Silica-bonded vanadic acid [SiO2-VO(OH)2] as a heterogeneous and recyclable catalyst for thiocyanation of organic compounds in aqueous media at room temperature

Article abstract of DOI:10.1016/j.catcom.2012.04.015

Silica vanadic acid (in which vanadium oxytrichloride has been supported on silica) is applied as an efficient, reusable and heterogeneous catalyst for the thiocyanation of heterocycles, substituted anilines (electron-rich and electron-deficient) and N-su

Full text of DOI:10.1016/j.catcom.2012.04.015

Catalysis Communications 26 (2012) 34–38
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Short Communication
Silica-bonded vanadic acid [SiO2–VO(OH)2] as a heterogeneous and recyclable
catalyst for thiocyanation of organic compounds in aqueous media
at room temperature
a
b
a
Ardeshir Khazaei a, , Mohammad Ali Zolfigol , Maliheh Safaiee , Mohammad Mokhlesi ,
⁎
Elmira Donyadari a, Morteza Shiri c, Hendrik Gerhardus Kruger d
Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 6517838683, Hamedan, Iran
Department of Chemistry, Payame Noor University, P.O. Box 19395–4697, Tehran, Iran
Department of Chemistry, College of Science, Alzahra University, Vanak 1993893973, Tehran, Iran
a
b
c
d
School of Chemistry, University of KwaZulu-Natal, Durban 4001, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Silica vanadic acid (in which vanadium oxytrichloride has been supported on silica) is applied as an efficient,
reusable and heterogeneous catalyst for the thiocyanation of heterocycles, substituted anilines (electron-rich
and electron-deficient) and N-substituted aromatic amines using hydrogen peroxide in the water as a solvent
at room temperature. The catalyst can be easily recovered and reused for ten reaction cycles without consid-
erable loss of activity.
Received 13 February 2012
Received in revised form 6 April 2012
Accepted 12 April 2012
Available online 21 April 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Silica vanadic acid
Hydrogen peroxide
Heterogeneous
Thiocyanation
Water
1. Introduction
has been significantly applied in industry. Among them, V-containing
mesoporous materials are constitutes a very important group of catalysts
The thiocyanation reaction is one of the most useful carbon–sulfur
bond forming reactions. Thiocyanate is a versatile synthon which can
be readily transferred to other functional groups. Therefore, it is
important to find the new, safe, fast and useful method for the syn-
thesis of aryl thiocyanates. Various methods have been reported for
the thiocyanation of aromatic and heteroaromatic systems [1–3]
such as, N-thiocyanatosuccinimide [4], iodine/methanol [5], diethyl
azodicarboxylate [6], K-10 clay [7], I2O5 [8], pentavalent iodine [9],
IBX [10], Selectfluor™ [11]. However, most of the reported methods
for the synthesis of aryl thiocyanates are associated with some draw-
backs such as the use of toxic solvent, long reaction times, the use of
large amount of catalyst and low yields. Thus, search for finding an
efficient, inexpensive and nonpolluting method for the synthesis of
this class of compounds is still of practical importance.
that are industrially used. They are perspective catalysts for H2O2-based
oxidations. These catalysts are known to catalyze hydrocarbon oxida-
tions, oxidation of o-xylene to phthalic anhydride, ammoxidation of
alkyl aromatics to aromatic nitriles, selective catalytic reduction (SCR)
of NOx emissions with NH3 to N2 and sulfuric acid manufacture, oxida-
tion of methanol and carbon monoxide oxidation [12–18].
Water is a desirable solvent for chemical reactions for reasons of
cost, safety, and environmental concerns. Unfortunately, water is
still not commonly used as a sole solvent for organic reactions, be-
cause most organic compounds do not dissolve in water as a consid-
erable amount, and solubility is generally considered a prerequisite
for reactivity. Nevertheless, the currently burgeoning field of organic
synthesis in aqueous media encompasses a large family of reactions.
Thus, development of an efficient and convenient synthetic method-
ology in water will be one of the most suitable strategies [19,20].
In continuation of our ongoing research program on the applications
of heterogeneous and silica supported catalysts for organic functional
group transformations [21–23], we found vanadium oxytrichloride (I)
as a powerful oxidizing agent [24]. However, many reported methods
in which vanadium oxytrichloride has been used as catalyst or reagent
are associated with the some disadvantages such as toxicity of VOCl3,
production of harmful HCl, by-products formation, difficulty of the
Oxidation over metal catalysts using eco-friendly oxidants such as
hydrogen peroxide and molecular oxygen are preferred for environmen-
tal and economical benefits. Along this line, the design of soluble and
heterogenized metal catalysts represents a rapidly growing field that
⁎
Corresponding author.
E-mail address: akhazaei_1326@yahoo.com (A. Khazaei).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.04.015

A. Khazaei et al. / Catalysis Communications 26 (2012) 34–38
35
Scheme 1. Synthesis of silica vanadic acid (SVA).
procedure, and no agreement with the green chemistry protocols.
Therefore, it is important to find the new and useful method for using
vanadium oxytrichloride under safe condition. In the previous reports,
vanadium oxytrichloride (I) was reacted with silica gel to give silica
vanadic acid (III) during an easy and clean reaction (Scheme 1) [25].
In fact, when vanadium oxytrichloride was supported on silica, it was
easy to be applied as a catalyst or reagent in reactions. None of the
above disadvantages were seen in this catalyst.
evaporated and the residue (the red-brown powder (II) obtained after
the grafting process) was filtered, washed with dry n-hexane several
times to remove unreacted vanadium oxytrichloride, dried under vacu-
um, and stirred in the air for 72 h to promote the hydrolysis of V-Cl
bonds. At this time, the red-brown powder changed to dark green
powder (III) slowly (Scheme 1). Typical loading of vanadium was
determined using X-ray fluorescence (XRF) and shows a loading 8
0.1 mmol g−1
.
2. Experimental
2.3. Characterization of the catalyst
2.1. General
The XRD patterns of catalysts show partially amorphous nature of
silica Fig. 1(a). The EDS analysis of the silica vanadic acid is presented
in Fig. 1(b). In the EDS spectrum of the as-synthesized sample, the
peaks of Si and V are obviously observed. Thermal gravimetric analy-
sis (TGA and DTG) of SVA was also studied. The corresponding
diagrams are shown in (Fig. 1(c, d)). Analysis of these diagrams
strongly proposed that silica vanadic acid structure was stable and
no further weight loss occurs below 200 °C.
Amines, Hydrogen peroxide (H2O2, 30%) and solvents were pur-
chased from Merck and used without further purification. Vanadium
oxytrichloride (VOCl3, 28.5%) were purchased from fluka. Silica gel
(100–200 mesh) was performed on Merck. All the other solvents
and chemicals were obtained from commercial sources and purified
using standard methods.
2.2. Preparation of the catalyst
The IR spectra of this catalyst showed some peaks in region of
471, 800, 1097, 1632 and 3415 cm−1. The band at 804 cm−1 is char-
acteristic for a symmetric Si–O–Si stretching vibration, and the band
at 1080–1220 cm−1 is characteristic for an asymmetric Si–O–Si
stretching vibration (Fig. 2) [25].
In a 100 ml round-bottomed flask, n-hexane (25 mL) and silica gel
(4.0 g) were stirred for 10 min and then vanadium oxytrichloride
(4.0 g) was dissolved in n-hexane (25 mL) and added drop wise to silica
gel suspension (15–20 min). After addition of vanadium oxytrichloride,
the red-brown mixture was stirred for 10 h. Then, solvent was
The morphology of the silica vanadic acid was observed using an
SEM. Typical SEM images are shown in (Fig. 3).
Fig. 1. (a). XRD of catalyst, (b). EDS spectra of silica vanadic acid, (c). TGA and (d). DTG of catalyst.

36
A. Khazaei et al. / Catalysis Communications 26 (2012) 34–38
Fig. 2. IR spectra of catalyst.
Then, optimization of solvent was carried out. Water, methanol,
THF, ethyl acetate, CH2Cl2 and acetonitrile were used as solvents
and surprisingly the same results were obtained in the case of
water, methanol and acetonitrile. In other organic solvents reaction
were proceeded in longer reaction time. So, the water was chosen
as a solvent because it is safe, non-toxic and environmentally benign.
The stoichiometry of the reactants was also varied. A ratio of 1:3:3
(indole:KSCN:H2O2) was found to be the most suitable, and decreas-
ing the amount of H2O2 or potassium thiocyanate increased the reac-
tion time and lowered the yield.
In general, this reaction provided good-to-excellent yields with all
the substrates tested. under the optimal reaction conditions, various
organic compounds including indole derivatives, substituted anilines
(electron rich and electron deficient), N-substituted aromatic amines
were reacted with hydrogen peroxide and KSCN in the presence of
SiO2–VO(OH)2 to afford the corresponding organo thiocyanates in
high to excellent yields within relatively short reaction times. The
reactions were carried out efficiently within 0.5–2 h, and the products
were obtained in 82–96% yields (Table 2).
2.4. Typical procedure for the preparation of 3-thiocyanato indole
A suspension of indole (1 mmol, 0.117 g), potassium thiocyanate
(0.228 g, 3 mmol) and SVA (10 mg, 0.078 mol% of V) in H2O (2 mL)
was stirred at room temperature for 10 min. Then, a solution of
H2O2 30% (0.34 g, 3 mmol) was added dropwise (5–7 min). The pro-
gress of the reaction was monitored by TLC. After completion of the
reaction, the catalyst was filtered and washed with CH3CN (10 mL).
The solvent was evaporated and the pure products were obtained
by chromatography on a short column of silica gel. The products
were characterized by 1H and 13C NMR, FT-IR spectroscopy and
mass spectrometry.
3. Result and discussion
The catalytic activity of the catalyst was evaluated in thiocyanation
of organic compounds in the water as solvent at room temperature.
We began with an investigation of the conversion of indole into the
corresponding indole thiocyanate using SVA/H2O2/KSCN as a model
reaction and different reaction parameters such as amount of catalyst
and nature of solvent were studied (Scheme 2).
First of all, the reaction of indole with KSCN and H2O2 was tested
using different amounts of silica vanadic acid at room temperature
(Table 1, Entries 1–5). As it can be seen in Table 1, the best amount
of the catalyst was 0.01 g (0.078 mol% of V). Also, in the absence of
catalyst, the reaction was field.
Scheme 2. Thiocyanation of indole to indole-3-thiocyanate.
Table 1
Thiocyanation of indole in various reaction conditionsa.
Entry Amount of catalyst Solvent H2O2
KSCN
Time Yield
TOF
c
(g)
(mmol) (mmol) (h)
(%)b
(h−1
)
1
2
3
4
5
6
7
8
–
H2O
H2O
H2O
H2O
H2O
EtOAc
THF
CH2Cl2
CH3OH
CH3CN
H2O
3
3
3
3
3
3
3
3
3
3
2
1.5
3
3
3
3
3
3
3
3
3
3
3
3
3
5
5
5
Sluggish
25
38
–
0.001
0.005
0.01
0.05
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
641
194
1641
324
0.75 96
0.75 95
2.5
3
77
75
30
39.4
320.5
76.9
5
9
0.75 95
1623
1230
148.5
10
11
12
13
1
5
5
5
96
58
25
81
H2O
H2O
64
207.6
2.5
a
Reaction condition: indole (1 mmol), H2O2 (30%) (3 mmol), KSCN (3 mmol),
solvent 2 mL, room temperature, stir.
b
Isolated yields.
Turnover frequencies [mol product/mole V/time (h)].
c
Fig. 3. SEM image of catalyst.

A. Khazaei et al. / Catalysis Communications 26 (2012) 34–38
37
Table 3
For recycling test, the thiocyanation of indole was chosen as a
model substrate for checking the reusability of silica vanadic acid.
After completing each of the reactions, the solvent was evaporated
off under the reduced pressure. The product was removed by
extracting with ether from the reaction mixture and the reaction
was charge again with fresh substrate and H2O2 30%. The recovered
catalyst was reused ten times in the thiocyanation of indole (1a)
with H2O2 (Table 3).
To assess the capability and efficiency of our catalyst with respect
to the reported catalysts for thiocyanation reaction, the results of the
application of these catalysts for the preparation of some organo thio-
cyanate are tabulated in Table 4. As shown in Table 4, higher yield and
shorter reaction time were obtained using silica vanadic acid.
The catalytic activity of SVA in 10 cycles for the reaction of indole with hydrogen per-
oxide and KSCN.
Run
1
2
3
4
5
6
7
8
9
10
85
Yield (%)
96
93
90
92
90
94
91
89
88
Based on a variety of studies, a mechanism as depicted in
Scheme 3 has been proposed [23]. The catalytic cycle involves, as fun-
damental step, the formation in the active site of a peroxovanadium
species (IV) [26–28], which is a much stronger oxidant than H2O2
(as shown in Table 1, entry 1). The peroxoderivative oxidizes the
(SCN)− to produce thiocyanium ion (S–CN)+ [10,29–32]. In the last
Table 2
Thiocyanation of various organic compounds under mild conditions.
Entry
Product
Time
(h)
Yield
(%)a
Entry
9
Product
Time
(h)
Yield
(%)a
1
0.75
96
1.5
84
2
1.5
93
10
2
82
3
1.25
84
95
11
12
1.5
1.5
88
80
4
0.75
5
0.5
90
92
13
14
1.25
92
89
6
1
1
7
8
1
85
86
15
2
90
88
1.25
16
1.5
a
Isolated yields.

38
A. Khazaei et al. / Catalysis Communications 26 (2012) 34–38
Table 4
has many advantages, such as chemoselectivity, easy work-up and
short reaction time.
Comparison of the thiocyanation of indole using the reported catalysts versus silica
vanadic acid.
Entry
System
Time
(min)
Yield % [Ref.]
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4. Conclusion
In conclusion, we have shown that the silica-based vanadium cat-
alyst III can be easily prepared from commercially available materials.
This catalyst can be considered as a heterogeneous version of vanadi-
um oxytrichloride and efficiently affects the thiocyanation of a variety
of organic compounds in room temperature. The described method