Amine-functionalized metal-organic framework-based Pd nanoparticles: Highly efficient multifunctional catalysts for base-free aerobic oxidation of different alcohols

Article abstract of DOI:10.1039/d0nj04138f

A palladium (Pd) nanoparticle containing a metal organic framework (MOF) is described as a highly efficient multifunctional catalyst for the aerobic oxidation of different aliphatic, aromatic and heteroaromatic alcohols in toluene. The catalytic efficienc

Full text of DOI:10.1039/d0nj04138f

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Amine-functionalized metal–organic
framework-based Pd nanoparticles: highly
efficient multifunctional catalysts for base-free
aerobic oxidation of different alcohols†
Cite this:DOI: 10.1039/d0nj04138f
ab
c
Abu Taher,
Ik-Mo Lee
*
*
Md. Abu Bin Hasan Susan,
Noorjahan Begumd and
b
A palladium (Pd) nanoparticle containing a metal organic framework (MOF) is described as a highly efficient
multifunctional catalyst for the aerobic oxidation of different aliphatic, aromatic and heteroaromatic alcohols
in toluene. The catalytic efficiency was demonstrated by the high conversion of various reactants with 100%
selectivity under mild reaction conditions, where a wide range of reactants are selectively converted into
their corresponding products in excellent yields (up to 98%). The unique texture of the prepared catalyst was
well maintained after several recycling cycles of the catalyst. The catalyst could be reused multiple times
without further pre-treatment or any extra precautions for further catalytic cycles. Importantly, the recovery,
reusability and excellent selectivity of the prepared catalyst for aerobic oxidation are the remarkable
advantages of this method. More importantly, this study has demonstrated the great potential of MOF-based
palladium nanoparticles as efficient multifunctional catalysts, while the reaction proceeded smoothly without
any external base additives and co-catalysts.
Received 16th August 2020,
Accepted 19th October 2020
DOI: 10.1039/d0nj04138f
rsc.li/njc
corrosive, flammable, and difficult to dispose.2–4 Unfortunately,
these reactions result in large quantities of extremely
Introduction
The oxidation of alcohols to their corresponding aldehydes and hazardous substances and call for the use of stoichiometric
ketones is one of the most important reactions in organic quantities of highly moisture-sensitive, unrecoverable, and
synthesis due to the wide-ranging utility of these carbonyl expensive chemical reagents that may pose serious health
compounds for different purposes in various branches of risks.5 Importantly, satisfactory results were found in only a
pure and applied sciences.1 Modern organic chemistry typically few cases, in which a large number of expensive additives were
represents a number of methods to affect these alcohol required, and this was usually achieved under relatively harsh
oxidations, and among the most widely featured oxidants are reaction conditions. Conversely, air is an extremely convenient
unwanted heavy-metal oxides, such as potassium permanga- terminal oxidant due to its abundance, cost effectiveness, ease
nate, manganese dioxide, pyridinium chlorochromate, pyridi- of use and promising high oxidation potential,6 which gener-
nium dichromate, and Jones reagent.2 These chemical reagents ates no toxic by-products.7
have found widespread applications in pharmaceuticals,
On the other hand, metal–organic frameworks (MOFs) are
agrochemicals, and functional materials, but they are toxic, newly developed attractive functional crystalline porous
materials,8 which are successfully applied in versatile branches
of pure and applied sciences.9,10 The most attractive feature of
MOFs is their well-ordered porous network structures with a
a Department of Industrial and Production Engineering, European University of
Bangladesh, Mirpur, Dhaka-1216, Bangladesh. E-mail: drtaher@eub.edu.bd;
wide range of tunable pore sizes, extremely high surface areas,
Tel: +88-01712-681890
b Department of Chemistry, Inha University, Incheon 402-751, South Korea.
porosity, and surface functionality.11 These key features, along
with other topological properties, make them suitable candi-
dates for a broad range of uses in catalytic engineering.12 These
catalysts can be easily separated and reused after the reaction,13
which is highly desirable for cost-effectiveness and green
synthesis considerations. In addition, palladium, as a rare
earth transition metal, is considered to be one of the most
attractive catalysts for use in the oxidation of alcohols14 due to
E-mail: imlee@inha.ac.kr; Fax: +82-32-867-5604; Tel: +82-32-860-7682
c Department of Chemistry, University of Dhaka, Dhaka 100, Bangladesh
d Department of Agricultural Chemistry, Sher-e-Bangla Agricultural University,
Dhaka, Bangladesh
† Electronic supplementary information (ESI) available: Synthetic details, digital
images, EDX spectrum, N2 adsorption–desorption isotherms, pore volume vs.
diameter curve, XPS spectra, X-ray powder diffraction, effect of catalyst loading
and FTIR spectra. See DOI: 10.1039/d0nj04138f
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its nominal features of high catalytic efficiency. In particular,
MOF–metal complexes have been demonstrated to exhibit
excellent performance for various alcohol oxidations.15 Until
now, even though many excellent MOF–metal catalytic methods
have been published,16 there are only a very limited number of
studies on MOF–Pd catalytic methods for aerobic oxidation.17
In addition, several research groups have reported base-free
catalytic oxidation using a hydrogen acceptor (ligand).18 How-
ever, to the best of our knowledge, only a few MOF–metal
systems of base-free catalytic oxidation have been described
to perform this reaction.19
Importantly, aerobic oxidations catalysed by palladium (Pd)
have been known for decades, but in spite of the remarkable
progress proposed by these oxidations the relatively low catalyst
performance due to Pd deactivation still remains challenging in
most catalytic reactions.20 In appropriate ligation environ-
Fig. 1 Concept of amine-functionalized MOF-based Pd nanoparticles
(2B) as multifunctional catalysts.
facilitate catalytic activity and product selectivity, and (iii) the
MOF–Pd interaction would show a faster reaction rate due to its
synergistic effect (Fig. 1). In this study, we describe the pre-
paration of MOF–Pd NPs (2B) and demonstrate their excellent
multifunctional activity in the aerobic oxidation of different
aliphatic, aromatic and hetero-aromatic alcohols under mild
reaction conditions.
21
ments Pd0 can be reoxidized by O2 while suppressing the
Pd (black) formation in the catalytic cycle by ancillary air-stable
ligands such as amines, pyridines and sulfoxides.22 More
importantly, a lot of catalytic systems15–17 have been developed
after decades of research on catalysis, but there are still some
drawbacks including low product selectivity23 that restrict its
target product formation; the usage of other materials24,25 as a
cocatalyst makes it economically inefficient and the usage of a
base additive26–28 severely restricts its industrial applications
due to corrosion and scale formation. Therefore, a properly
Results and discussion
designed heterogeneous catalyst with a unique character could An amine-functionalized compound (1) was prepared from
not only benefit high catalytic activity and product selectivity the azide functional group containing an organic linker
but also suppress byproduct formation. Herein, a novel multi- (C22H16N6O4) with Zn(NO3)2Á6H2O in N,N-diethylformamide
functional MOF–Pd nanoparticle (NP)-based catalytic method (DEF). Subsequently, we attempted to reduce the azide to amine
was proposed for the aerobic oxidation of various alcohols functional groups using triphenylphosphine (PPh3) in DEF, as
under cocatalyst free and base free conditions (Scheme 1). In described in our earlier report.29 The XRD patterns and FTIR
this type of catalytic system, we expected that (i) the amine spectra of the as-synthesized compound 1 were quite indistin-
moiety would show a base effect, (ii) the controlled pore might guishable in appearance to those described in the literature29,30
(Fig. 3a and b, respectively). Furthermore, the elemental
analysis of compound 1 was in good agreement with the values
calculated from the unit cell dimensions (Table S1, ESI†).
Catalyst 2B was prepared by loading the Pd precursor,
palladium(II) acetate, into 1 in acetone. The colour of the
solution changed from yellow to completely colourless and
the digital microscope observation illustrated that there was
no apparent damage to the shape of the crystals after modifica-
tion (Fig. S1, ESI†). The final products were harvested by simple
filtration, washing, and drying to afford 2B as dark solids
(Fig. S1, ESI†). The palladium complexes were anchored on
the amine-functionalized MOFs through coordination.31 The
presence of the Pd complexes was confirmed via FTIR, XRD,
HRSEM, FETEM, XPS, EDS, TGA and ICP analyses. The loading
of Pd in 1 was 8.33 wt%, which was detected via inductively
coupled plasma atomic emission spectroscopy (ICP-AES).
The morphology of catalyst 2B was observed via SEM and
TEM analyses. The HRSEM images at different magnifications
clearly demonstrated the fine surface morphology in Fig. 2a, b
and Fig. S2 (ESI†) and the FETEM images at different magni-
fications of catalyst 2B demonstrated homogeneous distribu-
tion of Pd NPs on the surface in Fig. 2c and Fig. S2 (ESI†), while
Scheme 1 Post-functionalization route of MOF-based copper nano-
particles (2A)25 and palladium nanoparticles (2B).
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respectively, which can be attributed to Zn(II) ions bound to
O ions34 and it is identical to the parent compound. Furthermore,
the C–O bands still maintained their positions along with Zn(II)
metals, which strongly suggested that the functionalized MOFs
were constructed by Zn(II) ions and carboxylate anions with
structural similarity (Fig. S5, ESI†). The thermal stability of
compound 1 and 2B was evaluated by thermo-gravimetric
analysis (TGA), as depicted in Fig. 3(f). The initial weight loss
of both samples at 200 1C can be attributed to loss of water
molecules from pores, and the solvent molecules trapped in the
MOF structure during synthesis.35 For catalyst 2B, a sharp
decrease in weight was detected from 260 1C onward, which
could be assigned to the loss of loosely bound ligand.35c The
weight loss of both samples from 260 1C to 450 1C may be due
to the decomposition of the organic ligand and the breaking
of the MOF network structure.35c
N2 adsorption/desorption isotherm profiles of compounds 1
and 2B indicate a type I isotherm, which is a characteristic
feature of micro-porous materials36 (Fig. S6, ESI†). Remarkably,
the BET surface area and pore volume of 1 were estimated to be
516.19 m2 gÀ1 and 0.29 cm3 gÀ1, respectively. These results were
then compared to precursor 1 where the BET surface area
and pore volume of 2B were reduced to 131.04 m2 gÀ1 and
0.06 cm3 gÀ1, respectively, which may be attributed to the added
mass of palladium species to 1.25 In addition, it is observed that
after the yellow MOF materials were added into the orange
palladium(II) acetate containing acetone solution, the solution
turned completely colourless (Fig. S1, ESI†), which once again
confirmed the likely formation of the MOF–Pd complexes.
We also performed X-ray photoelectron spectroscopy (XPS)
analyses to gain further insights into the oxidation state of Pd
Fig. 2 HRSEM images of the surface of 2B at different magnifications
(scale bars are shown at the bottom) for 50 mm and 2 mm, respectively (a
and b); FETEM images of 2B fresh (c) and after the 5th run (d) in alcohol
oxidation; particle size distribution histograms of catalyst 2B and after the
5th run (e and f), respectively.
the average particle size of Pd nanoparticles (NPs) was 3.2 Æ 0.5 nm in compound 2B. The asymmetric broadening of Pd (3d) peaks
(Fig. 2e). Furthermore, the SEM image, and corresponding toward higher binding energies suggests that Pd (3d) possesses
elemental mapping by energy-dispersive X-ray (EDX) spectro- Pd species in various oxidation states. For quantification, the
scopic analysis (EDS) of 2B, clearly demonstrated the uniform XPS peaks of Pd (3d5/2,3/2) were fitted, resulting in two series of
arrangement of Pd NPs in the amine-functional MOF network contributing peaks: peaks with binding energies of 335.2 and
(Fig. S3, ESI†). Importantly, the successful loading of Pd in 2B 340.5 eV were assigned to Pd(0) metal,37 whereas the peaks at
was confirmed by EDX analysis (Fig. S4, ESI†). A clear Pd peak 336.8 and 342.2 eV were assigned to Pd(II)38 (3d5/2,3/2) (Fig. 3c).
was resolved when we probed selected area 1 on the Pd loaded Comparison of the intensities of the assigned peaks for Pd(0)
MOFs and we also observed a similar EDX spectrum when we and Pd(II) in the fitted spectra indicates relative contents of
probed area 2. It can be noted that the FTIR spectrum of 68% and 32%, respectively. Importantly, the results demon-
compound 2B exhibited a new band at 1611 cmÀ1, which may strated that the Pd species in 2B are mainly formed as Pd NPs
be attributed to vibration of the acetate anion32 (Fig. 3b). In and that the amine groups strongly coordinate with Pd NPs
particular, the benzylamine related peaks at 3408 cmÀ1 and after the reduction.16b
3366 cmÀ1 shifted to lower wavenumbers at 3317 cmÀ1 and
The catalytic activity of the as-synthesized catalyst 2B was
3273 cmÀ1, respectively, in 2B because of the electron donation examined with a representative set of different aliphatic, aro-
from the benzylamine to the Pd center, which resulted in the matic and hetero-aromatic alcohols. Initially, to investigate the
formation of the MOF–Pd complexes.25,31 The XRD pattern of catalytic activity of the prepared catalyst 2B, a series of con-
the as-prepared 2B materials demonstrated no apparent trolled experiments were carried out under exactly the same
changes of the pattern from the starting 1 (Fig. 3a). Signifi- reaction conditions (except for 2A) (Fig. 4). We attempted to
cantly, the characteristic XRD peak of Pd (111) at 2y = 401 is conduct the oxidation of benzyl alcohol using air as the oxygen
assigned due to the insertion of palladium in 1 as shown in source in the presence of 2B (1 mol%) under the same reaction
Fig. 3a, which means that the amine-functionalized MOF has conditions (at 80 1C for 3 h) and 100% conversion (GC yield) of
coordinated with Pd that is in consonance with earlier benzyl alcohol to benzaldehyde was achieved. We compared
reports.33 In Fig. 3(e), the peak value of Zn 2p3/2 and Zn 2p1/2 these results with those of the same reaction in the presence of
states that the Zn ions in 2B were found at 1021.3 eV and 1044.2 eV, (i) 2A, (ii) 1 + Pd(OAc)2, (iii) Pd(OAc)2, and (iv) 1. It was found
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Fig. 3 XRD patterns (a) and FT-IR spectra (b) of compounds 1 and 2B; XPS spectra of Pd (3d) peaks of catalyst 2B fresh (c) and after the 5th run (d); high-
resolution XPS spectra of Zn 2p peaks of the precursor compound, compound 1, and catalyst 2B (e); TGA of compounds 1 and catalyst 2B (f).
because the Cu(II) species can enhance the reactivity in the
presence of the co-catalyst, 2,2,6,6-tetramethyl-piperidyl-
1-oxy.24,25 Interestingly, the use of 2B allowed the oxidation to
proceed significantly faster compared with other catalytic com-
ponents. This result was consistent with our hypothesis and
suggested that the MOF–Pd interactions and controlled pore
size of the MOF, enhancing the reactivity as depicted in Fig. 1.
In addition, when a mixture of 1 + Pd(OAc)2 or only Pd(OAc)2
was used, relatively poor yields were obtained that may be due
to the absence of coordinating ligands.25,31,39 Importantly, the
results demonstrate that the connected ligand between the
MOF and the Pd/Cu moieties highly promoted the oxidation
of benzyl alcohol to benzaldehyde under base-free conditions.
More importantly, catalyst 2B demonstrated a higher conver-
sion rate on alcohol under the defined conditions without the
assistance of a co-catalyst as compared to the earlier reports24,25
such as catalyst 2A (Scheme 1). These results suggested that the
incorporation of metal Pd into the MOF material could signifi-
cantly enhance its catalytic activity toward alcohol oxidation.
In order to obtain better catalytic performance, various
Fig. 4 Aerobic oxidation of benzyl alcohol catalysed by 2B, 2A, 1 +
Pd(OAc)2, Pd(OAc)2, and 1; in all the cases the reaction conditions are
the same as those listed in Table 1 (entry 1) except 2A (as reported).25 The
quantity of the product was determined using GC. We calculated an equal
amount of Zn2+ (when comparing 2A and 2B with 1).
that the oxidation reaction in the presence of 2A proceeds reaction parameters have been studied to optimize the reaction
much faster than the same reaction in the presence of 1 conditions. In the present study, we examined the effects of the
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Table 1 Aerobic oxidation of various aliphatic, aromatic and hetero-
aromatic alcohols catalysed by 2B in toluenea
Time Yieldb
TOFd
TONc (hÀ1
Entry Substrate
Product
(h)
(%)
)
1
2
3
3
98
2642 881
2642 1149
2642 1067
2.3
2.5
98
98
Fig. 5 Aerobic oxidation of benzyl alcohol catalysed by 2B: (a) reaction in
different solvents, (b) effect of the base, (c) hot filtration test, and (d)
recyclability of catalyst 2B. In all cases, the reaction conditions are the
same as those listed in Table 1 (entry 1).
4
5
6
7
2.7
4.5
4.7
5
92
98
88
98
2480 919
2642 587
2372 505
2642 528
solvent and observed that toluene is an excellent solvent for
aerobic alcohol oxidation as shown in Fig. 5a. The results
revealed the superior catalytic activity of 2B in toluene medium
and it is worth noting that catalyst 2B demonstrated poor
catalytic activity in the water medium. Importantly, the crystal
structure of the catalyst deformed when water was used as the
solvent due to the highly polar nature of water40 (Fig. S8, ESI†).
We investigated the effect of different bases such as Na2CO3
and CsF under the same catalytic conditions, but there were no
effects in the presence of bases. In particular, catalyst 2B
catalysed the aerobic oxidation of alcohol in the absence of a
base, which was very similar in the presence of K2CO3 (Fig. 5b).
The MOF–Pd demonstrates a remarkable efficiency for alcohol
oxidation without the assistance of a base additive. It was
assumed that 2B plays an important role in oxidation as a
catalyst, as well as a base. This is mostly because we believe that
the amino group containing the MOF had adequate basicity for
deprotonation.41 Subsequently, we optimised the effect of
catalyst loading by varying the amount of catalyst 2B under
similar reaction conditions (Fig. S9, ESI†). The results showed
that 1 mol% of Pd displayed the best catalytic performance
(98% isolated yield) for alcohol oxidation, suggesting that 1
8
6.5
61
1644 253
9
4
4
4
98
96
96
2642 661
2588 647
2588 647
10
11
12
8.5
8
65
64
1752 206
1725 216
13
14
4.5
4.5
61
52
1644 365
1402 312
15
a
Reaction conditions: alcohol (1 mmol), toluene (6 mL) and air as the
mol% of 2B is required for successfully performing the reaction oxygen source. b Isolated yield. Selectivity 100%. c TON units (mol product/
d
mol Pd catalyst). TOF units ((mol product/mol Pd cat.) per hour).
under base-free conditions (Table 1).
To explore the substrate scope of our newly developed
catalytic protocol, we performed the 2B-catalyzed aerobic oxida- literature.42 By the attachment of a substituent group to the
tion using a range of different aliphatic, aromatic and hetero- benzene ring, the electron density and consequently the rates of
aromatic alcohols under defined conditions. We obtained a reactions were either fast or slow depending on the electronic
high turnover number (TON) and turnover frequency (TOF) of nature or the position of the substituent groups. However,
the catalyst and the results are summarized in Table 1. All the catalyst 2B was tested with electron-donating group (EDG)
reactions proceeded smoothly to give the corresponding alde- containing substrates, such as p-tolylmethanol, (4-methoxy-
hydes/ketones with good yields (up to 98%); for instance, for phenyl)methanol and (2-methoxyphenyl)methanol, which
benzyl alcohol the reaction proceeded smoothly to give the gave the corresponding aldehydes (entries 2–4) in good
corresponding benzaldehyde in high yield (98%) with 100% yields (92–98%). Subsequently, catalyst 2B was tested with
selectivity (TOF = 881 hÀ1) under base-free conditions (entry 1), electron-withdrawing group (EWG) containing substrates such
and the product peaks were compared to those reported in the as (4-chlorophenyl)methanol, (2-chlorophenyl)methanol and
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(4-nitrophenyl)methanol which also gave the corresponding reused catalysts after five consecutive runs; the TEM and SEM
aldehydes (entries 5–7). Even with the double aromatic ring images of the used catalysts only showed slight aggregation of
alcohol, naphthalen-1-ylmethanol, the product was obtained in Pd (Fig. 2d and Fig. S2, ESI†) and the average particle size of Pd NPs
61% yield (entry 8) which was substantially lower than those of was 3.8 Æ 0.9 nm (Fig. 2f), which does not affect the activity of the
the other primary alcohols. This could be caused by molecular catalyst. Moreover, the XPS and FTIR spectra, and XRD patterns of
size and steric hindrance effects. Importantly, the reactivity of the catalyst, 2B, exactly resembled those of the fresh catalyst after
the substituted benzene could be strongly influenced by the reusing several times (Fig. 3d and Fig. S5, S10, respectively, ESI†),
presence of the EDG on the aromatic ring as reported in the suggesting that the structural features of 2B were retained.
literature.25 It is noteworthy that the aerobic oxidation of
The structurally well-defined pores of MOFs may offer
secondary alcohols such as 1-phenylethanol, 1-p-tolylethanol unique advantages for catalytic applications compared with
and 1-(4-methoxyphenyl)ethanol also gave the corresponding others and thus can make processes more feasible, cost-
products (entries 9–11) in good yields (96–98%). Unfortunately, effective and environmentally friendly.45 However, the increase
with this catalytic system secondary alcohols were a little of the size of aromatic rings reduces the yields of the corres-
bit difficult to oxidize probably due to steric interactions.43 ponding products (Fig. 6). Particularly, for the substrate benzyl
Furthermore, heterocyclic alcohols such as (pyridin-3-yl) alcohol, the size of which (4.9 Å) is smaller than the pore size of
methanol and (furan-2-yl)methanol were oxidized to give the 2B (Fig. S6 and S7, ESI†), the yield was up to 98% after 3 h. On
corresponding products with moderate yields, while this study the other hand, by increasing the number of aromatic rings or
showed that reaction times were getting longer for heterocyclic size (8.1 Å), naphthalen-1-ylmethanol gave a much lower yield
alcohols (entries 12 and 13). In the case of aliphatic alcohols under the same reaction conditions, suggesting a certain size
such as hexan-2-ol and octan-2-ol (entries 14 and 15) that also and shape selectivity. Therefore, this high catalytic activity with
gave the corresponding product, the rate of reaction was lower excellent selectivity would be due to the large surface area,
than those of the benzylic alcohols that may be due to steric tunable pore size, and surface functionality, along with other
hindrance.44 It is clearly shown that these aerobic heteroge- topological properties of the MOF materials.
neous conditions are highly efficient for the oxidation of
different aliphatic, aromatic and hetero-aromatic alcohols to
the corresponding aldehydes or ketones with high yields, with
Experimental
Catalyst preparation
TOFs up to 1149 hÀ1 being reached.
Recently, the use of MOFs as heterogeneous catalysts has
received much attention in different areas of organic synthesis,
while heterogeneity is thus a critical and important issue for
catalysis. However, to assess whether our catalyst 2B is indeed
heterogeneous in nature, we carried out a leaching test by
performing the oxidation of benzyl alcohol under optimized
reaction conditions, in which the catalyst was removed after
1 h, and the reaction proceeded. As indicated in Fig. 5c, no
further reaction took place without 2B after the initiation of the
reaction for oxidation at 3 h, while the possible leaching of
Pd after the completion of the reaction was monitored by
ICP analysis. This finding indicates that no leaching of the
catalytically active sites occurs and that 2B exhibits a typical
heterogeneous catalyst nature, which is attributed to an excel-
lent coordination interaction between Pd metal and the amine
functionalized-MOF.25,31 As a heterogeneous catalyst, 2B can be
repeatedly recycled and reused under optimized conditions.
This new catalyst can be easily recovered after each reaction
cycle using a simple filtration technique and then recycled for
further reactions. We thus carefully carried out the recycling
reactions and observed a very similar yield (98–100%) on five
consecutive runs with 100% selectivity (Fig. 5d). The conversion
and especially the selectivity for the desired product were very
similar after 3 h. The TON and TOF at 3 h for benzaldehyde
formation from benzyl alcohol were 2642 and 881 hÀ1, respec-
tively, under the optimized conditions.
Compounds 1 and 2A were prepared according to our pre-
viously reported procedures.25 Typically, 1.60 g (6.0 mmol) of
triphenylphosphine was dissolved in N,N-diethylformamide
(30 mL) and 0.62 g (0.40 mmol) of the azide functional group
containing the MOF was added to the solution in a 50 mL vial.
The reaction was carried out without stirring at 80 1C for 10 h in
an oil bath. After the starting material had disappeared, com-
pound 1 was found as a light-yellow solid (Fig. S1, ESI†). Then,
0.08 g (0.60 mmol) of copper(II) chloride was dissolved in
ethanol (24 mL), and 0.60 g of compound 1 was added to the
copper solution. The vial, placed in the oil bath, was heated to
The results indicate that the as-prepared catalyst 2B is highly
active and stable against the migration of the active species
and suppresses the sintering of Pd NPs. We characterized the
Fig. 6 Size and shape selectivity in the aerobic oxidation of benzyl
alcohols and naphthalen-1-ylmethanol using 2B. In all cases, the reaction
conditions are the same as those listed in Table 1.
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45 1C for one day, and compound 2A was given as a blue solid. Acknowledgements
Similarly, 0.28 g (1.26 mmol) of palladium(II) acetate was
This work was supported by an Inha University Research Grant
(Inha 2017).
dissolved in acetone (20 mL) and 0.60 g (0.42 mmol) of
compound 1 was added to the solution. The reaction mixture
was placed in the oil bath and heated to 45 1C for 8 h without
stirring and the colour of the solution changed from orange to
colourless (Fig. S1, ESI†). The crystals were filtered off and
washed with fresh acetone (15 mL Â 4), and the MOF-based
catalyst 2B was obtained as a dark solid (Fig. S1, ESI†). The
catalyst was dried under vacuum and ICP analysis detected
8.33% (wt) Pd in the resulting compound, 2B.
Notes and references
1 (a) R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation
of Organic Compounds, Academic Press, New York, 1981;
(b) M. Hudlicky, Oxidations in Organic Chemistry, American
Chemical Society, Washington, D.C., 1990; (c) R. A. Sheldon,
I. W. C. E. Arends, G. J. Brink and A. Dijksman, Acc. Chem.
Res., 2002, 35, 774–781; (d) J. Muzart, Tetrahedron, 2003, 59,
5789–5816; (e) M. Baumann and I. R. Baxendale, Beilstein
J. Org. Chem., 2013, 9, 2265–2319.
2 (a) I. A. S. Shibley Jr., K. E. Amaral, D. J. Aurentz and
R. J. McCaully, J. Chem. Educ., 2010, 87, 1351–1354;
(b) I. W. C. E. Arends and R. A. Sheldon, in Modern Oxidation
Methods, ed. J.-E. Backvall, Wiley VCH, Weinheim, Germany,
2nd edn, 2010, pp. 147–185.
General procedure for an oxidation reaction
The oxidation of alcohols was carried out in a very simple and
easy way such as in a base free, open air system at atmospheric
pressure. Typically, to a solution of the substrate (1.00 mmol) in
toluene (6 mL) was added 2B (1 mol%) and the resulting
mixture was heated at 80 1C without any base additive under
atmospheric air (in an open vial system). The reaction time was
determined by checking TLC and GC (Agilent Technologies
7890A GC System). After completion of the reaction, the catalyst
was easily filtered off and the filtrate was evaporated under
reduced pressure. The separated catalyst was successively
reused and recycled for the next reaction without further
processing. The crude products were purified via column
chromatography on silica gel (ethyl acetate–hexane) to give
the corresponding product (Table 1) and all the desired
products were characterized via the comparison of their spectra
with those reported in the literature.42 A hot filtration experi-
ment was carried out according to our previous procedure.25
Typically, catalyst 2B was carefully removed from the reaction
mixture after 30 minutes of reaction, and the resulting filtrate
was heated at 80 1C for an additional 3 h.
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In summary, we have designed and developed a novel, highly
efficient Pd containing MOF-based multifunctional catalytic
system for the aerobic oxidation of various alcohols. In this
catalytic system, the catalyst significantly enhanced the reactivity
via the synergic effect of the MOF–palladium interaction with the
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reactants and maintained 100% selectivity of the products.
Importantly, this catalytic method exhibited exclusive practical
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There are no conflicts to declare.
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