Pd(0)@UiO-68-AP: Chelation-directed bifunctional heterogeneous catalyst for stepwise organic transformations

Article abstract of DOI:10.1039/c6cc01194b

A bifunctional heterogeneous catalyst Pd(0)@UiO-68-AP based on a chelation-directed post-synthetic approach is reported. It exhibits typical heterogeneous catalytic behaviour and can promote benzyl alcohol oxidiation-Knoevenagel condensation in a stepwise way.

Full text of DOI:10.1039/c6cc01194b

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Pd(0)@UiO-68-AP: chelating-directed bifunctional heterogeneous
catalyst for stepwise organic transformations
Received 00th January 20xx,
Accepted 00th January 20xx
Yan-An Li, Song Yang, Qi-Kui Liu, Gong-Jun Chen, Jian-Ping Ma and Yu-Bin Dong∗
DOI: 10.1039/x0xx00000x
www.rsc.org/
reduction. In addition, such chelating-directed approach would
allow the active metal presurors to be readily implanted at the
chelating positions to result in a well-proportioned metal
A bifunctional heterogeneous catalyst Pd(0)@UiO-68-AP based on
a
chelating-directed post-synthetic approach is reported. It
exhibits a typical heterogeneous catalytic nature and can promote
benzyl alcohol oxidiation-Knoevenagel condensation in a stepwise
way.
species binding, consequently,
metal@MOF matrix.
a uniformly distributed
More importantly, such MOFs-based composite catalyst
frication would allow for fine-tuning of solid materials with
multifunctional catalytic behaviors by combination of the
different types of active species. So such kind of
heterogeneous composite catalysts might be used to perform
several steps in a reaction sequence. Although great efforts
have been made in this field, general and facile method that
can easily introduce the active catalytic species into MOF
matrix are still imperative.
In this contribution, a new composite solid catalyst of
Pd(0)@UiO-68-AP which is prepared by chelating-directed
post-synthetic approach is reported. By combining the aerobic
oxidation activity of the encapsulated Pd NPs and Knoevenagel
condensation activity of the Zr-Lewis acid sites and generated
secondary amino moiety on UiO-MOF, the obtained
Pd(0)@UiO-68-AP exhibits a bifunctional catalytic behaviour
and can promote benzyl alcohol oxidation-Knoevenagel
condensation in a stepwise way.
Heterogeneous catalysis, owing to the features of cost savings
and eco-friendliness, has received more and more attention
during the last decade. Composite type of catalytic materials is
one of the most important solid catalysts for heterogeneous
catalysis.1 For creating composite catalysts, various inorganic
2
4
materials, such as CeO2 SiO2,3 ZrO2 and porous carbon,5 and
organic materials, such as polystyrene,6 amphiphilic resin7 and
copolymers,8 have been utilized as supports to immobilize
active catalytic metal species, especially the active catalytic Pd
nanoparticles (Pd NPs).9 In contrast, the use of inorganic-
organic hybrid materials as a supporting matrix to load active
catalytic entities have not yet attracted much attention.10
Among various heterogeneous catalyst carriers, metal-
organic frameworks (MOFs) can be considered a new type of
supports for the fabrication of heterogeneous composite
catalysts.11 It is well known that the catalytically active species,
such as active metal nanoparticles, could be incorporated into
MOF matrix by solid grinding,12 vapour deposition,13 solution
infiltration14 and microwave-assisted diffusion.15 Recently,
Lin16 and Cohen’s17 groups reported that the catalytic metal
centers can be introduced into MOFs by a chelation-assisted
approach. For example, direct assembly of the metalloligands
with bpy-metal or ppy-metal moieties, or encapsulation of
metal center at reservation open chelating sites on the
framework rungs by post-synthetic method. It is anticipated
that the bidentate chelating coordinating sites should tightly
bind and stabilize active metal precursors; thus, generating a
stable, but highly unsaturated nano metal particles after
Scheme 1. Synthesis of UiO-68-NH2, Pd(II)@UiO-68-IP (
).
1) and Pd(0)@UiO-68-AP
(
2
UiO-68-NH2 was prepared as yellowish crystalline solids
according to the reported method (ESI).18 Pd(II)@UiO-68-IP (
1
)
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was obtained by the amine-aldehyde condensation and post-
synthetic metalation. As shown in Scheme 1, UiO-68-NH2 was
treated with excess of pyridine-2-carboxaldehyde in EtOH at
70°C for 10 h to afford UiO-68-IP with 2-iminopyridine (IP)
chelating coordination sites (Yield, 80% based on elemental
analysis, ESI). After separating the solids by centrifugation and
washed with fresh EtOH, the precipitate was mixed with
Pd(NO3)2 by ultrasound in CH3CN, and the mixture was heated
at 70°C for additional 1 h to generate
crystalline solids. Pd(0)@UiO-68-AP ( ) was synthesized by the
reduction of with NaBH4 in aqueous solution along with a
1 as yellow brown
2
1
distinct colour change (from yellow-brown to dark brown)
(Scheme 1). The imino group was also reduced to the
corresponding secondary amino group along with the Pd(II)
reduction because of the large excess of NaBH4, which is
unambiguously demonstrated by the MS analysis (Fig. S1).
IR spectrum of UiO-68-NH2 shows that the weak double
peaks at 3336 and 3297 cm-1 are attributed to the symmetrical
and asymmetrical vibration adsorption of amino group. In
addition, the peaks at 1599 and 831 cm-1 corresponding to
primary amino out-of- and in-plane bending vibrations are also
observed.19 These characteristic amino peaks are basically
disappeared after the Schiff base condensation. Meanwhile, a
new sharp peak associated with N=C stretching vibration at
Fig. 2 Top: SEM and elemental maps of Zr and Pd images of 2. Bottom: EDS
pattern of 2.
Scanning electron microscope (SEM) shows that the
octahedral shape of UiO-68-NH2 crystals was maintained after
post-synthetic modification (Fig. 2). Inductively coupled
plasma (ICP) measurement of the Zr : Pd ratio of digested
metalated MOF is 1 : 0.68 (5.1 wt % Pd loading). The energy-
dispersive X-ray spectrum (EDS) measurement (Fig. 2) shows
that the uploaded zero-valent palladium homogeneously
distributes in 2. It is known that the active catalytic metal
1604 cm-1 appeared in
1, indicating the formation of
precursors loading based on traditional impregnation method
is variable, and it can be significantly affected by the soaking
time, temperature and many other subtle factors.21 This would
lead to an unrepeatable catalyst preparation process,
consequentially, an inaccurate the catalyst evaluation.
Chelation-assisted Pd loading herein might be an alternative
approach to overcome the drawback caused by the traditional
impregnation. On the other hand, the generated Pd NPs could
be stabilized by the introduced multitudinous nitrogen donors
in the framework. The Pd-loading on UiO-68-NH2 crystals by
traditional impregnation was also tested under the same
conditions. The ICP measurement indicates that the Zr : Pd
iminopyridyl moiety (Fig. S1). After reductive reaction, a new
weak single peak at 3380 cm-1, together with 1595 cm-1, clearly
indicates the formation of the secondary amino group (Fig. S1).
For further confirmation of this post-synthetic metalation,
the metalation of the esterified 2-iminopyridyl group attached
organic linker was examined before the synthesis of MOF. The
reaction proceeded very smoothly under the same reaction
conditions and the expected metalation product was well
confirmed using MS spectrum (Fig. S2).
ratio is only 1 : 0.08. The existence of Pd(0) in
2 was further
confirmed by the X-ray photoelectron spectroscopy (XPS) (Fig.
S3). It is similar to UiO-68-NH2, thermogravimetric analysis
(TGA) indicated that the initial weight loss of
1
and
2 occurred
at ca. 125°C, but with a more smoothly weight loss processes
(Fig. S4).
Selective catalytic oxidation of benzyl alcohols to
corresponding benzaldehydes is one of the most important
issue in modern synthetic chemistry and chemical industry.22
Hitherto a great deal of research effort has been devoted to
Fig. 1 Left: XRPD patterns of simulated17 and measured UiO-68-NH2,
Right: HRTEM image of
1 and 2.
2.
finding novel and efficient catalysts for this type reactions.
2
X-ray powder diffraction (XRPD) indicated that the
crystallinity of UiO-68-NH2 was maintained in both and
herein showed excellent activity in aerobic oxidation of
benzylic alcohol to benzaldehyde.
1
2
after the post-synthetic metalation reactions (Fig. 1), which
supports that UiO-68-NH2 was chemically stable during the
palladium loading and following reducing process. To get a
further insight into the structure of
2, high resolution
transmission electron microscopy (HRTEM) was used to
investigate the dispersion and size distribution of the Pd NPs in
2. HRTEM analysis revealed that the Pd NPs were crystalline
and highly dispersed with an average particle size of ≤2 nm (Fig.
1). The lattice fringes with interplanar spacing of 0.24 nm,
corresponding to 1/3 (4 2 2) fringes of face-centered cubic (fcc)
Pd.20
Fig. 3 Left: Reaction time examination (black line) and leaching test (red line) for
benzyl alcohol reaction catalyzed by 2. Right: recycling catalytic test.
2 | J. Name., 2012, 00, 1-3
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aReaction conditions: benzaldehyde (1mmol), malononitrile (1.2 mmol), catalyst
(1 % with respect to the amount of benzaldehyde). a based on GC analysis.
Table 1. Oxidation of benzyl alcohol in different solvent systems at different
temperature in air
On the other hand, the [Zr6O4(OH)4(CO2)12] cluster nodes in
2
possesses the coordination-unsaturated Zr atoms, together
with the generated amino moieties, they should both serves as
active catalytic sites to promote Knoevenagel reaction, which
is an important C-C coupling reaction and widely used in the
synthesis of fine chemicals.23 In a typical reaction, a MeOH
solution of benzaldehyde and malononitrile (1 : 1.2, molar
conv.b
(%)
select.b
(%)
>99
entry
T (°C)
80
cat./solv.
t (h)
32
32
24
20
12
24
30
1
1
2
3
4
5
6
UiO-68-NH2/xylene
∼2.6
ratio) in the presence of
2 (1 mol %) in MeOH was stirred at
60
2
2
2
2
/xylene
/xylene
/xylene
/xylene
∼2.3
>99
>99
>99
>99
>99
>99
room temperature, the GC analysis (ESI) indicated that the
condensation reaction was finished in 7 h to afford 2-
benzylidenemalononitrile as the expected product with the
80
>99
>99
>99
>99
>99
100
120
80
excellent conversion (> 99%) and selectivity (> 99%) (Table 2,
2
/toluene
entry 1).
80
2/CH3CN
a
Reaction conditions: air, benzyl alcohol (1.0 mmol), catalyst (1 mol % Pd),
solvent (2 mL). b based on GC analysis.
Using
2
(1 mol% in Pd), nearly quantitative yield of
C (Table 1,
benzaldehyde was achieved in 12-30 h at 80-120
°
entries 2-4) in different solvent systems. By comparison, UiO-
68-NH2 gave very low conversion under the same reaction
conditions (Table 1, entry 1). As shown in Table 1, it seems that
all the solvents used such as xylene, toluene and CH3CN are
suitable for this oxidation reaction, but took different reaction
time. Based on an overall consideration of various factors,
Fig. 4 Left: recycling catalytic test. Right: reaction time examination (black line)
and leaching test (red line) for Knoevenagel reaction catalysed by
2.
2
catalysed Knoevenagel reaction herein is very sensitive to
the solvent systems. As shown in Table 2, MeOH is the best
solvent medium for this reaction (entry 1). The conversions in
temperature (ESI). In order to confirm the heterogeneous weak polar solvent are very low or even no expected
toluene and 80°C were chosen as the reaction medium and
condensing product was detected (entries 2-6). It is
noteworthy to point out that UiO-68-NH2 exhibits the same
catalytic activity as that of 2 for Knoevenagel condensation
(entry 7 and ESI), indicating the effective catalytic site for this
reaction is Zr-cluster cores and amino groups instead of the
encapsulated Pd NPs. Again, 2 can be reused. As indicated in
nature of
2, a hot filtration experiment was performed,
removing the catalyst by filtration after 7.5 h of the reaction
(Fig. 3). Time-dependent gas chromatography (GS) indicated
no additional conversion to benzaldehyde was observed (up to
30 h) after filtration. In addition,
recyclability without a detectable decrease in yields (
over five catalytic cycles (Fig. 3). After each run, can be easily
2
exhibits excellent
>
99 %)
Fig. 4, after five consecutive catalytic runs, the conversion is
still above 99 %, indicating no degradation in catalytic activity
2
of
2 during the reusable process. Moreover, 2 was
recovered by centrifugation and directly reused in the next run
under the same reaction conditions. The crystallinity of the
MOF was maintained after each cycle, as confirmed by XRPD
pattern (Fig. S5).
demonstrated to be highly crystalline from XRPD patterns even
after five catalytic runs (Fig. S6). Besides, the heterogeneity of
2
solution was removed quickly from the solid catalyst and
transferred to another reaction vessel under the same
conditions at 2 h. No further increase in the conversion of
condensing product was observed during following 7 h in the
was also confirmed the hot filtration test. The reaction
Table 2. Catalytic data for the Knoevenagel condensation of malononitrile with
benzaldehyde in different solvent systemsa
absence of
2 (Fig. 4).
Based on above observation,
2
could be a bifunctional
catalyst24 to promote benzyl alcohol oxidation and the
following Knoevenagel condensation in due succession.
When a toluene solution (2 mL) of benzyl alcohol (1.0 mmol)
conversionb
selectivityb
entry
cat.
solvent
T (°C)
(%)
>99
0
(%)
>99
0
1
2
3
4
5
6
7
2
MeOH
CH2Cl2
CH3CN
toluene
DMF
r.t.
r.t.
r.t.
r.t.
r.t.
50
in the presence of 2 (1 mol%) was heated at 80°C for 30 h, a
MeOH solution (2 mL) of malononitrile (1.2 mmol) was added
to above system after cooling down to the room temperature.
2
2
0
0
2
0
0
The mixture was stirred for additional
temperature to afford 2-benzylidenemalononitrile with
excellent conversion ( 99%) and selectivity ( 99%) (Table 3,
8 h at room
2
88.5
25.4
>99
>99
>99
>99
2
toluene
MeOH
>
>
UiO-68-NH2
r.t.
entry 1, ESI). ICP measurement showed that the Zr : Pd ratio is
1 : 0.67 after the oxidation-condensation reactions, indicating
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no Pd leaching during the reaction process. Again, the 21475078 and 21271120), 973 Program (Grant Nos.
comparison of XRPD patterns before and after reaction 2012CB821705 and 2013CB933800) and the Taishan Scholar’s
supported that 2 is stable, as further confirmed by its SEM Construction Project.
image after catalysis.
Table 3. Benzyl alcohol o
xidation-Knoevenagel condensation catalysed by 2.a
Notes and references
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Selectivity
entry
1
R
product
CN
CN
(%)b
Gallego, M. Kidder, S. Dai, J. Am. Chem. Soc., 2014, 136
11260.
,
H
99
65
58
62
99
4
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CN
CN
CN
CN
CN
CN
2
3
4
Me
MeO
F
>99
>99
>99
Me
MeO
F
CN
CN
5
NO2
29
>99
O2N
aReaction conditions: catalyst (1 mol %), benzyl alcohol and substituted benzyl
alcohols (1.0 mmol), toluene (2 mL), 80°C, 30 h, in air; malononitrile (1.2 mmol), 8
h, r.t. b based on GC analysis (ESI).
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Fig. 5 Left: simulated (UiO-68-NH2) and measured XRPD patterns of
after catalysis. Right: SEM image of after catalysis. The morphology of the
microcrystals of is well remained after two step reaction.
2 before and
2
2
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The scope of the bifunctional catalytic system was
investigated by performing the oxidation-condensation
reaction of other various substituted benzyl alcohols. Table 3
summarized the results of these reactions. Compared to
benzyl alcohol, we found that benzyl alcohols with either
electron-donating or electron-withdrawing groups gave lower
overall conversions, which is resulted from the lower
conversion rates of the first oxidation step. The oxidation
conversions for CH3-, CH3O-, F- and NO2-substituted substrates
are 66, 60, 65, 29 % (ESI), respectively. However, the following
,
condensation step for the obtained aromatic aldehydes is a 21 X. Li, Z. Guo, C. Xiao, T. W. Goh, D. Tesfagaber, W. Huang,
ACS Catal., 2014, 4, 3490.
nearly quantitative catalytic process.
In summary, a new Pd NPs loaded Pd(0)@UiO-68-AP was
22 Fernandez, M. I.; Tojo, G. Oxidation of Alcohols to Aldehydes
and Ketones: A Guide to Current Common Practice, Springer,
New York, 2006.
prepared based on
a chelating-directed post-synthetic
approach. The obtained Pd NPs embedded Pd(0)@UiO-68-AP
can be a highly active bifunctional heterogeneous catalyst to
successively facilitate benzyl alcohol oxidation- condensation
reaction under relative mild conditions. The results herein
might provide an alternative way to access to versatile family
of the multifunctional catalysts of this type for a broad scope
of organic transformations in practical application. Further
work on exploring new catalytic reactions based on such
NPs@MOF catalytic systems is currently underway.
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M. E. Davis, J. Am. Chem. Soc., 2008, 130, 13442; (b) M. J.
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We are grateful for financial support from NSFC (Grant Nos.
4 | J. Name., 2012, 00, 1-3
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