Preparation of Mesoporous Basic Oxides through Assembly of Monodispersed Mg–Al Layered Double Hydroxide Nanoparticles

Article abstract of DOI:10.1002/chem.201701282

Mesoporous basic Mg–Al mixed metal oxides (MMOs) with a high surface area and large pore size have been prepared through the assembly of monodispersed layered double hydroxide nanoparticles (LDHNPs) with block copolymer templates. The particle sizes of th

Full text of DOI:10.1002/chem.201701282

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A Journal of
Accepted Article
Title: Preparation of Mesoporous Basic Mixed Metal Oxides through
Assembly of Monodispersed Mg-Al Layered Double Hydroxide
Nanoparticles
Authors: Yuya Oka, Yoshiyuki Kuroda, Takamichi Matsuno, Keigo
Kamata, Hiroaki Wada, Atsushi Shimojima, and Kazuyuki
Kuroda
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To be cited as: Chem. Eur. J. 10.1002/chem.201701282
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Preparation of Mesoporous Basic Mixed Metal Oxides through
Assembly of Monodispersed Mg–Al Layered Double Hydroxide
Nanoparticles
Yuya Oka,[a] Yoshiyuki Kuroda,*[b]† Takamichi Matsuno,[a] Keigo Kamata,[c, d] Hiroaki Wada,[a] Atsushi
Shimojima[a] and Kazuyuki Kuroda*[a, e]
Abstract: Mesoporous basic Mg–Al mixed metal oxides (MMOs) with
a high surface area and large pore size were prepared through the
assembly of monodispersed layered double hydroxide nanoparticles
(LDHNPs) with block copolymer templates. The particle sizes of the
LDHNPs were controlled mainly by varying the concentration of
tris(hydroxymethyl)aminomethane (THAM), which was used as a
surface stabilizing agent for the LDHNPs. LDHNPs and micelles of a
block copolymer (Pluronic F127) were assembled to form a composite.
The composites were calcined to transform them into mesoporous
MMOs and remove the templates. The Brunauer-Emmett-Teller
surface areas, mesopore sizes, and pore volumes increased as a
result of using the templates. Moreover, the pore sizes of the
mesoporous MMOs were controlled by using LDHNPs with different
sizes. The mesoporous MMOs prepared from the LDHNPs showed
much higher catalytic activity than a conventional MMO catalyst for
the Knövenagel condensation of ethyl cyanoacetate with
benzaldehyde. The mesoporous MMO catalyst prepared using the
smallest LDHNPs, ca. 12 nm in size, showed the highest activity.
Therefore, the use of monodispersed LDHNPs and templates is
effective for preparing highly active mesoporous solid base catalysts.
Introduction
Porous basic mixed metal oxides (MMOs) are one of the most
important types of materials applied for catalysts,[1] catalyst
supports,[2] anion scavengers,[3] CO2 adsorbents,[4] and so on.
There have been several issues in the synthesis of porous MMOs
with uniform distributions of metal cations, high surface areas, and
highly accessible active sites. Basic MMOs are synthesized via
the thermal decomposition of the corresponding metal salts or
metal hydroxides, though the control of the crystal growth of the
metal salts or metal hydroxides is quite difficult. Thus, effective
synthetic methods for porous basic MMOs with controlled porosity
are in high demand.
Layered
double
hydroxides
(LDHs;
[M2+
1–
xM3+x(OH)2]x+[(Ann–)x n·yH2O]x–), which consist of positively
/
charged brucite-type layers and interlayer anions, are promising
precursors for basic MMOs. LDHs contain various
homogeneously distributed divalent and trivalent cations within
their layers. The use of LDHs is advantageous for achieving
uniform distributions of divalent and trivalent cations and diverse
compositions.[5] MMOs prepared from LDHs have excellent
basicity, environmentally friendly compositions, high thermal
stability, and acid-base bifunctionality;[6] therefore, MMOs are
used as solid base catalysts for various organic reactions, such
as the Knövenagel condensation,[7] aldol condensation,[8] transfer
hydrogenation,[9] and transesterification,[10] and they are important
in the industrial preparation of fine chemicals, such as
pharmaceuticals and perfumes.[11]
[a]
Y. Oka, T. Matsuno, Prof. Dr. H. Wada, Prof. Dr. A. Shimojima,
Prof. Dr. K. Kuroda
Department of Applied Chemistry
Faculty of Science and Engineering
Waseda University
Okubo 3-4-1, Shinjuku-ku, Tokyo 169-8555 (Japan)
E-mail: kuroda@waseda.jp
Dr. Y. Kuroda
Waseda Institute for Advanced Study,
Waseda University
1-6-1 Nishiwaseda, Shinjuku-ku Tokyo 169-8050 (Japan)
E-mail: kuroda-yoshiyuki-ph@ynu.ac.jp
Prof. Dr. K. Kamata
Laboratory for Materials and Structures,
Institute of Innovative Research,
Tokyo Institute of Technology
[b]
[c]
For these reasons, there have been many reports on the
preparation of porous basic MMOs using LDHs as precursors.[12]
LDH platelets are decomposed into an aggregate of
nanoparticulate MMOs because of the dehydration and
decarbonation of the LDHs. As a result, the MMOs prepared from
LDHs usually show high surface areas;[6] however, such a top-
down process for the formation of interparticle pores is not
suitable for the control of the pore sizes and surface area of
MMOs. Templated synthesis of porous MMOs using polystyrene
particles (350–600 nm)[12a,12b,12j] or block copolymer micelles
(such as Pluronic F127[12c] and Pluronic P123[12d]) as templates is
more useful for controlling the properties of the porous structures.
In these templated syntheses, porous MMOs were prepared via
4259-R3-33 Nagatsuta, Midori-ku, Yokohama 226-8503 (Japan)
Prof. Dr. K. Kamata
PRESTO
Japan Science and Technology Agency (Japan)
Prof. Dr. K. Kuroda
Kagami Memorial Research Institute for Materials Science and
Technology
2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051 (Japan)
Present address: Green Hydrogen Research Center, Yokohama
National University
[d]
[e]
†
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501 (Japan).
Supporting information for this article is given via a link at the end
of the document.
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coprecipitation of Mg–Al LDH or Ni–Al LDH in the presence of
templates, followed by calcination for the removal of the templates
and for the thermal decomposition of LDHs. Because the crystal
growth of LDHs is rapid and uncontrollable, physical confinement
by such templates is insufficient to suppress it. Pore walls
consisting of LDHs/MMOs tend to be thick and uncontrollable
compared to mesoporous metal oxides (e.g., mesoporous silica
and titania). Thus, controlled synthesis of LDHs is essential for
the design of porous MMOs. Using very small LDH nanoparticles
as building blocks is one of the most promising ways to prepare
porous MMOs with very high surface areas and controlled pore
sizes.
Recently, we reported the synthesis of very small (ca. 10 nm)
and monodispersed Mg–Al LDH nanoparticles (LDHNPs) using
Figure 1. (a) Formation of a mesoporous MMO by the assembly of LDHNPs
with an F127 micelle. Proposed structural models of (b) LDHNP(12)-F127, (c)
LDHNP(26)-F127, (d) LDHNP(60)-F127.
tris(hydroxymethyl)aminomethane (THAM) as
a
surface
stabilizing agent for the suppression of crystal growth.[13] LDHNPs
are highly dispersible in water and their particle sizes can be
controlled mainly through the concentration of THAM. The size
reduction
was
effective
in
improving
their
anion
Results and discussion
exchangeabilities[13a] and ultrafast adsorption of anionic
species.[13b] Here, we focus on their use as building blocks for the
preparation of highly porous Mg–Al MMOs. In addition, Mg–Al
MMOs have shown the highest catalytic activities for aldol
condensation,[14] side chain methylation of toluene,[15] and the
Oppenauer oxidation[16] among representative strong bases (e.g.,
Mg–Al, Co–Al, Zn–Al, Li–Al, and Ni–Al MMOs); therefore, the use
of Mg–Al LDHNPs as a precursor for the preparation of MMOs
with high catalytic activity is promising.
Size control of monodispersed LDHNPs
Monodispersed LDHNPs with various sizes were prepared
according to the methods in our previous reports[13] by using
THAM as a surface stabilizer for LDHNPs. The particle sizes were
controlled simply by varying the concentration of THAM (0.1, 0.25,
and 0.5 M), which suppressed the crystal growth of the LDHNPs.
The LDHNPs were synthesized as colloidal dispersions by mixing
aqueous solutions of metal salts and THAM. The colloidal
dispersions of LDHNPs were transparent and showed the Tyndall
effect, indicating the presence of dispersed colloidal LDHNPs
(Figures 2a–c). The X-ray diffraction (XRD) patterns of these
samples showed diffraction peaks assignable to those of typical
Mg–Al LDH (Figure 3).[5] The diffraction peaks became broader
as the concentration of THAM increased, indicating the reduction
of the particle sizes of the LDHNPs. Scanning electron
microscopy (SEM) images clearly showed that the LDHNPs
became smaller as the concentration of THAM increased (Figures
2d–f). The average sizes of the LDHNPs changed from 12 nm to
60 nm, and the monodispersity of the particle sizes was confirmed
through SEM observations (Figures 2g–i and Table 1). Thus, the
use of THAM is quite effective for obtaining very small and uniform
LDHNPs with well-controlled particle sizes.
The compositions of the LDHNPs are indicated in Table S1.
The molar ratios of THAM and metal cations (Mg+Al) were
estimated from elemental analyses (Table 1). The amount of
attached THAM increased with the initial concentration of THAM.
The particle sizes of the LDHNPs were negatively correlated with
the amount of attached THAM. This tendency is consistent with
the assumption that the crystal growth of LDHs is suppressed by
surface modifications with THAM.[13a] The Mg/Al ratios of the
products were different from those of the metal salt solutions used
for their syntheses. The Mg/Al ratios were roughly correlated with
the concentration of THAM; thus, the variations in the Mg/Al ratios
should be affected by the presence of THAM.
There have also been reports on the preparation of very small
metal hydroxide nanoparticles (particle size: 5–8 nm) using
propylene oxide for pH control and organic additives at a low
temperature (20–40 °C).[17] On the basis of this method, single-
component metal hydroxide nanoparticles composed of various
metal cations[17a] and Ni–Al LDH nanoparticles[17b] have been
prepared; however, if this method is applied to the preparation of
Mg–Al LDH nanoparticles, Al(OH)3 may be produced as an
impurity during the gradual increase of the pH.[18] However, the
possibility of applying this method for the synthesis of Mg–Al LDH
nanoparticles has not been mentioned.
In this study, we demonstrate a rational synthetic method for
mesoporous Mg–Al MMOs using tripodal-ligand-stabilized Mg–Al
LDHNPs as building blocks. LDHNPs and micelles of a block
copolymer (Pluronic F127) were assembled to form a composite.
The composites were then calcined to transform them into
mesoporous MMOs and remove the template (Figure 1a). A
highly porous structure was achieved by using the block
copolymer micelles as templates. The properties of the porous
structures were controlled through the particle sizes of the
LDHNPs. The effects of the porous structures controlled by using
templates on the catalytic activities of the mesoporous MMOs
were examined in the Knövenagel condensation.
It was confirmed from the 13C cross-polarization magic angle
spinning nuclear magnetic resonance (CP/MAS NMR) and
Fourier transform infrared (FTIR) spectra of the LDHNPs that
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Figure 3. XRD patterns of (a) LDHNP(12), (b) LDHNP(26), and (c) LDHNP(60).
Table 1. Synthetic conditions, particle sizes, and molar compositions of
LDHNPs.
Sample
[THAM]/M
Average
size/nm[a]
Molar composition
Mg/Al
2.5
THAM/(Mg+Al)[b]
LDHNP(12)
LDHNP(26)
LDHNP(60)
0.50
0.25
0.10
12 (19%)
26 (16%)
60 (29%)
0.059
0.032
0.014
2.2
Figure 2. Photographs of the dispersions of (a) LDHNP(12), (b) LDHNP(26),
and (c) LDHNP(60). Red light from the left demonstrates the Tyndall effect. SEM
images of (d) LDHNP(12), (e) LDHNP(26), and (f) LDHNP(60). Particle size
distributions of (g) LDHNP(12), (h) LDHNP(26), and (i) LDHNP(60) (N = 100).
1.9
[a] Standard deviation is indicated in parentheses. [b] The amount of THAM
was calculated from the nitrogen content.
THAM was covalently attached to the surface of the LDHNPs
through M-O-C (M = Mg or Al) bonds (see Figures S1 and S2).
The assignments of the signals and bands are described in the
Supporting Information, and were discussed fully in our previous
reports.[13a,19] When the concentration of THAM is relatively low
(i.e., 0.1 M), it modified predominantly the outermost basal plane
of a platelet, resulting in the reduction of the number of layers.
Because of the structural matching between the triangular
configurations of hydroxymethyl groups in a THAM molecule and
the triangular arrangement of surface hydroxyl groups on an LDH
layer, it is reasonable for a THAM molecule to be connected
predominantly on the basal plane. When the concentration of
THAM is relatively high (i.e., 0.25 M and 0.5 M), THAM should
also be attached at the edge sites of the platelets, resulting in the
reduction of the lateral sizes of the LDHNPs.
When the LDHNPs were collected without aging (80 °C, 1 d),
their particle sizes were 8.3 nm (LDHNP(12)), 13.7 nm
(LDHNP(26)), and 6.8 nm (LDHNP(60)), which are much smaller
than the sizes of those collected after aging (Figure S3). Thus,
the LDHNPs grow during the heating (80 °C). This means that
THAM molecules with controlled concentrations determine the
limit of the particle growth of the LDHNPs. Such a system has not
been reported for the synthesis of LDHNPs, to the best of our
knowledge. This characteristic of this synthetic method is quite
useful for the precise control of particle sizes, uniformity of particle
size distributions, and reproducibility.
Assembly of LDHNPs with a block copolymer for the
preparation of mesoporous MMOs
The LDHNP-F127 composites were assembled from LDHNPs
and a block copolymer template (Pluronic F127, abbreviated as
F127 hereafter) to prepare mesoporous MMOs (hereafter
denoted as MMO(X)-F127; X = particle size of the LDHNPs used
as the precursor). LDHNP-F127 composites were prepared by
mixing an LDHNP dispersion and an aqueous solution of F127,
and then filtering and drying at 80 °C (hereafter denoted as
LDHNP(X)-F127; X = particle size of the LDHNPs). Because the
critical micellar concentration of F127 is 6.1 g/L at 25 °C,[20] F127
should form micelles under the present experimental conditions.
The FTIR spectra of LDHNP(X)-F127 showed bands at ca. 420
cm−1 and ca. 665 cm−1 owing to lattice vibrations of LDH layers.
The band at 1110 cm−1 arises from the C-O-C asymmetric
stretching vibration characteristic of F127.[21] Thus, the
composites contain both LDHNPs and F127 (Figure S4). The
band due to the CH2 rocking vibration of F127 shifted from 963
cm−1 for pure F127 to 950 cm−1 for the composites. Such a shift
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