Silicate anion-stabilized layered magnesium-aluminium hydrotalcite

Article abstract of DOI:10.1039/c3ra42563k

Layered magnesium-aluminum hydrotalcite (HT) were completely intercalated and stabilized with silicate anions. The silicate anion-stabilized HT retained the layered structure and possessed a high surface area of 530-540 m² g^-1 with needle-like particles 60 nm × 4 nm in dimension. The findings of FT-IR, powder XRD, TGA, DSC and ²⁹Si MAS-NMR revealed that silicate anions were intercalated and coated on the surface of layered HTs with the formation of a solid solution of magnesium silicate and HT. The resultant silicate-anion-intercalated materials are found to be promising catalysts for the synthesis of 1-phenoxy-2-propanol using phenol with propylene oxide. The Royal Society of Chemistry 2013.

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Silicate anion-stabilized layered magnesium–aluminium
hydrotalcite3
Cite this: DOI: 10.1039/c3ra42563k
Thangaraj Baskaran,a Raju Kumaravel,a Jayaraj Christopherb
and Ayyamperumal Sakthivel*a
Layered magnesium–aluminum hydrotalcite (HT) were completely intercalated and stabilized with silicate
anions. The silicate anion-stabilized HT retained the layered structure and possessed a high surface area of
530–540 m2 g21 with needle-like particles 60 nm 6 4 nm in dimension. The findings of FT-IR, powder XRD,
TGA, DSC and 29Si MAS-NMR revealed that silicate anions were intercalated and coated on the surface of
layered HTs with the formation of a solid solution of magnesium silicate and HT. The resultant silicate-
anion-intercalated materials are found to be promising catalysts for the synthesis of 1-phenoxy-2-propanol
using phenol with propylene oxide.
Received 24th May 2013,
Accepted 8th July 2013
DOI: 10.1039/c3ra42563k
www.rsc.org/advances
approach. The synthesis involved in situ intercalation of
silicate anions followed by their crystallisation using tetra-
Introduction
Double-layered hydroxides or hydrotalcite (HT)-like materials
are lamellar compounds represented by the general formula
[M(II)(1 2 x)M(III)x(OH)2]x+[An2x/n]mH2O.1–10 The anions present
in the interlayers are exchangeable, giving rise to elegant
intercalation chemistry.11–14 The introduction of various
anions, including silicate, phosphate or molybdate, into the
layered HT structure by ion-exchange or co-precipitation
methods widens the scope of its potential applications.11–14
High-temperature calcination of HT results in loss of water
and interlayer anions, forming a solid solution of mixed oxide.
These compounds vary in their composition and yield a variety
of tailor-made materials that demonstrate potential as
catalysts, catalytic supports, ion-exchangers, adsorbents, flame
retardants, corrosion inhibitors, and pigments.1–20 The repla-
cement of carbonate ions in anionic clays is more difficult
because of high charge density, the stability of the resultant
HT, and the high affinity of aqueous carbonate with the
positively charged cationic sheets.1–14 Though there are many
anions pillared between the layers, structural stability is only
achieved in the range 350–450 uC. In addition, there are few
reports published on surface properties.1,11–14 It is speculated
that stabilization of the HT layered structure by silicate anions
might result in a new class of porous layered multifunctional
materials, broadening their potential for heterogeneous
catalysis.
propyl-ammonium hydroxide (TPAOH) by a dry gel conversion
method.
The resultant HTSiS materials formed the solid solution of
magnesium silicate and MgAl-HT which retained their
nanoscale needle-shape layered structure (60 6 4 nm) after
calcination at 550 uC with high surface area (530–540 m2 g21
)
and pore volume (1.5 cm3 g21).
Experimental section
Hydrotalcite (HT) with
synthesised according to a method previously described in
the literature.20 Solution
was prepared by dissolving
Mg(NO3)2?6H2O (1 M) and Al(NO3)3?9H2O (0.33 M) in 2.6 L
deionised H2O. Solution was prepared by dissolving
a Mg/Al molar ratio 3 : 1 was
1
2
(NH4)2CO3 (0.67 M) and NH4OH (3.27 M) in 2.225 L deionised
H2O. Solution 2 was added drop wise to solution 1 with
constant stirring at 40 uC for 1 h, resulting in the precipitation
Herein, we report the first silicate anion-stabilized high
surface area Mg–Al HT (HTSiS), prepared using a new synthetic
aDepartment of Chemistry, Inorganic Materials and Catalysis Laboratory, University
of Delhi, Delhi, India. E-mail: sakthiveldu@gmail.com; Tel: 918527103259
bIndian Oil Corporation Ltd, R&D Centre, Faridabad, 121007, Delhi, India
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
Scheme 1 Schematic representation of silicate-stabilised HT.
c3ra42563k
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of HT, with a final gel pH of 8.5. The gel was filtered and
washed with deionised H2O until the pH of the filtrate reached
7.0. The sample was dried at 80 uC overnight and calcined at
400 uC in the presence of air for 6 h; this sample is represented
as HT-Si0. For comparison, silica-free HT was also calcined at
550 uC.
Silicate anion intercalated HT was produced by introducing
various concentrations of sodium silicate into pre-prepared
HT gel. Silicate solutions of appropriate concentrations were
added slowly to the HT gel at room temperature and stirred for
48 h. The silicate intercalated HT materials were then filtered
and washed with deionised H2O. The intercalated HT samples
prepared by using 0.41, 0.81, 1.42 and 2.03 M sodium silicate
solutions are represented by HT-Si1, HT-Si2, HT-Si4 and HT-
Si6, respectively.
Fig. 1 FT-IR spectra of as-synthesized HT materials.
Stabilization of the silicate anion-intercalated species was
carried out using a dry gel conversion method. Approximately
1.5 g of as-synthesised HT-Si4/HT-Si6 was wetted with the help
of a calculated amount of 40% tetrapropylammonium hydro-
xide (TPAOH) solution. The resultant solid cake was allowed to
crystallise at 175 uC for 24 h or 72 h. The TPAOH was removed
by calcination at 550 uC in air for 6 h to produce the silicate
anion-stabilized HT. The samples synthesised using HT-Si4
with crystallisation for 24 and 72 h are represented as HT-
Si4S1 and HT-Si4S2, respectively, and the samples prepared
from HT-Si6 are represented as HT-Si6S1 and HT-Si6S2,
respectively.
pulse, 8 ms contact time, a spinning rate of 10 KHz & 3 s
recycle delays.
Catalytic study
The catalytic activity of silicate-anion-intercalated hydrotalcite
(HT) was studied for the phenoxylation of propylene oxide with
the molar ratio of propylene oxide (0.06 M) to phenol (0.02 M)
of 3, at different temperatures for 10 h in the liquid phase
medium by adding 500 mg of the catalyst under constant
stirring without any solvent. The products were extracted with
propanol and analyzed by gas chromatography (Agilent
Technologies 7890A, USA) using a flame ionization detector
with a HP-5 column.
Characterization
Powder XRD was performed to determine the bulk crystalline
phases of the materials using a Bru¨ker D8 diffractometer with
Cu-Ka radiation (l = 1.54184 Å). The diffraction patterns were
recorded in the 2h range 5–70u, with a scan speed and step size
of 0.5u/min and 0.02u, respectively. FT-IR spectra were
obtained using a Perkin-Elmer 2000-FTIR in the range 400–
4000 cm21 using KBr pellets. The morphology and size of the
materials were analysed using a Phillips Technai G2T30 TEM
operated at 300 kV. The textural properties (BET surface area,
micropore area (t-plot method), BJH average pore volume) of
the samples were derived from N2 adsorption–desorption
measurements carried out at 2196 uC using an automatic
micropore physisorption analyser (Micromeritics ASAP 2020,
USA) after the samples were degassed at 250 uC for at least 8 h
under 1023 Torr pressure prior to each run.
Results and discussion
Fig. 1 depicts the FT-IR spectra of silicate-free, silicate-
intercalated and silicate-stabilized HT. The broad absorption
bands centered around 3450 cm21 (data not shown) and 1640
cm21 for all materials are attributed to stretching and bending
vibration modes of the OH groups coordinated to the metal
ions in the brucite sheets. The broadening of the vibrational
band at 3100 cm21 corresponds to absorbed water mole-
cules.2,9,11 The IR absorption band around 1370 cm21
corresponds to the interlayer carbonate stretching vibration
(Fig. 1).2,11
Thermogravimetry-analysis (TGA) was carried out in
a
The introduction of silicate anions into the synthesized gel
resulted in new bands at around 1014, 800 and 1200 cm21
Thermal Analyzer (Model-2950, TA Instruments, USA). About
5–10 mg of the sample was taken in the platinum pan and
heated in air at the heating rate of 10 uC min21 up to 800 uC.
Differential scanning calorimetry (DSC) analyses were per-
formed using DSC equipment (Model-2920) using 5–10 mg of
the sample at a heating rate 10 uC min21 under air. 29Si CP-
MAS NMR spectra were recorded at 79.4 MHz, with a Varian
Unity Inova-400 MHz NMR spectrometer, with 6.6 ms ’H 180u
,
which are characteristic of asymmetric, symmetric and
bending vibrations of Si–O–Si, respectively.11 An increase in
the concentration of silicate anions in the gel resulted in a
decrease in the intensity of the vibration band that corre-
22
sponds to CO3 ions. This indicates that the silicate anions
were successfully intercalated between the brucite-like
layers.11 In particular, the absence of a vibrational band
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Fig. 2 XRD patterns of various as-synthesized HT samples.
around 1370 cm21 for the HT-Si6 sample is evidence of the
complete intercalation of silicate anions between the brucite
layers. The vibrational bands at 800 and 1000 cm21, and the
broadening of the latter vibrational band till around 1200
cm21 for the HT-Si4 and HT-Si6 samples account for the
existence of silicate anions in various polymeric forms
between the interlayers.11
The vibrational band at 800 cm21 indicates the presence of
42
the lower-symmetry SiO4
tetrahedron,11c which resulted
42
from the condensation of a SiO4
polyhedron. Another
Fig. 3 (a) TGA and (b) DSC profile of various HT samples.
vibrational band observed around 1000 cm21, which broadens
till 1200 cm21, indicates an increase in the Si–O–Si bond angle
of the silicate anionic species present in interlayer. The
observed broadening of the vibrational band near 1000 cm21
is due to the presence of silicate anions in the form of layered
silicate anions in a distorted tetrahedral environment.21 The
polymeric silicate anions with the increase in the Si–O–Si bond
angle are also an indication of the presence of silicate anions
such as Si2O522, Si6O1562, etc.11 In addition, the FT-IR spectra
of the silicate anion-stabilized HT samples exhibited an
additional band at 537 cm21 (inset, Fig. 1), revealing the
characteristics of a five-membered silicate ring that is similar
to zeolite secondary building units.22 Further, the FT-IR
spectra of all the materials displayed a broad absorption band
at 650–550 cm21 and a broad band at 450 cm21, which are
typical of M–O and M–O–M stretching and bending vibrations
of brucite-like layers.18,23 The latter band is also perceived to
be due to the Si–O–H bending vibration.
The powder XRD patterns of as-synthesized and silicate-
free, intercalated and stabilized HT are shown in Fig. 2. All as-
synthesized samples exhibited characteristic XRD reflections
around 2h of 8u–11u and 23u, corresponding to (003) and (006)
of an HT-type layered structure, respectively.1 The peak that
corresponds to the distance along the z-axis (d003) of silicate-
free HT appeared at 2h of 11.4u and it shifted to 10.9u, 10.8u
and 10.6u for the silicate-anion-intercalated HT; this reveals
the expansion of interlayer distance.12c The interlayer expan-
sion was more predominant in HT-Si2. A further increase in
the silicate anion concentration resulted in broadening of the
peak due to the orientation of the anion in the interlayer
space12c and coating of excess silicate anions on the surface of
the layered HT. The broadening of the XRD peaks on the
silicate-anion-intercalated and stabilized samples is due to the
decrease in the particle sizes on the nano scale and also
because of the presence of polymeric silicate anions in the
interlayer domain of HT.11 The absence of any segregated
silicate species on the external surface of HT is clearly evident
from the TEM images of the HT-Si samples (Fig. S1, ESI and
3
Fig. 6). In addition, there are no XRD peaks corresponding to
any bulk oxides such as magnesium silicate and aluminum
silicate species.
TGA and DSC curves for silicate-free, silicate-anion-inter-
calated and silicate-anion-stabilized HT are shown in Fig. 3.
Silicate-free HT showed two major weight losses as also
indicated by two endotherms on the DSC. The first weight loss
corresponds to the removal of the physisorbed interlayer water
molecules and the second weight loss is attributed to the
removal of interlayer anions and water molecules from the
condensation of the layer hydroxyl groups.1c,12c The silicate-
anion-intercalated and silicate-anion-stabilized HT showed
relatively less weight loss and there are no endothermic peaks,
which indicates the absence of interlayer carbonate ions as
well as changes in the layered HT into a mixed magnesium–
aluminum oxide phase upon calcination.
The powder XRD patterns of pure HT calcined at 550 uC
showed the characteristics of Mg–Al double oxides (Fig. 4).24
Interestingly, the XRD patterns of the calcined silicate anion-
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Fig. 4 XRD patterns of various calcined HT samples.
intercalated and stabilized HT retain the layered HT structure
with very broad peaks corresponding to (003) and (006) planes.
Further, the absence of an XRD peak around 2h of 43u at high
silicate anion loading clearly reveals the absence of any bulk
Mg–Al mixed oxide species.24 When the silicate anion-
intercalated HT was calcined at 800 uC, the formation of
magnesium silicate phase (Fig. S2, ESI ) along with the layer
3
structure was evident.25 The formation of magnesium silicate
phase upon calcination at 800 uC and the presence of FT-IR
vibrational bands at 675 and 450 cm21 support the structure of
silicate anion-intercalated and stabilized HT by forming a
solid solution of HT and magnesium silicate nanoparticles. In
particular, higher loading of silicate anions might help in the
stabilization of the layered HT by intercalation and cementing
the surface by uniform coating. The presence of the intact HT
structure on the HT-Si4, HT-Si6 and HT-SiS2 samples is due to
the complete intercalation of silicate anions in the interlayer
space with the formation of a solid solution of nanosized HT
and magnesium silicates. The results are in good agreement
with DSC data, in which no endothermic peak corresponding
to the phase change in HT to MgO and MgAlO4 is observed.
N2 adsorption–desorption isotherms of all the HT samples
exhibited multilayer adsorption followed by capillary conden-
sation, typical of a type IV adsorption isotherm as per IUPAC
classification (Fig. 5).26 The BET surface area of Mg–Al HT
synthesized without silica (HT-Si0) remained constant, irre-
spective of the calcination temperature, because of sorption in
the HT mesopores (see Table 1).27 Importantly, the interlayer
microporosity was lost on calcination at 550 uC because of the
formation of a solid solution of magnesium oxide and
magnesium–aluminum oxide spinels by the decomposition
of the layered structure. The loss of microporosity upon
calcination at 550 uC does not affect the total surface area. This
is because the observed microporosity that arises from the
interlayer has a negligible contribution to the surface area due
to the accumulation of carbonate and water molecules in the
interlayer micropores.
Fig. 5 N2 sorption isotherms of (a) silicate anion-free, intercalated (calcined at
400 uC) and (b) stabilized Mg–Al HT (calcined at 550 uC).
silicate anion content in the range 0.81–1.4 M (Table 1; HT-Si1,
HT-Si2 and HT-Si4), reaching a maximum BET surface area of
over 400 m2 g21. The observed surface area of HT-Si2 is higher
than that of HT-Si1 because of a drastic decrease in the
concentration of the carbonate anion, which was replaced by
interlayer silicate anions, as evident from FT-IR spectra
(Fig. 1). A further increase in the anion concentration (2.03
M; HT-Si6) resulted in a decrease in the surface area (277 m2
g
21) and the pore volume (0.88 cm3 21). These reductions
g
were a result of the filling of the HT interlayers by condensed
polymeric silicates.11
Treatment with TPAOH followed by calcination at 550 uC, in
particular on the HT-Si6S sample, resulted in an increase in
the surface area and micropore volume, indicating the
formation of silicate species as porous layered pillars between
the HT layers. The BET, micropore surface area and pore
volume are higher for all the silicate-stabilized samples
compared to silicate-free and intercalated materials. This is
because of uniform condensation of silicates in the HT
interlayer domains and on the surface. The presence of
silicate species in the interlayer helped to stabilize the layered
HT structure, even after calcination at 550 uC. The sharp
increase in the uptake on the HT-Si4S1, HT-Si4S2, HT-Si6S1
and HT-Si6S2 (Fig. 5) at a low p/po of 0.1 was exclusively due to
the presence of microporous layered oxides and zeo-type
silicate species present in the interlayer domain.22,25
The BET surface area and pore volume of the silicate-
intercalated HT samples were found to be higher than those of
the pure HT sample. They increased with the increasing
The TEM images of HT-Si4S2 and HT-Si6S2 (Fig. 6) show
uniform plate-like needles with dimensions of 60 nm 6 4 nm,
typical of a layered HT structure, confirming the maintenance
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Table 1 Textural properties of the HT materials
Surface area (m2 g21
)
Pore volume (cm3 g21
)
Sample code
Calcination Temp. (uC)
BET
Micropore (t-plot)
Total
Micropore
HT-Si0
HT-Si0
HT-Si1
HT-Si2
HT-Si4
HT-Si4
HT-Si6
HT-Si6
HT-Si4S1
HT-Si4S2
HT-Si6S1
HT-Si6S2
400
550
400
400
400
550
400
550
550
550
550
550
224
225
235
415
404
389
277
265
420
453
532
546
37
0
0.57
0.35
0.74
1.39
1.59
1.63
0.88
0.88
1.10
1.20
1.54
1.54
0.019
0.000
0.013
0.013
0.012
0.016
0.016
0.012
0.026
0.025
0.026
0.021
27
30
28
37
32
25
60
60
57
48
of the layered structure by the silicate anions. The needle-like
layered HT structure even after calcination at 550 uC supports
the presence of HT–magnesium silicate solid solution, which
stabilizes the layered structure.
ppm indicative of the presence of a solid solution of
magnesium silicate and layered silicate-anion-stabilized HT
species.28
Thoroughly characterized silicate anion-intercalated HTs
were tested for phenoxylation of propylene oxide. The results
of initial studies carried out at various temperatures using HT-
Si4 are shown in Fig. 8. The phenol conversion was increased
with an increase in the reaction temperature and reached a
maximum at 150 uC. However, the conversion of phenol
decreased drastically with a further increase in the tempera-
ture owing to the decomposition of phenol, and thus the
The 29Si MAS NMR spectra of silicate-anion-intercalated
and silicate-anion-stabilized HTs are shown in Fig. 7. All the
samples showed NMR chemical shifts in the range of 260 to
2120 ppm, which is characteristic of Si in different tetrahedral
environments.28 The 29Si MAS-NMR spectrum of HT-Si6 shows
broad signals in the range of 285 to 2111 ppm centered
around 290, 2104, and 2111 ppm corresponding to
Si(OSi)x(OH)42x species, where x = 2 to 0 are assigned to Q2,
Q3 and Q4 sites.11b This indicates that silicate anions form as
short-chain silicate structures and extra-gallery silicate species,
similar to a previously reported study.11b The 29Si-NMR spectra
of silicate-anion-stabilized HT by TPAOH treatment (HT-Si4S2
and HT-Si6S2) showed well-resolved fine peaks at 2104,
298.4, 293 and 289.55 ppm. The observed NMR chemical
shift values correspond to Si(OSi)x(OH)42x species, where x = 1
to 2, which are similar to Si in phyllosilicates minerals (such as
talc, kaolinite and vermiculate).11c In contrast, the previously
reported LDH-silicate showed only a single Q3 resonance11a,b
at 2104 ppm, which is typical of the a-form-layered silicates.
However, this study showed multiple peaks that indicate
strong cross-linked layered silicates in the
d
form.11
Additionally, HT-Si6S2 showed a weak signal around 262
Fig. 7 29Si MAS-NMR spectra of (a) HT-Si6, (b) HT-Si4S2 and (c) HT-Si6S2.
Fig. 6 TEM images of calcined (a) HT-Si4S2 and (b) HT-Si6S2.
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silicate anions were crystallized into porous species in the
interlayer domains of the HT structure. To the best of our
knowledge, this is the first report on silicate-anion-stabilized
HT materials that have a high surface area and pore volume;
thus, this study open new avenues for heterogeneous catalysis.
The stabilization of the HT-layered structure with the help of
layered silicate anions was further evident from 29Si MAS-
NMR. Preliminary studies on the phenoxylation of propylene
oxide showed promising results.
Acknowledgements
CSIR and University of Delhi, India are acknowledged for their
financial support, and USIC, University of Delhi, India is
thanked for analytical support.
Fig. 8 Effect of reaction temperature on phenoxylation of propylene oxide
using HT-Si4.
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