In situ formation of well-dispersed palladium nanoparticles immobilized in imidazolium-based organic ionic polymers

Article abstract of DOI:10.1039/c4cc04662e

A new strategy for in situ generation of well-dispersed palladium nanoparticles (NPs) immobilized in imidazolium-based organic ionic polymers was presented. Without extra addition of palladium species, the as-synthesized ionic polymers showed excellent catalytic activity and good reusability in the hydrogenation of nitroarenes. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04662e

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In situ formation of well-dispersed palladium
nanoparticles immobilized in imidazolium-based
organic ionic polymers†
Huaixia Zhao,ab Yangxin Wangab and Ruihu Wang*a
Cite this: Chem. Commun., 2014,
50, 10871
Received 19th June 2014,
Accepted 24th July 2014
DOI: 10.1039/c4cc04662e
www.rsc.org/chemcomm
A new strategy for in situ generation of well-dispersed palladium POPs containing imidazolium linkages as the stabilizers of
nanoparticles (NPs) immobilized in imidazolium-based organic ionic palladium NPs has not been explored.
polymers was presented. Without extra addition of palladium species,
Suzuki–Miyaura cross-coupling reaction has been a common
the as-synthesized ionic polymers showed excellent catalytic activity method for the synthesis of POPs.9a,10 It has been reported that
and good reusability in the hydrogenation of nitroarenes.
palladium NPs may be in situ generated during catalytic reaction
and are subsequently stabilized by imidazolium groups,7a this
The synthesis of palladium nanoparticles (NPs) immobilized on process avoids the use of reductive reagents in comparison with
porous supports has attracted extensive interest in heterogeneous traditional impregnation and coprecipitation methods. However, in
catalysis.1 The catalytic activity of palladium NPs is known to depend most cases, palladium species are leached during the reaction and
on their size and degree of dispersion. Much effort has been devoted the purification process of POPs, which makes it difficult to take
to embedding palladium NPs into porous materials containing advantage of palladium species in the preparation of POPs. The
coordination groups, which can offer potential for the control of development of a new approach to make full use of palladium will
particle size and prevention of NPs aggregation.2 In this context, be of great significance owing to the high price and source scarcity
porous organic polymers (POPs) are a new promising class of porous of palladium. In continuation of our efforts to develop highly
materials owing to their unique features,3 such as high stability, efficient and recyclable catalytic protocols,11,12 herein we present
flexible synthetic strategy and ready functionality.4 Coordination a facile approach for the synthesis of well-dispersed palladium NPs
groups of metal precursors can be easily introduced into POPs to in organic ionic polymers (OIPs).
enhance the interactions between the polymers and metals. Very
Pd@OIP-1 and Pd@OIP-2 were in situ generated for the
recently, POPs containing metal-binding sites have been successfully preparation of OIPs through Suzuki–Miyaura cross-coupling
employed as stabilizers of palladium NPs in heterogeneous catalysis; reaction of 1,3-bis(4-bromophenyl)imidazolium bromide with
high catalytic activity and recyclability have been achieved because 1,3,5-tri(4-pinacholatoborolanephenyl)benzene in 1,4-dioxane and
the synergetic effects of coordination and steric interactions in POPs DMF, respectively (Scheme 1). After the reaction, the precipitate
endow palladium NPs with unique performances.5 Imidazolium- was filtered and successively washed with DMF, deionized water and
based ionic compounds are proposed to stabilize palladium NPs not methanol to remove any possible residues. The resultant reddish
only through electrostatic interaction, but also by binding to NPs brown powder was further treated by Soxhlet extraction in CH2Cl2
surface,6 resulting in the decrement of palladium leaching from the and dried in vacuo at 80 1C for 12 h to give the target product. Both
NPs surface.7 Recently, some imidazolium-based POPs have Pd@OIP-1 and Pd@OIP-2 are insoluble in water and common
been synthesized, and their application in gas absorption8 and organic solvents. It should be mentioned that Pd@OIP-1 and
heterogeneous catalysis9 was reported. However, the study of Pd@OIP-2 were synthesized under the identical conditions except
for the difference in reaction media, but ICP analyses show that the
palladium content in Pd@OIP-1 and Pd@OIP-2 is 0.39 and
0.95 mmol gÀ1, respectively. Higher palladium content in Pd@OIP-2
is probably ascribed to easier reduction of palladium(II) species in
DMF than in 1,4-dioxane during the cross-coupling reaction,13,14
which is favorable for the formation of palladium NPs.
To determine the chemical structure of Pd@OIP-1 and Pd@OIP-2,
solid-state 13C NMR and IR were conducted. In the solid-state
13C NMR spectrum of Pd@OIP-1 (Fig. S1, ESI†), the weak peak
a State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Key Laboratory of Coal to Ethylene Glycol and Its Related
Technology, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China.
E-mail: ruihu@fjirsm.ac.cn
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: Experimental details, solid-state
13C NMR spectra, IR spectra, TGA curves, SEM images and PXRD patterns of Pd@OIP-1
and Pd@OIP-2, as well as NMR spectra of catalytic products. See DOI: 10.1039/
c4cc04662e
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Scheme 1 Synthesis of Pd@OIP-1 and Pd@OIP-2 ( = palladium NPs).
at 114 ppm corresponds to the 4,5-positioned carbon of imidazolyl
rings, other peaks at 139, 135 and 127 ppm may be ascribed to the
carbon of phenyl rings and the 2-positioned carbon of imidazolyl
rings.10a,15 The solid-state 13C NMR spectrum of Pd@OIP-2 is similar
to that of Pd@OIP-1 except for the disappearance of the peak at
135 ppm, which is probably overlapped by the adjacent peaks. It was
reported that the interactions between supported palladium NPs
and aromatic groups can decrease the regularity of the polymer
Fig. 1 TEM images of (a and b) Pd@OIP-1 and (c and d) Pd@OIP-2.
networks, and ultimately result in broadening of the peaks in solid- 3.07 Æ 0.48 nm, respectively (Fig. 1). It should be mentioned
state 13C NMR spectra. As a result, the difference in solid-state NMR that their particle size is smaller than imidazolium-immobilized
spectra between Pd@OIP-1 and Pd@OIP-2 presumably results from palladium NPs in organic–inorganic hybrid materials.7 A high
different loading amounts of palladium NPs.16 However, the magnification TEM image of Pd@OIP-2 (Fig. 1(d), inset) shows
FTIR spectra of Pd@OIP-1 and Pd@OIP-2 are almost identical to visible lattice fringes of 0.23 nm, corresponding to the d-spacing of
each other (Fig. S2, ESI†). Thermogravimetric-mass spectrometric the Pd (111) plane. The diffuse rings in the selected-area electron
(TG-MS) analyses show that Pd@OIP-1 and Pd@OIP-2 are stable up diffraction (SAED) pattern of Pd@OIP-2 show the reflection of
to 200 1C (Fig. S3, ESI†). Initial weight losses of 2.8% and 3.2% crystalline palladium (Fig. 1(d), inset). The lack of the diffuse rings
before 150 1C in Pd@OIP-1 and Pd@OIP-2 are ascribed to the or spots in Pd@OIP-1 (Fig. 1(b), inset) illustrates the amorphous
presence of water molecules captured in the polymers, which are nature of palladium NPs. Powder X-ray diffraction (XRD) patterns
confirmed by ion current at m/z = 18, respectively.
(Fig. S7, ESI†) further confirm the existence of the palladium lattice
The porosities of Pd@OIP-1 and Pd@OIP-2 were studied by in Pd@OIP-2. The strong peak at 2y = 44.11 is ascribed to (111)
N2 adsorption–desorption at 77 K (Fig. S4, ESI†). The specific surface lattice spacing, while the weak peaks at 2y = 68.2 and 82.11 are
areas of Pd@OIP-1 and Pd@OIP-2 are 5.9 and 60.4 m2 gÀ1
,
assigned to (220) and (311) lattice spacing of palladium,
respectively, which are lower than the reported imidazolium-based respectively.7c In contrast, palladium NPs in Pd@OIP-1 turn
POPs.9 Scanning electron microscopy (SEM) images show that both out to be amorphous, suggesting that the reducing ability of
Pd@OIP-1 and Pd@OIP-2 are composed of irregular granular solvents is an important factor in the determination of nucleation
particles, and the granules of Pd@OIP-2 are much smaller than and crystal growth rates of palladium in catalytic reactions.19
those of Pd@OIP-1 (Fig. S5a and b, ESI†), which is ascribed to the Incidentally, the broad reflection peak at 2y = 10–351 indicates that
solubility difference of the ionic polymers in DMF and 1,4-dioxane. Pd@OIP-1 and Pd@OIP-2 have no long-range frameworks owing to
Ionic polymers are more inclined to precipitate in low polar the kinetically-controlled irreversibility of Suzuki–Miyaura reaction.
1,4-dioxane, finally resulting in the formation of large particles.12c,17 High-annular dark-field scanning TEM (HAADF-STEM) and energy-
The existing state of surface palladium in Pd@OIP-1 and dispersive X-ray (EDX) images clearly demonstrate the homo-
Pd@OIP-2 was investigated by X-Ray photoelectron spectroscopy geneous distribution of palladium NPs throughout Pd@OIP-1
(XPS). As shown in Fig. S6 in the ESI,† the Pd 3d region is divided and Pd@OIP-2 (Fig. S8 ESI†).
into two spin–orbital pairs, indicating the presence of two types of
The catalytic performances of Pd@OIP-1 and Pd@OIP-2 were
surface-bound palladium species. The binding energy peaks at evaluated using hydrogenation of nitrobenzene at 25 1C. As shown
340.8 (Pd 3d5/2) and 335.5 eV (Pd 3d3/2) are assigned to Pd(0) in Fig. S9 in the ESI,† Pd@OIP-1 afforded a full conversion of
species, while the peaks at 342.4 (Pd 3d5/2) and 337.1 eV (Pd 3d3/2
)
nitrobenzene to aniline in 20 min, while Pd@OIP-2 almost gave a
correspond to Pd(II) species, which probably result from the oxida- quantitative conversion to aniline in 10 min. The higher catalytic
tion of Pd(0) during air contact.18 The ratios of Pd(0)/Pd(II), which activity of Pd@OIP-2 is probably ascribed to its higher specific
are estimated by the ratio of relative peak areas, are 2.94 and 2.85 in surface area and smaller granular particles than Pd@OIP-1, which
Pd@OIP-1 and Pd@OIP-2, respectively.
provide more opportunities for contact between substrates and
Transmission electron microscope (TEM) images reveal that active sites in the polymers. On the other hand, the exposed (111)
palladium NPs possess uniform size distribution, and their average palladium facets in Pd@OIP-2 are also responsible for the higher
diameters in Pd@OIP-1 and Pd@OIP-2 are 2.80 Æ 0.41 nm and catalytic activity.20 The control experiment was also performed in
10872 | Chem. Commun., 2014, 50, 10871--10874
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Table 1 Reduction of nitroarenesa
Entry Reactant
1
Product
Conv.b (%) Select.b (%)
100 (99)
100 (91)
100
100
2
3
100 (98)
100 (98)
100
100
4
Fig. 2 Reusability of Pd@OIP-2. Reaction conditions: nitrobenzene
(0.5 mmol), NaBH4 (2.5 mmol), [Pd] (1 mol%), H2O (2.5 mL), 25 1C, 15 min.
5
6
7
8
99 (88)
100 (99)
100 (99)
100
96
100
100
90
the absence of Pd@OIP-1 and Pd@OIP-2, and no aniline was
detected under the same conditions.
Besides activity, reusability is also an important factor in
heterogeneous catalytic systems. The recycling experiment was
further conducted using Pd@OIP-2 as a precatalyst in the hydro-
genation of nitrobenzene without the addition of extra palladium.
After the first run of hydrogenation was finished, ICP analysis shows
that palladium leaching into the solvent is only 0.20 ppm. Interest-
ingly, no obvious efficiency loss was detected after the reaction was
performed for five runs (Fig. 2). However, the selectivity of aniline
decreased to 88% in the sixth run with the total conversion of
nitrobenzene. The SEM image reveals that granular particles of
Pd@OIP-2 are well maintained after six consecutive runs (Fig. S5c,
ESI†). TEM analysis shows that the average diameter of the palla-
dium NPs after sixth run reaction in Pd@OIP-2 was increased to
4.78 Æ 1.01 nm, but no obvious aggregation of NPs was observed
(Fig. 3). Noteworthily, d-spacing of the Pd(111) plane stays visible
after the recycling reaction (Fig. 3(b), inset), indicating that palla-
dium NPs are well stabilized in polymers.
The excellent catalytic activity of Pd@OIP-2 encouraged us to
further explore the generality of the catalytic system, and several
nitroarenes with different steric and electronic characters were
tested. To ensure high conversion and selectivity of most substrates,
the hydrogenation reactions were performed in a mixed solvent of
H2O and THF (9:1 in volume) for 1.5 h at 25 1C (Table 1). The
substrates bearing electron-donating groups, such as 4-nitroanisole,
4-nitrotoluene, 3-nitrotoluene and 2-nitrotoluene, provided the full
conversion and selectivity to corresponding products (entries 1–4).
The steric hindrance at ortho-, meta- and para-positions of nitroto-
luene has no obvious effect on the catalytic reaction. In contrast,
4-t-butylnitrobenzene gave rise to 4-t-butylaniline in 99% GC yield
9c
100 (94)
100 (98)
100
100
10
a
Reaction conditions: nitroarene (0.5 mmol), NaBH4 (2.5 mmol), [Pd]
b
(1 mol%), in 2.5 mL H2O/THF (9 : 1, v/v) at 25 1C, 1.5 h. Conversion
and selectivity were determined by GC, isolated yields are given in
parentheses. 2.5 h.
c
and 96% selectivity due to its bulky steric effect (entry 5). Functional
groups, such as –OH and –NH2, could be well tolerated in this
catalytic system (entries 6 and 7). However, 4-nitrochlorobenzene
gave a 90% yield for 4-chloroaniline and 10% yield for the
dechlorinated product of aniline (entry 8). This tendency toward
dehalogenation was often observed in palladium-catalyzed hydro-
genation of aryl halides (entry 9).2c Interestingly, 4-nitroaceto-
phenone gave 1-(4-aminophenyl)ethanol as the sole product in
2.5 h, in which the carbonyl group was reduced to hydroxyl (entry 9).
It should be mentioned that the catalytic system is also reactive
for hydrogenation of 1,3-dinitrobenzene, and the corresponding
dianiline was obtained in a 98% isolated yield (entry 10).
In summary, a facile approach for in situ generation of
palladium NPs supported on imidazolium-based porous
organic ionic polymers was developed. The ionic polymers were
synthesized through Suzuki cross-coupling reaction, in which
the concomitant formation of well-dispersed palladium NPs
with small size and narrow size distribution was observed.
Through judicial selection of reaction solvents, the size of the
polymers, palladium content and the size of palladium NPs
could be regulated. The excellent catalytic performances and
recyclability of Pd@OIP-2 were observed in the hydrogenation
of nitroarenes without extra addition of palladium species. This
study not only provides a new method for fabrication of well-
dispersed metal NPs supported on porous organic polymers,
Fig. 3 TEM images of Pd@OIP-2 after six consecutive runs.
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