Palladium Nanoparticles Immobilized on Magnetic Porous Carbon Derived from ZIF-67 as Efficient Catalysts for the Semihydrogenation of Phenylacetylene under Extremely Mild Conditions

Article abstract of DOI:10.1002/cctc.201501283

A new magnetic porous carbon (MDPC) derived from metal-organic frameworks (MOFs) was synthesized successfully through the direct carbonization of ZIF-67 crystals and utilized as a support for Pd nanocatalysts. The prepared MDPC not only retains the original morphology of ZIF-67 crystals but also provides a high surface area and excellent magnetic properties. The designed catalysts (Pd-MDPC) were tested in the semihydrogenation of phenylacetylene under extremely mild conditions. The Pd nanoparticles are highly dispersed on the MDPC matrix without aggregation and exhibited an excellent catalytic activity and high selectivity toward olefins under mild conditions (25 °C, H₂ 1 atm). The Pd-MDPC catalysts were further investigated in the semihydrogenation of several other terminal alkynes. Compared with most traditional carbon materials, such as activated carbon and carbon nanotubes, the MDPC possesses an admirable magnetism so that it could be separated magnetically from the reaction system. Our study indicates that MOF-derived porous carbon is a suitable support for noble-metal nanocatalysts and has promise in the field of nanocatalysis.

Full text of DOI:10.1002/cctc.201501283

DOI: 10.1002/cctc.201501283
Full Papers
Palladium Nanoparticles Immobilized on Magnetic Porous
Carbon Derived from ZIF-67 as Efficient Catalysts for the
Semihydrogenation of Phenylacetylene under Extremely
Mild Conditions
Xinlin Li, Wei Zhang, Yansheng Liu, and Rong Li*[a]
A new magnetic porous carbon (MDPC) derived from metal–
organic frameworks (MOFs) was synthesized successfully
through the direct carbonization of ZIF-67 crystals and utilized
as a support for Pd nanocatalysts. The prepared MDPC not
only retains the original morphology of ZIF-67 crystals but also
provides a high surface area and excellent magnetic proper-
ties. The designed catalysts (Pd-MDPC) were tested in the sem-
ihydrogenation of phenylacetylene under extremely mild con-
ditions. The Pd nanoparticles are highly dispersed on the
MDPC matrix without aggregation and exhibited an excellent
catalytic activity and high selectivity toward olefins under mild
conditions (258C, H2 1 atm). The Pd-MDPC catalysts were fur-
ther investigated in the semihydrogenation of several other
terminal alkynes. Compared with most traditional carbon mate-
rials, such as activated carbon and carbon nanotubes, the
MDPC possesses an admirable magnetism so that it could be
separated magnetically from the reaction system. Our study in-
dicates that MOF-derived porous carbon is a suitable support
for noble-metal nanocatalysts and has promise in the field of
nanocatalysis.
Introduction
In the polymer industry, phenylacetylene in the styrene feed-
stock poisons the polymerization catalysts, thus it should be
removed to levels below 10 ppm. The semihydrogenation of
phenylacetylene is considered as an efficient method that not
only eliminates catalyst poisons, but also increases the amount
of feedstock.[1] However, the selective hydrogenation of alkynes
to olefins is a challenging process as olefins are overhydrogen-
ated to alkanes easily. Therefore, in this reaction, the choice of
an efficient active component with high activity and selectivity
is significant. Pd-based catalysts are highly efficient in the par-
tial hydrogenation of triply bound compounds that combine
high catalytic activity and selectivity.[1,2]
been put into this reaction,[8] there is still room for improve-
ment under extremely mild conditions (298 K, H2 1 atm).
However, although Pd-NP-based catalysts have attractive se-
lective hydrogenation behavior, some limitations still exist that
prevent their wide use. First, noble-metal NPs tend to aggre-
gate during the catalytic process because of their high surface
energy.[9] Second, traditional separation methods such as filtra-
tion are not convenient for the recycling of catalysts. There-
fore, supports with good performance ought to be found to
disperse and stabilize Pd NPs, thus their activities could be
maintained during the reaction process. Furthermore, a ferro-
magnetic component needs to be introduced into the support
to enable the catalyst to be recycled more easily.
New nanomaterials such as mesoporous silica,[10] nanopo-
rous carbon,[11] graphene,[12] and metal–organic frameworks
(MOFs)[13] provide promising supports to disperse and stabilize
Pd NPs. As a result of their excellent physicochemical stability,
large surface area, affinity to organics, and low density, carbon
materials are used widely to support metal NPs in heterogene-
ous catalysts. Besides traditional activated carbon and carbon
nanotubes, new carbon materials synthesized by nanocasting,
which include hard and soft templating methods,[14] have been
studied in recent years. Despite their excellent properties, such
as large surface area, ordered morphology, and pore structures,
their shortcomings are clear: the preparation and elimination
of templates are time- and ingredient-consuming steps. Nowa-
days, MOFs are promising candidates as precursors of porous
carbon materials because of their highly ordered porous struc-
tures with abundant organic struts, high surface areas, and
large pore volumes. The direct carbonization of MOFs is
Many kinds of heterogeneous Pd-based catalysts have been
designed and utilized in the hydrogenation of phenylacety-
lene. For example, Pd nanoparticles (NPs) supported on func-
tionalized polymers[3] and polymer-based stereocomplexes,[4]
[7]
MCM-41,[5] carbon materials,[6] and a-Al2O3 have been used in
this reaction. Good conversions and selectivities were obtained
in most of these studies, however, the hydrogenation of phe-
nylacetylene at room temperature and atmospheric pressure is
reported rarely. Therefore, although numerous efforts have
[a] X. Li, W. Zhang, Y. Liu, Prof. R. Li
College of Chemistry and Chemical Engineering
Gansu Provincial Engineering Laboratory for Chemical Catalysis
Lanzhou University
No.222, Tianshui south road, Lanzhou (P.R. China)
E-mail: liyirong@lzu.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201501283.
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a simple and efficient approach to prepare porous carbons
without the elimination of templates.[15] The uniform morphol-
ogy and pore structure, and high surface area and pore
volume can be inherited from the MOFs. Moreover, to vary the
metal ions and ligands in the MOF structure, functional metal
particles such as ferromagnetic metal particles can be intro-
duced into the carbon matrix for magnetic separation. There-
fore, MOF-derived porous carbons are good candidates for
supports of Pd NPs.
In this study, we developed a new kind of MOF-derived
porous carbon (MDPC), which was utilized as a catalyst support
for Pd NPs. To the best of our knowledge, this is the first
report of carbon materials prepared from ZIF-67 utilized to
support noble-metal NPs in a hydrogenation reaction. A series
of catalysts with different Pd contents was tested and the best
immobilized content was screened. The catalysts with different
contents of Pd are denoted as Pd-MDPC(0.86), (2.39), and
(3.97). The new Pd-based catalysts exhibit a high catalytic ac-
tivity and selectivity toward the semihydrogenation of phenyl-
acetylene under mild conditions (298 K, p(H2)=1 atm). The
values of the total conversion and final percentage of styrene
are pretty high compared with those reported previously
under the same conditions. Furthermore, the designed cata-
lysts were studied in the hydrogenation of other terminal al-
kynes, such as 1-octyne and 4-pentyn-1-ol. The catalysts could
be easily separated magnetically from the liquid phase and re-
cycled several times without clear deactivation.
Figure 1. TEM images of a) ZIF-67, b) MDPC, c) Pd-MDPC(0.86), d) Pd-
MDPC(2.39), and e) Pd-MDPC(3.97), f) HRTEM image of Pd NPs crystals in Pd-
MDPC(2.39), and g) HAADF-STEM image of Pd-MDPC(2.39), C mapping
image (C-K), N mapping image (N-K), Co mapping image (Co-K), and Pd
mapping image (Pd-L).
ure 1 f. The particle size is around 6 nm, and the lattice fringes
are clearly visible, which demonstrates the existence of crystal-
line, zero-valent Pd NPs. Furthermore, the spacings of the lat-
tice fringes are measured as 0.23 nm, which correspond to the
lattice spacings of the (111) planes of Pd NPs. The high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) image gives a specific element distribution of
the Pd-MDPC(2.39) catalyst (Figure 1 g). C, N, and Co are ob-
served clearly and are well dispersed in the catalysts. The intro-
duced Pd NPs are dispersed uniformly in the carbon matrix,
which indicates the successful immobilization of Pd NPs.
XRD and X-ray photoelectron spectroscopy (XPS) provide
evidence for the composition and chemical state of the ele-
ments of the prepared catalysts. The spectra are displayed in
Figures 2 and 3, and Figure S4. The powder XRD pattern of
MDPC is shown in Figure 2. Three characteristic peaks located
at 2q=44.3, 51.6, and 75.98 are observed, which correspond
to the (111), (200), and (220) lattice planes of Co NPs, respec-
tively. Peaks located at around 2q=258 correspond to the
characteristic peaks of carbon. However, compared to the pat-
tern of MDPC, no clear characteristic peaks of Pd are observed
in the patterns of Pd-MDPC(2.39), (0.86), and (3.97). This is as-
cribed to the low content and high dispersion of the superfine
Pd NPs.[16] The XPS wide-scan spectrum of Pd-MDPC(2.39) is
displayed in Figure 3a. Peaks that correspond to C, N, Pd, and
Co are observed, which indicates the composition of the Pd-
MDPC catalysts.
Results and Discussion
Characterization of Pd-MDPC catalysts
The synthesis of the Pd-MDPC catalysts is displayed in
Scheme 1. The uniform morphology of ZIF-67 crystals is main-
tained after carbonization. After carbonization, the rhombic do-
decahedral shape and size of ZIF-67 are almost maintained
(Figure 1a and b). Moreover, the Co particles transformed from
Co2+ are dispersed uniformly and wrapped in the carbon ma-
trix.[15e] The average size of the ZIF-67 and MDPC particles is
around 2 mm. After the reduction of Pd2+ by H2, Pd NPs are in-
troduced onto the support. The structures of the series of pre-
pared Pd-MDPC catalysts are shown in Figure 1c–e. Compared
with that shown in Figure 1b, clear black dots are observed in
the support, which indicates the uniformly dispersed Pd NPs
without aggregation. A high-resolution transmission electron
microscopy (HRTEM) image of the Pd NPs is displayed in Fig-
Furthermore, the detailed XPS spectrum of the Pd level is
displayed in Figure 3b. Peaks observed at binding energies
(BEs) of 335.7 and 341.1 eV correspond to the 3d5/2 and 3d3/2
levels of metallic Pd0.[17] Thus the Pd in the catalyst exists
mainly as Pd0. The characteristic peaks of PdO located at BE=
Scheme 1. Fabrication of the Pd-MDPC catalysts.
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337.3 and 342.4 eV demonstrates that part of Pd NPs were oxi-
dized in air.[18] Energy-dispersive X-ray spectroscopy (EDS) was
performed to determine the chemical composition of the Pd-
MDPC catalysts. The EDS results of Pd-MDPC(2.39) are shown
in Figure S2. Clear peaks that correspond to C, N, Pd, and Co
reveal the composition of the Pd-MDPC catalysts. The propor-
tions of the elements in the Pd-MDPC catalysts are listed in
Table 1. Almost 50 wt% of C and 42 wt% of Co exist in the cat-
alysts. There is around 5 wt% of N in the catalysts, which is de-
rived from 2-methylimidazol (MeIm).
Table 1. BET surface area, total pore volume, and relative weight ratios of
elements in Pd-MDPC catalysts.
BET surface
area [m2 gÀ1
Pore volume
[cmÀ3 gÀ1
]
Relative weight
ratios [%]
]
C
N
Co
MDPC
224
207
159
167
0.22
0.22
0.22
0.24
51.30
50.91
47.51
48.68
5.78
5.71
5.40
5.09
42.92
42.51
44.70
41.26
Pd-MDPC(0.86)
Pd-MDPC(2.39)
Pd-MDPC(3.97)
Figure 2. Powder XRD patterns of MDPC and Pd-MDPC(2.39).
To explore the porosity of the Pd-MDPC catalysts, N2 physi-
sorption measurements were performed. The BET surface areas
and total pore volumes are summarized in Table 1. The BET sur-
face area of MDPC is calculated to be 224 m2 gÀ1. After the im-
mobilization of Pd, the BET surface areas decreased to 207,
159, and 167 m2 gÀ1 in Pd-MDPC(0.86), Pd-MDPC(2.39), and Pd-
MDPC(3.97), respectively. The N2 adsorption–desorption iso-
therms of MDPC and Pd-MDPC(2.39) are displayed in Figure 4.
The N2 adsorption–desorption isotherms of the two samples
could be classified as type IV, which indicates the mesoporous
structures of the two samples. The mesopore size distribution
curves (inset in Figure 4) give detailed information on the mes-
opores in the two samples. The mesopores are well ordered
Figure 3. a) XPS wide-scan spectrum of Pd-MDPC(2.39) and b) detailed XPS
Figure 4. N2 adsorption–desorption isotherms and mesopore size distribu-
spectrum of Pd in Pd-MDPC(2.39).
tion curves (inner graph) of MDPC and Pd-MDPC(2.39).
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and their width is around 4 nm. Compared with Pd-
MDPC(2.39), Pd-MDPC(3.97) possesses a larger surface area and
total pore volume. We infer that in Pd-MDPC(3.97) some of the
Pd NPs are immobilized on the outer surface of the supports,
which results in an increase of the surface areas and total pore
volumes. Furthermore, the mesopore size distribution curves
of Pd-MDPC(3.97) (Figure S1) also provide evidence for our de-
duction. The appearance of pores at 20 nm indicates the exis-
tence of Pd NPs on the surface of MDPC, which generates new
pores.
Table 2. Selectivity toward styrene and substrate conversion with the Pd-
MDPC catalysts in different solvents.[a]
Entry Pd
[wt%]
Solvent
t
Conversion Selectivity Styrene yield
[min] [%]
[%]
[%]
1
2
3
4
5
6
7
8
9
0
EtOH
MeOH
MeOH
MeOH
EtOH
EtOH
EtOH
iPrOH
180
20
70
180
60
180
180
240
2
94
98
4
98
69
2
100
72
88
100
83
90
100
80
62
2
68
86
4
81
62
2
3.97
2.39
0.86
3.97
2.39
0.86
3.97
3.97
As a result of the existence of the ferromagnetic Co compo-
nent in the as-synthesized catalysts, it could be recovered con-
veniently by an external magnetic force. The superparamag-
netic properties were demonstrated by using vibrating sample
magnetometry (VSM). The magnetic saturation value of MDPC
is 52.5 emugÀ1. If Pd NPs are loaded, a slight decline of the
magnetic saturation is observed. The values are reduced to
52.1, 51.7, and 49.7 emugÀ1, respectively, for Pd-MDPC(0.86),
Pd-MDPC(2.39), and Pd-MDPC(3.97). Nevertheless, a slight re-
duction of the magnetic saturation value does not impede
magnetic separation. The excellent magnetic properties of the
Pd-MDPC catalysts are shown in Figure 5 (inset).
95
97
76
60
toluene 180
[a] Substrate: 1 mmol; catalyst: 10 mg; solvent: 10 mL; 298 K; p(H2)=
1 atm.
curred, which indicates that the supports were not active.
Next, Pd-MDPC(3.97) was used to screen the solvent. The best
conversion and selectivity were 98% and 83%, respectively, in
60 min in the ethanol system. In toluene and isopropanol, Pd-
MDPC(3.97) does not exhibit a high activity and selectivity.
However, in the methanol system, even though the selectivity
does not reach that of the ethanol system, the results should
not be ignored. In the methanol system, the substrate is
almost completely converted in 25 min, although the selectivi-
ty is relatively low, which implies an excessive catalytic activity
of Pd-MDPC(3.97) in the methanol system. Therefore, catalysts
with a lower Pd content were synthesized and tested with the
aim to obtain a higher selectivity and lower cost.
As mentioned above, two catalysts with a lower Pd content,
Pd-MDPC(2.39) and Pd-MDPC(0.86), were tested in both the
methanol and ethanol systems to screen the best combination,
and the results are displayed in Table 2 (entries 3, 4, 6, and 7).
As observed in the methanol system, with a decrease of the
Pd content from 3.97 to 2.39%, the selectivity toward styrene
improves from 72 to 88%, and a conversion of 98% was ach-
ieved in 70 min. The lower selectivity of Pd-MDPC(3.97) is
probably because of the consecutive hydrogenation of styrene
molecules on the excess free active sites that are not occupied
by the alkynes. The improvement of selectivity demonstrates
the more appropriate catalytic activity of Pd-MDPC(2.39) than
Pd-MDPC(3.97). Nevertheless, in the ethanol system, with the
decline of the Pd content from 3.97 to 2.39%, only 69% con-
version is obtained in 180 min, which demonstrates that Pd-
MDPC(2.39) does not exhibit a high enough activity in ethanol.
The different catalytic activities of Pd-MDPC exhibited in meth-
anol and ethanol could be interpreted as the changes of the
adsorption behavior of the alkyne on the surface of Pd NPs,
which are caused by the solvent polarity-induced modifica-
tion.[19,21] We infer that compared with ethanol, the higher po-
larity of methanol results in a faster adsorption of phenylacety-
lene on the surface of the Pd NPs, which leads to a higher hy-
drogenation rate. A catalyst with a lower Pd content denoted
as Pd-MDPC(0.86) was tested next. No clear conversion is ob-
served in either methanol or ethanol, which suggests that Pd-
MDPC(0.86) does not exhibit an adequate activity for the reac-
Figure 5. Room-temperature magnetization curves of MDPC, Pd-MDPC(0.86),
Pd-MDPC(2.39), and Pd-MDPC(3.97).
Semihydrogenation of phenylacetylene
The series of catalysts with different Pd contents was used for
the semihydrogenation of phenylacetylene under mild condi-
tions of 298 K and p(H2)=1 atm. On account of its high elec-
tron density and restricted rotation, a triple bond exhibits
a stronger adsorption on active centers than a double bond,
which is the cause of the selectivity of Pd.[5,19]
In the hydrogenation of phenylacetylene, the most efficient
solvent was screened for Pd-MDPC(3.97), and the results are
displayed in Table 2 (entries 2, 5, 8, and 9). The selectivity to
styrene and the total conversion were recorded under the
maximum concentration of styrene.[20] First, the supports with-
out Pd NPs were tested (entry 1), and no clear reaction oc-
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tion because of the low Pd content. According to the above
elaboration, the system composed of Pd-MDPC(2.39) with
methanol, which exhibits a high activity and selectivity, is the
most efficient reaction system in our study.
To study the evolution of selectivity throughout the reaction
process, the selectivity versus conversion under the most effi-
cient conditions was plotted (Figure 6). In the Pd-MDPC(2.39)/
methanol system, a clear decline of the selectivity can be ob-
Figure 7. Time evolution curves of the conversion of phenylacetylene with
Pd-MDPC(2.39) and Pd-MDPC(3.97) in methanol.
Semihydrogenation of other terminal alkynes
The catalysts with different Pd contents were further used in
the semihydrogenation of other terminal alkynes in methanol.
The results for the partial hydrogenation of 1-octyne are dis-
played in Table 3. The conversion and selectivity reach 89%
Table 3. Semihydrogenation of 1-octyne with the Pd-MDPC catalysts.[a]
Entry
Pd
[wt%]
t
Conversion
[%]
Selectivity
[%]
1-Octene yield
[%]
Figure 6. Selectivity versus conversion curve of Pd-MDPC(2.39) in methanol.
[min]
1
2
3
3.97
2.39
0.86
5
6
20
90
89
28
54
70
89
49
62
25
served as the reaction progress. The selectivity decreased from
95 to 88% as the conversion increased from 27 to 98%. This
decrease can be interpreted as follows: in this reaction, the
surface of the active sites is occupied initially by the alkynes
that could be regarded as surface poisons. As the reaction pro-
ceeds, the adsorption of phenylacetylene on the surface of the
active sites decreases thus some surface sites become avail-
able. Therefore, more styrene molecules are further hydrogen-
ated on these free surfaces to result in a decreased selectivi-
ty.[22] The conversion versus selectivity curves of Pd-MDPC(3.97)
in different solvents are displayed in Figure S3. The decrease of
selectivity is similar in all kinds of solvents. Compared with that
of Pd-MDPC(2.39), a much faster selectivity decrease is ob-
tained in the Pd-MDPC(3.97)/methanol system. This result
proves that the Pd-MDPC(2.39)/methanol system is more effi-
cient for another reason.
[a] Substrate: 1 mmol; catalyst: 10 mg; MeOH: 10 mL; 298 K; p(H2)=
1 atm.
and 70%, respectively, with Pd-MDPC(2.39) in 6 min, which is
the best result among the three samples. Compared with phe-
nylacetylene, 1-octyne exhibits a higher reactivity under the
same conditions as much less time is consumed to achieve
a high conversion for all three catalysts. The higher reactivity
of 1-octyne in the semihydrogenation reaction is in accordance
with results reported previously.[24] This may be because 1-
octyne is more easily and quickly adsorbed on the surface of
Pd than phenylacetylene, which may be attributed to the non-
conjugated structure and the rotation of the molecule.[20] Nev-
ertheless, the selectivity toward olefin is clearly decreased com-
pared with that of phenylacetylene in the same system. The
optimum yield of 1-octene is 57% if Pd-MDPC(2.39) is utilized,
and 43 and 25% yields of 1-octene are obtained with Pd-
MDPC(3.97) and Pd-MDPC(0.86), respectively. The decline of se-
lectivity compared with the partial hydrogenation of phenyla-
cetylene demonstrates a weaker trend of the desorption of 1-
octyne from the Pd surface to result in overhydrogenation. Pd-
MDPC(0.89) still exhibits a low catalytic activity in the above re-
action (Table 3). Similar to phenylacetylene, it follows that Pd-
MDPC(2.39) is a more efficient catalyst for the partial hydroge-
nation of 1-octyne.
Furthermore, the time evolution curves that correspond to
the conversion of phenylacetylene using Pd-MDPC(3.97) and
Pd-MDPC(2.39) in the methanol system were plotted (Figure 7).
An apparently linear trend for the conversion versus time in
the Pd-MDPC(3.97)/methanol system is observed, which indi-
cates a zeroth-order reaction and has been demonstrated in
previous studies.[6a,b,d,f,23] However, in the Pd-MDPC(2.39)/meth-
anol system, the initial linear trend was not maintained
throughout the reaction,[5] which could be explained by the
competitive adsorption between phenylacetylene and styrene
on the surfaces of Pd in the later stage of the reaction.
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The selective hydrogenation of acetylenic alcohols to allyic
alcohols is a challenging process because of the possible hy-
drogenolysis of the hydroxyl group before the desorption of
the allyic alcohols from the active sites.[19] However, no hydro-
genolysis was observed in our work (Table 4), which indicates
a stable level through the four cycles, which was around 88%.
Inductively coupled plasma (ICP) analysis of Pd-MDPC(2.39)
after four reaction cycles was performed to determine whether
the lixiviation of Pd existed. The result shows that 2.13 wt% of
Pd remained after four cycles. The slight lixiviation of Pd is
probably the main reason for the decrease of the conversion.
Table 4. Semihydrogenation of 4-pentyn-1-ol with the Pd-MDPC
catalysts.[a]
Conclusions
In this study, different contents of Pd nanoparticles were im-
mobilized successfully onto a new magnetic carbon material
(MDPC). The series of catalysts synthesized was utilized in the
semihydrogenation of phenylacetylene under mild conditions
(258C, 1 atm of H2). Methanol is a more efficient solvent for
this reaction than ethanol, toluene, and isopropanol. In the
methanol system, catalysts with a Pd content of 3.97, 2.39, and
0.86% were tested, and the best metal loading was screened.
Pd-MDPC(2.39) was the most efficient catalyst, which exhibits
a high catalytic activity and selectivity.
Entry
Pd
t
Conversion Selectivity 4-Penten-1-ol yield
[wt%] [min] [%]
[%]
[%]
1
2
3
3.97
2.39
0.86
5
5
97
94
4
81
87
100
79
82
4
60
[a] Substrate: 1 mmol; catalyst: 10 mg; MeOH: 10 mL; 298 K; p(H2)=
1 atm.
that the semihydrogenated product desorbs from the surface
of Pd NPs before hydrogenolysis occurs,[5] and a rapid desorp-
tion might be the reason for the improvement of selectivity
compared to that of 1-octyne.The conversion of substrate can
reach 94% in 5 min over Pd-MDPC(2.39), and the selectivity of
Pd-MDPC(2.39) is higher than that of Pd-MDPC(3.97) to some
extent (87 vs. 81%). Based on this, we conclude that 2.39% is
a more efficient content for the active component. This phe-
nomenon is in accordance with that of phenylacetylene, which
could also be explained by the adsorption and consecutive hy-
drogenation of the semihydrogenated product by the exces-
sive active sites. Moreover, in this reaction, Pd-MDPC(0.86)
does not exhibit a sufficient catalytic activity.
The variation of selectivity was investigated in this study.
Curves of selectivity versus conversion were plotted and
a clear downtrend was observed with the progress of the reac-
tion. This is because of the release of some surfaces from the
occupation of pheneacetylene as the reaction progressed. As
more surfaces are released, more styrene molecules are ad-
sorbed on these free surfaces and thus overhydrogenated,
which results in the decline of the selectivity.
The series of catalysts was further investigated in the semi-
hydrogenation of other terminal alkynes, such as 1-octyne and
4-pentyn-1-ol. 1-Octyne exhibits a much higher reactivity than
phenylacetylene in the same reaction system, and an almost
complete conversion was observed in 5 min, however, the se-
lectivity was lower. This could be explained by the nonconju-
gated structure and the rotation of the molecule, which result-
ed in a faster adsorption on the surfaces of the Pd nanoparti-
cles. In the hydrogenation of 4-pentyn-1-ol, the hydrolysis of
the hydroxyl group is not observed, which indicates the rapid
desorption of the semihydrogenated product from the surfaces
of the active sites before hydrolysis, and the timely desorption
leads to less excessive hydrogenation, which resulted in a high
selectivity. In the recycling test, no apparent deactivation oc-
curred after at least four cycles.
The results of the recycling of Pd-MDPC are displayed in
Figure 8. The semihydrogenation of phenylacetylene in metha-
nol was selected to test the reusability of the most efficient
catalyst, Pd-MDPC(2.39). After the reaction, the catalyst was
separated magnetically from the mixture, washed with metha-
nol, and dried thoroughly before the next cycle. No clear de-
crease in the conversion and selectivity is observed after four
cycles, which indicates the good recycling performance of Pd-
MDPC(2.39) (Figure 8). A slight decrease of conversion occurred
from 98 to 89% after four cycles. The selectivity reaches
This study proves the high catalytic activities of the Pd-
MDPC catalysts in the semihydrogenation of phenylacetylene
and shows the potential of metal–organic frameworks to act
as precursors of carbon materials with good performance.
Experimental Section
Materials
Cobalt chloride(CoCl2·6H2O), 1-octyne, and 2-methylimidazole were
purchased from Aladdin Chemical Co. Ltd. Palladium acetate and
phenylacetylene were purchased from Sigma Aldrich and used as
received. 4-Pentyn-1-ol was obtained from Alfa Aesar Chemical Co.
Methanol, ethanol, toluene, isopropanol, and acetonitrile were pur-
chased from Tianjin Rionlon reagent Co. All chemicals were used
without further purification.
Figure 8. Reusability of Pd-MDPC(2.39) for the semihydrogenation of phenyl-
acetylene in methanol.
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1 mmol substrate, 10 mL solvent, and 10 mg catalyst was added.
Typically, appropriate amount of catalysts, substrate, and solvent
were added in order to a round-bottomed flask. The materials
were dispersed completely under ultrasound conditions and then
evacuated. After evacuation, H2 was introduced into the flask, and
the reaction was started immediately. During the reaction, samples
were collected and filtered to be analyzed by GC–MS later.
Synthesis of MDPC
MDPC was obtained by the carbonization of ZIF-67 crystals. ZIF-67
crystals were synthesized according to the following steps:
CoCl2·6H2O (1.173 g) was dissolved in methanol (100 mL) to gener-
ate a clear solution. 2-Methylimidazole (3.241 g) was also dissolved
in methanol (100 mL). Then the two solutions were mixed together
under vigorous stirring. After complete mixing, the stirring was
stopped, and the mixture was aged at RT for 24 h. The precipitated
purple powder was obtained by centrifugation. The powder was
washed with methanol several times and dried completely under
vacuum. The prepared ZIF-67 crystals were carbonized at 7008C
for 2 h under a N2 flow to generate MDPC.
Acknowledgements
This work was supported by the Key projects in the National Sci-
ence and Technology Pillar Program of China (2014 4BAK16B01).
Preparation of Pd-MDPC catalysts
Keywords: alkynes · carbon · hydrogenation · metal–organic
frameworks · palladium
A series of Pd-MDPC catalysts with different amounts of Pd was
prepared by the wetness impregnation method: first, as-prepared
MDPC (500 mg) was immersed in a calculated volume of palladium
acetate solution in acetonitrile (2 mgmLÀ1) and completely dis-
persed in the liquid phase under ultrasonic conditions. The mixture
was stirred continuously for 24 h. The MDPC that had adsorbed
Pd2+ was separated magnetically from the liquid phase and dried
completely under mild conditions. The dried MDPC was heated to
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Published online on February 17, 2016
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