7
mg), the hydrogen source was 50 mg and reaction conducted at
[b]
[c]
room temperature.
Mass ratio of Pd(acac)2/Hacac.
The
pressure of H2 was 1 atm. [d] Conversion was determined by TLC and
[e]
HPLC.
Selectivity was determined by GC-MS and UV-Vis
spectroscopy.
S3 in the ESM) of the Pd@PCF-600 and Pd@PCF-800. The
results of mass ratios (Pd(acac)2/Hacac) shown that the
reaction can completed in only 60 seconds as increasing
thedosage of the Pd(acac)2 to 2/500. When the mass ratios of
Pd(acac)2/Hacac is 1/1000 and 1/5000, the reaction time was
30 and 120 min respectively (entries 13-15). During the
reaction, we observed that the reaction catalyzed by the
Pd@PCF (Pd(acac)2 /Hacac=2/500), the reaction was violently.
After evaluated the safe and catalyst activity, we used the
Pd@PCF (Pd(acac)2 /Hacac=1/500) for the further research.
The solvent was a significant factor for the reaction, from the
solvent optimization, we found that even the nitroarene was
more solubility in the alcohol solution, neither ethanol nor
methanol were satisfied this catalytic system and the reaction
need 120 min (entries 16-17). The hydrogen sources were also
studied. Sodium borohydride was the best reductive agents for
this catalytic system (entries 18-21).
In order to real-time monitor the reaction procedure, the UV-
Vis spectroscopy was used to detect 4-NP and 4-AP. From the
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Table 2. Hydrogenation of nitroarenes with NaBH4 and catalyzed by
View Article Online
DOI: 10.1039/C8TA05369C
Pd@PCF nanocatalyst.[a]
Entry
Substrate
Product
T (min)
4
Conv. (%)
>99
Sel. (%)
>99
1
2
1
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
3
1.5
1.5
3
4
5
6
4
Figure 7. Mechanism for the hydrogenation of nitroarenes over
Pd@PCF nanocatalysts.
7
4
We explored the catalytic hydrogenation of a series of
structurally diverse nitroarenes to manifest the applied scope
of this optimized catalytic system. As shown in Table 2, all
substrates completed the reactions with >99 % conversion and
>99 % chemoselectivity. As the previous works reported, the
dehalogenation was usually occurred in the transformation.
Thus, we firstly used several halo-nitroarenes to test the
selectivity of the Pd@PCF. To our satisfaction, the
dehalogenation was successfully avoided in this reaction
system and all of the halo -nitroarenes were transformed to
the corresponding anilines in 4 min (entries 2-7). This catalyst
system was also used to the reduction of nitroarenes with
other functional groups, such as, methyl, alcohol, olefin, ester
group, ether, amide and amino group (entries 8–14). The
reaction results demonstrated that this catalyst was selective
and efficient. In particular, owing to amino-substituted
heterocycles and diphenyl are important intermediates in the
pharmaceuticals and agrochemicals producing industries, we
tested the nitro compounds bearing heterocyclic ring moieties
or other complex group (entries 15–20). The reaction results
indicated that in few minutes all the catalytic hydrogenations
proceeded smoothly and the substrates were converted into
the corresponding hetero aromatic amines with high yields. In
addition, in all cases, the azoxy, azo, and hydrazo compounds,
which were usually generated as by-products in the catalytic
hydrogen transfer nitroarenes, were not detected.
8
1
9
2
10
11
12
13
14
15
16
17
18
19
20
3
2
1
2
2
3
3
2
3
2
Based on previous literatures data and the reaction
phenomenon,55 a possible reaction mechanism for the
hydrogenation of nitroarene to anilines over the Pd@PCF
nanocatalyst was proposed. As shown in Figure 7, it is
following along the typical lines and involving a palladium
hydride (Pd-H@PCF) intermediate species. Initially, as the
NaBH4 added into the reaction blends, the NaBH4 contacted
with water and released active H* species. The H* species was
clipping stabilized by the catalyst and fabricated the Pd-
H@PCF intermediates. As the nitroarenes rapidly snatch the
H* species from the Pd-H@PCF, the nitroarenes was gradually
hydrogenated to corresponding anilines and the catalyst was
2
[a]Reaction condition: 0.1 mmol nitroarenes, 2 mL water, 7 mg
catalyst, NaBH4 50 mg; the conversion and selectivity were
determined by HPLC and GC-MS.
local work function on the carbon surface that increased the
catalytic activity. Thus, the above reasons synergetic effect the
catalyst system and lead the catalyst maintains its’ initial
activity without any deactivation after reused for ten recycles.
The HRTEM and XPS characterization (Figures S16-S17 in the
ESM) indicated that the Pd@PCF catalyst was stable.
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Journal Name
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turned into Pd@PCF. The general pathway for the
View Article Online
Hahn, J. F. Nierengarten, A. DemessenceD, OChI:e1m0.1.0C3o9m/Cm8TuAn0.52306195C,
51, 6730.
chemoselective hydrogenation reactions was: Ar-NO2
NOOH* Ar-NOH* Ar-NHOH* Ar-NH2.
→Ar-
→
→
→
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Conclusions
In conclusion, we have developed an innovative strategy for
the scalable synthesis of stable and highly dispersed pores
carbon framework encapsulated ultra-small metal (oxides)
nanoparticles. The sub 3 nm ultra-small nanoparticles were
fabricated by carbonization of the metallo-supramolecular
polymer precursors that were produced by the dexterous aldol
condensation of metal-acetylacetonate with acetylacetonate.
This size-controlling method was applied to a variety of metal,
including Fe, Co, Ni, Mo, Ru, Rh, Pt and Pd. The porous carbon
encapsulated sub-3 nm ultra-small Pd nanoparticles shows
excellent catalytic activity and chemoselectivity for the
hydrogenation of nitroarene to the corresponding anilines. As
the ultrafine NPs were encapsulated by the PCF, the catalysts
can be kept the active phase structurally stable and reused for
more ten cycles.
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Conflicts of interest
There are no conflicts to declare
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Acknowledgements
This work was financially supported by the Ministry of Science
and Technology (Nos. 2017YFC0906902 and 2017ZX09301032),
National Natural Science Foundation of China (No. 21621003) [31] P. Tomkins, E. G. Henke, W. Leitner, T. E. Müller, ACS Catal.
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA05369C
ToC
We developed a organometallics conjunction strategy for the scalable fabrication of porous carbon frameworks
encapsulated sub-3 nm ultra-small nanoparticles.