Solvent-free and melt aerobic oxidation of benzyl alcohols using Pd/Cu2(BDC)2DABCO-MOF prepared by one-step and through reduction by dimethylformamide

Article abstract of DOI:10.1039/c7ra07209k

In this paper we report an efficient method for the aerobic oxidation of benzyl alcohols to aldehydes by supported palladium nanoparticles in a nanoporous metal-organic framework Cu₂(BDC)₂(DABCO) (BDC = 1,4-benzenedicarboxylate, DABC

Full text of DOI:10.1039/c7ra07209k

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Solvent-free and melt aerobic oxidation of benzyl
alcohols using Pd/Cu2(BDC)2DABCO–MOF
prepared by one-step and through reduction by
dimethylformamide†
Cite this: RSC Adv., 2017, 7, 40881
*
Sara Akbari, Javad Mokhtari
and Zohreh Mirjafari
In this paper we report an efficient method for the aerobic oxidation of benzyl alcohols to aldehydes by
supported palladium nanoparticles in a nanoporous metal–organic framework Cu2(BDC)2(DABCO) (BDC
¼ 1,4-benzenedicarboxylate, DABCO ¼ 1,4-diazabicyclo[2.2.2]octane) under a temperature control
program and through reduction by dimethylformamide. The Pd-NPs/Cu2(BDC)2(DABCO) offered higher
catalytic activity and recyclability than reported in literature. The supported Pd nanoparticles (PdNPs/
Cu2(BDC)2(DABCO)) were characterized by X-ray diffraction, BET analysis, field emission scanning
electron microscopy, transmission electron microscopy, inductively coupled plasma atomic emission
spectroscopy and X-ray photoelectron spectroscopy (XPS).
Received 29th June 2017
Accepted 10th August 2017
DOI: 10.1039/c7ra07209k
rsc.li/rsc-advances
The oxidation of organic compounds, such as alcohols is catalytic sites.24 Also MOFs have a wide range of potential uses
a primary and essential transformation in organic synthesis and including gas storage, separations, solar cells and catalysis.25 In
industrial chemistry.1 Among the alcohols, benzyl alcohol is one particular, applications in energy technologies such as fuel
of the most studied substrates.2–8 Traditionally, various cells, supercapacitors, and catalytic conversions have made
oxidants, such as peroxides,9,10 toxic metal oxides,11 halides,12 them objects of extensive study, industrial-scale production,
ozone,13 etc., were used for the oxidation of alcohols. Compared and application.26 Recently, metal–organic frameworks (MOFs),
with those oxidants, undoubtedly, air is the most atom- as a new class of organic inorganic hybrid porous materials,
economical and environmentally benign oxidant. Catalysis have attracted considerable attention for stabilization and
using metal nanoparticles (MNPs) has attracted great interest in supporting of metal nanoparticles as a novel class of functional
recent years. Considering the current impetus of nanoscience, it materials due to their high surface areas, tunable pore sizes,
is understandable that all aspects related to the preparation of and thermal stability.27–29 The employment of MOFs as the
small nanoparticles (NPs) with narrow size distribution, their supports for PdNPs has attracted considerable interest.30–32 For
stabilization and their unique properties compared to large catalytic applications, PdNPs have been successfully deposited
particles.14–17 The liquid-phase oxidation of alcohols over sup- at the outer surface of the MOF supports (Pd/MOFs)33–38 or
ported metal catalysts has been extensively studied in the last loaded inside the cavities of the MOFs (Pd@MOFs) via various
decade.18–21 Crystalline microporous and mesoporous metal– methods such as impregnation and chemical vapor deposi-
organic frameworks (MOFs) are constructed by joining metal- tion.39–43 Although great efforts have been made in this eld,
containing units [secondary building units (SBUs)] with a general and facile method is needed that can easily introduce
organic linkers such as dicarboxylate or other organic rings, the palladium(0) precursor inside the cavities of MOFs in one-
using strong bonds (reticular synthesis) to create open crystal- pot and without any reducing agent and control the formation
line frameworks with permanent porosity.22 During the past of PdNPs with a small particle size, which is benecial in the
century, extensive work was done on crystalline extended cases such as catalysis. Herein, we report the one-step and
structures in which metal ions are joined by organic linkers reducing-agent-free encapsulation of ultrane PdNPs in to
containing Lewis base-binding atoms such as nitriles and nanopores Cu2(BDC)2(DABCO) without Pd(0) NPs aggregation
bipyridines.23 MOFs have low density, very high surface area on the external surface of framework and application of this
with a large percentage of transition metals that can act as catalyst in aerobic oxidation of benzyl alcohols.
Cu2(BDC)2(DABCO) was synthesized and characterized by
that are reported in our previous work.44 At the rst we evalu-
ated a simple and straightforward one-step protocol to encap-
Department of Chemistry, Science and Research Branch, Islamic Azad University, P.O.
Box 14515/775, Tehran, Iran. E-mail: j.mokhtari@srbiau.ac.ir
sulate PdNPs inside MOFsꢀthrough temperature programming
† Electronic supplementary information (ESI) available. See DOI:
ꢀ
control from 80 C to 130 C for 20 h. Cu2(BDC)2(DABCO) has
10.1039/c7ra07209k
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been shown to act as a proper support for deposition of PdNPs.
Using temperature controlling method aer sonication, Pd(II)
cations reduced to Pd(0) nanoparticles deposed on the surface
of activated Cu2(BDC)2(DABCO) carriers. The actual loading of
Pd(0) was characterized by ICP analysis and found to be 0.9%.
To investigate the oxidation state information of palladium,
XPS analysis of Pd 3d binding energies was performed.
As a shown in Fig. 1 the peaks at 341.5 (Pd-3d3/2) and 336.2 (Pd-
3d5/2) eV are due to the binding energy corresponds to the Pd(0),
and the peaks at 342.7 (Pd-3d3/2) and 337.6 (Pd-3d5/2) eV are due
to the binding energy corresponds to the Pd(II). Based on the
peak areas of Pd-3d5/2 at 336.2 and 337.6 eV, the ratio of Pd(0) to
Pd(II) in Pd/Cu2(BDC)2DABCO is calculated to be 1.5 (Fig. 1a).
XPS analysis of recycling Pd/Cu2(BDC)2DABCO did not show any
signicant change in palladium oxidation state (Fig. 1b).
The XRD pattern of PdNPs/Cu2(BDC)2(DABCO) shows that
the crystalline structure of Cu2(BDC)2(DABCO) is maintained
aer deposition of PdNPs. The absence of a Pd diffraction
pattern relates to the low Pd contents in the materials (Fig. 2).
There have theretofore been many processes reported for the
encapsulation of Pd(0) NPs in MOFs but in most of the methods
used a reducing agent for reduction of Pd(II) to Pd(0), while in
this work we encapsulated and supported Pd(0) NPs in MOFs by
temperature control program and without using any reducing
agent through reduction by DMF.45–47 The DMF organic solvent
is a powerful reducing agent to reduce different ions such as
Ag+, Au3+ and etc.48–50 The possible reaction mechanism for the
reduction of Pd2+ incorporated in MOF by DMF is as the
following equation:
Fig. 2 (a) XRD pattern of prepared Cu2(BDC)2(DABCO). (b) XRD
pattern of prepared PdNPs@Cu2(BDC)2(DABCO).
Table 1 Textural properties of [Cu2(BDC)2(DABCO)] and PdNPs@
[Cu2(BDC)2(DABCO)] from N2 adsorption–desorption experiments
BET surface
area (m2 gꢂ1
Pore volume
Material
)
(cm3 gꢂ1
)
[Cu2(BDC)2(DABCO)]
PdNPs@[Cu2(BDC)2(DABCO)]
1496
873
0.48
0.475
HCONMe2 + Pd2+ + H2O / Pdo + Me2NCOOH + 2H+
This mechanism is supported by a measured increase of samples were noticeably reduced compared to Cu2(BDC)2
conductivity as the reaction proceeds, which indicates that the (DABCO), indicating that the cavities of Cu2(BDC)2(DABCO) may
larger Pd2+ ions are progressively exchanged for the more be occupied by highly dispersed Pd nanoparticles (Table 1).
mobile H+ ions. Accordingly, a decrease of the solution pH was
TEM micrograph (Fig. 3), conrmed that a crystalline and
observed. Based on a literature report48–50 we also proposed that nano-scaled material was produced. It is also to emphasize that
carbamic acid produced during the reduction was easily Pd nanoparticle has resulted in producing uniformly distrib-
decomposed to dimethyl amine and gaseous CO2 in high uted nano-scale. It is also to emphasize that Pd nanoparticle has
temperature.
resulted in producing uniformly distributed nano-scale and
Specic surface areas of the samples were measured by N2 with a mean particle diameter of 2.9 ꢁ 0.2 nm as estimated from
adsorption at 77 K, with results presented in Table 1. Nitrogen particle size distribution.
adsorption and surface areas of Pdo-in-Cu2(BDC)2(DABCO)
The synthetic PdNPs/Cu2(BDC)2DABCO was employed as
a catalyst in the aerobic oxidation of benzyl alcohols (Scheme 1).
Fig. 1 (a) XPS spectrum of synthesized Pd-NPs/Cu2(BDC)2DABCO and
(b) recycled Pd-NPs/Cu2(BDC)2DABCO.
Fig. 3 TEM micrograph of the PdNPs/Cu2(BDC)2DABCO.
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(20 mol% Na2CO3 or K2CO3) was used. As was expected the rates
and yields improved.
For the optimization of loading of the catalyst, different
amount of catalyst was used. As shown in Table 2, with
increasing the amount of catalyst, yield was improved (Table 2,
entry 5–7). In another attempt, when the reaction was carried
out in the absence of PdNPs/Cu2(BDC)2DABCO or Cu2(BDC)2-
DABCO, no product was formed aer 24 hours at 120 ꢀC. It has
been observed that PdNPs/Cu2(BDC)2DABCO promotes the
oxidation of benzyl alcohol under solvent-free condition with
Scheme 1 Schematic representation of PdNPs@Cu2(BDC)2DABCO
catalyzed oxidation of benzyl alcohols.
Table 3 Oxidation of different benzyl alcohols
Benzyl alcohol was chosen as a model substrate to study the
catalytic activity of PdNPs/Cu2(BDC)2DABCO to identify suitable
reaction conditions using atmospheric air as the terminal
oxidant. The results are presented in Table 1. Initiaꢀlly, the
oxidation of benzyl alcohol was tested in CH3CN at 80 C with
20 mg of PdNPs/Cu2(BDC)2DABCO (0.9% Pd) and a stream of air
produced by air pump was conducted into the reaction mixture,
in this condition negligible yield of benzaldehyde was attained.
When oxidation of benzyl alcohol was tested in toluene at
Entry
R
Time (h)
Yielda
1
2
3
4
5
6
7
8
H
24
24
24
24
24
24
24
24
99
97
96
93
96
95
96
97
4-Me
2-Me
4-OMe
4-Cl
4-Br
4-NO2
2-NO2
ꢀ
110 C, conversion of benzyl alcohol to the benzaldehyde was
increased. These tests shown that the nature of solvent and
temperature greatly affect the efficiency of oxidation. So, in
another attempts for conrmation of this hypothesis, reaction
a
Isolated yield.
ꢀ
was performed in xylene at 135 C. As expected, conversion of
benzyl alcohol was increased. From the viewpoint of green
chemistry and toxicity of above solvent, in another attempt,
when oxidation of benzyl alcohols was performed in absences of
Table 4 Reusability test of catalyst
Experiment no.
Yield
ꢀ
solvent and under solvent-free condition at 120 C, conversion
1
2
3
4
98
95
95
93
increased signicantly.
In another attempt to investigate the role of the base in the
rate and yield of oxidation, a catalytic amount of mineral base
Table 2 Optimization of reaction conditions
Base (20
mol%)
Run
Catalyst (mg)
Solvent
Time (h)
T (ꢀC)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
15
15
15
15
20
25
30
—
CH3CN
Toluene
Xylene
—
—
—
—
—
—
—
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
—
24
24
24
24
24
24
24
24
24
24
80
110
135
120
120
120
120
120
120
120
5
20
40
89
99
90
95
0
20
51
15
Cu–MOF (20 mg)
Na2CO3
a
Isolated yield.
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Table 5 Comparison of activity for different catalytic systems in the oxidation of benzyl alcohols
Entry
Catalyst
Reaction condition
Yield
(Sel.%)
Ref. no.
1
2
3
4
5
6
7
Pd@C–GluA-550
Pd@C–GluA-550
Pd/NaX zeolite
Pd/Al2O3
Pd cluster colloid
MNP–Pd
Substrate 4 mL/air 1 atm/120 ꢀ
C
48.9
48.9
66
80
86
63
80
97
94
100
93
53
53
54
55
56
57
Solvent-free/O2 (1 MPa)/120 ꢀ
C
Toluene/100 ꢀC/O2 (3 mL minꢂ1
)
Solvent free/120 ꢀC/O2 (50 mL minꢂ1
Water/32 ꢀC/air bubbling/(pH 3.5)
Toluene/K2CO3/85–90 ꢀC/air
)
89
99
Pd@Cu2(BDC)2DABCO
Solvent free/Na2CO3 (20 mol%)/air
100
Present work
atmospheric air. So, as shown in Table 2 the best result the reaction was completed aer 24 hour and the conversion
observed in entry 5. and isolated yield of the reaction was 100 and 98%, respectively
For review of the catalytic performance of Cu2(BDC)2 (experimental section). This reusability test was repeated for
(DABCO) on the aerobic oxidation of benzyl alcohol an experi- three times and the result summarized in Table 4.
ment was performed as a shown in Table 2 entry 10. As ex-
pected, little efficiency was achieved. It can be concluded that benzyl alcohol demonstrated that our present Pd/MOF catalyst
palladium is very effective in this study. system exhibited a higher conversion and yield under milder
A comparison with other catalytic systems in the oxidation of
Encouraged by this success, we next investigated the scope of conditions (Table 5). In the literature has been reported that the
the oxidation of benzyl alcohols derivatives (Table 3). The oxidative dehydrogenation of alcohols to the corresponding
aerobic oxidation reactions of the benzylic alcohols with aldehydes happened on all exposed metallic palladium faces
different substituted groups under solvent-free were carried out and the undesired products such as toluene, benzyl ether oen
in air (monitored by GC).
produce on hollow sites of Pd particle faces.51 However, in our
We found that all benzylic alcohols with both electron- work, undesired products not were formed, so we proposed that
withdrawing (–NO2) and electron-donating groups (–OMe, the sites on face centered cubic Pd are the primary sites for the
–Me) at either para- or ortho-position gave excellent conversions. oxidation of benzyl alcohols.52 The proposed mechanism for
To determine the degree of leaching of the palladium from the this reaction is given below (Scheme 2).
PdNPs/Cu2(BDC)2DABCO catalyst, aer the ltration of catalyst
the palladium content of the ltrate was determined by ICP. It melting point (for the solid product) and H-NMR spectra.
was shown that less than 0.1% of the total amount of the
The structures of all products were elucidated from their
1
original palladium species was lost into solution during the
reaction. This leaching level was negligible which was also
conrmed by the excellent reusability of this heterogeneous Materials and instruments
catalyst.
All reagents including organic linker H2BDC, metal salt
One of the important properties of the heterogeneous cata-
Cu(OAc)2$H2O, 1,4-benzenedicarboxylate (BDC, 99%), 1,4-dia-
lyst is the reusability, so to conrm this for our catalyst, we
zabicyclo[2.2.2]octane (DABCO), palladium(II) chloride (PdCl2),
performed recycling experiment; so the oxidation of benzyl
benzyl alcohols and solvent were obtained from commercially
alcohol was chosen as a model reaction with recycled catalyst;
available sources such as Sigma-Aldrich and Merck without any
purication. X-ray powder diffraction (XRD) measurements
were performed using an X'pert MPD. Philips diffractometer
ꢀ
with Cu radiation source (l ¼ 1.54050 A) at 40 kV voltage and 40
mA current. BET (Brunauer–Emmett–Teller) surface area of the
samples was determined from N2 adsorption–desorption
isotherms using a micromeritics ASAP 2020 analyzer. The
sample was characterized using a scanning electron microscope
(SEM) with a ZEISS at 30 kV with gold coating. Transmission
electron microscopy (TEM) was carried out using an EM10C-100
kV series microscope from the Zeiss Company, Germany. The
actual loading of palladium was determined by Inductively
Coupled Plasma (ICP) analysis on sequential plasma spec-
trometer, Shimadzu (ICPS-7000 ver. 2). X-ray photoelectron
spectroscopy (XPS) measurements were conducted on a PHI
Quantum 2000 XPS system equipped with an Al X-ray source
(1486.6 eV). 1H-NMR spectra were measured (CDCl3) with
a Bruker DRX-500 AVANCE spectrometer at 400.13. Melting
Scheme 2 Proposed mechanism for the oxidation of benzyl alcohols. points were measured on an Electrothermal 9100 apparatus and
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GC analysis of determination of conversions and selectivities control program without adding any external reduction agent.
was performed on an Agilent 6890N.
The Pd/MOFs offered higher catalytic activity than common
palladium salts for the quantitative and selective oxidation of
benzyl alcohols. Further studies on encapsulation of other
useful PdNPs in MOFs and development of its catalytic scope
are in progress and would be presented in due course.
One-step supporting of palladium
nanoparticle in Cu2(BDC)2(DABCO)
(Pd/Cu2(BDC)2(DABCO))
Conflicts of interest
[Cu2(BDC)2DABCO] was synthesized by that are reported in our
previous work.40 A mixture of Cu(OAc)2$H2O (0.6 mmol), H2BDC
(0.6 mmol) and DABCO (0.3 mmol) with molar ratio of 2 : 2 : 1 were
ball-milled vigorously at 28 Hz at room temperature for 2 hours.
The obtained green powder was washed with DMF (3 ꢃ 10 mL).
Solvent exchange was carried out with methanol (3 ꢃ 10 mL) at
room temperature. To remove the guest molecules of MOFs,
obtained powder was treated by heating under vacuum at 130 ꢀC
for 12 hours, resulting in 1.6 g of pure product which corre-
sponds to 94% isolated yield.
There are no conicts to declare.
Acknowledgements
The authors gratefully acknowledge from Science and Research
Branch, Islamic Azad University for partial nancial support of
this work.
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(BDC)2(DABCO) was centrifuged, washed with DMF and
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