Silylated layered double hydroxide nanosheets prepared by a large-scale synthesis method as hosts for intercalation of metal complexes

Article abstract of DOI:10.1016/j.apcata.2016.05.001

We present a novel strategy for preparing silylated MgAl layered double hydroxide (LDH) intercalated with metal complexes with the purpose of improving the catalytic activity of the intercalated active species. This strategy involves preparing the exfolia

Full text of DOI:10.1016/j.apcata.2016.05.001

Applied Catalysis A: General 522 (2016) 101–108
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Silylated layered double hydroxide nanosheets prepared by a
large-scale synthesis method as hosts for intercalation of metal
complexes
Wenya Guoa, Yuan Zhaoa, Fan Zhoua, Xiaoliang Yana, Binbin Fana,∗, Ruifeng Lia,b,∗∗
a College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
b Key Laboratory of Coal Science and Technology MOE, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We present a novel strategy for preparing silylated MgAl layered double hydroxide (LDH) interca-
lated with metal complexes with the purpose of improving the catalytic activity of the intercalated
active species. This strategy involves preparing the exfoliated LDH nanosheets by a large-scale synthesis
method, grafting phenyltriethoxysilane (PTES) on the exfoliated LDH nanosheets followed by interca-
lating the anionic metal complexes such as CuBDACl2 anions (H2BDA, 2, 2ꢀ-bipyridine-5, 5ꢀ-dicarboxylic
acid). The physicochemical properties of the hybrid materials were characterized by X-ray diffraction
(XRD), Fourier-transformed infrared spectroscopy (FT-IR), diffuse reflectance ultraviolet-visible spec-
troscopy (DR UV–-vis), N2 adsorption-desorption, scanning electron microscopy (SEM), and inductively
coupled plasma (ICP) techniques. The silane grafted on LDH nanosheets was found to affect the single
layer restack while intercalating anionic Cu complexes in water-containing systems. Thus, the resultant
hybrid material (PTES-LDH-BDA/Cu) exhibited higher surface area, pore volume and hydrophobicity than
its corresponding non-silylated counterpart (LDH-BDA/Cu). These characteristics improved the accessi-
bility of the intercalated CuBDACl2 anions, thus increased their catalytic efficiency in alkene oxidation
with tert-butyl hydroperoxide (TBHP). In addition, the prepared PTES-LDH-BDA/Cu material exhibited
higher reusability as compared to the non-silylated LDH-BDA/Cu material.
Received 22 December 2015
Received in revised form 29 April 2016
Accepted 1 May 2016
Available online 2 May 2016
Keywords:
Layered double hydroxide
Nanosheets
Silylation
Metal complexes
Alkene oxidation
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
been employed as supports for the immobilization of a variety
of metal complexes. Nevertheless, heterogeneous catalysts usu-
With strong striving for an economical and green chemical
transformation, considerable attention has been paid on immo-
bilization of catalytically relevant metal complexes on suitable
supports [1–3]. The immobilization of homogeneous metal com-
plexes is an interesting method in that it allows easy separation
from the reaction media, efficient recycling, superior handling con-
trol, and minimization of metal traces in reaction products. Many
metal complexes have been successfully immobilized on various
polymers or inorganic supports. Compared with organic polymers,
inorganic supports are preferred because of their mechanical and
thermal stability. Thus, many inorganic materials such as activated
carbons [4–6], zeolites [7], and mesoporous materials [8,9] have
ally exhibit lower activity and selectivity than their homogeneous
counterparts because of poor dispersion of active sites, steric hin-
drance effects, and changes in the chemical microenvironment
of the active sites. Therefore, novel supports and immobilization
strategies are required for the development of efficient heteroge-
neous catalysts.
Layered double hydroxides (LDHs) are a class of layered materi-
als with the general formula of [M1-x2+Mx3+(OH)2]x+[An−
]x/n·yH2O
formed by brucite-type octahedral layers in which M2+ ions are
partially substituted by M3+ ions. The net positive charge result-
ing from this substitution is counterbalanced by exchangeable
anions (i.e., interlayer anions) [10–12]. LDHs are composed of two-
dimensional positively charged host lattices with excellent anion
exchange capacity. These unique structural properties (not shown
by conventional zeolite and mesoporous materials) make LDHs
promising intercalation hosts for anionic metal complexes. Thus,
Fe(III) porphyrin [13–16], Ti(IV)-Schiff base [17], chiral sulphonato-
salen-manganese(III) [18], and Ti(IV) tartrate [19,20] have been
∗
Corresponding author.
Corresponding author at: Key Laboratory of Coal Science and Technology MOE,
∗∗
Taiyuan University of Technology, Taiyuan 030024, PR China.
E-mail addresses: fanbinbin@tyut.edu.cn, fanbinbin@tyut.edu.cn (B. Fan),
rfli@tyut.edu.cn (R. Li).
http://dx.doi.org/10.1016/j.apcata.2016.05.001
0926-860X/© 2016 Elsevier B.V. All rights reserved.

102
W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
successfully intercalated into LDHs and subsequently used as
catalysts in a number of reactions (e.g., cyclohexane oxidation,
enantioselective hydrogenation, sulfoxidation). Remarkably, the
intercalation of metal complexes has been demonstrated to result
in materials with enhanced catalytic performance (i.e., activity,
selectivity, and stability). However, the strong interaction between
the metal hydroxide layers and the interlayer anions can often
hinder bulky metal complex anions from gaining access to the inter-
layer space. This issue can be solved by exfoliation of LDHs into a
completely open single interlayer showing no steric hindrance for
the immobilization of macromolecules. However, LDHs are exfo-
liated with difficulty because of the high formal charge density
and the strong interaction between adjacent layers [21,22]. Exfolia-
tion of LDHs generally involves the following steps: (i) inserting an
anion into the interlayer space; (ii) increasing the distance between
the adjacent LDH layers by swelling with an appropriate solvent;
and (iii) exfoliating the swollen LDH structure by energy input in
an appropriate solvent/dispersant medium [22,23]. This method
typically suffers from high solvent consumption and difficult isola-
tion of the exfoliated LDH nanosheets from the solvent dispersion
while avoiding aggregation. These issues have dramatically limited
the commercial exploitation of LDH nanosheets. Recently, a new
aqueous miscible organic solvent treatment (AMOST) method has
been developed for the large-scale high yield synthesis of highly
porous dispersed LDH powders containing exfoliated nanosheets
[24]. The AMOST synthesis involves initial formation of LDHs by
conventional co-precipitation and subsequent re-dispersion in an
aqueous miscible organic solvent before the final isolation of the
solid. This method provides a new opportunity for the application
of LDH nanosheets as catalyst supports. However, exfoliated LDHs
readily reconstruct into their original layered structure while inter-
calating anionic metal complexes, leading to hybrid materials with
low surface area, pore volume, and low accessibility of the inter-
calated active species. Hence, it is desirable to find a solution to
prevent the exfoliated LDH layers from reconstructing during the
immobilization of the anionic metal complexes.
Silylated LDH nanosheets were prepared according to the liter-
ature with some modification [25]. 30 mL of toluene was added to
1 g of LDH-del under flowing Ar. After stirring for 0.5 h, 2.95 mmol
of phenyltriethoxysilane (PTES) dissolved in 20 mL of toluene was
added and the mixture was refluxed for 12 h. The solid product
was Soxhlet-extracted with ethanol for 24 h and then dried at 80 ◦C
overnight. The obtained sample was denoted as PTES-LDH.
The intercalation of CuBDACl2 anions was carried out following
the procedure reported in the literature [26]. Typically, two equiv-
alents of KOH in DD water (20 mL) were added to a suspension of
H2BDA (2.5 mmol) in 25 mL of N,N-dimethylformamide (DMF) at
70 ◦C with stirring under flowing Ar. Subsequently, 1.0 g of LDH-
del or PTES-LDH was added and the resulting mixture was stirred
at 50 ◦C for 48 h under flowing Ar. The generated solid was sep-
arated by centrifugation, washed several times with ethanol, and
vacuum dried at 40 ◦C for 5 h. The obtained products were denoted
as LDH-BDA and PTES-LDH-BDA, respectively.
LDH-BDA or PTES-LDH-BDA (0.8 g) was added to 40 mL ethanol
solution of CuCl2·2H2O (0.025 M) and the resulting suspension
was stirred at 30 ◦C for 19 h. The pale bluish green solid obtained
by centrifugation was Soxhlet-extracted with ethanol for 24 h
and then dried at 70 ◦C under vacuum. The obtained catalysts
were denoted as LDH-BDA/Cu and PTES-LDH-BDA/Cu, respec-
tively.
The Cu(H2BDA)Cl2 complex was synthesized as described else-
where [27]. Briefly, 1 equivalent of H2BDA (2.5 mmol) and a few
drops of hydrochloric acid were added to 50 mL of DMF, and then
the mixture was stirred at 70 ◦C until H2BDA was completely dis-
solved. The as-prepared solution was added to 15 mL of ethanol
containing 1 equivalent of CuCl2·2H2O (2.5 mmol) and the resulting
mixture was stirred for 2 h at room temperature (RT). The solu-
tion was kept at −10 ◦C until formation of a blue precipitate. After
removal of the solvent, the solid was washed with ethanol and
vacuum dried at 40 ◦C for 5 h.
In this work, a novel strategy for preparing silylated LDH interca-
lated with metal complexes is proposed with the aim of enhancing
the catalytic activity of the intercalated active species. This strat-
egy involves initial grafting phenyltriethoxysilane (PTES) on the
surface of the exfoliated LDH nanosheets (LHD-del) to gener-
ate silylated material (PTES-LDH) and subsequent intercalation
of anionic metal complexes such as CuBDACl2 anions (H2BDA:
2, 2ꢀ-bipyridine-5, 5ꢀ-dicarboxylic acid). The as-prepared hybrid
materials were characterized by various techniques and their cat-
alytic performances were investigated in alkene oxidation with
tert-butylhydroperoxide (TBHP) as the oxidant.
2.2. Characterization
Powder X-ray diffraction (XRD) measurement was performed
on a Shimazu XRD-6000 diffractometer by using Cu Kɑ radia-
tion over a 2ꢀ range from 2 to 65◦. Fourier-transformed infrared
spectroscopy (FT-IR) spectra were recorded by a SHIMADZU FTIR-
Affinity-1 spectrometer using the conventional KBr pellet method.
Diffuse reflectance UV–vis (DR UV–vis) spectra were recorded using
a Cary-300 Absorption Spectrometer. Physisorption of N2 was per-
formed at 77 K using a Quantachrone Nova 1200e and the sample
was evacuated at 150 ◦C for 3 h before measurement. The surface
area was calculated by the BET method and the pore distribution
was calculated by the BJH method. The Cu and Si contents of the
samples were measured by IRIS intrepid II inductively coupled
plasma (ICP) atomic emission spectrometer. Mg and Al contents
of the samples were measured by energy dispersive X-ray (EDX)
on a field-emitting HITACHI S-4800 scanning electron microscope.
Thermogravimetrical analysis (TGA) was carried out on a STA 449
F3 thermogravimetrical analyzer from NETZSCH with the sample
held in a ceramic pan in a flowing air (10 mL/min). Samples were
heated from RT up to 650 ◦C with a heating rate of 10 ◦C/min. The
contact angles were measured using a KINO SL200B standard opti-
cal contact angle measurer. All samples were pressed into disks
with a diameter of 14 mm and a thickness of 0.4 mm. The deionized
water drops with 5 mL were used in the measurement. Observ-
ing all water drops maintained for 20 s, and the measurement
for each sample was repeated 3 times. C and N contents in the
samples were measured by a Vario EL/micro cube elemental ana-
lyzer.
2. Experimantal
All chemicals were used as received without further purification.
2.1. Sample preparation
LDH nanosheets were prepared according to the AMOST method
reported in the literature [24]. Typically, 40 mL of aqueous solution
of Mg(NO3)2·6H2O (30 mmol) and Al(NO3)3·9H2O (10 mmol) was
dropped into 100 mL of H3BO3 solution under vigorous stirring. The
pH of the mixture was maintained at 9 by 1.5 M NaOH solution.
The resultant gel-like slurry was stirred at room temperature for
10 min and then aged at 65 ◦C for 12 h. After filtration, the obtained
solid was washed with distilled deionized (DD) water until neutral,
and then the white slurry was washed by acetone several times
and dried at 65 ◦C overnight. The obtained sample was denoted as
LDH-del.

W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
103
003
006
PTES-LDH-BDA/Cu
110/113
PTES-LDH-BDA
LDH-BDA/Cu
LDH-BDA
003
LDH-re-H
O
2
Scheme 1. The strategy for the preparation of silylated LDHs intercalated with
anionic Cu complexes.
PTES-LDH
LDH-del
012
2.3. Catalytic reaction
Alkene oxidation was carried out in a 25 mL round bottle
flask fitted with water cooled condenser. tert-butyl hydroperoxide
(TBHP, 70% in aqueous solution) was used as the oxidant. Typically,
10 mmol of substrate was mixed with 5 mL of acetonitrile solvent,
and to that 25 mg of catalyst was added. After the reaction mixture
was heated to 35 ◦C in a water bath with vigorous stirring, 1.4 mL
of TBHP (10 mmol) was then added and kept at this temperature
for 12 h. The liquid products were identified by a GCMS-QP2010
SE and analyzed using a SHIMADZU GC-2014 equipped with a FID
detector and a RTX-1 capillary column.
10
20
30
40
50
60
2 Theta (degree)
Fig. 1. XRD patterns of the prepared different samples.
from: (i) BDA anions slightly tilted (i.e., not perpendicular) to the
host layers; (ii) a strong hydrogen bond between the carboxylate
groups in the BDA anions and the brucite-like layers (as revealed by
TG results below). An unexpectedly sharp and intense reflection at
2ꢀ = 10◦ was also observed in the case of LDH-BDA. Since LDH-del
was synthesized in the system containing NO3− and [B4O5(OH)4]2−
anions, the resulting LDH-BDA may contain two segregated phases,
3. Results and discussion
3.1. Characterization
one containing only NO3 (LDH-NO3−) and the other containing
−
BDA anions and NO3−/[B4O5(OH)4]2−. Accordingly, the sharp and
intense reflection at 2ꢀ = 10◦ may result from the overlap of the
(003) and (006) reflections from the two segregated phases. Neatu
et al. observed a similar phenomenon during immobilization of
Rh(TPPTS)3Cl on LDHs by anionic exchange [29]. After coordinating
with CuCl2, the obtained LDH-BDA/Cu showed nearly similar XRD
pattern to LDH-BDA. Remarkably, the PTES-LDH sample, prepared
by refluxing LDH-del with a PTES toluene solution, displayed nearly
identical XRD pattern as compared to LDH-del, thereby indicat-
ing that the silane modification had little influence on the layered
structure of the original LDH nanosheets. In the same way, PTES-
LDH-BDA exhibited similar XRD pattern to the LDH-BDA sample.
Moreover, unlike LDH-BDA, PTES-LDH-BDA showed the reflection
at 2ꢀ = 10◦ with normal intensity compared to other reflections,
revealing that PTES-LDH-BDA mostly consists of the phase contain-
ing BDA anions and NO3−/[B4O5(OH)4]2− and the dispersion of BDA
anions in PTES-LDH-BDA is higher than that in LDH-BDA. After coor-
dinating with CuCl2, the obtained PTES-LDH-BDA/Cu also showed
nearly identical XRD pattern as compared to PTES-LDH-BDA.
3.1.1. XRD
MgAl LDH nanosheets were prepared by the reported AMOST
method. Intercalation of anionic Cu complexes was carried out
by the following two steps: (i) BDA anions were absorbed on the
prepared or silylated MgAl LDH nanosheets by ion-exchange; (ii)
the resulted LDH-BDA or PTES-LDH-BDA was used as solid lig-
ands to coordinate with CuCl2 in ethanol. The detailed preparation
route is illustrated in Scheme1. Fig. 1 shows the XRD patterns of
the prepared different samples. No 00l reflections were observed
in the XRD pattern of LDH-del but 012 and 110/113 reflections
with low intensity appeared. These results are in good agree-
ment with previous reports [24] and indicate that LDH nanosheets
were successfully prepared. Remarkably, a new reflection corre-
sponding to the (003) reflection appeared upon addition of BDA
anions (LDH-BDA sample) thereby revealing recombination of LDH
nanosheets during the intercalation process. The (003) reflection
of LDH-BDA, originated from the brucite-like layer stacking, was
detected at 2ꢀ value significantly lower as compared to LDHs
with tetraborate and nitrate anions (2ꢀ = 8 and 10◦, respectively)
[28,29]. This shift originated from the intercalation of bulky BDA
anions into the restructured LDH interlayer. In order to further
confirm the successful intercalation of BDA anions, LDH-re-H2O
was prepared by immersing of LDH-del in water for 24 h. The
water-treated sample showed the (003) reflection at 2ꢀ = 9.5 and
the corresponding basal spacing was 0.93 nm. Subtracting the
layer thickness (0.48 nm) from the basal spacing, its interlayer
spacing was only 0.45 nm. In contrast, the (003) reflection of
LDH-BDA was detected at 2ꢀ = 4.8◦, resulting in an interlayer spac-
ing (1.36 nm = 1.84 nm–0.48 nm) slightly lower than the longest
dimension of the BDA anions (1.42 nm) [30,31]. This lower inter-
layer spacing (as compared to BDA anion dimension) may arise
3.1.2. FTIR and DR UV-vis
The successful graft of PTES and the intercalation of the
CuBDACl2 anions into LDHs can be further testified by FTIR and
DR UV–vis. The FTIR spectra of the prepared samples are shown in
Fig. 2. The broad band at 3446 cm−1 was attributed to the O
H
stretching vibration of hydroxyl groups in the brucite-like lay-
ers and water in the galleries. LDH-del and PTES-LDH showed an
intense sharp band at 1385 cm−1 attributed to the ꢁ3 stretching
vibration of nitrate [32]. Compared with LDH-del, some new bands
were observed in the FTIR spectrum of the PTES-LDH, in which the
band at 1040 cm−1 is attributed to the stretching vibration of Si-

104
W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
Table 1
1040
The surface areas and pore volumes of the prepared different samples.
PTES-LDH-BDA/Cu
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
3446
LDH-del
LDH-BDA/Cu
PTES-LDH
233
61
242
199
0.617
0.309
0.934
0.766
1619
PTES-LDH-BDA/Cu
PTES-LDH
2914
2967
LDH-BDA/Cu
calation of CuBDACl2 anions was accompanied by the onset of two
new bands at 256 and 314 nm ascribed to the ␲-␲* transition of the
ligand BDA [37]. These bands were observed for both LDH-BDA/Cu
and PTES-LDH-BDA/Cu samples, indicating that BDA anions were
ion-exchanged into the interlayer of the restacked LDH. Addition-
ally, these two samples showed a weak shoulder at ca 398 nm and a
broad band at ca 688 nm that were ascribed to the ligand-to-metal
charge transfer and d–d electronic transitions, respectively [26],
thereby demonstrating that intercalated BDA anions were success-
fully coordinated with CuCl2.
1619
LDH-del
1385
3.1.3. N2 adsorption isotherms
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber ( cm-1)
The textural properties of the samples under study were inves-
tigated by N2 adsorption-desorption isotherms (Fig. 4). From
Fig. 4(A), it can be seen that all the samples showed a type IV adsorp-
tion isotherm with a H3 hysteresis loop indicating the presence
of plate-like particles giving rise to slit-shaped pores. The onset
of the hysteresis loop started at ca 0.8 and 0.6 relative pressure
for PTES-LDH and LDH-del, respectively. Thus, the grafted silane
influenced the pores between the exfoliated layers with PTES-LDH
showing larger pore size than LDH-del (Fig. 4(B)). After intercala-
tion of CuBDACl2 anions, the specific surface area and pore volume
of LDH-BDA/Cu decreased significantly as compared to its precur-
sor host (Table 1). This decrease in textural properties is mainly
explained by the reconstruction of the exfoliated LDH nanosheets
while intercalating anionic BDA ligands in the water-containing
system. The same trend was observed for PTES-BDA/Cu to a much
lower extent as compared to LDH-BDA/Cu (Table 1). This behavior
might be associated to: (i) grafted PTES increasing the hydrophobic-
ity of LDH nanosheets and thereby partially hindering their restack;
(ii) grafted PTES increasing the extent of particle crosslinking with
the corresponding increase in mesopore volume [33]; and (iii)
Fig. 2. FTIR spectra of the prepared different samples.
256
314
398
688
PTES-LDH-BDA/Cu
LDH-BDA/Cu
PTES-LDH
263
−
LDH-BDA/Cu being enriched in LDH-NO3 phase as compared to
PTES-LDH-BDA/Cu. Remarkably, the hypothesis (i) was confirmed
by water contact angles measurements (LDH-BDA/Cu: 57.3◦, while
PTES-BDA/Cu: 79.3◦).
LDH-del
3.1.4. TG analysis
200 300 400 500 600 700 800
Wavelength (nm)
As shown in Fig. 5, the TG curves of LDHs are typically com-
posed of three losses, namely, desorption of physically adsorbed
water or acetone (from RT to 150 ◦C), dehydroxylation of the
brucite-like sheets (150–290 ◦C), and decomposition of the inter-
layer anions/grafted organic silane (T > 290 ◦C). Dehydroxylation
accounted for lower mass losses in silylated samples (5.5 and 5.8%
for PTES-LDH and PTES-LDH-BDA/Cu, respectively) as compared
to LDH-del (6.7%), potentially revealing lower amounts of OH
groups in the PTES-LDH nanosheets as a result of the condensation
reaction [35]. The neat Cu(H2BDA)Cl2 complex decomposed at ca
300 ◦C (insert graph), while this temperature shifted to higher val-
ues (350 ◦C) after intercalation. The formation of strong hydrogen
bonds between the carbonyl oxygen atoms of coordinated carboxy-
late groups and the LDH layers can explain this phenomenon [38].
Fig. 3. DR UV–vis spectra of the prepared different samples.
O bond and the bands at 2967 cm−1 and 2914 cm−1 are ascribed to
the vibrations of CH3-, −CH2- groups from EtO- in the PTES [33–35].
These results reveal that PTES was successfully grafted on the LDH
nanosheet surface by Mg(Al)
O Si bonds (i.e., single, double, or
triple bond). Intercalation of CuBDACl2 anions resulted in the onset
of a new band at 1619 cm−1 (detected in both LDH-BDA/Cu and
PTES-LDH-BDA/Cu samples) attributed to the asymmetric stretch-
ing vibrations of the carboxylate groups. These results confirmed
the presence of BDA anions in the LDH materials.
The DR UV–vis spectra of the different samples under study are
shown in Fig. 3. LDH-BDA/Cu and PTES-BDA/Cu were light green
whereas the LDH support was white colored. Unlike LDH-del, PTES-
LDH showed an intense peak at 263 nm assigned to the benzoic ring
of PTES [36] thereby confirming the successful silane graft. Inter-
3.1.5. Elemental analysis
As measured by SEM coupled with energy-dispersive X-ray
spectroscopy (EDX), the prepared LDH nanosheets showed a Mg/Al
molar ratio of 2.2 (theoretical value: 3) that remained nearly

W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
105
A
LDH-del
B
LDH-del
1.4
600
500
LDH-BDA/Cu
PTES-LDH
LDH-BDA/Cu
PTES-LDH
PTES-LDH-BDA/Cu
PTES-LDH-BDA/C
u
1.2
1.0
0.8
0.6
0.4
0.2
0.0
400
300
200
100
0
0.0
0.2
0.4
0.6
P/PO
0.8
1.0
0
20
40
60
80
100
Pore size (nm)
Fig. 4. N2 adsorption/desorption isotherms (A) and pore size distributions (B) of the prepared different samples.
Table 2
The Si and Cu contents of the samples prepared under different conditions.
Entry
Sample
PTES/LDH
(mmol/g)
BDA/LDH
(mmol/g)
Si
Cu
(mmol/g)
C (wt%)
N (wt%)
(mmol/g)
1
2
3
4
5
6
7
8
9
10
LDH-BDA/Cu
–
2.5
2.5
2.5
2.5
1.0
5.0
10.0
–
0
1.133
0.704
0.656
0.636
0.457
1.055
1.278
–
16.32
12.35
13.17
15.61
10.39
15.84
22.2
0.45
16.53
3.86
3.59
2.17
1.95
1.84
1.31
3.38
3.67
1.81
3.61
1.02
PTES-LDH-BDA/Cu-1
PTES-LDH-BDA/Cu
PTES-LDH-BDA/Cu-2
PTES-LDH-BDA/Cu-3
PTES-LDH-BDA/Cu-4
PTES-LDH-BDA/Cu-5
LDH-del
1.48
2.95
5.91
2.95
2.95
2.95
–
0.189
0.384
0.643
0.384
0.384
0.384
–
LDH-BDA
PTES-LDH
–
2.95
2.5
–
–
–
–
0.384
100
80
60
40
20
BDA concentration in toluene or DMF solution. From Table 2, it
can be clearly seen that the silylation process strongly affected
the amount of intercalated CuBDACl2 anions. The Cu content in
the PTES-LDH-BDA/Cu was lower than that in the corresponding
LDH-BDA/Cu, and the Cu content decreased with the PTES loading
(Table 2, Entry 1–4). As indicated above, intercalation of CuBDACl2
anions involves the following steps: (i) adsorption of BDA anions on
the prepared or silylated MgAl LDH nanosheets via ion-exchange
of the nitrate or polyborate anions; and (ii) coordination of CuCl2
with intercalated BDA anions. The first step is crucial and it deter-
mines the final amount of intercalated CuBDACl2 anions. Thus, the
higher hydrophobicity of the silylated samples might influence the
interaction of BDA anions with the LDH layers, thereby resulting
in less BDA anions being incorporated into the reconstructed LDH
interlayer. In addition, the concentration of BDA anions in the ion-
exchange solution can also influence the amount of intercalated
CuBDACl2 anions. Thus, the Cu content was tailored to some extent
by varying the BDA concentration of the ion-exchange solution
(Table 2, Entry 4–7).
100
Cu(H BDA)Cl2
2
90
80
70
60
0
300 C
100 200 300 400 500 600
0
Temperature
(
C)
MgAl-LDH-del
MgAl-PTES-LDH
MgAl-PTES-LDH-BDA/Cu
100
200
300
400
500
600
Temperature
(
0C
)
The C and N microanalysis for the different samples are also
listed in Table 2. The C (from atmospheric CO2 adsorption) and
N (from nitrate ions) contents in LDH-del were 0.45 and 1.81%,
respectively. PTES-LDH showed higher C content as compared to
LDH-del, with the amount of C exceeding the theoretical value
(based on Si content and no residual EtO- groups). This result
reveals some PTES to be grafted on LDH nanosheets by single
Fig. 5. TG curves of the prepared different catalysts.
unchanged after PTES grafting and CuBDACl2 intercalating pro-
cesses. The Cu and Si contents of the samples were measured by
ICP (Table 2). PTES-LDH-BDA/Cu samples with varying amounts
of PTES and BDA anions were prepared by varying the PTES or
or double Mg(Al)
O Si bonds. The N contents in all CuBDACl2-

106
W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
Table 3
Conversion of styrene
Catalytic results obtained over the different samples in styrene oxidation with TBHP.
70
60
50
40
30
20
Selectivity to benzaldehyde
Selectivity to styrene oxide
Entry Sample
Conversion (%) Selectivity (%)
BAa SOa
TONb
others
1
2
3
4
5
6
7
8
9
10
PTES-LDH
CuBDA
CuBDA+ PTES-LDH
LDH-BDA/Cu
1.6
3.9
4.5
76.2 19.9
73.1 21.5
70.6 25.3
3.9
5.4
4.1
–
14.2
14.7
66.4
111.9
157.3
159.7
158.0
108.4
99.5
18.8
64.6 25.0 10.4
PTES-LDH-BDA/Cu-1 19.7
PTES-LDH-BDA/Cu 25.8
PTES-LDH-BDA/Cu-2 25.4
PTES-LDH-BDA/Cu-3 17.9
PTES-LDH-BDA/Cu-4 28.6
PTES-LDH-BDA/Cu-5 31.8
65.4 31.4
65.9 31.5
65.0 31.3
65.3 31.8
63.4 34.1
58.1 37.9
3.2
2.6
3.7
2.9
2.5
4.0
Reaction condition: catalyst 25 mg, styrene 10 mmol, TBHP aqueous 10 mmol, ace-
tonitrile 5 mL, 35 ◦C, 12 h.
a
BA: benzaldehyde, SO: styrene oxide.
TON: mmol of converted substrate per mmol of copper content in the catalyst
b
for 12 h.
20
30
40
50
60
70
0C
)
containing samples were slightly higher than those calculated
based on the Cu content. This is due to that ion-exchange of nitrate
anions in LDH-del with BDA anions was incomplete. Therefore, the
C and N contents are not proper indication of the Cu/BDA ratio in
the samples. The BDA loading can be roughly estimated in PTES-
LDH-BDA/Cu by deducting the C content of PTES-LDH, resulting in
a Cu/BDA molar ratio close to 1.
Reaction temperature
(
Fig. 6. Effect of reaction temperature on the catalytic performance of PTES-LDH-
BDA/Cu (Reaction conditions: catalyst 25 mg, styrene 10 mmol, TBHP 10 mmol,
acetonitrile 5 mL, 12 h).
Table 4
Catalytic results for alkene oxidation over the different catalysts.
Substrate
Catalyst
Conversion (%)
Selectivitya (%)
TONb
3.2. Catalytic reaction
Cu(H BDA)Cl
LDH-BDA/Cu
PTES-LDH-BDA/Cu
Cu(H2BDA)Cl2
LDH-BDA/Cu
1.8
12.8
24.1
0.6
7.9
14.3
14.8
81.2
91.9
62.7
70.3
77.2
6.6
45.2
147.0
2.2
27.9
87.2
Cyclohexene
2
2
3.2.1. Catalytic performance of the prepared samples
Cyclooctene
Table 3 summarizes the catalytic performance of the prepared
samples in the oxidation of styrene with TBHP. Benzaldehyde was
the main product along with styrene oxide. Minor amounts of
benzoic acid and phenylacetaldehyde were produced during the
course of the reaction. PTES-LDH showed very low styrene conver-
sion (1.6%, Table 3, Entry 1) as compared to CuBDACl2-containing
samples (LDH-BDA/Cu: 18.8%; PTES-LDH-BDA/Cu: 25.8% (Table 3,
Entry 4–6), thereby indicating that CuBDACl2 anions are the active
species for styrene oxidation. Neat Cu(H2BDA)Cl2 and a physical
mixture of silylated LDHs and Cu(H2BDA)Cl2 (containing the same
amount of CuBDACl2 as the heterogeneous counterpart) showed
very low conversions (3.9 and 4.5%, respectively, Table 3, entry
2 and 3), as expected by the insolubility of the Cu complex in
reaction medium. In contrast, intercalation of CuBDACl2 anions
into LDH effectively increased its dispersion thus greatly improv-
ing the catalytic performance. LDHs are weak bases and can thus
assist in the cleavage of the peroxide bond in TBHP [36,39] thereby
enhancing the oxidation capacity of TBHP. Compared with LDH-
BDA/Cu, PTES-LDH-BDA/Cu showed higher activity and TON value.
Based on the characterization results, this result can be explained
by: (i) the higher dispersion of CuBDACl2 anions in PTES-LDH-
BDA/Cu as compared to LDH-BDA/Cu increased the accessibility
of the intercalated CuBDACl2 to the substrate; (ii) the higher sur-
face area and pore volume of silylated samples were beneficial
to the diffusion of the substrates and reaction products; and (iii)
the higher hydrophobicity of PTES-LDH-BDA/Cu was favorable for
styrene adsorption and the main product desorption, because ben-
zaldehyde is more polar than styrene. The amount of grafted PTES
and intercalated CuBDACl2 anions also influenced the catalytic per-
formance (Table 3). The TON value enhanced with the PTES loading
(Table 3, Entry 4–7) up to 0.384 mmol/g and leveled off there-
after (Table 3, Entry 7). The styrene conversion increased with the
amount of intercalated CuBDACl2 anions, although the correspond-
ing TON values decreased (Table 3, Entry 6, 8–10).
PTES-LDH-BDA/Cu
Reaction conditions: catalyst 25 mg, substrate 10 mmol, TBHP 10 mmol, acetonitrile
5 mL, 35 ◦C, 12 h.
The corresponding product selectivity are 2-cyclohexanone and cyclooctene
oxide, respectively.
a
b
TON: mmol of converted substrate per mmol of copper content in the catalyst
for 12 h.
3.2.2. Influence of the reaction temperature
The influence of the reaction temperature over the styrene
conversion and product distribution was investigated for PTES-
LDH-BDA/Cu. The styrene conversion increased significantly with
temperature (Fig. 6). Unlike styrene oxide, the selectivity to ben-
zaldehyde decreased with temperature, being consistent with the
reported result [40]. Thus, epoxidation of the C C bond to styrene
oxide is favoured at higher temperatures than the C C cleavage
process leading to benzaldehyde.
3.2.3. Influence of the reaction time
The styrene conversion over LDH-BDA/Cu and PTES-LDH-
BDA/Cu gradually increased with time (19.3 and 29.7%, respectively
at 24 h and 35 ◦C, Fig. 7). The product distribution (basically
benzaldehyde and styrene oxide, not shown) remained constant
throughout the experiment. These results indicate that the two
competitive reaction mechanisms (i.e., C C cleavage and epoxida-
tion) proceeded in parallel at the mild reaction conditions herein
used.
3.2.4. Influence of the reaction substrates
The oxidation of cyclohexene and cyclooctene were also inves-
tigated and the corresponding results are given in Table 4. As
expected, LDH-BDA/Cu and PTES-LDH-BDA/Cu exhibited much
higher catalytic activity and TON values than neat Cu(H2BDA)Cl2.

W. Guo et al. / Applied Catalysis A: General 522 (2016) 101–108
107
zoic acid in reaction. ICP measurements showed that ca 6% of the
Cu loading in LDH-BDA/Cu leached after the first run versus only
ca 2.5% for PTES-BDA/Cu. Thus, PTES modification can effectively
improve the stability of the intercalated CuBDACl2 anions. To fur-
ther check the contribution of the leached Cu species, the following
experiments were carried out. After finishing the first run, the PTES-
LDH-BDA/Cu catalyst was separated from the reaction mixture, and
then the substrate and TBHP were added to the filtrate to perform
the second run. It was found that the leached Cu species from PTES-
LDH-BDA/Cu were nearly inactive during the oxidation of fresh
styrene with TBHP (ca 2% conversion).
35
30
25
20
15
10
5
LDH-BDA/Cu
PTES-LDH-BDA/Cu
4. Conclusions
Silylated LDH nanosheets were successfully employed as inter-
calation hosts for metal complexes. The grafted silane greatly
affected the restack of the single layer while intercalating
Cu(BDA)Cl2 anions in aqueous solution, and the resultant PTES-
LDH-BDA/Cu exhibited higher surface and pore volume than its
corresponding non-silylated LDH-BDA/Cu counterpart. PTES-LDH-
BDA/Cu showed both higher catalytic activity and stability than
LDH-BDA/Cu for the styrene oxidation with TBHP. This work pro-
vides a novel method for the preparation of heterogeneous catalysts
based on LDH hosts.
6
12
18
24
Reaction time (h)
Fig. 7. Effect of reaction time on the catalytic performance over LDH-BDA/Cu and
PTES-LDH-BDA/Cu (Reaction conditions: catalyst 25 mg, styrene 10 mmol, TBHP
10 mmol, acetonitrile 5 mL, 35 ◦C).
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (No. 20971095, 21576177) and Research Project
Supported by Shanxi Scholarship Council of China (2013-047).
LDH-BDA/Cu
28
24
20
16
12
8
PTES-LDH-BDA/Cu
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