The direct assembly of Mg-Al LDH nanosheets and Mn(ii)-salen complex into sandwich-structured materials and their enhanced catalytic properties

Article abstract of DOI:10.1039/c3ra42837k

The direct assembly of LDH nanosheets and organic molecules, such as Mn(ii)-salen complex, into a sandwich-structured nanocomposite has been achieved via a modified flocculation method. When used as a catalyst, the obtained material has superior thermal s

Full text of DOI:10.1039/c3ra42837k

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The direct assembly of Mg–Al LDH nanosheets and
Mn(II)–salen complex into sandwich-structured
materials and their enhanced catalytic properties†
Cite this: RSC Adv., 2013, 3, 19885
Received 8th June 2013
*
Liyan Dai, Jing Zhang, Xiaozhong Wang and Yingqi Chen
Accepted 16th August 2013
DOI: 10.1039/c3ra42837k
www.rsc.org/advances
The direct assembly of LDH nanosheets and organic molecules, such via occulation. If all of the organic particles were exposed on
as Mn(II)–salen complex, into a sandwich-structured nanocomposite the surface of the solid, it would reduce the stability of the
has been achieved via a modified flocculation method. When used as materials and make only limited use of the native structural
a catalyst, the obtained material has superior thermal stability and properties of LDHs. Currently, most reports have been con-
catalytic properties over a material prepared using a traditional ion cerned with the intercalation chemistry of LDHs using ion
exchange method in the oxidation of 4-picoline.
exchange methods and stepwise LBL,8 however, the direct
assembly of LDH nanosheets and ionic organic particles has
still not been achieved. In contrast, this method is so intriguing
because not only is the interlayer of the LDH completely open,
resulting in no steric hindrance for the immobilization of
macromolecules, but it also avoids the complex operations of
other processes. In this regard, it is desirable to modify existing
occulation methods to intercalate organic molecules into LDH
nanosheet galleries. This process will denitely provide a much
simpler and more widely applicable strategy for material
syntheses.
In this work, we have reported the direct combination of
LDH nanosheets and an ionic Mn(II)–salen complex to form a
nano-sized sandwich-structured material for the rst time and
then explored its catalytic properties. In order to prepare a
suspension of LDH nanosheets, a new way to exfoliate LDH was
developed that incorporated benzoate. The well-established
nanosheets were combined with an ionic Mn(II)–salen complex
through a modied occulation process yielding ordered
sandwich-structured particles. We then applied the obtained
materials to the N-oxidation of pyridines, which resulted in
enhanced catalytic properties over a material prepared using a
traditional ion exchange method and its homogeneous
analogue.
The preparation of clay mineral particles on the nanoscale is an
active area of academic and, more importantly, application
research in the elds of electronics, photonics, magnetism and
catalysis.1 In particular, LDHs (layered double hydroxides,
3+
[M2+
M ]x/n$zH2O) that consist of stacks of posi-
x(OH)2][Anꢀ
1ꢀx
tively charged metal hydroxide layers and exchangeable inter-
layer anions have been extensively studied, particularly their
delamination processes.2 The resulting nanosheets can be used
as exible building blocks to create novel organic–inorganic or
inorganic–inorganic nanomaterials. Different forms of LDH
nanocomplex have been developed as catalysts, using electro-
static sequential deposition, occulation or layer-by-layer (LBL)
methods over the last few years.3 Among these methods, oc-
culation is the most convenient and provides a rational way to
design novel materials with controlled nanostructures.4 Using a
occulation process, target species can be easily and rapidly
loaded into the nanocomposite materials. As reported, inor-
ganic nano-colloids such as AuNPs5 and oxide nanosheets6 have
been assembled with LDH nanosheets via spontaneous occu-
lation, forming sandwich-structured materials. However, the
arrangement is different when organic macromolecules are
involved. Anionic porphyrins, rst reported by Nakagaki and
coworkers,7 were simply adsorbed on to the surface of LDH
crystals instead of intercalating into the interlayers of the LDHs
The LDH incorporated with benzoate (LDH-BA) was synthe-
sized according to a coprecipitation method from the literature9
and then delaminated in formamide to obtain a stable and
transparent colloid suspension using an ultrasonic treatment.
Dispersions of LDH-BA were prepared with concentrations of 10
g Lꢀ1, 20 g Lꢀ1, 30 g Lꢀ1 and 40 g Lꢀ1 using an ultrasonic
treatment (see Scheme S1†). Gelation happened at high
concentrations (30 g Lꢀ1 and 40 g Lꢀ1) aer resting for few days.
However, the transparent and colourless sol of the lower
Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou,
P.R. China. E-mail: dailiyan@zju.edu.cn; Fax: +86-571-87952693; Tel: +86-571-
87952693
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra42837k
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concentration dispersions (10 g Lꢀ1) could be maintained, even
for months. The delamination process was further investigated
using the LDH-BA dispersion with a concentration of 10 g Lꢀ1
.
As illustrated in Fig. S1a,† a clear Tyndall effect can be observed,
which provides evidence for the formation of exfoliated LDH-BA
nanosheets. Disk-like particles were seen using TEM with
particle sizes ranging from 20 nm to 100 nm (Fig. S2b†). Zeta
potential measurements demonstrated that the LDH-BA nano-
sheets were in positively charged state (Fig. S1c†). The average
particle size is ꢁ100 nm, which can be further characterised
using DLS measurements (see Fig. S1d†). The AFM analysis,
shown in Fig. S1e,† further conrmed that LDH-BA delami-
nated into single sheets. The height measurement curves show
that the completely delaminated LDH nanosheets possess a
height of ꢁ0.7 nm (see the green curve), while some particles
are thicker, with heights of up to ꢁ2 nm (see the red and blue
curves).
The preparation of Cat.1 was conducted by exchanging BA
with an anionic salen complex at rt as reported in the litera-
ture.10 Cat.2 was prepared by adding exfoliated LDH-BA nano-
sheets, at a speed of 0.1 mL minꢀ1, into a salen–water solution
at a high stirring speed. A slow feeding rate and fast stirring
speed are key to controlling the crystal size of the nanoparticles.
Restacking of the LDH nanosheets occurred as soon as they
came into contact with the salen–water solution. Formamide
molecules were replaced with H2O and the ionic salen complex
acted as a bridge, combining the separate nanosheets layers
together to form the sandwich-structured Cat.2. Elemental
analysis (C, H and N) and ICP-AES clearly demonstrated that the
specic formula of LDH-BA is Mg0.66Al0.32(OH)2(BA)0.3$H2O,
Fig. 1 (a) XRD patterns of LDH-BA, Cat.1 and Cat.2. SEM images of (b) LDH-BA,
(c) Cat.1 and (d) Cat.2.
and the weight percentages of salen complex in Cat.1 and Cat.2 opened up via delamination and no barrier existed for the
were 18.3 and 20.1 wt%, respectively (see Table S1†). intercalation of the salen complex. Interestingly, adding the
The XRD patterns of LDH-BA, Cat.1 and Cat.2 are shown in salen complex solution dropwise into the LDH-BA nanosheet
Fig. 1. All of the present samples possessed the same reection suspension as in the literature,7 results in the anionic salen
(010) in the high 2q region, indicating that the Mg/Al ratio was molecules being simply adsorbed on to the surfaces of the LDH
unchanged and the characteristic LDH structure was well crystals (see Fig. S2†). No reections appeared in the low 2q
maintained aer the ionic exchange (Cat.1) or restacking region (<10ꢂ), conrming that: (1) BA molecules were fully
˚
process (Cat.2). A basal spacing of 15.2 A for LDH-BA dissolved in the formamide solution during delamination and
was calculated from the (003) reection, which corresponds to (2) no salen molecules were inserted into the interlayers aer
the formation of a benzoate bilayer-like structure within the restacking. Therefore, the results of XRD determined that our
gallery.11,12 Given that the hydrophobic groups stay in the modied occulation process was effective, providing Cat.2
middle of the layer much like fatty acids, it is easy for delam- with a clear sandwich-structure. SEM images of LDH-BA, Cat.1
ination to occur in a polar solvent. The (003) reection is and Cat.2 are shown in Fig. 1. Hexagonal platelets of LDH with
retained in Cat.1, although its intensity is decreased and a typical size of 3 mm were observed. Aer ion exchange, Cat.1
several overlapped reections appear in the low 2q region, features a much larger particle size and more agglomeration
indicating that LDH-BA is partly intercalated with the salen than LDH-BA. For Cat.2, regular rod-like particles with a
complex. The basal spacings of the two main reections (*) are micrometer long and a nanometer wide were formed as
˚
˚
18.9 A and 31.5 A, suggesting that the arrangement of the salen expected. FT-IR spectra of the obtained samples are shown in
and BA layers is regular (see Scheme S2†). Notably, a new Fig. S3.† The proles of curves c and d are nearly identical,
reection at 3.5ꢂ, which is characteristic of Cat.2, conrmed the indicating that the compositions of Cat.1 and Cat.2 are similar.
successful intercalation of the salen complex into the interlayer The imine C]N stretching vibration of the free ligand at 1623
of LDH. The d-spacing was 24.5 A, which is larger than the cmꢀ1 shis to 1640 cmꢀ1 for Cat.1 and 1633 cmꢀ1 for Cat.2. The
˚
salen-layer in Cat.1 (18.9 A) and the previously reported value bands at 1115 and 1032 cmꢀ1, corresponding to the sulfonato
˚
13
˚
(18.78 A). The calculated size of the Mn(II)–salen complex is groups further conrm the successful intercalation of the
˚
17–18 A. The salen complex in Cat.1 might be oriented diago- anionic salen complex.
nally at an angle less than 90ꢂ to the layers,14 while it is
The thermal stabilities of LDH, Cat.1 and Cat.2 were studied
perpendicular in Cat.2 as the layers of LDH-BA were totally using simultaneous thermal analysis (TGA–DTA) under a N2
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Table 1 The N-oxidation of 4-picoline using Cat.1, Cat.2 and their homogeneous
atmosphere. Fig. S4(a)† shows a total weight loss of up to ꢁ48 wt
% and ꢁ44 wt% for LDH and Cat.1 and Cat.2, respectively, at
analoguea
ꢂ
700 C, which could be due to the loaded salen complex. For
Entry
Catalyst
Oxidant
Time/h
Yieldb (%)
TOFc
instance, in order to reach the same decomposition level of
25%, the required temperature increased from 350 ꢂC for LDH-
1
2
3
4
5
6
7
8
Mono-salen
Mono-salen
Cat.1
Cat.1
Cat.2
Cat.2
LDH
—
H2O2
O2
H2O2
O2
H2O2
O2
H2O2
H2O2
5
24
8
24
3.5
24
24
24
77
56
42
ꢂ
ꢂ
6.2
176
—
415
14
BA to 380 C for Cat.1 and to 515 C for Cat.2. Obviously, the
thermal stability of Cat.2, which was prepared aer rebuilding
the LDH nanosheets, is far better than that of Cat.1, prepared
using the traditional ion exchange method. The exothermic and
endothermic conditions are other important pieces of evidence
related to crystallization and decomposition, which can be
detected using DTA. As shown in Fig. S4(b),† the rst endo-
thermic peaks for all three samples are observed at ꢁ70 ꢂC and
can be attributed to the elimination of water molecules
including physisorbed and crystallized water.15 However, the
second endothermic peak, associated with layer dehydrox-
ylation and interlayer anion decomposition, appears at different
positions: the ion exchange of benzoate with salen complex
84 (83)d
—
95 (93)d
22
—
—
—
—
a
All reactions were carried out using 10 mmol 4-picoline and 20 mg
catalyst in 20 mL DD water with 0.68 g (30 mmol) H2O2 or molecular
b
oxygen at room temperature. The yield was determined using HPLC.
c
TOF (turnover frequency) ¼ mmol of product per mmol of catalyst
per hour. d Product yields aer 6 recycles.
improves the alignment of the crystal resulting in a 40 ꢂC specic surface area, but also remarkably decreases the diffu-
increase, while aer assembly of the LDH nanosheets the crystal sion distance for the substrate from the reaction system to the
alignment is even better, which is in agreement with the results active site in the material due to its nanoscale thickness.
of TGA. Thus, the excellent thermal stability of Cat.2 can expand However, the catalytic results were very different when molec-
its application to different oxidation reactions with broad ular oxygen was used as the oxidant (entry 2, 4, 6). The reaction
temperature requirements.
rate decreased rapidly and the yields were not good, even aer
Since Mn(II)–salen complexes are well known for their effi- 24 hours for all of the catalysts. In addition, the homogeneous
cient reactivity in oxygenation reactions, we decided to apply analogue, contrary to the previous results, showed higher
our catalyst to the N-oxidation of pyridines, which is the rst catalytic activity than Cat.1 and Cat.2. This might be due to the
example of applying a Mn(II)–salen complex in this oxidation. poor solubility of molecular oxygen in water, which inhibits its
High temperature (90 ꢂC) and acidic conditions are always combination with the active center in the interlayer of the LDH.
required to obtain N-oxides of pyridines during industrial Furthermore, solvent effects have also been discussed, which
production, however our catalyst was able to provide an efficient show that water is the best solvent for this reaction, while the
and mild route, facilitating the oxidation of pyridines at room yield is poor in other solvents such as methanol, ethanol,
temperature. The catalytic performance of Cat.1, Cat.2 and the acetonitrile and dichloromethane (see Table S2†). The recycla-
mono complex was investigated using 30% H2O2 or molecular bility of Cat.1 and Cat.2 was tested under the same conditions.
oxygen as the oxidant in water, with 4-picoline used as the raw The results demonstrated that both heterogeneous catalysts
material (Scheme 1). As can be seen in Table 1, when obtaining were stable for 6 cycles with a slight loss of catalytic activity. The
the target N-oxide of 4-picoline using 30% H2O2 (entry 1, 3, 5), high stability of Cat.2 over these cycles is mainly due to the
Cat.2 displayed superior activity over the other catalysts, strong electrostatic interaction between the negatively charged
including Cat.1 and their homogeneous analogue. The catalyst Mn(II)–salen complex and the positively charged LDH nano-
turnover frequency of Cat.2 was the highest, showing 2–10 fold sheet layers.
activity over the others. Blank experiments demonstrated that
To further investigate the stability of our catalyst, further
with LDH (entry 7) or only the oxidant (30% H2O2, entry 8) in the tests have been conducted. Electron spin resonance (ESR)
system the reaction did not proceed. We believe that the spectra of Cat.2 and Cat.20 (aer 6 recycles) are shown in Fig. 2.
structural properties of LDH are responsible for the enhance- As can be seen, the typical six hyperne splitting lines were well
ment of the catalytic performance (Cat.1 and 2). LDHs are weak resolved around g ¼ 2.0, corresponding to the hyperne
bases and can help to cleave the peroxide bond in H2O2 due to coupling of the low spin state (S ¼ 1/2) of the Mn2+ ion’s elec-
the formation of H-bonding between the OHꢀ of LDH and a tronic spin with its 5/2 nuclear spin.16 Aer being recycled 6
hydrogen atom of H2O2. Therefore, the capacity of H2O2 was times in the reaction, the intensity decreased a little, but there is
improved. A comparison between Cat.1 and Cat.2 also reveals no change in the hyperne splittings of the six-line pattern. This
that the size of the nanoparticles of Cat.2, not only increases the result conrms that the Mn(II) species remains the same aer
oxidation. The result of ICP-AES analysis showed no trace of Mn
in the ltrate, inferring that no demetallation occurred. The
main reason for the yield decrease might be contamination of
the active center by the substrate.
In conclusion, we report the delamination of LDH and
incorporation of benzoate in formamide. In addition, we have
Scheme 1 The oxidation of 4-picoline to give its N-oxide.
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demonstrated the successful direct assembly of LDH and
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Mn(II)–salen complex using a modied occulation process,
thus providing a practical solution to introduce organic mole-
cules into LDH galleries. The particles, when used as catalysts,
showed remarkable catalytic properties and thermal stability
over that prepared using a traditional ion exchange process. The
yield of the oxidation of 4-picoline to its N-oxide was over 90%,
even aer the catalyst was recycled 6 times. The reaction was
both green and low cost, and the preparation was simple, thus
offering an effective strategy for industrial production.
This work was supported by the NSFC (no: 21176213), and
Zhejiang Key Innovation Team of Green Pharmaceutical Tech-
nology (no: 2010R50043).
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19888 | RSC Adv., 2013, 3, 19885–19888
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