Highly Dispersed Ultrafine Palladium Nanoparticles Enabled by Functionalized Porous Organic Polymer for Additive-Free Dehydrogenation of Formic Acid

Article abstract of DOI:10.1002/cctc.201701805

Formic acid (FA) is one of the most favorable chemical hydrogen storage materials for renewable energy transformation, the development of efficient heterogeneous catalysts for ultraclean H₂ generation from FA in the absence of any alkalis or additives under mild conditions remains a major challenge. Here, we present a porous organic polymer containing triphenylamine (TPA) and 2,6-bis(1,2,3-triazol-4-yl)pyridyl (BTP) units (TB-POP). The ultrafine palladium nanoparticles (NPs) with an average size of 1.5±0.6 nm can be facilely generated using convenient methods and are uniformly dispersed over the TB-POP support. TPA and BTP units are homogeneously arranged in the host framework of TB-POP; they serve as effective alternatives of basic sites to deprotonate FA into formate intermediates. Pd@TB-POP has demonstrated remarkable catalytic activity and high selectivity in additive-free dehydrogenation of aqueous FA solution. The value of initial turnover frequency (TOF) at 50 °C is as high as 1344 h^?1. In sharp contrast, the use of sodium formate as an additive in the catalytic system results in the decrement of catalytic activity. The ultrafine palladium NPs are effectively immobilized during dehydrogenation of FA, and no aggregation of palladium NPs is observed after consecutive catalytic runs.

Full text of DOI:10.1002/cctc.201701805

Accepted Article
Title: Highly Dispersed Ultrafine Palladium Nanoparticles Enabled
by Functionalized Porous Organic Polymer for Additive-free
Dehydrogenation of Formic Acid
Authors: Caiyan Cui, Yujiao Tang, Muhammad Asad Ziaee, Ruihu
Wang, and Dongxu Tian
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ChemCatChem
10.1002/cctc.201701805
FULL PAPER
Highly Dispersed Ultrafine Palladium Nanoparticles Enabled by
Functionalized Porous Organic Polymer for Additive-free
Dehydrogenation of Formic Acid
Caiyan Cui,[a, c] Yujiao Tang, Muhammad Asad Ziaee, [a, c] Dongxu Tian,* and Ruihu Wang*
[b]
[b]
[a]
Abstract: Formic acid (FA) is one of the most favorable chemical
to its high energy density, non-toxicity and excellent stability.
hydrogen storage materials for renewable energy transformation, the
Hydrogen generated from FA holds great promises for application
in hydrogen-powered fuel cells. Hydrogen stored in FA can be
catalytically released via dehydrogenation reaction,[3] however,
undesirable dehydration pathway usually leads to the poisoning
of fuel cell catalysts.[ The developments of highly active catalysts
for FA to dehydrogenate selectively under mild conditions are
development of efficient heterogeneous catalysts for ultraclean H
2
generation from FA in the absence of any alkalis or additives under
mild conditions remains a major challenge. Here, we present a porous
organic polymer containing triphenylamine (TPA) and 2,6-bis(1,2,3-
triazol-4-yl)pyridyl (BTP) units (TB-POP). The ultrafine palladium
nanoparticles (NPs) with an average size of 1.5±0.6 nm can be facilely
generated using convenient methods, and are uniformly dispersed
over TB-POP support. TPA and BTP units are homogeneously
arranged in the host framework of TB-POP, they serve as effective
alternatives of basic sites to deprotonate FA into formate
intermediates. Pd@TB-POP has demonstrated remarkable catalytic
activity and high selectivity in additive-free dehydrogenation of
aqueous FA solution. The value of initial turnover frequency (TOF) at
4]
2
pivotal to give ultrapure H for subsequent use in fuel cells.
The dehydrogenation of FA using homogeneous catalysts in
organic solvents has been widely investigated,[ and their
catalytic performances are greatly enhanced by adding additives,
such as sodium formate (SF)[6] and triethylamine,[7] but the use of
organic solvents and/or additives greatly mitigates the conception
5]
2
of green chemistry for H generation from FA. Heterogeneous
catalysts based on metal nanoparticles (NPs) are generally more
practical catalysts because of facile handling and outstanding
reusability associated with heterogeneous materials. Moreover,
the supports of metal NPs can be functionalized through
o
-1
5
0 C is as high as 1344 h . In sharp contrast, the use of sodium
formate as an additive in the catalytic system results in the decrement
of catalytic activity. The ultrafine palladium NPs are effectively
immobilized during dehydrogenation of FA, and no obvious
aggregation of palladium NPs is observed after consecutive catalytic
runs.
[
8]
introducing coordination or basic groups, such as amine and
[
9]
imine , which not only contribute to synergetic stabilization and
activation of metal NPs, but also serve as proton scavengers to
replace the additives during the formation of formate
intermediates, thus promoting FA dehydrogenation in the
absence of any additives. However, most heterogeneous catalytic
systems show much less activity than homogeneous counterparts.
Great achievements have been made for controllable synthesis of
Introduction
With ever-increasing use of limited fossil fuels and worldwide
concerns on environmental pollution and energy crisis,
considerable efforts have been devoted to the exploration of
green and sustainable energy sources.[1] Hydrogen is a globally
accepted energy carrier for future applications. The widespread
utilization of hydrogen could facilitate the transition from fossil
fuels to renewable energy sources. However, controlled storage
and release of hydrogen are still key issues in fuel-cell-based
hydrogen economy. Among various kinds of hydrogen storage
materials,[2] formic acid (FA, HCOOH), a product of biomass
processing and reduction of carbon dioxide, has been identified
as a safe and convenient liquid energy carrier for hydrogen due
metal NPs and deliberate modification of supports to improve
catalytic activity of heterogeneous catalytic systems.[3a, 10]
Generally, the catalytic activities of metal NPs are closely
associated with particle size and dispersion of the active sites,
while the reusability and catalytic lifetime of metal NPs are
influenced by their stability against agglomeration. In the context,
the supports of metal NPs play a crucial role in improving their
dispersion and restraining their agglomeration. Therefore, it is
highly desirable to develop new type of support materials of metal
2
NPs for additive-free ultrapure H production from FA.
As one type of emerging organic network materials, porous
organic polymers (POPs) have recently attracted tremendous
interest in heterogeneous catalysis owing to their large surface
area, flexible synthetic methods and high stability.[10d, 11] The
permanent porosities and homogeneous distribution of
functionalized groups in POPs can provide an ideal platform for
effective immobilization and uniform dispersion of metal NPs over
POPs. Noticeably, the modular nature of POPs synthesis has
allowed various coordination groups to be incorporated
homogeneously into organic networks by deliberately designing
building blocks at molecular level.[9, 12] Besides synergetic
immobilization of metal NPs and FA deprotonation to form formate
intermediates, the coordination groups also modify the surface
electronic structure of supported palladium NPs for promoting the
cleavage of C–H bond in formate decomposition. In our
continuous efforts to develop highly efficient heterogeneous
catalytic systems based on palladium NPs,[10a, 13] herein, we report
[
a]
C. Cui, M. A. Ziaee, 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
E-mail: ruihu@fjirsm.ac.cn
[
b]
c]
Y. Tang, Prof. D. Tian
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian 116024, China
[
C. Cui, M. A. Ziaee
University of Chinese Academy of Sciences
Beijing 100049, China
Supporting information for this article is given via a link at the end
of the document.
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10.1002/cctc.201701805
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a POP containing 2,6-bis(1,2,3-triazol-4-yl)pyridyl (BTP) and
triphenylamine (TPA) units (TB-POP). The TB-POP-supported
palladium NPs possess an ultrafine average size of 1.5±0.6 nm
with high dispersion over TB-POP, and exhibit remarkable
catalytic activity, high selectivity, excellent stability and good
reusability in the additive-free dehydrogenation of aqueous FA
solution.
Pd@TB-POP show excellent hydrophilicity with water contact
angles being 67 and 62 , respectively (Figure 1c and 1d).
o
o
Results and Discussion
Figure 2. (a) FTIR spectra of DEP, TAPA, TB-POP and Pd@TB-POP; (b) Solid
state 13C CP/MAS NMR, (c) N
adsorption-desorption isotherms and (d) pore
2
size distributions for TB-POP and Pd@TB-POP.
The chemical structure of TB-POP was characterized by
Fourier transform infrared spectroscopy (FTIR) and Solid-state
1
3C cross-polarization magic angle spinning nuclear magnetic
Scheme 1. Schematic illustration for the synthesis of TB-POP and Pd@TB-
1
3
resonance ( C CP/MAS NMR). As shown in FTIR spectrum of
TB-POP (Figure 2a), the characteristic peaks for terminal alkynyl
group in DEP at 3275 cm-1 and the azide group in TAPA at 2123
cm-1 disappear, and the typical peak of the triazolyl group occurs
POP.
As shown in Scheme 1, TB-POP was facilely prepared through
conventional click reaction between tris(4-azidophenyl)amine
-1
at 1607 cm , confirming the formation of the 1,2,3-triazolyl
linkage.[10a, 13b] In solid-state 13C CP/MAS NMR spectrum of TB-
POP (Figure 2b), no distinguishing peaks of alkynyl in DEP are
observed in the range of 70 to 100 ppm, indicative of complete
polymerization of DEP and TAPA. The resonance peak at 148
ppm corresponds to the carbon atoms adjacent to pyridine and
triphenylamine nitrogen atoms. The broad peak from 100 to 139
ppm is assigned to the rest of carbon atoms. FTIR and solid-state
13C CP/MAS NMR spectrum of Pd@TB-POP are identical with
those of TB-POP, which indicates high chemical and thermal
stability of TB-POP in the process of palladium loading and
subsequent thermal reduction by hydrogen. Thermogravimetric
analysis (TGA) curve of TB-POP shows about 5% weight loss
(
TAPA) and 2,6-diethynylpyridine (DEP) in the presence of
copper sulfate pentahydrate and sodium ascorbate in DMF.
Simple treatment of TB-POP with Pd(OAc) in CH Cl gave rise
/N at
2
2
2
to Pd(II)@TB-POP, and subsequent reduction in 5 vol% H
2
plasma (ICP) analysis shows the palladium content in Pd@TB-
POP is 0.244 mmol g .
2
2
o
00 C generated black Pd@TB-POP. Inductively coupled
-1
o
before 100 C, which is assigned to the removal of trapped guest
molecules (Figure S1). TB-POP starts to decompose gradually
o
after 270 C. The thermal stability of Pd@TB-POP is slightly lower
Figure 1. Photographs for dispersion in water (10 mg sample in 4 mL H2O) and
than that of TB-POP, which is probably ascribed to incomplete
reduction of
supported
Pd(OAc) [
2
14]
and/or
catalytic
water contact angles of TB-POP (a, c) and Pd@TB-POP (b, d).
decomposition caused by palladium nanoparticles. Powder X-ray
diffraction (PXRD) pattern indicates that TB-POP is amorphous
due to the kinetic-controlled irreversibility of click reaction (Figure
TB-POP and Pd@TB-POP are insoluble in water and common
organic solvents. Both show excellent water dispersibility (Figure
o
1a and 1b), which is an important issue for water-phased
S2). Notably, an extra diffraction peak at 40 is observed in PXRD
heterogeneous catalytic systems because of the improvement of
mass transfer of aqueous substrates. The wettability of TB-POP
and Pd@TB-POP were further studied by water contact angle
measurement using the pellet of each material. Both TB-POP and
of Pd@TB-POP, which corresponds to the (111) crystal plane of
palladium NPs. Relatively weak intensity of the diffraction peak
and the absence of other diffraction lines are probably attributable
to both small size and low loading of palladium NPs.[15]
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The porosities of TB-POP and Pd@TB-POP were examined by
physisorption of nitrogen at 77 K (Figure 2c). TB-POP exhibits a
nitrogen adsorption/desorption isotherm of type I, the hysteresis
Pd@TB-POP reveal that palladium NPs are uniformly dispersed
over the support with an average diameter of 1.5±0.6 nm (Figure
3c). Notably, the ultrafine size of palladium NPs is close to
reported metal nanoclusters,[ which means more surface active
palladium species are available for substrates in heterogeneous
catalysis.[18] The ultrafine size and homogeneous distribution of
palladium NPs probably result from cooperative coordination
interaction of BTP and TPA units. In addition, the confinement
effect of micropores also effectively suppresses the growth and
agglomeration of palladium NPs, which is consistent with the
result of micropore diminishment in Pd@TB-POP. As shown in
high-resolution transmission electron microscopy (HRTEM)
image of Pd@TB-POP (Figure 3d), the lattice fringe of palladium
NPs displays an interlayer spacing of 0.223 nm corresponding to
Pd (111) plane[19] which matches well with PXRD result. High-
annular dark-field scanning transmission electron microscope
17]
loop at high P/P
contrast, Pd@TB-POP
0
probably results from void space of particles. In
shows relatively good
adsorption/desorption reversibility with a minor hysteresis loop at
high relative pressure. Brunauer-Emmett-Teller (BET) specific
2
-1
surface areas of TB-POP and Pd@TB-POP are 252 m g , which
is comparable to that of reported planar POPs.[16] TB-POP
possesses a wide pore size distribution in the range from 0.37 to
86 nm based on the nonlocal density functional theory (NLDFT)
method (Figure 2d), while Pd@TB-POP mainly consists of
mesopore and macropore. The decrease of BET specific surface
area (80 m2 g-1) and micropore in Pd@TB-POP is probably
attributed to both partial pore filling and mass increment after
palladium loading.
(
HAADF-STEM) and energy-dispersive X-ray (EDX) mapping
images clearly reveal homogeneous distribution of carbon,
nitrogen and palladium in a wide selected region of Pd@TB-POP
(
Figure 3e).
Figure 4. (a) N 1s XPS spectra for TB-POP and Pd@TB-POP; (b) Pd 3d XPS
spectrum for Pd@TB-POP.
X-ray photoelectron spectroscopy (XPS) analysis was
conducted to gain further insight into the interaction between
palladium NPs and TB-POP. XPS survey spectrum of TB-POP
possesses three peaks around 284.8, 400.1 and 532.1 eV,
corresponding to C 1s, N 1s and O 1s, respectively (Figure S3).
An extra peak of Pd 3d around 336.77 eV is observed in XPS
survey spectrum of Pd@TB-POP. The high-resolution N 1s XPS
spectrum of TB-POP can be resolved into three peaks
corresponding to 2-positioned nitrogen atom of the triazolyl ring
(
401.41 eV), 1- and 3-positioned nitrogen atoms of the triazolyl
ring (399.88 eV), pyridyl and TPA nitrogen atoms (398.91 eV),
respectively.[10a, 13b] Their intensity ratio, determined by the ratio of
relative areas, is 1.00: 2.00: 0.83, which is almost in agreement
with theoretical values. The peaks in N 1s XPS spectrum of
Pd@TB-POP shift to 401.56, 400.02 and 399.04 eV, respectively,
with corresponding positive shifts of 0.15, 0.14 and 0.13 eV,
respectively, which is indicative of strong coordination interaction
between BTP/TPA and palladium NPs (Figure 4a). Pd 3d XPS
spectrum of Pd@TB-POP presents two sets of doublet peaks
Figure 3. SEM images for (a) TB-POP and (b) Pd@TB-POP; (c) TEM image,
particle size distribution of palladium NPs in Pd@TB-POP; (d) HRTEM image
and corresponding FFT pattern of Pd@TB-POP; (e) HAADF-STEM and EDX
mapping images of Pd@TB-POP.
corresponding to Pd 3d5/2 and Pd 3d3/2
electronically different palladium species (Figure 4b). The peaks
at 341.14 (3d3/2) and 335.88 eV (3d5/2) are attributed to Pd0
species, and the peaks at 342.91 (3d3/2) and 337.65 eV (3d5/2) are
related to Pd2+ species. The existence of Pd2+ probably results
from incomplete reduction and/or reoxidation of surface Pd . The
, associated with
The morphologies of TB-POP and Pd@TB-POP were
investigated by scanning electron microscope (SEM). SEM image
shows that TB-POP is composed of granular particles (Figure 3a),
and the morphology remains intact after loading of palladium NPs
(
Figure 3b). Transmission electron microscope (TEM) images of
0
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ratio of surface Pd0 to Pd2+ in Pd@TB-POP is 1.34. Electron
transfer from BTP and TPA units to palladium induces a negative
To study the effect of FA concentration on catalytic
performance, FA conversion and H
2
selectivity versus FA
2
+
shift of 0.75 eV when compared with Pd 3d5/2 peak (338.4 eV)
concentration were evaluated in the presence of 1.41 mol%
Pd@TB-POP at 50 oC for 15 min. As shown in Figure 5c and
Figure S4, the gas generation rate increases slowly as FA
concentration increases in the range of 0.1-1.0 M, and reaches
the maximum value at 1.0 M, and then drops rapidly with further
increment of FA concentration, only 36.9% and 13.4% FA
conversions are achieved at 4.0 M and 8.0 M, respectively. These
results reveal that a large amount of water plays an indispensable
role in the catalytic dehydrogenation of FA. Notably, selectivity for
the dehydrogenation reaction remains 100% in the whole range
of FA concentration.
[20]
2
for free Pd(OAc) .
The catalytic activity of Pd@TB-POP was evaluated by FA
dehydrogenation in the absence of any additives under ambient
atmosphere. As shown in Figure 5a, the complete conversion of
aqueous FA (1 M) is achieved within 9 min in the presence of only
o
1
.41 mol% Pd@TB-POP at 50 C. The initial TOF value is 1344
-
1
[15,
h , as determined by the reaction time of FA conversion at 20%.
2
1]
The catalytic activity of Pd@TB-POP is comparable with
reported additive-free heterogeneous systems based on
palladium NPs[10c, 22] (Table S1). Notably, no CO was detected
when the compositions of the produced gas were analyzed by gas
chromatography (GC), suggesting excellent selectivity of FA
dehydrogenation in this catalytic system (Figure 5b). As a
comparison, Pd(II)@TB-POP and commercial Pd/C were also
examined in FA dehydrogenation under the same conditions
As a pivotal factor in catalytic reactions, temperature effect
was also investigated. Time-dependent H generation at different
2
temperatures was also tested in 1.0 M aqueous FA solution. As
shown in Figure 5d, the gas generation rate increases sharply
o
with the increase of reaction temperature from 30 to 60 C, which
(
Figure 5a), but much lower catalytic activities were achieved,
which indicates that Pd(0) species are the active components for
catalytic H generation from aqueous FA. It should be mentioned
indicates that high reaction temperature is beneficial for
accelerating FA dehydrogenation. Interestingly, a complete
conversion of FA was observed within only 6 min at 60 C. The
o
2
that only 20% conversion of aqueous FA was achieved when
commercial Pd/C was used as catalyst, which is close to that for
Pd(II)@TB-POP.
apparent activation energy (Ea) estimated from Arrhenius plot is
41.8 kJ mol-1 (Figure 5e), which is lower than those in most
[
9,
reported additive-free catalytic systems of FA dehydrogenation.
1
0b]
Figure 6. (a) TEM image and particle size distributions of palladium NPs (inset),
b) HRTEM image and corresponding FFT pattern, (c) High-annular dark-field
scanning TEM and energy-dispersive X-ray mapping images for Pd@TB-POP-
run.
(
5
Figure 5. (a) Plot of H
GC signals for pure CO (blue), pure H
FA solution catalyzed by Pd@TB-POP (red); (c) FA conversion (black) and H
selectivity (red) versus FA concentration; (d) Plot of H generation from FA as a
2
generation from FA as a function of time at 50 oC; (b)
(black), the produced gas from aqueous
2
The reusability of Pd@TB-POP was evaluated using 1.0 M
2
o
aqueous FA at 50 C (Figure 5f). After the reaction was finished,
the recovered catalyst was used for the next run through recharge
of new aqueous FA solution. Notably, the catalytic system can be
used at least five runs with a slight depression in its catalytic
activity. The catalyst after five consecutive catalytic runs is
2
function of time at different temperatures; (e) Initial TOF values at different
temperatures and Arrhenius plot of FA dehydrogenation; (f) Recyclability of
Pd@TB-POP for FA dehydrogenation.
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denoted as Pd@TB-POP-5run. In order to further understand this
catalytic system, Pd@TB-POP-5run was characterized by FTIR,
SEM, TEM and PXRD. FTIR (Figure S5) and SEM images (Figure
S6) show that the original structural framework and morphology
of Pd@TB-POP are intact after consecutive FA dehydrogenation.
A new characteristic peak of carbonyl group at 1680 cm-1 is
free dehydrogenation of aqueous FA solution, but also develops
a new approach to customize heterogeneous catalysts for
additive-free ultrapure
conditions.
H
2
generation from FA under mild
observed,[8] which may result from the adsorption of FA molecule. Experimental Section
TEM images clearly show that palladium NPs in Pd@TB-POP-
5
6
2
run remain highly dispersed without obvious aggregation (Figure
a), while the diameter of palladium NP slightly increases to
.1±0.7 nm, suggesting palladium NPs are effectively stabilized
Preparation of TB-POP: To a solution of 2,6-diethynylpyridine (0.255 g,
2.0 mmol), tris(4-azidophenyl)amine (490 mg, 1.33 mmol) and
CuSO ·5H O (50 mg, 0.2 mmol) in DMF (28 mL), sodium ascorbate (198
4 2
mg, 0.2 mmol) was added under N
at 100 C for 24 h to obtain brown powder. The powder was separated by
vacuum filtration and then rinsed thoroughly with DMF, ethanol, THF and
2
atmosphere. The mixture was heated
by TB-POP. HR-TEM image and corresponding FFT pattern show
the interval between lattice fringes in Pd@TB-POP-5run is the
same as that in Pd@TB-POP (Figure 6b), which is consistent with
the result of PXRD pattern of Pd@TB-POP-5run with a diffraction
o
2 2
CH Cl to remove any unreacted monomers or residues. The resultant
solid was then redispersed in saturated aqueous EDTA-2Na solution (200
mL) at room temperature overnight. After separated and washed with
o
peak at 40 (Figure S2), indicative of good stability of palladium
NPs in consecutive catalytic FA dehydrogenation. HAADF-STEM
and EDX mapping images further testify the size and distribution
of Pd NPs after catalytic recycles (Figure 6c), which is agreement
with TEM results.
Based on aforementioned analyses and previous related
reports,[8-9, 10b] a plausible mechanism of FA dehydrogenation over
Pd@TB-POP has been proposed. Periodically distributed TPA
and BTP units in Pd@TB-POP serve as efficient Lewis basic sites
to facilitate the O–H bond dissociation of FA, generating formate
intermediates. It should be mentioned that the addition of
equivalent SF as an additive results in the decrement of catalytic
activity under the same condition, suggesting that TPA and BTP
units are effective to deprotonate FA in the catalytic system
excess H O and ethanol, the resultant material was purified by Soxhlet
2
extraction using MeOH for 24 h and dried under vacuum. Yield: 730 mg,
(98%). Elemental analysis: Calculated (%): C, 67.69; H, 3.51; N, 28.8;
-
1
Found (%): C, 60.93; H, 4.07; N, 24.75. IR (KBr, cm ): 3429 (s), 1607 (s),
574 (s), 1510 (vs), 1451 (s), 1404 (m), 1319 (s), 1267 (s), 1234 (s), 1109
w), 1035 (s), 989 (m), 834 (s), 804 (s), 738 (w), 775 (m), 536 (m).
1
(
2 2
Synthesis of Pd@TB-POP: TB-POP (0.15 g) was added to CH Cl
solution (150 mL) of palladium acetate (0.045 g, 0.2 mmol), and stirred
o
vigorously at 60 C for 24 h. The resulting mixture was washed with CH
2
Cl
2
o
to remove excess palladium precursor and dried under vacuum at 80 C
to give Pd(II)@TB-POP. Pd(II)@TB-POP was reduced in a stream of H /N
, 80 mL min ) at 200 C for 4 h to give rise to Pd@TB-POP as
a black solid. The palladium content in Pd@TB-POP was 0.244 mmol g-
as determined by ICP analysis.
2
2
-
1
o
(5 vol% H
2
1
(
Figure S7). Besides stabilizing ultrafine palladium NPs, TPA and
BTP units can donate electron to palladium NPs, which
contributes to β-hydride elimination in palladium-formate species,
resulting in rapid release of CO and H under additive-free mild
2 2
conditions. The nitrogen-containing moieties and palladium NPs
are then recovered and available to protonate and catalyze
dehydrogenation of next FA molecule.
General procedure for hydrogen generation from aqueous FA
solution: The volume of released gas during the reaction was monitored
using a gas burette by water displacement. In a general procedure, pre-
2
weighed Pd@TB-POP catalyst and 1 mL H O was added to a two-necked
reaction flask (25.0 mL), and the mixture was sonicated for 2 min. After
this mixture was stirred for 5 min to achieve thermal equilibrium, 1.0 mL
aqueous FA solution (2.0 M) was added into the reaction flask and the
catalytic reaction was started (t = 0 min) by stirring the mixture at 800 rpm.
The catalytic activity of Pd@TB-POP in dehydrogenation of FA was
determined by measuring the rate of hydrogen generation.
Conclusions
A new porous organic polymer containing TPA and BTP units has
been presented, which can serve as an effective heterogeneous
support for ultrafine palladium NPs. Pd@TB-POP shows
remarkable catalytic activity, high selectivity, excellent stability
and good reusability in additive-free dehydrogenation of aqueous
FA solution under mild conditions. The promising catalytic
performances are mainly attributed to the co-existence of BTP
and TPA units in Pd@TB-POP: 1) They endow the catalyst
Reusability of Pd@TB-POP in the additive-free dehydrogenation of
FA: After the catalytic dehydrogenation of aqueous FA solution at 50 oC,
Pd@TB-POP was isolated from reaction mixture by centrifugation and
washed thoroughly with water and ethanol. After dried under ambient
conditions, the recovered catalyst was directly used for the next run with
the addition of fresh aqueous FA solution.
material with excellent hydrophilicity and high water dispersibility, Acknowledgements
which increases the interfacial contact area between catalytic
active sites and aqueous substrates; 2) They cooperatively
This work was financially supported by National Natural Science
stabilize ultrafine palladium NPs through coordination interaction,
and ultrasmall size of palladium NPs renders more reactive sites
available for substrates; 3) They corporately deprotonate FA to
Foundation of China (21673241 and 21471151) and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(
XDB20000000).
2
form formate intermediates, which facilitates H release from FA
under additive-free conditions. In summary, this study not only
provides one type of new materials for highly efficient additive-
Keywords: porous organic polymer • hydrogen generation •
heterogeneous catalysis • nanoparticles • sustainable chemistry
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ChemCatChem
10.1002/cctc.201701805
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Porous organic polymer containing
triphenylamine and 2,6-bis(1,2,3-
triazol-4-yl)pyridyl units can serve as
an effective heterogeneous support
for ultrafine palladium nanoparticles.
The catalytic system shows
remarkable catalytic activity, high
selectivity, excellent stability and good
reusability in additive-free hydrogen
generation from aqueous FA solution
under mild conditions.
Caiyan Cui, Yujiao Tang, Muhammad
Asad Ziaee, Dongxu Tian,* and Ruihu
Wang*
Page No. – Page No.
Highly Dispersed Ultrafine Palladium
Nanoparticles Enabled by
Functionalized Porous Organic
Polymer for Additive-free
Dehydrogenation of Formic Acid
This article is protected by copyright. All rights reserved.