Prefunctionalized Porous Organic Polymers: Effective Supports of Surface Palladium Nanoparticles for the Enhancement of Catalytic Performances in Dehalogenation

Article abstract of DOI:10.1002/chem.201601956

Three porous organic polymers (POPs) containing H, COOMe, and COO^?groups at 2,6-bis(1,2,3-triazol-4-yl)pyridyl (BTP) units (i.e., POP-1, POP-2, and POP-3, respectively) were prepared for the immobilization of metal nanoparticles (NPs). The ultrafine palladium NPs are uniformly encapsulated in the interior pores of POP-1, whereas uniform- and dual-distributed palladium NPs are located on the external surface of POP-2 and POP-3, respectively. The presence of carboxylate groups not only endows POP-3 an outstanding dispersibility in H₂O/EtOH, but also enables the palladium NPs at the surface to show the highest catalytic activity, stability, and recyclability in dehalogenation reactions of chlorobenzene at 25 °C. The palladium NPs on the external surface are effectively stabilized by the functionalized POPs containing BTP units and carboxylate groups, which provides a new insight for highly efficient catalytic systems based on surface metal NPs of porous materials.

Full text of DOI:10.1002/chem.201601956

DOI: 10.1002/chem.201601956
Full Paper
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Organic Polymers
Prefunctionalized Porous Organic Polymers: Effective Supports of
Surface Palladium Nanoparticles for the Enhancement of Catalytic
Performances in Dehalogenation
Hong Zhong, Caiping Liu, Hanghui Zhou, Yangxin Wang, and Ruihu Wang*[a]
Abstract: Three porous organic polymers (POPs) containing
H, COOMe, and COOÀ groups at 2,6-bis(1,2,3-triazol-4-yl)pyr-
idyl (BTP) units (i.e., POP-1, POP-2, and POP-3, respectively)
were prepared for the immobilization of metal nanoparticles
(NPs). The ultrafine palladium NPs are uniformly encapsulat-
ed in the interior pores of POP-1, whereas uniform- and
dual-distributed palladium NPs are located on the external
surface of POP-2 and POP-3, respectively. The presence of
carboxylate groups not only endows POP-3 an outstanding
dispersibility in H2O/EtOH, but also enables the palladium
NPs at the surface to show the highest catalytic activity, sta-
bility, and recyclability in dehalogenation reactions of chloro-
benzene at 258C. The palladium NPs on the external surface
are effectively stabilized by the functionalized POPs contain-
ing BTP units and carboxylate groups, which provides a new
insight for highly efficient catalytic systems based on surface
metal NPs of porous materials.
Introduction
As a new class of important porous materials, porous organ-
ic polymers (POPs) have captured widespread interest in gas
storage and separation, sensing, proton conductivity, drug de-
livery, and catalysis.[19–23] It has been demonstrated that the
structures and porous properties of POPs could be elegantly
tuned through selecting building units with different linking
groups.[23–25] In addition, the modular nature of the synthesis of
POPs has allowed the incorporation of various functionalities
for a specific purpose.[26,27] Relative to an extensive research in
the functionality of POPs directed at gas sorption and separa-
tion,[28–30] the pre-designable functionalities for immobilization
of metal NPs have seldom been explored.
Metal nanoparticles (NPs) supported on porous materials have
been widely applied in energy conversion, environmental pro-
tection, and catalysis.[1–4] Considerable efforts have been devot-
ed to the fabrication of ultrafine metal NPs in the interior
pores based on powerful confinement effect of pore struc-
tures,[5–7] but most of the synthetic methods show a lack of
generality for porous materials with different structures and
functions, because the size, distribution, and location of metal
NPs strongly depend on the porous nature of the materials
and the reduction method of the metal precursors.[8–11] The in-
corporation of functional groups for the customized perform-
ances usually decreases the surface area and the pore volume
of the porous materials, resulting in the diffusion of metal NPs
and their precursors from interior pores to the external surface
during preparation and/or catalytic reactions.[12–14] In contrast,
metal NPs on the external surface can provide higher catalytic
activity than NPs in the interior cavities, owing to acceleration
of mass transfer and easy availability of active sites.[15–18] How-
ever, these surface metal NPs have received little attention be-
cause they are instable and prone to aggregation. It is desira-
ble but challenging to seek for an effective strategy to immo-
bilize metal NPs on the external surface of porous materials.
The introduction of suitable functional groups into building
units is one of the feasible routes for tailor-made synthesis of
functionalized POPs at the molecular level.[31,32] Carboxylate
has been known to possess an outstanding coordination abili-
ty to metal species, the incorporation of carboxylate into POPs
not only may improve the stability of metal NPs through coor-
dination and electrostatic interactions, but is also favorable for
catalytic reactions in polar solvents through increasing the po-
larity and dispersibility of the POPs. To search for stable palladi-
um NPs on the external surface of POPs, herein, we reported
three POPs containing H, COOMe, and COOÀ groups at 2,6-
bis(1,2,3-triazol-4-yl)pyridyl (BTP) units, that is, POP-1, POP-2
and POP-3, respectively. The introduction of COOMe and car-
boxylate groups has exerted crucial effects on the porous
nature and functions of the POPs, resulting in variation of the
size and location of the palladium NPs from interior pores to
the external surface. In comparison with palladium NPs in the
interior cavities, a superior catalytic activity, stability, and recy-
clability in dehalogenation reactions have been demonstrated
by the external surface palladium NPs in Pd/POP-3.
[a] H. Zhong, Dr. C. Liu, H. Zhou, Y. Wang, Prof. R. Wang
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences, Fuzhou, Fujian 350002 (China)
Fax: (+86)591-8371-1028
E-mail: ruihu@fjirsm.ac.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201601956.
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and azido groups at n˜ =3262 and 2126 cmÀ1, respectively, the
characteristic peaks of the triazolyl group occur at n˜ =1610
and 3142 cmÀ1, which suggests complete azide–alkyne cyclo-
addition (Figure 1a–c).[33,38] Furthermore, the characteristic
bands of the COOMe group at n˜ =1726 cmÀ1 and the carboxyl-
ate group at n˜ =1604 cmÀ1 were observed in the FTIR spectra
of POP-2 and POP-3, respectively. In the solid-state 13C NMR
spectra of POP-1, POP-2, and POP-3 the present resonance of
the C4-triazolyl carbon atom around d=148 ppm and the
absent characteristic peaks of alkynyl carbon atoms in the
range of d=70–100 ppm further confirm the successful poly-
merization (Figures S2a–c in the Supporting Information).[11,19]
The broad signals at d=118–139 ppm correspond to the aro-
matic carbon atoms. The resonance peaks at d=166 and
169 ppm are assigned to carbonyl carbon atoms of POP-2 and
POP-3, respectively. Scanning electron microscopy (SEM)
images show that POP-1, POP-2, and POP-3 are composed of
granular particles with a size of 50–100 nm (Figure S3 in the
Supporting Information), which is in agreement with most of
the reported POPs.[39–41] It should be mentioned that the FTIR
(Figure 1) and solid-state 13C NMR spectra (Figure S2 in the
Supporting Information) as well as the SEM images (Fig-
ures S3b, e, and h in the Supporting Information) of Pd/POP-1,
Pd/POP-2, and Pd/POP-3 are almost identical with those of
POP-1, POP-2, and POP-3, respectively, indicating that the
structural polymers are well maintained after palladium load-
ing.
Results and Discussion
As show in Scheme 1, POP-1 and POP-2 were readily synthe-
sized through a facile click reaction of 1,3,5-tris(4-azidophenyl)-
benzene with 2,6-diethynylpyridine and methyl 2,6-diethynyli-
sonicotinate, respectively, under standard click reaction condi-
Scheme 1. Schematic illustration for the synthesis of POP-1, POP-2, POP-3,
Pd/POP-1, Pd/POP-2, and Pd/POP-3.
tions.[33–37] Subsequent hydrolysis of POP-2 in aqueous LiOH so-
lution gave rise to POP-3. As expected, POP-1 and POP-2 are
hydrophobic, whereas POP-3 shows superior dispersibility in
water/ethanol owing to the presence of the carboxylate group
(Figure S1 in the Supporting Information). The treatment of
POP-1, POP-2, and POP-3 with Pd(OAc)2 with a 1:1 molar ratio
of BTP to PdII, and subsequent reduction by NaBH4 gave rise to
the Pd/POP-1, Pd/POP-2, and Pd/POP-3 systems, respectively.
Inductively coupled plasma (ICP) analyses show palladium con-
tents in Pd/POP-1, Pd/POP-2, and Pd/POP-3 of 0.22, 0.36, and
0.39 mmolgÀ1, respectively.
In the thermal gravimetric analysis (TGA) curves of POP-
1 and POP-2 an initial weight loss of 5.5 and 5.6%, respectively,
before 1308C was observed, whereas POP-3 shows a weight
loss of 18.4% before 1708C (Figure S4 in the Supporting Infor-
mation), which is attributed to the hydrophilicity enhancement
of the structural framework. The thermal stability of Pd/POP-1,
Pd/POP-2, and Pd/POP-3 is lower than that of POP-1, POP-2,
and POP-3, respectively (Figure S4 in the Supporting Informa-
tion). Powder X-ray diffraction (PXRD) analyses indicate that
POP-1, POP-2, and POP-3 are amorphous (Figure S5 in the Sup-
porting Information) owing to the irreversibility of the click re-
action.[34,35] No obvious characteristic peaks of palladium parti-
cles were observed in the PXRD patterns of Pd/POP-1, Pd/POP-
2, and Pd/POP-3.
The FTIR spectra of POP-1, POP-2, and POP-3 indicate the
total consumption of the starting materials due to the disap-
pearance of stretching vibration peaks of the terminal alkynyl
Figure 1. FTIR spectra of a) the starting materials, POP-1, and Pd/POP-1, b) the starting materials, POP-2, and Pd/POP-2, and c) the starting materials, POP-3,
and Pd/POP-3. (DEP=2,6-diethynylpyridine, DEP-COOMe=methyl-2,6-diethynylisonicotinate, TAPB=1,3,5-tris(4-azidophenyl)benzene).
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The porous properties of POP-1, POP-2, and POP-3 were in-
vestigated by physisorption of nitrogen at 77 K. The nitrogen
adsorption/desorption isotherm of POP-1 exhibits a combina-
tion of type I and IV (Figure 2a),[11] which indicates the exis-
tence of both micropores and mesopores in POP-1. However,
POP-2 and POP-3 give an isotherm of type IV.[11] The Brunauer–
Emmett–Teller (BET) surface areas of POP-1, POP-2, and POP-3
are 358, 106, and 79 m2 gÀ1, respectively. POP-1 possesses
a narrow pore size distribution of around 1.50 nm (Figure 2b),
whereas the micropores in POP-2 and POP-3 are negligible
(Figure S6 in the Supporting Information). The contribution of
microporosity to these POPs can be calculated as the ratio of
the micropore volume (V0.1) to the total pore volume (Vtot).[19]
The V0.1/Vtot value of POP-1 is 0.160, which is decreased sharply
to 0.035 and 0.030 in POP-2 and POP-3, respectively (Table S1
in the Supporting Information). The shapes of the N2 adsorp-
tion/desorption isotherms of Pd/POP-1, Pd/POP-2, and Pd/POP-
3 are well preserved (Figure 2a), indicating that the pore sys-
tems have not been altered substantially by the palladium
NPs. The BET surface areas of Pd/POP-1, Pd/POP-2, and Pd/
POP-3 are decreased to 263, 72, and 50 m2 gÀ1, respectively,
which is attributed to both partial pore filling and mass incre-
ment after palladium loading.
Considering significant influences of BTP functionalization
on the porous properties of POPs, their effects on the palladi-
um NPs and the catalytic performances were further examined.
Transmission electron microscope (TEM) analyses show that ul-
trafine palladium NPs in Pd/POP-1 are uniformly encapsulated
in the interior cavities (Figures 3a and b), and that their aver-
Figure 3. TEM images of a,b) Pd/POP-1, d,e) Pd/POP-2, and g,h) Pd/POP-3.
HAADF-STEM and EDX mapping images of c) Pd/POP-1, f) Pd/POP-2, and
i) Pd/POP-3.
age size is (1.5Æ0.3) nm, which is similar to the micropore size
of POP-1. It should be mentioned that the average size of pal-
ladium NPs in Pd/POP-1 is smaller than those supported on
POPs containing mono- or bi-dentate coordination groups.[5]
Moreover, the uniform distribution of the palladium NPs is
much different from the reported dual distribution both in the
interior pores and on the external surface of POPs when NaBH4
is used as a reduction agent of the palladium precursors.[19]
These observations are probably ascribed to the strong chelat-
Figure 2. a) N2 adsorption/desorption isotherms for POP-1, POP-2, POP-3,
Pd/POP-1, Pd/POP-2, and Pd/POP-3. b) Pore size distribution for POP-1 and
Pd/POP-1. c) Pd 3d XPS spectra for Pd/POP-1, Pd/POP-2, and Pd/POP-3. d) N
1s XPS spectra for POP-1, POP-2, POP-3, Pd/POP-1, Pd/POP-2, and Pd/POP-3.
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ing coordination ability of the ter-dentate BTP units and a supe-
rior confinement effect of the micropores, which restrict the
diffusion of the palladium NPs and their precursors from interi-
or pores to the external surface of POP-1 during the prepara-
tion of the palladium NPs. In contrast, the palladium NPs in
Pd/POP-2 are uniformly located on the external surface with
an average size of (3.3Æ0.3) nm (Figures 3d and e), which is
probably ascribed to the filling of the interior cavities by the
COOMe groups. Interestingly, the change of the functional
groups from the COOMe to the carboxylate group results in
the formation of dual-distributed palladium NPs in the interior
pores and on the external surface of POP-3 (Figures 3g and h),
and the average diameters of the palladium NPs in Pd/POP-3
are (1.6Æ0.35) and (3.5Æ0.35) nm, respectively, which are
comparable with those in Pd/POP-1 and Pd/POP-2, respective-
ly. The dual distribution of the palladium NPs in POP-3 is prob-
ably ascribed to a synergetic effect of the carboxylate groups
by electrostatic and/or coordination interactions. The size, loca-
tion, and distribution of the palladium NPs in the interior pores
and the external surface were further demonstrated by high-
annular dark-field scanning TEM (HAADF-STEM) and energy-
dispersive X-ray (EDX) mapping images of Pd/POP-1, Pd/POP-2,
and Pd/POP-3 (Figures 3c, f, and i as well as Figure S7 in the
Supporting Information).
The existing state of the surface palladium in Pd/POP-1, Pd/
POP-2, and Pd/POP-3 was investigated by X-ray photoelectron
spectroscopy (XPS). As shown in Figure 2c, the Pd 3d spectra
present two sets of doublet peaks corresponding to Pd 3d5/2
and Pd 3d3/2. The Pd 3d5/2 peaks at 335.75, 335.90, and
335.40 eV are attributed to Pd0 species of Pd/POP-1, Pd/POP-2,
and Pd/POP-3, respectively. In comparison with Pd/POP-1,
a positive shift of 0.15 eV and a negative shift of 0.35 eV was
observed for Pd/POP-2 and Pd/POP-3, respectively, which ex-
hibits that Pd0 species in Pd/POP-2 are more electron-deficient
than in Pd/POP-1 because of an electron-withdrawing effect of
the COOMe group, whereas an electron-donating effect of the
carboxylate group results in more electron-rich Pd0 species in
Pd/POP-3.[11,34] The ratio of Pd0 to PdII in Pd/POP-1, Pd/POP-2,
and Pd/POP-3 are 0.49, 0.52, and 0.41, respectively. It should
be mentioned that the Pd 3d5/2 binding energy peaks for PdII
species in Pd/POP-1, Pd/POP-2, and Pd/POP-3 shift negatively
by 0.65, 0.45, and 0.70 eV, respectively, in comparison with that
of 338.4 eV for free Pd(OAc)2,[4] the negative shift is ascribed to
the strong coordination interaction of PdII with the chelating
ter-dentate BTP units, in which election donation from BTP to
PdII makes the PdII species less electron-deficient.
In order to further confirm the interactions between the
supports and the incorporated palladium species, XPS spectra
of N 1s in the POPs and the Pd/POPs systems were implement-
ed (Figure 2d). The peaks at 399.45, 399.60, and 399.70 eV cor-
respond to the N 1s of POP-1, POP-2, and POP-3, respectively.
The peaks in Pd/POP-1, Pd/POP-2, and Pd/POP-3 are shifted to
399.65, 399.70, and 399.75 eV, respectively. The shift of the N
1s toward higher binding energies results from coordination of
nitrogen atoms to palladium,[42–44] which further suggests the
electron donation from the BTP units to the palladium species
after palladium loading.
It has been reported that the loading amount of palladium
precursors and the reduction methods may influence the struc-
tures and properties of the palladium NPs,[2] however, we
failed to obtain uniformly distributed palladium NPs even
when half the amount of Pd(OAc)2 with respect to Pd/POP-3
was used under the same conditions, no apparent variation
was observed in their TEM images (Figures 4a and d). In addi-
tion, the variation of the reduction agent of Pd(OAc)2 from
NaBH4 to H2 or N2H4·H2O has no detectable effect on the dual
distribution of the palladium NPs on the external surface of
POP-3, either (Figures 4b, e, c, and f). These results reveal that
the introduction of carboxylate groups not only can modulate
the properties of the POPs, but also exerts an important effect
on the formation and structure of the palladium NPs.
To have a clear insight into the effects of the functional
groups on the electronic properties of POP-1, POP-2, and POP-
3, the hybrid Becke three-parameter Lee–Yang–Parr (B3LYP)
density functional method in combination with the 6-311+
G(d) basis set were performed based on their model com-
pounds. As shown in Figure 5 and Table S2 in the Supporting
Information, the H, COOMe, and COOÀ groups at the BTP units
significantly influence their charge distribution. The charges of
the pyridyl nitrogen atom in POP-1, POP-2, and POP-3 are
À0.415, À0.393, and À0.438 e, respectively, which are more
negative than the nitrogen atoms in 1,2,3-triazolyl. In compari-
son with POP-1, the electron-withdrawing COOMe group in
POP-2 and the electron-donating carboxylate group in POP-3
result in a positive shift of 0.022 e and a negative shift of
0.023 e, respectively, which is consistent with the results of the
XPS analyses. The nitrogen atoms of the 1,2,3-triazolyl groups
in POP-1, POP-2, and POP-3 all possess negative charges, the
charges of the N1 and N3 atoms are almost equal, and are
more negative than that of the N2 atom. In view of the charge
density of the pyridyl nitrogen atom and the nitrogen atomic
arrangement of the 1,2,3-triazolyl group, the BTP unit prefers
to coordinate with palladium in a N3-N4-N3’ ter-dentate che-
lating mode, which is the same as those in the BTP-based co-
ordination compounds.[45–49] HOMO and LUMO analyses reveal
Figure 4. TEM images of Pd/POP-3 prepared a,d) by using half of the theo-
retical amount of Pd(OAc)2, b,e) by using H2 as a reducing agent, and c,f) by
using N2H4 as reducing agent.
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Figure 6. a) Dehalogenation reaction of chlorobenzene as a function of the
time by using 0.5 mmol chlorobenzene and 0.5 mol% palladium at 258C.
b) Recyclability in the dehalogenation reaction of chlorobenzene for Pd/
POP-1, Pd/POP-2, Pd/POP-3, and Pd/C by using 0.5 mmol chlorobenzene
and 1 mol% palladium for 6 h at 258C.
Figure 5. Electrostatic potential (ESP)-mapped molecular van der Waals sur-
faces, highest occupied molecular orbitals, and the lowest unoccupied mo-
lecular orbitals for the model compounds of a,b) POP-1, c,d) POP-2, and
e,f) POP-3.
forded full conversion of chlorobenzene to benzene in 20 h,
whereas the use of Pd/POP-1 and Pd/POP-2 gave 70 and 93%
GC yield, respectively. The superior catalytic activity of Pd/POP-
3 is attributed to its superior dispersibility in H2O/EtOH (Fig-
ure S1 in the Supporting Information), which facilitates the
contact between the active sites and the substrates. The elec-
tron-donating effect of the carboxylate group also increases
the electron density of the Pd0 species in Pd/POP-3, which suf-
ficiently facilitate oxidative addition of the CÀX bond.[60] Appa-
rently, the palladium NPs on the external surface in Pd/POP-2
and Pd/POP-3 possess higher catalytic activity than those en-
capsulated in the interior cavities in Pd/POP-1. As a comparison,
commercial Pd/C was also tested under the same conditions
and a conversion of 85% of chlorobenzene was achieved
within 20 h, which is lower than that of the palladium NPs on
the external surface.
that the HOMO and LUMO in POP-1 and the HOMO in POP-2
are shared by BTP and phenyl units, whereas the HOMO in
POP-3 is mainly occupied by the carboxylate group. The LUMO
in POP-2 is mainly focused on the pyridyl unit, and the LUMO
in POP-3 is mainly occupied by the triazolyl and phenyl groups
(Figure 5).
Aryl halides are a high risk to environment and our health
due to their toxicity and strong bioaccumulation potential,
however they are widely used as insect repellents, fungicides,
and organic intermediates.[50–52] The dehalogenation of aryl hal-
ides is a significant process for the removal of halogenated or-
ganic pollutants and for the synthesis of fine chemicals. Various
catalysts, such as TiO2–Fe2O3 mixed-oxides,[53] phosphinorhodi-
um catalysts,[54] organometallic complexes,[55] and palladium
NPs,[56,57] have been reported for dehalogenation reactions.
Among them, supported palladium NPs are one of the
common catalysts. The use of suitable supports not only can
inhibit the agglomeration of palladium NPs through coordina-
tion, electrostatic, and/or confinement interactions, but also
may accelerate the rate-determining oxidative addition of aryl
halides through endowing palladium NPs with steric and elec-
tronic effects, resulting in an improvement of the catalytic ac-
tivity.[58–60] However, the use of POPs as heterogeneous sup-
ports of palladium NPs in this reaction has not been reported
hitherto. To distinguish the influences of the COOMe and car-
boxylate groups on the catalytic performances, the Pd/POP-1,
Pd/POP-2, and Pd/POP-3 systems were initially evaluated by
the dehalogenation of chlorobenzene with 0.5 mol% palladi-
um in H2O/EtOH at 258C. As shown in Figure 6a, Pd/POP-3 af-
To further explore the catalytic efficiency of this system,
a more versatile and practical method was investigated in the
dehalogenation reaction by using Pd/POP-3 as a catalyst in the
presence of HCOONH4 at 258C. As shown in Table 1, the
amount of the catalyst is very important for this reaction. The
use of 0.25 mol% palladium catalyst gave only 4% conversion
within 24 h (Table 1, entry 1), whereas a quantitative yield was
observed when the palladium amount was increased to
0.5 mol% (Table 1, entry 2) with a TON value up to 200, which
exceeds those from palladium NPs supported by metal–organ-
ic frameworks[53] and amphiphilic polymers.[57] Notably, chloro-
benzene can be completely transferred into benzene in the
presence of 1.0 mol% Pd/POP-3 in 6 h (Table 1, entry 3). The
dependence on the catalyst loading amount is probably as-
cribed to a long induction period of the dehalogenation reac-
tion.[51] A control experiment was also performed in the pres-
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of the target product was obtained when the reaction time
was prolonged to 12 h (Table 1, entry 14).
Table 1. Dehalogenation reaction of aryl halides catalyzed by Pd/POP-3.[a]
The high catalytic activity of Pd/POP-1, Pd/POP-2, and Pd/
POP-3 prompted us to examine their stability and recyclability,
which are also important factors in heterogeneous systems.
The catalytic recyclability was evaluated by the dehalogenation
reaction of chlorobenzene by using 1.0 mol% palladium at
258C. As shown in Figure 6b, Pd/POP-3 may be used for at
least four runs with 100% conversion of chlorobenzene,
whereas the conversion decreased to 80 and 43% for Pd/POP-
1 and Pd/POP-2, respectively, in the fourth run. When Pd/C
was used, the conversion of chlorobenzene decreased to 16
and 5% in the third and fourth run, respectively. The palladium
leaching after the first run for Pd/POP-1, Pd/POP-2, and Pd/
POP-3 was 0.78, 1.62, and 0.93 ppm, respectively. Notably, after
the reaction was run for 1 h, the palladium-containing catalytic
species was quickly removed by filtration, and the filtrate was
continued to react for additional 23 h, and no change in the
conversion was detected (Table 1, entries 15 and 16), which in-
dicates that the dehalogenation reaction proceeds in a hetero-
geneous manner in the catalytic system. The recyclability was
also investigated by using 0.5 mol% palladium at 258C for
12 h. As shown in Figure S8 in the Supporting Information,
a conversion of 45% of chlorobenzene was maintained for at
least five runs for Pd/POP-3, whereas the conversion of chloro-
benzene for Pd/POP-1 and Pd/POP-2 was decreased from 30 to
19 and from 40 to 9%, respectively, under the same condi-
tions, which further reveals a superior stability and recyclability
of Pd/POP-3.
Entry Substrate
[Pd] [mol%] t [h] Conversion [%][b] TON[c]
1
0.25
0.5
1
24
24
6
4
100
100
0
16
200
100
0
2
3
4[d]
5
–
6
1
6
100
93
100
93
6
1
6
7
1
6
100
100
100
100
100
100
8
1
6
9
1
6
10
1
6
87
87
11
12
1
1
6
6
100
100
100
100
13
14
1
1
6
92
92
12
100
100
After consecutive reaction, the resultant powders were iso-
lated and examined by SEM and TEM. The SEM images (Fig-
ure S3 in the Supporting Information) show that the original
morphologies of Pd/POP-1, Pd/POP-2, and Pd/POP-3 are intact
after consecutive dehalogenation for four runs. However, the
average size of the palladium NPs in Pd/POP-1 increases to
(2.5Æ0.35) nm with concomitant formation of a few large pal-
ladium NPs after the fourth run (Figures 7a and S9a in the
Supporting Information), which may be ascribed to the migra-
tion of the palladium NPs from the interior pores and the accu-
mulation on the external surface during the consecutive deha-
logenation reaction. In contrast, an obvious agglomeration was
observed in Pd/POP-2 after the fourth run (Figures 7d and S9b
in the Supporting Information). However, the palladium NPs in
Pd/POP-3 are uniformly dispersed on the external surface with
a mean size of (4.5Æ0.35) nm after the fourth run (Figures 7g
and S9c in the Supporting Information). These results signify
that the carboxylate group is favorable for the stabilization of
the palladium NPs through electrostatic and/or coordination
interactions. HAADF-STEM and EDX mapping images further
testify the location and distribution of the palladium NPs on
the external surface after catalytic recycles (Figure 7).
15
1
–
1
32
32
32
32
16[e]
24
[a] Reaction conditions: aryl halide (0.5 mmol), HCOONH4 (2.5 mmol),
H2O/EtOH (1:1, 2 mL), 258C. [b] GC yield. [c] TON=turnover number, that
is, mol product per mol catalyst. [d] POP-3 was used in the absence of
palladium. [e] Filtration experiment.
ence of POP-3 only, no dehalogenation product was detected
under the same conditions (Table 1, entry 4). It is known that
the activation of aryl iodides is easier than of aryl bromides or
chlorides due to the order of the CÀX bond energy. In fact,
chlorobenzene and bromobenzene are more reactive than io-
dobenzene (Table 1, entries 3, 5, and 6).[53,59] When aryl chlor-
ides bearing electron-withdrawing and electron-donating
groups, such as ÀCF3, ÀMe, and ÀOMe, were used, the corre-
sponding products were obtained in quantitative yields
(Table 1, entries 7–9). The use of 2,6-dimethyl-chlorobenzene
with bulkier steric hindrance gave an GC yield of 87% (Table 1,
entry 10). Interestingly, 1,2-dichlorobenzene and 1,4-dichloro-
benzene can be completely dehalogenated under the same
conditions (Table 1, entries 11 and 12). The dehalogenation re-
action of 1,3,5-trichlorobenzene afforded a conversion of 92%
with chlorobenzene (1.8%) and 1,2-dichlorobenzene (3.0%)
residues in 6 h (Table 1, entry 13), whereas a GC yield of 100%
Conclusion
An effective strategy for the functionalization of POPs at the
molecular level and for the stabilization of palladium NPs on
the external surface of POPs has been presented. The function-
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adsorption and desorption isotherms were measured at 77 K by
using a Micromeritics ASAP2020 system. The samples were de-
gassed at 1008C for 10 h before the measurements. Surface areas
were calculated from the adsorption data by using Brunauer–
Emmett–Teller (BET) methods. The pore size distribution curves
were obtained from the adsorption branches by using the non-
local density functional theory (NLDFT) method. Field-emission
scanning electron microscopy (SEM) was performed on a JEOL
JSM-7500F instrument operated at an accelerating voltage of
3.0 kV. Transmission electron microscope (TEM) images were ob-
tained with a JEOL JEM-2010 instrument operated at 200 kV. X-ray
photoelectron spectroscopy (XPS) measurements were performed
on a Thermo ESCALAB250 spectrometer by using non-monochro-
matic AlKa X-rays as the excitation source and choosing C 1s
(284.6 eV) as the reference line. Elemental analyses were performed
on an Elementar Vario MICRO elemental analyzer. Inductively cou-
pled plasma spectroscopy (ICP) was measured on a JobinYvonUlti-
ma2 spectrometer. Gas chromatography (GC) was performed on
a Shimadzu GC-2014 equipped with a capillary column (RTX-5,
30 mꢂ0.25 mm) by using a flame ionization detector.
Computational methods: The hybrid Becke three-parameter Lee
Yang Parr (B3LYP)[63,64] density functional method in combination
with 6-311+G(d) basis set was employed to study the equilibrium
geometries and electronic properties for POP-1, POP-2, and POP-3.
All geometries were fully optimized and confirmed as minima by
analytical frequency calculations at the same level of theory. The
natural population analyses were performed by the NBO 3.1
module embedded in the Gaussian 09 program.[65] The atomic
charges produced by fitting to the electrostatic potential at points
were evaluated by using the ChelpG scheme.[66] The wavefunction
analyses were performed by using the Multiwfn software, which is
a multifunctional wavefunction analysis program developed by Lu
and Chen, and can be freely downloaded.[67] During the calculation
processes, the default settings were used for all programs. All iso-
surface maps including the frontier molecular orbitals and the elec-
trostatic potential on van der Waals surface were rendered by
using the VMD program.[68]
Figure 7. TEM and HAADF-STEM images for a–c) Pd/POP-1, d–f) Pd/POP-2,
and g–i) Pd/POP-3) after recycling four runs.
alization of BTP unit by using COOMe and carboxylate groups
has exerted dominant effects on the structures and properties
of the resulting POPs and the palladium NPs. The introduction
of carboxylate groups engenders a dual functionality; it not
only endows POP-3 am outstanding dispersibility in H2O/EtOH,
but also improves the stabilization of palladium NPs on the ex-
ternal surface of POP-3 through its electrostatic and coordina-
tion interactions. The use of POPs as heterogeneous supports
of palladium NPs in dehalogenation reactions has been evalu-
ated. Pd/POP-3 shows the highest catalytic activity, recyclabili-
ty, and no obvious agglomeration of the palladium NPs at the
surface was observed after consecutive four runs. The remark-
able performances of Pd/POP-3 provide a new and general
avenue for the customerization of metal NPs with high catalyt-
ic activity and recyclability. Further investigation into the pre-
designable synthesis of metal NPs on the external surface of
functionalized POPs for specific applications is in progress.
Synthesis of POP-1: A mixture of 2,6-diethynylpyridine (0.191 g,
1.50 mmol), 1,3,5-tris(4-azidophenyl)benzene (TAPB) (0.429 g,
1.00 mmol), CuSO4·5H2O (0.038 g, 0.150 mmol), and sodium ascor-
bate (0.030 g, 0.15 mmol) in dry DMF (60 mL) was stirred under
a nitrogen atmosphere at 1008C for 72 h to afford a dark yellow
powder. The solid was isolated by filtration, and subsequently
washed with an aqueous solution of EDTA-2Na (EDTA=ethylene-
diaminetetraacetic acid) (0.250 g in 200 mL H2O), ethanol, and
CH2Cl2 to remove any unreacted monomers or residues. The deep
yellow powder was further treated by Soxhlet extraction in CH2Cl2
overnight and dried in vacuo at 608C for 12 h. Yield: 0.603 g
(97%); FTIR (KBr): n˜ =3430 (s), 3142 (m), 1608 (s), 1574 (s), 1517 (s),
1446 (m), 1392 (m), 1232 (m), 1035 (s), 829 cmÀ1 (s); elemental
analysis calcd (%) for C25H19N7: C 71.19, H 4.56, N 23.50; found: C
64.73, H 4.57, N 20.38.
Experimental Section
General information: 2,6-Diethynylpyridine,[61] methyl-2,6-diethy-
nylisonicotinate,[61] and 1,3,5-tris(4-azidophenyl)benzene[62] were
prepared according to literature methods. Other chemicals were
commercially available and used without further purification. Solid-
state 13C cross-polarization/magic angle spinning (CP/MAS) NMR
spectrometry was performed on a Bruker SB Avance III 500 MHz
spectrometer with a 4 mm double resonance MAS probe, a sample
spinning rate of 7.0 kHz, a contact time of 2 ms, and a pulse delay
of 5 s. FTIR spectra were recorded with KBr pellets by using
a Perkin–Elmer instrument. Thermal gravimetric analysis (TGA) was
carried out on NETZSCH STA 449C by heating samples from 30 to
8008C under a dynamic nitrogen atmosphere with a heating rate
of 108CminÀ1. Powder X-ray diffraction (PXRD) patterns were re-
corded in the range of 2q=5–858 on a desktop X-ray diffractome-
ter (RIGAKU-MiniflexII) with CuKa radiation (l=1.5406 ꢁ). Nitrogen
Synthesis of POP-2: A mixture of methyl-2,6-diethynylisonicotinate
(0.278 g, 1.50 mmol), 1,3,5-tris(4-azidophenyl)benzene (0.429 g,
1.00 mmol), CuSO4·5H2O (0.038 g, 0.15 mmol), and sodium ascor-
bate (0.030 g, 0.15 mmol) in dry DMF (70 mL) was stirred under
a nitrogen atmosphere at 1008C for 72 h to afford a dark yellow
powder. The solid was isolated by filtration, and subsequently
washed with an aqueous solution of EDTA-2Na (0.250 g in 200 mL
H2O), ethanol, and CH2Cl2 to remove any unreacted monomers or
residues. The deep yellow powder was further treated by Soxhlet
extraction in CH2Cl2 overnight and dried in vacuo at 608C for 12 h.
Yield: 0.678 g (96%); FTIR (KBr): n˜ =3356 (s), 3077 (w), 1726 (m),
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829 cmÀ1 (s).
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Keywords: chelating effect · dehalogenation · heterogeneous
catalysis · palladium nanoparticles · porous organic polymers
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Received: April 27, 2016
Published online on && &&, 0000
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FULL PAPER
POP out: The introduction of carboxyl-
ate groups not only endows porous or-
ganic polymers (POPs) a superior disper-
sibility in H2O/EtOH, but also results in
the variation of the size and location of
palladium nanoparticles (NPs) from the
interior pores to the external surface
(see figure). Surface palladium NPs
show a superior catalytic activity, stabili-
ty, and recyclability in the dehalogena-
tion of aryl halides.
&
Organic Polymers
H. Zhong, C. Liu, H. Zhou, Y. Wang,
R. Wang*
&& – &&
Prefunctionalized Porous Organic
Polymers: Effective Supports of
Surface Palladium Nanoparticles for
the Enhancement of Catalytic
Performances in Dehalogenation
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&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!