Diaminosilane-functionalized on silicate-stabilised hydrotalcite (MA-HTSi-DA): As potential catalyst for nitro-aldol condensation

Article abstract of DOI:10.1016/j.molcata.2015.08.003

Diaminosilane was introduced into silicate-anion-intercalated hydrotalcite (HT-Si) using a post-synthesis method. The resulting functionalized materials possess surface basicity. The textural properties of the functionalized materials were systematically investigated through powder XRD, FTIR and MAS-NMR studies. The diamine-functionalized materials showed very good catalytic activity for the nitro-aldol condensation of benzaldehyde and nitromethane under solvent-free conditions. The materials showed comparable catalytic activity upon recycling.

Full text of DOI:10.1016/j.molcata.2015.08.003

Accepted Manuscript
Title: Diaminosilane-functionalized on silicate-stabilised
hydrotalcite (MA-HTSi-DA): As potential catalyst for
nitro-aldol condensation
Author: Thangaraj Baskaran Jayaraj Christopher Srinivasan
Radhakrishnan Ayyamperumal Sakthivel
PII:
S1381-1169(15)30050-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.08.003
MOLCAA 9590
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
20-6-2015
4-8-2015
4-8-2015
Please cite this article as: Thangaraj Baskaran, Jayaraj Christopher, Srinivasan
Radhakrishnan, Ayyamperumal Sakthivel, Diaminosilane-functionalized on
silicate-stabilised hydrotalcite (MA-HTSi-DA): As potential catalyst for
nitro-aldol condensation, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.08.003
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Diaminosilane-functionalized on silicate-stabilised hydrotalcite
(MA-HTSi-DA): As potential catalyst for nitro-aldol condensation
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Thangaraj Baskaran,[a] Jayaraj Christopher,[b] SrinivasanRadhakrishnan,[c]
Ayyamperumal Sakthivel*[a]
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[a] Department of Chemistry, Inorganic Materials and Catalysis Laboratory,
University of Delhi, New Delhi-110007, India.
[b]Indian Oil Corporation Ltd, R&D Centre, Faridabad-121007, India.
[c]NMR Lab, CSIR-Central Electrochemical Research Institute, Karaikudi-630 006, India.
E-mail: sakthiveldu@gmail.com / asakthivel@chemistry.du.ac.in
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Graphical abstract
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Highlights
 First report on diaminosilane functionalized silicate-anion-intercalated hydrotalcite (HT).
 The basicity increased by introducing diaminosilane on HT materials.
 The resultant materials showed good conversion with formation of dehydrated product.
 Catalyst yield excellent conversion for variety of aldehydes
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Abstract: Diaminosilane was introduced into silicate-anion-intercalated hydrotalcite (HT-Si)
using a post-synthesis method. The resulting functionalized materials possess surface basicity.
The textural properties of the functionalized materials were systematically investigated through
powder XRD, FTIR and MAS–NMR studies. The diamine-functionalized materials showed very
good catalytic activity for the nitro-aldol condensation of benzaldehyde and nitromethane under
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solvent-free conditions. The materials showed comparable catalytic activity upon recycling.
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Key words: Hydrotalcite, Diamine functionalization, Silicate intercalated hydrotalcite, Nitro-
aldol condensation, functionalization.
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1. Introduction
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The functionalization of inorganic support materials using various functional ligands such as
mercaptyl, amino propyl and diamine has received great research interest in recent decades,
owing to the formation of hybrid materials, which may be useful for diverse applications [1–10].
Among the various materials, amine-functionalized silica, polymers, carbon materials and so
forth, have shown a strong ability to bond with metals ions, to capture CO2 and have been
important in base-catalyzed reactions [11, 12]. In particular, amine-functionalized mesoporous
silicate materials such as SBA-15, MCM and HMS (hexagonal mesoporous silica) were used in
various applications, such as condensation reactions (aldol, nitro-aldol, Knoevenagel and
Claisen–Schmidt), Michael additions, C–C bond formation and as adsorbents, etc.[5-10, 13, 14].
The double layered hydroxides or hydrotalcite like materials (HT) are another class of
supporting materials [15-19], which yield active mixed oxides through the calcination. The
resultant mixed oxides derived from HT are used in various fields, such as adsorbents, drug
delivery, sensors, ion exchangers, electrode modifiers, catalysts and so forth [20-22]. Modified
hydrotalcite materials were used for various base-catalyzed condensation and organic
transformation reactions [23-27]. However, reproduction of catalytic activity on multi-metal
oxide derived from HT is a challenging task. The layered structure of HT collapses upon
calcinationbetween 200–400 °C owing to dehydration, dehydroxylation and decarboxylation. For
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example, calcination and activation of hydrotalcite materials results thermal migration of cations
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into tetrahedral positions and formation of spinel type oxide which is disable to reconstruction of
layered structure [28-29].
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In this regards, recently, we have shown that these interlayer spaces can be rejoined by
introducing hard anions (such as silicate) and they have been shown as promising catalysts and
catalytic supports, owing to the their intrinsic properties [30-32]. The nitro-aldol condensation is
an important base catalyzed reaction used in pharmaceuticals as well in various organic
syntheses. In particular, nitro-alkenes are versatile intermediates in organic synthesis and in the
chemical industry [33-35], Conventionally these reactions were studied using various catalysts
such as NaY zeolite [36], magnesium aluminium oxide derived from HT [37], hydrotalcite in
ionic liquid medium [38], MgAl-O-tBu [39], functionalized SBA-15 [40], SBA-15/Cu-complex
[41], hetero bi-metallic complexes [42], Co(II) Salen catalyst [43], etc.
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We believe that the introduction of reactive diamine functionality on silicate-intercalated stable
layered MA-HT materials (MA-HTSi-DA) might enhance the strong basicity, which are derived
from both HT support and the diamine functionality.Here with we report the synthesis of
diamine functionalized silicate-anion-intercalated hydrotalcite (Scheme 1).The synthesized
materials were systematically characterized by various analytical and spectroscopic techniques.
The basicity of the materials is utilized for nitro-aldol condensation reaction in a liquid phase
medium.
2. Experimental Section
Preparation of HT and diamine functionalization
Magnesium aluminium-hydrotalcite was synthesized by reported procedure [30, 32, 44] with
molar composition of 1 Mg: 0.33 Al: 0.67 (NH4)2CO3: 3.27 NH4OH: 125 H2O and denoted as
MA-HT.Silicate intercalated hydrotalcite was obtained as per the procedure mentioned earlier
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[32] by introducing 1.42 M sodium silicate solution into the hydrotalcite gel having molar
composition of 1 Mg: 0.33 Al: 0.67 (NH4)2CO3: 3.27 NH4OH: 125 H2O at room temperature
with constant stirring. The resultant silicate intercalated hydrotalcite materials was filtered and
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washed with deionized water. The final materials were dried at 100 C and represented as MA-
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HTSi. For comparison, cobalt aluminium and zinc aluminum containing hydrotalcite were
prepared with molar gel composition of 1 Co: 0.33 Al: 0.67 (NH4)2CO3: 3.27 NH4OH: 125 H2O
and 1 Zn: 0.33 Al: 0.67 (NH4)2CO3: 3.27 NH4OH: 125 H2O respectively [32, 44], which are
represent as CA-HT and ZA-HT. Further, the silicate intercalated CA-HT and ZA-HT having
similar silicate concentration (1.42 M) was prepared in a similar procedure described as above
and final samples are represented as CA-HTSi and ZA-HTSi respectively.
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Various siliane such as (N-[3-(trimethoxysilyl) propyl] ethylenediamine, 3-aminopropyltri-
ethoxysilane, 3-anilinopropyltri-methoxysilane are obtained from Sigma-Aldrich and utilized as
received form without any further purification. Diamine functionalized MA-HTSi was prepared
by treating MA-HTSi sample with N-[3-(Trimethoxysilyl) propyl] ethylenediamine [45]. First,
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MA-HTSi was pre-activated at 110 C for overnight under vacuum in a Schlenk round bottom
flask. Then the activated material was cooled to room temperature and different concentration of
N-[3-(Trimethoxysilyl) propyl] ethylenediamine was introduced and refluxed using dry toluene
(30 ml/g support) for 24 h in nitrogen atmosphere. The different amount of diamine 0.0012,
0.0046, 0.0092 and 0.0138 mol (0.25, 1, 2, and 3ml respectively) per gram has been
functionalized on MA-HTSi materials. The final reaction mixture was cooled down to room
temperature and toluene was removed by vacuum filtration. The excess diamine was removed by
washing with dichloromethane (DCM) several times and the solid was finally dried under
vacuum. The diamine functionalized MA-HTSi materials are labelled as MA-HTSi-DA1, MA-
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HTSi-DA2, MA-HTSi-DA3 and MA-HTSi-DA4 (range from 0.0012-0.0138 mol/g) respectively.
For comparison, aliphatic aminosilane (3-aminopropyl)tri-ethoxysilane (0.0138 mol/g) and
aromatic aminosilane(3-anilinopropyl)tri-methoxysilane(0.0138 mol/g) were grafted on MgAl-
hydrotalcite as per the above described method and are represented as MA-HTSi-PA4 and MA-
HTSi-PAn4 respectively. Also N-[3-(Trimethoxysilyl) propyl] ethylenediamine(0.0138 mol/g)
were graftedon silicate intercalated different hydrotalcite materials such as CA-HTSi and ZA-
HTSi using similar procedure mentioned above and represented as CA-HTSi-DA4 and ZA-
HTSi-DA4 respectively.
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Characterization
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Powder XRD was performed to determine the crystallinity of the materials using a 8 KW
(Rigaku, Japan) diffractometer with Cu-Kα radiation (λ = 1.5418 Å). The diffraction patterns
were recorded in the 2θ range 5–70°, with a scan speed and step size of 0.5°/min and 0.02°,
respectively. FT-IR spectra were obtained with KBr pellets a Brüker, TENSOR 27 FT-IR
Spectrometer. The textural properties such as BET surface area, pore volumeand pore diameter
of the samples were derived from N2sorption analysis. The measurement was carried out at -196
°C out using automatic micropore physisorption analyzer (Micromeritics ASAP 2020, USA). The
samples were degassed at 110 °C for at least 8 h under 10-3Torr pressure prior to each run. The
solid-state CP-MAS NMR experiments were carried out on a Bruker AVANCE 400 narrow bore
spectrometer equipped with a superconducting magnet with a field of 9.4 T using a 4 mm double
resonance cross-polarized magic angle spinning (CP-MAS) probe operating at resonating
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frequencies of 79.4 and 100.6 MHz for Si and C respectively. The samples were packed in 4
mm zirconia rotors and subjected to a spinning speed of 5 kHz: single pulse experiment with
pulse duration of 4 s and a relaxation delay time of 5 s were used for recording 29Si and 13C MAS
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NMR pattern with 2000 number of scans. Thermogravimetry-analysis (TGA) was carried out in
a 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 oC min-1 up to 800 oC.
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Catalytic Studies
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Amino silane functionalized materials were studied for liquid phase nitro-aldol condensation in
around bottom flask. About 100 mg of catalyst was loaded into the reaction mixture containing
1:3 mole ratio of aldehyde (5 mmol) and nitro methane (15 mmol). The flask was charged into
required temperature for an appropriate time. After completion of the reaction, the reaction
mixture was filtered and analyzed using gas chromatography equipped with FID (Agilent 7890A
Series) connected to a HP-5 capillary column (30 m; HP-5).The products were confirmed
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through H NMR. For the comparison, the reaction was also studied on various HT, the amine
functionalized HT and without catalyst under identical conditions.
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3. Results and discussion
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FTIR spectra of pure and diamine-functionalized silicate-intercalated hydrotalcite materials are
shown in Fig. 1 and other amine functionalized materials are displayed in Fig. S1. All the spectra
show vibrational bands around 1070 and 800 cm-1, which correspond to Si–O–Si stretching of
silicate species present on the interlayer [30-32, 44-47]. A weak band observed around 1350 cm-1
is related to the carbonate stretching frequency, which is also present in the interlayer [15, 30-
32]. The presence of aminosilane functionality on MA-HTSi materials can be evidenced from the
observed new bands around 2950–2740 and 1480 cm-1. These vibrational bands are attributed to
C–H stretching and bending vibration of the propyl group present on the silane linker attached to
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the MA-HTSi samples. An additional peak observed around 1570 cm-1 confirms the presence of
the amine functionality on the surface of the MA-HTSi-DA samples. The relative intensity of the
peaks corresponds to –CH and –NH vibration is increases with increasing loading of aminosilane
[45, 48]. Furthermore, all of the materials show a band at 600 cm-1, owing to the M–O or M–O–
M vibrations in the layered framework [15,30-32].
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The powder XRD patterns of silicate-intercalated and diaminosilane-functionalized materials are
shown in Fig.2. In all cases, the peaks evident around 2θ of 12°, 23° and 33° are characteristic of
the (003) (006) and (009) planes of layered hydrotalcite materials, respectively [15, 16, 30-32,
44]. The broadening of the XRD peaks was observed on both silicate-intercalated and diamine-
functionalized materials, which is attributed to the orientation of polymeric silicate anions and to
the aminosilane group present in the interlayer of hydrotalcite [30-32, 44-47].Similar trend was
evident on the HT materials obtained with different amine functionality (Fig. 2).The interlayer
distance of grafted hydrotalcite was calculated based on the (003) plane and the results are
summarized in Table 1. The interlayer distance of MA-HTSi is 8.2 Å, which is higher than the
parent hydrotalcite (MA-HT; 7.85 Å), owing to slight lattice expansion of the HT layers along
the c axis. The introduction of diaminosilane functionality on various HT materials does not
change the unit cell parameter, which indicates diamine functionality on the exposed surface
through interlayer silicates.
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The N2 sorption isotherms of silicate-intercalated and amine-functionalized MA-HTSi materials
are shown in Fig. 3. All of the materials exhibited multilayer and type IV isotherms [30-32]. The
isotherms of diamine functionalized various HT materials and various amine functionalized
materials are shown in Fig S2. The silicate-intercalated material showed a BET surface area of
380 m2.g-1 and a pore volume of 1.0 cm3g-1. The reduction in surface area and pore volume of
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amine-functionalized MA-HTSi materials signifies successful grafting of the diaminosilane
moiety (Table 1).
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The silicon and carbon environments of the amine-functionalized materials were studied by 29Si
and 13C CP-MAS NMR spectroscopy. The 29Si MAS NMR spectrum of MA-HTSi-DA4 (Fig.
4A) shows chemical shift values around –115, –106 and –99 ppm, corresponding to
Si(OSi)n(OH)4–n species where n = 0–2. These peaks are assigned as Q4, Q3 and Q2 sites, and in
case of MA-HTSi-DA4, it is predominantly the Q4 and Q3 sites that are evident [30, 32, 45-47].
Further, MA-HTSi-DA4 exhibits an additional peak around –53 ppm, which is characteristic of
an organosilica environment (T sites) obtained by the covalent grafting of diaminosilane on the
surface of MA-HTSi. In the 13C CP-MAS NMR spectrum (Fig. 4B), MA-HTSi-DA4 showed
signals at 9, 20, 37 and 49 ppm, which are characteristic of an alkyl linker being present in the
diaminosilane [45, 49, 50]. Additionally, an unexpected peak was observed around 163 ppm for
the MA-HTSi-DA4 sample, corresponding to carbamic acid functionality. The observed
carbamic acid carbonyl peak may be attributed to atmospheric CO2 interactions with the diamine
functionality present on the MA-HTSi-DA4 surface [45]. The above fact confirms that
organosilica groups are covalently bonded with silica moieties of intercalated HT materials.
The thermal stability of amine-functionalized HT materials was studied by thermo-gravimetric
analysis (TGA). The parent (MA-HT) material shows (Fig. S3) typical weight losses
corresponding to physisorbed water molecules and the decomposition of carbonate anions
present in the interlayer [16, 38]. The silicate-intercalated material (MA-HTSi) showed a
relatively low weight loss (about 35%) compared to the parent HT material (51%), which
supports the hypothesis that the silicate species were successfully intercalated between the inter-
layers. The amine-functionalized material (MA-HTSi-DA4) showed little difference in weight
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loss (about 37%) to MA-HTSi (35%), owing to the decomposition of amine functional groups in
the materials.
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The catalytic activity of the diamine-functionalised material (MA-HTSi-DA) was evaluated in
the nitro-aldol condensation reaction in liquid-phase medium. A representation of the nitro-aldol
condensation is shown in Scheme 2. Various reaction parameters, such as temperature, time,
aminosilane loading, various HT support and different amine functional groups present on the
support, were systematically studied. In all cases, dehydrated 1-(2-nitro-vinyl) benzene (1) and
the aldol 2-nitro 1-phenyl ethanol (2) were formed as major and minor products, respectively.
Figure 5a displays the effect of the reaction time at various temperatures. At low temperature
(40oC), the conversion gradually increases with time, reaching a maximum of about 70% after
around 6 h (Fig.5a). An increase in reaction temperature (60–80oC) leads to about 50–70%
conversion being obtained within 1h, and complete conversion was observed at 2h. In all cases,
1-(2-nitro-vinyl) benzene was obtained as the major product (more than 99%). The best catalytic
activity was achieved at 70oC within 2h; thus, all further studies were carried out at 70oC for 2h.
Kinetic study on nitro-aldol condensation reaction was studied at different temperature and
results are shown in Fig 5b. The rate of reaction was steadily increases with increase in reaction
temperature. The reaction appears to be first order with respect to benzaldehyde since a
logarithmic plot of concentration [A] versus time is linear. The activation energy for the nitro-
aldol condensation is obtained using Arrhenius plot, by plotting the log k versus 1/T (Fig. 5c).
The activation energy of nitro-aldol condensation reaction was found to be 48 KJ mol-1 which is
comparable to literature value [51] and indicates that the reaction doesn’t have any diffusion
limitation.
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The effect of the concentration of the diaminosilane loading on the surface and its catalytic
activities over catalysts possessing different diamine concentration (0.0012, 0.0046, 0.0092 and
0.0138 mol g‒1) was investigated and the results are displayed in Fig. 6. As the loading of
diamine functionality increased, the conversion increased steadily and a maximum conversion
and selectivity was evident for MA-HTSi-DA4. The selectivity of 1-(2-nitro-vinyl) benzene
remains more than 90% in all cases.
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In order to estimate the turn over frequency (TOF) diamine functionalized catalysts were studied
the catalytic reaction till the conversion level reaches 90 % using 5 mmol of substrate and 100
mg of catalysts. The catalysts with diammine concentration of 0.0012, 0.0046 and 0.0092 M
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yielded 90 % conversion of nitro-aldol at 70 C, with the reaction duration of 36, 12 and 6 h
respectively. Based on the above results, the corresponding TOF was found to be around 1 h‒1
for the catalyst possessing lower diamine loading (between 0.0012 to 0.0092 mol.g‒1). The
catalyst with maximum diammine loaded (0.013 M) sample showed complete conversion at 2 h,
with the TOF of 1.8 h‒1. The presence of a maximum number of active sites (0.013 mol.g‒1) and
the high surface area (Table 1) in MA-HTSi-DA4 facilitate mono-layer distribution of active
sites and favor the maximum conversion with better TOF. Furthermore, the reaction was also
studied on various supports, with and without diamine functionality, and the results are
summarized in Table 2. The reaction did not proceed without a catalyst (GC data Fig. S4).
Further, the pure as-prepared hydrotalcite showed about 31 % conversion with equal distribution
of products 1 and 2. The presence of weak basicity derived from surface hydroxyl group on
parent support material responsible for poor selectivity of 1 observed on pure HT. Furthermore,
silicate-intercalated hydrotalcite MA-HTSi showed low conversions of only about 1 %. The
reactivity enhances only when amine functionality was present on the layered stable hydrotalcite
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support. The presence of amine functionality on hydrotalcite materials improves catalytic activity
on nitro-aldol condensation reaction.
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To understand the role of the diaminosilane support, it was also functionalized on different
silicate-intercalated HTs such as cobalt-aluminium (CA-HTSi-DA4) and zinc aluminium (ZA-
HTSi-DA4), and the catalytic activities were evaluated. CA-HTSi-DA4 and ZA-HTSi-DA4
showed about 70 and 36% conversion, respectively. The presence of surface redox-active species
with cobalt and a Lewis acidic Zn center do not favor the catalytic activity. Furthermore, the
reaction was also tested with different aminosilane-functionalized MA-HTSi, such as aliphatic
amine (3-aminopropyl)tri-ethoxysilane (MA-HTSi-PA4) and aromatic amine (3-
anilinopropyl)tri-methoxysilane (MA-HTSi-PAn4). The catalytic results are shown in Table 3.
MA-HTSi-PAn4 does not show any conversion under identical conditions and only it shows 45
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% conversion at 90 C, which may be attributed to the delocalization of electron inside the ring,
and does not facilitate the formation of imine intermediate, thus showing a lower conversion.
The mono-amine-functionalized material MA-HTSi-PA4 showed only 61% conversion. Further,
diaminosilane functionalized silica material (SBA-15) was tested for catalytic activity under
identical conditions and showed only 32 % conversion, which is lower than diaminosilane
functionalized hydrotalcite materials. The diaminosilane-functionalized materials (MA-HTSi-
DA4) yielded a maximum conversion of 96% with a selectivity of 99.5% 1-(2-nitro-vinyl)
benzene. The observed higher conversion could be attributed to the strong basicity derived from
the diamine functionality associate with HT support on the present materials.
It is known from the previously published reports that the nitro-aldol condensation can follow
two types of mechanism, that is, the imine or ion-pair mechanism to yield dehydrated and
alcohol products, respectively [50-53]. The possible mechanism of the nitro-aldol condensation
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is proposed and shown in Scheme 3. In the first step, the aminespresent on the surface interacting
with the carbonyl groups of the aldehyde. In the second step, an imine intermediate is formed
and followed by dehydration; subsequently, nitro methane reacts with intermediate imine to
produce dehydrated products. The higher conversion was observeddue to the close proximity of
the hydroxyl group and amine sites in the functionalized materials.
Table 4. Nitro-aldol condensation using MA-HTSi-DA4 as catalyst for different aldehydes.[a]
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The reaction was further studied using different substituted aromatic, aliphatic and heterocyclic
aldehydes under optimum conditions. The results are summarized in Table 4. In all the cases, a
conversion of 85–90 % was evident; however, the reactions of substituted aldehydes proceeded
for longer durations compared to non-substituted aldehydes. In all cases, a conversion level of
more than 80 % was achieved with exclusive formation of the dehydrated product. The nature of
the substituent on the benzene ring and the nature of the ring (aromatic or heterocyclic) in the
aldehydes did not affect the conversion. Heterocyclic aldehydes also (indole-3-carboxaldehyde;
Table 3, entry 8) shows 88% conversion with 98% selectivity of dehydrated product.The use of
aliphatic aldehyde result in relative less conversion compare to aromatic aldehydes. The low
reactivity of aliphatic aldehyde may results in relatively lower conversion.
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After each reaction, the catalyst was separated from the reaction mixture and washed with
dichloromethane and dried wellbefore the recycling studies. The catalyst retains about 70% of its
initial activity even after three recycles; the slight decrease in conversion is due to the blocking
of active sites by the chemisorbed products. In addition the considerable decrease in product 1
selectivity on recycled catalyst might also due to masking of some of the basic sites by
chemisorbed organic (reactant and product) molecules and thus conversion of product 2 into 1 is
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limited. The FT-IR spectrum of a spent catalyst was (Fig. S5) indicated that the increase in
relative intensity of peaks corresponds to CH vibrations (1500–1400 cm‒1 range), derived from
chemisorbed organic present in the support. Further, weak symmetric and asymmetric vibrational
bands of nitro group also evident around 1300 cm‒1, which comes from products. An additional
peak appeared around 1517 cm‒1, which corresponds to the bending –NH vibration [54] also
supports the presence of functional group present in the spent catalyst [55].
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4. Conclusions
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In summary, silicate-anion-intercalated hydrotalcite was synthesized and functionalized with
diaminosilane groups. The composition of the functionalized materials was confirmed by FT-IR
and MAS-NMR techniques. The materials show better conversion than the parent hydrotalcite
materials in the nitro-aldol condensation reaction in liquid-phase medium. The catalysts contain
the amine with silanol groups, enhancing the catalytic efficiency on nitro-aldol condensation.
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Acknowledgements
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Authors thank DU-DST Purse grants (Ref. No. CD/2015/1534), University of Delhi, India for
their financial support, and USIC, University of Delhi, India for analytical support. We thank
Prof. Ramesh C. Deka for his valuable scientific discussion.
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2
3
4
5
6
7
Scheme 1. Schematic representation of silicate intercalation and aminosilane functionalization of
hydrotalcite materials.
8
9
10
11
12
13
14
15
16
17
18

1
2
3
4
5
6
7
8
9
Scheme 2. Nitro-aldol condensation of benzaldehyde with nitromethane.
10
19

1
2
3
4
5
Scheme 3. Plausible mechanism for the nitro-aldol condensation of benzaldehyde with nitro
methane.
6
20

1
2
Figure Captions:
3
4
Fig. 1. FTIR spectra of (a) MA-HT, (b) MA-HTSi, (c) MA-HTSi-DA1, (d) MA-HTSi-DA2 and
(e) MA-HTSi-DA4.
e
d
C-H
c
N-H
b
a
Si-O-Si
4000
3000
2000
1000
Wavenumer (cm-1)
5
6
7
Figure 1. Baskaran et al.
8
9
10
11
12
13
14
15
21

1
2
Fig. 2. XRD patterns of (a) MA-HT, (b) MA-HTSi, (c) MA-HTSi-DA1, (d) MA-HTSi-DA4, (e)
MA-HTSi-PA4 and (f) MA-HTSi- PAn4.
006
003
f
e
d
c
b
a
10
20
30
40
50
60
2degree
3
4
5
6
Figure 2. Baskaran et al.
7
8
9
10
11
12
13
14
22

1
Fig. 3. Nitrogen sorption isotherms of the silica-intercalated and aminosilane-grafted materials.
200
MA-HTSi-DA4
MA-HTSi-DA3
MA-HTSi-DA2
MA-HTSi-DA1
MA-HTSi
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/po)
2
3
4
Figure 3. Baskaran et al.
5
6
7
8
9
10
11
12
13
14
15
23

1
Fig. 4. Solid-state NMR spectra (A) 29Si-NMR and (B) 13C- NMR of MA-HTSi-DA4.
2
3
4
Figure 4. Baskaran et al.
5
6
7
8
24

1
2
3
Fig. 5. (a) Effects of the reaction temperature and time on nitroaldol condensation onMA-HTSi-
DA4; (b) kinetics study at different temperature and (c) Arrhenius plot on nitroaldol
condensation.
100
4
5
40 oC
60 oC
70 oC
80 oC
(a)
80
60
40
20
6
7
8
9
50
100
150
200
250
300
350
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Reaction time (min)
(b
(c)
25

1
2
3
4
Figure 5. Baskaran et al.
5
6
Fig. 6. Effect of diaminosilane loading in the nitroaldol condensation reaction.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Figure 6. Baskaran et al.
21
22
23
24
25
26

1
Table 1. Textural properties of various HT materials.
Surface area
Average BJH
desorption Pore
diameter (nm)
Total Pore
volume (cm3g-1)
d (003)
Å
Sample code
(m2g-1)
BET
MA-HT
MA-HTSi
022
364
141
0.03
1.14
0.67
--
7.8
8.2
--
13.6
20.3
MA-HTSi-DA1
MA-HTSi-DA2
131
122
193
0.79
0.48
0.96
25.5
17.8
21.4
7.4
--
MA-HTSi-DA3
MA-HTSi-DA4
8.2
CA-HTSi-DA4
161
77
0.72
0.57
17.3
31.0
--
--
ZA-HTSi-DA4
MA-HTSi- PAn4
MA-HTSi-PA4
205
116
0.78
0.52
15.3
18.4
8.5
8.5
2
3
4
5
6
7
8
9
10
11
12
13
27

1
Table 2. Effect of the various catalyst on nitro-aldol condensation reactions.[a]
Conversion
S.no
Catalyst
Selectivity (%)
2
(%)
1
1
2
Without catalyst
MA-HTSia
00
1.0
96
65
31
-
-
100
99.5
74.0
52
-
3
MA-HTSi-DA4
MA-HTSi-DA4
MA-HTa
0.5
26
48
4[b]
5
2
3
4
[a]
o
Benzaldehyde (5mmol): nitromethane (15 mmol), catalyst: 100 mg, Temperature: 70 C, Time: 2h, a-as-
synthesized, [b]-recycle-catalyst.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
28

1
2
3
Table 3. Effect of aminosilane grafted on different hydrotalcite materials and effect of various
aminosilane groups on MgAl-HT for nitro-aldol condensation reactions.[a]
[a]
4
5
Benzaldehyde (5 mmol): nitromethane (15 mmol), catalyst: 100 mg, Temperature: 70 oC, Time: 2 h.
‡Temperature: 90 oC, Time: 2h.
Selectivity (%)
Conversion
(%)
S.No
Catalyst
12
1
2
3
4
5
6
MA-HTSi-DA4
CA-HTSi-DA4
ZA-HTSi-DA4
MA-HTSi-PAn4
MA-HTSi-PA4‡
SBA-DA
96
70
36
45
61
32
99.5
97.0
98.0
80.5
100
0.5
3.0
2.0
19.5
-
94.0
06
6
7
8
9
10
11
12
13
14
15
16
17
18
19
29