Improved activity of palladium nanoparticles using a sulfur-containing metal-organic framework as an efficient catalyst for selective aerobic oxidation in water

Article abstract of DOI:10.1039/c7nj00709d

A simple and efficient nanostructured catalyst system was developed, comprising Pd nanoparticles stabilized by thiophene groups in a metal-organic framework (MOF). Pd species were deposited into the DUT-67(Zr) structure and reduced to nanoparticles that could be stabilized by the synergistic effect of thiophene groups and confinement in the MOF pores. Increasing the interaction time for palladium entry into the MOF cavities led to more effective incorporation into the metal-organic framework, which produced a more efficient catalytic system. Reduction was performed using NaBH₄ or N₂H₄. The catalysts were characterized by N₂ adsorption-desorption analysis, infrared spectroscopy, powder X-ray diffraction, scanning electronic microscopy, and transmission electron microscopy. Selective aerobic oxidation of alcohols in neat water under an O₂ atmosphere was achieved with 0.5%Pd/DUT-67(Zr) containing Pd nanoparticles created using NaBH₄. Furthermore, the catalyst could be easily separated from the reaction mixture by simple filtration and reused at least ten times without any significant loss in catalytic efficiency under the investigated conditions.

Full text of DOI:10.1039/c7nj00709d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Improvement activity of palladium nanoparticles using a sulfur-
containing metal–organic framework as an efficient catalyst for
selective aerobic oxidation in water
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
DOI: 10.1039/x0xx00000x
S. Abedi, and A. Morsali*
www.rsc.org/
Simple and efficient nanostructured catalyst system comprising Pd nanoparticles stabilized by thiophene-groups of a
metal-organic framework (MOF) was developed. The Pd species were deposited into the DUT-67(Zr) structure and reduced
to nanoparticles that can be stabilized by the synergism effect of thiophene groups and confinement effect of the MOF
pores. Increasing the interaction time of palladium entrance into the MOF cavities leads to more effective connecting
them to the metal-organic framework and produced more efficient catalytic system. Reduction process was performed
using NaBH
powder X-ray diffraction (PXRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM).
Selective aerobic oxidation of alcohols in neat water under O atmosphere was addressed with 0.5%Pd/DUT-67(Zr)
including Pd nanoparticles created using NaBH . Furthermore, the catalyst could be easily separated from the reaction
4 2 4 2
and N H . The catalysts were characterized by N adsorption-desorption analysis, infrared spectroscopy (IR),
2
4
mixture by simple filtration and reused at least ten times without significant loss in catalytic efficiency under the
investigated conditions.
Introduction
Although palladium catalyzed oxidation reaction of alcohols
has a long and ancient history, but the capabilities and
expertise of the metal and the importance of this type of
reaction among chemical reactions, has led to continue efforts
to perform more efficient oxidation reaction. Among the
catalyst systems have been proposed to date, the design of
heterogeneous systems to support the palladium
nanoparticles have been notable successes in this field. During
the past decade, great progress has been made in
heterogeneous Pd catalysts for oxidation processes, for
1
2,
3
example, Pd/hydroxyapatite , Pd/mesoporous silicas
,
,
4
polymer-supported Pd , carbon nanotubesupported Pd
, 5
6, 7
Scheme 1. Entrapment of Pd NPs into the sulfur-containing pores of
DUT-67(Zr) for aerobic oxidation reaction
8
Pd supported on metal oxide . Recently applications of metal-
organic frameworks (MOFs) in heterogeneous catalysis
especially those as a support for metal nanoparticles (e.g., Pd,
Au, Ru, and Pt) have attracted extensive attention owing to
workers developed a heterogeneous Pd catalyst deposited on
2
a zeolite-type MOF, MIL-101, using a simple colloid method.
2
9
-20
The 0.35% Pd/MIL-101 has been shown to be highly efficient in
the liquid-phase aerobic oxidation of a variety of alcohols in
their high surface area, porosity, and chemical tunability.
The most important challenge of these heterogeneous systems
is to form palladium nanoparticle in the right active sizes and
their stabilization during relatively harsh oxidation reaction
conditions. Structural diversity of MOFs provides the
opportunity of designing more improved base-free systems
the absence of base in toluene at 80
affirmed that the Co/C-N composite prepared by calcination of
triazine containing Co-MOF can be an efficient
heterogeneous catalyst for base-free aerobic alcohol oxidation
in water at 110 C. Moreover, using Zr-MOF containing thiolic
̊C . This group also
a
2
1, 22
̊
bearing amine functional organic linkers
. Li and co-
group linkers, Xu and co-workers developed a heterogeneous
system in which Pd (II) species anchor to the framework by
poisoning effect of thiol groups dangling within the MOF
2
3
pores. The Suzuki-Miyaura couping reaction was performed
with the catalyst in EtOH medium at 80 C. To create larger
̊
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pore opening that causes less diffusion limitations, DMTA thiophenedicarboxylic acid (H
Journal Name
2
TDC, 0.67 mmol) was added to the
(dimercapthoterphenyldicaboxylate) was used as elongated mixture and sonicated for another 5 min. Then, acetic acid (3.5 mL)
building block. It has been proved that while Pd(0) atoms was added to the solution and it was sonicated for a further 10 min
formed during the reaction, they combine to make less active and then transferred to a 100 mL Tefon-lined autoclave and heated
Pd nanoparticles when the catalyst was placed in air. In at 120
another work by Wang and co-workers, Zr-MOF bearing was filtered off and washed several times with DMF and CH
thiophenedicarboxylic acid known as DUT-67, used for resultant product was dried at 80 C under in air.
̊
3
̊
2
4
supporting Pd nanoparticles. 0.5%Pd@DUT was introduced
as the most suitable Pd loading for the Suzuki coupling
Synthesis of Pd/DUT-67 (1). Loading of Pd was performed
24, 28
according to previous report
by some changes. Typically, for
(1), 8.5 mg (0.05 mmol) PdCl
2
reaction in the mixed water-ethanol solvents, at 70
Nitrobenzene hydrogenation at 60 C. Pd NPs was formed
before the catalytic reaction using NaBH , as reducing agent.
̊C , and
preparation of 0.5%Pd/DUT-NaBH
4
̊
powder and 1.5 mg of NaCl was dispersed in 10 mL DMF through
sonication for 10 min. Then, MeOH (1 ml) and H O (0.5 ml) was
4
2
The catalyst was recycled 4 times with slight decrease in its
reactivity for coupling reaction, but there is no report of hot
filtration or ICP analysis to clarify the exact fate of Pd NPs.
Principally, water is an environmentally benign medium
especially for safety and scalable catalytic systems. It also can
introduce many other benefits such as reaction and selectivity
improvements, simplification of the work-up procedures and
added and the mixture was further sonicated and stirred till a clear
orang solution was obtained. In another container the as-prepared
DUT-67 (0.5 g) was dispersed in 8 mL DMF and stirred for 20 min in
a 25 mL beaker. Then, both solutions were mixed in a flask and
stirred for a further 1h. Subsequently, NaBH4 (20 mg for 0.5%
Pd/DUT-67) was added dropwise to the mixture. Immediately, the
colour of the solution changed from orange to dark grey. The
product was collected by filtration and washed with DMF several
25
mild reaction conditions. Generally, gold based catalysts are
the ideal choice in water, since they do not suffer from
deactivation in the presence of oxygen. Some oxidations of
benzylic alcohols in water as solvent were also reported using
3
times and finally with CH CN. The resultant products were dried
using a desiccator under vacuum for 3 days. The Pd content in the
sample was ca. 0.47 wt% based on ICP analysis.
2
6
copper-based catalysts. Here, we try to investigate Pd
NPs/DUT-67 as the catalyst for aerobic oxidation reaction of
alcohols. Step by step survey of the conditions for Pd loading
on the thiophene-containing MOF, led to develop a seldom
heterogeneous Pd catalyst system for selective aerobic
oxidation of alcohols in water. According to our studies, there
is no report for water-tolerant oxidation catalyst of Pd NPs on
MOF support for this reaction in water.
Identical procedures were followed to deposit Pd nanoparticles
4
onto the DUT-67 for the synthesis of 2%Pd/DUT-NaBH (1) unless 4
times amount of all materials was used. The Pd content in the
sample was ca. 1.8 wt% based on ICP analysis.
0
.7%Pd/DUT-N
(0.5 ml) was used as reducing agent instead of NaBH
material produced in this way was denoted as 0.7%Pd/DUT-N
2
H
4
(1): All details were the same as above unless
N
2
H
4
4
. The
2
H
4
(
1), because the ICP analysis confirmed the 0.74 wt% of Pd content
deposited onto the MOF structure.
Experimental Section
Synthesis of Pd/DUT-67 (2). For preparation of 0.5%Pd/DUT-NaBH
2), 8.5 mg (0.05 mmol) PdCl
dispersed in 10 mL DMF through sonication for 10 min. Then, MeOH
1 ml) and H O (0.5 ml) was added and the mixture was further
4
(
2
powder and 1.5 mg of NaCl were
Materials
2
,5-thiophenedicarboxylic acid (H
chloride (ZrCl from Aldrich) and all alcohols (from Merck) N,N-
dimethylformamide (DMF, from DAE JUNG Reagents Chemicals),
CH CN (from Merck) were used without additional purification.
2
TDC, from Aldrich), Zirconium(IV)
(
2
4
sonicated and stirred till a clear orang solution was obtained. In
another container the as-prepared DUT-67 (0.5 g) was dispersed in
3
8
mL DMF and stirred for 20 min in a 25 mL beaker. Then, both
Toluene (from Merck) was distilled, and dried by refluxing in the
presence of sodium under the argon atmosphere and transferred
into the reaction bottle through a syringe.
solutions were mixed in a flask and stirred for a further 1h. The
argon gas was purged into the solution and the beaker was sealed
4
and stirred for another 24 h. Subsequently, NaBH (20 mg for 0.5%
Instrumentation
Pd/DUT-67) was added dropwise to the mixture. The dark grey
mixture was stirred for 1h. The product was collected by filtration
Transmission electron microscopy (TEM) analyses were performed
on a Philips CM 200FEG microscope. Powder X-ray diffraction
and washed with DMF several times and finally with CH
resultant products were dried using a desiccator under vacuum for
days. The Pd content in the sample was ca. 0.51 wt% based on ICP
analysis.
3
CN. The
(PXRD) data were acquired on a Philips X-Pert diffractometer using
3
Cu Kα radiation. SEM images were captured on Philips XL-300
instrument. The nitrogen sorption isotherms were collected by
Belsorb (BELMAX, Japan). The GC analyses were performed on Aerobic oxidation reaction using Pd/DUT-67 catalysts
Echrom A90 using a flame ionization detector (FID).
A mixture of alcohol (0.5 mmol), base (0.5 mmol) and catalyst (0.04
Catalyst preparation
4
g, ca. 0.3 mol% of Pd for 0.5%Pd/DUT-NaBH (1) and 0.4 mol% of Pd
for 0.5%Pd/DUT-NaBH (2) ) in dry toluene (or other solvents, 4 ml)
4
Synthesis of DUT-67. DUT-67 was synthesized according to the
2
3, 24, 27
was prepared in a two-necked Schlenk flask. The flask was then
evacuated, refilled with pure oxygen. The resulting mixture was
references.
4
Typically, 230 mg solid powder of ZrCl (1 mmol)
was dissolved in a mixture of DMF (12.5 mL) and NMP (12.5 mL)
and was sonicated for 10 min. Subsequently, 110 mg of 2,5-
stirred at 80 ̊C under an oxygen atmosphere. Before analysing with
2
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Table 1. Lattice parameters of the catalyst structures prepared in the
absence or presence of different amounts of Pd species on DUT-67.
S
BET
2
D
micro
D
meso
V
meso
3
Sample
-1
a
b
c
-1 c
(m g )
(nm)
0.34
0.34
0.35
(nm)
(cm g )
DUT-67
642(748)
593(701)
702(842)
1.5 (2.5)
1.5 (2.7)
1.5 (2.8)
1.8 (2.9)
1.6 (2.7)
1.5 (2.6)
0.27
0
2
0
0
0
.5% Pd/DUT-NaBH
4
(1)
0.27
% Pd/DUT-NaBH (1)
4
0.33
.5% Pd/DUT-NaBH
4
4
(2)
650 (762) 0.32
187 (236) 0.36
0.29
.5% Pd/DUT-NaBH
(2R)
0.09
.5% Pd/DUT-N
2
H
4
(1)
736(865)
0.34
0.31
(a) The Brunauer-Emmett-Teller (BET) surface area. (b) The misopore size
distributions were calculated using MP plot. (c) The mesopore pore volume
and the mesopore size distributions were calculated using the Barrett-
Joyner-Halenda (BJH) method of adsorbtion branch; data in parentheses are
calculated from desorption branch.
Fig. 1 PXRD pattern of the prepared DUT-67 after Pd loading
GC, 100 ml of cyclohexane as the internal standard was added.
Then, the organic layer was extracted with ethyl acetate,
subsequently subjected to GC analysis.
Results and discussion
DUT-67 (Zr) was prepared based on Kaskel report and its structure
confirmed by powder X-ray diffraction, PXRD, and scanning electron
27
microscope, SEM analyses. The powder XRD of the prepared
sample has been shown in Figure 1, accordingly indicated the high
agreement of the planes in as-synthesized MOF with the simulated
pattern of DUT-67. SEM images, Figure 2a, provided the both
hexagon and square shape planes of the polyhedral microcrystals
that are characteristics of DUT-67 (Zr). Two previous methods
mixed together and changed a little to use for Pd deposition onto
24, 28
the MOF structure.
section, PdCl powder was used as precursor of Pd in a mixture of
DMF, H O and MeOH solvents. In protocol (1), after addition of Pd
source into the mixture of MOF stirring in DMF, reducing agent,
NaBH was added one hour later. Two types of Pd-supported MOF
were prepared at the ratio of 0.5% and 2% for Pd loading which was Fig. 2 Scanning electron microscope (SEM) images of a) DUT-67 (Zr); b) 0.5%
measured by ICP analysis, denoted as 0.5%Pd/DUT-NaBH
(1) and Pd/DUT-NaBH (1); c) 0.5% Pd/DUT-N (1); d) 2% Pd/DUT-NaBH (1); e)
%Pd/DUT-NaBH
(1). To investigate the effect of reducing agent, 0.5% Pd/DUT-NaBH (2); f) 0.5% Pd/DUT-NaBH (2R).
one another catalyst was prepared with 0.5% of Pd ratio (which
known as the best loading of Pd on DUt-67 for coupling reaction), The lattice parameters of the materials was examined by N
while N was used as reducing agent instead of NaBH
. It was adsorption-desorption analysis. As summarized in Table 1, the as-
named as 0.5%Pd/DUT-N
(1). Finally, we prepared 0.5%Pd/DUT- prepared DUT-67 has a BET surface area of 642 m g , which has
NaBH
(2) by considering the results of aerobic oxidation reaction been changed randomly for the prepared catalysts when Pd loaded
which will explain later. In this case, addition of the reducing agent, onto the net. In the case of 0.5%Pd/DUT-NaBH (1), insertion of the
NaBH
, was performed 24 h after the mixing of Pd source with the Pd species led to reducing the specific surface area to 592 m g .
MOF support under the inert atmosphere. All these four material Increasing the amount of Pd component to four times unexpectedly
were identified completely. Figure S1 (Electronic Supplementary augmented the average surface area in 2%Pd/DUT-NaBH (1).
As described in details in experimental
2
2
4
4
H
2 4
4
4
2
4
4
4
2
2
H
4
4
2
-1
2 4
H
4
4
2
-1
4
4
Information) showed the IR spectrum of the original DUT-67 and Comparison of SEM images of these two materials with bare MOF
the prepared materials after Pd loading. As it is clear, no represented in Figure 2, demonstrated this discrepancy. As it
remarkable changes was determined after Pd loading in the whole shown, when Pd content increased, some Pd species have been
samples. PXRD patterns of the materials compared in Figure 1, integrated with each other and a new separated cluster phase of Pd
confirmed that the crystallinity of the MOF remains intact after Pd was formed besides the MOF structure. The construction of this
loading using both procedures and both reducing agents. SEM new phase of Pd caused the augmentation of surface area for
images were also averred these analyses, Figure 2. The polyhedral 2%Pd/DUT-NaBH
microcrystals of the MOF support stayed stable during the Pd of Pd species in this case. Increasing the amount of palladium
(1) and actually indicated low efficient deposition
4
loading even in the case of 0.5%Pd/DUT-NaBH
little more, Figure 2e.
4
(2) that changed a content only creates larger particles that accumulate during the
reaction and inactive clusters are obtained. Moreover, reducing the
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4
catalytic activity of 2%Pd/DUT-NaBH (1) points to the crucial role of catalyst. Interestingly, oxidation reaction of benzylalcohol using this
palladium nanoparticles in the catalyst system. Change the method catalyst led to producing 75% of benzaldehyde in the best reaction
and further timing of the loading process of palladium ions onto the conditions that resulted 57% aldehyde yield in the presence of
support, resulting in better dispersion of Pd particles into the MOF 0.5%Pd/DUT-NaBH
and their more effective interaction with sulfur atoms. As it is clear to examine the more reaction conditions in the presence of the new
in SEM images of 0.5%Pd/DUT-NaBH (2), no Pd nanoparticles was prepared catalyst. As displayed in Table 2, replacing K CO with
accordingly observed on microcrystal surfaces of DUT-67, while Na CO and/or Et N greatly diminished the yield of aldehyde
more palladium content was detected by ICP analysis, 4 mmol/g Pd (entries 11 and 12). Using the solvent mixture of PhCH /PEG did not
for 0.5%Pd/DUT-NaBH (2) instead of 3 mmol/g for 0.5%Pd/DUT- also develop the catalytic system (13% yield, not shown in the Table
NaBH (1). Meanwhile, TEM image of the sample interestingly 2).
4
(1). This notable improvement encouraged us
4
2
3
2
3
3
3
4
4
displayed that Pd NPs significantly penetrated into the cavities and
deposited within the MOF interfaces, Figure 3b. A little increasing
the surface area of the MOF after the Pd loading confirmed that
these most likely sources of catalytic activity entered into
polyhedral cages of DUT-67. It will be showed that this small
amount of Pd NPs do not separate from the bed even after several
reaction runs causing the possibility of repeating the reaction in
several stages. An important point is that the average size of Pd NPs
deposited in these two samples is not different, 12-17 nm.
Meanwhile, comparison of TEM images of these samples clearly
indicated more uniformity for nanoparticles in the case of
0
.5%Pd/DUT-NaBH (2), Figure 3a and 3b.
4
The type of reducing agent is also very important. 0.5%Pd/DUT-
N H (1) was just prepared in a similar procedure to 0.5%Pd/DUT-
2
4
NaBH (1). The lower catalytic reactivity of the former is may be due
4
to different kinetics of the reduction process by these two reagents
NaBH Vs N H . In general, the type of reducing agent affected the
4
2 4
final properties of the nanoparticles. The two reducing agents have
different reduction potentials in solution, i.e. 1.24 V for NaBH Vs
4
29-32
1
.15 V for N H .
Higher reduction power of NaBH4 than
2
4
hydrazine probably increases the formation of smaller catalytically
active Pd nanoparticles versus larger inactive Pd clusters using the
former reagent.
Aerobic oxidation reaction with these Pd supported DUT-67
catalysts was performed using benzyl alcohol as substrate. The
results of experimental reactions summarized in Table 2. As it
shows surprisingly, no detectable oxidation product was formed
using 2%Pd/DUT-NaBH4 (1) that confirmed the obtained
observation based on the construction of deactivated Pd clusters
(
Table 2, entry 2). Moreover, 0.7%Pd/DUT-N H (1) also indicated a
2 4
very weak catalytic performance. While about 57% of benzaldehyde
produced using 0.5%Pd/DUT-NaBH (1), only 3% of this product was
4
formed using 0.7%Pd/DUT-N H (1) in the same reaction conditions
2
4
(
Table 2, entries 1 and 3). This data actually introduced the former
as the best catalyst among three samples synthesized via protocol
1). Therefore, we tried to optimize the reaction conditions using Fig. 3 Transmission electron microscope (TEM) images of a) 0.5%Pd/DUT-
.5%Pd/DUT-NaBH (1). However, no remarkable improvement was NaBH (1), b) 0.5%Pd/DUT-NaBH (2) and c) the recovered catalyst,
happened using Et N and NaOH as base or replacing the solvent, 0.5%Pd/DUT-NaBH (2R).
with CH CN, DMF or NMP (Table 2, entries 4-8). Using the mixture Surprisingly, in the presence of water as solvent 82% of
of solvents, PhCH /PEG, 1:1, did not also improve the catalyst benzaldehyde was formed without any detectable benzoic acid
performance of the heterogeneous system (Table 2, entry 9). (more than 99% selectivity, entry 15). This significant result in the
Subsequently, we decided to produce some changes in deposition greenest solvent, O, has not been yet observed for
stage of Pd NPs onto DUT-67. So, as indicated before, using the heterogeneous oxidation of alcohols with Pd nanoparticles under
optimum amount of Pd precursor, 0.5%Pd/DUT-NaBH (2) was the O atmosphere with this kind of MOF catalysts. Substrate scope
synthesized via timing the mixture of MOF and Pd source to be of the reaction was indicated in Table 2 and demonstrated the
stirred for 24 h in DMF. After that, reducing agent, NaBH was reactivity of the other substrates according to their electronic
added and the same procedure was followed to get the final and/or hindrance character. The substrates bearing electron
(
0
4
4
4
3
4
3
3
H
2
4
2
4
4
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Table 2. aerobic oxidation reaction using the prepared Pd/DUT-67(Zr) samples in different conditions
Pd/DUT-67 (0.04 g)
CH OH
CHO
2
Solv., base, 75 °C
O2 (1 atm.)
X
X
a
Entry
1
Cat.
0.5%Pd/DUT-NaBH
Substrate
Solvent
Product
Time: Yield (%)
6
h: 23
4
(1)
PhCH
3
2
4h: 57
2
3
4
5
6
7
8
9
2%Pd/DUT-NaBH
0.7%Pd/DUT-N
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-NaBH
0.5%Pd/DUT-67(Zr)
0.5%Pd/DUT-NaBH (2)
0.5%Pd/DUT-NaBH (1)-PEG
2%Pd/DUT-NaBH (1)
4
(1)
(1)
(1)
PhCH
PhCH
PhCH
PhCH
CH CN
3
24h: ---
24h: 3
2
H
4
3
b
4
3
23h: 38
23h: 53
c
4
(1)
(1)
(1)
(1)
(1)
(2)
(2)
(2)
3
4
3
24h: 9.8
24h: 21
24h: 12
4
DMF
NMP
CH OH
2
4
CHO
d
4
PhCH
PhCH
PhCH
PhCH
PhCH
PhCH
3
-PEG
24h: 6
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
4
3
22h: 75
e
4
3
24: 53
f
4
3
24h: 8.2
24h: 2.5
24h: 8.3
24h: 82
24h: 4
4
(1)-PEG
3
3
4
H
2
H
2
H
2
O
O
O
4
4
24h: 2.5
H
3
C
CHO
H
3
3
C
2
CH OH
1
1
2
2
2
8
9
0
1
2
0.5%Pd/DUT-NaBH
4
(2)
(2)
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
10h: 100
8h: 100
24h: 3.5
15h: 74
15h: 7
H
CO
CH
2
OH
H
3
CO
CHO
0.5%Pd/DUT-NaBH
4
CH
2
OH
CHO
2 4
0.5%Pd/DUT-N H (1)
Cl
CH
2
OH
Cl
CHO
0.5%Pd/DUT-NaBH
4
(2)
(2)
O N
CH OH
O
2
N
CHO
2
2
0.5%Pd/DUT-NaBH
4
CH OH
CHO
2
Ph
Ph
2
2
2
3
4
5
0.5%Pd/DUT-NaBH
4
(2)
(2)
(2)
H
2
H
2
H
2
O
O
O
10h: 73
10h: 54
OH
O
0.5%Pd/DUT-NaBH
4
Ph
CH3
Ph
CH3
OH
O
0.5%Pd/DUT-NaBH
4
24h: 15
Ph
Ph
Ph
Ph
a
b
c
d
e
GC Yield. 0.5 mmol K
2
CO
3
as base. Using 0.5 mmol Et
3
N as base. Using 0.5 mmol NaOH as base. PhCH
3
/ PGE (200), 1:1. Using 0.5
f
mmol Na
2
CO
3
as base. Using 0.5 mmol Et
3
N as base; 3% of benzoic acid was also detected.
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ARTICLE
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micro/mesochannels of the catalyst system during the reaction
even after reducing the MOF crystallinity indicated by PXRD
pattern, Figure 1 and SEM image, Figure 2f.
The appealing point in this investigation is that no over oxidation to
acid formation was observed even after prolonged reaction time. In
addition, aerobic oxidation of alcohols using heterogeneous
3
5
systems comprising transition metal nanoparticles such as cobalt
3
6-38
39-41
platinum
and gold
nanoparticles in neat water solvent has
Fig. 4 The catalyst, 0.5%Pd/DUT-NaBH
frequent aerobic oxidation of p-methylbenzyl alcohol in water with high nanoparticles for aerobic oxidation of alcohols in neat water and O
4
(2) was recycled and reused in ten been developed. Heterogeneous catalyst based on palladium
2
conversion and selectivity.
atmosphere has been also developed using inorganic and organic
4
2
supports such as mesoporous carbon
and functionalized
4
3, 44
donating group react more efficiently than their containing polymers.
However, there is no report of this worth reaction in
electron-withdrawing group.
water using MOF structures.
What makes this catalyst as a specific MOF-base Pd heterogeneous
catalyst is its capability to proceed the partial oxidation reaction of
alcohols in water with high selectivity. As discussed above,
comparing the SEM and TEM images of this catalyst showed that
the Pd species could permeate into the cavities of MOF and Pd NPs
were formed within the interfaces of polyhedral microcrystals of
MOF. Furthermore, the light brown colour of the reaction solution
in the presence of 0.5%Pd/DUT-NaBH4 (1) compared with
Conclusions
Pd-catalyzed aerobic oxidation of alcohols has been addressed
using Pd nanoparticles entrapped into the thiophene-
containing MOF structure. DUT-67 (Zr) was loaded with
4 2 4
different amounts of Pd sources. Utilizing NaBH and N H as
reducing agent indicated that the best results were obtained in
the presence of 0.04 g of the catalysts (ca 0.4 mol% of the Pd)
prepared by the former. In addition, increasing the time of Pd
deposition into MOF structure afforded an efficient and
recoverable heterogeneous Pd catalyst which developed
selective aerobic oxidation in water.
0
.5%Pd/DUT-NaBH (2) is obviously evidence for the more effective
4
anchoring of Pd species into thiophene groups of the MOF
framework in the later, Figure S2 (Electronic Supplementary
Information).
For more investigation about the catalyst system, hot filtration test
was performed, consequently. The reaction of p-methylbenzyl
alcohol was run in the optimum conditions described in Table 2
using 0.5%Pd/DUT-NaBH (2). After 4 h of the reaction (47% yield), Acknowledgements
4
the solid phase of the reaction mixture was hot-filtered off. Then
This work was supported by Tarbiat Modares University.
the reaction was run again to continue without catalyst. Further
reaction took place after another 10 h and progressed only 6%
more (total conversion: 53%). Analysing the reaction solution by ICP
indicated that less than 0.02% of the Pd species had leached into
Notes and references
4
5
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
the reaction mixture. Hence, the contribution of local leaching
3
3, 34
induced homogeneous catalysis
and heterogeneous character
4
6
of the catalyst both play for the catalyst reactivity.
Disclosing the recyclability of 0.5%Pd/DUT-NaBH4 (2) were then
served using of p-methylbenzyl alcohol in the presence of twice
amount of the catalyst (The reaction time dropped to 6 h) in the
same reaction conditions indicated in Table 2. After each run, the
reaction mixture was let to settle down and the supernatant was
sintered off. The flask was charged by substrate, base and solvent
and the reaction let to run again. Surprisingly, the recovered
catalyst was successfully used in 10 subsequent reactions without
loss of catalytic activity and selectivity. Quantitative amount of the
corresponding aldehyde was produced with more than 99%
selectivity in all recycling runs, determined by GC analysis, Figure 4.
Figure 3c is a typical TEM image of the recovered 0.5%Pd/DUT-
1
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Table of Contents
Highlight
Synergism of thiophene and confinement of MOF with time of Pd entrance leads an efficient
catalyst for oxidation in water.