Reversible Dehydrogenation and Hydrogenation of N-Heterocycles Catalyzed by Bimetallic Nanoparticles Encapsulated in MIL-100(Fe)

Article abstract of DOI:10.1002/cctc.201801311

Pd?Ni bimetallic nanoparticles (BMNPs) encapsulated in MIL-100(Fe) (Pd-Ni@MIL-100(Fe) was fabricated and employed as an efficient catalyst for the reversible dehydrogenation/hydrogenation of N-Heterocycles derivatives in water under mild conditions. Excellent catalytic performance for both reactions endows Pd-Ni@MIL-100(Fe) great potential value in organic chemistry. Alloying Pd with Ni can enhance the catalytic performance due to the bimetallic synergy. Both Lewis acidity and ordered mesoporous structure of MIL-100(Fe) are beneficial to the performance of the catalyst owing to its stabilization of BMNPs, reduction of the electron density of Pd atoms and enhancement of substrates adsorption capacity.

Full text of DOI:10.1002/cctc.201801311

Accepted Article
Title: Reversible Dehydrogenation and Hydrogenation of N-
Heterocycles Catalyzed by Bimetallic Nanoparticles
Encapsulated in MIL-100(Fe)
Authors: Jia-wei Zhang, Dan-dan Li, Guo-ping Lu, Tao Deng, and
Chun Cai
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201801311
Link to VoR: http://dx.doi.org/10.1002/cctc.201801311
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ChemCatChem
10.1002/cctc.201801311
FULL PAPER
Reversible Dehydrogenation and Hydrogenation of N-Heterocycl-
es Catalyzed by Bimetallic Nanoparticles Encapsulated in MIL-
100(Fe)
[
a]
[a]
[a]
[b]
[a]
Jia-wei Zhang, Dan-dan Li, Guo-ping Lu , Tao Deng and Chun Cai*
Abstract: Pd-Ni bimetallic nanoparticles (BMNPs) encapsulated in
MIL-100(Fe) (Pd-Ni@MIL-100(Fe) was fabricated and employed as
an efficient catalyst for the reversible dehydrogenation/hydrogenation
of N-Heterocycles derivatives in water under mild conditions.
Excellent catalytic performance for both reactions endows Pd-
Ni@MIL-100(Fe) great potential value in organic chemistry. Alloying
Pd with Ni can enhance the catalytic performance due to the bimetallic
synergy. Both Lewis acidity and ordered mesoporous structure of MIL-
required to fulfill the dehydrogenation process.[6] More recently,
Somorjai et al. reported a catalytic system based on the metallic
nanoparticles stabilized by G4OH PAMAM dendrimer for the
acceptorless reversible dehydrogenation and hydrogenation of
tetrahydroquinoline and indoline derivatives.[7] Despite those
impressive progresses, it is still urgent to develop innovative
catalytic systems for de/hydrogenation transformations consider-
ing the vital role of quinoline and its derivatives.
1
00(Fe) are beneficial to the performance of the catalyst owing to its
With our interest on the bimetallic nanoparticles (BMNPs)
catalysts and Metal Organic Frameworks (MOFs),[8,9] we envisage
to design a new and efficient BMNPs catalyst immobilized on
MOFs for the de/hydrogenation transformations with assistance
of “synergistic effects” of BMNPs[ and host-guest cooperation
between MOFs and nanoparticles[ . Herein, reversible dehydro-
genation/hydrogenation of N-Heterocycles could be realized in
the presence of Pd-Ni@MIL-100(Fe) catalyst, and we disclose
that Lewis acid sites on the MIL-100(Fe) is likely attributable to
the weak electron-donating capability of Pd particles, which
reinforce the catalytic performance of Pd-Ni@MIL-100(Fe). To the
best of our knowledge, this is the first report on catalytic system
based on MOFs supported BMNPs for reversible dehydro-
genation and hydrogenation of N-Heterocycles.
stabilization of BMNPs, reduction of the electron density of Pd atoms
and enhancement of substrates adsorption capacity.
10]
11]
Introduction
Catalytic dehydrogenation and hydrogenation of N-
heterocycles are fundamental processes to produce quinolines
and tetrahydroquinolines (THQs), which are important scaffolds
due to their wide existence in biologically active natural products,
synthetic pharmaceuticals and material science.[1] Moreover,
switchable transformation between THQs and quinolines also
attracted considerable attention from the viewpoint of hydrogen
storage.[2] Considering the importance of this two reactions,
various catalytic systems have been established, hydrogenation
process of N-heterocycles could be achieved under relatively mild
reaction conditions.[3] While the dehydrogenation process in the
absence of hydrogen scavenger is still challenging despite
considerable efforts in this field,[1a,4] efficient catalyst for both
dehydrogenation and hydrogenation reactions are still urgent.
Few catalytic systems based on homogeneous metal
complexes for the reversible dehydrogenation and hydrogenation
of N-Heterocycles have been reported, the reaction proceeds well
but the catalyst separation is problematic.[2c,5] Several hetero-
geneous catalysts have also been established for this process,
Gu et al. used Pt nanowire as the catalyst to realize the reversible
hydrogenation and dehydrogenation of N-heterocycles. The
reaction proceeds smoothly under mild condition and the catalyst
could be recycled, but an atmosphere of oxygen (1 bar) was
Results and Discussion
Our study commenced with dehydrogenation of 1,2,3,4-
tetrahydroquinoline 1a, 130 °C and 12 h were chosen accordingly
4b,7]
(
Table 1) as the reaction itself is thermodynamically uphill.[
To
our delight, complete transformation into the dehydrogenated
product (quinoline) was observed after 12 h when using 1mol%
Pd
was obtained when monometallic Pd was applied as the catalyst
entry 2). Contrast experiment using 4 mol% Ni/SiO as the
catalyst was also carried out, but no product was detected (entry
). The conversion decreased to 62% when lowering the catalyst
1 4 2
Ni /SiO as the catalyst (Table 1, entry 1). Much lower yield
(
2
3
loading to 0.5 mol% (entry 4), and no target product was detected
in the absence of catalyst (entry 5). o-Dichlorobenzene (o-DCB)
was proved to be an excellent solvent for this reaction, but it didn‘t
fulfill the requirements of green and sustainable society due to its
high toxicity. Therefore, water was attempted as solvent for this
reaction (entries 6-23). Replacing Ni with Co or Cu to alloy with
Pd were also tried, but no enhancement on catalytic activity was
observed (entries 6~8).
[
[
a]
b]
J. Zhang, D. Li, Prof. G. Lu and Prof. C. Cai
Chemical Engineering College
Nanjing University of Science & Technology
Xiaolingwei 200, Nanjing 210094, P. R. China
E-mail:c.cai@njust.edu.cn
Dr. T. Deng
Institute of Tropical Medicine
Guangzhou University of Chinese Medicine
Guangzhou 510405, P. R. China
To further improve the unsatisfactory yields, catalyst
supports were screened. Initially, alkaline oxides (Al
were attempted as the catalyst carriers instead of SiO
dehydrogenation process was dramatically retarded (entries 9,
0), while much better catalytic efficiency was achieved when
Nb was used as the support (entries 8 vs 11). Better catalytic
2
O
3
and MgO)
2
, the
1
Supporting information for this article is given via a link at the end of
the document.
2
O
5
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ChemCatChem
10.1002/cctc.201801311
FULL PAPER
performance of Pd Ni
1
4
/Nb
2
O
5
could be ascribed to the strong
surface area and pore volume of the synthesized MIL-100(Fe) are
1826 m g-1 and 1.02 cm g , respectively, very similiar to the
publiahsed data.[15] Better catalytic performance of MIL-100(Fe)
(entries 13 vs 14) maybe ascribed to its larger pores, which
2
3 -1
Lewis acidity of Nb O , which provides a strong absorption for the
2
5
reactant molecules, reduction of the electron density of Pd NPs
could strengthen its absorbability for quinolines.[12] Subsequently,
Table
1
Optimization of the conditions for dehydrogenation of 1,2,3,4-
guarantee a better contract between the reactant molecules and
the BMNPs.[16]
a
tetrahydroquinoline into quinoline.
Lowering the reaction temperature caused obvious decrease
in yield, and only 25% yield was obtained when the reaction
o
temperature was decreased to 90 C (entries 15~16). To further
Catalyst
amount
Yield
(%)
clarify the contribution of “synergistic effects” between the BMNPs
and MOFs, unsupported Pd-Ni BMNPs was attempted in this
reaction. Poor yield of the reaction could be a result of serious
agglomeration of the catalyst during the reaction process (entry
Entry
Catalyst
Solvent
b
1
2
3
4
5
6
7
8
9
Pd
Pd/SiO
Ni/SiO
Pd Ni /SiO
1
Ni
4
/SiO
2
2
1%
1%
4%
0.5%
0
o-DCB
o-DCB
o-DCB
o-DCB
o-DCB
99
75
ND
62
0
2
2
17), which further confirm the vital role of MIL-100(Fe).
1
4
Monometallic Pd NPs encapsulated in MIL-100(Fe) was also
attempted, moderate yield was obtained (entry 18), indicating that
Pd was the main active site for the dehydrogenation reaction.
No Catalyst
Pd
1
Cu
4
/SiO
/SiO
2
2
1%
1%
1%
1%
1%
1%
1%
1%
1%
1%
1%
1%
1%
0.5%
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
23
34
47
26
7
Pd
1
Co
4
1 4
Furthermore, the ratio of Pd and Ni was also optimized, Pd Ni
Pd
1
Ni
Ni
Ni
Ni
Ni @UiO-66
@UiO-66-NH
4
/SiO
/Al
/MgO
/Nb
2
was proved to be the best option (entries 14, 19-21), which clearly
indicated that both Pd and Ni in the BMNPs are essential for the
enhanced catalytic performance. A control experiment using
pyridine as a probe was conducted, much lower yield was
observed upon introduction of pyridine (0.2 mL) into the system to
poison the Lewis acid sites (entry 22 vs 23), which demonstrate
the significant influence of Lewis acidity on the catalytic
performance.[17] o-DCB, DMF, p-xylene, Toluene were used as
the solvent in this reaction to investigate the solvent's effect on
this reaction, excellent yields were observed (entries 24-27). But
when the reaction was carried out with no solvent, a significant
decrease on yield was observed (entry 28), which might be
caused by the viscosity of quinoline. Furthermore, to prove the
Pd
1
4
2
O
3
10
11
12
13
14
Pd
1
4
Pd
Pd
Pd Ni
1
4
2
O
5
84
50
95
100
92
25
15
78
73
1
4
1
4
2
Pd
Pd
Pd
1
Ni
Ni
Ni
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
1
1
5c
6d
1
1
1
7
8
9
Pd
Pd@MIL-100(Fe)
@MIL-100(Fe)
Pd0.5Ni16@MIL-
00(Fe)
1 4
Ni
1
1
2
2
Pd0.5Ni
8
0
0.5%
H
2
O
82
1
2
production of H , the produced gas was collected in a eudiometer
and subjected to GC analysis and the retention time matched with
a sample collected from a hydrogen cylinder of 99.9% purity
1
Pd
1
Pd
1
Pd
1
Pd
1
Pd
1
Pd
1
Pd
1
Pd
1
Ni
Ni
Ni
Ni
Ni
Ni
Ni
Ni
2
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
4
@MIL-100(Fe)
1%
1%
1%
1%
1%
1%
1%
1%
H
H
H
2
2
2
O
O
O
87
38
82
100
91
96
94
63
2
2e,f
3e
2
(
Figure S3, SI). Volume of the produced gas was measured on a
2
4
5
6
7
8
o-DCB
DMF
gas buret setup (Figures S4,SI)
2
2
2
2
p-xylene
Toluene
Neat
[
a] Reaction conditions: substrate (0.1 mmol), catalyst amount was
measured based on Pd, solvent (1 mL) (o-DCB = o-Dichlorobenzene),
stirred in a sealed tube at corresponding temperature for 12 h. [b]
Conversion and yield were determined by GC and GC-MS using biphenyl
o
o
as the internal standard. [c] At 110 C. [d] At 90 C. [e] The reaction time
was 6 h. [f] 0.2 mL pyridine was added as a probe.
UiO-66 was chosen as the catalyst matrix because of its high
surface area, intersecting 3D structure and high stability,[13] no
significant enhancement in catalytic performance was observed
(
entry 12). While much better catalytic activity was achieved when
UiO-66-NH was applied as the catalyst carrier (entry 13), which
may be caused by the increased adsorptive capacity of the UiO-
2
Figure 1. FT-IR spectra of THQ and THQ absorbed on different catalysts
6
6(NH
2
) through hydrogen bonding and increased Van der Waals
During the conditions optimization process, we noticed that
properties of catalyst carrier have a huge impact on the catalytic
performance, so further efforts were made to understand the roles
of carriers on the catalytic performance. FT-IR experiments were
performed to explore the absorptivity of catalysts (same ratio of
forces from the -NH
2
group.[14] MOFs based on Fe were also
attempted as carriers for BMNPs due to its strong Lewis acidity,[
MIL-100(Fe) was emerged as the most suitable matrix (entry 14).
MIL-100(Fe) was prepared according to a reported method, BET
15]
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ChemCatChem
10.1002/cctc.201801311
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of Fe spectrum; (e) high resolution of Pd spectrum; (f) high resolution of Ni
spectrum
Ni:Pd loaded on different supports) to THQ (Figure 1), obvious
decay of signal around 3400 cm-1 (N-H stretching vibration) was
observed on Pd-Ni@MIL-100(Fe), indicating the strong binding of
THQ to Pd-Ni@MIL-100(Fe) catalyst through amino groups,
which also confirmed our speculation that strong Lewis acidity of
the catalyst support have a strong beneficial for the catalytic
Well-defined peaks corresponding to metallic Pd and Ni can
also be detected (Figure 2e-2f), the doublet with binding energies
of 335.5 (Pd 3d5/2) and 340.9 eV (Pd 3d3/2) (Figure 2e) can be
0
[24]
assigned to metallic Pd . Ni 2p XPS (Figure 2f) displays two
spin−orbit doublets at 855.6 and 873.2 eV and two shakeup
satellites at 861.1 and 879.6 eV, which are consistent with Ni
oxide and Ni hydroxide.[25] To further verify the electronic
interaction between MIL-100(Fe) and BMNPs, XPS spectrum of
performance.[18] What’s more, MIL-100(Fe) has
a strong
[
13]
absorption ability for gas, hydrogen extracted from THQs could
be quickly dispread in the micropores of MIL-100(Fe), promoting
the equilibrium toward dehydrogenation pathway.
To ascertain the details of the surface composition and
chemical states of the, X-ray photoelectron spectroscopy (XPS)
measurement was carried out. Surface concentrations of major
1 4 1 4 2 5
the unsupported Pd Ni BMNPs and Pd Ni /Nb O were also
performed (Figure S2, SI). As shown in Figure 3, binding values
of Pd in unsupported Pd-Ni BMNPs are relatively higher than
corresponding values in Pd-Ni@MIL-100(Fe), indicating that MIL-
elements in the Pd
in Table S2 (ESI). Figure 2a presents overall surveys of the as-
synthesized Pd Ni @MIL-100(Fe) nanocomposites, which clearly
1 4
Ni @MIL-100(Fe) catalyst were summarized
1
00(Fe) can decrease the electron density of Pd as an acidic
material. And more obvious shift of Pd peak was observed in
Pd Ni /Nb catalyst (Figure 3), which reveals the stronger
Lewis acidity of Nb than MIL-100(Fe). Moreover, serious
1
4
reveal Fe, O, C, Ni and Pd element signals. High-resolution XPS
spectrum of C 1s was shown in Figure 2b, which could be fitted
by three peaks at binding energies of around 284.6, 288.7 and
1
4
2 5
O
2
O
5
oxidation of Pd was found in the unsupported Pd-Ni BMNPs,
which showcased that carrier with Lewis acidity may prevent
BMNPs from oxidation.
285.0 eV, peaks at 284.6 and 288.7 eV could be assigned to
phenyl and carboxyl signals, while peak at about 285.0 eV may
be due to the carbon on the surface of the sample.[19] As shown
in Figure 2c, XPS spectrum of the O 1s could be fitted by two
peaks, peak at 532.2 is the signal of Fe-O-C species,[20] while
peak at 531.6 eV was attributed to the oxygen components in the
BTC linkers.[21] Observation of the Fe 2p spectrum (Figure 2d),
peaks centered at 725.4, 711.6 eV are characteristic of Fe(III) in
MIL-100(Fe),[ and peaks around 717.9 and 729.7 eV could be
22]
ascribed to the two satellite peaks of Fe 2p3/2 and Fe 2p1/2
respectively.[23]
,
Figure 3. Comparison of Pd 3d scan of the different Pd-Ni catalyst
Based on these experimental results, we could propose
the following conclusions: i) Lewis acidity of the carrier could
decrease the electron density of the active sites, thus enhancing
2 5
the absorbency for N-heterocycles; ii) Although Nb O has
stronger Lewis acidity than MIL-100(Fe), better catalytic efficiency
of Pd-Ni@MIL-100(Fe) was observed, which could be a result of
the cooperation between Lewis acidity and excellent texture
properties; iii) No obvious shift in binding values of Pd was
observed when compared with Pd@MIL-100(Fe),[24c] which
means that the “electronic effect” between Pd and Ni is not
obvious, and the “synergistic effects” between Pd and Ni could be
assigned to the “ensemble effect” to stabilize Pd atoms and the
excellent affinity of Ni to hydrogen.
Figure 2. XPS patterns of Pd
1 4
Ni @MIL-100(Fe): (a) survey spectrum; (b) high
resolution of C spectrum; (c) high resolution of O spectrum; (d) high resolution
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Gratifyingly, THQs with different substituents proceeded
efficiently under the reaction conditions and gave corresponding
quinolines in good to excellent yields (entries 1~5).
Dehydrogenation of other heterocycle compounds (like indoline,
1,2,3,4-tetrahydro-isoquinoline and 2,3-dihydrobenzofuran under
the optimized conditions gave successfully conversion into
corresponding products (entries 7~9). Moreover, although
electron-deficient group was reported to impair the electron
density of the aromatic ring which may result in more difficult
coordination of the substrates with the catalyst.[2a,2b] 6-Cl
Substituted tetrahydroquinoline was also tolerated in our catalytic
system (entry 6), which are appealing functional groups for late-
stage derivatization.[1] Furthermore, dehydrogenation of
cycloalkane was performed in our system, but the conversion was
nil (entry 10), because dehydrogenation of cycloalkane is more
endothermic than N-heterocycles,[ and absence of an additional
nitrogen atom also results in the decrease of absorption strength
1 4
Figure 4. (a), b) TEM images of Pd Ni @MIL-100(Fe) catalyst; (c) HRTEM
image of Pd
1 4
Ni @MIL-100(Fe) catalyst; (d) Particle size distributions of
Pd Ni @MIL-100(Fe) catalysts
1
4
1 4
Morphology of Pd Ni @MIL-100(Fe) was directly observed
2e]
by transmission electron microscopy (TEM). The TEM images
reveals that anomalously spherical nano-particles were mainly
encapsulated in the pores of MIL-100(Fe). BET surface area of
[
7]
onto the Pd Ni @MIL-100(Fe) catalyst.
1
4
1 4
the synthesized MIL-100(Fe) and Pd Ni @MIL-100(Fe) were
Table 2. Substrate scope of the dehydrogenation of THQs and other
heterocycle derivatives.
messured to be 1826 m2g-1 and 553 m g , drastic decrease in
BET surface area also confirmed that BMNPs were encapsulated
in the pores of MIL-100(Fe). Cavities of the MIL-100(Fe) can act
as nanoreactors, in which the encapsulated BMNPs and
reactants confined in close proximity could ensure a rapid electron
transfer and a facile contact access.[26] No obvious agglomeration
of BMNPs was observed and the average diameter of the BMNPs
is 2.6 nm (a statistical result of about 100 nanoparticles), the
ultrafine BMNPs provide more active unsaturated metal atoms
and more contact opportunities between the active sites and
substrates, which means higher catalytic efficiency.
2 -1
a
Entry
1
R
Yield (%)b
1
1a
H
100 (2a)
83 (2b)
87 (2c)
76 (2d)
81 (2e)
95 (2f)
2
3
4
5
6
1b
1c
1d
1e
1f
2-Me
8-Me
8-NH2
6-OH
6-Cl
7
1g
100 (2g)
8
9
1h
1i
83 (2h)
91 (2i)
10
1j
ND (2j)
[
a] Reaction conditions: substrate (0.1 mmol),
Pd Ni @MIL-100(Fe) (catalyst amount measured based on Pd),
O (1 mL), stirred in a sealed tube at 130 C for 12 h. [b] Yields
1 mol%
1
4
o
H
2
were determined by GC and GC-MS using biphenyl as the
internal standard.
1 4
Figure 5. TEM-EDS mapping images of Pd Ni @MIL-100(Fe) catalyst
Having verified the excellent performance of Pd
00(Fe) for the dehydrogenation reaction, we then turn to explore
the potential of our bimetallic catalyst system for the reverse
reaction (hydrogenation of quinoline). Initially, hydrogenation of
quinoline was performed under 1 atm hydrogen, and only
moderate yield was achieved (Table 3, entry 1), increasing the
1 4
Ni @MIL-
Furthermore, representative high-resolution TEM (HRTEM)
image shows that BMNPs have clear crystalline fringe patterns
and the lattice fringes align parallel to each other (Figure 4c). The
1
1 4
lattice spacing of Pd Ni @MIL-100(Fe) was measured to be 0.21
[
27]
nm, corresponding to the lattice distance of the {111} plane of
Pd-Ni alloy, which clearly confirmed the formation of Pd-Ni alloy.
Table 3. Optimization of the conditions for hydrogenation of quinoline.a
TEM-EDS mapping of the Pd
carried out (Figure 5), which clearly indicates the presence of Pd
and Ni in the catalyst. The amount of Pd in the Pd Ni @MIL-
00(Fe) catalyst was determined to be 0.54 wt.% by ICP-MS,
Pd/Ni ratio was measured to be 1/3.8 (Table S2, SI).
1 4
Ni @MIL-100(Fe) catalyst was also
1
4
1
Yield
o
After the promising initial results, subsequent efforts were
devoted to examining the applicability of this protocol (Table 2).
Entry
Catalyst
Tem. ( C)
H
2
(atm)
b
(%)
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1
1
Pd Ni
4
@MIL-100(Fe)
60
1
75
6
2g
100 (1g)
2
3
4
5
6
7
8
9
Pd
1
Ni
Ni
4
@MIL-100(Fe)
@MIL-100(Fe)
80
100
80
80
80
60
60
60
1
1
1
1
1
4
4
4
76
81
5
7
2h
2i
87 (1h)
100 (1i)
Pd
1
4
8b
Pd
Pd
Pd Ni
Ni @MIL-100(Fe)
1
Ni
Ni
/Nb
4
/Al
2
O
3
a
Reaction conditions: substrate (0.1 mmol),
Pd Ni @MIL-100(Fe) (catalyst amount was measured
2
based on Pd), H O (1 mL), stirred under 4 atm H at 60 C
1 mol%
1
4
/SiO
2
59
83
100
82
<5
1
4
o
2
1
4
2 5
O
for 12 h. Yields were determined by GC and GC-MS using
biphenyl as the internal standard the reaction temperature
b
Pd
1
4
o
was 80 C.
Pd@MIL-100(Fe)
Ni@MIL-100(Fe)
1 4
To clarify the heterogeneous nature of Pd Ni @MIL-100(Fe)
catalyst, a hot filtration test was performed during dehydro-
genation process (Figure 6). The catalyst was filtered in the
middle of the reaction, and it was confirmed that no reaction
proceeded in the filtrate that was obtained at two different times.
No obvious Pd leaching was observed by ICP analysis of the
filtrate, which clearly confirmed that active specie in the catalytic
system is the heterogeneous metallic nanoparticles.
[
(
1
a] Reaction conditions: quinoline (0.1 mmol), 1 mol% Pd
catalyst amount measured based on Pd), H
2 h. [b] Yields were determined by GC and GC-MS using biphenyl as the
internal standard
1
Ni
4
@MIL-100(Fe)
2 2
O (1 mL), stirred under H for
reaction temperature do not find obvious improvement of the
reaction yield (entries 2~3). As a comparison, Pd-Ni BMNPs
supported on other supports were also tested in the
hydrogenation reaction (entries 4~6). The reaction results clearly
indicates that Lewis acidity of supports have an obvious
enhancement on the catalytic performance. Similar catalytic
efficiency of Pd-Ni@MIL-100(Fe) and Pd-Ni/Nb
again confirmed our proposal that excellent catalytic
performance of the Pd Ni @MIL-100(Fe) catalyst could be
2 5
O (entries 2 vs
6)
1
4
assigned to the cooperation between Lewis acidity and excellent
texture properties of MIL-100(Fe).
Further reaction optimization revealed that full conversion
was reached when increasing the hydrogen pressure to 4atm
(
entry 7), implying that our Pd-Ni bimetallic catalytic system is able
to do both the dehydrogenation and the opposite reactions in the
same solvent. Hydrogenation reaction catalyzed by monometallic
nanoparticles were also carried out, and it was confirmed that Pd
was the active site for both dehydrogenation and hydrogenation
reaction (entries 8~9). Scope of the hydrogenation reaction was
Figure 6. Hot filtration test results. Red line, time=2 h, and blue line, time=4 h.
Finally, studies were also carried out to assess the reusability
of the catalyst. Catalyst was filtrated from the reaction mixture
upon completion of the reaction, washed with ethanol and water
and then directly reused in the next cycle. As shown in Figure 7,
the catalyst can be reused for at least six times to fulfill three
dehydrogenation and hydrogenation cycles without significant
loss in catalytic performance. No obvious agglomeration was
observed in the TEM imagines of the recycled catalyst (Figure S1,
SI), excellent catalytic stability of the catalyst could be ascribed to
the porous structure and the strong water stability of MIL-100(Fe).
also studied in the presence of Pd
1 4
Ni @MIL-100(Fe) catalyst
(
Table 4), quantitative hydrogenation of quinoline derivatives was
realized under optimized conditions (entries 1~5). What’s more,
this catalytic system was also suitable for the hydrogenation of
other heterocycle compounds (entries 6~8), which may also find
widely applications in the production of commodity chemicals,
pharmaceuticals, and agrochemicals.[28]
Table 4. Substrate scope of the hydrogenation of quinoline and other
a
heterocycle derivatives.
Entry
2
R
Yield (%)
1
2
3
4
5
2a
2b
2c
2d
2e
H
100 (1a)
91 (1b)
94 (1c)
87 (1d)
85 (1e)
2-Me
8-Me
8-NH
2
6-OH
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ChemCatChem
10.1002/cctc.201801311
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solution was heated to 60 oC under H
for 12 h. After the reaction, the
Figure 7. Recycling of the Pd
1
Ni
4
@MIL-100(Fe) catalyst during the dehydro-
2
gennation and hydrogenation process.
solution was allowed to cool to room temperature and extracted with ethyl
acetate (3*1 mL), GC and GC-MS spectrum were recorded to determine
the yield of the products using biphenyl as the internal standard.
Conversions for each reaction were calculated from the relative GC peak
area integrations.
Conclusions
In summary, we firstly combine BMNPs and MOFs for efficiently
reversible dehydrogenation and hydrogenation of quinoline
derivatives in the absence of external additive using water as a
sustainable solvent. Switchable transformation between THQs
and quinoline could be achieved via tuning the temperature and
hydrogen pressure in the presence of the same catalyst, which
endows great potential value of Pd-Ni@MIL-100(Fe) catalyst in
synthetic chemistry. An excellent catalyst stability was also
observed during the reaction process, no evident decrease in
catalytic activity was approached after six cycles. “Synergistic
effects” between Pd and Ni could be assigned to the “ensemble
effect” to stabilize Pd atoms and the excellent affinity of Ni to
hydrogen. Functional synergy between BMNPs and MIL-100(Fe)
could be ascribed to stabilization of MIL-100(Fe) for BMNPs,
electronic interaction between MIL-100(Fe) and BMNPs and
Acknowledgements
We gratefully acknowledge Chinese Postdoctoral Science
Foundation (2015M571761, 2016T90465) for financial support.
This work was a project funded by the Priority Academic Program
development of Jiangsu Higher Education Institution. We also
thank Wan-yin Tang at Analysis and Test Center of Nanjing
University & Technology for the help in obtaining the FT-IR data.
Conflict of interest
2
strong absorbability of MIL-100(Fe) for substrates and H . With
the investigation on the host−guest cooperation, especially
electronic effects between BMNPs and MOFs, we believe that the
combination of BMNPs and MOFs will bring more possibilities for
catalyst development.
The authors declare no conflict of interest.
Keywords: Bimetallic nanoparticles • Metal–organic frameworks
•
Reversible • Dehydrogenation • Hydrogenation
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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A recyclable catalyst, Pd-Ni bimetallic
nanopar-ticles encapsulated in MIL-
Jia-wei Zhang, Dan-dan Li, Guo-ping
Lu*, Tao Deng and Chun Cai*
100(Fe), is fabricated and applied in
acceptorless reversible dehydrogena-
tion and hydrogenation of quinoline
derivatives. Stabilization of MIL-
Page No. – Page No.
Reversible Dehydrogenation
and
Hydrogenation of N-Heterocycles
Catalysed by BMNPs Encapsulated in
MIL-100(Fe)
100(Fe) for BMNPs, absorptivity of
MIL-100(Fe) for reactants and gas
together with the electronic effect
between BMNPs and MIL-100(Fe) all
contribute to outstanding catalytc
performance of Pd
catalyst.
1 4
Ni @MIL-100(Fe)
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