Mesoporous silica polymer nanocomposites encapsulated with polyphosphoric acid for the synthesis of β-amino carbonyl compounds under environmental benign conditions

Article abstract of DOI:10.1039/c4nj01748j

Mesoporous silica-polymer nanocomposites (SBA/PS) synthesized via wet impregnation method were functionalized by mild polyphosphoric acid (PPA) for catalytic applications. The catalysts were characterized and analysed by powder X-ray diffraction (PXRD), solid state nuclear magnetic resonance (³¹P-SSNMR), N₂-adsorption-desorption isotherm, Fourier transform-infrared (FT-IR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and thermogravimetric analysis (TGA/DTA). The catalytic studies of a multicomponent Mannich reaction was conducted wherein the hydrophobic nature of these nanocomposites showed an enhancement in the product activity and selectivity of β-amino carbonyl (Mannich base) compounds. The probable reaction mechanism elucidates the role of well-distributed active sites on the hydrophobic surface and the stability of the catalyst for good catalytic activity.

Full text of DOI:10.1039/c4nj01748j

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ARTICLE TYPE
Mesoporous silica polymer nanocomposites encapsulated with
polyphosphoric acid for synthesis of βꢀamino carbonyl compounds
under environmental benign conditions
a
b
b
b
Divya Sachdev *, Amit Dubey , G. Robin Wilson and Ankita Sharma
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Mesoporous silicaꢀpolymer nanocomposites (SBA/PS) synthesized via wet impregnation method was
functionalized by mild polyphosphoric acid (PPA) for catalytic application. The catalysts were
characterized and analysed by Powder Xꢀray Diffraction (PXRD), Solid State Nuclear Magnetic
31
1
0
Resonance ( PꢀSSNMR), N ꢀAdsorptionꢀDesorption Isotherm, Fourier Transformꢀ Infrared (FTꢀIR),
2
Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRꢀTEM)
and Thermogravimetric Analysis (TGA/DTA). The catalytic studies of a multicomponent Mannich
reaction was conducted wherein the hydrophobic nature of these nanocomposites showed an enhancement
in the product activity and selectivity of βꢀamino carbonyl (Mannich base) compounds. Probable reaction
mechanism elucidates the role of well distributed active sites on the hydrophobic surface and the stability
of the catalyst for good catalytic activity.
1
5
for catalytic applications. Therefore, in order to synthesize stable
Introduction
nanoꢀcatalyst effectively, the catalytic active moieties are
dispersed in the embedded polymer matrix to form one of the
structural components of the framework . Furthermore, the
The discovery of mesoporous materials have gained tremendous
interest since it acts as an excellent medium for embedding
polymers; especially ordered mesoporous silica (OMS) materials
are an ideal tethering agent for entrapping the polymers at the
4
5
5
6
6
7
7
0
5
0
5
0
5
catalytic moieties can be incorporated inside the framework by
cross linking the monomers to the silica surface directly via
impregnation or grafting method for their subsequent controlled
2
2
3
3
4
4
0
5
0
5
0
5
1
surface strongly . The flexible pore architecture and high surface
5ꢀ6
radical polymerization . This provides uniform functional
area of OMS allows fabrication of organicꢀinorganic
nanocomposites with different polymer functionalities for desired
applications. This penetrating network of polymer inside the
silica improves the flexibility, mechanical strength and generates
density of the monomers on the surface of the silica framework
for catalytic, sensors, ion exchange and adsorption applications.
Multicomponent reaction has emerged as a proficient, oneꢀpot
synthesis of complexꢀorganic molecules from readily available
substrates without isolating any intermediate; this reduces time
and energy as compared to the conventional reaction procedures
modulation
in
controlling
the
degree
of
hydrophilicity/hydrophobicity on the surface property. Such
properties are essential for tuning and transforming these
nanocomposites into a highly efficient catalyst. Though many
researchers have already incorporated the polymers directly on
the surface of silica however in most of the cases the structural
order either collapsed completely or became almost nonporous.
Therefore methods were devised for uniform distribution of
polymer over the inner pore surface while retaining the
7ꢀ9
. Various biologically active compounds especially βꢀamino
10
carbonyl compounds act as a precursor for antibacterial ,
antitumor, anticancer, antiꢀinflammatory drugs
11ꢀ13
;
for which
amino alcohols, peptides, lactams, optically active amino acids
respectively have been synthesized via Mannich reactions. So far
14
15
several catalysts such as ionic liquid , Cuꢀnanoparticles ,
16
17
SnTUD1 nanoparticles , phosphonic acids , metal salts,
2
mesoporosity and enhancing the active surface area . This
18
19
18
heteropoly acids , polyaniline , SiꢀAl O have been employed
2
3
enables the synthesized nanoꢀcatalyst to exhibit better stability,
uniformity and strength with an appropriate spatial distribution.
In recent years, heterogeneous catalysts have been introduced
to address the sustainability issues and have emerged as an
alternative approach for limiting the homogeneous acid catalyst
which are often highly toxic, environmentally hazardous, and
corrosive and produce lot of wastes . Particularly, the
introduction of controlled hydrophobicity on highly hydrophilic
silica surface could be interesting for achieving higher selectivity
for Mannich
reaction but silica polymer nanocomposites
functionalized with polyphosphoric acid (PPA), a mild catalyst
have not been employed yet. Moreover the present catalyst has a
greener role to play since it can overcome the constraints related
to the need of strong acidic sites in many selective catalytic
reactions. Recently, we have reported the acylation of
naphthalene over SBAꢀPolyphosphoric acid (SBA/PPA)
nanocomposites in liquid phase with very high selectivity of 2ꢀ
3
20
acetylnaphthalene . In the present studies, efforts are devoted to
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modify the surface properties via inꢀsitu polymerization of vinyl
temperature. The reaction mixture was cooled, filtered and
monomers followed by incorporation of PPA. The endeavor of 55 washed with excess solvent to remove the residual PPA and dried
o
incorporating vinyl monomers is to enhance the degree of
hydrophobicity on the highly hydrophilic silica surface for varied
structural properties particularly for higher selectivity in organic
transformations and PPA was chosen because of its mild acidic
at 80 C for 12h.
Finally the samples were named as
SBA/aPS/xPPA, where a and x represents the wt% of polymer
and PPA incorporated respectively. Similarly PPA of (5, 10. 20,
30 wt%,) was impregnated over SBAꢀ15 without surface
5
behavior (pKa= 2.1).Moreover devising such ways for a multiꢀ 60 modification and named as SBA/xPPA (imp) where “imp”
component reaction (MCR) in an environmentally benign way
would result in enhanced and improved atom economy.
stands for impregnation.
Characterization details
PXRD pattern was recorded on Hecus XꢀRay Systems S3 Model
using Cu Kα radiation (k = 1.5404 A). Nitrogen adsorption–
desorption isotherms were measured by using Quantachrome
NOVA series. Prior to the measurements, the materials were
outgassed at 80°C under vacuum for 12 h. The specific surface
area was calculated by the BET method and the pore size
1
0
Experimental
Synthesis of SBAꢀ15
6
5
All the chemicals were procured from SigmaꢀAldrich and used
without further purification. The synthesis of SBAꢀ15 is carried
out using Pluronic (P123) (EO20PO70EO20,MW = 5800,
Aldrich) as the surfactant and tetra ethoxy orthoꢀsilicate (TEOS)
15
20
25
distribution was calculated from the desorption isotherms by the
2
1
31
as a silica source, respectively . In a typical synthesis batch with 70 Barrett–Joyner–Halenda (BJH) method. Solid state NMR P of
TEOS, 3 g of P123 was dissolved in 100 g of distilled water and
.9 g of conc. HCl (35%). After stirring for 1 h, 7.3 g of TEOS
samples was recorded on AV500S ꢀ 500 MHz High Resolution
Multinuclear FTꢀNMR Spectrometer at 121.4 MHz with a 5mm
CP/MAS probe and magic spinning frequency of 5 KHz and
adenosine triphosphate was chosen as a standard. The IR spectra
5
o
(ACROS, 98%) was added at 35 C while maintaining the molar
ratio of P123: H O: HCl: TEOS ratio as 1: 5562.9: 86.29: 42.51
2
and continued stirring for 24 h. Subsequently the mixture was 75 of samples (as KBr pellets) were recorded using a Schimadzu
o
ꢀ1
heated for 24 h at 100 C under static conditions in a closed
polypropylene bottle. The solid product obtained after the
hydrothermal treatment was filtered and dried at 80 C. The
template was removed by calcinations at 550 C for 6 h.
FTIR spectrophotometer in the range of 400–4000 cm . The
number of acidic sites on the catalyst surface was determined by
titration method. Scanning Electron Microscopy was used to
confirm the structural morphology of PPA functionalized silica
polymer nanocomposites, Energy Dispersive Xꢀray Analysis
o
o
8
8
9
0
5
0
Synthesis of PPA functionalized silica polymer nanocomposite
(
EDX, Oxford INCA250 energy, UK) was also performed.
(
SBA/PS/PPA)
The synthesis methodology of polymer supported silica
SBA/PS) involves the incorporation of vinyl monomers (styrene
2
Microstructural characterization was conducted using Tecnai G
F30 SꢀTwin (FEI; Super Twin lens with Cs =1.2 mm) instrument
operated at an accelerating voltage at 300 kV, having a point
resolution of 0.2nm and lattice resolution of 0.14nm. Program
Digital Micrograph (Gatan) was used for image processing. The
TEM samples were prepared by drying a droplet of PPA
functionalized silica polymer nanocomposites on a carbon grid.
Thermogravimetric measurements (TGA) were performed under
air atmosphere on equipment Netzch in the temperature range
from 30 to 1000°C at 20°C/min.
(
30
35
40
45
50
and divinyl benzene), cross linkers and radical initiators into the
SBAꢀ15 mesopore walls via the wetꢀimpregnation method and
equilibrated under reduced pressure to achieve a uniform
distribution. The monomers adsorbed on the mesopore walls were
subsequently polymerised with temperature programmed heating.
Typically, for 30 wt% polymer loading of polymer on SBAꢀ15,
6
0
.114 g of styrene (80 mol%), 0.0358 g divinylbenzene (20
mol%), 0.0065 g of AIBN, a,a9ꢀazoisobutyronitrile (3% relative
to the total vinyl group) were dissolved into 2 ml of solvent
Synthesis of βꢀamino carbonyl compunds
(dichloromethane). The resulting mixture was impregnated
The catalytic activity studies were carried out for one pot three
component reaction between aniline, benzaldehyde and
solution, the sample was heated to 40 C to remove the 95 acetophenone to achieve the synthesis of Mannich base under
uniformly into the SBAꢀ15 framework. After impregnating the
o
dichlorobenzene and subjected to freeze–vacuum–thaw to remove
the residual solvent and air. The sample was subjected to
controlled temperature programming for polymerization. The
solvent free conditions (Scheme.1). Typically, 1mmol of all the
three reactants were taken in round bottom flask under stirring
conditions at 52°C without any solvent. After 15 minutes, the
catalyst was added into the reaction mixture and the reaction was
o
o
o
temperature scheme follows 45 C for 24 h, 60 C for 4 h, 100 C,
o
o
1
20 C and 150 C for 1 h. Finally the polymer was washed with 100 monitored by TLC for 24hrs. The products of the reaction were
ethanol to remove excess of monomers. Samples with 10 and 20
wt% polymer loading were also synthesized under similar
conditions and named as SBA/PS. The role of styrene is to make
the surface more hydrophobic. Different amount (5, 10, 20, 30
wt%) of PPA was incorporated on SBA/PS using ethanol as a
solvent. Typically for 20wt% incorporation of PPA, 600mg of
PPA in ethanol was impregnated on SBA/PS for 2h at reflux
isolated and compared with the authentic samples. Finally, the
yield of the product was calculated after purification of the
product.
1
05
2
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ARTICLE TYPE
R
O
O
C
C
Ph
CHO
NH
2
COCH3
SBA/PS/PPA
no solvent
CH
CH
C
NH
CH
O
C
X
Ph
R
N
+
Ph
+
+
+
R
X
R
X
O
R
Ph
mannich base
aldol product
side product)
B
bis mannich base
(
(side product)
A
C
Scheme1 Mannich Reaction
N Adsorption studies
Results and Discussions
2
N adsorption isotherms were measured by volumetric adsorption
2
Characterization of PPA functionalized silica
polymer nanoꢀcomposite(SBA/PS/PPA).
3
3
4
0
5
0
5
analyzer and showed a Type IV adsorption isotherm indicating
the mesoporous nature of the samples (Fig.2) according to the
PXRD studies
21
IUPAC classification . A sharp capillary condensation step at
Powder Xꢀray diffraction pattern of the samples (Fig.1) at low
angles shows the diffraction pattern similar to SBAꢀ15. The
diffraction at (100), (110), and (200) planes can be indexed to
reflections comprising of hexagonal p6mm space group
relatively high pressure p/p = 0.5ꢀ0.7 with an H1 hysteresis loop
0
indicates the presence of large mesopores with narrow pore size
distributions. The surface area, pore volume and pore diameter
decreased with the incorporation of polymer and PPA indicating
that the polymer and PPA has been incorporated inside the SBA
framework. Also the structural parameters of only PPA
incorporated SBAꢀ15 (SBA/30PPA) showed variations from
polymer coated silica (SBA/PS/PPA) incorporated with PPA as
given in Table 1.
1
1
2
0
5
0
21
indicating that the materials possess an ordered mesoporous
structure and polyphosphoric acid has been incorporated inside
the framework. A careful examination of the spectrum showed
increase in the intensity of the base peak in SBA/30PS/10PPA
catalyst compared to the SBA/PPA catalyst due to the polymer
formation inside the mesopores of silica materials resulting in an
increase in the apparent density of the polymer on the mesopore
walls than the pores of the nanocomposite resulting in
distinguished scattering contrast. This increased intensity
probably gives an indication towards a uniform coating of the
4
0
% (SBA/PS(30))
%
%
(SBA/PS(30)/PPA(30))
(SBA/PS(30)/PPA(10))
35
6
polymer on the mesopore walls .
3
2
2
1
0
5
0
5
a-SBA-15
b-SBA/(10)PPA
c-SBA/(30)PS/10-PPA
10
5
0
a
b
c
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
45
Fig.2 Nitrogen adsorptionꢀdesorption isotherms of SBA/PS,
SBA/PS/PPA with different concentration of polymer and
polyphosphoric acid. (PSD analysis BJH method)
0
1
2
3
4
5
2
Theta (degrees)
13
C CP-MAS NMR
50
Incorporation of PPA inside the silica framework was further
confirmed by C CPꢀMAS SSNMR. The spectrum of SBA/PS
Fig.1 PXRD pattern of mesoporous silica functionalized
nanocomposites;(a)SBAꢀ15, (b)SBA/PPA, (c) SBA/PS/PPA.
13
2
5
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(
Fig.3(a)) showed peaks at 128, 139 and 120 ppm indicating the
The SEM images of SBA/PS/PPA with different polymer and
PPA weights are shown in the Fig.4, the rod shaped particles are
evenly dispersed with relatively uniform size of 1µm. The
polymer and the impregnated polyphosphoric acid on ordered
mesoporous silica material are uniformly dispersed which results
in better catalytic activity and selectivity. SEM/EDX was also
done which showed the presence of Si, O, C and P in the
framework. The average amount of P atoms in SBA/PS matrix
was estimated to be about in (0.37%), (0.62%), (0.8%)
(atomic wt%) for SBA/30PS/10PPA, SBA/30PS/20PPA,
SBA/30PS/30PPA which is in correlation with the theoretical
value.
presence of aromatic carbons of the polymer resulted from the
monomers styrene and divinyl benzene . The peak at 25 ppm is
little upfield showing the presence of the carbon directly
2
2
2
5
5
Table 1 Structural Parameters of functionalized mesoporous silica
nanocmposites (SBA/PS/PPA)
1
2
Materials
Surfac
Pore
a
0
0
h (nm Por
e area
Volum
(nm)
)
e
2
30
(
m /g)
e
Size
(D)
3
(cm /g)
(nm
)
HRTEM/EDX
35
SBAꢀ15
678
1.2
10.8
3.4
7.4
The silica polymer nanoꢀcomposites showed large domains of
highly ordered stripeꢀlike hexagonally arranged images. The
images shown exhibits wellꢀordered hexagonal arrays of
mesopores. HRꢀTEM images (Fig. 5) of SBA/PS/PPA clearly
indicate (shown by arrows) the presence of the polymer coating
on the mesoporous material, in addition to this spots seen in the
image showed adsorbed PPA. The comparative TEM images of
SBAꢀ15 and SBA/PS/PPA clearly depicts the polymer
incorporation in mesopores (given in supplementary
information). The amount of carbon and phosphorus was also
verified by HRTEM/EDX (which was found to be consistent with
the theoretical one. Moreover the diffraction pattern (image given
in supplementary information) clearly indicates the presence of
amorphous nature.
SBA/30PPA
SBA/30PS
391
312
0.43
0.59
10.7
ꢀ
6.0
ꢀ
4.7
4.0
SBA/30PS/30PP
ꢀ
ꢀ
2
1
21
14
0.46
0.20
3.8
2.8
4
4
5
0
5
0
A
SBA/30PS/10PP
A
11.0
8.2
a
pore size distribution analysisꢀ BJH method
1
2
hexagonal unitꢀcell dimension (a
wall thickness = a ꢀD
0
)= 2d100/√3
h
0
0
10
linked to mesoporous silica (ꢀCH ꢀSiꢀ). However incorporation of
PPA inside the silica framework was further identified by
2
3
1
P
magic angle spinning (MAS) NMR.. NMR spectrum of the
samples (Fig.3(b)) exhibits two different peaks at ꢀ0.3 and ꢀ10.85
ppm indicating the distinct nature of phosphorus incorporated
inside the silica framework . The peak in the spectra clearly
indicates the presence of PPA over the polymeric surface similar
to our previous studies.
15
2
3
20
SEM/EDX
55
3
1
Fig.3 P spectra of mesoporous silica polymer nanocomposite (SBA/PS) and polyphosphoric acid functionalized polymeric silica (SBA/PS/PPA)
4
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Fig .4 SEM image of polyphosphoric acid functionalized silica polymer nanocomposite(SBA/PS/PPA) with EDX.
5
Fig.5 TEM image of polyphosphoric acid functionalized silica polymer nanocomposite(SBA/PS/PPA) and EDX
35
shown in Table 2.
1
1
2
2
3
0
5
0
5
0
FT-IR and Acidity studies
(
a)
Fig 6 shows vibrational frequencies of neat PPA, SBA/PS,
ꢀ
1
SBA/PS/PPA. A broad peak from 3612 to 3120 cm for neat
PPA can be assigned to POꢀH stretching since PPA is an
oligomer of (OH) ꢀPꢀ(O) ꢀP(OH) units. The infrared spectrum of
(b)
n
n
n
(
c)
SBA/PS/PPA looks similar to neat PPA and polymer supported
24
Polymeric C
SBA spectra . The intensity of OꢀH stretching from 3300 to3450
ꢀ
1
cm in case of SBA/PS/PPA is more than SBA/PS which clearly
indicates the incorporation of PPA on the polymeric surface.
C=C
C-Cl
SBA/PS
SBA/PS/PPA
PPA
ꢀ
1
Moreover the vibrational frequencies for C=C at 1690 cm and
O-H stre
ꢀ
1
for CꢀCl at 798 cm for SBA/PS/PPA are consistent with
functionalized SBA/PS.
Acidity of SBA/PS/PPA was determined by titration
method, wherein 500mg of the SBA/PS/PPA with varied
concentrations of PPA as 30%, 20% and 10% was stirred with
4000 3500 3000 2500 2000 1500 1000
500
-1
wavenumber cm
0
.01M sodium carbonate solution for 12 h and finally the left
Fig.6 FTꢀIR spectra of polyphosphoric acid and functionalized
mesoporous silica;(a) PPA, (b) SBA/PS and (c) SBA/PS/PPA
over basic solution was filtered, washed and titrated with 0.01 M
oxalic acid. The titrant value gives the volume of the unused
sodium carbonate which was not adsorbed and through this
volume the strength of the SBA/PS/PPA acidic sites was
calculated. The number of acidic sites for SBA/30PS/30PPA,
SBA/30PS/20PPA and SBA/30PS/10PPA was found to be
40
Catalytic Studies
Effect of different variants for optimization
3
8.5mmol/g, 27.36 mmol/g and 13.51 mmol/g respectively also
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Synthesis of βꢀamino carbonyl compound over SBA/PS/PPA for
obtaining Mannich base was optimized on the model reaction
1
00
% conversion
Mannich base
(Scheme 1, R=H).Various reaction parameters were studied to
8
0
5
0
5
0
5
0
obtain the suitable conditions for the synthesis of βꢀamino
carbonyl compounds (Table 2). The results indicate 94%
conversion with 98% selectivity of Mannich base over
SBA/PS/PPA compared to 60% conversion and 82% selectivity
over SBA/PPA; the catalytic activity of SBA/PS/PPA increased
with increase in PPA content. Also variations in the polymer
concentration indicate that 30 wt% polymer loading with
equivalent amount of impregnated PPA (about 30 wt %) gave
60
40
1
1
2
2
3
20
9
4% of Mannich base with 98% selectivity.
0
This may be due to high surface area and better dispersion of
active sites over SBA/PS/PPA than SBA/PPA catalyst. Although
there was no pronounced difference in the catalytic behavior with
increase in the polymer content but maximum activity was
achieved with 30 wt% polymer. It is important to mention here
that 60% conversion and 65% selectivity was obtained on
homogeneous PPA, indicating the necessity of heterogeneous
support for better dispersion of PPA.
The best catalyst SBA/30PS/30PPA was selected for further
studies. An increasing trend of conversion of Mannich base with
respect to the weight of both the catalyst SBA/PS/PPA and
SBA/PPA was observed. Fig.7 shows an increase in product
conversion with increase in catalyst weight uptill 90mg and then
became almost constant with further increase in the catalytic
sites, depicting the optimum amount of catalytic centers required
for bringing about the reaction. However no conversion with
10mg of the catalyst suggests that minimum concentration of the
active site is required for bringing out the conversion.
0
20
40
60
80
100
120
wt of catalyst (mg)
Fig.7 Effect of catalyst weight on the conversion of βꢀamino
carbonyl compound, conditions as given in Table 2
3
4
4
5
0
5
In order to optimize the solvent, different solvents were tried such
as water, ethanol, methanol, acetonitrile, chloroform,
dichloromethane, toluene, ethylacetate but the best results were
achieved, without solvent.
The initial kinetics in the synthesis of βꢀamino carbonyl
compound is a second order nucleophilic reaction (SN ) wherein
the imine ion formation (Scheme2) is a rate determining step and
the stabilization of this intermediate in different solvents leads to
this distinct behavior. Percentage conversion in a polar protic
solvent (such as ethanol, methanol) was low, probably due to the
solvation of nucleophile (aniline) in these medium and as a result
the overall reaction rate gets retarded.
2
Table 2 Effect of varied catalyst on the conversion of βꢀamino carbonyl compound (Mannich base)
Catalyst
βꢀamino carbonyl compound TON
Acidity
(
mmol/g)
a
b
Conversion(%) Selectivity(%)
PPA
60
65
82
80
80
98
96
94
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
SBA/30PPA
60
ꢀ
SBA/20PPA
54
ꢀ
SBA/10PPA
50
ꢀ
SBA/30PS/30PPA
SBA/30PS/20PPA
SBA/30PS/10PPA
94 (90)
90 (85)
84(77)
241
310
514
38.5
27.36
13.51
a
Benzaldehyde (1mmol), Aniline (1mmol), acetophenone (1mmol), no solvent, catalyst (90mg), Tempꢀ 52°C, conversion as detected by GC
b
selectivity of the product only, () parenthesis in the column (%) conversion indicates % yield, TON= no of moles of product/no of moles of active sites x
%
yield
c
TON is the number of moles of desired product formed per number of active centres in a catalyst.
More importantly there was no conversion in water, since the
reactants were immiscible. However the polar aprotic solvent
acetonitrile gave high conversion around 75% which may be
due to the stabilization of charged intermediate (imine, shown in
50
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New Journal of Chemistry
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DOI: 10.1039/C4NJ01748J
scheme 2) with lone pairs of acetonitrile but the overall
conversion was highest when there was no solvent, this may be
due to the low dissolving property of the product Mannich base in
this medium. Moreover the dielectric constant of all the three
reactants aniline=7.3, benzaldehyde= 17, acetophenone=17.4 is
high enough for stabilization of iminium ion (Schiff base).
In addition, the water formed during the reaction is repelled due
to the presence of hydrophobic catalyst SBA/PS/PPA, and this
ultimately does not allow any hindrance in product formation
(thereby facilitates the stability of desired product) and
consequently stabilizes the product (B). Fig.8 showed the kinetic
effect of temperature on % conversion and selectivity of βꢀamino
carbonyl compound. The results concluded that maximum
conversion (94%) of the product is achieved at 52°C in 8h, yet
the reaction was under observation for 24 h and it was found that
no side products were obtained i.e. high selelctivity was achieved
during the course of the reaction. However further increase in
temperature at 80°C leads to decrease in conversion and
selectivity of the product which may be due to the formation of
the side products (other Mannich bases ) via replacement of
proton on N (given in scheme 2). Thus the favorable temperature
was chosen for the reaction.
100
90
5
80
70
60
50
0
1
5 C
1
1
2
0
5
0
O
40
4
0 C
0
52 C
30
O
selectivity at 52 C
0
6
0 C
20
0
80 C
0
selectivity at 80 C
10
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26
Time (h)
2
5
Fig 8. Effect of time and reaction temperature on the conversion of
βꢀamino carbonyl compound, conditions as given in Table 2
30
O
O
O
CHO
Si
+
O
H N
2
P
OH
O
HC
H
k1
O
HO
+
P
O
O
k '
1
O
OH
O
O
O
P
Si
O
SN2
P
HO
O
HO
HO
O
O
O
-
+
P
O
OH2
Si
P
O
O
O
HO
H
N
+
k1
H O
3
N
H
+
+
k '
1
imine
A
H C
2
acetophenone
H-O
H O
2
k2
Path2
O
Path-1
N
Ph
H
O
O
O
O
Ph
N
P
OH
O
+
O
HO
P
Mannich base
aldol product
Si
O
B
(side product)
O
O
O
Ph
C
Bis Mannich base
(side product)
D
Scheme 2 Plausible Reaction Mechanism for synthesis of βꢀamino carbonyl compound (Mannich base).
3
5
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unreacted reactants were found, this might be the reason for the
formation of schiff base (A) in low concentration and 94%
conversion for Mannich base (B). In addition to this,
SBA/PS/PPA catalyst plays a strong role in enveloping the
nitrogen site which ultimately retards the side reaction via path 2.
Moreover the water removed during the formation of an
intermediate reacts with SBA/PS/PPA to form hydronium ion
which also contributes in the catalytic activity of the reaction and
enhances the % conversion of the product (B). Overall the
behavioral properties of SBA/PS/PPA catalyst such as
dehydration, enhanced acidity and regeneration of catalyst during
the course of reaction play an important role in procuring the
higher conversion and selectivity of the product.
Structure Activity relationship
5
0
5
0
Careful analysis of the results indicate that polymer coating has a
profound role in activity and selectivity of the product formation.
The polymer creates the hydrophobic surface and ultimately
enhances the diffusion of the products which leads to higher
conversion and good selectivity of the βꢀamino carbonyl
compound. The significant participation of the hydrophobic
surface of SBA/PS/PPA during the course of the reaction can be
explained via the mechanism of the reaction. During the initial
course of synthesis (Scheme 2); a schiff base (an imine, A) via
4
5
1
1
2
50
SN mechanism is formed via a nucleophilic reaction between a
2
protonated benzaldehyde (carbocation) and aniline that further
reacts with acetophenone to form Mannich base (B) (shown in
Scheme2).
It is important to mention here that the reaction at the initial
stages provides maximum schiff base (~70%) with in 2h (Fig.9)
55
Reusability and the stability of the catalyst
In order to see the reusability of the catalysts, the catalyst after
the reaction was over was filtered thoroughly first with methanol,
water and then with acetone. The catalyst was dried at 120°C and
subjected for fresh reaction. Recycling of the catalyst was
which gets subsequently consumed during the course of reaction 60 repeated for 5 cycles without significant loss in the activity and
with acetophenone to form product (B). The percentage of
product (B) subsequently grows with time while the % of schiff
base (A) drops down to almost zero, however the selectivity of
the reaction remains same.
selectivity indicating the nonꢀleaching behavior of the acidic
contents from the catalyst during the course of the reaction. The
optimized conditions were employed for the synthesis of
substituted βꢀamino carbonyl (Table 3) and it was observed that
the yield of βꢀamino carbonyl increased when electron
withdrawing groups were attached to the carbonyl groups.
Furthermore the thermogravimetric analyses Fig10 of the
polymer supported PPA (SBA/PS/PPA) demonstrated a good
thermal stability up till 600°C than only mesoporous silica
(SBAꢀ15) at 365°C and polymer supported silica (SBA/PS) at
410°C.
6
5
100
90
80
70
7
0
%
%
schiff base
Mannichbase
60
5
0
%Selectivity
1
1
05
00
4
TGA-SBA-15
TGA-SBA/PS
TGA SBA/PS/PPA
DTG-SBA-15
40
3
30
DTG-SBA/PS
DTG-SBA/PS/PPA
9
9
8
8
7
5
0
5
0
5
2
2
0
10
1
0
0
0
5
10
15
20
25
-1
-2
-3
-4
-5
Time(h)
25
30
35
40
Fig.9 Comparative study of rate of formation of Schiff base to
Mannich base for explaining the route of probable product
formation, conditions as in Table 2
70
6
6
5
0
However we noticed that the maximum concentration obtained
for schiff base (A) is only ~70% , this can be explained due to the
three equilibriums participating in the reaction (as shown in
scheme 2), although the results for initial kinetics indicate that k1
0
200
400
600
800
1000
0
Temp C
Fig10. Thermogravimetric analysis of mesoporous silica and
functionalized SBAꢀ15, SBA/PS, SBA/PS/PPA materials.
is > k ´ , this is because, the water is constantly removed by the
1
75
catalyst SBA/PS/PPA (water repelling property due to
hydrophobicity) and therefore the reaction moves majorly in
forward direction and follows path 1 (k > k ) however we cannot
Conclusions
2
1
exclude completely the probability of reaction in backward
Polyphosphoric acid functionalized mesoporous silica polymer
nanocomposites were synthesized successfully via wet
impregnation method. The precise control of the polymeric
direction (k ´) which is also supported by the observation made
1
towards the end of the reaction wherein some amount of
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This journal is © The Royal Society of Chemistry [year]

Page 9 of 9
New Journal of Chemistry
View Article Online
DOI: 10.1039/C4NJ01748J
moiety along with the successful dispersion of polyphosphoric
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4
5
5
6
6
7
7
0
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0
5
0
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0
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Table 3 Effect of substituent on the yield of βꢀamino carbonyl
compound
4
5
4
.
4
Aldehyde (R)
Aniline
X)
%yield
6
.
M. Choi, F. Kleitz, D. Liu, H. Y. Lee, W.ꢀS. Ahn and R. Ryoo, J.
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(
7
.
K. Namitharan and K. Pitchumani, Eur. J. Org. Chem., 2010, 2010,
H
pꢀNO
2
97
4
11ꢀ415.
8. B. Ganem, Acc. Chem. Res., 2009, 42, 463ꢀ472.
I. Ugi, Pure Appl. Chem., 2001, 73, 187ꢀ191.
H
H
pꢀOMe
pꢀCl
72
82
9
.
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Perumal, Eur. J. Med. Chem., 2012, 51, 79ꢀ91.
H
H
pꢀOH
pꢀBr
63
84
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Bioorg. Med. Chem. Lett., 2010, 20, 5054ꢀ5061.
1
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H
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76
73
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H
pꢀBr
pꢀCH
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2
1
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a
The reaction conditions are similar to Table 2 except there is change in
the reactants
1
6. M. P. Pachamuthu, K. Shanthi, R. Luque and A. Ramanathan, Green
Chem., 2013, 15, 2158ꢀ2166.
acid showed mild acidic behavior. The SBA/PS/PPA gave 98%
selectivity with 94% conversion of the Mannich base, which is
quite an achievement in respect to only 65% selectivity of the βꢀ
amino carbonyl product with neat PPA. This can be attributed to
the combination of hydrophobicity, textural properties and the
presence of bronsted acidic sites on SBA/PS/PPA. Moreover the
probable mechanism explains the role of hydrophobicity in
constantly removing the water and in turn increasing the rate of
the reaction which leads to good selectivity and activity. Overall
PPA functionalized polymer impregnated silica has a potential to
show promising results with reactions involving water as one of
the product.
1
1
2
2
0
5
0
5
1
1
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Acknowledgements
24. J.ꢀB. Baek and L.ꢀS. Tan, Polymer, 2003, 44, 4135ꢀ4147.
DS and AD thanks Department of Science and Technology and
CSIR for the financial support. We thank NPLꢀDelhi for
conducting SEM and TEM analysis, IIScꢀBangalore for SSNMR,
Delhi University for adsorption isotherm of the samples.
a*
Author of Correspondence
Department of Agriculture and Environment Sciences
National Institute of Food Technology Entrepeneurship and
Management-Kundli, (Haryana) India
E-mail: divya19sachdev@gmail.com
Maulana Azad National Institute of Technology (MANIT) - Bhopal
M.P.)-462051
3
3
0
5
,
b
(
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