β-Diketimine appended periodic mesoporous organosilica as a scaffold for immobilization of palladium acetate: An efficient green catalyst for Wacker type reaction

Article abstract of DOI:10.1016/j.jcat.2014.07.019

A series of β-diketimine appended periodic mesoporous organosilicas (PMOs) were prepared and characterized. These PMOs were found to be an excellent scaffold for the immobilization of palladium acetate [Pd(OAc)₂] through the complexation between Pd(OAc)₂ and iminic nitrogen atoms of β-diketiminate, leading to the formation of efficient heterogeneous catalysts (Pd/PMOs) for the oxidation of styrene. Catalytic evaluations show that, in liquid phase using H₂O₂ as oxidant, Pd/PMOs could promote styrene to undergo Wacker type reaction with a high selectivity of up to 100% toward the formation of acetophenone, a useful chemical with high demand in the global market. The novel solid catalyst offers a good catalytic recyclability with consistently high catalytic activity. In addition, it was experimentally demonstrated that H₂O₂ plays an important role to prevent unwanted catalyst deactivation. Thus, the current work presents a convenient approach for the direct production of acetophenone from styrene.

Full text of DOI:10.1016/j.jcat.2014.07.019

Journal of Catalysis 318 (2014) 43–52
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
b-Diketimine appended periodic mesoporous organosilica as a scaffold
for immobilization of palladium acetate: An efficient green catalyst for
Wacker type reaction
a
a,b,
⇑
Parijat Borah , Yanli Zhao
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore
School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of b-diketimine appended periodic mesoporous organosilicas (PMOs) were prepared and
Received 12 March 2014
Revised 13 June 2014
Accepted 27 July 2014
characterized. These PMOs were found to be an excellent scaffold for the immobilization of palladium
acetate [Pd(OAc)
nate, leading to the formation of efficient heterogeneous catalysts (Pd/PMOs) for the oxidation of styrene.
Catalytic evaluations show that, in liquid phase using H as oxidant, Pd/PMOs could promote styrene to
2 2
] through the complexation between Pd(OAc) and iminic nitrogen atoms of b-diketimi-
2 2
O
undergo Wacker type reaction with a high selectivity of up to 100% toward the formation of
acetophenone, a useful chemical with high demand in the global market. The novel solid catalyst offers
a good catalytic recyclability with consistently high catalytic activity. In addition, it was experimentally
demonstrated that H O plays an important role to prevent unwanted catalyst deactivation. Thus, the
2 2
current work presents a convenient approach for the direct production of acetophenone from styrene.
Keywords:
b-Diketimine
Heterogeneous catalysis
Palladium acetate
Periodic mesoporous silica
Wacker reaction
Ó 2014 Elsevier Inc. All rights reserved.
1
. Introduction
In 1999, seven years after Mobil reported MCM 41-type meso-
and so on [10–14]. For instance, Jin et. al. have reported air and
moisture stable b-diketiminato-Pd complex for catalytic C-C
coupling reactions [12–14]. In this work, we capitalized the
porous silica materials [1], researchers have developed a novel
concept to prepare mesoporous materials in which an organosilane
containing at least two terminal trialkoxysilane groups was used as
a silica source to form organic/inorganic hybrid materials, termed
as periodic mesoporous organosilica (PMO) [2–4]. The presence
of organic functionality within the inorganic silica framework not
only provides the robustness to the structure, but also makes
PMO potentially useful for various applications. Designing catalyt-
ically active PMO, one of the most important applications, is a
novel strategy for transforming a catalytically important transition
metal complex into a heterogeneous catalyst [5–9].
In the current research, we synthesized a new type of PMO
immobilized with b-diketimine moiety bound onto the framework
walls through a distribution at molecular level. b-Diketimine has
gained a lot of attention as a versatile ligand for the formations
of various transition metal complexes that have been proven to
be useful in catalysis, metallo-organic chemical vapor deposition,
embedded b-diketiminate centers in PMO to coordinate with
2
Pd(OAc) , leading to a heterogeneous architecture consisting of
2+
stable PMO immobilized with b-diketiminato-acetate Pd
complex for catalytic applications.
In the area of catalysis, one of the important research directions
is the direct and selective oxidation of various hydrocarbons on
account of their industrial as well as academic significance. Among
various commercially imperative oxidation processes, Wacker
reaction is always a popular topic especially in the functionaliza-
tion of terminal olefins [15–17]. An excellent representation of
Wacker reaction is the selective oxidation of styrene to acetophe-
none, an important raw material in petrochemical, pharmaceuti-
cals, resin, and fragrance industries [18]. A commonly utilized
industrial process for the production of acetophenone is the acety-
lation of benzene using acetyl chloride/acetic anhydride, which is
considered as an inefficient process [19]. On the contrary, Pd cata-
lyzed Wacker reaction is a clean process to produce acetophenone
as compared to the Friedel–Crafts route. However, Wacker reaction
for the styrene oxidation is often associated with various problems,
such as low selectivity toward the acetophenone production, diffi-
⇑
Corresponding author at: Division of Chemistry and Biological Chemistry,
School of Physical and Mathematical Sciences, Nanyang Technological University,
culties in the product separation, uses of acetic acid and CuCl
2
to
2
1 Nanyang Link, Singapore 637371, Singapore.
promote the reoxidation of Pd(0) into highly active Pd(II) resulting
E-mail address: zhaoyanli@ntu.edu.sg (Y. Zhao).
http://dx.doi.org/10.1016/j.jcat.2014.07.019
021-9517/Ó 2014 Elsevier Inc. All rights reserved.
0

4
4
P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
in the corrosion hazards of the reactor, and generation of large
amounts of toxic and corrosive wastes [20]. In order to circumvent
these major drawbacks associated with the Wacker process, many
catalytic systems have been investigated for the selective oxidation
of styrene to acetophenone. Hitherto, improved efficiency of the
catalytic systems has been attempted by avoiding copper(II) salts
Na
three times (20 mL each). The organic layer was dried through
MgSO and then concentrated under reduced pressure. The solid
product was washed with cold ethanol and dried under vacuum
2 3
CO . The solution was extracted with dichloromethane for
4
1
to give compound 1. Product yield 34%.
3
H NMR (CD OD,
300 MHz, ppm) d 7.30 (d, 4H, J = 9 MHz), 6.72 (d, 4H, J = 9 MHz),
1
3
[
20], using supported catalysts [21–23], surfactants [24], cyclodex-
2.08 (s, 6H), 1.29 (s, 2H); C NMR (400 MHz, CDCl
3
, ppm) d
trins [25], and calixarenes [25]. In addition, making use of various
alternative solvents such as fluorous phase [26], polyethylene gly-
158.9, 153.4, 142.3, 121.2, 118.6, 40.0, 23.2; IR:
m
3430, 3062,
À1
2852, 1680, 1610, 1594 cm
17 18 2 2
; MS: calcd for C H N O
cols [27], supercritical CO
2
[21,28], and ionic liquids [19,29] have
m/z = 282, found: m/z = 282. Elemental analysis: calcd%: C 72.32,
been adopted to reduce the main drawbacks of the Wacker oxida-
tion process. However, the development of a simple, efficient, and
environmentally benign method for selective synthesis of styrene
to acetophenone is still of great challenge.
H 6.43, N 9.92, found%: C 72.88, H 6.08, N 10.1.
2.2. Synthesis of b-diketiminato bis(4,1-phenylene) bis(3-
triethoxysilylpropylcarbamate) (2)
In order to address the challenge, herein, we developed a novel
2
strategy to immobilize Pd(OAc) onto the PMO frameworks via the
In a typical synthesis of compound 2 (Scheme 1), synthesized
compound 1 (1.0 g, 3.5 mmol) was dissolved in dry THF (50 mL)
under N in a two-necked round bottom flask equipped with a con-
2
denser. 3-Isocyanatopropyltriethoxysilane (1.8 g, 7.0 mmol) was
then added dropwise to the solution over 30 min. After the reac-
tion mixture was refluxed for 24 h, it was cooled down to room
temperature. THF was removed in vacuo, leaving a highly viscous
liquid. Hexane was added to the viscous liquid, and the precipitates
were formed to afford compound 2. Product yield 80%. 1H NMR
2+
formation of b-diketiminato-acetate Pd complex. Heterogeneous
catalysis of the styrene oxidation in liquid phase using PMO based
catalysts have been studied in few instances, where epoxide and
benzaldehyde were formed as selective products [30–33]. To our
best knowledge, this is the first demonstration of employing
PMO based catalytic system for selective oxidation of styrene to
2 2
acetophenone through Wacker reaction using H O as oxidant. In
the present work, various control reactions were carried out in
order to establish a green catalytic process in terms of high selec-
tivity as well as high catalyst reusability.
(
CD
J = 12 MHz), 3.84 (q, 12H, J = 8 MHz), 3.16 (t, 4H, J = 8 MHz), 2.11
s, 6H), 1.64 (s, 2H), 1.22 (t, 18H, J = 8 MHz), 0.66 (t, 4H,
3
OD, 400 MHz, ppm) d 7.54 (d, 4H, J = 8 MHz), 7.04 (d, 4H,
(
1
3
2
. Experimental
J = 8 MHz); C NMR (400 MHz, CDCl
3
, ppm) d 159.4, 151.8 149.4,
1
44.2, 124.2, 121.0, 57.8, 45.0, 40.2, 24.8, 22.9, 18.4, 11.2; IR:
m
À1
All the chemicals and materials used in the synthesis and catal-
3323, 2976–2887, 1714, 1670, 1551, 1506, 1207, 1082 cm ; MS:
calcd for C37H N O10Si m/z = 777, found: m/z = 778 (MH ). Ele-
60 4 2
+
ysis were purchased from Sigma-Aldrich or Alfa Aesar and used
without further purifications.
mental analysis: calcd%: C 57.19, H 7.78, N 7.21, found%: C 57.01,
H 8.19, N 7.43.
2.1. Synthesis of b-diketimine (1)
2.3. Synthesis of PMOs by hydrothermal process
b-Diketimine (1) was synthesized according to a reported
method with some modifications [12]. In a typical synthesis
PMO was prepared according to a reported method with some
(
(
Scheme 1), acetylacetone (1.0 g, 10 mmol) and 4-aminophenol
2.2 g, 20 mmol) were dissolved in absolute ethanol (50 mL) in a
modifications [8]. Compound 2 and tetraethylorthosilicate (TEOS)
were used as silica sources, and cetyltrimethylammonium bromide
(CTAB) was used as the structure-directing agent. A gel was formed
round bottom flask (100 mL) containing a magnetic stir bar. Conc.
HCl (0.5 g) was added into the mixture solution. After the reaction
mixture was refluxed for 3 days, the mixture was cooled down to
room temperature and treated with water (20 mL) containing
3 2
with a particular molar ratio of Si: CTAB: NH (25%): H O:
EtOH = 1.0: 0.12: 8.0: 114: 10. Three different molar ratios of 2 to
TEOS including 2%, 5%, and 10% were used, leading to PMO-1,
Scheme 1. Schematic representation for the synthesis for compounds 1 and 2.

P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
45
PMO-2, and PMO-3, respectively. In a typical synthesis of PMO
with 2% of 2, CTAB (1.5 g, 4.1 mmol) was dissolved in H
using an autosorp-IQ instrument from Quantachrome Instruments
Corporation. The BET method was utilized to calculate the specific
surface areas of the PMO materials, whereas the NLDFT equilib-
2
O
(
70 mL), and then NH
3
solution (20 mL of 25% NH
3
solution,
À1
0
.9 g mL , 265 mmol) was added. The mixture solution was stir-
2
rium model for cylindrical pores considering N as adsorbate and
red at 40 °C for 30 min to give a clear solution. A premixed solution
of 2 (0.5 g, 0.6 mmol) and TEOS (5.8 g, 28.0 mmol) in EtOH (18 mL)
was added to the above solution under vigorous stirring. After 2 h,
the resulted gel was transferred to a polyethylene container, which
was then heated to 90 °C for 4 days. The solid obtained was washed
with H O and dried in air at 60 °C. The structure-directing agent
2
was finally removed by extraction of the solid with dilute ethanolic
HCl solution (20 mL of 0.5 M ethanolic HCl for 0.5 g of solid) at
silica as adsorbent at 77 K was adopted for measuring the pore size
distributions of the PMOs. Elemental analyses (EA) were obtained
from EuroVector Euro EA Elemental Analyzer. Thermo Fischer iCAP
6000 series inductively coupled plasma-atomic emission spectros-
copy (ICP-OES) was used for performing trace metal analysis. X-ray
photoelectron spectroscopy (XPS) analysis was carried out by a
SPECS HSA3500 plus spectrometer using Mg X-ray source. Quadru-
pole ion trap mass spectrometer equipped with Thermo Finnigan
Trace GC ultra was used for the identification of various products
from the catalytic reactions.
4
0 °C for 2 h. PMO was obtained by filtration and then dried in
vacuo.
2.4. Synthesis of PMOs containing Pd complex (Pd/PMO-n)
3
. Results and discussion
In a typical synthesis of Pd/PMO, Pd(OAc)
was dissolved in dry dichloromethane (10 mL) and then PMO-n
200 mg) was added. The mixture was stirred at room temperature
for 24 h. The resulted solid was isolated by centrifugation and
washed with dichloromethane using Soxhlet extraction for another
2
(23 mg, 0.1 mmol)
3
.1. Synthesis of organo alkoxysilane
(
PMO is a unique hybrid material with a uniform molecular dis-
tribution of organic bridging units within inorganic silica network.
In the current work, an essential organic precursor with two termi-
nal alkoxysilanes, compound 2, was synthesized through two-step
reactions (Scheme 1). Throughout the synthesis including the iso-
lation and storage of compound 2, moisture free and inert condi-
tions were adopted in order to avoid unwanted polymerization of
silane. Compound 2 was characterized by common analytical tools
as mentioned in the experimental section. The FT-IR spectrum
2
4 h. The solid was then dried at 80 °C under vacuum for 12 h to
yield a brown powder. After the incorporation of Pd(OAc) into
2
PMO-1, PMO-2, and PMO-3, the resulted materials were denoted
as Pd/PMO-1, Pd/PMO-2, and Pd/PMO-3, respectively.
2.5. Catalytic tests
(
Fig. S2 in the SI) of compound 2 shows characteristic signals
The catalytic tests were carried out in a two-necked round
À1
À1
attributed from C@O (1714 cm ), NAH (3323 and 1670 cm ),
C@N (1551 cm ), CAN (1506 cm ), Si-C (1207 cm ), SiAO
(
vides solid evidence for the successful formation of compound 2.
The H NMR (Fig. S6 in the SI) and C NMR (Fig. 2a) spectra were
also obtained to characterize the compound.
bottom flask. In a typical experiment, styrene (1.0 g, 10 mmol)
was added to an acetonitrile solution (30 mL) containing Pd/
PMO-n as catalyst. In order to avoid immediate decomposition of
À1
À1
À1
À1
À1
1082 cm ), and CAH (2976–2887 cm ) vibrations, which pro-
H
2
O
2
and its strong effervescence, 30% H
2
O
2
(3.4 g, 30 mmol) was
1
13
added slowly to the mixture solution over a period of 3 h. The
resulting mixture was stirred under 400 rpm at 60 °C for 5 h. The
product was analyzed qualitatively using gas chromatography–
mass spectrometry (GC–MS). Then, the solvent from the reaction
mixture was evaporated under vacuum, and the crude product
was purified by column chromatograph using silica gel with n-hex-
ane/ethyl acetate (v/v 5:1) as eluent. The acetophenone product
was obtained as a colorless liquid. Isolated product was also
weighed to validate the GC–MS result and further characterized
by MS and NMR. A similar procedure was adopted to carry out
3
.2. Synthesis and characterization of b-diketiminate incorporated
PMOs
3
.2.1. FT-IR study of PMOs
The FT-IR spectra (Fig. 1) of PMO-1, PMO-2, and PMO-3 show
characteristic peaks of typical siloxane network in mesoporous sil-
ica materials [34,35]. The peaks in the range from 1550 to
1400 cm
À1
2
+
(inset in Fig. 1) in all the three FT-IR spectra are
the catalysis using Pd(OAc)
2
and b-diketiminato-acetate Pd com-
assigned to the organic unit in compound 2, providing clear evi-
dence of immobilized compound 2 into the organosilica frame-
work. It can also been seen from the spectra that the intensities
of the peaks attributed by 2 gradually increased upon the loading
amount increase of compound 2 during the preparation. The pre-
dominant band of physisorbed water centered at 1630 cm over-
laps completely with the peaks of the C@O and C@N bonds, while
the signals attributed by the C@N, CANH, and CAO bonds could be
plex (L-Pd(OAc) ) as the catalyst (see the Section S8 in the Support-
ing Information (SI) for more details).
2
In order to examine the recyclability of the Pd/PMO catalyst, the
catalyst from the reaction mixture was recovered by centrifugation
followed by washing with acetonitrile for several times. The recov-
ered catalyst was kept under vacuum oven for complete dryness
beforeusing for the subsequentcatalysis or various characterizations.
À1
observed in the spectra (inset in Fig. 1). A broad band around
2.6. Characterizations
À1
3
426 cm
is attributed to the OAH stretching vibration of
1H NMR spectra were recorded on a Bruker spectrometer
hydrogen bonded silanols on PMOs [36].
operating at 400 MHz. The solid-state NMR spectra were
performed on an ECA400 NMR spectrometer (JEOL, 400 MHz) using
a 4 mm CP/MAS probe at room temperature. Scanning was carried
out with spinning rate of 10000 Hz and 5000 scans. Transmission
electron microscopy (TEM) images were collected on a JEOL JEM-
1
3
29
3.2.2. C and Si solid-state NMR study of PMOs
The 13C cross-polarization magic angle spinning (CP MAS) NMR
spectra of PMO-1, PMO-2, and PMO-3 (Fig. 2) exhibit strong signals
ranging from 9 to 42 ppm, which can be attributed to different ali-
phatic primary and secondary carbon atoms from the organic
bridging unit. Strong signals from aromatic carbons on the phenyl
ring can be identified at ꢀ105 ppm as well as in the range from 140
to 150 ppm. In addition to these signals, two significant peaks at
151 and 157 ppm are assigned to the carbons from carbamate
1
400 instrument operated at 100 kV. FT-IR spectra were recorded
as KBr pellets on a SHIMADZU IRPrestige-21 spectrometer with
the sample powder diluted in KBr (1%). N adsorption/desorption
2
measurements for Brunauer–Emmett–Teller (BET) and non-local
density functional theory (NLDFT) calculations were performed

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6
P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
Fig. 1. FT-IR spectra of (a) PMO-1, (b) PMO-2, (c) PMO-3, and (d) compound 2.
Fig. 3. 29Si CP MAS NMR spectra of (a) PMO-1, (b) PMO-2, and (c) PMO-3.
Fig. 2. 13C NMR spectrum of (a) compound 2 and 13C CP MAS NMR spectra of (b)
PMO-1, (c) PMO-2, and (d) PMO-3.
organic bridging unit into the silica framework through the hydro-
lysis of terminal alkoxysilanes.
and imine groups of the incorporated organic bridging unit,
respectively. Since these signals can be easily correlated with the
3.2.3. Isothermal N
2
adsorption/desorption measurements of PMOs
adsorption/desorption measurements were
1
3
C NMR spectrum of compound 2 (Fig. 2a), the CP MAS NMR data
provide a solid support for the successful incorporation of com-
The isothermal N
carried out for the three PMOs to determine their surface areas
and pore size distributions (Figs. S7 and S8 in the SI). From the
2
2
9
pound 2 into the silica framework in the synthesized PMOs. Si
CP MAS NMR analysis was also carried out for all the three PMOs
in order to realize the covalent incorporation of the organic bridg-
ing units into the siloxane network. 29Si NMR spectra of PMO-1,
PMO-2, and PMO-3 (Fig. 3) show the signals with major intensities
N
2
adsorption isotherm under the lowest pressure, the BET surface
areas of PMO-1, PMO-2, and PMO-3 were measured to be 720, 676,
2
À1
and 712 m g , respectively (Table 1). The NLDFT equilibrium
centered at À110 and À100 ppm and a small shoulder at À90 ppm,
4
Si], Q3 [(„SiO)
Table 1
which can be assigned to Q [(„SiO)
4
3
Si(OR)], and
BET surface area and NLDFT pore diameter of PMOs.
2
Q
3 2
[(„SiO) Si(OR) ], respectively, where R = Et or H. Along with
BET surface area (m2
g
À1
)
NLDFT pore diameter (nm)
No
Materials
these signals, all the spectra displayed one more peak at
À67 ppm, corresponding to T resonance from silicon species
1
2
3
PMO-1
PMO-2
PMO-3
720
676
713
2.9
3.0
3.7
attached to the organic group. 29Si CP MAS NMR spectra also pro-
vide solid evidence in support of complete immobilization of

P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
47
model for the pore size distributions indicates the presence of mes-
opores in all the PMOs, and the average pore diameters for PMO-1,
PMO-2, and PMO-3 were found to be 2.9, 3.0, and 3.7 nm, respec-
tively (Table 1). A profound effect was observed between the load-
ing percentages of compound 2 and the mesopore size, which can
be attributed to the swelling of the pore channels caused by the
addition of organic bridging unit with high degree of complexity.
discussed in the experimental section (Scheme 2). The Soxhlet
extraction method was adopted to ensure the removal of free
2
+
2+
Pd from the materials, leaving behind only the Pd immobilized
2
+
PMOs. Successful incorporation of Pd was confirmed quantita-
tively through the ICP analysis for each Pd/PMO, and the obtained
results show that the Pd contents in Pd/PMO-1, Pd/PMO-2, and
À1
Pd/PMO-3 are 0.13, 0.16, and 0.24 mmol g , respectively (Table
2). From EA, it was found that the amount of b-diketiminato unit
3
.2.4. Mesostructural morphology of PMOs
TEM analysis (Fig. 4a–c) shows mesostructural morphologies
in each Pd/PMO is almost equal to the Pd content in the Pd/PMO,
indicating the 1:1 coordination between them (Table 2).
for PMO-1, PMO-2, and PMO-3, which are in accordance with the
NLDFT calculation from N adsorption/desorption measurements.
2
3.3.1. TEM study of Pd/PMOs
PMO-1 and PMO-2 exhibit the presence of regular mesoporous
channels. However, the periodicity of the mesoporous channels
gradually declines upon increasing the loading percentage of
organic unit, which becomes significant in the case of PMO-3.
The TEM images (Fig. 4d–f) show the retention of the
mesostructural morphologies for Pd/PMO-1, Pd/PMO-2, and
Pd/PMO-3, which are apparently similar to that of PMO-1,
PMO-2, and PMO-3, respectively. The observations indicate that
the synthesized PMOs possess enough structural stability in order
to maintain their morphological integrity even after the metal
3.3. Synthesis and characterization of Pd/PMOs
incorporation. In many previous reports where Pd(OAc)
2
was
The incorporation of Pd(OAc)
generating b-diketiminato-acetate Pd complex through a method
2
into PMOs was achieved by
attempted to load into porous materials, black dots were observed
in the TEM images of obtained materials, which may be attributed
2+
Fig. 4. TEM images of (a) PMO-1, (b) PMO-2, (c) PMO-3, (d) Pd/PMO-1, (e) Pd/PMO-2, and (f) Pd/PMO-3.

4
8
P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
2
Scheme 2. Schematic representation for the incorporation of Pd(OAc) into the PMOs.
Table 2
Analytical, textural, and porosity data of different Pd/PMO catalysts.
No Catalyst
CHN analysis
wt%)
Amount of b-diketiminato
Pd content from ICP
2
N adsorption/desorption measurements
À1
À1
(
(mmol g
)
(mmol g
)
C
N
H
Before
catalysis
After
catalysis
BET surface area
(m g )
NLDFT pore diameter
(nm)
2
À1
1
2
3
Pd/PMO-
1
Pd/PMO-
2
Pd/PMO-
3
3.66 0.89 3.02 0.159
5.84 1.00 2.52 0.179
6.74 1.43 3.21 0.25
0.133
0.159
0.238
0.131a
0.157a
0.232b
655
625
570
2.65
2.77
3.18
a
Catalyst recovered from single catalytic cycle.
Catalyst recovered from the 5th catalytic cycle.
b
to the formation of Pd(0) nanoparticles [37–40]. The formation of
the nanoparticles has been described as an outcome of electron-
beam induced reduction of original Pd(II) species during the TEM
measurements [40]. In our Pd-loaded PMOs, however, there was
no evidence for the formation of Pd nanoparticles on PMOs, which
Pd/PMO-1, Pd/PMO-2, and Pd/PMO-3 are presented in Figs. S11–
S13 in the SI. The high resolution XPS spectra of Pd/PMO-1, Pd/
PMO-2, and Pd/PMO-3 (Fig. 6) exhibit characteristic 3d5/2 peak
with the binding energy (BE) of 337.8 eV, indicating the presence
of Pd in PMOs [41]. However, the BE value of the 3d5/2 peak in
the cases of Pd/PMOs shows a decline as compared to that
2
+
2
+
endorses the molecular distribution of b-diketiminato-acetate Pd
complex into PMOs.
(338.7 eV) of Pd(OAc)
peak can be attributed to the strong coordination of Pd(OAc)
with the iminic nitrogen atoms of the b-diketiminate moiety,
which eventually makes the Pd species less electron deficient.
On account of this observation, a rational conclusion regarding
the successful immobilization of b-diketiminato-acetate Pd2+ com-
plex within Pd/PMOs can be reached.
2
. This negative shift in the BE value of 3d5/
2
2
3
.3.2. Isothermal N
A significant reduction in the BET surface area was observed for
all Pd/PMOs upon the incorporation of Pd(OAc) (Table 2 and Fig. 9
in the SI). This change can be ascribed to the fact that incorporated
Pd(OAc) occupies certain amount of space inside the pores,
2
adsorption/desorption measurements of Pd/PMOs
2
+
2
2
resulting in a contraction of the pore diameter. The decrease in
the average pore diameter of Pd/PMOs after the introduction of
Pd is also evidenced from the NLDFT average pore size distribu-
tion analysis (Table 2 and Fig. S10 in the SI).
3.4. Catalytic test
2
+
Catalytic activity of different Pd/PMO catalysts for the oxidation
of styrene was evaluated at 60 °C using acetonitrile as solvent and
H O as oxidant. As seen from the results (Table 3), it is evident
3.3.3. FT-IR study of Pd/PMOs
2
2
The FTIR spectra (Fig. 5) of Pd/PMO-1, Pd/PMO-2, and Pd/PMO-3
that all the Pd/PMOs offer nearly 100% selectivity toward the pro-
duction of acetophenone, which is in contrast to many other
reports where benzaldehyde was the predominant product by
show all the characteristic peaks of PMOs as well as the peaks of
acetate groups from Pd(OAc) . These IR signals provide solid evi-
dence in support of the formation of b-diketiminato-acetate Pd
2
2
+
2+
using Pd as a catalyst without cocatalyst [22]. Among three Pd/
complex within Pd/PMOs. The intensities of the peaks contributed
by the acetate groups increase as a direct function of the loading
amount of compound 2 in PMOs. This is an obvious phenomenon,
since the more b-diketimine units are incorporated within PMOs,
PMO catalysts, Pd/PMO-3 demonstrated the highest catalytic activ-
ity (Table 3, entry 4) under the given reaction conditions. Due to
the low Pd loading in Pd/PMO-1 and Pd/PMO-2, the required
amount of solid catalysts is much higher, making the catalyst-sus-
pended reaction mixture very dense. Consequently, the mobility of
the reaction substrate in solution is reduced markedly, which is
difficult for the substrate to defuse effectively into the pore chan-
nels for reaction. Moreover, most of the substrate got adsorbed
on the surface of Pd/PMOs and remained unreacted due to a low
density of catalytic site on the surface of Pd/PMOs. These are the
2+
the more b-diketiminato-acetate Pd complex is formed.
3
.3.4. XPS study of Pd/PMOs
The incorporation of Pd2 into PMOs through the complexation
+
with the iminic nitrogen atoms of the b-diketiminate moiety was
further investigated by high resolution XPS. The XPS spectra of

P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
49
Fig. 7. Catalytic activities of Pd/PMO-3 in terms of styrene conversion and
selectivity of different products as a function of reaction time. Reaction conditions:
styrene/H O = 1:3 (mole ratio), styrene/catalyst = 200:1 (mole ratio), 30 mL of
2 2
acetonitrile as solvent at 60 °C.
Fig. 5. FT-IR spectra of (a) Pd/PMO-1, (b) Pd/PMO-2, (c) Pd/PMO-3, and (d)
Pd(OAc)
2
.
solvent was as low as 20 mL, which can be improved by increasing
the volume. However, a slight decrease of activity in the case of Pd/
PMO-3 was also observed upon the increase of solvent volume,
mainly due to the high dilution effect. On the other hand, very poor
conversion of styrene was observed in the absence of any Pd/PMO
catalyst even after 12 h of reaction, resulting in preferential pro-
duction of benzaldehyde and benzoic acid (Table 3, entry 1). The
catalytic activity of prepared Pd/PMOs was also compared with
their homogeneous counterparts such as Pd(OAc)
Since the compound 2 has higher propensity to undergo the self-
polymerization due to its terminal alkoxysilane, L-Pd(OAc) was
prepared (Scheme S1 in the SI) through the complexation between
Pd(OAc) and compound 1. Owing to their homogeneity in nature,
both Pd(OAc) and L-Pd(OAc) showed slightly higher conversion
of styrene under given conditions (Table 3, entries 5 and 6). How-
ever, Pd(OAc) offered significantly low production yield of aceto-
phenone. Although the production yield of acetophenone in the
case of L-Pd(OAc) was found to be better than Pd(OAc) , it was
2 2
and L-Pd(OAc) .
2
2
2
2
2
2
2
still much lower than that of the developed Pd/PMOs. The compar-
ison studies clearly indicate better performance of Pd/PMOs than
the homogeneous counterparts in terms of the selectivity and
recyclability.
A series of catalytic experiments using the best catalyst, i.e. Pd/
PMO-3, were carried out in order to understand the variation from
styrene to acetophenone as a function of the reaction time (Fig. 7).
The results indicate that the conversion rate increases quite
remarkably upon increasing the reaction time, showing the con-
version enhancement from 40% for 3 h of reaction to 100% for
Fig. 6. High-resolution XPS spectra at the 3d region for (a) Pd/PMO-1, (b) Pd/PMO-
, (c) Pd/PMO-3, and (d) Pd(OAc)
2
2
.
primary reasons responsible for low conversion and low TOF
turnover frequency) observed in the case of Pd/PMO-1 and Pd/
A
(
A
10 h of reaction. The highest TOF of the catalyst under the given
PMO-2. A series of reactions were carried out by varying the sol-
vent volume, and a profound effect was observed (Fig. S14 in the
SI). Poor catalytic activity was observed when the volume of
reaction conditions was observed at 5 h, which was recorded as
À1
29.7 h . Apparent decrease of TOF
A
upon increasing the reaction
time is due to the formations of benzaldehyde and benzoic acid,
Table 3
a
Catalytic activities of different catalysts in terms of styrene conversion and selectivity of different products.
À1
(h )b
No
Catalyst
Reaction time (h)
Conversion of styrene (mol%)
Selectivity of products (mol%)
TOF
0
20.2
26.1
29.7
7.0
A
Acetophenone
Benzaldehyde
Benzoic acid
1
2
3
4
5
6
–
10
5
5
5
5
3
51
66
75
82
79
–
65
35
–
–
–
34
11
Pd/PMO-1
Pd/PMO-2
Pd/PMO-3
Pd(OAc)
L-Pd(OAc)
>99
>99
>99
18
Trace
Trace
Trace
48
2
2
5
73
16
28.9
a
Reaction conditions: styrene/H
2
O
2
= 1:3 (mole ratio), styrene/catalyst = 200:1 (mole ratio), acetonitrile as solvent at 60 °C.
b
TOF
A
: turnover frequency, i.e., moles of desired product (acetophenone) formed per mole of active catalytic center per hour.

5
0
P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
Scheme 3. Possible catalytic products from the oxidation of styrene.
2 2
Scheme 4. Proposed catalytic mechanism of using Pd/PMO catalysts for selective oxidation of styrene to acetophenone with H O as oxidant.
which were also observed in the absence of the catalyst. When the
reaction continued up to 12 h, the yield of acetophenone was fur-
ther decreased to 92%, giving rise to few other byproducts.
Pd/PMOs prefer Path 2 over Path 1. As a result, markedly high ace-
tophenone production was able to achieve.
Based upon previously reported work [29,43], we proposed an
analogous catalytic mechanism using Pd/PMO catalysts for selec-
2 2
It is well-known that catalytic oxidation of styrene using H O
as oxidant can undergo different pathways to afford various possi-
ble oxidized products, where styrene oxide, benzaldehyde, benzoic
acid, 1-phenylethane-1,2-diol, and phenylacetaldehyde are pri-
marily via Path 1, as well as acetophenone and 1,3-diphenyl-1-
butene are mainly via Path 2 (Scheme 3) [29,42]. Based on the
obtained result (Table 3, entry 1), in the absence of any catalyst,
styrene undergoes Path 1 to afford benzaldehyde, which is further
oxidized to produce benzoic acid. Upon the introduction of Pd/PMO
catalysts, styrene adopts Path 2 to give acetophenone. All the three
2 2
tive oxidation of styrene to acetophenone with H O as oxidant
in the absence of copper or indium salts as cocatalyst (Scheme
4). According to the proposed mechanism, immobilized Pd2+ forms
2 2
a palladium hydroperoxidic species by the addition of H O
(Scheme 4, step 1). This species is responsible for transferring oxy-
gen to the olefin through a pseudocyclic hydroperoxypalladation
process of the coordinated olefin, leading to the formations of cor-
responding ketone and palladium hydroxyl species (Scheme 4, step
2 2
3). In the presence of an additional H O , this palladium hydroxyl

P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
51
Fig. 9. Catalytic recycling experiments of Pd/PMO-3 by maintaining the reaction
time of 5 h in each cycle. Reaction conditions: styrene/H = 1:3 (mole ratio) with
2 2
O
a slow addition of oxidant, styrene/catalyst = 200:1 (mole ratio), 30 mL of aceto-
nitrile as solvent at 60 °C.
Fig. 8. Catalytic activities of Pd/PMO-3 in terms of styrene conversion and
selectivity of different products as a function of H :styrene ratios. Reaction
conditions: styrene/catalyst = 200:1 (mole ratio), 30 mL of acetonitrile as solvent at
0 °C.
2 2
O
6
species regenerates the palladium hydroperoxidic species, com-
pleting the catalytic cycle (Scheme 4, step 4). The fate of the inter-
mediate, palladium hydroxyl species, could be different in the
presence of low amount of H
fin will take place, affording Pd(0) and causing the deactivation of
2 2
O , where hydroxypalladation of ole-
Fig. 10. Catalytic recycling experiments of Pd/PMO-3 by maintaining the reaction
time of 10 h in each cycle. Reaction conditions: styrene/H O2 = 1:3 (mole ratio) with
2
the catalysts (Scheme 4, step 5). Thus, low amount of H
disfavors the regeneration of active Pd2+ species.
2
O
2
always
a slow addition of oxidant, styrene/catalyst = 200:1 (mole ratio), 30 mL of aceto-
nitrile as solvent at 60 °C.
In order to prove the hypothesized mechanism, another series
of catalytic reactions were carried out to realize the variation of
the conversion and selectivity as a function of different H O :sty-
2 2
portion addition of H
be observed in the catalysts used in the previous cases with several
portion additions of H (Figs. S16D and S16E in the SI). Thus, it
can be concluded that prolonged addition of H supplies acces-
sible peroxide during the course of the reaction, inhibiting the cat-
alyst deactivation.
In the catalytic recycling experiments, the reactions were car-
ried out for 5 h and 10 h, respectively, by maintaining a slow addi-
2 2
tion of H O over a period of 3 h with an H:S ratio of 3:1. In the case
of 5 h reactions (Fig. 9), Pd/PMO-3 presents a good recyclability
toward the acetophenone formation with over 99% of styrene con-
version up to 3 cycles. A slight catalytic deactivation was observed
in the 4th and 5th catalytic cycles. The ICP analysis of the reaction
mixture after the 5th catalytic cycle reveals no metal leaching dur-
ing the catalysis. Similarly, the ICP analysis of Pd/PMO-3 recovered
after the 5th catalytic cycle eliminates the metal leaching from the
solid catalyst (Table 2). The hot filtration test was also carried out,
and no detectable leaching of Pd was observed by the ICP analysis
(See the section 10 in the SI for more details). Moreover, the forma-
tion of unwanted Pd(0) nanoparticles was ruled out on the basis of
the evidence obtained from the TEM and XPS experiments of the
used catalyst (Fig. S16B and S16C in the SI). On the other hand, pro-
gressive decreases in the conversion as well as in the production
yield of acetophenone were observed when the catalytic recycling
experiment of Pd/PMO-3 was carried out for 10 h (Fig. 10). The
obvious deactivation of Pd/PMO-3 can be attributed to the progres-
sive formation of Pd(0) nanoparticles, which were observed in the
TEM images (Fig. S16F, S16G, and S16H in the SI). The reason is
that, when the reaction was carried out for 10 h, the amount of
2 2
O , whereas similar nanoparticles cannot
rene (H:S) molar ratios. The obtained results (Fig. 8) indicate that
the conversion rate increases quite remarkably when the ratio
increases, showing the conversion enhancement from 26% for a
ratio of 1:1 to 84% for a ratio of 5:1. A consistent yield of 100%
for the acetophenone production was observed when the H:S
was up to 3:1. Furthermore, the production yield of acetophenone
decreases to 93% and 90% for the H:S of 4:1 and 5:1, respectively.
The apparent decrease in the production yield upon increasing
the H:S molar ratio is due to the formation of benzaldehyde and
benzoic acid through Path 1 (Scheme 3) under the influence of high
2 2
O
2 2
O
concentration of oxidant. On the other hand, TOF
A
follows an
increasing trend upon the increase of H:S under given reaction
time, mainly due to the increase in the total conversion of styrene
to acetophenone (Fig. S15 in the SI), So far, all experiments were
carried out by adding H O in several portions over a long period
2 2
of time, preventing its rapid decomposition.
Similar catalytic tests were performed for the H:S ratios of 3:1,
4
2 2
:1, and 5:1, where H O was added in a single portion at the
beginning of the reactions (Fig. 8). A significant decrease in the
conversion of styrene was observed, which can be attributed to
the unwanted decomposition of H
yield of acetophenone also decreased significantly, resulting in
much lower TOF as compared to the previous cases, where H
was added in several portions over a long period of time
Fig. S15 in the SI). The deactivation of catalytic activity can be
ascribed to the formation of Pd(0) species on account of rapid
decomposition of H to result in incomplete catalytic cycle
Scheme 4, step 5). The formation of metallic Pd(0) nanoparticles
was observed from the catalysts used in the reactions with one
2 2
O . In addition, the production
A
2 2
O
(
2 2
O
(
2
H O
2
decreased remarkably, resulting in the formation of Pd(0)
nanoparticles.

5
2
P. Borah, Y. Zhao / Journal of Catalysis 318 (2014) 43–52
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This research is supported by the National Research Foundation
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