Synthesis and characterization of Trichloroisocyanouric acid functionalized mesoporous silica nanocomposite (SBA/TCCA) for the Acylation of Indole

Article abstract of DOI:10.1007/s12039-016-1131-z

Trichloroisocyanouric acid (TCCA)-functionalized mesoporous silica nanocomposites (SBA/ TCCA) were synthesized and characterized for the acylation of indole. The uniform incorporation of TCCA inside the SBA-15 matrix was confirmed by standard characteriza

Full text of DOI:10.1007/s12039-016-1131-z

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1131-z
Synthesis and characterization of Trichloroisocyanouric acid
functionalized mesoporous silica nanocomposite (SBA/TCCA)
for the Acylation of Indole
G ROBIN WILSON and AMIT DUBEY∗
Department of Chemistry, Maulana Azad National Institute of Technology (MANIT), Bhopal,
Madhya Pradesh, 462 003 India
e-mail: dramit.dubey@gmail.com; amitdubey75@yahoo.com
MS received 7 January 2016; revised 21 June 2016; accepted 21 June 2016
Abstract. Trichloroisocyanouric acid (TCCA)-functionalized mesoporous silica nanocomposites (SBA/
TCCA) were synthesized and characterized for the acylation of indole. The uniform incorporation of TCCA
inside the SBA-15 matrix was confirmed by standard characterization techniques (PXRD, Adsorption studies,
FT-IR, etc.). The catalytic activity studies of SBA/TCCA nanocomposites for acylation of indole showed high
selectivity (60–90%) of the 3-acetyl indole compared to homogeneous TCCA (50%). The advantage of solid
support for higher selectivity is also explained.
Keywords. Trichloroisocyanouric acid; mesoporous silica; acylation; indole.
1. Introduction
compounds, mononitration and dinitration of phenols
have been reported over TCCA/NH4SCN/ wet SiO2
mixture.10,11 However, based on the literature review,
TCCA-functionalized SBA-15 via co-condensation
method has not been reported for acylation of indole.
Therefore, in the present investigation, our interest
emerged to synthesize SBA/TCCA nanocomposites for
the first time in the liquid phase acylation of indole. Par-
ticularly, 3-substituted indoles are industrially impor-
tant compounds in diverse biological activities such as
in platelet activating factor receptor (PAFR) antago-
nists for septic shock,12 anticonvulsant,13 for fine chem-
ical industries.13 16 Various reagents such as indole salts
with acyl chlorides,17,18 Vilsmeir-Hack acylations,19
Friedel-Crafts acylations,20 regioselective synthesis of
3-acyl indoles with anhydrous SnCl2 using nitroben-
zene and acetyl chloride as a co-solvent and acylating
agent, respectively21 have been used for this interest-
ing conversion. Recently, the use of tungstophosphoric
acid-modified H-β zeolite was reported for acylation
of indole under microwave conditions.22 However; all
these methods suffer from low yield and lower selec-
tivity of 3-acylated product. Although, the electrophilic
attack at 3-position is more facilitated, yet the lower
selectivity was usually observed due to the competitive
formation of 1-acylated and 1, 3-diacylated diindolyl-
methanes, and self-polymerization of indoles under
acidic conditions.23 Hence, the 3-acylated activity and
selectivity of 3-acylated indole still remains a challenge
in this interesting reaction.
The rigorous environmental concerns have forced the
manufacturing industry across the globe to replace the
hazardous homogeneous reagents with recyclable, cost
effective and environmental friendly processes.1 The
development of new heterogeneous systems such as
zeolites, clays, heteropoly acids, mesoporous silica and
carbon have opened the new avenues for many catalytic
applications.2,3 Systematic variation in the surface area,
pore size and pore volume of these materials gen-
erates the flexibility to impregnate many acidic/basic
moieties for various applications.4 In our continuing
research efforts to develop mild acid catalysts, different
functionalized mesoporous silica materials were pre-
pared for their effective usages in catalytic and adsorp-
tion applications.5 7 Trichloroisocyanuric acid (TCCA),
one of the highly accepted homogeneous catalyst, acts
as a sensitizer to kill bacteria, control algae in water in
swimming pools and hot tubs, and has also been used
in acidic conversions.8,9 However, its corrosive nature
to eyes, skin, toxicity and burning effects within a short
period of its exposure and the difficulties associated in
product recovery limit its practical utilization.9 There-
fore, heterogenization or functionalization of TCCA
into a suitable solid matrix may be beneficial for
many important transformations to overcome the above
cited difficulties. Recently, thiocyanation of aromatic
∗For correspondence

G Robin Wilson and Amit Dubey
2. Experimental
3. Results and Discussion
2.1 Synthesis of SBA-15/TCCA nanocomposites
3.1 Physico-chemical Characterization
by co-condensation method
Powder X-ray diffraction (PXRD) pattern was recorded
SBA/TCCA materials were synthesized by non- on Hecus X-Ray Systems S3 Model using Cu-Kα radi-
ionic surfactant (poly(ethylene glycol)-poly(propylene ation (λ = 1.5404 Å) from 0◦–5◦. The adsorption stud-
glycol)-poly(ethylene glycol), MW = 5800), from ies were performed by using Micromeritics ASAP 2010
Sigma Aldrich), silica source (tetra ethoxy ortho- analyzer at −196◦C. BET (Brunauer–Emmett–Teller)
silicate (TEOS), with simultaneous addition of TCCA. method was used to measure surface area whereas BJH
Initially, 3 g of the surfactant was mixed with 80 g of (Barrett–Joyner–Halenda) method was used to calculate
distilled water followed by hydrolysis with 5.9 g of the pore size distribution from the desorption isotherm
conc. HCl (35%) under stirring condition for 1 h until of the sample. The IR spectra (400–4000 cm−1) of the
the solution becomes homogeneous. After the addition samples were measured with FTIR spectrophotometer
of 7.3 g of TEOS at 35◦C for 6 h, desired amount of (Perkin Elmer) using KBr pellets. The titration method
TCCA was added into the reaction mixture was main- was used to calculate the number of acidic sites on the
tained at the molar ratio of P123: H2O: HCl: TEOS: catalyst surface. The surface morphology of the cata-
TCCA as 1: 8546.9: 306.6: 67.54: 17.65. The solu- lysts was confirmed by Scanning Electron Microscopy.
tion was kept for 24 h under stirring condition and the
The indexing of the diffraction peaks of SBA/xTCCA
mixture was again subjected to hydrothermal treatment at (100), (110), and (200) planes (Figure 1) showed two
at 100◦C for 24 h (without stirring) in a polypropy- dimensional hexagonal p6mm symmetry identical to
lene bottle. The solid product was filtered and dried at SBA-15 materials.24 These results pointed out that the
80◦C. The surfactant was removed by extraction with materials could retain the ordered mesoporous structure
ethanol/HCl mixture. The synthesized samples were even after the addition of 2 g of TCCA which is incor-
designated as SBA/xTCCA, where x refers the amount porated uniformly inside the mesoporous framework of
of TCCA. To compare the activity of TCCA on other SBA-15. PXRD pattern of the samples was diminished
solid supports, equivalent amount of TCCA (5 wt%) with increase in the amount of TCCA in SBA/3TCCA
was impregnated on silica gel (SiO2) and Al2O3 using nanocomposite (Figure S1 in Supplementary Informa-
ethanol as solvent. However, efforts were not devoted to tion (SI)). This is due to the rupture of silica wall thick-
characterize SiO2/TCCA and Al2O3/TCCA catalysts as ness at higher concentration of TCCA. Careful analysis
they were chosen as references to compare the activity of the results indicates that the peak positions are
with almost same amount of acidity.
shifted towards lower angle because the simultaneous
addition of the hydrophilic ligand TCCA prevents the
2.2 Catalytic studies
For acylation of indole (Scheme 1), 1 mmol of indole,
1 mmol of acetic anhydride and a suitable solvent were
taken in a two-neck round bottom flask. The calcu-
lated weight of SBA/TCCA catalyst (10–500 mg) was
mixed with the reaction mixture under stirring condi-
tion at different temperatures (30–90◦C). The reaction
was observed for 24 h by TLC and the products of the
reaction were analyzed by gas chromatography by cal-
culating the response factors of the standard samples
using Perkin Elmer gas chromatograph (model Clarus
480) using 10% OV-17 column.
(a)
(b)
(c)
(d)
O
O
O
O
SBA/TCCA
0
1
2
3
4
5
N
+
+
Other products
C
O
+
2 Theta (degrees)
N
H
N
H
1-acylindole
B
3-acylindole
A
Figure 1. PXRD pattern of (a) SBA-15 (b) SBA/1.5TCCA
(c) SBA/2TCCA and (d) SBA/1.5TCCA (after first cycle)
nanocomposites.
Scheme 1. Acylation of Indole.

Trichloroisocyanouric acid functionalized SBA-15
Table 1. Structural parameters of the samples.
hydrophobic interactions between the terminal moiety
of the silica source and the long alkyl chain surfactant
during the formation of silica-surfactant composite in
the miceller solution. Therefore, the terminal organic
moiety may not be able to penetrate easily into the
micelle which leads to the increase in d(100) spacing.
This is in agreement with the previous reports for the
co-condensation method.25,26
Materials
Surface area Pore Volume Pore size
(m2/g)
(cm3/g)
(nm)
SBA-15
626
463
153
68
0.93
0.64
0.29
0.13
8.4
5.5
4.6
2.8
SBA/1.5TCCA
SBA/2TCCA
SBA/3TCCA
The adsorption studies (N2 adsorption-desorption)
indicated type-IV adsorption isotherm according to
IUPAC classification (Figure 2) with an H1 hystere-
sis loop similar to SBA-15.24 These results indicate
the presence of large mesopores with narrow pore size
distributions (Figure 2 (inset), and Figure S2 in SI).
The shape of the adsorption isotherm of SBA/
2TCCA is slightly bent due to the greater strain created
on the mesoporous silica surface with higher concen-
tration of TCCA. The structural parameters (Table 1)
showed that the surface properties were lowered with
the increase in the concentration of TCCA in SBA/
xTCCA materials, which further indicate that TCCA
has been incorporated inside the mesopores. Further-
more, the mesoporosity of the SBA/3TCCA nanocom-
posite was completely destroyed (Figure S3 in SI)
with significant reduction in the surface area (Table 1).
Scanning electron microscopy images (Figure S4 in
SI) showed the rod type geometry of SBA/1.5TCCA
nanocomposite, similar to SBA-15, which further indi-
cates the uniform distribution of TCCA.
(a)
(b)
(c)
(d)
C=Ostre
N-Cl stre
C-N stre
4000 3600 3200 2800 2400 2000 1600 1200
800
400
Wavenumber cm-1
Figure 3. FTIR spectra of (a) Neat TCCA (b) SBA-15 (c)
SBA/1.5TCCA and (d) SBA/2TCCA nanocomposites.
the presence of SBA-15 material. A broad peak at
1700–1600 cm−1, 1250–1200 cm−1 and 770–800 cm−1
can be attributed to C=O stretching, C-N stretch-
ing and N-Cl stretching, respectively, for neat TCCA
and SBA/TCCA nanocomposites. These results clearly
showed the uniform incorporation of TCCA inside
the mesoporous matrix. Acidity of the SBA/1.5TCCA,
SiO2/TCCA and Al2O3/TCCA catalysts, calculated by
titration method, was observed to be 1.9, 2.1 and
1.7 mmol/L, respectively.7
FT-IR spectra of SBA/1.5TCCA and SBA/2TCCA
(Figure 3) showed the characteristic peaks similar to
homogeneous TCCA. However, presence of some extra
peaks for SBA/TCCA nanocomposites may be due to
(a)
(a)
(b)
(b)
(c)
3.2 Catalytic activity studies
0
5
10
15
20
Pore diameter (nm)
3.2a Variation of catalysts on the acylation of indole:
The catalytic activity studies showed considerably high
activity (45–60%) of the total products with (60–90%)
selectivity of the 3-acylated product in all SBA/TCCA
catalysts compared to homogeneous TCCA catalyst
under our experimental conditions (Table 2). The
results indicated lesser selectivity of the 3-acylated
product for SBA/3TCCA catalyst. This may be due
to the loss of long range ordering of the mesoporous
structure at higher concentration of TCCA. In order
to check the effect of solid supports on activity and
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
Figure 2. N2 adsorption-desorption isotherms and pore
size distribution curves (inset) of (a) SBA-15 and (b)
SBA/1.5TCCA and (c) SBA/2TCCA nanocomposites.

G Robin Wilson and Amit Dubey
Table 2. Variation of different catalysts on the conversion and the selectivity of
the product.
Selectivity (%)
Catalyst
aConversion (%) 3-acetyl indole 1-acetyl indole Other products
SBA-15
TCCA
0
0
0
0
10
0
0
0
75
45
40
38
55
60
50
45
50
90
90
90
85
60
70
65
40
10
10
10
15
20
15
10
SBA/1.5TCCA
SBA/1.5TCCA (2)b
SBA/1.5TCCA (3)b
SBA/2TCCA
SBA/3TCCA
SiO2/TCCA
0
20
15
25
Al2O3/TCCA
Reaction conditions: indole (1mmol), Ac2O (1 mmol), solvent: hexane, Temp.: 60◦C.
aconversion is based on the product.
brefers to 2nd and 3rd cycle.
selectivity of the 3-acylated product, SiO2/TCCA and products was also observed in polar solvents under sim-
Al2O3/TCCA catalysts were also tested for this reaction ilar reaction conditions. In contrast, slightly lower con-
under similar reaction conditions. The results showed version but very high selectivity (90%) of 3-acylated
almost same conversion but with very less selectivity product was observed using hexane as solvent without
of the 3-acylated product corroborating the necessity of formation of any side products or polymerized prod-
ordered mesoporous support. No conversion was noted ucts. These observations may be attributed to the non-
using SBA-15 materials inferring that certain acidity polar nature of hexane which maintains the optimum
is required to achieve the conversion of indole. These acidity in the reaction medium to stabilize the transi-
observations suggest that the higher selectivity of 3- tion state more to achieve the formation of 3-acylated
acylated product is due to large surface area, uniform product. The high selectivity of 3-acylated product and
dispersion of acidic centers (SBA/TCCA) and presence the lesser requirement of the catalyst are industrially
of ordered mesoporous support (SBA-15) which was beneficial under environmentally friendly conditions.
absent in homogeneous medium and other supports.
Hence, SBA/1.5TCCA is selected for further studies.
3.2c Effect of indole: acetic anhydride ratio: The
results of catalytic activity (Figure 4) showed that an
3.2b Effect of the solvent(s): Screening of different
solvents showed nearly the same conversion over all
the polar solvents with less selectivity of the 3-acylated
products (Table 3). The formation of some polymerized
increasing trend of the conversion with increase in the
ratio of indole: acetic anhydride. However, the selectiv-
ity of the 3-acylated indole was lowered at higher con-
centration of acetic anhydride due to the formation of
more side products. This may be due to excess acylat-
ing agent which enhances the rate of the reaction and
the unused acetic anhydride facilitates the formation of
over-acylated products (diacylated products and poly-
merized products). Therefore, indole: acetic anhydride
ratio (1:1) was selected for further studies.
Table 3. Influence of different solvents on conversion and
selectivity of the product over SBA/1.5TCCA catalyst.
Product selectivity (%)
Solvent
Conversion 3-Acetyl 1-Acetyl
Other
(%)
Indole
Indole products
CH2Cl2
CHCl3
CCl4
CH3OH
C2H5OH
CH3CN
CH3COCH3
C6H14
50
55
50
70
75
65
60
45
60
65
60
65
60
55
50
90
25
20
40
15
15
30
35
10
15
15
0
20
25
15
15
0
3.2d Variation of the catalyst weight: Results of
variation of the catalyst weight (Figure 5) showed that
the total conversion increased initially up to 250 mg of
the catalyst with (85–90%) selectivity of the 3-acylated
product. The conversion of the 3-acylated product was
reduced with further increase in the catalyst weight
up to 500 mg. This lowering of activity is because of
the blockage of the active sites and possible repulsion
between the adsorbed species on mesoporous silica
Reaction conditions: indole (1 mmol), Ac2O (1 mmol),
SBA/1.5TCCA (100 mg), Temp.: 60◦C.

Trichloroisocyanouric acid functionalized SBA-15
60
100
90
80
70
60
50
40
30
20
10
0
50
(a)
40
30
(b)
300C
400C
500C
600C
900C
20
(c)
10
0
-10
0
5
10
15
20
25
30
1:0.5
1:1
1:1.5
1:2
Time (h)
Indole : Ac2O (mmol)
Figure 6. Variation of the reaction temperature and time on
the conversion of the product over SBA/1.5TCCA catalyst.
(Reaction conditions: indole (1 mmol), Ac2O (1 mmol),
SBA/1.5TCCA (100 mg), solvent: hexane, Temp.: 60◦C).
Figure 4. Effect of indole to acetic anhydride ratio on (a)
conversion, (b) selectivity of 3-acetyl indole and (c) selec-
tivity of other products over SBA/1.5TCCA catalyst. (Reac-
tion conditions: SBA/1.5TCCA (100 mg), Solvent: hexane,
Temp.: 60◦C).
lowering in conversion at higher temperatures can be
due to the polymerization of the reactant (indole).
Hence, the optimum reaction temperature of 60◦C was
chosen for the reaction. Time on stream studies indi-
cated that total conversion of the products increased
with the increase in the reaction time and the maximum
conversion could be achieved within 12 h but the reac-
tion was monitored for 24 h to see the possibility of the
formation of any over-acylated or inter-converted prod-
ucts. No change in the conversion and selectivity was
noted inferring the stability of SBA/TCCA with time.
100
90
(b)
80
70
60
50
40
(a)
30
20
10
0
3.3 Reusability of catalysts
The reusability and heterogeneity of the catalyst were
tested with 250 mg of the catalyst with the PXRD anal-
ysis after the first cycle. The catalyst was filtered and
washed with methanol, water and finally with acetone.
-10
0
100
200
300
400
500
Weight of catalyst (mg)
Figure 5. Variation of weight of catalyst on (a) con-
version and (b) product selectivity (for 3-acetyl indole) The dried catalyst (120◦C) was tested again for the sec-
over SBA/1.5TCCA catalyst. (Reaction conditions: indole
ond cycle. A small reduction in the conversion (5–7%,
Table 2 and Figure S5 in SI) but with no change in
the selectivity of 3-acetyl indole was noted up to third
cycle. The mesoporous nature of the used catalyst was
confirmed by PXRD (Figure 1).
(1 mmol), Ac2O (1 mmol), solvent: hexane, Temp.: 60◦C).
surface. However, selectivity of 3-acylated product
remained constant irrespective of the catalyst weight.
Therefore, the catalyst weight of 100 mg and hexane as
solvent was selected for further studies.
4. Conclusions
3.2e Effect of the reaction temperature and time: SBA/TCCA nanocomposites were synthesized by co-
The effect of the reaction time (Figure 6) was car- condensation method and characterized for the acyla-
ried out at different temperatures (30–90◦C) for 24 h. tion of indole. Very high selectivity of 3-acetyl indole
The conversion of the products increased up to 60◦C (60-90%) was observed over SBA/TCCA catalysts
and then showed decreasing trend up to (90◦C). This using hexane as solvent at 60◦C. Different reaction

G Robin Wilson and Amit Dubey
8. Fujioka H, Murai K, Kubo O, Ohba Y and Kita Y 2007
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Supplementary information includes the PXRD pattern
of SBA/3TCCA (Figure S1), pore size distribution
curve of (a) SBA-15 and (b) SBA/1.5TCCA & (c) SBA/
2TCCA nanocomposites (Figure S2), N2 adsorption-
desorption isotherms (Figure S3) of SBA/3TCCA
nanocomposites, SEM images (Figure S4) of (a)
SBA-15 & (b) SBA/1.5TCCA nanocomposites and
reusability data (Figure S5) of SBA/1.5TCCA catalyst.
Supplementary information is available at www.ias.ac.
in/chemsci.
Acknowledgment
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AD thanks MANIT-Bhopal for help and support.
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