The ionic liquid microphase enhances the catalytic activity of Pd nanoparticles supported by a metal-organic framework

Article abstract of DOI:10.1039/c5gc01333j

Here we demonstrate the utilization of the ionic liquid (IL) microphase for enhancing the catalytic activities of the metal nanoparticles supported on a MOF. The IL microphase offers an excellent environment for stabilizing metal nanoparticles. A new heterogeneous catalyst Pd/IL/MOF is developed, which combines the advantages of highly dispersed small Pd nanoparticles, the IL microphase and a porous MOF. The as-synthesized Pd/IL/MOF catalysts have shown high catalytic activity and reusability for selective hydrogenation under mild conditions.

Full text of DOI:10.1039/c5gc01333j

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Ionic liquid microphase enhances the catalytic activity of Pd
nanoparticles supported by metal-organic framework
Received 00th January 20xx,
Accepted 00th January 20xx
Li Peng, Jianling Zhang,* Shuliang Yang, Buxing Han, Xinxin Sang, Chengcheng Liu and Guanying
Yang
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here we demonstrate the utilization of ionic liquid (IL) microphase solve the recycle problem and make it convenient to be applied in
for enhancing the catalytic activities of the metal nanoparticles industrial processes.21
supported on MOF. The IL microphase offers excellent
Here we propose for the first time the utilization of IL microphase
environment for stabilizing metal nanoparticles. new for enhancing the catalytic activities of the metal nanoparticles
A
heterogeneous catalyst Pd/IL/MOF is developed, which combines supported on MOF. The IL microphase offers excellent environment
the advantages of highly dispersed small Pd nanoparticles, IL for stabilizing metal nanoparticles and thus highly dispersed small
microphase and porous MOF. The as-synthesized Pd/IL/MOF metal nanoparticles are expected to be immobilized on MOF. By
catalysts have shown high catalytic activity and reusability for this strategy,
a new heterogeneous catalyst Pd/IL/MOF is
selective hydrogenation under mild condition.
developed, which combines the advantages of the three
components, i.e. small Pd nanoparticles, IL microphase, and porous
MOF. The as-synthesized Pd/IL/MOF catalysts have shown high
activities in catalyzing the selective hydrogenation of acetylene
hydrocarbon under mild condition.
In recent years, metal-organic frameworks (MOFs), also known as
porous coordination polymers (PCPs), have became one kind of the
most researched porous materials.1-5 MOFs have found wide
applications in different fields, including catalysis,6 gas storage and
separation,7-8 sensing,9 drug delivery,10 etc. Among these
applications, depositing metal nanoparticles onto MOFs as
heterogeneous catalysts is an emerging field because of the large
surface area, tunable topologies and designable surface properties
of MOFs.11-12 However, the aggregation of metal nanoparticles
supported on MOF is often observed and hardly prevented, which
greatly reduces their catalytic activities. To limit the aggregation of
metal nanoparticles, the functionalized MOFs or surfactants are
requisite for stabilizing metal nanoparticles.13-14
For a typical synthesis of Pd/IL/ MOF catalyst, the precursors of
Cu3(BTC)2 MOF and Pd precursor were loaded into the IL 1,1,3,3-
tetramethylguanidinium trifluoroacetate (TMGT). After stirring for a
certain time, the MOF was gained and a desired amount of reducing
agent was added into the reaction system to produce Pd/IL/MOF
catalyst (denoted as Catalyst-1, detailed synthesis procedures are
shown in ESI†). FT-IR spectra reveal that the carboxylate groups of
1,3,5-tricarboxybenzene (H3BTC) are coordinated to Cu (II) ions (Fig.
S1†). The powder XRD pattern of Catalyst-1 is shown in Fig. S2. The
XRD peak positions and relative intensities of the as-synthesized
22
Ionic liquids (ILs), as tunable and environmentally friendly
solvents, have attracted tremendous attention owing to their very
low vapor pressures, high chemical and thermal stability, wide
electrochemical windows, and excellent solvency for both organic
and inorganic compounds.15 Now ILs have been used to replace
volatile organic solvents for different processes such as material
synthesis,16 chemical reactions,17-18 gas adsorptions19 and energy
sample are in accordance with those of the reported Cu3(BTC)2
,
revealing the successful synthesis of Cu3(BTC)2 MOF. No obvious
diffractions of Pd were observed in the XRD pattern, suggesting the
formation of ultra-small Pd nanoparticles23 or/and the low content
of Pd in the catalyst.24 The weight percentage of Pd in Catalyst-1
was measured to be 1.27 wt% by inductively coupled plasma atomic
emission spectroscopy (ICP-AES). According to the TGA analysis (Fig.
S3†), the IL content in Catalyst-1 is 13.2 wt%.
production20
.
Despite many advantages, ILs have some
shortcomings such as high viscosity, difficulties in product
purification, recycling, and so on, which hinder their chemical
industry applications. Immobilizing ILs onto solid supports could
The SEM images show that Catalyst-1 displays a porous structure,
formed by stacked MOF nanoparticles in size of several dozen
nanometers (Figs. 1a and 1b). The surface of the MOF was coated
with small particles, as evidenced by TEM images (Figs. 1c and 1d).
From the high-resolution TEM (HRTEM) images, the lattice fringe of
Pd (2 0 0) can be clearly seen and the size of Pd nanoparticles is
about 2 nm (Figs. 1e and 1f). Electron energy loss spectroscopy
reveals that Pd and N (resulting from IL) are uniformly dispersed on
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid
and Interface and Thermodynamics, Institute of Chemistry Chinese Academy of
Sciences. Beijing 100190, P.R.China. E-mail: zhangjl@iccas.ac.cn
†Electronic Supplementary Information (ESI) available: detailed procedure of
experiments, Figs S1-S11 and Table S1. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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the surface of MOF (Fig. 1g). Energy-dispersive X-ray (EDX) further Catalyst-1 shows outstanding activity and excellent selectivity
confirms the presence of Pd in the prepared Catalyst-1 (Fig. S4†). among the tested catalysts and the reported catalysts under the
Catalyst-1 has a BET surface area of 708 m2/g, as determined by N2 same reaction conditions. It has been widely reported that the N
adsorption-desorption method. The surface area of Catalyst-1 is containing compounds can be used as surface modifiers to improve
lower than that of pure Cu3(BTC)2 (1026.7 m2/g), resulting from the the selectivity of the catalyst for alkyne hydrogenation to alkene.27-
30
incorporation of IL and Pd into Cu3(BTC)2.
Herein the ligand effect of N element in the IL integrated in the
catalyst is believed to be responsible for the high selectivity.31-32
Table 1 Hydrogenation of phenylacetylene with different catalysts.
[a] reaction condition for Entries 1-6: catalyst (5 mg, Pd loading: 1.27 wt%
for entries 1-3, 1.35 wt% for entries 4-5 and
5 wt% for entry 6),
phenylacetylene (0.91 mmol) in 2.0 mL acetonitrile using the 1,3,5-
trimethylbenzene as the internal standard, atmospheric H2 balloon, 30 oC. [b]
5.85 mmol, 10 mg catalyst (5.64 wt% Pd), 50 mL solvent, atmospheric H2
balloon, 30 oC.26 [c] Turnover number (TON)=mol of product per mole of Pd;
TOF=TON h-1.
The catalytic activities of Catalyst-1 were tested for the selective
hydrogenation of other alkynes. The reaction conditions are the
same with those for the hydrogenation of phenylacetylene. The
phenylacetylenes with electron-donating and electron-withdrawing
groups are all selectively hydrogenated to the corresponding
terminal alkenes with high activity (Table 2, Entries 1-4). Catalyst-1
also shows high activity and good selectivity for the hydrogenation
of aliphatic alkynes that 4-octyne and 1-octyne could be converted
completely in 45 and 50 minutes with >96% selectivity (Table 2,
Fig. 1 (a and b) SEM; (c and d) TEM; (e and f) HRTEM images and (g) element
mapping images of Pd/IL/MOF (Catalyst-1).
The catalytic performance of Catalyst-1 for the selective
hydrogenation of phenylacetylene was tested. Selective
hydrogenation of alkynes to alkenes, without further reduction to
less valuable alkanes, is of great importance in the polymer and fine
chemical industries.25 The phenylacetylene was almost completely
converted (>99 %) in 40 min with high selectivity (>99 %) toward
styrene (Table 1, Entry 3). A hot extraction experiment confirmed
that the catalysis was exclusively heterogeneous (Fig. S5†). For
comparison, the IL-free Pd/MOF (Catalyst-2) was synthesized in
ethanol/water system and its catalytic activity for the selective
hydrogenation of alkynes was detected under the same
experimental conditions. Catalyst-2 shows much slower reaction
rate, i.e. a 83.1% conversion was obtained even as the reaction time
was prolonged to 12 hours (Table 1, Entries 4-5). The main reason
may be that the Pd nanoparticles obtained in the absence of IL are
aggregated (Fig. S6†). Further, the catalytic performance of
Catalyst-1 was compared with those of commercial Pd/C catalyst
and the reported Pd catalysts26 (Table 1, Entries 6-10). Apparently,
Entries
5 and 6). The reaction rates of hydrogenation of
diphenylacetylene and 1-phenyl-1-butyne (Table 2, Entries 7 and 8)
are slower than those of the above, which could be ascribed to the
steric effect.
Table 2 Hydrogenation of alkynes with Catalyst-1.[a]
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trifluoromethanesulfonate
([BMIm]OTf),
1-butyl-3-
methylimidazolium hexafluorophosphate ([BMIm]PF6), 1-hexyl-3-
methylimidazolium hexafluorophosphate ([HMIm]PF6) and 1-octyl-
3-methylimidazolium hexafluorophosphate ([OMIm]PF6) were used
to synthesize Pd/IL/MOF catalysts, the experimental conditions
being the same with those for synthesizing Catalyst-1. See XRD
patterns, FT-IR spectra and TGA analysis of the catalysts in Figs. S8†,
S9† and S10†. All these Pd/IL/MOF catalysts show high activity and
selectivity in catalyzing the hydrogenation of phenylacetylene to
styrene (Table S1). It indicates that the IL microphase is versatile in
enhancing the activity of Pd nanoparticles supported on MOF.
To understand the IL-promoted catalytic activity of Pd
nanoparticles supported on MOF, the interactions between IL, Pd
and MOF were investigated by XPS spectra (Fig. S11†). As shown in
Fig. 3A, the binding energies of Pd3d5/2 in Catalyst-1 (335.5 eV) shift
down slightly compared with that of Catalyst-2 (335.7 eV). It
indicates that there is interaction between IL and Pd nanoparticles,
possibly through the nitrogen in IL.33-34 Moreover, Pd 3d XPS spectra
of Catalyst-1 show Pd2+ characteristic peaks, which is absent in
Catalyst-2, implying that IL TMGT participated in the bonding of
Pd.35 As seen from Fig. 3B, the Cu 2p2/3 binding energy of Catalyst-1
(934.5 eV) is lower than that of Catalyst-2 (934.8 eV). It is indicative
of the interactions between IL and Cu-MOF, which might be derived
[a] reaction condition: Catalyst-1 (5 mg, Pd loading: 1.27 wt%), Substrate from N⋅⋅⋅Cu interaction between N of TMGT and Cu2+ of Cu-MOF.36-
37 The XPS results prove that IL interacts with both Pd nanoparticles
(100 μL for Entries 1-7, 0.91 mmol for Entry 8) in 2.0 mL acetonitrile using
the 1,3,5-trimethylbenzene as the internal standard, atmospheric H2 balloon,
and MOF support, through which the small Pd nanoparticles can be
30 oC.
well stabilized on MOF support.
The reusability of Catalyst-1 for the hydrogenation of
A
phenylacetylene was investigated. After the reaction, Catalyst-1
Pd2+ 3d5/2
Pd 3d5/2
was recovered by simple centrifugation and washing. There is no
any loss of activity and selectivity after the catalyst was reused for 4
cycles (Fig. 2). In the fifth run, a little loss of activity was observed,
which could be attributed to the unavoidable catalyst loss during
the washing process. The TEM image of the catalyst after used for
five runs shows no particle aggregation and no major difference
was observed for the XRD patterns of the fresh and recovered
catalyst (Fig. S7†). ICP-AES analysis showed Pd loading of the
recovered catalyst was 1.23 wt%, which was very close to the
original value (1.27 wt%). This result implied the Pd leaching during
the cycle process was negligible.
Pd 3d3/2
Pd2+ 3d3/2
Catalyst-1
Catalyst-2
345
340
335
330
Binding energy (eV)
B
Cu 2p3/2
Cu2p1/2
Catalyst-2
Catalyst-1
955
950
945
940
935
930
Binding energy(eV)
Fig. 3 Pd 3d (A) and Cu 2p (B) XPS spectra of Catalyst-1 and Catalyst-2.
Fig. 2 Reusability of Catalyst-1 for hydrogenation of phenylacetylene.
Based on the above results, the structure of the as-synthesized
Pd/IL/MOF catalyst and its catalytic activity for the selective
hydrogenation of phenylacetylene is illustrated in Scheme 1. The IL
could anchor on the MOF surface through the interaction between
Different
ILs
including
1-butyl-3-methylimidazolium
1-butyl-3-methylimidazolium
tetrafluoroborate
([BMIm]BF4),
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of the IL and the unsaturated Cu2+ sites of the MOF.
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Scheme 1 Structure of Pd/IL/MOF catalyst and its catalytic activity for the
selective hydrogenation of phenylacetylene.
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In summary, we designed
a Pd/IL/MOF catalyst by taking
advantage of the IL microphase. IL could stabilize the Pd
nanoparticles and highly dispersed small metal nanoparticles are
immobilized on MOF. The Pd/IL/MOF catalyst shows excellent
catalytic activity and high selectivity for the hydrogenation of
terminal alkynes. The synthetic route for the Pd/IL/MOF catalysts is
simple and can be applied to the synthesis of different kinds of
MOF-supported metal nanoparticles. We anticipate that other high
efficient heterogeneous catalysts stabilized by IL microphase can be
designed for various catalytic reactions.
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The authors thank the National Natural Science Foundation of
China (21173238, 21133009, U1232203, 21021003), Chinese
Academy of Sciences (KJCX2.YW.H16).
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Here Pd/ionic liquid/metal-organic framework catalyst is developed, which shows high activities
in catalyzing the selective hydrogenation of alkynes under mild condition.