Nano-titania/Preyssler acid–catalysed synthesis of chromens and pyrano[2,3-c]pyrazoles
97
2
.4 Recycling potential of the catalyst
These results are consistent with the BET-specific
surface areas of titania of different particle sizes, and
indicate that increasing the surface area of titania as cata-
lyst can cause the improvement of reaction rate and
yield as well. The BET results obtained from different
particle sizes of nano-titania are shown in table 1.
In order to study the stability and potential reusabil-
ity of the catalyst, the reaction mixture was centrifuged
after the completion. The separated solid was washed
with ethyl acetate (2 × 5 mL) and dried under reduced
◦
pressure (20 C). The recovered catalyst was reused for
three consecutive fresh runs without any significant loss
of activity (see tables 3 and 4, entry 1).
3
.2 Effect of supporting Preyssler acid on
nano-titania
As previously reported,31,32 supporting the homoge-
neous Preyssler acid on the titania can alter the effi-
ciency of the catalyst to improve the reaction yield.
Here, we studied the effect of supporting Preyssler acid
on nano-titania of 15 nm particle size as the favourite
particle size. To find the most effective molar ratio of
the Preyssler acid/nano-titania, we performed thermo-
dynamic study of the adsorption of Preyssler acid on
3
. Results and discussion
3.1 Effect of the titania particle size
In the present study, various particle sizes of titania
were prepared via sol–gel method according to the
reported procedure.3
1,33
The resulted particle size distri-
bution was estimated to be 15, 30 and 50 nm as deter-
mined by transmission electron microscopy (TEM)
images. To investigate the effect of minimizing the par-
ticle size of the titania on the rate and yield of the titled
reactions, initially we studied the reaction between
benzaldehyde (1 mmol), malononitrile (1 mmol) and
1
5 nm titania. The amount of the adsorbed Preyssler
acid per unit mass of the adsorbent (mg/g) as defined by
q ) was calculated using the equation (1), where C and
(
e
o
C are initial and final concentrations of Preyssler acid,
e
respectively (mg/L=ppm), V is the volume of water
as solvent (Lit) and m is the weight of nano-titania as
5,5-dimethylcyclohexan-1,3-dione (1 mmol) in EtOH
32
adsorbent (g).
(5 mL) as the model reaction under reflux condition
using various sizes of titania (25 mg) as the catalyst, and
the results are schematically presented in figure 1.
As shown in figure 1, comparison of the reaction
rates and yields resulted in the presence of various parti-
cle sizes of titania indicates that these two factors of the
reaction are both improved with decreasing the size of
titania nanoparticles. Moreover, no completion of con-
version occurs when the reaction is carried out using
micro-titania or in the absence of the catalyst.
q = (C − C ) V/m.
(1)
e
0
e
3
.3 Optimizing the reaction conditions
To establish the reaction conditions, the effect of molar
ratio of the Preyssler acid/nano-titania on the aforemen-
tioned model reaction was studied under various condi-
tions using the neat nano-titania as well as nano-titania-
supported Preyssler-type heteropolyacid with different
molar ratios (table 2).
The results summarized in table 2 clearly indicate the
compelling effect of the Preyssler acid on the yield of
the reaction (95%, entry 8) when supported on nano-
titania in comparison with the yield (72%) obtained
from the neat nano-titania as the catalyst (entry 3). In
addition, the important role of nano-titania as the cat-
1
00
1
5 nm titania
75
50
25
0
30 nm titania
5
0 nm titania alyst was approved by conducting the reaction under
Micro-titania & No Catalyst
Table 1. BET results for various n-TiO2particle sizes.
2
n-TiO2 particle size (nm)
Specific surface area (m /g)
0
1
2
3
4
5
Time (h)
1
3
5
5
0
0
175
110
55
Figure 1. Effect of titania particle size on the rate and
yield of the model synthesis of 2-amino-7,7-dimethyl-5-oxo-
-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile.
Micro size
15
4