PVP-Pd@ZIF-8 as highly efficient and stable catalysts for selective hydrogenation of 1,4-butynediol

Article abstract of DOI:10.1039/c3cy00873h

Cubic Pd nanoparticles were rapidly encapsulated in ZIF-8 through a PVP-assisted synthetic method at room temperature. The obtained PVP-Pd@ZIF-8 exhibited high activity and stability for hydrogenation of 1,4-butynediol, with excellent selectivity to 1,4-b

Full text of DOI:10.1039/c3cy00873h

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PVP–Pd@ZIF-8 as highly efficient and stable
catalysts for selective hydrogenation of
,4-butynediol†
Cite this: Catal. Sci. Technol., 2014,
, 329
1
4
Received 1st November 2013,
Accepted 22nd November 2013
Mingming Zhang, Yuebin Yang, Chuang Li, Qian Liu, Christopher T. Williams
and Changhai Liang*
DOI: 10.1039/c3cy00873h
www.rsc.org/catalysis
Cubic Pd nanoparticles were rapidly encapsulated in ZIF-8 through a
PVP-assisted synthetic method at room temperature. The obtained
PVP–Pd@ZIF-8 exhibited high activity and stability for hydrogenation
of 1,4-butynediol, with excellent selectivity to 1,4-butenediol.
Our investigation aims at the elucidation of the main
characteristics of ZIF-8 as a support for polyvinyl-pyrrolidone
(PVP) protected Pd catalysts for selective hydrogenation of
BYD. The PVP–Pd@ZIF-8 catalysts were synthesized through a
1
4
PVP-assisted process described by Lu et al., except for using
water as the solvent instead of methanol and a much shorter
time. The strong adsorption between the PVP and ZIF-8 frame-
works resulted in the insertion of PVP–Pd into the lattice of
ZIF-8. The loading of Pd can be tuned by adding different
amounts of PVP–Pd colloid during the formation of ZIF-8.
XRD patterns (Fig. S1†) showed that the rapidly produced
ZIF-8 and PVP–Pd@ZIF-8 materials have very high crystallinity.
The diffraction peak of metallic palladium at 40° cannot be
detected until the Pd loading is up to 5 wt%, indicating a high
dispersion of Pd, which can be further confirmed from TEM
images as shown in Fig. 1. The TEM images show that the orig-
inal cubic Pd nano-sol has almost no change after being encapsu-
lated in ZIF-8 crystals. The nanocubic Pd particles with the size
of 4–6 nm are randomly dispersed without any aggregation.
Selective hydrogenation of carbon–carbon triple bonds to
double bonds is a challenge relevant to commodity chemical
production as well as for synthesis of fine and special
1
,2
chemicals. The partial hydrogenation of 2-butyne-1,4-diol
BYD) is especially interesting because the product 2-butene-
,4-diol (BED) is an important intermediate in the production
(
1
3
4
of endosulfan and vitamins A and B. It is also used as an
additive in resin manufacturing. Lindlar catalysts (Pd on
CaCO poisoned with lead) are the prototypical examples of a
5
6
3
highly selective supported metal catalyst for liquid phase
hydrogenation of BYD created by addition of an appropriate
surface modifier. For this catalyst, the lead is used as an
irreversible dopant to promote the selective reduction of BYD
to cis-BED, while a basic compound such as quinoline is
often added during the reaction to slow the subsequent
hydrogenation of the desired product to 1,4-butanediol
1
(
BDO). The outstanding selectivity is thus associated with the
use of undesirable, toxic lead compounds and other soluble
additives. Therefore, it is necessary to develop alternative,
green and stable catalysts for this transformation.
In recent years, metal–organic frameworks (MOFs) have
been extensively explored as metal supports for heterogeneous
7
-11
catalysts.
In particular, using MOFs as the support allows
stabilization of catalytically active metal nanoparticles that are
1
2
too labile under other conditions. The high surface area
and narrow pore distribution of MOFs may lead to uniform
distribution of highly dispersed NPs, which is of great interest
1
3
for controlling catalytic activity and selectivity.
Laboratory of Advanced Materials and Catalytic Engineering, Dalian University of
Technology, Dalian 116024, China. E-mail: changhai@dlut.edu.cn;
Fax: +86 411 84986353; Tel: +86 411 84986353
Fig. 1 TEM images of PVP–Pd colloid (a) and PVP–Pd@ZIF-8 (0.3 wt% (b),
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
1 wt% (c), 5 wt% (d)) and N
catalysts.
2
adsorption results (e, f) for PVP–Pd@ZIF-8
c3cy00873h
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This agrees well with what has been reported in a study involv-
ing similar synthesis methodology but with much shorter syn-
thetic time and lack of control over the time for adding PVP–Pd.
The size of the ZIF-8 framework is about 100 nm, which can
also be confirmed from SEM results (Fig. S3†). It can be
seen that most of the Pd particles are embedded inside of the
ZIF-8 crystal lattice and almost no particles on the edges of
ZIF-8 can be seen when Pd loading is low. However, when Pd
loading is up to 5 wt%, some Pd particles can be occasionally
seen on the edges of the ZIF-8 lattice, but still with no aggrega-
catalysts. The selectivity to monoene products can often be
improved by poisoning step or terrace sites of catalysts with
14
15
16
17
18
bismuth, sulfur adatoms, lead, Ag, etc. These catalytic
systems usually provide high selectivity to partially hydro-
genated products with the hazards of toxic heavy metals. A
residual bacterial biomass poisoned Pt catalyst had been
1
9
reported by Bennett et al. The bio-Pt catalyst had a better
selectivity to butenediol than the 5 wt% Pt/graphite catalyst.
However, the selectivity was still relatively low. Some other
catalytic systems for hydrogenation of 2-butyne-1,4-diol are
summarized in Table S2† in which most catalysts had
good selectivity only when BYD was incompletely converted.
Pd@MOFs catalysts have also been applied in the hydro-
tion. The N adsorption isotherms (Fig. 1 (e, f)) of the ZIF-8 sup-
2
port and PVP–Pd@ZIF-8 catalysts show typical type I isotherms
as classified by IUPAC, which match well with the microporous
structure of ZIF-8. A narrow pore size distribution (1–1.7 nm)
was observed. No obvious differences can be detected between
the support and the catalysts, which indicate the intactness of
ZIF-8 after even high Pd loading. The encapsulation of ZIF-8 to
PVP–Pd can also be indirectly proven by the hydrogenation of
n-hexylene and cyclooctene, in which PVP–Pd@ZIF-8 had
extremely high selectivity to n-hexylene for its linear structure
making it possible to pass through the narrow canal of ZIF-8
2
0
genation of BYD. The Pd@MOF-5 had the highest selectivity
to BED at the first hydrogenation stage (BYD conversion
~95%), and the butenediol products could be consecutively
hydrogenated with longer reaction time. Unfortunately,
the Pd@MOFs showed low stability during the reaction
process. In contrast, the PVP–Pd@ZIF-8 catalysts here almost
completely blocked the second hydrogenation stage, leading
to excellent selectivity to cis-BED even with very long reaction
time or high temperature.
(Table S1†). As a consequence, a surface area and pore volume
change happened. Though the BET surface area does not make
so much sense for MOFs, for reference only, the results are
listed in Table 1. The total surface areas of these three samples
are almost the same. However, as the Pd loading increases, the
micropore area decreases and the external surface area
increases. In addition, the micropore volumes also decrease
when Pd amount increases.
The catalytic activities and selectivities under different
conditions are shown in Table 2, in which the reaction time
is 0.5 h. The activity increased with the rise of Pd loading.
Full conversion accompanied with 97.5% selectivity to
cis-BED was obtained at 50 °C and 2 MPa when the Pd loading
was about 1 wt%. It can be seen that a few more products of
the second hydrogenation stage (BDO) were obtained at 5 wt%
Pd loading, where some Pd particles were embedded on the
edge of ZIF-8. Higher temperatures often lead to deep hydroge-
nation of BED to BDO. However, very high selectivity to BED
(94.5) was still obtained when the temperature was raised to
The catalytic performance of PVP–Pd@ZIF-8 catalysts was
tested in a batch reactor with H O as solvent. The hydro-
2
genation pathways of BYD are shown in Scheme S1. BYD can
be easily fully hydrogenated to BDO by monometallic Pd or Pt
100 °C. The pressure also affects the activity, and the decrease
in conversion at lower pressure was expected. The unexpected
phenomenon was that excessive pressure also reduced the
activity. This is attributed to the high internal surface area and
Table 1
2
N physisorption results of PVP–Pd@ZIF-8 catalysts
a
b
c
d
SA
MA
EA
MV
3
2
−1
2
−1
2
−1
−1
21
Sample
(m g
)
(m g
)
(m g
)
(cm g
)
metal cation sites of ZIF-8 that facilitate hydrogen adsorption.
With higher hydrogen pressure (over 2 MPa), increased adsorp-
0
1
5
.3 wt% PVP–Pd@ZIF-8 1757
1717
1666
1488
39
88
112
0.64
0.62
0.57
wt% PVP–Pd@ZIF-8
wt% PVP–Pd@ZIF-8
1754
1600
tion of H molecules into the micropore of ZIF-8 limited the
2
substrate to reach the Pd encapsulated in the ZIF-8 framework.
Another possible reason for this behavior might be the compet-
itive adsorption of the substrate with surface Pd hydride.
a
d
b
c
BET surface area.
Micropore volume.
Micropore area.
External surface area.
Table 2 Hydrogenation of 1,4-butynediol on the PVP–Pd@ZIF-8 catalysts with different Pd loadings
Selectivity (%)
T
(°C)
P
Conversion
(%)
a
Catalyst
(MPa)
cis-BED
BDO
n-Butanol
Side products
0.3 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
1 wt% PVP–Pd@ZIF-8
5 wt% PVP–Pd@ZIF-8
50
20
50
100
50
50
2
2
2
2
1
3
4
2
66.9
26.6
99.7
94.5
77.1
87.8
79.6
98.2
98.8
98.0
97.5
94.5
97.2
98.0
97.9
95.5
0.7
1.3
1.5
3.5
1.2
1.3
1.2
4.0
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
0.5
0.7
1.0
2.0
1.6
0.7
0.9
0.5
50
50
a
Isomerization products such as γ-oxybutyraldehyde, 2-hydroxytetrahydrofuran, etc.
330 | Catal. Sci. Technol., 2014, 4, 329–332
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Fig. 2 Kinetic data for hydrogenation of 1,4-butynediol obtained for
Fig. 3 Recyclability of 1 wt% PVP–Pd@ZIF-8 catalyst (a) and XRD
1
wt% PVP–Pd@ZIF-8 (a) and PVP–Pd colloid (b).
(b)/TEM (c) characterization for the recycled catalyst.
The kinetic data for BYD hydrogenation for 1 wt% PVP–
an N containing compound that may play a role in adjusting
2
3
Pd@ZIF-8 are shown in Fig. 2a. BYD was almost completely
converted in 30 min, and the extremely high selectivity kept
constant during the whole reaction process. Even with another
the selectivity of the catalysts. To confirm the effects of
the encapsulation of ZIF-8, the same synthesis method was
extended to another acetate protected Pt nano-sol. Once again,
in this case the Pt@ZIF-8 also had a similar outstanding
performance in this reaction (Fig. S4†).
3
.5 h after the complete consumption of BYD, there was sur-
prisingly still no hydrogenation of cis-BED or isomerization.
The selectivity to BED was still high (97.7) after 4 h of reaction,
indicating that the reaction stopped after the first hydro-
genation stage. These facts suggest that the selectivity of our
catalysts should be mainly derived from their structural
properties rather than controlling their kinetics. In compari-
son, the original cubic PVP–Pd nano-sol was directly applied
in the same reaction under the same conditions and the
relevant kinetic data are shown in Fig. 2b. The PVP–Pd
nano-sol had a lower activity, needing 2.5 h to fully convert
BYD because of the aggregation of Pd nano-sol as observed
during the reaction process. A relatively high selectivity to
BED (93%) can also be gained at the point of 2.5 h and then
the further hydrogenation and isomerization began to hap-
pen as the reaction continues. The results above clarified
that the outstanding partial hydrogenation performance of
PVP–Pd@ZIF-8 was not just because of the adsorption
of PVP around Pd particles. The protection of long-chain
PVP definitely reduces the activity of Pd, but obviously
the PVP–Pd particles were still active enough for the hydro-
genation of BED.
The PVP–Pd@ZIF-8 catalysts can be reused without any
reactivation treatments except for two-time washing with
H O. The recyclability data for 1.0 wt% PVP–Pd@ZIF-8 are
2
shown in Fig. 3a. It shows that the sample had reproducible
performance over five runs, sustaining very good conversion
of BYD (97%) and excellent selectivity of BED (98%) even
after the fifth run. The XRD pattern of PVP–Pd@ZIF-8 after
reaction is shown in Fig. 3b. No obvious differences can be
detected compared with the fresh sample, indicating the
intactness of the ZIF-8 crystal structure during the reaction.
The N
2
adsorption data (Fig. S5†) also confirmed the
unchanged pore structure of the recycled catalyst. The TEM
image (Fig. 3c) clearly showed that the ZIF-8 framework and
the cubic Pd particles are all preserved during the reaction
runs. No aggregation/growth of Pd particles or collapse of the
ZIF-8 structure occurred.
In summary, we have demonstrated the successful prepa-
ration of PVP–Pd catalysts encapsulated by ZIF-8 through
a PVP-assisted rapid dynamic crystallization method at
room temperature. The monometallic PVP–Pd@ZIF-8 catalysts
showed prominently enhanced selectivity to cis-1,4-butenediol
for hydrogenation of 1,4-butynediol, due to edge site
blocking by ZIF-8 and assistance of the N containing organic
ligand in ZIF-8. At the same time, high activity was obtained
due to the high catalyst surface area and high dispersion of
Pd particles. Notably, the PVP–Pd@ZIF-8 not only had an
excellent performance but also had a great stability in the
reaction. The method can be extended to other metal
particles (e.g. Pt), which can find wide potential applications
in catalytic reactions.
The most probable reason for this interesting performance
is the encapsulation within ZIF-8. It is known that the step
and edge atoms are less coordinated than those atoms on
2
2
faces of metal particles. Thus, there must be more PVP
molecules adsorbed on the vertex and edge sites on the
present Pd particles, which would in turn have more
tendency to adsorb on the ZIF-8 framework during the
synthetic process. Based on the above reasons, the edge and
vertex sites of Pd were much more blocked by ZIF-8 than
the terrace sites (Scheme S2†). It has been proven that the C–C
triple bonds are hydrogenated upon terrace sites of Pd
catalysts, while the resulting double-bond products are further
1
5,20
hydrogenated on the edge and vertex sites.
Thus, the linear
Acknowledgements
BYD could only be hydrogenated upon the terrace sites but
couldn't be further hydrogenated due to the blockage of the
edge sites, leading to a high selectivity to BED. At the same
time, the narrow pore canal of ZIF-8 may limit the isomeriza-
tion process. In addition, the 2-methylimidazole in ZIF-8 is
This work was supported by the National Natural Science
Foundation of China (21073023 and 21373038) and the
Fundamental Research Funds for the Central Universities
(DUT12YQ03 and DUT13RC(3)41).
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