Nano fibrous silica sulphuric acid as an efficient catalyst for the synthesis of β-enaminone

Article abstract of DOI:10.1007/s10562-013-1190-4

Synthesis of KCC-1 silica sulphuric acid as a new catalyst has been reported. The catalyst was characterized using XRD, SEM, TEM, BET, FT-IR, TGA-DTA, and EDX analysis. The catalytic activity was evaluated for the synthesis of enaminones under solvent-free conditions. The catalyst was easily recovered and reused up to seven cycles with slight loss of activity. The attractive features of this protocol are low amount of catalyst loading, short reaction time, mild reaction conditions, good to excellent yield of the products, easy handling of the catalyst and simple operational procedure. Graphical Abstract: [InlineMediaObject not available: see fulltext.]A highly efficient Sulphated nanosilica sulphuric acid has been synthesized and applied for the synthesis of enaminones. This catalytic protocol is better for the synthesis of enaminone than previously reported procedures in terms of catalyst loading, reaction condition, reaction time and yields of the products.

Full text of DOI:10.1007/s10562-013-1190-4

Catal Lett
DOI 10.1007/s10562-013-1190-4
Nano Fibrous Silica Sulphuric Acid as an Efficient Catalyst
for the Synthesis of b-Enaminone
•
Zeba N. Siddiqui Kulsum Khan Nayeem Ahmed
•
Received: 28 October 2013 / Accepted: 25 December 2013
Ó Springer Science+Business Media New York 2014
Abstract Synthesis of KCC-1 silica sulphuric acid as a
new catalyst has been reported. The catalyst was charac-
terized using XRD, SEM, TEM, BET, FT-IR, TGA-DTA,
and EDX analysis. The catalytic activity was evaluated for
the synthesis of enaminones under solvent-free conditions.
The catalyst was easily recovered and reused up to seven
cycles with slight loss of activity. The attractive features of
this protocol are low amount of catalyst loading, short
reaction time, mild reaction conditions, good to excellent
yield of the products, easy handling of the catalyst and
simple operational procedure.
offer simpler, less costly, more reactive and more envi-
ronmentally benign alternatives than their homogeneous
counterparts [2–6]. While these differently functionalised
nanosilica provide large specific surface areas [7, 8], but
their surface accessibility could become an issue limiting
their applications in certain cases, since any molecule
involved in adsorption or reaction have to travel through a
long pathway and therefore the actual surface area may not
be fully utilized. To overcome this problem, we have used
high-surface area fibrous SiO2 spheres with much more
open porosity for the synthesis of sulphuric acid func-
tionalised nanosilica. These SiO2 spheres (KCC-1) with a
very large specific surface area have promising applica-
tions as catalyst supports, where diffusion is no longer a
limiting step in heterogeneous catalysis. The high surface
area of KCC-1 is attributable to fibres and not to pores,
which dramatically increase its accessibility of active sites
[9]. Moreover, this material exhibits excellent physical
properties, including a fibrous surface morphology, good
thermal, hydrothermal and mechanical stability. Therefore,
it has been used as a support for many catalysts in organic
transformations [10–14]. However, so far synthesis of
b-enaminones using sulphuric acid modified nano fibrous
silica as catalyst has not been studied.
Keywords KCC-1-SA Á b-Enaminones Á Solvent-free
condition
1 Introduction
Nanocatalysis adds greenness to chemical process through
higher activity with less amount of catalysts. It also avoids
drastic reaction conditions and increases energy efficiency
and selectivity, which reduces by-products and allows to
perform chemical reactions in a selective manner with the
least possible consumption of substances [1]. This, in turn,
improves atom economy and waste prevention. Nanosilica
functionalized by incorporating organic and inorganic
functional groups within their mesopores are of great
interest for their use as heterogeneous catalysts. Various
mesoporous silica compounds such as SBA, MCM and
FSM that have been functionalized by sulfonic acid groups
The b-enaminones having the general formula of –NH–
CH=CH–(CH2)x–C=O are bioactive compounds used as
key intermediates for the synthesis of various heterocyclic
and naturally occurring alkaloids [15–17]. The conven-
tional approach for the preparation of b-enaminones and
b-enamino esters is by direct condensation of 1,3-dicar-
bonyl compounds with amines in refluxing aromatic sol-
vents with azeotropic removal of water. A variety of
catalysts such as B2O3/Al2O3 [18], SbCl3/SiO2 [19],
HClO4/SiO2 [20], P2O5/SiO2 [21], silica chloride [22],
sulfamic acid [23], L-proline [24], phosphotungstic acid
Z. N. Siddiqui (&) Á K. Khan Á N. Ahmed
Department of Chemistry, Aligarh Muslim University,
Aligarh 202002, India
e-mail: siddiqui_zeba@yahoo.co.in
123

Z. N. Siddiqui et al.
2.4 Catalytic Activity Determination
[25], and Ag [26] or Cu nanoparticles [27] and Ga(OTf)3
[28] have been used to promote this transformation. Pre-
vious investigations required prolonged reaction times, use
of hazardous reagents, tedious workup procedures, rigorous
reaction conditions, unsatisfactory yields, low selectivity,
or the use of toxic solvents which limit these methods to
small-scale synthesis. Therefore, there is a need for
development of more efficient and eco-friendly catalysts
for the synthesis of enaminones. In the present work, we
have developed a mild route to synthesize b-enaminones
under solvent free conditions taking into consideration the
green principle of chemistry.
A mixture of primary aromatic amine (1 mmol), 1,3-dicar-
bonyl compound (1 mmol), and the catalyst KCC-1-SA
(5 mg)wastakenina25 mLroundbottomflask. Thisreaction
mixture was stirred at 80 °C and the progress of reaction was
monitored by TLC after every 30 s, till the completion of the
reaction. After completion of the reaction, ethyl acetate was
added to the reaction mixture and the catalyst was filtered off.
The solvent of the filtrate was evaporated and the crude
product purified by recrystallization from appropriate solvent.
The products obtained were identified by IR and comparing
themeltingpointswithauthenticsamples. Thefilteredcatalyst
waswashedwithethyl acetateandacetone, driedat110 °Cfor
2 h and used for the next catalytic cycle.
2 Experimental
2.1 Materials
3 Results and Discussion
All reagents were purchased from Merck and Aldrich and
used without further purification. All yields refer to isolated
products after purification. Products were characterized by
comparison of melting points with authentic samples.
Melting points were taken in Riechert Thermover instru-
ment and are uncorrected. The IR spectra were recorded on
Perkin Elmer RXI spectrometer. All of the compounds
were solid and solid state IR spectra were recorded using
the KBr disk technique.
3.1 Amount of Acid Loading
In order to find most efficient and recyclable nanosilica
bonded sulphuric acid a series of KCC-1 bonded sulphuric
acid with different acid loading x mmol (x = 4, 6, 8, 10) in
1 g of KCC-1 were prepared according to the Scheme 1.
The effect of acid loading on the model reaction using
aniline and acetylacetone as model substrates was studied
(Fig. 1). It was found that catalyst with 8 mmol of acid
loading performed most efficiently.
2.2 Catalyst Synthesis
3.2 Structure and Morphology of the KCC-1-SA
2.2.1 Fibrous Nano Silica Support KCC-1 is Synthesized
According to the Reported Procedure [9]
To gather insight about the surface area, pore volume, pore
diameter and morphology of KCC-1-SA catalyst with
8 mmol of acid loading, different analyses were performed
such as N2 adsorption analysis, powder X-ray diffraction,
transmission electron microscopy (TEM), EDX, thermo-
gravimetric analysis and FTIR spectroscopy.
2.2.1.1 Synthesis of Catalyst Fibrous Nano Silica Sulfuric
Acid (KCC-1-SA) KCC-1 (1.0 g) was dispersed in
CH2C12 (10 mL) in a flask. Chlorosulfonic acid (8 mmol)
was dissolved in CH2C12 (10 mL) and added to KCC-1
silica suspension through a constant-pressure dropping
funnel under stirring, over a period of 30 min at room
temperature. After the addition was completed, the mixture
was stirred for another 30 min at room temperature. The
brown solid was collected by filtration and washed with
ether (50 mL). Finally it was dried at room temperature.
3.2.1 H? Ion Concentration of the Catalyst
The amount of H? in KCC-1-SA with 8 mmol of acid
loading was calculated through back titration and was
calculated to be 9 mmol H?/g.
2.3 H? Ion concentration Determination of the Catalyst
3.2.2 XRD
H? ion concentration of the catalyst was determined by
back titration analysis of KCC-1-SA. NaOH solution
(20 mL, 0.1 M) was added to the catalyst (100 mg) in an
Erlenmeyer flask. This solution was stirred for 10 min.
Excess amount of the base was neutralized by addition of
HCl solution (0.1 M) to the equivalence point of titration.
X-ray diffractograms (XRD) of the samples were recorded
in the 2h range of 0–80° with scan rate of 4°/min on a
Rigaku Minifax X-ray diffractometer with Ni-filtered Cu
˚
Ka radiation at a wavelength of 1.54060 A. Diffraction
peak at around 2h = 22° (Fig. 2) was characteristic of
123

Nano Fibrous Silica Sulphuric Acid as an Efficient Catalyst
Scheme 1 Preparation of fibrous nano silica sulphuric acid (KCC-1-SA)
3.2.3 FTIR
The FT-IR spectrum of the catalyst (Fig. 3) showed broad
antisymmetric Si–O–Si stretching from 1,200 to
1,000 cm-1 and symmetric Si–O–Si stretching near
800 cm-1 for SiO2. For sulfonic acid functional group, the
absorption range of the O=S=O asymmetric and symmetric
stretching modes were present at 1,110–1,240 and
1,020–1,080 cm-1 respectively, and that of the S–O
stretching mode at 600–700 cm-1 respectively. The spec-
trum also exhibited overlapping of asymmetric and sym-
metric stretching bands of SO2 with Si–O–Si stretching
bands in the silica sulfuric acid catalyst whereas broad OH
stretching absorption were present at 3,500 cm-1.
3.2.4 SEM and TEM Images
Fig. 1 Effect of acid loading on KCC-1 for catalysing model reaction
Scanning electron microscopy (SEM) image of KCC-1
(Fig. 4a) revealed that the material consisted of colloidal
spheres. Close inspection of TEM image of KCC-1
(Fig. 4b) showed that the material possessed dendrimers
which were arranged in 3D to form spheres of diameter
100–500 nm.
TEM images (Fig. 5) of KCC-1 after functionalization
with chlorosulfonic acid showed that dendrimeric fibers
were intact arranged in three dimensions to form spheres
with diameters 150–500 nm. Thus, functionalization did
not alter the basic morphology of the KCC-1.
3.2.5 EDX
The EDX spectrum of KCC-1-SA (Fig. 6), showed the
peaks for S, Si and O confirming the formation of expected
catalytic system.
3.2.6 Thermogravimetric Analysis
Fig. 2 XRD (a) KCC-1-SA (b) recovered catalyst
TGA and DTA data were obtained with DSC-60 Shimadzu
instrument. DTA and TGA analysis were performed in the
temperature range of 0–600 °C at a constant heating rate of
20 °C/min in the nitrogen atmosphere. The TGA curve
furnished information about the thermal stability of loaded
silica. Moreover, the XRD peaks of KCC-1-SA and
recovered catalysts showed the same positions with dif-
ferent intensities.
123

Z. N. Siddiqui et al.
Fig. 3 FT-IR spectra of fibrous -
silica sulfuric acid (KCC-1-SA)
Fig. 4 SEM and TEM images of KCC-1
Fig. 5 TEM images of KCC-1-SA
3.2.7 BET Surface Area Analysis
sulphuric acid on silica. TG (Fig. 7) curve indicated two-
stage decomposition. The weight loss of about 30 %
around 76 °C can be attributed to the loss of surface
physisorbed water. A further weight loss of about 28 % at
225 °C was due to decomposition of the SO3H group
bonded to KCC-1. DTA measurement also confirmed the
loss of physiosorbed water and SO3 group (Fig. 7).
The surface area of KCC-1 and KCC-1-SA were obtained
by Brunauer–Emmett–Teller (BET) method, which
employed nitrogen adsorption at different pressures at the
liquid nitrogen temperature (77 K). The surface area of
KCC-1 (Fig. 8) was found to be 386.16 m2/g [10–14].
123

Nano Fibrous Silica Sulphuric Acid as an Efficient Catalyst
respectively. Shape of N2 adsorption–desorption isotherm
(Fig. 9) was a typical type IV isotherm [14]. Large N2
uptakes and hysteresis at the relative pressure near 1.0 was
due to fibrous morphology of silica nanoparticles. Due to
this fibrous nature of the nanosilica there was easy acces-
sibility of surface area hence increasing the activity of the
catalyst.
3.3 Catalytic Activity of KCC-1-SA
Fig. 6 EDX spectra of fibrous-silica sulfuric acid (KCC-1-SA)
After characterizations of sulphuric acid functionalized
KCC-1, experiments were performed to optimize the reac-
tion conditions for synthesis of b-enaminones from 1,3-
dicarbonyl compounds and primary amines (Scheme 2). In
our preliminary experiments, we investigated the optimiza-
tion of reaction conditions regarding the solvent, amount of
catalyst and temperature of the reaction. For this purpose,
aniline and acetylacetone were chosen as model substrates
for the synthesis of representative compound 4a.
3.4 Effect of Solvents on Catalysis
To study the effect of solvent on the reaction, we performed
the reaction using model substrates and 5 mg of KCC-1-SA
as catalyst in different solvents (Table 1) under reflux con-
ditions. In MeOH, EtOH and (CH3)2CHOH (Table 1, entry
1, 2 and 3), the reaction took longer time (1.2–2 h) with
moderate yield of the products whereas in water (Table 1,
entry 4), the product was obtained in low yield (25 % yield)
only after 2.5 h. In CH3CN (Table 1, entry 5) lower yield of
the product was obtained with long reaction time. Con-
ducting the reaction in THF and C6H5CH3 (Table 1, entry 6,
7), again only trace amount of the product was obtained.
However, when the model reaction was carried out under
solvent free conditions (Table 1, entry 8) there was signifi-
cant increase in the yield of the product in shorter time
period. Thus, it is concluded that solvent-free condition is
the best condition for the synthesis of enaminones.
Fig. 7 TG and DTG analysis of the KCC-1-SA
After functionalization of KCC-1 the surface area reduced
to 91.21 m2/g (Fig. 9). The reduction in surface area was
due to encapsulation of dendrimeric fibres of silica (KCC-
1) by SO3H groups. The pore size distributions were also
obtained from these adsorption isotherms by employing
Barrett–Joyner–Halenda (BJH) method. The total pore
volume was estimated from the amount of nitrogen
adsorbed at the highest relative pressure. Pore volume and
˚
pore diameter were found to be 0.1155 cc/g and 25 A
BET surface area = 386.16 m2/gm
400
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
-0.001
Pore volume = 0.5792 cc/gm
4.9 nm
350
300
250
200
150
100
Adsorption
Desorption
50
0
0
100
200
300
400
500
600
0.2
0.4
0.6
0.8
1.0
Pore width[Å]
)
Relative pressure (P/P
0
Fig. 8 N2 adsorption–desorption isotherm of KCC-1
123

Z. N. Siddiqui et al.
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
2.5 nm
Pore volume = 0.1155 cc/gm
BET surface area = 91.21 m2/gm
100
80
60
40
20
0
Adsorption
Desorption
10
50
100
150
200
250
0.2
0.4
0.6
0.8
1.0
Pore width[Å]
Relative pressure (P/P )
0
Fig. 9 N2 adsorption–desorption isotherm of KCC-1-SA
O
O
H3C
CH3
1
NH
O
R
NH2
3a-f
KCC-1-SA
solvent-free
80 ° C
O
O
O
R
HN
R
2
4a-j
R= H, p-OMe, p-NO2, m-NO2, p-Cl, p-OH, p-CH3,
Scheme 2 Synthesis of b-enaminones
Table 1 Effect of various solvents on the model reaction
S. No. Solvent
Temperature Time (min)a Yield (%)b
Fig. 10 The effect of temperature on the model reaction
1.
2.
3.
4.
5.
6.
7.
8.
MeOH
EtOH
Reflux
Reflux
1.5 h
1.2 h
2 h
68
71
(CH3)2CHOH Reflux
57
H2O
Reflux
Reflux
Reflux
Reflux
80 °C
2.5 h
3 h
25
Table 2 Effect of catalyst loading on the synthesis of model reaction
CH3CN
THF
41
S. No.
Amount of catalyst
Time (min)a
Yield (%)b
4 h
Trace
Trace
98
1.
2.
3.
4.
5.
6.
–
2
2
2
2
2
2
–
C6H5CH3
Solvent-free
4.5 h
2
2
70
98
98
98
98
5
Model reaction: 1 mmol acetyl acetone and 1 mmol aniline
a
Reaction progress monitored by TLC
10
15
20
b
Isolated yield
Model reaction: 1 mmol acetyl acetone and 1 mmol aniline
a
Reaction progress monitored by TLC
3.5 Effect of Temperature on Catalysis
b
Isolated yield
To get optimum temperature, the model reaction was carried
out at different temperatures (Fig. 10). The yield of product
increased when the reaction temperature was raised from
room temperature to 80 °C. However, no increase in the yield
of product was observed when the reaction temperature was
raised from 80 to 110 °C. Therefore, all further reactions were
carried out at 80 °C.
3.6 Effect of Catalyst Loading
Effect of amount of catalyst on model reaction was
investigated to get the optimum concentration of catalyst.
123

Nano Fibrous Silica Sulphuric Acid as an Efficient Catalyst
Table 3 KCC-1-SA catalyzed synthesis of b-enaminones
S. No.
4a.
Products
Time (min)a
2
Yield (%)b
98
M.P Observed (8C)c
45–46 [29]
4b.
4c.
4d.
4e.
2
3
2
1
97
95
96
98
144–146 [30]
60–62 [29]
58–59 [29]
41–43 [18]
4f.
4
1
92
99
61–63 [31]
5a.
184–186 [31]
5b.
4
94
163–164 [31]
5c.
3
94
174–175 [31]
123

Z. N. Siddiqui et al.
Table 3 continued
S. No.
Products
Time (min)a
2
Yield (%)b
96
M.P Observed (8C)c
5d.
246–247 [31]
5e.
3
92
209–210 [31]
5f.
4
91
236–237 [31]
5g.
3
92
221–224 [31]
5h.
2
94
250–251 [31]
5i.
1
97
192–194 [31]
5j.
2
97
203–204 [31]
a
Reaction progress monitored by TLC
Isolated yield
b
c
Reference for melting point of products
123

Nano Fibrous Silica Sulphuric Acid as an Efficient Catalyst
of b-enaminones using different amines and different 1,3-
dicarbonyl compounds. The obtained results are summa-
rized in Table 3. The products obtained were confirmed by
comparing its melting point with authentic samples.
Table 4 Comparison of the efficiency of KCC-1-SA for synthesis of
4a
S.
No.
Catalyst
Amount of Timea Yieldb Ref.
catalyst
To show the superiority of KCC-1-SA, the model
reaction was also carried out in the presence of other het-
erogeneous catalysts including nanocatalysts. The data of
comparative study is summarized in Table 4. As we can
see (Table 4 entry 1–4) that in silica based heterogeneous
catalysts, excellent yield of products were obtained with
relatively shorter time period but amount of catalyst
required as compared to our catalyst is higher. When we
carried out the reaction in L-proline (Table 4, entry 5)
under solvent-free conditions good yield of products were
obtained with prolonged reaction time. We also carried out
the model reaction using nanocatalysts such as Ag and Cu
(Table 4 entry 6, 7). These catalyst took very long time
period ranging from 2.5 to 8 h for completion of reaction
with an additional requirement of solvents. Thus it can be
concluded that KCC-1-SA (Table 4, entry 8) is the best
catalyst as it took only 2 min for completion of reaction
with excellent yield of product. Moreover only 5 mg of
catalyst is required. This enhanced activity was explained
by the excellent accessibility of these sites due to the open
and flexible fibrous structure of KCC-1 which helped
substrates to easily penetrate and interact with acidic sites
and, in turn, accelerated the overall reactions.
1.
2.
3.
4.
5.
6.
7.
8.
HClO4-SiO2/
solvent-free
50 mg/mol 14 min 98
[20]
[22]
[32]
[33]
[24]
[26]
[27]
Silica chloride/
solvent-free
10 % (w/w) 5 min
25 mol% 7 min
91
89
Fe(HSO4)3ÁSiO2/
solvent free
Silica sulfuric acid/ 0.4 g
solvent-free
10 min 89
L-Proline/solvent-
free
5 mol%
4 h
85
90
92
98
Ag nanoparticles/
MeOH
20 mol%
10 mol%
5 mg
8 h
Cu nanoparticles/
MeOH
2.5 h
2 min
KCC-1-SA
Present
work
Reaction conditions: acetyl acetone (1 mmol), aniline (1 mmol) and
catalyst
a
Reaction progress monitored by TLC
b
Isolated yield
3.7.1 Recycling Study
The reusability of the catalyst is a significant advantage
particularly for commercial applications. Thus, the recov-
ery and reusability of the catalyst were also investigated
(Fig. 11). After the completion of the reaction, products
were extracted by ethyl acetate and the recovered catalyst
was washed with ethyl acetate and then with acetone and
dried in oven at 110 °C for 2 h. The recovered catalyst was
reused at least seven times without any appreciable
decrease in yield.
Fig. 11 The reusability of the catalyst in the synthesis b-enaminones
For this purpose model reaction was carried out using 2, 5,
10, 15 and 20 mg (Table 2 entry 1, 2, 3, 4, 5 and 6) of
KCC-1-SA at 80 °C under solvent-free conditions. The
product was obtained in 0, 70, 98, 98, 98 and 98 % yield,
respectively. Thus it is concluded that 5 mg of the catalyst
is sufficient for the best result. Therefore, all reactions were
carried out at 80 °C in the presence of 5 mg of KCC-1-SA
under solvent-free conditions.
4 Conclusion
In conclusion we have developed an efficient protocol for
the preparation b-enaminones. The reactions are carried out
under thermal solvent-free conditions within few minutes
yielding corresponding products with excellent yields. The
requirement of catalyst is also very low. The methodology
is environmentally benign as compared to H2SO4 with
respect to the amount, hazard and reaction conditions. The
catalyst could be successfully recovered and is recyclable
up to seven cycles without significant loss in activity.
3.7 Synthetic Application of KCC-1-SA
Using these optimized reaction conditions, the scope and
efficiency of this approach was explored for the synthesis
123

Z. N. Siddiqui et al.
Acknowledgments The authors are thankful to the SAP scheme
(DRS-I) from the University Grants Commission, New Delhi. One
of the authors KK is thankful to UGC for providing financial assis-
tance in the form of MANF[No.F.40-3(M/S)/2009(SA-III/MANF)].
Authors are also thankful to Centre of nanotechnology, Department of
Applied Physics, AMU, for providing powder XRD, University
Sophisticated Instrument Facility (USIF), AMU, Aligarh for provid-
ing SEM, TEM facilities and SAIF Punjab University, Chandigarh for
providing NMR and Mass spectra.
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