Mg/Al-CO3 layered double hydroxide nanorings

Article abstract of DOI:10.1039/c1jm12129d

Mg/Al-CO₃ Layered Double Hydroxide (LDH) nanorings with a 750 nm exterior diameter and 250 nm interior diameter were synthesized in an organic/water solvent system via a urea hydrolysis method, using Mg ₁₀(OH)₁₈Cl₂

Full text of DOI:10.1039/c1jm12129d

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PAPER
Mg/Al-CO layered double hydroxide nanorings†
3
a
a
a
bc
b
a
Miaosen Yang, Junfeng Liu, Zheng Chang,* Gareth R. Williams, Dermot O’Hare, Xuehan Zheng,
a
a
Xiaoming Sun* and Xue Duan
Received 13th May 2011, Accepted 7th July 2011
DOI: 10.1039/c1jm12129d
Mg/Al-CO Layered Double Hydroxide (LDH) nanorings with a 750 nm exterior diameter and 250 nm
3
interior diameter were synthesized in an organic/water solvent system via a urea hydrolysis method,
using Mg (OH) Cl $5H O nanowires as precursors. X-Ray diffraction and Fourier-transform
1
0
18
2
2
3
infrared spectroscopy clearly revealed the nanorings to comprise Mg/Al-CO LDH. The character of
the precursor materials and the ratio of organic solvents to water used for reaction played crucial roles
in determining the final ring-like morphology. A possible mechanism was proposed for the LDH
nanoring formation. The unique LDH nanorings exposed a higher specific surface area and larger pore
volume than plate-like and flower-like analogues, and thus were further explored for their catalytic
activity using the Knoevenagel reaction between benzaldehyde and diethyl malonate as a probe
reaction. The ring-like structure displayed a significantly enhanced catalytic performance above other
analogues.
Introduction
growth tendency in LDHs, and results in almost exclusively
21
hexagonal plate-like morphologies. Synthesis of LDHs with
other well defined morphologies is rare and challenging. Previ-
ously, hard or soft templating methods have been used for the
morphology control of LDHs. For instance, hollow spheres have
been obtained by growing the LDH on subsequently sacrificing
Layered Double Hydroxides (LDHs) can be described using the
II
III
nꢀ
II
general formula M 1ꢀx
x 2 2
M (OH) (A )x/n$mH O (where M and
III
nꢀ
M
are metal cations, A is an n-valent anion). They have
a brucite-like structure and positive charged layers owing to
II
partial substitution of M cations in the parent Mg(OH)
2
struc-
22
hard templates such as carbon microspheres or polystyrene
20,23
III
ture by M cations. These materials have applications in a wide
1
colloid crystal templates.
One-dimensional LDH nano-
2–4
range of fields, including as catalysts or catalyst precursors,
structures (e.g. nanobelt or nanorod) have also been synthesized
24,25
using surfactants or copolymer molecules as soft templates.
5
absorbents, anion exchangers, electro- and photo-active mate-
6
7–11
12–15
rials,
and bioactive nanocomposites.
However, the range of morphologies that can be induced is still
limited. Furthermore, using templates or surfactants to tailor the
LDH morphology often increases the risk of impurity intro-
duction, and even causes structure collapse during template
The morphological tailoring of LDHs is of significant scientific
importance because it could influence their optical, electronic,
16–19
magnetic, and catalytic properties.
For example, three-
dimensional ordered macroporous LDHs intercalated by deca-
removal. Ring-like nanostructures have attracted particular
26–30
tungstate anions exhibit a higher photocatalytic activity than the
attention,
because they have relatively larger surface areas,
20
standard plate-like counterparts. However, the brucite-like
two-dimensional (2D) crystal structure drives a strong 2D
higher chemical accessibility and more active sites than their
31–33
plate-like counterparts.
Yet no ring-like structures of LDHs
have been reported. Recently, LDHs and their oxides derived via
thermal treatment have attracted remarkable interest as effective,
environment-friendly solid base catalysts for many reactions
a
State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Beijing, 100029, P. R. China.
E-mail: sunxm@mail.buct.edu.cn; changzheng@mail.buct.edu.cn
34–36
b
such as aldol condensation, alkylation and isomerization.
So
Chemistry Research Laboratory, Department of Chemistry, University of
Oxford, Mansfield Road, Oxford, UK OX1 3TA
the study of the catalytic performance of LDH materials with
different morphologies is important.
c
School of Human Sciences, London Metropolitan University, Holloway
Road, London, UK N7 8DB
Herein, we demonstrate the facile synthesis of Mg/Al-CO
nanorings via a solvothermal treatment of Mg10(OH)18Cl $5H
nanowires in an organic/water mixed solvent system.
Mg10(OH)18Cl $5H O was employed both as the magnesium
3
LDH
† Electronic supplementary information (ESI) available: FT-IR pattern
of MAC-R (Fig. S1), XRD pattern and TEM image of MAC-R
calcined at 500 C for 6 h (Fig. S2,S3), XRD pattern and FT-IR
pattern of MAC-F (Fig. S4,S5), TEM image of MAC-R prepared after
a reaction time of 40 min (Fig. S6), 3D stacked plot recorded for the
transition of Mg10-NW into MAC-R (Fig. S7),TEM image of MAC-R
prepared in pure water (Fig. S8), XRD pattern and TEM image of
MAC-P (Fig. S9,S10). See DOI: 10.1039/c1jm12129d
2
2
O
ꢁ
2
2
source and also as a sacrificial hard template. This allowed us to
exercise morphological control on the LDH products while
ensuring phase purity. No surfactants were used in the synthesis of
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J. Mater. Chem., 2011, 21, 14741–14746 | 14741

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either precursors or products, with the aim of reducing processing
dissolved into 25 mL aqueous solution, then transferred into a 40
ꢁ
complexity. The Mg/Al-CO LDH nanorings exposed high specific
3
mL Teflon-lined stainless autoclave and heated at 150 C for 6 h.
surface area, large pore volume, and thus displayed an enhanced
catalytic performance compared to their plate-like and flower-like
analogues in the Knoevenagel reaction. This unique structure
might offer potential applications in catalysts, adsorbents, and
drug delivery systems.
The product was filtered, rinsed and dried, and denoted MAC-P.
Characterization
Fourier transfer infrared (FT-IR) spectra were recorded on
a Bruker Vector 22 using the KBr disk technique in the range
ꢀ1
ꢀ1
4
000–400 cm , with a resolution of 1 cm . Scanning Electron
Experimental section
Microscopy (SEM, Zeiss SUPRA 55) was operated at an accel-
eration voltage of 20 kV. Transmission Electron Microscopy
Materials
(
TEM) was performed on Hitachi 800 and JEM JEOL3010
microscopes equipped with energy dispersive X-ray spectroscopy
EDS) running at 200 kV. The low-temperature nitrogen
Dodecyl mercaptan (98%) was purchased from Alfa Aesar
Chemical. Co. Ltd. Urea (AR), MgCl
2 2 3 2
$6H O (AR), AlCl $6H O
(
(
AR), MgO (AR), 1-hexanol (99%), benzaldehyde (98.5%),
adsorption–desorption experiments were carried out using
a Quantachrome Autosorb-1C-VP system. Specific surface area
was calculated from the adsorption branch according to the
Brunnauer–Emmett–Teller (BET) method. The pore size distri-
bution was determined using the Barrett–Joyner–Halenda (BJH)
model on the desorption branch. Prior to the measurements,
diethyl malonate (99.5%), ethanol (99.7%), and acetone (HPLC
grade) were purchased from Beijing Yili Fine Chemical. Co. Ltd.,
and used without further purification. Deionized water was used
throughout the experimental processes.
ꢁ
Synthesis of Mg10(OH)18Cl
2
$5H O nanowires
2
samples were outgassed at 200 C under vacuum for 4 h. In situ
energy dispersive X-ray diffraction (EDXRD) data were
Mg10(OH)18Cl
the method previously described. In a typical procedure,
MgCl $6H O (16.2640 g) was dissolved with 20 mL deionized
2 2
$5H O nanowires were synthesized according to
38
obtained at the Diamond Light Source using Beamline I12. In
order to access the low-q space (q ¼ 2p/d) necessary to follow
LDH formation reactions, the 23-element detector was shifted
horizontally with respect to the beam, and only one of the
detector elements (number 23) employed for data collection. The
relationship between q and the energy of diffracted X-rays was
calculated from a standard sample ([LiAl (OH) ]Cl$H O). X-
37
2
2
water in a 50 mL beaker. The beaker was placed in a constant-
ꢁ
temperature oil bath at 70 C, and MgO powder (0.3224 g) was
slowly added under vigorous stirring. The mixture was contin-
ually stirred for 1.5 h after the addition of MgO. Then the
solution was left to stand at room temperature for 24 h. A white
viscous colloidal precipitation was obtained. The resulting
precipitation was poured into a 40 mL Teflon-lined stainless
2
6
2
Ray diffraction patterns were recorded at 65 s intervals for
approximately 2.5 h.
ꢁ
autoclave, sealed, and heated at 160 C for 6 h, then allowed to
Catalytic experiment
cool to room temperature naturally. The product was washed
ꢁ
with deionized water and pure ethanol, dried at 50 C for 6 h, and
The Knoevenagel reaction between benzaldehyde and diethyl
malonate was used as a probe reaction to examine the catalytic
performance of MAC-R. Benzaldehyde (7.24 mL), Diethyl
malonate (10.80 mL) and the catalyst (MAC-R, 0.19 g) were
added together into a 25 mL three-necked flask. The reaction was
denoted Mg10-NW.
3
Synthesis of Mg/Al-CO LDH nanostructures
Mg/Al-CO
a simple urea hydrolysis method with Mg10(OH)18Cl
nanowires as precursors. Urea (0.4054 g), AlCl $6H O (0.3018 g)
and Mg (OH) Cl $5H O (0.2662 g) were dispersed in 25 mL of an
3
LDH ring-like nanostructures were synthesized using
ꢁ
carried out at 140 C for 14 h with vigorous agitation. For
2
$5H
2
O
comparison, parallel reactions with MAC-F (0.19 g) or MAC-P
3
2
(
0.19 g) as catalyst were carried out in the same conditions.
10
18
2
2
Aliquots (0.3 mL) were aspirated using an micropipette at 2 h
intervals, and then diluted to 30 mL in acetone (HPLC grade).
The liquids were injected into a GC-Mass (Shimadzu QP2010)
with FID detector on a RTX-5 column (30 m, 0.53 mm i.d.). The
organic/water mixed solvent. The volume of organic solvent
dodecyl mercaptan or 1-hexanol) was increased gradually from 0 to
0 mL, and water added to make the solvent volume up to 25 mL.
(
2
The resulting suspension was stirred or agitated by ultrasonication
for 10 min, then transferred into a 40 mL Teflon-lined stainless
ꢁ
analysis programme held the samples for 3 min at 50 C, then
ꢁ
ꢀ1
ꢁ
heated them at 10 C min to 250 C with a 10 min dwell. The
conversion was calculated on the basis of the initial and final
concentrations of diethyl malonate.
ꢁ
autoclave which was sealed and heated at 150 C for times between
40 min and 6 h. The white precipitate formed after natural cooling in
air was filtered, washed with deionized water and pure ethanol, and
finally dried at room temperature. This product was denoted
MAC-R.
Results and discussion
Mg/Al-CO3 LDH flower-like nanostructures were prepared
using a similar method, but using MgCl $6H O (0.7624 g)
Morphological and textural characterization of
2
2
Mg (OH) Cl $5H O nanowires and Mg/Al-CO LDH
1
0
18
2
2
3
2+
instead of Mg (OH) Cl $5H O nanowire as the Mg source;
1
0
18
2
2
nanorings
this product was denoted MAC-F.
For comparison, Mg/Al-CO LDH plate-like nanostructures
were also prepared using a traditional method. Urea (0.4054 g),
AlCl $6H O (0.3018 g) and MgCl $6H O (0.7624 g) were
3
The XRD pattern and TEM image of Mg10-NW are given in
Fig. 1. As shown in curve A in Fig. 1a, the precursors exhibited
a set of characteristic diffraction peaks which could be indexed as
3
2
2
2
1
4742 | J. Mater. Chem., 2011, 21, 14741–14746
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ꢁ
The XRD pattern of MAC-R calcined at 500 C for 6 h (Fig. S2
in ESI†) indicated that the characteristic reflections of MgO
(
JCPDF 74-1225) were present while of LDH crystals completely
disappeared. The TEM image of calcined MAC-R (Fig. S3 in
ESI†) revealed that the ring-like morphology was mostly inheri-
ted, especially the obvious holes in the particle centers. It was clear
that the ring-like morphology of MAC-R was stable because
ꢁ
calcination at 500 C completely destroyed the layered structure
but retained the morphology of the LDH precursors.
Morphologic and textural characterization of Mg/Al-CO LDH
3
nanoflowers
To estimate the influence of the precursor on the LDH products,
2 2
we used MgCl $6H O instead of Mg10-NW as the magnesium
2
source. Synthesis in the same conditions (dodecyl mercaptan/H O
ꢁ
mixed solvents, 150 C, 6 h, etc.) yielded a Mg/Al-CO
3
LDH
nanostructure with rose flower-like morphology (Fig. 2). The top
view image showed polygon patterns with diameters in the range
of 1.5–3 mm. Each polygon had a bright spot in the center and
exhibited gradually increasing contrast from the center to edge,
which implied a multivalve structure constructed with regularly
stacked nanoplatelets around a center with discrete angles. The
side view TEM image clearly exhibited a patchwork of several
platelets, which looked exactly like rose flowers (as shown in the
inset). The multivalve structures were also confirmed by SEM
observation (Fig. 2c, 2d). The observation of a typical LDH XRD
pattern with an interlayer spacing of 0.77 nm (Fig. S4 in ESI†)
Fig. 1 (a) XRD pattern of Mg10-NW (A) and MAC-R (B), (b) TEM
image of Mg10-NW, (c) low-magnification TEM image of MAC-R
(insert is the homologous high-magnification TEM image), (d) SEM
image of MAC-R.
2 2
monoclinic Mg10(OH)18Cl $5H O (JCPDF No. 07-0409). The
36
2ꢀ
evidenced the phase purity of MAC-F. The presence of CO3
was further supported by the FT-IR spectrum (Fig. S5 in ESI†).
TEM image (Fig. 1b) of Mg10-NW depicted uniform wire-like
nanostructures with a width typically in the range of 20–40 nm
and length from tens to hundreds of micrometres.
36
Discussion on the formation mechanism
The Mg10-NW were used as precursors and treated in dodecyl
ꢁ
mercaptan/H O (20 mL/5 mL) at 150 C for 6 h. The XRD
2
It was clear that unique ring-like structures were closely related
to the Mg10-NW precursor. A possible formation mechanism of
pattern of the resulting samples (curve B in Fig. 1a) strongly
indicated a crystal structure transformation from the precursor
nanowires. All reflections could be indexed to a (00l) diffraction
line series and (110), (113) diffraction peaks of a rhombohedral
ꢀ
symmetry (R3m) LDH (JCPDF 54-1030). The basal spacing of
0
.75 nm, calculated from the characteristic (003) reflection, was
2ꢀ
in agreement with that reported for CO
3
intercalated
39
x 2 3 2
Mg1xAl (OH) (CO )x/2$mH O. Complete absence of reflec-
tions corresponding to the Mg10-NW precursors suggested that
the final product comprised a pure LDH phase, with complete
conversion of the nanowires.The FT-IR absorption spectrum of
2ꢀ
MAC-R (Fig. S1 in ESI†) evidenced that a characteristic CO3
intercalated LDH was formed. The bands observed in the low-
ꢀ1
frequency region 400–900 cm corresponded to lattice vibration
modes of the LDH structure, and the most intense band at about
ꢀ
1
2ꢀ
3
39
1
364 cm could be assigned to the y
ꢀ
3
vibration of CO
anions,
which can easily replace Cl between the LDH layers.
Fig. 1c and 1d respectively give typical TEM and SEM images
of MAC-R at different magnifications. The LDH samples clearly
displayed a unique hexagonal ring-like nanostructure, with
a uniform size in the range of 700–800 nm but with obvious holes
of average diameter about 250 nm in their centers, different from
conventional plate-like and any other morphologies reported to
date. The above results unambiguously revealed that the Mg/Al-
Fig. 2 (a) and (b) TEM images of the top and cross views of MAC-F, (c)
SEM image of the top view of MAC-F, (d) high-magnification SEM
image of a single flower-like structure, (e) photo of a rose flower.
3
CO LDH nanoring structure was synthesized successfully
without any surfactants involved.
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the LDH nanorings is illustrated in Fig. 3. Experimental evidence
for this mechanism was obtained by quenching the reaction at
various time points, and investigating the materials recovered by
TEM and XRD. At Stage I (the first ꢂ40 min), the Mg10-NW
were partially dissolved in an acidic environment due to the
and resulted in rose flower-like structures, which had been
43
reported previously in the literature. Moreover, the urea
hydrolysis was indispensable for generation of the ring-like
LDH, because the gradually increased pH was likely to benefit
the Mg10-NW dissolution and LDH heterogeneous nucleation
at the initial stages.
3+
hydrolysis of Al cations, which resulted in irregular fragments
of the nanowires (see TEM image in Fig. S6 in ESI†) and
significant weight loss (ꢂ60%) of the solid materials. The pH
value of the solution gradually increased at the consequent stage
The mixed solvent was found to be critical for generation of
the ring-like structures. A sample prepared in the same condi-
ꢁ
tions (Mg10-NW, 150 C, 6 h) with pure water as solvent only
(
Stage II, 40ꢂ80 min) with urea hydrolysis and more
yielded regular hexagonal platelets (Fig. S8 in ESI†), which
might be due to a very rapid dissolution of the Mg-rich precur-
sors. However, surprisingly, when using organic molecules such
as 1-hexanol to replace dodecyl mercaptan, nanorings still
formed (Fig. 4a, 4b). Repeating the experiment in the revised
Mg (OH) Cl $5H O dissolution. The induction time was
1
0
18
2
2
relatively long because of slow dissolution of the nanowire
precursors and hydrolysis of urea before the LDH formed.
2+
3+
At Stage II, Mg and Al began to co-precipitate as the LDH,
which grew around the residual Mg10(OH)18Cl $5H O fragments
2+
2
2
solvent system but using Mg salt as precursor resulted in
flower-like structures (Fig. 4c, 4d), too. These two organic
solvents might work in some similar ways, for instance tailoring
the solubility or dissolution speed of the Mg-rich precursors,
increasing the inner pressure and viscosity, and limiting the
diffusion speed of the ions.
and resulted in the regain of mass and formation of transitional
core/shell structures. Formation of ring-like structures at the
consequent stage (Stage III, 80ꢂ480 min) could be understood by
the nanoscale Kirkendall effect, which leads to formation of kinds
40–42
2+
of hollow nanocrystals.
That is, concomitant with Mg and
3+
Al co-precipitation and growth of LDH platelets on the surface
of the heterogeneous structures, the Mg-rich cores gradually
dissolved, were completely digested, and resulted in hollow ring-
like structures, replicas of the templating fragments. A prelimi-
nary EDXRD study was conducted and supported the proposed
mechanism. A three-dimensional plot of the data collected is given
in Fig. S7 in the ESI.† No reflections were seen until ca. 40 min
after the commencement of reaction, after which a strong reflec-
Catalytic application
3
To estimate the catalytic performance of Mg/Al-CO LDH
nanorings, the Knoevenagel reaction between benzaldehyde and
diethyl malonate was used as a probe reaction. The plate-like
sample (MAC-P) prepared via a traditional co-precipitation
method and the flower-like counterpart (MAC-F) were used for
comparison. The phase purity and size similarity of the plate-like
sample were evidenced by XRD and TEM (Fig. S9, S10 in ESI†).
The corresponding conversions of diethyl malonate over MAC-
R, MAC-P and MAC-F are plotted in Fig. 5a. The catalytic
ꢁ
ꢀ1
tion at q z 0.8 A corresponding to the (003) reflection of the Mg/
Al-CO LDH was observed. The intensity increased gradually
3
with reaction time. This confirmed the idea that M10-NW
underwent significant dissolution before the LDH formed.
When using MgCl $6H O as precursor, however, the nanoring
2
2
structure could not form, which might be related to a relatively
quicker homogenous nucleation behavior, and absence of Mg-
rich cores. Another self-assembled growth process took place
Fig. 4 (a) TEM and (b) SEM images of MAC-R prepared in 1-hexanol/
H
2
O mixed solvent using Mg10-NW as precursors (insert is the homol-
ogous high-magnification TEM image), (c) TEM and (d) SEM images of
2 2 2
MAC-F prepared in 1-hexanol/H O mixed solvent using MgCl $6H O as
Fig. 3 Scheme of the formation mechanism of MAC-R.
precursors.
1
4744 | J. Mater. Chem., 2011, 21, 14741–14746
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activity of MAC-R was significantly higher than that of the other
morphologies: conversion reached 45.8% after achieving an
approximate reaction balance (about 12 h), but conversion with
MAC-P was only 34.3% and that with MAC-F was even lower.
Moreover, the curve slope of conversion with MAC-R was
largest, especially in the first 8 h, suggesting that its catalytic
efficiency was the highest among the analogues. It was clear that
the LDH morphology significantly affected the catalytic activity.
To understand the difference in catalytic performance, the
specific surface areas and pore size distributions of MAC-R,
MAC-F and MAC-P were studied via the N2 adsorption/
desorption method. The results are displayed in Fig. 5b. The
isotherms were of type IV, which is characteristic of mesoporous
materials could expose more active sites located in the exterior
and interior of the rings in accompany with the increase of
46
specific surface areas, as supported by the work of de Jong et al.
It was proposed that, compared to the other morphologies,
a ring-like morphology had the highest specific surface area,
largest pore volume and its hollow interior structure allowed
external reagents to approach, which all benefited the catalytic
performance.
Conclusions
Unique Mg/Al-CO LDH nanorings were synthesized by a urea
3
hydrolysis method in organic/water mixed solvents. Using solid
2+
44
materials. The insert in Fig. 5b suggested a significant differ-
precursors (Mg -source), Mg (OH) Cl $5H O nanowires,
rather than conventional Mg salts, was found critical for the
1
0
18
2
2
ence in pore size distributions. The pore volumes of MAC-R,
3
ꢀ1
MAC-F and MAC-P were 0.2529, 0.1051 and 0.1012 cm g ,
respectively. The pore volume of nanorings was significantly
greater than that of the other morphologies. Generally, LDH
materials possess poor porosity characteristics. The difference
for MAC-R may be explained because the center holes in the
LDH nanostructures helped to create more channels during the
irregular stacking of different arrangements of parallel-assem-
bled layers, resulting in an increase of porosity and pore volume.
nanoring formation, which is likely to be a result of their alter-
2+
nating the Mg -release and LDH nucleation/growth process.
Comparison of the catalytic activities of ring-, plate- and flower-
like LDH nanostructures via solid base catalyzed Knoevenagel
reaction indicated that the ring-like structure had better catalytic
performance than its plate- or flower-like counterparts. The
enhanced catalytic activity is thought to be related to the higher
specific surface area and larger pore volume induced by the
hollow structure. It is expected that LDH nanorings will provide
advantageous materials for the future which not only in catalysis
but also as adsorbents, drug delivery systems, etc.
2
ꢀ1
The specific surface area of MAC-R was 48.54 m g , almost
2
three-times as large as its flower-like (18.04 m g ) and plate-like
ꢀ1
2
20.54 m g ) analogues. The ring-like morphology introduced
ꢀ1
(
more irregular channels and subsequently increased the surface
area of the LDH.
Acknowledgements
Moreover, according to a previous report, reactions using
solid base catalysts usually occur on the entire outer crystal
This work was supported by the National Natural Science
Foundation of China, the 111 Project (B07004), the 973 Program
45
surface of the catalysts. Therefore, the hollow-structured
(
2011CBA00503), the Program for New Century Excellent
Talents in Universities, and the Project sponsored by SRF for
ROCS, SEM.
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1
1
1
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