Ultralow-content palladium dispersed in covalent organic framework for highly efficient and selective semihydrogenation of alkynes

Article abstract of DOI:10.1021/acs.inorgchem.9b01117

Developing noble-metal-based catalysts with ultralow loading to achieve excellent performance for selective hydrogenation of alkynes under mild reaction conditions is highly desirable but still faces huge challenges. To this end, a SO₃H-anchored covalent organic framework (COF-SO₃H) as the support was deliberately designed, and then ultralow-content Pd (0.38 wt %) was loaded by a wet-chemistry immersion dispersion method. The resulting Pd^0.38/COF-SO₃H composite exhibits outstanding performance for the selective hydrogenation of phenylacetylene with 97.06% conversion and 93.15% selectivity to styrene under mild reaction conditions (1 bar of H₂, 25 °C). Noticeably, the turnover frequency value reaches as high as 3888 h^-1, which outperforms most of reported catalysts for such use. Moreover, such a catalyst also exhibits excellent activity for a series of other alkynes and high stability without obvious loss of catalytic performance after five consecutive cycles.

Full text of DOI:10.1021/acs.inorgchem.9b01117

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ultralow-Content Palladium Dispersed in Covalent Organic
Framework for Highly Efficient and Selective Semihydrogenation of
Alkynes
Jian Hong Li,†,‡,⊥ Zhi Wu Yu,§,⊥ Zhi Gao,‡ Jian Qiang Li,‡ Yuan Tao,‡ Yu Xin Xiao,‡ Wen Hui Yin,‡
Ya Ling Fan,‡ Chao Jiang,‡ Li Jun Sun,‡ and Feng Luo*,‡
†China Institute of Atomic Energy, Beijing 100822, P. R. China
‡State Key Laboratory of Nuclear Resources and Environment, School of Chemistry, Biology and Materials Science, East China
University of Technology, Nanchang 330013, P. R. China
§High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, Anhui, P. R. China
S
* Supporting Information
ABSTRACT: Developing noble-metal-based catalysts with ultralow
loading to achieve excellent performance for selective hydrogenation of
alkynes under mild reaction conditions is highly desirable but still faces
huge challenges. To this end, a SO3H-anchored covalent organic
framework (COF-SO3H) as the support was deliberately designed, and
then ultralow-content Pd (0.38 wt %) was loaded by a wet-chemistry
immersion dispersion method. The resulting Pd0.38/COF-SO3H compo-
site exhibits outstanding performance for the selective hydrogenation of
phenylacetylene with 97.06% conversion and 93.15% selectivity to styrene
under mild reaction conditions (1 bar of H2, 25 °C). Noticeably, the
turnover frequency value reaches as high as 3888 h−1, which outperforms
most of reported catalysts for such use. Moreover, such a catalyst also
exhibits excellent activity for a series of other alkynes and high stability
without obvious loss of catalytic performance after five consecutive cycles.
load noble-metal Pd for the selective hydrogenation of alkynes.
For example, Ulan et al. reported a CaCO3-supported Pd
catalyst (Pd/CaCO3 treated by Pb salts) for the selective
hydrogenation of 2-hexyne.17 Unfortunately, this catalyst
suffers low selectivity to 2-hexene (about 60%) and poor
stability, as well as high toxicity because of the existence of
toxic Pb, which seriously limits its further large-scale
application. Considering that bimetallic catalysts generally
exhibit more excellent catalytic activity compared to
monometallic catalysts because of the distinct electronic
interaction between different metals, an active carbon-
supported Pd-based bimetallic catalyst (Pd0.33Pb0.67/C) was
constructed for the selective hydrogenation of phenylacetylene,
which exhibits high conversion (98%) and selectivity to styrene
(99%).18 However, the content of Pd in Pd0.33Pb0.67/C is as
high as 10.64 wt %, resulting in high cost. In recent years,
developing a Pd-based catalyst with extremely low loading
attracted significant attention.19,20 For example, Pd/Ni(OH)2
with an ultralow content of Pd (only 0.005 wt %) was
INTRODUCTION
■
The selective hydrogenation of alkynes to alkenes without
deep hydrogenation to alkanes lies at the heart of the industrial
manufacturing of fine chemicals, pharmaceuticals, nutraceut-
icals, and agrochemicals.1−4 However, their efficiency is often
seriously hindered by the inevitable deep hydrogenation of
alkynes into undesirable alkanes.5 Up to now, extensive efforts
have been devoted to improving the catalytic efficiency of
alkyne hydrogenation. Among different metal-based catalysts,
noble-metal (Pt, Pd, Au, etc.) catalysts are the more ideal
choice because of their high catalytic activity and durable
stability. Compared to other noble-metal catalysts, Pd-based
catalysts are considered to be more suitable alternatives owing
to their strong dissociative ability for H2 and relatively cheap
cost.6
As is well-known, support materials play a crucial role in the
catalytic performance because a suitable support often leads to
strong interaction between the active metal and support, which
is good for enhancing the catalytic activity.7−10 Thus, a variety
of different supports including porous silica,11 carbon
materials,12 graphene,13 metal oxide,14 polymers,15 and
metal−organic frameworks (MOFs)16 have been used to
Received: April 17, 2019
© XXXX American Chemical Society
A
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Scheme 1. Procedure for the Synthesis of Pd/COF-SO3H
successfully prepared by Hu et al. for the selective hydro-
genation of acetylene.21 However, the conversion of acetylene
and selectivity for ethylene are not satisfied, at only 80% for
both of them. Moreover, the Pd@Zn-MOF-74 catalyst with a
low Pd loading of 1.67 wt % was prepared and exhibits
extremely high phenylacetylene conversion (100%) and
styrene selectivity (92%).22 However, a long reaction time
and a complex preparation process are necessary, which
hinders its large-scale application. Thus, it is highly desirable to
develop a promising catalyst with an ultralow content of Pd
based on a facile and convenient synthesis strategy for a highly
efficient catalytic hydrogenation of alkynes to the desirable
olefin.
In the recent years, covalent organic frameworks (COFs),
established as an emerging class of crystalline porous polymers
with high surface area, structural diversity, and designability,
attracted much interest and exhibited huge potential in the
catalytic field. The distinct advantage is the atomically precise
integration of pure organic units to create predesigned
skeletons and nanopores based on reticular chemistry,23−30
which can significantly enhance the metal−support interaction,
thus improving the catalytic activity. For example, Banerjee
and co-workers reported COF (TpPa-1)-supported Au and Pd
nanoparticle hybrid materials, which show high catalytic
activity in nitrophenol reduction and Sonogashira coupling
because of the strong synergistic effect, respectively.31−33
In this work, COF-SO3H was synthesized by reacting 2,4,6-
triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid
under solvothermal conditions, and then Pd nanoparticles were
loaded onto it to obtain Pd/COF-SO3H with different Pd
loadings by a wet-chemistry immersion dispersion method.
The as-prepared Pd/COF-SO3H with a Pd loading of only
0.38 wt % exhibits excellent performance for the selective
hydrogenation of phenylacetylene with 97.06% phenylacety-
lene conversion and 93.15% styrene selectivity, as well as
durable stability. Characterization results reveal that the
outstanding performance is ascribed to the strong synergistic
effect between the Pd nanoparticles and COF-SO3H support.
To the best of our knowledge, this is the first report about such
highly efficient COF-supported Pd-based catalysts with
ultralow Pd loading for the selective hydrogenation of alkynes
to olefin.
B
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Figure 1. Space-filling view of the structure of COF-SO3H.
Figure 2. (a and b) SEM images and (c) SEM−EDS mapping of the as-prepared COF-SO3H.
the structural information. The FT-IR spectrum of COF-SO3H
(Figure S1) shows the characteristic SO3− stretching vibrations
of a sulfonic acid at ∼1048 and ∼1103 cm−1, which match well
with those of the sulfonic acid of 2,5-diaminobenzenesulfonic
acid, confirming the successful introduction of −SO3H.
Meanwhile, the N−H (3444 cm−1) stretch and the character-
istic band related to CO (1700 cm−1) were also obviously
detected. On the basis of the above IR spectrum results, we can
better understand the structure of COF-SO3H and determine
its successful synthesis. Furthermore, EDS analysis indicates
that the element content agrees well with the values obtained
from elemental analysis (Table S1). The SEM images show
uniform filiform morphology (Figure 2a,b), and the SEM−
EDS mapping images indicate that the as-prepared COF-SO3H
is composed of C, N, O, and S elements (Figure 2c). As
demonstrated by the X-ray diffraction (XRD) patterns in
Figure 3, the as-synthesized COF-SO3H agrees with the
simulated COF-SO3H data, strongly demonstrating the
successful preparation of COF-SO3H. TGA reveals that
COF-SO3H possesses good thermal stability up to 290 °C
(Figure S2a), the weight loss of which is less than 15%.
Furthermore, the crystalline structures of COF-SO3H after
immersion in a strong acid, boiling water, and a strong base,
respectively, were evaluated by powder XRD (PXRD) studies
(Figure S2b). The results demonstrate that COF-SO3H has
ultrahigh chemical stability, exhibiting a huge industrial
application prospect. From cross-polarized magic-angle-spin-
ning solid-state 13C NMR spectroscopy (Figure S3), the β-
RESULTS AND DISCUSSION
■
The synthesis process of Pd/COF-SO3H is schematically
illustrated in Scheme 1. The whole catalyst synthesis process is
divided into three steps. The first step is to synthesize a 2,4,6-
triformylphloroglucinol organic unit based on hexamethylene-
tetramine and phloroglucinol using p-phenylenediamine and
fuming sulfuric acid to prepare 2,5-diaminobenzenesulfonic
acid. Then a SO3H-anchored covalent organic framework
(COF-SO3H) was synthesized based on the organic units
2,4,6-triformylphloroglucinol and 2,5-diaminobenzenesulfonic
acid under solvothermal conditions. As shown in Figure 1,
COF-SO3H shows the eclipsed stacking ABA mode with a
stacking distance of 3.4 Å, in which −SO3H units among
adjacent layers hold the opposite-side arrangement rather than
the common same-side mode. The lattice parameters of a, b,
and c in COF-SO3H are 22.499, 22.499, and 6.8 Å,
respectively. The as-prepared COF-SO3H is not soluble in
water and common organic solvents such as dichloromethane,
acetone, ethanol, and tetrahydrofuran, exhibiting promising
application as a support. Moreover, COF-SO3H can promote
ion exchange between H+ in −SO3H and Pd2+, thus enhancing
the interaction of Pd2+ and COF-SO3H and promoting the
dispersion of Pd species. Finally, Pd/COF-SO3H was obtained
using NaBH4 as a reductant.
Fourier transform infrared (FT-IR) spectroscopy, scanning
electron microscopy (SEM), energy-dispersive X-ray spectros-
copy (EDS), thermogravimetric analysis (TGA), and N2
adsorption−desorption were investigated to gain insight into
C
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of Pd with different contents. The slight decrease of the
diffraction intensity related to the (100) plane is ascribed to
blockage of the pore channel in COF-SO3H by Pd nano-
particles, as demonstrated by the smaller BET surface area after
loading of Pd. TGA proves that Pd0.38/COF-SO3H also
possesses excellent thermal stabilities up to 350 °C (Figure
S5), which meets the demands for potential applications in
catalysis. The SEM image discloses that Pd/COF-SO3H still
can retain the uniform fiber morphology (Figures 5, S6, and
S7). The porosities of these Pd-loaded samples were explored
by N2 adsorption at 77 K, and we found that the loading of Pd
onto COF-SO3H can significantly reduce its BET surface area
(Figure S8). Relative to the BET surface area in COF-SO3H
(306 m2/g), the N2 adsorption capacity at 77 K is largely
reduced in these Pd-loaded samples, giving BET surface areas
of 200, 99.45, and 66.7 m2/g in Pd0.38/SO3H, Pd0.69/COF-
SO3H, and Pd1.66/COF-SO3H, respectively, most likely due to
the occupation of pores by Pd nanoparticles. Correspondingly,
Pd0.38/SO3H delivers a smaller pore size (0.97 nm) compared
to Pd0.69/COF-SO3H (1.05 nm) and Pd1.66/COF-SO3H (1.3
nm; Figure S9).
Typical transmission electron microscopy (TEM) images of
Pd/COF-SO3H (Figures 6 and S10) confirm that the sizes of
Pd nanoparticles vary with the content of Pd, viz., 2.63 nm for
0.38 wt % Pd/COF-SO3H, 2.98 nm for 0.69 wt % Pd/COF-
SO3H, and 3.24 nm for 1.66 wt % Pd/COF-SO3H, as revealed
by particle size distribution histograms (Figure S11).
X-ray photoelectron spectroscopy (XPS) measurements
were performed to investigate the electronic state of Pd
nanoparticles in Pd/COF-SO3H. As shown in Figure 7, the
binding energies of Pd0 related to the typical 3d5/2 peak at
335.68 eV and 3d3/2 peak at 341.03 eV were detected, while
Pd2+ with the typical 3d5/2 peak at 337.8 eV and 3d3/2 peak at
342.1 eV were also observed in these Pd-loaded samples.35−38
The presence of Pd0 indicates the successful preparation of Pd
nanoparticles, while the formation of Pd2+ may be ascribed to
the reoxidation of Pd0 to Pd2+ by air.
To verify the catalytic activity of Pd/COF-SO3H, we
explored the semihydrogenation reaction. The activity of Pd/
COF-SO3H for the selective hydrogenation of phenylacetylene
was performed. Obviously, Pd0.38/COF-SO3H possesses the
best performance with phenylacetylene conversion and styrene
selectivity compared to Pd0.69/COF-SO3H and Pd1.66/COF-
SO3H under mild reaction conditions (1 bar of H2, 25 °C, 28
min), which should be attributed to the smallest Pd
nanoparticles. Moreover, the activity of Pd-free COF-SO3H
was tested under the same reaction conditions, which only
exhibits 2.4% conversion even after a reaction of 1440 min,
demonstrating that Pd nanoparticles are the key active sites
(Table 1). In addition, from Table 2, by a comparison with the
reported semihydrogenation experiments of alkynes in the
literature, it is clearly suggested that our catalyst presents a
highly rare example, showing both high selectivity and
conversion for semihydrogenation just based on a single Pd
metal with ultralow content. Correspondingly, the ultrahigh
catalytic efficiency evaluated by the turnover frequency (TOF)
value is up to 3888 h−1, which outperforms most of reported
catalysts for such use.
Figure 3. PXRD patterns of the as-synthesized and simulated COF-
SO3H.
ketoenamine-linked structure in COF-SO3H is further
proven.34 What is more, the 77 K N2 adsorption−desorption
tests (Figure S4) indicate that the Brunauer−Emmett−Teller
(BET) surface area of COF-SO3H is 306 m2/g. Subsequently,
a series of Pd/COF-SO3H structures with different Pd
contents were prepared by a wet-chemistry immersion
dispersion approach. The metal precursor was deposited in
the COF-SO3H material by mixing a Pd(NO3)2·xH2O solution
with a suspension of COF-SO3H and stirring for 2 h. The
mixture was washed by water and methyl alcohol and then
reduced with NaBH4 in methanol under a N2 atmosphere to
obtain Pd/COF-SO3H. Finally, the mixture was thoroughly
washed with water and methyl alcohol and dried before
characterization and further use. Inductively coupled plasma
analysis demonstrates that the Pd contents in these samples are
0.38, 0.69, and 1.66 wt %, respectively, well consistent with the
results obtained by SEM−EDS (Table S2).
In order to understand the morphology, chemical
composition, and porosities of Pd/COF-SO3H, a series of
characterizations were carried out. From the XRD patterns
(Figure 4) of COF-SO3H and Pd/COF-SO3H, it can be found
that the structure of COF-SO3H was maintained after loading
On the basis of the catalytic and characterization results
presented above, the enhanced performance for Pd0.38/COF-
SO3H is believed to be related to the following factors. First,
the highly dispersed Pd nanoparticles in Pd0.38/COF-SO3H
can provide more active sites. Second, the strong interactions
Figure 4. PXRD patterns of COF-SO3H and Pd/COF-SO3H with
different Pd loadings.
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Figure 5. SEM images of (a) Pd0.38/COF-SO3H, (b) Pd0.69/COF-SO3H, and (c) Pd1.66/COF-SO3H. (d) SEM−EDS mapping of Pd0.38/COF-
SO3H.
Table 1. Activity for the Selective Hydrogenation of
a
Phenylacetylene Using Different Catalysts
catalyst
t (min) conversion (%) selectivity to styrene (%)
COF-SO3H
1440
360
28
28
28
2.424
99.78
97.06
15.69
29.99
100
8.5
93.15
92.89
93.64
Pd2+/COF-SO3H
Pd0.38/COF-SO3H
Pd0.69/COF-SO3H
Pd1.66/COF-SO3H
a
Reaction conditions: phenylacetylene, 1 mmol; ethanol, 15 mL; H2,
1 bar, 25 °C; catalysts, 15 mg.
Figure 6. TEM images of Pd0.38/COF-SO3H.
As is well-known, the stability and reusability of catalysts are
the key parameters to determine the prospect of industrial
application. The repeated experiment is carried out as follows.
The recycled Pd0.38/COF-SO3H catalyst is obtained by
centrifugation and dried after the end of the previous reaction.
As presented in Figure 8, the conversion of phenylacetylene
and the selectivity to styrene exhibit negligible loss even after
five successive runs. Further confirmation on the stability is
demonstrated by XRD and XPS tests (Figures S14 and S15).
Moreover, the BET measurements of Pd/COF-SO3H after the
first and fifth cycles were performed. Negligible change of the
surface area and pore-size distribution was found (Figures S16
and S17). Also, the morphology was not destroyed after the
fifth cycles (Figure S18). The above results demonstrate the
high stability of Pd0.38/COF-SO3H.
between the Pd nanoparticles and COF-SO3H support can
strengthen the intrinsic activity of a single site, as demonstrated
by the high TOF value in Pd0.38/COF-SO3H.46
To further demonstrate the general availability of Pd0.38
/
COF-SO3H for semihydrogenation reactions, a series of alkyne
substrates were employed as the feedstock. As shown in Table
3, Pd0.38/COF-SO3H also presents an excellent catalytic
performance under mild reaction conditions. The alkyne
conversion and selectivity to the the target olefin are more than
92%, suggesting its superior application in the field of
1
heterogeneous hydrogenation. As demonstrated by H NMR
(Figures S12 and S13), the main products for the selective
hydrogenation of 2-pentyne and 4-octyne are (Z)-2-pentene
and (Z)-4-octene, respectively.
Figure 7. Fine Pd 3d XPS spectra of Pd/COF-SO3H with different Pd contents.
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Table 2. Comparison with Other Reported Catalysts for the Hydrogenation of Alkynes
content (%)
catalyst
bar of H2
T (°C)
t (min)
C
S
TOF (h−1
)
ref
Pd0.38/COF-SO3H
Pd@mpg-C3N4
C−Pd@C
PdCo@AC
Pd@Zn-MOF-74
Pd/SiC
Pd/ZnO@C
Pd9Au1/ZnTi
Pd/pph3@FDU-12
Pd/CeO2
PdC0.18
Pd/boehmite/Al2O3
1
1
1
1
1
1
1
1
1
1
1
1
25
30
30
30
40
70
30
45
30
60
30
60
28
85
30
97.06
99
62
93
100
90
96
100
92
60
99
93.1
97
95
94
90
90
99
90
94
3888
72
1128
224
2284
1000
733
2284
560
this work
9
19
20
23
39
40
41
42
43
44
45
75
720
2880
60
460
480
68
99.2
90
2639
7896
11.5
80
90
a
Table 3. Activity for the Selective Hydrogenation of Alkynes Using the Pd0.38/COF-SO3H Catalyst
a
Reaction conditions: alkynes, 1 mmol; ethanol, 15 mL; H2, 1 bar, 25 °C; catalyst, 15 mg.
CONCLUSIONS
■
In conclusion, COF-SO3H was strategically fabricated based
on a cheap raw material, and then ultralow-content Pd was
loaded onto it to obtain Pd/COF-SO3H. The resulting Pd0.38
/
COF-SO3H with a Pd loading of only 0.38 wt % exhibited high
crystallinity and thermal stability up to 350 °C and was
employed for the selective hydrogenation of a series of alkynes
to produce olefins. As expected, different alkynes could be
totally converted to the desired olefins with almost 95%
selectivity in Pd0.38/COF-SO3H. Impressively, the TOF value
for the semihydrogenation of phenylacetylene to produce
styrene can reach as high as 3888 h−1, which outperforms most
of the reported excellent catalysts. A series of characterizations
and experimental results revealed that the excellent catalytic
performance in Pd0.38/COF-SO3H is mainly ascribed to small
Pd nanoparticles with an average size of 2.2 nm, which can
provide more accessible active sites and enhance the metal−
support interaction. Moreover, the as-fabricated Pd0.38/COF-
Figure 8. Recyclability of the Pd0.38/COF-SO3H catalyst: conversion,
black; selectivity, red.
F
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01117.
■
S
Figures S1−S11 and Tables S1 and S2 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: ecitluofeng@163.com (F.L.).
ORCID
■
Feng Luo: 0000-0001-6380-2754
Author Contributions
⊥These authors are the cofirst authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation of China (Grants
21871047, 21661001, and U1832148), the Natural Science
Foundation of Jiangxi Province of China (Grant
20181ACB20003), and the Technology Program of Education
Department of Jiangxi Province of China (Grant GJJ1505).
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