Pd(II) loaded on diamine functionalized LDH for oxidation of primary alcohol using water as solvent

Article abstract of DOI:10.1016/j.apcata.2013.04.003

Hybrid inorganic-organic materials which comprise organic functional groups tethered from layered double hydroxide surfaces are versatile, heterogeneous catalysts. This functional group can bind with palladium that provides catalytic activity superior to what can be obtained from homogeneous catalysts. This work reports synthesis of heterogeneous and recyclable Pd(II) (supported on diamine functionalized layered double hydroxide) catalyst for oxidation of primary alcohol using water as solvent and molecular O₂ as a terminal oxidant. The organometallic-derived catalyst has been studied by XRD, FTIR, UV-vis DRS, solid-state NMR (¹³C, ²⁹Si), XPS, TGDTA, SEM and EDX. Here the ligand N-[3-(trimethoxysilyl)-propyl] ethylenediamine (TPED) is covalently attached to the surface of LDH, which is shown by NMR spectroscopy. XPS results strongly suggest that it involves the formation of NPd coordination bonds. EDX confirmed that all the elements were present in the LDH/Pd(II) complex. The heterogeneous catalyst can be easily recovered and reused multiple times without significant loss of catalytic activity and selectivity, which is a better green alternative for practical applications.

Full text of DOI:10.1016/j.apcata.2013.04.003

Applied Catalysis A: General 460–461 (2013) 36–45
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Applied Catalysis A: General
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Pd(II) loaded on diamine functionalized LDH for oxidation of primary
alcohol using water as solvent
Mitarani Sahooa,b, K.M. Paridaa,b,∗
a Academy of Scientific and Innovative Research, New Delhi, India
b Colloids and Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar 751013, Odisha, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Hybrid inorganic–organic materials which comprise organic functional groups tethered from layered
double hydroxide surfaces are versatile, heterogeneous catalysts. This functional group can bind with
palladium that provides catalytic activity superior to what can be obtained from homogeneous catalysts.
This work reports synthesis of heterogeneous and recyclable Pd(II) (supported on diamine functionalized
layered double hydroxide) catalyst for oxidation of primary alcohol using water as solvent and molecular
O2 as a terminal oxidant. The organometallic-derived catalyst has been studied by XRD, FTIR, UV–vis
DRS, solid-state NMR (13C, 29Si), XPS, TGDTA, SEM and EDX. Here the ligand N-[3-(trimethoxysilyl)-
propyl] ethylenediamine (TPED) is covalently attached to the surface of LDH, which is shown by NMR
spectroscopy. XPS results strongly suggest that it involves the formation of N Pd coordination bonds. EDX
confirmed that all the elements were present in the LDH/Pd(II) complex. The heterogeneous catalyst can
be easily recovered and reused multiple times without significant loss of catalytic activity and selectivity,
which is a better green alternative for practical applications.
Received 30 November 2012
Received in revised form 22 February 2013
Accepted 2 April 2013
Available online 25 April 2013
Keywords:
Layered double hydroxide
Oxidation reaction
Pd(II)-complex
Water
Functionalization
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
usage of organic or inorganic solvent results in unnecessary waste
formation, which is not acceptable from an environmental point of
Selective oxidation of alcohols to the corresponding aldehydes
or ketones is one of the most fundamental reactions in organic
chemistry [1,2]. Many oxidations of these types are carried out
using a stoichiometric amount of oxidizing agent (i.e. KMnO4,
MnO2, CrO3, Br2, etc.) with considerable drawbacks such as high
cost, by-products and serious environmental issues [3]. From the
standpoint of green and sustainable chemistry, the development
of catalytic systems that use molecular oxygen as the only oxi-
dant has attracted great interest [4,5]. This has many remarkable
advantages, including abundance, low cost, and the by-products
are benign. Palladium-catalyzed oxidation reactions rank among
the most versatile strategies for the selective oxidation of organic
molecules with molecular oxygen. Heterogeneous catalyst may
bring several advantages such as facile separation from the reac-
tion mixture and the possibility of recycling. However, there are
few examples involved on Pd catalyst such as Sheldon et al. who
reported a quite sophisticated catalytic system for oxidation of
alcohols in water using palladium salt as a catalyst and air as a
sole oxidant, where the oxidation was performed at a high tem-
perature, i.e. 100 ◦C under 30 bar pressure of air [6]. Furthermore
LDH supported Pd(II) catalyst for aerobic oxidation of alcohol using
toluene as solvent has been reported by Kakiuchi et al. [7]. The
view [8–11]. Water is the most innocuous substance on Earth and,
hence it is the safest solvent possible. Therefore, there is a clear
need to study and develop new LDH supported diamine function-
alized Pd-catalysts, in particular heterogeneous catalysts that will
work under low temperature, molecular O2 as oxidant and water
as solvent.
Hydrotalcite of layered materials consists of cationic brucite
layers and anionic compounds in the interlayer in which a frac-
tion of the divalent cations have been replaced isomorphously
by trivalent cations giving positively charged sheets with charge-
balancing anions in hydrated galleries between the layers [12].
3+
Their general formula can be expressed as [M2+
M
x(OH)2]x+
1−x
(An−
)
x/n·mH2O, where M2+ and M3+ are di- and trivalent metal
cations; An− denotes an organic or inorganic anion with nega-
tive charge n; and x (=[M3+]/([M2+] + [M3+])) is the value of the
stoichiometric coefficient [13]. The properties of the layered dou-
ble hydroxide can be altered by incorporating different anionic
complexes and enzymes through electrostatic interaction [14].
However, recently our group has successfully demonstrated the
covalent attachment of organic moiety functionalized on layered
double hydroxide for aldol condensation reaction [15]. Moreover
surface modification of the LDH with (3-aminopropyl) triethoxysi-
lane by covalent attachment has been reported by Park et al. [16],
by which unprecedented properties akin to that of a matrix, and
affinities for various organic and inorganic species other than con-
ventional anionic guest can be obtained. Different methods have
∗
Corresponding author. Fax: +91 674 2581637.
E-mail address: paridakulamani@yahoo.com (K.M. Parida).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.04.003

M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
37
Scheme 1. Schematic pathway for the synthesis of LDH/pd(II) catalyst.
been adopted for incorporating Pd in between brucite like layers
of LDH [7,17]. However, here the ligand (TPED) is terminated by
the diamine group pendant from the surface, which is useful in
surface modification strategies due to its diversified reaction. In
order to achieve environmentally benign oxidation reactions, pre-
cise control of the local structure of the catalytically active species
is required. To facilitate achievement of green organic synthesis
of high-value chemicals, the use of functional materials as cata-
lyst support abates the need for organic ligands which are often
required to stabilize Pd(II) species [18].
We present herein a prominent feature of the more recent dis-
coveries related to the use of robust ␲-acidic bidentate nitrogen
ligands, where the ligands are covalently anchored with LDH to sta-
bilize the active palladium catalyst in alcohol oxidation. The use of
water as a solvent and molecular oxygen as an oxidant together
represent a greener system. To our knowledge this work is the
first example of the application of a palladium complex co-valently
anchored to LDH for the oxidation of primary alcohols. Moreover,
in this context benzyl alcohol was selected as model compound for
activity measurement.
The functionalized product was dried at room temperature and
abbreviated as LDH/TPED.
2.1.2. Synthesis of LDH/Pd(II) catalyst
0.2 g of PdCl2 was dissolved in 10 ml of 2 M HCl solution of
methanol. The whole Pd solution was transferred to a solution of
0.5 g functionalized LDH in 20 ml of methanol. The mixed solution
was stirred for 2 h at room temperature and cooled overnight at
4 ◦C. The coloured precipitate was filtered, washed several times
with methanol and dried in air. The final product was abbreviated
as LDH/Pd(II) and was shown in Scheme 1.
2.2. Physico-chemical characterization of the catalyst
Powder XRD measurements were performed on a Rigaku XRD-
˚
600 diffractometer, using Cu K␣ radiation (ꢀ = 1.5418 A) and a 2ꢁ
angle ranging from 3◦ to 80◦. The Fourier transform infrared (FTIR)
spectra of the samples were recorded using a Varian 800-FTIR
spectrometer. The powdered samples were mixed with KBr and
pressed in form of pellets for measurement of FTIR analysis in the
range 4000–400 cm−1. The coordination environments of the sam-
ples were examined by diffuse reflectance UV–vis spectroscopy.
The spectra were recorded using a Varian Cary-100 spectropho-
tometer in the wavelength range of 200–800 nm, with boric acid
as the reference. X-ray photoelectron spectroscopy (XPS) mea-
surement was performed on a VG Microtech Multilab ESCA 3000
spectrometer with a non-monochromatised Mg-K␣ X-ray source.
Energy resolution of the spectrometer was set at 0.8 eV. The binding
energy correction was performed using the C 1s peak of carbon at
284.9 eV as a reference. The TG–DTA thermograms were recorded
on Perkin-Elmer thermal analyser in the temperature range from
30 to 800 ◦C at a heating rate of 10 ◦C/min in nitrogen atmosphere.
Scanning electron microscopy (SEM) images were obtained using
HITACHI 3400N microscope. The samples were placed on a cop-
per tape and then coated with a thin layer of gold (layer thickness
3 nm) using a sputter coater. The chemical composition of the
products was confirmed quantitatively and qualitatively by energy
dispersive X-ray (EDX). 13C CP MAS NMR spectra were obtained
at 100.63 MHz by applying 7 kHz pulses at 90◦ pulses with 2.0 s
pulse delays. 29Si CP MAS spectra were recorded at 79.49 MHz by
applying pulses at 90◦ along with 300 s pulse delays and 5.0 ms
contact time. Both the spectra were performed by using a Bruker
2. Experimental
2.1. Preparation of the catalysts
2.1.1. Synthesis of LDH/TPED
The layered double hydroxide containing Zn–Al with Zn:Al
molar ratio 3:1 was prepared by the co-precipitation method at
a constant pH [19]. ZnAl-LDH/SDS precursor was synthesized with
3:1 ratio of Zn(II) and Al(III) salts together with sodium dodecyl sul-
fate (SDS) as an intercalated anion. The synthesis was carried out
by the co precipitation method under a constant pH of ∼7 similar to
that reported previously [15]. LDH/SDS and CTAB (cetyl trimethyl
ammonium bromide) were dried at 80 ◦C overnight to maintain
an anhydrous condition. 30 ml of methylene dichloride was added
to 1.75 g of dried CTAB (0.16 M) under nitrogen atmosphere. 4.6 ml
of N-[3-(trimethoxysilyl)-propyl] ethylenediamine (abbreviated as
TPED) was added to the dried LDH precursor containing SDS. Then
the CTAB solution containing methylene dichloride was added to
the mixture of TPED and LDH/SDS. The whole mixture was allowed
to react for two days at room temperature. Finally the product was
filtered, and then washed thoroughly with methylene dichloride.

38
M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
O
even after multistep synthesis. In case of recycled catalyst all
the peaks are retained in their respective positions with respect
to LDH/Pd(II)-complex, suggesting properties of recycled catalyst
remain unaltered.
LDH/Pd(II)
OH
R
Mol.O2, Pyridine,
R
H
Water,65oC, 6h
Scheme 2. Catalytic reaction pathway.
3.1.2. FTIR studies
Avance 400 MHz spectrometer. Surface area was determined by N2
adsorption–desorption at liquid nitrogen temperature (77 K) using
ASAP 2020 (Micromeritics).
FTIR spectra of (a) LDH, (b) LDH/SDS, (c) LDH/TPED, (d)
LDH/Pd(II) and (e) recycled catalyst are shown in Fig. 2. In case
of LDH, bands around 443 and 673 cm−1 are due to Al O and Zn
O
lattice vibrations. Sharp band around 1384 cm−1 is assigned to
stretching vibration of interlayer NO3−. Interlayer water also gives
rise to medium intensity absorption close to 1620 cm−1 ␦(H2O).
Broad bands in between 3300 and 3600 cm−1 (as a result of H-
bonding) are due to the ␯(OH) modes of the hydroxyl groups (both
from brucite like layers and from water molecules). The character-
2.3. Catalytic oxidation reaction
Benzyl alcohol (1 mmol), pyridine (5 mmol, 0.4 ml) as base,
LDH/Pd(II) catalyst (0.03 g), and 10 ml of water as solvent were
charged in a round bottom flask. The suspension was purged with
molecular oxygen. The reaction mixture was stirred at 65 ◦C under
1 atm of O2 (O2 bubbling rate, 30 mL min−1) for 6 h under stirring
(500 rpm). After completion of the reaction (Scheme 2), the cat-
alyst was separated by filtration/centrifugation. Organic part was
extracted with diethyl ether and the final product was analyzed
by GC-2010 (Shimadzu) equipped with capillary column (ZB-WAX,
30 m, 0.25 mm I.D.) and flame ionization detector (FID).
−
istic bands are detected for SO4 group in LDH/SDS. Bands around
1047 an−d 1202 cm−1 are due to asymmetric stretching of C
O
and SO4 bonds of surfactant. Typical C H stretching modes for
antisymmetric stretching of CH3 and CH2 at 2962 and 2927 cm−1
and symmetric stretching of CH3 and CH2 at 2873 and 2852 cm−1
respectively are observed. For TPED intercalated LDH, the intensi-
ties of the characteristic bands due to SDS molecules are generally
weakened as compared with that of LDH/SDS. This is a clear
indication of replacement of SDS molecules by TPED moiety and
stabilized by Br− anions. In the spectra of LDH/TPED, the bands
for amine group of TPED molecule overlap with O H stretching
vibration of LDH to produce a merged spectrum in the range of
3300–3500 cm−1. Band around 1645 and 1020 cm−1 is attributed
to the N H bending and C N bending vibration respectively. More-
over the spectra shows band around 1000–1300 cm−1 (ꢂas,Si O) and
456 cm−1 (ıSi O) respectively [22,23]. All the above results provide
an indication of the presence of TPED in the LDH host. In case of
LDH/Pd(II) the less intensity of amine bands is due to the coor-
dination of Pd to amine host. The recycled catalyst shows similar
spectra as that of a fresh catalyst, which indicates that the structural
integrity is maintained in case of the recycled catalyst.
3. Results and discussion
3.1. Characterization techniques
3.1.1. X-ray diffraction (XRD)
The XRD patter◦n of LDH-NO3 (Fig. 1) shows a basal reflection
˚
at 8.62 A (2ꢁ = 10.2 ), which is similar to that reported in the liter-
ature [20]. In case of SDS intercalated LDH the position of (0 0 3)
peak shifted to lower 2ꢁ = 3.6◦ corresponds to the interlayer dis-
◦
tance d003 ∼ 26.3 A. The four harmonics, i.e. at 2ꢁ = 4.8 (0 0 3), 10◦
(0 0 6), 15◦ (0 0 9) and 21◦ (0 1 2), attributed for LDH/Pd(II) species.
The observed shift of the (0 0 3) reflection to lower 2ꢁ values cor-
˚
˚
responds to an increase of the d003 basal spacing from ∼8.62 A for
˚
parent LDH to 22.8 A for LDH/Pd(II) sample. From these diffraction
peaks we conclude that functionalized Pd is present in the cat-
alyst. The retention of characteristic peak (1 1 0) around 2ꢁ = 60◦
indicated that the structure of the layer has been retained [21]
3.1.3. UV–vis spectral studies
The UV–vis DRS spectra of LDH, LDH/SDS, LDH/TPED, LDH/Pd(II)
complex are shown in Fig. 3. The spectra of LDH/Pd(II) complex has a
Fig. 1. XRD patterns of (a) LDH-NO3, (b) LDH/SDS, (c) LDH/TPED, (d) LDH/Pd(II) and (e) recycled catalyst.

M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
39
Fig. 2. FT-IR spectra of (a) LDH, (b) LDH/SDS, (c) LDH/TPED, (d) LDH/Pd(II) and (e) recycled catalyst.
strong absorption band at 232 nm and in the region of 340–365 nm,
it can be assigned to typical metal–ligand charge transfer (MLCT)
transition in Pd(II) co-ordinated to diamine complex [24]. The peak
around 400 nm refers to the existence of Pd(II), which is in agree-
ment with the literature [25].
structure [27]. By comparing N 1s XPS spectrum of LDH/TPED
and LDH/TPED/Pd(II) complex, it is obvious that N 1s XPS spectra
of LDH/TPED/Pd(II) complex is shifted to higher binding energy.
The positive shift indicates the decrease of electron cloud den-
sity around N atom, which suggests formation of chemical bonds
between Pd-ion and N atom of TPED. The amine group is the strong
electron donor to transition metal ion due to lone pair of electrons
of nitrogen atom. On the other hand Pd(II), as a soft metal ion, pos-
sesses empty lower energy orbit for accepting electron, as a result
of which strong bond formation takes place between N and Pd.
So, XPS results strongly suggest that there is a formation of N Pd
coordination bond.
3.1.4. XPS analysis
The presence of the catalytic groups (Pd and amine) was charac-
terized by X-ray photoelectron spectroscopy. The Pd(3d) spectrum
has two peaks at 337.3 and 342.4 eV which correspond to Pd 3d5/2
and 3d3/2 respectively of Pd(II) complex. The peak of Pd 3d5/2 at
337.3 eV can be attributed to Pd(II) in Pd Cl bond [26]. Binding
energy at 287.8 eV is due to the presence of carbon in the cat-
alyst as shown in Fig. 4. Moreover Fig. 5 shows that the binding
energy at 399.3 eV of N(1s) is attributed to amine such as (–NH–)
3.1.5. Thermal analysis
Fig. 6 shows the thermal analysis of LDH functionalized Pd(II)
complex. Weight loss is mainly divided into 3 regions: below 100 ◦C,
150–400 ◦C and 400–620 ◦C. Weight loss 30–150 ◦C is assigned due
to the loss of physically adsorbed water (1.8%). The large weight
loss between 480 and 620 ◦C is owing to the decomposition of co-
valently bonded organic groups (4.6%). A small amount of loosely
bound water causes weight loss (less than 2%) below 200 ◦C. This
is followed by a weight loss of about 9.1% between 350 and 500 ◦C
due to decomposition of organic ligand bound palladium complex.
3.1.6. EDX and SEM
The SEM image (Fig. 7) showed that the LDH prepared by co-
precipitation method has a good crystallinity. In addition, Pd loaded
in functionalized LDH has tetrahedral and plate like triangular
structure. The catalytic activity and selectivity of metallic catalyst
are strongly dependent on the shape of the active particle [28].
Energy dispersive X-ray spectroscopy (EDX) analysis provides local
information of different elements. EDX confirmed that Pd is present
in LDH/Pd(II) complex, which is not seen in case of LDH.
The exact amount of diaminopropyl groups and palladium in the
materials was determined by elemental analysis. Elemental anal-
ysis of LDH/TPED gave, C 12.93, H 2.92, N 6.08% and LDH/Pd(II)
complex was found to contain C 9.39, H 2.15, N 4.37%. ICP-AES
Fig. 3. UV–vis DRS spectra of LDH, LDH + SDS, LDH + diamine, and LDH + Pd complex.

40
M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
Fig. 4. XPS spectrum of final catalyst (i.e. TPED/Pd(II)-LDH): (a) wide scan spectrum and high resolution XPS spectra of (b) Pd 3d and (c) C 1s peaks.
analysis of LDH/Pd(II) complex gave Pd 16.63% which confirmed
the presence of palladium in the Pd/LDH catalyst.
3.1.7. NMR analysis
29Si CP MAS NMR spectra (Table 1) provide significant infor-
mation about the degree of functionalization and the nature of
the M
O Si bond around Si atom. Peaks in the T region are con-
tributed by Si atoms of the triethoxy silane and are denoted by Tn
Table 1
Solid state 13C and 29Si CP MAS NMR chemical shifts (ppm); the peak assignment
for different type of Ca and Tb notations.
System
Chemical shifts (ppm)
29Si
13C
LDH/SDS
71.9(C1), 61.8(C2),
42.7(C3), 36.4(C4–C10),
28.7(C11), 19.2(C12
)
Position
% integral
8.65
LDH/TPED
9.8(C1), 21.3(C2),
38.8(C3), 47.2(C4),
52.3(C5)
−46.57(T1)
−57.69(T2)
−66.87(T3)
38.53
52.82
a
C notations (C1–12) are used to describe the numbering of carbon atoms.
b
T notations (T1–3) are used to describe the extent of cross linking M
O Si bond.
Fig. 5. N (1s) XPS spectrum of (a) LDH + TPED and (b) LDH/TPED/Pd(II) complex.

M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
41
(n = 0, 1, 2, 3), which are used to describe the extent of cross-linking
depending on M
Si bond types. T3 represents the formation of
three Si M bonds, which indicates complete cross-linking in the
hybrid materials, while T2 represents formation of two Si
O
O
O
M
bonds bearing one uncoordinated Si-OEt group. High intensity of
T3 peak reveals the high degree of condensation of the original tri-
ethoxy silane with LDH. As shown in Fig. 8, 29Si CP MAS NMR spectra
of both LDH/TPED and LDH/TPED/Pd(II) sample display 3 peaks
downfield corresponding to T3 (ı = −66 ppm), T2 (ı = −57 ppm)
and T1 (ı = −46 ppm). This result confirms the successful covalent
attachment of TPED through Si
O
M linkage. Moreover the 13C CP
MAS NMR spectrum in TPED functionalized LDH shows five differ-
ent electronic environments at 9.8(C1), 21.3(C2), 38.8(C3), 47.2(C4),
52.3(C5) ppm corresponding to the different carbon atoms (Fig. 9).
C1, C2, C3 are ascribed to propyl spacer, C4 corresponds to carbon
atom attached to NH and C5 attributed to carbon atom attached
to NH2 and the chemical shifts match well with earlier literature
reports on similar systems [29–31]. All the peak positions for dif-
ferent types of environments (29Si and 13C) are summarized in
Table 1.
Fig. 6. TGDTA diagram of LDH/TPED/Pd(II) complex.
On the basis of the N2 adsorption–desorption isotherms, the
textural measurements are listed in Table 2. The textural property
of LDH/TPED and LDH/TPED/Pd(II) complex decrease in surface area
and pore volume than that of LDH. This decrease in surface area
confirmed that Pd(II) organometallic species was immobilized onto
LDH. One more decrease in pore volume on Pd loading species sug-
gests that they were not only deposited on the external surface, but
also entered inside the hydrotalcite species.
Table 2
Textural measurement of the LDH and Pd catalysts.
Catalyst
Surface area BET (m2 g−1
)
Pore volume (cm3 g−1
)
ZnAl-LDH
LDH/TPED
120
48
0.62
0.32
0.25
LDH/Pd(II) complex
48
Fig. 7. SEM image and EDX analysis of (a) Zn/Al LDH and (b) LDH/Pd(II).

42
M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
Fig. 8. 29Si MAS NMR spectra of (a) LDH/TPED and (b) LDH/TPED/Pd(II) complex.
3.2. Catalytic performance in oxidation reaction
catalysts [41,42]. In contrast to traditional physisorbed heteroge-
neous catalysts, in the interphase counterparts, the organic spacer
provides sufficient mobility of the reactive centre and diminishes
any desired steric effect of the matrix over the accessibility of the
reactive centre, which enhances the catalytic activity.
The use of palladium catalysts for the oxidation of alcohols to
aldehydes and ketones in the presence of various types of oxidants
is well known [32–34]. The advantages of using molecular oxy-
gen as the oxidant in the Pd-catalyzed oxidation of alcohols has
also been explored [35,36]. Moreover it has been reported that a
Pd(II) complex physisorbed onto hydrotalcite, effects the aerobic
oxidation of alcohols to the corresponding carbonyl compounds
in the presence of pyridine as an exogenous base [37,7]. Recently,
many researchers including our group have developed a variety of
catalytic system based on palladium loaded LDH for various cat-
alytic applications [38–40]. However reports on Pd(II) loaded on
diamine functionalized LDH for oxidation of primary alcohol are
rather scarce. An important strategy to transform a homogeneous
process into a heterogeneous one is to covalently incorporate the
active site onto a large solid surface area through an organic entity
(flexible spacer) to create organic inorganic hybrid (interphase)
The oxidation of benzyl alcohol over Pd(II)–LDH in various sys-
tems was carried out and the result is illustrated in Table 3. In this
present investigation, oxidation of benzyl alcohol was used as a
standard substrate in presence of catalytic amount of LDH/Pd(II)
taking water as solvent in presence of molecular O2. The oxida-
tion of benzyl alcohol (entry no. 1, Table 4) in water at 65 ◦C
for 6 h in the presence of catalytic amount of LDH/Pd(II) under
O2 afforded 94% conversion of benzaldehyde and 100% selectiv-
ity towards benzaldehyde. Here substituted benzyl alcohols with
either electron-donating or electron-withdrawing substituents in
benzene ring were all less reactive than benzyl alcohol over our
catalyst (Table 4), which is in good agreement with the literature
[43]. In our present study, the electronic influence of substitution
Fig. 9. 13C NMR spectra of LDH/SDS and LDH/TPED.

M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
43
Table 3
Oxidation of benzyl alcohol catalyzed by different heterogeneous catalysts supported on LDH.a
Catalyst
Solvent
Temperature (◦C)
Timeb (h)
Conv. (%)
Reference
Pd(II) complex in functionalized LDHa
ZnAl-LDHa
Pd(II)-HTLc
Pd(II)/NiZn HTLc
Pd(II)-HTLc
Water
Water
Toluene
Trifluoro toluene
Toluene
65
65
80
80
80
6
20
2
1
2
94
8
100
99
74
This work
This work
[7]
[17]
[37]
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.03 g, 0.5 mol% of Pd), pyridine (5 mmol), p = 1 atm, O2 bubbling rate (30 mL min−1), water as solvent (10 mL),
conversion and selectivity were determined by gas chromatography.
b
Time required for maximum conversion.
on phenyl ring has also been studied (Table 4, entries 5 and 6).
Primary benzyl alcohol with electron donating group substituent
(Table 4, entry 5) accelerates the reaction rate, while withdraw-
ing group substituent (Table 4, entry 6) retards the reaction rate.
This is presumable due to its electron deficiency in case of electron
withdrawing group, similar to the results with other homogeneous
Pd(II) catalyst [7]. The oxidation of straight chain aliphatic alco-
hols (Table 4, entries 2 and 3) under standard reaction conditions
took place effectively over this catalyst. This process reported here
allows a convenient method for oxidation of primary alcohols to
aldehydes.
In addition, pyridine is an oxidatively stable ligand which
modulate catalyst stability and activity [44]. Addition of pyridine
(5 mmol) to the reaction media leads to increase in the efficacy of
the catalyst. On the other hand if the amount of the catalyst was
reduced to 0.01 g, longer reaction time was required to achieve the
complete conversion of benzaldehyde (Fig. 11). An interphase cata-
lyst is composed of three parts: an inert matrix (support), a flexible
organic spacer, and an active centre. The high conversion and selec-
tivity towards benzaldehyde demonstrate the beneficial influence
of designing a complex with sufficient large flexibility, which allows
the substrate–complex adduct to adopt a spatial disposition that is
similar to that in the homogeneous phase. The coordination envi-
ronment around Pd in our catalyst is less crowded in comparison to
the bulkier metal-porphyrins or metal-phthalocyanines [45]. Prob-
ably due to this, molecular O2 has enough space to bind Pd in the
intermediate stage of the catalytic reaction and thereby facilitating
the oxidation reaction.
A remarkable piece of work has recently been done by Shi et al.
[46], where LDH-supported Au–Pd bimetallic nanoparticle cata-
lysts have been employed for oxidation of various alcohols in which
LDH is synthesized by urea hydrolysis method where reaction
was performed at high temperature (i.e. at 100 ◦C). In compari-
son to their work, in our catalytic system, LDH has a functionalized
hydrophilic surface and alkyl group end capping that renders sur-
face hydrophobicity. These properties make it possible to use water
as a solvent in the reaction. It was anticipated that water would
interact with the hydrophilic surface and facilitate the transport of
the hydrophobic substrate towards the hydrophobic surface, ren-
dering the substrate and the catalyst to be in close proximity, thus
leading to enhanced reactivity [47]. Meanwhile, the strong surface
hydrophobicity could prevent the entrancing of water molecules
into the pore channels and thus, could enhance the hydrothermal
stability of the layered structure [48]. Furthermore relatively hard
nitrogen donor ligand plays an important role in enhancing the rate
of catalyst in oxidation reaction. This reaction generally exhibits
high catalytic stability because it has a palladium (II) catalyst in the
resting state, which is less prone to decompose by aggregation into
palladium black. Previous studies have shown that the presence of
an aromatic nitrogen ligand capable of ␲-back-bonding from palla-
dium to the ligand is required to prevent the irreversible reduction
of Pd(II) to Pd black by the alcohol substrate [49,50,6]. Hence in the
Table 4
Oxidation of primary alcohols using LDH/Pd(II) as catalyst.a
Entry
1
Substrate
Product
Time (h)b
6
Conv. (%)
94
Selectivity (%)
TOFc (h−1
)
CHO
CH2OH
100
31
CHO
CHO
OH
OH
2
3
12
12
88
88
100
100
14
14
(CH )CHO
(CH2)2OH
2
4
5
10
8
91
92
98
18
23
CHO
CHO
OH
100
MeO
NO2
MeO
NO2
OH
6
10
89
100
17
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.03 g, 0.5 mol% of Pd), temperature 65 ◦C, pyridine (5 mmol), p = 1 atm, O2 bubbling rate (30 mL min−1), water as
solvent (10 mL), conversion and selectivity were determined by gas chromatography.
b
Time required for maximum conversion.
Turnover frequency (TOF) = Turnover number (TON) per hour; TON calculated as (mol% of product)/(mol% of metal catalyst).
c

44
M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
OH
I
H2O2
IV
R
R'
Ln Pd(II)
O
+
R'
2H+
2H+
R
III
-nL, deactivation
O
O
Ln Pd(0)
[Pdo]m
aggregated
Ln Pd
II
O2
Scheme 3. Possible mechanistic pathway for alcohol oxidation.
Fig. 11. Effect of amount of catalyst with time on oxidation of benzyl alcohol.
absence of ␲-acidic N-based ligand, the aqueous palladium (II) salts
are irreversibly reduced to heterogeneous palladium black.
However, the mechanism is still a matter of discussion and
unambiguous proof of the nature of the active noble metal sites
appears to be of fundamental importance [51,52]. Hence in this
piece of work, Pd(II) catalyzed aerobic oxidation of primary alcohol
is likely to proceed through a reduction of Pd(II) to Pd(0) by the
alcohol (Scheme 3). Pd(0) then reacts with dioxygen to produce a
palladium peroxo intermediate (step II). The reaction rates are fur-
ther improved by adding pyridine, which might prevent the Pd(0)
aggregation [53] to form palladium metal (step III).
To examine the effect of reaction time on the conversion per-
centage we have performed the reaction by increasing the time by
1 h. As expected, the conversion rate increases with time up to 6 h
(Fig. 10) and then maintained at a steady rate, i.e. no significant
change in conversion is observed with further increase in reaction
time.
Table 5
Recyclability test up to four cycles.a
No. of cycles
Conversion (%)
Selectivity (%)b
Fresh
94
93
93
91
88
100
100
100
100
100
1
2
3
4
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.03 g, 0.5 mol% of Pd),
pyridine (5 mmol), temperature 65 ◦C, p = 1 atm, O2 bubbling rate (30 mL min−1),
water as solvent (10 mL), time = 6 h.
b
Benzaldehyde.
which indicates that Pd species was not leached into the reaction
mixture, which confirmed the heterogeneity of the catalyst.
3.4. Reusability test
3.3. Heterogeneity test
For the recyclability tests, the reactions were performed under
the same reaction conditions as described above. Each time, the
catalyst was isolated from the reaction solution at the end of the
catalytic reaction, washed with ethanol, and heated at 100 ◦C. The
dried catalyst was then reused in a next run. The catalyst was reused
up to four cycles, which shows almost the same rate as that of the
first run (Table 5). In contrast to traditional physisorbed heteroge-
neous catalysts, the interphase counterparts have the advantage of
easy separation and recovery of the heterogeneous catalysts.
To study the leaching of Pd during the reaction, a hot filtration
test was performed for oxidation reaction. During the catalytic oxi-
dation of benzyl alcohol, the solid catalyst was separated from the
reaction mixture by filtration after 3 h of the reaction and the filtrate
obtained was continuously stirred under same reaction conditions
for further 3 h. GC analysis showed no increment in the conversion,
4. Conclusions
In summary, we have developed a simple method for tethering
a palladium complex onto diamine-functionalized LDH, in which
the active palladium catalyst is stabilized by the ␲-acidic nitrogen
ligand in aqueous solution and has performed full characteriza-
tion of the resultant material. This material is effective enough to
show high performance towards oxidation of primary alcohol using
water as a solvent. XRD, FTIR, NMR and XPS analyses indicate the
successful formation of palladium complex, in which the ligand is
covalently anchored with LDH. The present hybrid heterogeneous
catalysts had the advantages of good crystallinity, recyclability and
facile separation as well as providing the ability to monitor. The
results are quite good using water as a solvent and molecular O2
as an oxidant without leaching of Pd species. Furthermore the cat-
alyst was efficient enough, so that it could be reused several times
without losing catalytic activity and structural integrity.
Fig. 10. Effect of time on oxidation of benzyl alcohol.

M. Sahoo, K.M. Parida / Applied Catalysis A: General 460–461 (2013) 36–45
45
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