Well ordered two-dimensional SnSBA-15 catalysts synthesized with high levels of tetrahedral tin for highly efficient and clean synthesis of nopol

Article abstract of DOI:10.1016/j.apcata.2009.11.014

Highly ordered two-dimensional SnSBA-15 molecular sieve catalysts with different concentrations of tin species have been synthesized using Pluronic P123 as a structure directing agent under pH-adjusting direct hydrothermal method and have been used for synthesis of nopol by Prins condensation of β-pinene with paraformaldehyde. The influence of various reaction parameters on this reaction has been investigated. The Prins condensation reaction has also been carried out with different solvents for finding the best solvent with a good catalytic activity. In addition, the Prins condensation of β-pinene with paraformaldehyde in the presence of propyl cyanide with water has been carried out for the synthesis of nopol, and in this condition the catalytic activity of the catalyst used was not seriously affected. On the basis of all catalytic results, SnSBA-15(5) is found to be a highly active, water-resistant and recyclable heterogeneous catalyst for selective synthesis of nopol.

Full text of DOI:10.1016/j.apcata.2009.11.014

Applied Catalysis A: General 373 (2010) 186–191
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Well ordered two-dimensional SnSBA-15 catalysts synthesized with high levels of
tetrahedral tin for highly efficient and clean synthesis of nopol
M. Selvaraj *, Y. Choe
Department of Chemical Engineering, Pusan National University, Busan 609-735, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Highly ordered two-dimensional SnSBA-15 molecular sieve catalysts with different concentrations of tin
species have been synthesized using Pluronic P123 as a structure directing agent under pH-adjusting
Received 31 August 2009
Received in revised form 28 October 2009
Accepted 11 November 2009
Available online 17 November 2009
direct hydrothermal method and have been used for synthesis of nopol by Prins condensation of
b-
pinene with paraformaldehyde. The influence of various reaction parameters on this reaction has been
investigated. The Prins condensation reaction has also been carried out with different solvents for finding
the best solvent with a good catalytic activity. In addition, the Prins condensation of
b-pinene with
Keywords:
paraformaldehyde in the presence of propyl cyanide with water has been carried out for the synthesis of
nopol, and in this condition the catalytic activity of the catalyst used was not seriously affected. On the
basis of all catalytic results, SnSBA-15(5) is found to be a highly active, water-resistant and recyclable
heterogeneous catalyst for selective synthesis of nopol.
Mesoporous molecular sieves
pH-adjusting direct hydrothermal method
Two-dimensional SnSBA-15
Prins condensation
b
-Pinene conversion
ß 2009 Elsevier B.V. All rights reserved.
Nopol selectivity
Recyclability
Catalytic stability
1
. Introduction
-(7,7-Dimethyl-4-bicyclo[3.1.1]hept-3-enyl)ethanol, C11
Nowadays, in industries, mesoporous heterogeneous catalysts
are widely used to prepare large scale products with higher
2
H
18
O
selectivity. To prepare MCM-41 mesoporous catalysts with high
mild acidity under basic hydrothermal conditions, several good
candidates featured the loading of metal species on the silica pore
walls of MCM-41 [4–7]. These mesostructured catalysts have
several attractive properties such as the uniform pore size and
large surface area. Particularly, Villa de et al. reported that SnMCM-
41 catalyst prepared using chemical vapor deposition (CVD)
method can be used for obtaining nopol with a good selectivity by
is called nopol. It is widely employed to produce a variety of
products such as aromas, food additives, agrochemicals and
pharmaceuticals.
The nopol is synthesized [1] using three general methods: (1)
reacting a mixture of
acid solution at 120 8C, yielding nopyl acetate which is saponified
to nopol; (2) heating a mixture of -pinene and paraformaldehyde
b-pinene and paraformaldehyde in an acetic
b
in the presence of a small quantity of a homogeneous catalyst, such
the Prins condensation of b-pinene with paraformaldehyde [8].
as zinc chloride at 115–120 8C for several hours, yielding about 57%
Corma et al. reported the synthesis and characterization of Sn(IV)
Lewis acid centers incorporated into MCM-41 catalysts with
different concentration of tin species [9]. The mesoporous SnMCM-
41 catalysts are found to be very active for the Baeyer–Villiger
oxidation of ketones with hydrogen peroxide [10] and can also be
used for the preparation of nopol with a better selectivity [11].
Mesoporous iron phosphate is used for the synthesis of nopol with
a higher selectivity by the Prins condensation reaction using
different solvents [12]. Well ordered ZnAlMCM-41 catalysts are
used for obtaining highly selective synthesis of nopol by Prins
condensation reaction [13,14], and the ZnAlMCM-41 catalysts give
higher selectivity of nopol than microporous catalysts [14].
In 1998, mesoporous SBA-15 molecular sieves were first
synthesized newly with tunable uniform hexagonal channels
ranging from 5 to 30 nm [15]. The SBA-15 mesoporous materials
have thick framework walls and higher flexibility between the
nopol; (3) autoclaving paraformaldehyde and
b-pinene at 150–
2
30 8C for several hours yielding a substantial amount of pure
nopol. However, the monocyclic isomers and other side products
form along with nopol in all the above methods. Moreover, the
current need for heterogeneous catalysts in processing hydro-
carbons with high molecular weights has led researchers to better
catalyst systems. These limitations have been overcome with the
discovery of mesoporous catalysts [2] which have been widely
used for the production of large amounts of organic fine chemicals
by several catalytic reactions [3].
*
Corresponding author. Tel.: +82 51 510 2397; fax: +82 51 512 8563.
E-mail address: chems@pnu.edu (M. Selvaraj).
0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2009.11.014

M. Selvaraj, Y. Choe / Applied Catalysis A: General 373 (2010) 186–191
187
silanol groups than MCM-41. These materials also have higher
hydrothermal and thermal stabilities than MCM-41. This evidence
strongly suggests that the higher metal species can be incorporated
on the surface pore walls for the introduction of catalytic active
sites. The synthesized MSBA-15 (M = metal species) mesoporous
materials have been widely used as catalysts for the several
catalytic studies [16–18]. Particularly, Selvaraj et al. reported that
the high amounts of metal species such as Cr, Mn, Sn, Ga, Al and Nb,
have been successfully incorporated into SBA-15 mesoporous
materials under pH-adjusting direct hydrothermal (pH-aDH)
method for the introduction of catalytic active sites [19–26]. From
these characterization results, one can easily conclude that MSBA-
H O
(n =nHCl of 295) and stirred for 24 h at 313 K. The solid products
2
prepared with the molar gel composition of 1 TEOS:0.01–0.2
SnO :0.016 P123:0.43–5.2 HCl:127–210 H O, were recovered by
2
2
filtration, washed with water several times, and dried overnight at
373 K. Finally, the samples were calcined at 813 K in air at 6 h for
complete removal of the template.
The mesoporous SnSBA-15 molecular sieve with a nSi/nSn ratio
of 10 was synthesized using SA solution as a tin precursor under
pH-aDH method according to the above published procedure [22].
The synthesized catalyst was denoted as SnSBA-15(SA).
The mesoporous SiSBA-15 molecular sieve has been synthe-
sized without tin source under pH-aDH method according to the
above published procedure [22].
1
5 catalysts have higher hydrothermal stability than MMCM-41
catalysts.
Recently, there has been an increasing interest in developing
2.3. Synthesis of SnMCM-41(40)
catalytic reaction processes with minimal environmental threats
and maximal economical benefits. Hence, there has been a great
demand to develop highly selective heterogeneous catalysts with a
large number of Lewis acid sites for mild reaction conditions
without employing toxic materials. To the best of our knowledge,
there are currently no reports in the open literature on the
Mesoporous SnMCM-41(40) was synthesized using cetyltri-
methylammonium bromide as a template with a molar ratio, 1
2 2 2
SiO :0.025 SnO :0.25 CTMABr:100 H O under hydrothermal
condition according to the published procedure [6].
heterogeneous catalytic reaction for the Prins condensation of
b
-
2.4. Characterization
pinene over SnSBA-15 catalysts, with higher
and nopol selectivity.
b-pinene conversion
The calcined catalytic materials (SnSBA-15 and SnMCM-41(40))
were characterized using several sophisticated instrumental
techniques, according to the published procedure [6,22].
Herein we report the selective synthesis of nopol by the Prins
condensation of -pinene with paraformaldehyde using different
b
SnSBA-15 catalysts in the liquid phase reaction conditions. This
reaction has been conducted with different optimal conditions:
2.5. Prins condensation reaction
viz. amount of catalyst, temperature, time, ratios of reactant (b-
pinene/paraformaldehyde), solvents and recycles. This reaction
has been investigated in the presence of propyl cyanide with water.
The catalytic results of SnSBA-15 have been also correlated and
Liquid phase Prins condensation reaction for the synthesis of
nopol was performed using the required amount of the desired
reactants viz., b-pinene and paraformaldehyde in the presence of
compared in the production of nopol by Prins condensation of
pinene with paraformaldehyde.
b
-
a solvent (e.g. toluene) with an appropriate amount of catalyst
(e.g. SnSBA-15(5)) under a vigorous stirring thermostatted glass
vessel rector. After the reaction, the mixture was extracted with
acetone and analyzed by a gas chromatography (GC) with
authentic samples. The major product of nopol was again
confirmed with a Hewlett-Packard 6890 Gas Chromatograph
using an HP-Innowax polyethylene glycol capillary column
2
. Experimental
.1. Materials
All chemicals for the synthesis of tin-containing mesoporous
2
(30 m  320
meter equipped with a HP 5973 mass-selective detector. Very
trace amounts of byproducts viz., -pinene, camphene and
m
m  0.25
mm) and a quadrupole mass spectro-
catalytic materials viz. triblock copolymer poly(ethylene glycol)-
block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic
P123, molecular weight = 5800, EO20PO70EO20), cetyltrimethylam-
monium bromide (CTMABr), tetraethylorthosilicate (TEOS),
sodium metasilicate, hydrochloric acid (HCl), tin (IV) chloride
pentahydrate (SC) and tin (IV) acetate (SA), were purchased from
Aldrich Chemical Inc. All chemicals were used as received without
further purification. Millipore water was used in all experiments.
a
limonene were analyzed by GC with authentic samples. The
conversion and selectivity were calculated from the GC results
using standard formulas.
3. Results and discussion
All chemicals for the synthesis of nopol viz.
b
-pinene,
3.1. Synthesis of SnSBA-15 by pH-aDH method
paraformaldehyde, toluene, acetone, acetonitrile, methanol, n-
butyronitrile (PrCN), t-butanol, ethyl acetate and methyl ethyl
ketone (MEK), were also purchased from Aldrich Chemical Inc. and
used as received without further purification.
As already reported by Selvaraj and Kawi [22], mesoporous
SnSBA-15 synthesized under highly acidic condition (pH < 1) has a
low amount of Sn species incorporated into SBA-15, possibly due to
the high solubility of tin precursors, which hinder their incorpora-
tion into the silica walls of SBA-15. However the hydrolysis rate of
both TEOS, used as the silica precursor, and SC, used as the tin
precursor, may not be equally matched with each other. Therefore,
an attempt to increase the amount of Sn species (nSi/nSn ratio = 5 in
gel) incorporated in the framework of silica walls has been done in
this study by simply adjusting the gel pH using the aqueous HCl
2.2. Synthesis of SnSBA-15
The mesoporous SnSBA-15 molecular sieve materials with nSi
/
n
Sn = 5, 10, 30, 60 and 100, were synthesized under pH-aDH
method according to the previous published procedure [22]. In a
typical synthesis of SnSBA-15, 4 g of Pluronic P123 were stirred
with 25 ml of water to produce a clear solution. Thereafter, the
required amount of dilute HCl solution was added, and the solution
was again stirred for another 1 h to associate the hydronium ions
with the alkylene oxide units. Then, 9 g of TEOS and the required
amount of the desired SC solution were added, and the resulting
mixture was adjusted to pH > 2 using diluted HCl solution
solution prepared with a n
H O
=nHCl molar ratio of 295 (Fig. 1)
2
without changing the structural integrity of the parent SBA-15
materials. This mechanism is explained by Selvaraj and Kawi [22]:
the hexagonal phase is initially formed by adjusting the pH of the
synthesis gel between 0 and 1.6. Once the surfactant and silica
species have been protonated, the cationic silica species undergo

1
88
M. Selvaraj, Y. Choe / Applied Catalysis A: General 373 (2010) 186–191
Fig. 2. XRD powder patterns of calcined (a) SnSBA-15(60) and (b) SnSBA-15(5).
increasing tin species content (Table 1). On increasing the tin
species content, one finds that N adsorption measurements show
that the pore diameter of SnSBA-15 can be tuned from 87.8 to
Fig. 1. Synthesis of SnSBA-15 using various molar ratios of n
2
H O
=nHCl.
2
3
˚
8
9.3 A, with an increase of pore volume from 1.08 to 1.17 cm /g
2
partial condensation and form mesostructures through the
accompanied by the increase of surface area from 907 to 1048 m /
g. All these results are shown in Table 1. As shown in Fig. 3, the UV–
vis DRS spectra at 213 Æ 2 show that the Sn -ions are tetrahedrally
À
counteranions (X ) with the cationic surfactant species; as a
4+
result, the pH of the synthesis medium increases. When the pH of
the synthesis gel reaches around at ꢀ1.8 even after the addition of
the silicon source and a few hours of stirring, the Sn precursor is
then added while the pH of the synthesis gel is adjusted by aqueous
4+
coordinated to Si on the pore walls [22], as similarly reported for
SnMCM-41 materials [27]. TEM images show that SnSBA-15 has the
uniform pore diameter as shown in Fig. 4, and FE-SEM images show
the rope-like hexagonal mesoporous structure of SnSBA-15 [22].
Particularly, SnSBA-15(10) has higher mesostructured properties
than SnSBA-15(SA) synthesized with nSi/nSn = 10, as noted in Table 1.
The tin content in SnSBA-15(SA) is low because it has less production
of tin-oxo-complex. The formation of tin-oxo-complex depends upon
H O
HCl solution (n 2 =nHCl molar ratio = 295) to around 2.3, which is
above the zero net charge of silica. If the pH of the synthesis
medium rises above the zero net charge of silica, the silica species
are negatively charged, which enhances the interaction with the
+
+
Sn(OH)
3
species. With decreasing H concentration in the
À À À
synthesis gel, the concentration of tin hydroxyl species increases.
the counter ions (Cl and CH COO ). Moreover, CH COO slowly
3
3
Hence, the partially condensed silica species are able to form Sn–
reacts with the silica gel for the production of tin-oxo-complex in the
acidic hydrothermal condition, because of less attraction of the
surfactant head group [22].
+
O–Si bond with Sn(OH)
3
species at high pH. Moreover, the
structural order of the mesoporous material is maintained when it
is formed at a pH of ca. 2.4.
The synthesized SnSBA-15 materials have been characterized
3.2. Prins condensation of b-pinene
2
by ICP–AES, XRD, N adsorption, UV–vis DRS, FE-SEM, and TEM,
according to the published procedure [22]. ICP-AES studies show
that the higher amount of tin species is incorporated on the silica
pore walls. The elemental compositions of SnSBA-15 materials
synthesized with different nSi/nSn ratios are listed in Table 1. The
XRD patterns of calcined SnSBA-15 materials exhibit five well-
resolved peaks which are indexed to the (1 0 0), (1 1 0), (2 0 0),
The Prins condensation of
b
-pinene was carried out with
-pinene
1:2 mmol ratio of nb-pinene/nparaformaldehyde (4 mmol of
b
and 8 mmol of paraformaldehyde), as the reactants and 9 ml of
toluene, as a solvent at 363 K for 6 h over several catalysts viz.,
SiSBA-15, SnSBA-15(5), SnSBA-15(10), SnSBA-15(30), SnSBA-
15(60), SnSBA-15(100), SnSBA-15(SA) and SnMCM-41(40). The
catalytic results of this reaction are shown in Table 2. The order of
catalytic activity for selectivity of nopol is found as follows: SnSBA-
15(5) > SnSBA-15(10) ꢁ SnMCM-41(40) > SnSBA-
(2 1 0), and (3 0 0) reflections of the hexagonal p6mm space group
(Fig. 2). The observed d-spacing and unit cell parameter results
(Table 1) are well-matched with the hexagonal p6mm space group
[15]. The unit cell parameter and d-spacing values increase with
15(SA) > SnSBA-15(30) > SnSBA-15(60) > SnSBA-
Table 1
Physico-chemical properties and synthesis conditions of SnSBA-15.
Sample
n
H
O
=nHCl
n
Si/nSn ratio
d
1 0 0 ( A˚ )
a
o
( A˚ )
A
BET (m /g)
2
d
p
(A)
˚
V
p
(cm /g)
3
t
w
= a
o
À d
p
(A)
˚
2
a
Gel
Product
SiSBA-15
40
–
–
102.4
104.9
106.4
108.3
110.0
110.5
109.3
38.6
118.2
121.2
122.9
125.1
127.1
127.6
123.2
44.57
908
907
87.4
87.8
88.2
88.5
89.1
89.3
87.9
30.4
1.07
1.08
1.10
1.13
1.15
1.17
1.12
0.77
30.3
33.4
34.7
36.6
38.0
38.3
35.3
SnSBA-15(100)
SnSBA-15(60)
SnSBA-15(30)
SnSBA-15(10)
SnSBA-15(5)
SnSBA-15(SA)
SnMCM-41(40)
295
295
295
295
295
295
–
100
60
30
10
5
99.6
53.6
26.3
15.7
13.5
18.6
37.2
943
975
1001
1048
957
10
40
1080
14.17
a
0
a
, unit cell parameter; ABET, specific surface area; d
p
, pore diameter; V
p
, pore volume; pore wall thickness (t
w
) = unit cell parameter (a
0
) À pore diameter (d
p
).
The results of nSi/nSn ratios in the products are determined by ICP-AES.

M. Selvaraj, Y. Choe / Applied Catalysis A: General 373 (2010) 186–191
189
Table 2
Condensation of
b-pinene with paraformaldehyde over various types of
a
mesoporous Sn-containing SBA-15 catalysts .
b
Catalysts
Conversion of
-pinene (%)
Selectivity of
nopol (%)
TON
b
SiSBA-15
15.4
99.3
92.8
85.4
63.4
49.5
94.8
99.3
99.4
99.5
49.4
49.2
49.3
91.3
–
60.0
95.4
90.5
80.6
76.5
71.5
80.8
95.5
95.2
95.4
71.5
71.3
71.4
82.3
–
–
SnSBA-15(5)
SnSBA-15(10)
SnSBA-15(30)
SnSBA-15(60)
SnSBA-15(100)
SnSBA-15(SA)
1473
1596
2457
3735
5876
1931
1473
1474
1476
5455
5433
5444
1734
–
c
Recycling 1
c
Recycling 2
c
Recycling 3
d
Recycling 1
d
Recycling 2
d
Recycling 3
SnMCM-41(40)
Absence of catalyst
a
Reaction conditions: 0.1 g of catalyst; 4 mmol of
b-pinene and 8 mmol of
paraformaldehyde (1:2 mmol ratio) in the presence of 9 ml of toluene were
introduced into a reactor; reaction temperature, 363 K; reaction time, 6 h.
b
Turnover number (TON) =mole of
b
-pinene converted/mol of active site.
c
Recycling SnSBA-15(5) after exhaustive washing with acetone 40 8C and drying
at 120 8C overnight.
d
Recycling SnSBA-15(100) after exhaustive washing with acetone 40 8C and
drying at 120 8C overnight.
The
b-pinene conversion and nopol selectivity in SnSBA-15(5)
are also significantly higher as compared to SnMCM-41(40)
catalyst because SnSBA-15(5) has higher structural and textural
properties than SnMCM-41(40). It is interesting to note that the
well ordered mesostructured catalyst also plays a very important
catalytic role in the production of nopol with a good selectivity.
Fig. 3. UV–vis DR spectra of calcined (a) SnSBA-15(5) and (b) SnSBA-15(100).
1
5(100) > SiSBA-15. SnSBA-15(5) exhibits the best performance
Although SnSBA-15(100) has the b-pinene conversion and
with a
5.4%. Very trace amounts of byproduct selectivities viz.,
camphene and limonene form during the -pinene isomerization
in the presence of weak and strong Lewis acid sites of SnSBA-15
catalysts (not shown). SnSBA-15(5) has higher -pinene conver-
b
-pinene conversion of 99.3% and a nopol selectivity of
nopol selectivity values that are much lower than those of the
other SnSBA-15 catalysts, it has a higher turnover number (TON)
than the other SnSBA-15 and SnMCM-41(40). This evidence
strongly supports the conclusion that a low amount of tetrahedral
tin species containing SBA-15 gives good catalytic activity for this
reaction. However, SnSBA-15(5) gives superior catalytic activity
than other SnSBA-15 catalysts because of the higher tetrahedral tin
coordinated into silica surfaces.
For recyclable studies, SnSBA-15(5) and SnSBA-15(100) were
reused. Initially, the used mesoporous SnSBA-15 catalysts usually
suffer from lower catalytic activity values, and hence the catalysts
need to be regenerated by calcination. The recycled catalysts such
as SnSBA-15(5) and SnSBA-15(100) were washed four times with
acetone and dried at 120 8C overnight in order to remove the
organics and unreacted paraformaldehyde. No losses of the
catalytic activities and tin species (confirmed by ICP-AES, not
shown) on the inner side surface of pore walls in the SnSBA-15(5)
9
a
-pinene,
b
b
sion and nopol selectivity than other SnSBA-15 catalysts. The
observed higher activity of SnSBA-1(5) is tentatively ascribed to its
4
+
two-dimensional space and high loading of tetrahedral Sn
species on the inner pore walls of SBA-15, resulting in a higher
4+
number of accessible active sites because the Sn -ion produces a
high number of Lewis acid sites on the inner surface of pore walls.
and SnSBA-15(100) are observed after three recycles. Their
b-
pinene conversion and nopol selectivity remain constant with each
cycle (Table 2), while these catalysts have mostly the similar TON
values. Especially, for the Prins condensation reaction, the SnSBA-
15(5) is a more active and suitable catalyst than SnSBA-15(100)
due to greater tin species incorporated on the silica walls. This
reaction has also been performed in the absence of catalyst. There
is no formed nopol product (Table 2). Since SnSBA-15(5) is the
most active catalyst in this work, it was chosen for finding the best
optimal conditions.
To find the best optimal conditions for highly selective
synthesis of nopol over SnSBA-15(5), the Prins condensation
reaction was carried out using the different optimal conditions
such as amount of catalyst, reaction temperature, reaction time,
Fig. 4. TEM micrograph of calcined SnSBA-15(5).
ratios of reactant (b-pinene:paraformaldehyde) and solvents.

1
90
M. Selvaraj, Y. Choe / Applied Catalysis A: General 373 (2010) 186–191
Table 3
a
An optimal condensation of
b
-pinene with paraformaldehyde over SnSBA-15(5) catalyst .
S. No.
Amount of SnSBA-15(5) (g)
Amount of toluene (ml)
Temp. (K)
Time (h)
Conversion of
b-pinene (%)
Selectivity. of nopol (%)
TON
1
2
3
4
5
6
7
8
9
0.1
0.15
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
9
9
363
363
363
363
363
353
333
313
363
363
363
363
363
363
6
6
99.3
99.5
99.7
55.4
72.3
85.3
67.3
38.9
96.5
99.6
99.7
99.8
56.8
67.5
95.4
95.7
96.7
51.4
68.1
79.3
62.4
33.4
82.0
95.7
96.5
98.9
51.4
62.7
1473
984
9
6
739
5
6
822
14
9
2
1073
1265
998
6
9
6
9
6
577
9
3
1431
1477
1479
1480
843
1
1
1
1
1
0
1
2
3
4
9
8
9
12
24
6
9
b
0.1
c
9
0.1
9
6
2003
a
b
c
Reaction conditions: reactants, 4 mmol of
b
-pinene and 8 mmol of paraformaldehyde (1:2 mmol ratio).
-pinene and 4 mmol of paraformaldehyde (1:1 mmol ratio).
-pinene and 4 mmol of paraformaldehyde (2:1mmol ratio).
4
8
mmol of
mmol of
b
b
The Prins condensation reaction was conducted with different
amount of SnSBA-15(5) catalyst using 1:2 mmol ratio of nb-pinene
paraformaldehyde in the presence of 9 ml of toluene at 363 K for 6 h,
as shown in Table 3 (S. No. 1–3). When the amount of catalyst is
varied from 0.1 to 0.2 g, the -pinene conversion and nopol
selectivity slightly increase with the decrease of TON. This
evidence strongly supports the conclusion that a low tetrahedral
tin-containing SBA-15 gives good nopol selectivity in the similar
reaction condition.
different times using 1:2 mmol ratio of nb-pinene/nparaformaldehyde
/
and 9 ml of toluene at 363 K over SnSBA-15(5), as shown in Table 3
(S. No. 1, 9–12). At 3 h, the b-pinene conversion and nopol
selectivity reach 96.5% and 82.0%, respectively. The nopol
selectivity increases from 82% to 95.7% when the reaction time
is increased from 3 to 8 h. TON of this reaction also increases with
increasing the time from 3 to 8 h. When the reaction time is
n
b
increased from 12 to 24 h, the
b-pinene conversion and nopol
selectivity could not be largely varied, and also the TON could not
be changed very much. Based on the catalytic results, one can
clearly observe that a low (3 h) reaction time is also good for
For this Prins condensation reaction, the amount of solvent
(
toluene) was varied with 1:2 mmol ratio of nb-pinene/nparaformalde-
hyde at 363 K for 6 h over SnSBA-15(5), as shown in Table 3 (S. No. 1,
–5). When this reaction carried out with 9 ml of toluene under the
similar reaction condition, it produces the -pinene conversion of
9.3% and nopol selectivity of 95.4%. But the conversion and
obtaining a higher
b-pinene conversion. However, the nopol
4
selectivity majorly decreases. Overall, it is concluded that the
optimum time for the highly selective synthesis of nopol is 6 h.
To find the best reactant ratio for the highly selective synthesis
of nopol, the Prins condensation reaction was conducted with
different mmol ratio of nb-pinene/nparaformaldehyde in the presence of
9 ml of toluene at 363 K and 6 h over SnSBA-15(5) as shown in
Table 3 (S. No. 1, 13, 14). When the catalytic reaction is carried out
with 1:1 and 2:1 mmol ratios of nb-pinene/nparaformaldehyde, it shows
b
9
selectivity decrease during this reaction with 5 ml of toluene
because the paraformaldehyde may not be completely dissolved
and reacted with
pinene, camphene and limonene form during the isomerization of
-pinene in the presence of weak or strong acid sites (not shown).
Moreover, the -pinene conversion and nopol selectivity decrease
b-pinene. Additionally, the trace amount of a-
b
b
a decrease of b-pinene conversion and nopol selectivity in 1:1 with
with the increase of TON, when the Prins condensation reaction is
carried out with 14 ml of toluene for 2 h. However, it might be
the decrease of TON but it produces a slight increase of conversion
and selectivity in 2:1 with the increase of TON. In these ratios, one
reason may be that the quantity of reactants is insufficient to mix
with each other as a homogeneous solution. Although 2:1 mmol
noted that the
68.1%) at 2 h are better than those of standard reactions. Then, the
obtained results clearly show that the -pinene conversion and
b-pinene conversion (72.3%) and nopol selectivity
(
b
ratio has high TON, it gives much less b-pinene conversion and
nopol selectivity were not affected during this reaction with higher
amount of solvent (up to 15 ml).
For the production of highly selective synthesizes of nopol, the
Prins condensation reaction was carried out in different reaction
temperatures with 1:2 mmol ratio of nb-pinene/nparaformaldehyde and
nopol selectivity than 1:2 mmol ratio. Moreover, in 1:2 mmol ratio
the reactant mutually react with each other on the catalytic
surface. This evidence strongly supports that 1:2 mmol ratio is the
best ratio for obtaining higher nopol selectivity.
The Prins condensation was conducted with different solvents,
time and temperature using 1:2 mmol ratio of nb-pinene/nparafor-
maldehyde over SnSBA-15(5), as shown in Table 4. Toluene and ethyl
acetate are apolar aprotic solvents. When these solvents are used
9
1
3
ml of toluene for 6 h over SnSBA-15(5), as shown in Table 3 (S. No.
, 6–8). When the reaction temperature is decreased from 363 to
13 K, the
b-pinene conversion and nopol selectivity decrease
with the decrease of TON. The reason may be that the SnSBA-15(5)
with its mild Lewis acid sites may not be supported to react
at 353 K and 2 h, the b-pinene conversion and nopol selectivity
increase because of increasing the catalytic activity. Toluene has
higher conversion and selectivity than ethyl acetate. Moreover, a
small amount of nopyl acetate forms during the reaction with ethyl
acetate. t-Butanol, MEK and acetonitrile are dipolar solvents; when
(
reactant of
b
-pinene and paraformaldehyde) at the lower reaction
-pinene conversion and nopol
temperature. Moreover, the
b
selectivity also decrease with increasing the number of isomers
and other side products when the reaction temperature is
increased above 363 K (not shown). This evidence strongly
supports the conclusion that the lower (<313 K) or higher
these solvents are used at 353 K and for 2 h, the
b-pinene
conversion increases in this order: MEK > acetonitrile > t-butanol;
nopol selectivity also increases in this order: acetonitrile > t-
(
>363 K) reaction temperature is not suitable to produce highly
butanol > MEK. It might be noted that the converted b-pinene in
selective synthesis of nopol.
To find optimum reaction time for the highly selective synthesis
of nopol, we carried out the Prins condensation reaction with
the two solvents viz. t-butanol and acetonitrile, is almost
completely formed as nopol, but in MEK a small amount of
byproducts form with nopol. Methanol is a protic solvent, which

M. Selvaraj, Y. Choe / Applied Catalysis A: General 373 (2010) 186–191
191
Table 4
a
Condensation of
b
-pinene and paraformaldehyde with different solvents over SnSBA-15(5) .
S. No.
Name of solvents
Temp. (K)
Time (h)
Conversion of
b-pinene (%)
Selectivity of nopol (%)
TON
1
2
3
4
5
6
7
8
9
Acetonitrile
Methanol
353
353
353
353
353
353
363
373
373
373
2
2
2
2
2
2
6
8
8
8
65.4
10.5
50.4
87.7
83.4
88.5
96.5
98.4
95.4
98.3
97.8
6.5
970
156
t-Butanol
91.3
78.9
89.5
81.5
92.3
94.5
93.4
94.2
748
Ethyl acetate
MEK
1301
1237
1313
1431
1460
1416
1459
Toluene
PrCN
PrCN
PrCN + 50 mg of water
PrCN + 20 mg of water
1
0
a
Reaction conditions: catalyst, 0.1 g of SnSBA-15(5); reactants, 4 mmol of
b-pinene and 8 mmol of paraformaldehyde (1:2 mmol ratio); 9 ml of solvent.
gives low
h because of less support with reactants and on the surfaces of
the catalyst. When the Prins condensation reaction is conducted at
63 K and 6 h in the presence of PrCN, the -pinene and nopol
b
-pinene conversion and nopol selectivity at 353 K and
Acknowledgment
2
This work was supported for 2 years by a Pusan National
University Research Grant.
3
b
selectivity could not be largely changed, but the conversion and
nopol selectivity increase when the reaction is carried out at higher
temperature (373 K) and time (8 h). It is found that the catalytic
activity slowly improves during the reaction with this solvent. On
the basis of catalytic activity, one can clearly observe that toluene
is the best solvent among the other solvents, for getting higher
nopol selectivity in a low reaction time and temperature.
To find the stability of SnSBA-15(5) against water, the Prins
condensation reaction was carried out with different amounts of
water using 1:2 mmol ratio of nb-pinene/nparaformaldehyde and 9 ml of
PrCN at 363 K and 8 h over SnSBA-15(5), as shown in Table 4 (S. No.
References
[
[
1] J.P. Bain, J. Am. Chem. Soc. 68 (1946) 638–641.
2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992)
710–712.
[3] A. Corma, Chem. Rev. 97 (1997) 2373–2420.
[
[
4] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, V. Krishnasamy, K.B. Lal, J.
Mol. Catal. A: Chem. 186 (2002) 173–186.
5] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, K.B. Lal, Appl. Catal. A: Gen.
242 (2003) 347–364.
[
[
[
6] M. Selvaraj, B.H. Kim, T.G. Lee, Chem. Lett. 34 (2005) 1290–1291.
7] Y.J. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56–77.
8] A.L. Villa de, P.E. Alarcon, C.M. de Correa, Chem. Commun. (2002) 2654–2655.
9
, 10). The b-pinene conversion and nopol selectivity increase
when the water content is increased in the range of 50–100 mg.
This evidence strongly shows that the catalytic activity and
stability of SnSBA-15 could not be affected when the Prins
condensation is carried out with water suitable content of reactant.
[9] A. Corma, M.T. Navarro, L. Nemeth, M. Renz, Chem. Commun. (2001) 2190–2191.
10] A. Corma, S. Iborra, M. Mifud, M. Renz, J. Catal. 234 (2005) 96–100.
11] A. Corma, M. Renz, ARKIVOC (2007) 40–48.
[
[
[
12] U.R. Pillai, E. Sahle-Demessie, Chem. Commun. (2004) 826–827.
[13] M. Selvaraj, B.R. Min, Y.G. Shul, T.G. Lee, Micropor. Mesopor. Mater. 74 (2004)
43–155.
1
[
[
14] M. Selvaraj, S. Kawi, J. Mol. Catal. A 246 (2006) 218–222.
15] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky,
Science 279 (1998) 548–552.
4
. Conclusions
The pH-aDH method is an environmentally friendly method, and
[16] S. Wu, Y. Han, Y.-C. Zou, J.-W. Song, L. Zhao, Y. Di, S.-Z. Liu, F.-S. Xiao, Chem. Mater.
6 (2004) 486–492.
1
it has been successfully used for two-dimensional mesostructured
SnSBA-15 catalysts with very high loadings of tetrahedral tin
species. The SnSBA-15 catalysts have been successfully used in the
[17] Y. Yue, A. Gideon, J.-L. Bonardet, N. Melosh, J.-B. D’Espinose, J. Fraissard, Chem.
Commun. (1999) 1967–1968.
[18] W.-H. Zhang, J. Lu, B. Han, M. Li, J. Xiu, P. Ying, C. Li, Chem. Mater. 14 (2002) 3413–
421.
3
selective synthesis of nopol by Prins condensation of b-pinene with
[
[
19] M. Selvaraj, S. Kawi, Chem. Mater. 19 (2007) 509–519.
20] M. Selvaraj, S. Kawi, Micropor. Mesopor. Mater. 101 (2007) 240–249.
paraformaldehyde. The nopol selectivity in SnSBA-15(5) is higher
than that of other SnSBA-15 because of the higher number of
tetrahedral tin atoms coordinated onto the silica pore walls. On the
basis of all catalytic experimental results, SnSBA-15(5) is found to be
a highly active, reusable, water-resistant and environmentally
friendly heterogeneous catalyst for the highly selective synthesis of
nopol. For the Prins condensation reaction, well mesostructured
SnSBA-15(5) catalyst has superior catalytic activity than the other
tin species that contain mesoporous catalysts.
[21] M. Selvaraj, T.G. Lee, J. Phys. Chem. B 110 (2006) 21793–21802.
[
[
22] M. Selvaraj, S. Kawi, J. Mater. Chem. 17 (2007) 3610–3621.
23] M. Selvaraj, S. Kawi, D.-W. Park, C.S. Ha, Micropor. Mesopor. Mater. 117 (2009)
586–595.
[24] M. Selvaraj, S. Kawi, Catal. Today 131 (2008) 82–89.
[
[
25] M. Selvaraj, S. Kawi, Stud. Surf. Sci. Catal. 165 (2007) 219–222.
26] M. Selvaraj, S. Kawi, D.-W. Park, C.S. Ha, J. Phys. Chem. C 113 (2009) 7743–
7749.
[27] T.K. Das, K. Chaudhari, A.J. Chandwadkar, Sivasanker, J. Chem. Soc., Chem. Com-
mun. (1995) 2495–2496.