The application of spontaneous flocculation for the preparation of lanthanide-containing polyoxometalates intercalated layered double hydroxides: highly efficient heterogeneous catalysts for cyanosilylation

Article abstract of DOI:10.1016/j.apcata.2014.09.005

The lanthanide-containing polyoxometalates (POMs) have been intercalated into layered double hydroxides (LDHs) successfully through the spontaneous flocculation method, resulting in the formation of new heterogeneous LDHs–POMs catalysts Mg₃Al–L

Full text of DOI:10.1016/j.apcata.2014.09.005

Applied Catalysis A: General 487 (2014) 172–180
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
The application of spontaneous flocculation for the preparation of
lanthanide-containing polyoxometalates intercalated layered double
hydroxides: highly efficient heterogeneous catalysts for
cyanosilylation
Yueqing Jia, Shen Zhao, Yu-Fei Song∗
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 June 2014
Received in revised form 3 September 2014
Accepted 7 September 2014
Available online 16 September 2014
The lanthanide-containing polyoxometalates (POMs) have been intercalated into layered double hydrox-
ides (LDHs) successfully through the spontaneous flocculation method, resulting in the formation of new
heterogeneous LDHs–POMs catalysts Mg3Al–LnW10 (Ln = Eu, Tb and Dy). The synthetic method can pre-
vent Mg2+/Al3+ cations leaching out of the LDHs during intercalation and successfully control the final
Mg2+/Al3+ ratio in the corresponding Mg3Al–LnW10. Most importantly, the as-prepared Mg3Al–LnW10
can be obtained without the formation of the impurity phase. The heterogeneous Mg3Al–EuW10 exhibits
highly efficient catalytic reactivity for cyanosilylation of various aldehydes and ketones under solvent-
free conditions. The Mg3Al–EuW10 can be easily recovered and reused for at least 10 times without
obvious decrease of catalytic activity and the composition and the structure of the Mg3Al–EuW10 cata-
lyst remain stable. Moreover, the Mg3Al–EuW10-catalyzed cyanosilylation of benzaldehyde and hexanal
gives the highest TON of 119,950 and 119,906, respectively.
Keywords:
Polyoxometalates
Lanthanide
Layered double hydroxides
Flocculation
Cyanosilylation
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
of electrostatic force and hydrogen bonding interactions through
the water molecule of the interlayer or the hydroxyl group on the
Polyoxometalates (POMs) are a large class of discrete anionic
metal oxides consisting of group V and VI metals in their highest
oxidation states. POMs are thermally and oxidatively stable, and
their chemical properties can finely be tuned by adjusting the metal
ions, the heteroatoms and the counter ions [1]. These properties
endow POMs superior catalytic materials that have been widely
applied in industry [2]. In the case of POMs-based heterogeneous
catalysts, two strategies including solidification and immobiliza-
tion have been adopted up to now [3]. One of the efficient ways is
the intercalation of POMs into layered double hydroxides (LDHs) to
develop the heterogenized LDHs–POMs catalysts [4].
layers [5c,5d]. LDHs can provide flexible confining space that can
be adjusted by changing the size and the arrangement of guest
molecules. The flexible interlayer space can not only fit small-
sized moieties, but also accommodate bulky catalytic sites that
are difficult or even impossible to enter the rigid pores with fixed
dimension [5e,5f]. Therefore, LDHs are ideal support for bulky POMs
catalysts.
Generally, the LDHs–POMs catalysts can be prepared using
the traditional co-precipitation, ion-exchange, and two-step ion-
exchange method [6]. However, these methods show the following
disadvantages that largely restrict their further application: (1) the
Layered double hydroxides (LDHs) are a class of layered materi-
als with positively charged layers and the balancing anions located
M
2+ and/or M3+ cations can leach out of the LDHs under the neutral
to slightly acidic reaction conditions; (2) some POMs are hydrolyt-
ically unstable at weakly acidic to basic pH and the intercalation
reactions are often accompanied by the co-formation of an impu-
rity phase; (3) the intercalation of POMs can block the micropores
of LDHs, and thereby results in low surface areas [7]; (4) the adapt-
ability of the above-mentioned methods is in general quite poor; (5)
most importantly, XRD patterns of the as-synthesized LDHs–POMs
materials contain the impurity phase, which can be observed after
(003) diffraction in most cases. As such, it is significant to develop
new methods for the preparation of POMs intercalated LDHs.
in the interlayer region. The general molecular formula of LDHs
3+
is [M2+
M
x(OH)2](An−
)
x/n·mH2O, where M2+ and M3+ are di-
1 – x
and trivalent metal cations and An− is a counteranion, x = 0.17–0.33
and it is defined as the M3+/(M2+ + M3+) ratio [5a,5b]. The inter-
layer anions are linked with positively charged host layer by means
∗
Corresponding author. Tel.: +86 10 64431832; fax: +86 10 64431832.
E-mail addresses: songyufei@hotmail.com, songyf@mail.buct.edu.cn (Y.-F. Song).
http://dx.doi.org/10.1016/j.apcata.2014.09.005
0926-860X/© 2014 Elsevier B.V. All rights reserved.

Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
173
Recently, the exfoliated LDHs have been utilized success-
fully for the development of advanced functional materials.
For example, Kim and co-workers [8] reported the successful
assembly of negative charged polymer-encapsulated quantum
dots (QDs) with the exfoliated LDH nanosheets in formamide by
electrostatic interactions. Hwang and co-workers [9] reported
the layer-by-layer assembly of positively charged Zn–Cr–LDH
2D nanosheets and negatively charged layered metal oxide of
titanate 2D nanosheets yields mesoporous nanohybrids with an
excellent photocatalytic activity. It can be naturally conceived
that POMs, as a class of important metal oxides, can be assembled
with LDHs in a similar way as the above-mentioned examples.
Inspired by these studies [8–10], we report herein a series of
new heterogeneous LDHs–POMs with the molecular formula
ICPS-7500 spectrometer. BET measurements were performed at
77 K on a QuantachromeAutosorb-1C analyzer. The samples were
degassed at 110 ◦C for 6 h before the measurements. X-ray photo-
electron spectroscopy (XPS) measurements were performed with
monochromatized Al K␣ exciting X-radiation (PHI Quantera SXM).
The temperature-programmed desorption of ammonia and car-
bon dioxides (NH3/CO2-TPD) were obtained using Auto Chem. II
2920 equipment to examine acid and basic properties of the cata-
lysts surface, respectively. The measurements were accomplished
with 0.050 g of a sample in the temperature range from 100 to
700 ◦C, with helium (He) as a carrier gas and NH3/CO2 as adsorb-
ing gas. Prior to the measurements, the samples were heated in
the flow of He at 500 ◦C. After cooling the samples in He to 100 ◦C,
the heating rate of 20 ◦C min−1 was applied. The corresponding
cyanohydrins were analyzed by Agilent 7820A gas chromatogra-
phy (GC) system using a 30 m 5% phenylmethyl silicone capillary
column with an ID of 0.32 mm and 0.25 ␮m coating (HP-5).
Yields were determined by GC analysis using reference standards.
Assignments of corresponding cyanohydrins were analyzed by 1H-
NMR. 1H-NMR spectra was recorded on a Bruker 400 MHz NMR
spectrometer.
of Mg0.77Al0.23(OH)2[EuW10O36
Mg0.76Al0.24(OH)2[TbW10O36
0.027·0.64H2O (Mg3Al–TbW10
Mg0.76Al0.24(OH)2[DyW10O36 (Mg3Al–DyW10
0.027·0.70H2O
]
0.026·0.62H2O (Mg3Al–EuW10),
]
]
)
and
)
through the spontaneous flocculation method. It is worthwhile
noting that the flocculation process provides a new and rational
way to design new materials with a precisely controlled nano-
structure. Moreover, this method can prevent Mg2+/Al3+ cations
leaching out of the LDHs during the intercalation and thereby,
successfully control the final Mg2+/Al3+ ratio in the Mg3Al–LnW10
(Ln = Eu, Tb and Dy). Most importantly, the Mg3Al–LnW10 materi-
als have been obtained without the co-formation of an impurity
phase. Application of the heterogeneous catalyst of Mg3Al–EuW10
for cyanosilylation of various aldehydes and ketones has been
carried out under solvent-free conditions, and it shows excellent
cyanosilylation of benzaldehyde and hexanal with the highest TON
of 119,950 and 119,906, respectively.
2.3. Preparation of the Na–LnW10 and Mg3Al–NO3
Na9LnW10O36 (Na–LnW10
;
Ln=Eu, Tb and Dy) [11],
[Mg0.75Al0.25(OH)2](CO3)0.125·2H2O (denoted as Mg3Al–CO3)
[12], [Mg0.75Al0.25(OH)2](NO3)0.25·2H2O (Mg3Al–NO3) [12] can be
prepared according to literature methods.
2. Experimental
2.4. Preparation of the Mg3Al–LnW10
2.1. Chemical materials
An amount of 0.10 g of Mg3Al–NO3–LDHs was mixed with
100 ml of formamide in a flask, which was tightly sealed after
purging with N2 to avoid carbonate contamination. The mixture
was vigorously stirring for 2 days. The nanosheet suspension was
obtained after separating an un-exfoliated component by centrifu-
gation for 10 min [13].
Na–LnW10 (0.35 g, 0.11 mmol) was dissolved in 10 ml H2O
(Ln = Eu, Tb and Dy). The Mg3Al–LnW10 was synthesized by mix-
ing the nanosheets suspension of Mg3Al–NO3 and the solution
of Na–LnW10 with stirring under N2 atmosphere for 1 day. The
restacked Mg3Al–LnW10 was obtained by centrifugation, washed
with ethanol and water thoroughly, and then dried under vacuum.
For Mg3Al–EuW10, 0.11 g (yield = 89%); XRD (Cu K␣, ): 2ꢁ = 6.958
(003), 16.046 (006), 28.665 (012), 36.553 (015), 61.279 (110). FT-IR
(KBr, cm−1): ꢂ = 3423, 947, 884, 699, 447. XPS (eV): 35.6 (W4f7/2),
37.7 (W4f5/2), 74.7 (Al2p), 1134.3 (Eu3d5), 1303.7 (Mg1s). Elemen-
tal analysis (%)—found: Mg 13.81, Al 4.39, W 35.01, Eu 2.93; Cacld.
Cyclohexanone (99%), cyclooctanone (99%), 2-adamantanone
(98%), 2-heptanone (97%), 2-octanone (98%), 2-nonanone (98%),
2-decanone (98%), acetophenone (99%), hexanal (99%), heptanal
(99%), octanal (98%), nonanal (99%), decanal (96%), all the used
aldehydes and solvents were purchased from Alfa Acesa. Analyti-
cal europium (III) chloride hexahydrate (EuCl3·6H2O), terbium (III)
chloride hexahydrate (TbCl3·6H2O), dysprosium (III) chloride hexa-
hydrate (DyCl3·6H2O), acetic acid (CH3COOH), sodium tungstate
(Na2WO4·2H2O), magnesium nitrate (Mg(NO3)2·6H2O), aluminum
nitrate (Al(NO3)3·6H2O), hexamethylenetetramine (HMT) and
nitric acid (65 wt.% HNO3) were obtained from Energy Chemical in
Shanghai. All the chemicals and solvents were used without further
purification.
2.2. Measurements
For Mg0.77Al0.23(OH)2[EuW10O36
]
0.026·0.62H2O: Mg 13.68, Al 4.54,
W 34.93, Eu 2.89. For Mg3Al–TbW10, 0.11 g (yield = 88%); XRD (Cu
K␣,◦): 2ꢁ = 7.082 (003), 16.012 (006), 28.724 (012), 36.777 (015),
61.329 (110). FT-IR (KBr, cm−1): ꢂ = 3442, 949, 882, 699, 448. XPS
(eV): 35.6 (W4f7/2), 37.7 (W4f5/2), 74.6 (Al2p), 1244.4 (Tb3d5), 1303.6
(Mg1s). Elemental analysis (%)—found: Mg 13.27, Al 4.49, W 35.52,
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku XRD-6000 diffractometer under the following conditions:
40 kV, 30 mA, Cu K␣ radiation (ꢀ = 0.154 nm). Fourier transform
infrared (FT-IR) spectra were recorded on a Bruker Vector 22
infrared spectrometer using KBr pellet method. Thermogravimet-
ric and differential thermal analyses (TG-DTA) were performed
on a TGA/DSC 1/1100 SF from Mettler Toledo in flowing N2
with a heating rate of 10 ◦C min−1 from 25 to 1000 ◦C. Scan-
ning electron microscopy (SEM) images and energy dispersive
X-ray (EDX) analytical data were obtained using a Zeiss Supra
55 SEM equipped with an EDX detector. Transmission electron
microscopy (TEM) micrographs were recorded using a Hitachi
H-800 instrument. Inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) analysis was performed using a Shimadzu
Tb 3.11; Cacld. For Mg0.76Al0.24(OH)2[TbW10O36
]
0.027·0.64H2O:
Mg 13.20, Al 4.63, W 35.46, Tb 3.07. For Mg3Al–DyW10, 0.10 g
(yield = 82%); XRD (Cu K␣,◦): 2ꢁ = 7.082 (003), 16.090 (006), 28.411
(012), 36.465 (015), 61.172 (110). FT-IR (KBr, cm−1): ꢂ = 3424,
947, 880, 698, 447. XPS (eV): 35.6 (W4f7/2), 37.7 (W4f5/2), 74.6
(Al2p), 1295.0 (Dy3d5), 1303.6 (Mg1s). Elemental analysis for
Mg0.76Al0.24(OH)2 [DyW10O36
Al 4.52, W 35.21, Dy 3.13; Cacld: Mg 13.09, Al 4.59, W 35.17, Dy
3.11.
]
0.027·0.70H2O (%)—found: Mg 13.15,

174
Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
Scheme 1. The preparation process of Mg3Al–LnW10 using the spontaneous flocculation method.
2.5. Procedure for cyanosilylation
3.2. Characterization of the Mg3Al–LnW10
In a typical experiment, 1 mmol aldehyde or ketone, 1.5 mmol
TMSCN and Mg3Al–LnW10 (0.25 mol% LnW10 to substrate, Ln = Eu,
Tb and Dy) as catalyst were placed in a 20 ml glass bottle at
25 ◦C and the reaction mixture was kept stirring vigorously. The
yield of cyanohydrin was periodically determined by GC analy-
sis by reference standards. After the reaction was completed, the
resulting oily product was extracted by diethyl ether. The cat-
alyst of Mg3Al–LnW10 was recovered by centrifugation, washed
with acetone, and dried in air. The corresponding cyanohy-
drin was obtained by column chromatography on silica gel, and
the isolated yield could be calculated based on the obtained
cyanohydrin.
The powder XRD patterns of Mg3Al–EuW10, Mg3Al–TbW10
and Mg3Al–DyW10 (Fig. 1A) show five Bragg diffraction peaks
in the range of 5◦ < 2ꢁ < 70◦. XRD patterns of Mg3Al–EuW10
,
Mg3Al–TbW10 and Mg3Al–DyW10 are fully indexed to a hexago-
nal unit cell belonging to the R-3m space group. The corresponding
basal spacing of d(003) has been summarized in Table S1. Taking
Mg3Al–EuW10 as an example, the gallery height value of 0.79 nm
can be obtained by subtracting the thickness of host layer (0.48 nm)
from the value of d(003) spacing of Mg3Al–EuW10. The gallery
height is nearly the same as the diameter of the short axis of
EuW10 [11], which indicates that the intercalated EuW10 anions
are arranged in an orientation angle of ꢁ (ꢁ = 90◦) with respect
to the Mg3Al–LDHs layers. The other Mg3Al–LnW10 catalysts of
Mg3Al–TbW10 and Mg3Al–DyW10 are in the similar case.
It is very common to observe the impurity phase neighboring
(003) diffraction in the XRD patterns of LDHs–POMs by tradi-
tional synthetic methods such as ion exchange, co-precipitation,
etc. [4,16] and this impurity phase is often attributed to the co-
formation of the salt of the POMs [7]. Interestingly, the XRD patterns
of Mg3Al–LnW10 do not exhibit such impurity phase.
3. Results and discussion
3.1. Preparation of the Mg3Al–LnW10
Scheme 1 shows the process of the preparation of Mg3Al–LnW10
using the spontaneous flocculation method. First of all, the pre-
cursor of [Mg0.75Al0.25(OH)2](NO3)0.250·2H2O (Mg3Al–NO3) can be
prepared from [Mg0.75Al0.25(OH)2](CO3)0.125·2H2O (Mg3Al–CO3)
through acid–alcohol mixed method [14]. The positive nanosheets
of [Mg0.75Al0.25(OH)2]0.25+ can be obtained by exfoliation of
Mg3Al–NO3 in formamide. The positive Mg3Al–NO3 nanosheets
can be confirmed to be unilamellar with lateral dimensions of
2–3 ␮m, which are consistent with previous studies [15]. Sec-
ondly, the aqueous solution of POMs is added to the positive
Mg3Al–NO3 nanosheets suspension. They are reconstructed into
the Mg3Al–LnW10 nanohybrids that are precipitated from the
reaction mixture. Finally, the Mg3Al–LnW10 nanohybrids can be
obtained by centrifugation through the spontaneous flocculation of
the positive Mg3Al–NO3 nanosheets and the negative POMs clus-
ters. Furthermore, the acid resistance of LDHs is much higher in
organic solvents than that in water [14], which can reduce largely
the co-formation of the impurity phase the Mg2+ rich salt of the
POMs.
FT-IR spectra of Mg3Al–EuW10, Na–EuW10 and Mg3Al–NO3
are shown in Fig. 2. The sharp and narrow absorption band at
1378 cm−1 in the spectrum of Mg3Al–NO3 is due to the charac-
−
teristic stretching vibration of NO3 groups [17]. In the case of
Na–EuW10, the strong absorption band at 946, 880 and 697 cm−1
can be assigned to the W–Ot, W–Oc–W and W–Oe–W asymmet-
ric stretching vibrations, respectively. For the FT-IR spectrum of
Mg3Al–EuW10, the broad band at ca. 3423 cm−1 is assigned to the
O–H stretching vibration in the brucite-like layers, and it is noted
that this absorption is located at much lower frequency than that of
free water at 3600 cm−1 [18], which might result from the hydro-
gen bonding between interlayer water and hydroxyl groups of the
host layers [19]. The characteristic absorption bands for the W–O
shift from 946, 880 and 697 cm−1 in Na–EuW10 to higher frequency
at 947, 884 and 699 cm−1 in Mg3Al–EuW10, indicating the presence
of strong electrostatic interactions between the host layers and the
guest anions [4]. The absorption band at 447 cm−1 can be ascribed
Figure 1. (A) Powder XRD patterns of Mg3Al–EuW10, Mg3Al–TbW10 and Mg3Al–DyW10; (B) FT-IR spectra of Mg3Al–NO3, Mg3Al–EuW10 and Na–EuW10
.

Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
175
the octahedral structural geometry. In the case of Mg3Al–EuW10
,
only one peak at 9.07 ppm is visible in the 27Al-MAS-NMR spec-
trum, suggestiong the Al3+ in Mg3Al–EuW10 also possess the
octahedral structural geometry. The above results reveal that the
structure of LDHs remains unchanged after the interacalation of
EuW10. The peak shift in the 27Al-MAS-NMR spectra is due to
the intercalation of EuW10 into LDHs [22]. Similar explanation
can be applied to the 27Al-MAS-NMR results of Mg3Al–TbW10
and Mg3Al–DyW10. The assignments of 27Al-MAS-NMR spectra of
Mg3Al–NO3, Mg3Al–EuW10, Mg3Al–TbW10 and Mg3Al–DyW10 can
be found in Table S7.
The XPS spectra corresponding to Mg core levels of the
Mg3Al–EuW10 and Mg3Al–NO3 are shown in Fig. 3A. The bind-
ing energy of Mg1s is 1303.7 eV in the Mg3Al–EuW10, and that is
1303.6 eV in the Mg3Al–NO3. The XPS spectra corresponding to
Al core levels of the Mg3Al–EuW10 and Mg3Al–NO3 are shown in
Fig. 3B. The binding energy of Al2p is 74.7 eV in the Mg3Al–EuW10
,
and 74.6 eV in the Mg3Al–NO3. The binding energy of W4f7/2 and
W4f5/2 are 35.6 and 37.7 eV in the Mg3Al–EuW10, while 35.5 and
37.6 eV in the Na–EuW10 (Fig. 3C). Similarly, it can be observed that
the binding energy of Eu3d5 is 1134.3 eV in the Mg3Al–EuW10, and
1134.1 eV in the Na–EuW10 (Fig. 3D). The above results reveal that
the binding energy of Mg1s and Al2p in the Mg3Al–EuW10 are almost
the same as those of the Mg3Al–NO3. The W4f7/2 and W4f5/2 peak
Figure 2. The 27Al-MAS-NMR spectra of Mg3Al–NO3, Mg3Al–EuW10, Mg3Al–TbW10
and Mg3Al–DyW10
.
to O–M–O (M = Mg and Al) vibrations in the brucite-like layers of
the LDH [20]. Compared with that of Mg3Al–NO3, the disappear-
ance of the absorption band at 1378 cm−1 for nitrate anions and the
presence of the stretching bands of EuW10 in the FT-IR spectrum of
Mg3Al–NO3 indicates the successful formation of the LDHs–POMs.
FT-IR spectra of Mg3Al–TbW10 and Mg3Al–DyW10 can be explained
in a similar way (Figs. S2 and S3).
width of Mg3Al–EuW10 is slightly different from that of Na–EuW10
,
and such difference is dues to the interactions between the LDHs
layers and EuW10 in Mg3Al–EuW10. XPS results of Mg3Al–TbW10
and Mg3Al–DyW10 can be explained in a similar way (Figs. S4 and
S5).
TG-DTA curve for Mg3Al–EuW10 is shown in Fig. S6. Three
weight-loss stages can be observed with the increase of the temper-
ature from 25 to 1000 ◦C. The first weight loss is 7.98% between 30
and 242 ◦C, which is attributed to the removal of water molecules
(Calcd. 0.62H2O per Mg3Al–EuW10). The second weight loss step
of 8.94% at 242–476 ◦C can be attributed to the collapse of the
Solid state 27Al-MAS-NMR has been used to estimate the local
environments of Al3+ in the corresponding LDHs–POMs. As is
known, the 27Al resonance line positions are very sensitive to
the coordination number and are expected to occupy the −5 to
15 ppm range for the octahedral geometry (AlO6) sites [21]. The
27Al-MAS-NMR spectrum of Mg3Al–NO3 shows only one peak at
10.50 ppm (Fig. 2), indicating all the Al3+ in Mg3Al–NO3 possess
Figure 3. The XPS spectra of Mg3Al–EuW10
.

176
Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
Table 1
BET and TPD results of Mg3Al–EuW10, Mg3Al–TbW10 and Mg3Al–DyW10
.
Entry
Catalyst
Surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm)
Total acidity
Total basicity
a
b
(mmol NH3 gcat−1
)
(mmol CO2 gcat−1
)
1
2
3
4
Mg3Al–EuW10
Reused Mg3Al–EuW10
Mg3Al–TbW10
33
32
42
29
0.09
0.09
0.17
0.06
4.2
4.2
3.8
4.2
1.09
1.08
1.05
1.08
1.31
1.29
1.24
1.27
Mg3Al–DyW10
a
The amount of acid sites were calculated by quantifying the desorbed NH3 from NH3-TPD.
The amount of basic sites were calculated by quantifying the desorbed CO2 from CO2-TPD.
b
layered structure. The third weight loss of 1.12% at 476–591 ◦C
corresponds to partial decomposition of interlayer EuW10. Similar
explanation can be applied to the TG-DTA results of Mg3Al–TbW10
and Mg3Al–DyW10 (Figs. S7 and S8). Combining the TG-DTA and ICP
analysis (Table S9), the molecular formula of Mg3Al–LnW10 can be
determined.
Table 2
The effect of different catalysts on cyanosilylation of acetophenone with TMSCN at
25 ◦C under solvent-free condition.
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
Mg3Al–EuW10
Mg3Al–TbW10
Mg3Al–DyW10
Na–EuW10
95
72
48
7
7
6
6
5
5
4
7
8
9
5
6
7
SEM and TEM images of Mg3Al–EuW10, Mg3Al–TbW10 and
Mg3Al–DyW10 (Fig. 4) show the porous stacking of sheet-like crys-
tallites. The particle sizes of these crystallites are 300–400 nm.
Mg3Al–EuW10 shows a type I adsorption isotherm at relative lower
pressure (P/P0 < 0.1), and a H3 type N2 hysteresis loop at rel-
ative higher pressure (P/P0 > 0.5) according to BDDT (Brunauer,
Deming, Deming and Teller) classification (Fig. S9) [23], indicat-
ing the presence of both interlayer micropores and inter-particle
mesopores. Mg3Al–TbW10 and Mg3Al–DyW10 exhibit similar situ-
ation (Figs. S10 and S11). Temperature-programmed desorption of
ammonia (NH3-TPD) demonstrate that the Mg3Al–EuW10 exhibit
a main NH3 desorption peak at 378.4 ◦C in Fig. 5A, which corre-
sponds to the NH3 molecules adsorbed on medium acid sites (both
Brónsted and Lewis acid sites) of lanthanide-containing POMs of
EuW10. Temperature-programmed desorption of carbon dioxide
(CO2-TPD) demonstrate that the Mg3Al–EuW10 exhibit a main CO2
desorption peak at 369.3 ◦C in Fig. 5B, which corresponds to the
CO2 molecules adsorbed on medium basic sites of Mg3Al–LDHs. The
total acidity and basicity of the Mg3Al–EuW10, Mg3Al–TbW10 and
Mg3Al–DyW10 determined by NH3-TPD and CO2-TPD have been
summarized in Table 1. The results reveal that the Mg3Al–LnW10
catalysts can provide both acidity and basicity.
K–Eu(PW11
)
)
2
K–Eu(SiW11
K–Eu(P2W17
H–PW12
Na–PW12
H–SiW12
2
)
2
9
10
11
12
13
14
15
16
17
18
19
20
21
K–PW11
Na–A–PW9
Na–B–PW9
K–SiW11
Na–SiW9
Mg3Al–NO3
Mg3Al–NO3 + Na–EuW10
Na2WO4
EuCl3
EuCl3 + Na2WO4
None
33
8
48
49
7
Reaction conditions: acetophenone (1 mmol), catalyst (2.5 mol% W to acetophe-
none), 25 ◦C, 6 h. Yields were determined by GC analysis using reference standards.
Assignments of corresponding cyanohydrins were analyzed by 1H- and 13C-NMR.
K17[Eu(P2W17O61)2] (Na–EuW10, K–Eu(PW11)2, K–Eu(SiW11
K–Eu(P2W17)2) are not effective (Entries 4–7). The Keggin-
type POMs of H3[PW12O40], Na3[PW12O40], H4[SiW12O40
)2 and
]
(H–PW12, Na–PW12 and H–SiW12; Entries 8–10), and the lacunary
Keggin-type POMs of K7[PW11O39], Na8H[A-PW9O34], Na8H[B-
PW9O34], K8[SiW11O39] and Na10[SiW9O34] (K–PW11, Na–A–PW9,
Na–B–PW9, K–SiW11 and Na–SiW9, Entries 11–15) show almost
no catalytic efficiency.
Contrast experiments suggest that the LDHs precursor of
Mg3Al–NO3 is not effective (Entry 16). The physical mixture of
Mg3Al–NO3 and Na–EuW10 gives the yield of 33%, which is much
lower than that of Mg3Al–EuW10 (Entry 17). Na2WO4 is not effec-
tive (Entry 18). EuCl3 and the physical mixture of EuCl3 and
Na2WO4 give the yields of 48 and 49%, respectively (Entries 19
3.3. Catalytic performance of the Mg3Al–LnW10
The cyanosilylation of aldehydes and ketones with trialkylsi-
lyl cyanides (TMSCNs) is of significance in organic synthesis
because this reaction not only creates a new C–C bond, but
also protects an alcohol function. Cyanohydrins produced by
this method can be transformed into
a variety of building
blocks, such as ˛-hydroxy acids, ˛-hydroxy ketones and ˇ-
amino alcohols [24], which are widely applied in chemical
industry. Among the catalysts developed for the cyanosilyla-
tion reaction, both acid catalysts [25] and base catalysts [26]
can act as catalysts to promote cyanosilylation. Therefore, from
molecular structural viewpoint, design of acid–base catalysts
is a promising pathway to develop efficient catalysts for such
cyanosilylation reaction. As such, the Mg3Al–LnW10 catalysts
with both acidity and basicity have been applied for cyanosi-
lylation of various aldehydes and ketones under solvent-free
conditions.
Cyanosilylation of acetophenone catalyzed by Mg3Al–EuW10
has been carried out at 25 ◦C under solvent-free conditions,
and excellent yield of 95% with >99.9% selectivity of corre-
sponding cyanohydrin can be obtained (Table 2, Entry 1). Other
LDHs–POMs of Mg3Al–TbW10 and Mg3Al–DyW10 only give the
yields of 72 and 48% (Entries 2 and 3), which are much lower
than that of Mg3Al–EuW10. The lanthanide-containing POMs of
Na9[EuW10O36], K11[Eu(PW11O39)2], K13[Eu(SiW11O39)2] and
and 20). These yields are much lower than that of Mg3Al–EuW10
.
The reaction cannot proceed in the absence of catalyst (Entry 21).
The above results reveal that Mg3Al–EuW10 gives the best cat-
alytic activity, and the catalytic activity decreases with the decrease
´
3+
of the ionic radii of Ln3+ in Mg3Al–LnW10 (Eu3+ (1.21 A) > Tb
˚
´
´
3+
˚
˚
(1.18 A) > Dy (1.17 A) for eight coordination). The above results
reveal that interactions between the Ln3+ cations and the two sand-
6−
wiching [W5O18
]
POM units weaken with an increase of ionic
radii of the Ln3+ cations, which suggests that the steric crowd-
ing around the Ln3+ centers decreases with an increase in ionic
radii of Ln3+ cations [29]. During the reaction, the weaker interac-
tion between the Ln3+ cations and the two sandwiching [W5O18
6−
]
POM units results in the stronger interaction between the Ln3+
cations and carbonyl groups. Therefore, the carbonyl groups can
be better activated by the Ln3+ cations with larger radii, and the

Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
177
Figure 4. SEM and TEM images of Mg3Al–EuW10 (A and D), Mg3Al–TbW10 (B and E) and Mg3Al–DyW10 (C and F).
reaction will be better promoted by the Ln3+ cations with
larger radii. In conclusion, though the acidity and basicity of
Mg3Al–LnW10 (Ln = Eu, Tb and Dy) are similar, the different ionic
radii of Ln3+ in Mg3Al–LnW10 leads to different catalytic activities
in cyanosilylation reaction.
Effect of solvents on cyanosilylation of acetophenone has been
investigated catalyzed by Mg3Al–EuW10 (Fig. S12). The results
reveal that the yield under solvent-free conditions is higher than
those in solvents. Therefore, to obtain high yields, cyanosilylation
of various aldehydes and ketones will proceed under solvent-free
conditions.
To obtain the kinetic parameters for cyanosilylation of
acetophenone, experiments have been performed with the
acetophenone:TMSCN:EuW10 = 401:602:1 at 25 ◦C under solvent-
free conditions. Yield and ln(Ct/C0) are plotted against reaction
time in Fig. 6A, in which C0 and Ct are initial acetophenone
concentration and acetophenone concentration at time t, respec-
tively. The linear fit of the data reveals that the catalytic reaction
exhibits pseudo-first-order kinetics for the cyanosilylation reac-
tion (R2 = 0.9933). The rate constant k of cyanosilylation can be
determined to be 0.0067 min−1 on the basis of Eqs. (1) and (2).
Cyanosilylation of acetophenone could be completed with the
yield of 95% in 6 h. Thus, the catalyst of Mg3Al–EuW10 exhibits high
catalytic efficiency for cyanosilylation, and the catalytic reaction
strictly obey pseudo-first-order kinetics with >99.9% selectivity of
corresponding cyanohydrins.
−dCt
= k
(1)
(2)
dt
C0 − Ct = kt
Cyanosilylation of various aldehydes and ketones catalyzed by
Mg3Al–EuW10 has been carried out at 25 ◦C under solvent-free
conditions. All the corresponding cyanohydrins can be afforded in
good yields with >99.9% selectivity of corresponding cyanohydrins
(Table 3, Entries 1–32). The results indicate that cyanosilylation of
electron-donating substituted benzaldehydes such as methyl- and
methoxy-substituted benzaldehydes (Entries 2–7) proceed easier
than that of electron-withdrawing substituted benzaldehydes such
as chloro-, bromo- and nitro-substituted benzaldehydes (Entries
8–16). TheHammettplotforthecyanosilylationof para-substituted
benzaldehyde derivatives gave the negative ꢃ value of −0.7582
(Fig. 6B), suggesting the formation of positive transition state cat-
alyzed by Mg3Al–EuW10 is part of the rate-limiting step.
It is worth noting that the cyanosilylation of benzalde-
hyde proceeds efficiently in >99.9% yield with Mg3Al–EuW10
(benzaldehyde:TMSCN:EuW10 = 23,990:35,985:1)
at
25 ◦C
Figure 5. NH3 (A) and CO2 (B) TPD profiles of Mg3Al–EuW10, Mg3Al–TbW10 and Mg3Al–DyW10
.

178
Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
Table 3
Cyanosilylation of various aldehydes and ketones with TMSCN catalyzed by Mg3Al–EuW10 at 25 ◦C under solvent-free conditions.
Entry
Acceptor
Product
Time (h)
Yield (%)a
98 (97)
1
X = H
X = H
2
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
X = 2-Me
X = 3-Me
X = 4-Me
X = 2-MeO
X = 3-MeO
X = 4-MeO
X = 2-Cl
X = 3-Cl
X = 4-Cl
X = 2-Br
X = 3-Br
X = 2-Me
X = 3-Me
X = 4-Me
X = 2-MeO
X = 3-MeO
X = 4-MeO
X = 2-Cl
X = 3-Cl
X = 4-Cl
X = 2-Br
X = 3-Br
1.5
1.5
1.5
1.5
1.5
1.5
3.5
4
4
3.5
4
4
5
>99.9 (98)
>99.9 (98)
>99.9 (99)
>99.9 (98)
>99.9 (99)
>99.9 (98)
>99.9 (99)
97 (95)
98 (96)
>99.9 (99)
97 (95)
>99.9 (99)
>99.9 (99)
>99.9 (98)
98 (96)
X = 4-Br
X = 4-Br
X = 2-NO2
X = 3-NO2
X = 4-NO2
X = 2-NO2
X = 3-NO2
X = 4-NO2
5
6
17
3
6
>99.9 (98)
18
82 (78)
19
20
21
22
23
R = n-C5H11
R = n-C6H13
R = n-C7H15
R = n-C8H17
R = n-C9H19
R = n-C5H11
R = n-C6H13
R = n-C7H15
R = n-C8H17
R = n-C9H19
2
2
2.5
2.5
2.5
97 (94)
96 (92)
97 (93)
98 (95)
99 (98)
24
2
>99.9 (99)
25
6
95 (92)
26
27
28
29
R = n-C5H11
R = n-C6H13
R = n-C7H15
R = n-C8H17
R = n-C5H11
R = n-C6H13
R = n-C7H15
R = n-C8H17
3
3
3
3
>99.9 (97)
>99.9 (98)
99 (97)
99 (98)
30
1
>99.9 (98)
31
32
6
3
76 (71)
>99.9 (98)
Reaction conditions: substrate (1 mmol), Mg3Al–EuW10 (0.25 mol% to substrate based on EuW10), TMSCN (1.5 mmol), 25 ◦C. Yields were determined by GC analysis using
reference standards. Assignments of corresponding cyanohydrins were analyzed by 1H- and 13C-NMR.
a
The values in parentheses are the isolated yields.
nanoparticles in chloroform (TON = 869, Entry 9) [30], Eu3+-MCM-
41 under solvent-free conditions (TON = 137, Entry 10) [25a],
supported OH− ionic liquid in dichloromethane (TON = 35.7,
Entry 11) [31], supported Sc triflate in dichloromethane
(TON = 20.4, Entry 12) [32], Yb(OTf)2 in dichloromethane
(TON = 17.2, Entry13) [33], Cu(OTf)2 in dichloromethane (TON
=16.2, Entry 15) [34], P(MeNMCH2CH3)3N in THF (TON = 9.2,
Entry 16) [35], Bu2SnCl2 under solvent-free conditions (TON = 9,
Entry 17) [36], and other catalysts (Entries 18–20 and 22–26)
[37–39].
under solvent-free conditions. The turnover number (TON)
can reach as high as 23,990 (Table 4, Entry 1). Furthermore,
the 10 g scale experiment of cyanosilylation of benzalde-
hyde can also proceed in >99.9 yield with the Mg3Al–EuW10
(benzaldehyde:TMSCN:EuW10 = 119,950:179,925:1)
at
25 ◦C
under solvent-free conditions. The TON can reach as high as
119,950 (Entry 2), which is higher than that of LiCl in the 10 g scale
experiment of cyanosilylation of benzaldehyde (TON = 100,000,
Entry 5) at 25 ◦C in THF [27]. To the best of our knowledge, the
TON for Mg3Al–EuW10-catalyzed cyanosilylation of benzalde-
hyde is the highest among previously reported values, which is
higher than other catalysts including LiCl in THF (TON = 100,000,
Entry 3) [27], Sc(OTf)3 in [bmim][SbF6] (TON =10,000, Entry
Furthermore,
the
cyanosilylation
of
hexanal
pro-
ceeds efficiently in >99.9% yield with Mg3Al–EuW10
(hexanal:TMSCN:EuW10 = 23,981:35,972:1) under solvent-free
conditions at 25 ◦C. The TON can reach as high as 23,981 (Entry
3). The 10 g scale experiment of cyanosilylation of hexanal
6)
[28],
TBA8H2[(ꢄ-SiYW10O36)2]·7H2O
(TBA–Y2(SiW10)2)
in 1,2-dichloroethane (TON = 9400, Entry 7) [29], Au

Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
179
Figure 6. (A) Kinetic profiles of cyanosilylation of acetophenone catalyzed by Mg3Al–EuW10 [square dot: yield of cyanohydrins; round dot: ln(Ct /C0)]; (B) Hammett plot for the
cyanosilylation of benzaldehyde and p-substituted benzaldehyde derivatives. Reaction conditions: benzaldehyde or p-substituted benzaldehyde (1.0 mmol), Mg3Al–EuW10
(0.25 mol% to substrate based on EuW10), TMSCN (1.5 mmol), 25 ◦C.
can also proceed in >99.9 yield with the Mg3Al–EuW10
(hexanal:TMSCN:EuW10 = 119,906:179,859:1) at 25 ◦C under
solvent-free conditions. The TON can reach as high as 119,906
at 25 ◦C and under solvent-free conditions. Moreover, the heteroge-
neous catalyst of Mg3Al–EuW10 can be easily recovered and reused.
To confirm the catalysis is truly heterogeneous, cyanosilylation
of acetophenone with TMSCN has been selected as an example by
applying Mg3Al–EuW10 as catalyst at 25 ◦C under solvent-free con-
ditions. When the product yield reaches about 50%, Mg3Al–EuW10
is removed from the reaction mixture by filtration, and the reaction
is allowed to proceed with the filtrate under the same conditions.
It can be found that no product can be obtained by adding more
TMSCN. ICP-AES measurement reveals there is no Mg, Al, W and
Eu content in the filtrate. These results rule out the contribution of
Mg, Al, W and Eu species leached into the reaction solution for the
observed catalytic results. Therefore, the catalytic reaction is truly
heterogeneous.
(Entry 4). To the best of our knowledge, the TON for Mg3Al–EuW10
-
catalyzed cyanosilylation of hexanal is the highest among
previously reported values, and is higher than other catalysts
such as TBA8H2[(ꢄ-SiYW10O36)2]·7H2O (TBA–Y2(SiW10)2) in
1,2-dichloroethane (TON = 18,000, Entry 8) [29], Yb(OTf)2 in
dichloromethane (TON = 47.5, Entry 14) [33], and Zr(KPO4)2 in
dichloromethane (Entry 21) [38].
From the above results, it can be seen clearly that to the best of
our knowledge, Mg3Al–EuW10-catalyzed cyanosilylation of benzal-
dehyde and hexanal gives the highest TON of 119,950 and 119,906,
respectively. Such cyanosilylation reaction can proceed efficiently
Table 4
Comparison of yields and TONs for the cyanosilylation catalyzed by different catalysts.
Entry
Catalyst
Substrate
Solvent
T (◦C)
t (h)
Yield (%)
TONa
Ref.
1
2
3
4
5
6
7
8
Mg3Al–EuW10
Mg3Al–EuW10
Mg3Al–EuW10
Mg3Al–EuW10
LiCl
Sc(OTf)3
TBA–Y2(SiW10
TBA–Y2(SiW10
Au nanoparticles
Eu3+-MCM-41
Supported OH− ionic liquid
Supported Sc triflate
Yb(OTf)2
Benzaldehydeb
Benzaldehydec
Hexanald
None
None
None
None
25
25
25
25
25
25
25
25
25
25
32
25
25
25
25
0
25
23
25
Reflux
Reflux
0
0
0
0
0
6
8
6
8
48
0.5
0.25
0.03
4
1
0.5
3
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
94
90
93
90
>99.9
95
86
95
81
92
90
99
>99.9
98
83
23,990
119,950
23,981
119,906
100,000
10,000
9400
18,000
869
137
This work
This work
This work
This work
[27]
[28]
[29]
[29]
[30]
[25a]
[31]
[32]
[33]
[33]
[34]
[35]
[36]
[37]
[25b]
[38]
[38]
[39]
[39]
[39]
[39]
[39]
Hexanale
Benzaldehydef
Benzaldehyde
Benzaldehyde
Hexanal
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Hexanal
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Hexanal
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
THF
[bmim][SbF6]
1,2-Dichloroethane
1,2-Dichloroethane
Chloroform
g
)
)
2
2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
None
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
THF
35.7
20.4
17.2
47.5
16.2
9.2
9
–
–
–
–
15
2
3
1
Yb(OTf)2
Cu(OTf)2
P(MeNMCH2CH3)3N
Bu2SnCl2
Sn–W mixed oxide
Al3+-MCM-41
Zr(KPO4)2
Zr(KPO4)2
Fe-Mont
CaF2
HAp
None
0.5
0.5
0.08
0.3
0.2
0.2
0.5
0.3
0.2
0.2
1,2-Dichloroethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
Dichloromethane
96
99
95
96
–
–
–
–
CaO
MgO
95
–
a
TON (turnover number) = mole of corresponding product/mole of catalyst used.
Benzaldehyde (5.3 g, 50 mmol), benzaldehyde:TMSCN:EuW10 = 23,990:35,985:1.
Benzaldehyde (10.6 g, 100 mmol), benzaldehyde:TMSCN:EuW10 = 119,950:179,925:1.
Hexanal (5.0 g, 50 mmol), benzaldehyde:TMSCN:EuW10 = 23,981:35,972:1.
Hexanal (10.0 g, 100 mmol), benzaldehyde:TMSCN:EuW10 = 119,906:179,859:1.
Benzaldehyde (10.6 g, 100 mmol), benzaldehyde:TMSCN:LiCl = 100,000:100,000:1.
TBA–Y2(SiW10)2: TBA8H2[(ꢄ-SiYW10O36)2]·7H2O.
b
c
d
e
f
g

180
Y. Jia et al. / Applied Catalysis A: General 487 (2014) 172–180
(c) A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh, G. Izzet, Chem.
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Soc. Rev. 41 (2012) 7605–7622;
be achieved in the cyanosilylation of acetophenone through cen-
trifugation after the reaction. The powder XRD, FT-IR, XPS, TPD
(Fig. S14) and BET results (Table 1) reveal that the structure
of the Mg3Al–EuW10 remains unchanged. The yields of corre-
sponding products remain unchanged and the yields for catalyst
recovery of Mg3Al–EuW10 are above 96% (Fig. S15). The ICP
results (Table S10) also reveal that the composition of reused the
Mg3Al–EuW10 remains unchanged. All these results reveal that
the structure and composition of Mg3Al–EuW10 is not affected by
the cyanosilylation and Mg3Al–EuW10 is stable before and after
cyanosilylation.
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To summarize, the intercalation of the lanthanide-containing
POMs into LDHs by the spontaneous flocculation method has led
to the formation of a series of new Mg3Al–EuW10, Mg3Al–TbW10
and Mg3Al–DyW10. The spontaneous flocculation method not only
prevents Mg2+/Al3+ cations leaching out of the LDHs during the
intercalation reaction and successfully control the final Mg2+/Al3+
ratio in the Mg3Al–POMs, but also shows no co-formation of the
impurity phase.
The application of Mg3Al–EuW10 for cyanosilylation of vari-
ous aldehydes and ketones under solvent-free conditions shows
that the acid and base cooperative effect of Mg3Al–EuW10 plays
a significant role for the highly efficient catalytic activity in
cyanosilylation of various aldehydes and ketones. As far as we
know, Mg3Al–EuW10-catalyzed cyanosilylation of benzaldehyde
and hexanal give the highest TON of 119,950 and 119,906, respec-
tively; cyanosilylation catalyzed by Mg3Al–EuW10 can proceed
efficiently at 25 ◦C and under solvent-free conditions without
the environmental-unfriendly solvents of THF, dichloromethane,
1,2-dichloroethane and chloroform; Mg3Al–EuW10 can be eas-
ily recovered and reused for at least 10 times without obvious
decrease of catalytic activities. The structure and the composition
of the Mg3Al–POMs catalysts remain untouched. The easy prepa-
ration, reusability, and efficiency of the heterogeneous catalysts of
Mg3Al–EuW10 provide great potential for industrial application.
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This research was supported by National Basic Research
Program of China (973 program, 2014CB932104), National
Science Foundation of China (21222104), Key Laboratory of
Polyoxometalate Science of Ministry of Education (Northeast
Normal University), Fundamental Research Funds for the Central
Universities (RC1302) and the Program for Changjiang Scholars and
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Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
apcata.2014.09.005.
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