Tin oxide nanoparticles (NP-SnO2): Preparation, characterization and their catalytic application in the Knoevenagel condensation

Article abstract of DOI:10.1007/s13738-012-0135-y

Tin oxide (SnO₂) nanoparticles were synthesized by modified thermal decomposition process. Taguchi analysis was used and three important synthetic factors, molar concentration ratio of [NaNO₃]/[SnCl ₄], temperature and tim

Full text of DOI:10.1007/s13738-012-0135-y

J IRAN CHEM SOC (2013) 10:141–149
DOI 10.1007/s13738-012-0135-y
ORIGINAL PAPER
Tin oxide nanoparticles (NP-SnO2): preparation, characterization
and their catalytic application in the Knoevenagel condensation
•
Hashem Sharghi Sakineh Ebrahimpourmoghaddam
•
•
Raheleh Memarzadeh Sirous Javadpour
Received: 7 March 2012 / Accepted: 5 July 2012 / Published online: 26 July 2012
Ó Iranian Chemical Society 2012
Abstract Tin oxide (SnO2) nanoparticles were synthe-
sized by modified thermal decomposition process. Taguchi
analysis was used and three important synthetic factors,
molar concentration ratio of [NaNO3]/[SnCl4], temperature
and time of calcinations, which affect the size of SnO2
particles, were studied. The optimal conditions were
determined using Taguchi robust design method and nano-
sized SnO2 particles (*2 nm) were obtained. Nanoparti-
cles were characterized by X-ray diffraction, transmission
electron microscopy and UV–visible spectroscopy tech-
niques. The results show that tin oxide nanoparticles could
be one of the most active and reusable catalysts in the
Knoevenagel condensation. Different active methylene
group compounds and diverse range of aldehydes were
chosen to react in the presence of tin oxide nanoparticles at
ambient temperature at solvent-free condition (SFC) with
excellent isolated yields.
Introduction
In recent years, green chemistry has attracted people’s
great attention as a result of increasing environmental
pollution. Therefore, improved synthesis routes in terms of
minimal waste, product purity and catalysts recovery are
highly desirable in the synthesis [1]. With regard to this,
the development of solvent-free organic synthesis using
reusable heterogeneous catalysts has become highly
desirable [2–5]. Although these heterogeneous catalysts are
more environmentally safe and possess advantages such as
ease of application capabilities, decreased reactor and plant
corrosion problems [6], their activity and selectivity are
frequently diminished, because of the decrease in the dif-
fusion rate of reactant to the surface of catalyst [7]. It
seems that, nanoparticles can be a good candidate as a solid
catalyst, since their nanometric size causes a dramatic
increase in the surface area and results in an increase in the
substrates diffusional rate [8].
Keywords Knoevenagel condensation ꢀ Nanoparticles ꢀ
Taguchi ꢀ Tin oxide
Tin oxide is an n-type semiconductor with wide band
gap that has many applications including gas sensors [9],
oxidation catalysis [10], barrier layers for solar cells [11],
optoelectronic devices, liquid crystal displays and as anode
material in lithium-based batteries [12]. The synthesis of
metal oxide nanoparticles smaller than about 20 nm has
become a highly developed scientific and technological
field owing to the structural peculiarities and unusual
physical and chemical properties they may lead to [13, 14].
Crystalline structure, size and shape of the particles and
superficial characteristics are highly dependent on the route
followed for the synthesis.
H. Sharghi (&) ꢀ S. Ebrahimpourmoghaddam
Department of Chemistry, College of Science,
Shiraz University, 71454 Shiraz, Iran
e-mail: shashem@chem.susc.ac.ir
R. Memarzadeh
Department of Materials Science and Engineering,
Marvdasht Branch, Islamic Azad University, Marvdasht, Iran
In general, SnO2 nanoparticles have been produced by
various physical and chemical methods including precipi-
tation [15], hydrothermal [16], sol–gel [17], hydrolytic
[18], carbothermal reduction [19], polymeric precursor [20]
S. Javadpour
Materials and Science Engineering Department,
Shiraz University, Shiraz, Iran
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J IRAN CHEM SOC (2013) 10:141–149
and thermal decomposition methods [12]. Among these
methods, the thermal decomposition route is an efficient
method that allows the preparation of uniform and small
size tin dioxide particles. However, in the formation of
SnO2 nanoparticles by this method, there are many
important parameters that should be controlled during the
preparation of nano-sized powder. The experiments can be
planned in four ways: (1) trial-and-error, (2) one-factor-at-
a-time experiments, (3) full-factorial experiments and (4)
Taguchi’s orthogonal arrays (OA). Hence, the interrela-
tionships between the parameters are complex and the
analysis of the process to optimize the factors is a time- and
labor-consuming work, the analyses using conventional
experimental methods are inefficient. Orthogonal arrays
can eliminate the bias produced by one-factor-at-a-time
experiments and improve the experimental efficiency
of full-factorial experiments. Based on the principle of
maintaining the accuracy of experiment results, the use of
orthogonal arrays can considerably reduce the time
required to perform the experiments and increase the
reproducibility of the experiment results [21]. Therefore,
Taguchi orthogonal arrays are applied herein to perform
experimental planning [22–24]. Taguchi method is a
combination of mathematical and statistical techniques
used in an empirical study. It can determine the experi-
mental condition having the least variability as the
optimum condition. The variability can be expressed by
signal-to-noise (S/N) ratio [25–27]. The experimental
condition leading to maximum S/N ratio is considered as
the optimal condition.
with amino groups [44], iridium or platinum hydroxyapa-
tites [45], polyvinyl pyrrolidone nickel nanoparticles [46],
nitrogen containing carbon nanotubes [47] and zirconium-
substituted aminoethyl phosphonate [48] have been repor-
ted in the literature. Although, some of these approaches
are satisfactory and effective for the synthesis of KC
products, many of them are expensive, highly corrosive,
need use of toxic organic solvents and involve tedious
separation procedures and also require prolonged reaction
times. Hence, it is of great practical importance to develop
a more efficient, convenient and also an environmentally
benign method for the synthesis of these scaffolds. As
shown in this study, tin oxide nanoparticles display high
reactivity in the KC reaction process in the absence of any
solvent, and it can be reused for at least seven runs. The
important reaction parameters affecting the yield, including
size and crystal structure of NP-SnO2, catalyst amount,
substituent and reactants molar ratio were investigated in
detail.
Experimental procedures
Materials and characterization techniques
Reagents used for obtaining SnO2 nanoparticles were tin
(IV) chloride pentahydrate and sodium nitrate. Chemicals
were provided from Merck and Aldrich Chemical Com-
panies. Spectrophotometric analysis of SnO2 nanoparti-
cles were carried out using a Perkin-Elemer UV–visible
spectrometer. X-ray diffraction (XRD) and transmission
electron microscopy (TEM) were employed for character-
ization of the SnO2 nanoparticles (Philips CM10). XRD
The objectives of this work are: applying the Taguchi
robust design method to evaluate the effect of three
parameters on the particle size and the size distribution of
nanoparticles, obtaining the SnO2 nanoparticles using
optimal conditions based on the thermal decomposition
method and the application of SnO2 nanoparticles in
catalysis.
˚
with fixed Cu-Ka line (k = 1.54 A) was used for recording
XRD pattern operating at steps 0.05°. The size of the
crystals was analyzed from the measured width of their
diffraction pattern. An exact treatment gives D = 0.9k/b
cos h, which is known as Scherrer equation for hexagonal
peaks, where k is the wavelength of the copper Ka radia-
tion, b is the full-width at half-maximum (FWHM) of the
X-ray line and h is Bragg’s diffraction angle for the line.
Samples for TEM were prepared by dispersing the product
in ethanol using ultrasonic irradiation and placing a droplet
on carbon-coated Cu grids. NMR spectra were recorded
on a Bruker Avance DPX-250 (1H-NMR 250 MHz) in
pure deuterated solvents with tetramethylsilane (TMS) as
internal standards. The purity determination of the sub-
strates and reaction monitoring were accomplished by a
Shimadzu Gas Chromatograph (GC-10 A) instrument with
a flame-ionization detector using a column of 15 % car-
bowax 20M on acid washed, 60–80 mesh chromosorb W.
Melting points were determined in open capillary tubes in a
Buchi-535 circulating oil melting point apparatus.
To evaluate the catalytic performance of these nano-
particles, we checked the solvent-free Knoevenagel con-
densation (KC) [28–30] reaction between aldehydes and
active methylene compounds in the presence of catalytic
amount of NP-SnO2. KC has been receiving considerable
attention, due to its broad spectrum of uses including
perfume [31], cosmetic, pharmaceutical [32] and polymer
applications [33]. With regard to this, lots of methods to
achieve KC are known. The reaction is catalyzed by bases,
acids or catalysts containing acid–base sites [34]. Numer-
ous types of heterogeneous acidic and basic systems
including Al2O3 [35] bases derived from originally modi-
fied silicas and micelle-templated silicas [36], anionic
resins [37], alkaline earth oxides [38], clays [39], calcined
hydrotalcites [40], aluminophosphate oxynitrides [41],
basic zeolites [42], titanium dioxide [43], modified PEG
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143
Table 1 Parameters and levels used in this experiment
Parameters
Levels
(1)
(2)
(3)
A
B
C
[NaNO3]/[SnCl4]
1
200
2
2
3
Temperature of calcinations (°C)
Time of calcinations (h)
400
4
600
6
Preparation of tin oxide
Samples were prepared by thermal decomposition of mixed
NaNO3 solution. For preparation of solution, 3 ml
(0.25 mol) of SnCl4 was dissolved in 100 ml of deionised
water and then mixed with a nitrate solution in the different
molar ratios to obtain a transparent solution. The trans-
parent solution was then heated at 150 °C for 16 h in a
Pyrex vessel and cooled to room temperature. The thermal
decomposition reaction is represented in equation 1 [22].
Fig. 1 UV–visible absorption spectrum of the SnO2 nanoparticles
SnCl4 þ 4NaNO3 ! SnO2 þ 4NO2 þ 4NaCl þ O2
ð1Þ
after the reaction was complete, acetone was added to the
reaction mixture and centrifuged to separate the catalyst.
The organic solvent was removed under reduced pressure.
After purification by plate chromatography, the Knoeve-
nagel adduct was obtained. The structure of the products
After this stage, a white SnO2 powder comprised of
nano-sized particles was thus obtained. NaCl is produced in
minor quantity in the above reaction. The prepared powder
was washed thoroughly with deionized water until it
became chloride salt free (silver nitrate test). Finally, the
sample was obtained by calcining the powders at
200–600 °C.
1
was confirmed by melting point, H-NMR spectrum and
comparison with authentic samples prepared by reported
methods.
Orthogonal array and characterization
Results and discussion
A widerange ofcalcination temperatureshasbeen reported in
the literature that shows strong influence of heat on the par-
ticle size [49]. Another important factor is the concentration
ratio of the reactants [49, 50]. Therefore, we studied the effect
of concentration, calcination temperature and time as the
most important factors on the particle size of tin oxide for-
mation. The molar concentration ratio of [NaNO3]/[SnCl4]
was varied by 1 to 3. In addition, the calcination temperatures
and time were studied in the range of 200–600 °C and 2–6 h,
respectively. Table 1 shows the parameters and levels used in
this experiment. The particle size and crystal structure of
nanoparticles were observed using TEM and XRD, respec-
tively. In addition, the SnO2 nanoparticles were subjected to
UV–visible spectrophotometer.
Characterization of SnO2 nanoparticles
As shown in Fig. 1, band gap of the SnO2 nanoparticle is
310 nm (4 eV); this exhibits a 0.4 eV blue shift from that
of the bulk SnO2 (3.6 eV) [51]. The band edge or
absorption edge is defined as the transition between the
strong short-wavelength and the weak long-wavelength
absorption in the spectrum of a solid, generally a semi-
conductor. The band gap increases with decreasing particle
size and the absorption edge is shifted to a higher wave-
length with decreasing particle size [51].
Figure 2 shows the XRD pattern of preparing a sample
of SnO2 obtained at optimal conditions. All the diffraction
lines are assigned [(110), (101) and (211)] well to tetrag-
onal rutile crystalline phase of tin oxide with a reference
pattern (JCPDS 41-1445).
General procedure for NP-SnO2-catalyzed
Knoevenagel condensation
The TEM micrograph and selected area diffraction
(SAD) patterns of the tin dioxide nanoparticles obtained at
optimal conditions are shown in Fig. 3. TEM data show the
formation of SnO2 nanocrystals of the size 2–5 nm
(Fig. 3a). The diffused rings in Fig. 3b determine the poor
Knoevenagel condensation of benzaldehyde (1.0 mmol)
and activated methylene compounds (1.2 mmol) was per-
formed in a mortar. NP-SnO2 (10 mol%) was added in the
reaction mixture. The mixture was ground by a pestle and
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J IRAN CHEM SOC (2013) 10:141–149
NC
O
H
NP-SNO2
rt/SFC
CN
NC
CN
H
Scheme 1 The model reaction
crystalline structure. Although sample has the least size, it
does not have full crystalline structure.
Catalytic performance
To measure the catalytic performance of NP-SnO2 obtained
at optimal conditions, the KC reaction in single step and
solvent-free condition was selected as a model test reac-
tion. The condensation of benzaldehyde and malononitrile
with NP-SnO2 catalyst was first carried out at ambient
temperature (Scheme 1), taking benzaldehyde/mal-
ononitrile molar ratio 1.0:1.2 and benzaldehyde to catalyst
weight ratio of 8.0 (10 mol% NP-SnO2). After 120 min the
conversion of benzaldehyde to 2-(phenyl methylene)mal-
ononitrile (1) was 80 %. The reaction needs prolonged
reaction time and higher temperature in the presence of
bulk SnO2 which showed that NP-SnO2 possesses suffi-
cient catalytic activity for the reaction. Parameters such as
size and crystal structure of NP-SnO2, amount of catalyst,
reactant molar ratio and substituents were optimized in
order to achieve the maximum catalytic activity and good
conversion of the desired product.
Fig. 2 XRD pattern of the SnO2 nanoparticles
Effect of size and crystal structure of NP-SnO2
Initially SnO2 nanoparticles obtained at optimal condition
were screened in KC reaction between benzaldehyde and
malononitrile. The conversion of benzaldehyde increased
up to 80 % after 2 h at ambient temperature under SFC
(Table 2, entry 1). Spurred with the success of KC reac-
tions, the other nanoparticles obtained over optimizing
SnO2 synthesis were used for the model reaction (entries
2–10). Among the SnO2 samples screened in the KC
reaction, the NP-SnO2 with sizes 39.25 and 6.5 nm was
found to be superior to the other samples in terms of yields
and time of the reaction.
If SnO2 crystallites contained hydroxyl groups in its
lattice and around it, tin vacancies would be generated into
the structure. When samples were calcined, the tin vacancy
sites decreased as a consequence of the dehydroxylation
process. The presence of hydroxyls in the material con-
tributed to an enhancement of the lattice parameters and
also to change oxygen positions in the SnO2 rutile struc-
ture, which changed the symmetry of the representative
Fig. 3 a TEM images. b Selected area electron diffraction patterns of
the SnO2 nanoparticles
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145
Table 2 Effect of size and crystal structure of NP-SnO2
Entry
Sample size
Time (min)
Conversion (%)
1
2
3
4
5
6
7
8
9
10
*2.00a
2.90
120
120
10
5
80
70
90
95
75
92
95
50
95
89
24.50
39.25
2.80
120
8
4.46
4.20
7
2.80
120
5
6.50
Fig. 4 Effect of catalyst amount (mol%) on conversion of benzal-
dehyde to (1)
3.70
10
Reaction condition: benzaldehyde (1 mmol); malononitrile (1.2 mmol);
r.t.; 10 mol% catalyst
Table 3 Effect of molar ratios of benzaldehyde/malononitrile
a
NP-SnO2 obtained at optimal condition
Entry
Molar ratio
Conversion (%)
1
2
3
4
5
6
7
1:2
93
91
95
82
80
81
75
tin–oxygen octahedron [52]. For SnO2 single crystals, the
electron concentration depends on the stoichiometry devia-
tion determined by oxygen vacancies (VO), which are pre-
dominant atomic defects [53]. These results indicate that the
efficiency of the catalytic activity shows dependence on
particle size, and comparing different samples with nano-
metric sizes revealed that the structure and shape of crystals
have important role in progressing the reaction. With regard
to this, the reaction rate could be further increased when SnO2
nanoparticles were in perfect crystal structure.
1:1.5
1:1.2
1:1
1.2:1
1.5:1
2:1
Reaction condition: r.t.; 5 min; 10 mol% catalyst
the product yields. Even on increasing the molar ratios to
2:1, the conversion of the main product was decreased.
Effect of catalyst amount
Effect of substituents
The effect of catalyst amount on conversion of benzalde-
hyde was studied by varying the mol percentage of catalyst
under optimized reaction conditions. As shown in Fig. 4, at
lower catalyst amount i.e., 2 mol%, only 53 % conversion
of benzaldehyde was observed after 5.0 min. On increasing
the catalyst weight to 0.015 g (10 mol%), conversion of
benzaldehyde increased up to 95 %. The increase in the
conversion with an increase in the catalyst weight can be
attributed to an increase in the availability of number of
catalytically active sites required for this reaction. On
further increasing the amount of catalyst, there is no
appreciable change in conversion.
To widen the generality and scope, NP-SnO2 was tested in the
KC reaction of aldehydes with activated methylenes
(Table 4). Various aldehydes were reacted with malononit-
rile under this condition (entries 1–16). Several functional-
itiespresent inthearomaticaldehydes, such as alkyl, halogen,
methoxy, hydroxyl and nitro group were tolerated (entries
2–10). Furthermore, heteroaromatic aldehydes, such as thi-
ophene-2-carbaldehyde
and
pyridine-4-carbaldehyde
(entries 11, 12) were all effective substrates to successfully
execute the solvent-free KC reactions by NP-SnO2. We also
found that the reaction of other activated methylenes with
benzaldehyde under these conditions was good enough, but
rates for the condensation on NP-SnO2 decreased with the
less reactive methylenes (entries 13–16).
Effect of reactants molar ratio
Reuse of catalyst NP-SnO2
The effect of benzaldehyde to malononitrile molar ratio on
conversion was monitored at different molar ratios from
2:1 to 1:2 as given in Table 3. The conversion was found
maximum at 1:1.2 molar ratios with 95 % conversion of
benzaldehyde to the product. Further increase in concen-
tration of benzaldehyde or malononitrile did not improve
To study the stability and reusability of the catalyst, the
solid was recovered after centrifuging the reaction mixture
with acetone and calcined at 400 °C for 1 h. Thus, obtained
catalyst was reused in the next reaction cycle. The catalyst
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J IRAN CHEM SOC (2013) 10:141–149
Table 4 Knoevenagel condensation of different substrates
NP-SnO2
X
O
Y
Ar
X
Ar
rt / SFC
H
Y
H
Entry Ar
X/Y
Yields
(%)a
MP (Lit)
Fig. 5 Reusability of catalyst
1
2
3
C6H5
4-Me-C6H5
CN/CN
CN/CN
CN/CN
95
95
90
82 (79–80) [54]
135 (140) [43]
132 (130) [43]
It seems that, the KC reactions proceed via dual acti-
vation of both substrates (nucleophiles and electrophiles)
by NP-SnO2. Tin oxide has a Lewis acid site Sn4? and
Lewis basic sites O2- [60]. Thus, the Lewis base (O2-) of
the catalyst produces carbanion from malononitrile as
shown in Scheme 2, which forms a complex with the Sn4?
site of NP-SnO2. The carbanion attacks on the carbonyls,
which have been activated with the Lewis acid moiety
(Sn4?) and forms condensation product by simple nucleo-
philic substitution reaction.
4-Isopropyl-
C6H4
4
5
4-Cl-C6H5
CN/CN
CN/CN
95
91
161 (158–160)
[55]
4-OMe-C6H5
115 (110–112)
[55]
6
7
8
4-OH-C6H5
3-OH-C6H5
4-NO2-C6H5
CN/CN
CN/CN
CN/CN
87
90
95
186 (180) [43]
90 (91) [43]
159 (159–160)
[55]
9
3-NO2-C6H5
2-NO2-C6H5
CN/CN
CN/CN
90
90
119 (102–103)
[55]
Comparison of activities in the KC of different catalysts
10
139 (141–143)
[56]
In Table 5, a comparison of activities in the KC of benz-
aldehyde and malononitrile with other catalysts is dis-
played. The activity of NP-SnO2 is higher than some other
catalytic systems like 2-hydroxy ethylammonium formate,
Znb and TiO2 (entries 1, 2 and 7). It may be caused by the
number or accessibility of the active sites of tin oxide
nanoparticles. Homogeneous catalysts such as diethyl-
amine modified PEG 600 and ionic liquid [2-aemim][PF6]
react with less amount of catalyst (entries 3 and 6).
11
2-Thienyl
3-Pyridyl
C6H5
CN/CN
87
92
87
85
92 (90) [55]
12
CN/CN
89 (80–83) [54]
51 (50–52) [55]
35 (33) [57]
13b
14b
CN/CO2Et
C6H5
CO2Et/
CO2Et
15b
16b
C6H5
C6H5
CO2Me/
CO2Me
88
89
31 (27–28) [58]
248(239) [47]
Barbituric
acid
Reaction condition: r.t.; 5–30 min; 10 mol% catalyst
a
Isolated yield
Taguchi design and analysis of variance
b
Prolonged reaction time of 1–3 h is needed
Taguchi robust design method was used to identify the
optimal conditions and to calculate the correlation coeffi-
cient of each parameter affecting the particle size. The
structure of Taguchi’s orthogonal design and the results of
measurement are shown in Table 6, and the smallest values
of particle size were shown in experimental Nos. 4 and 7,
respectively. The values of S/N ratio of ‘‘Taguchi’’ method
were used to measure the quality characteristic deviating
from the desired value. Different values of S/N ratios are
obtained consistent with the type of characterization.
Where smaller characteristics are desired, the S/N ratio is
defined according to equation 2 [26].
was successfully reused consecutively seven times under
similar reaction conditions giving high conversion in the
range 87–95 %, which shows that there is no considerable
change in the catalytic activity of the catalysts even after
five reaction cycles (Fig. 5). The conversion was decreased
after the seventh cycle, which may be due to the aggre-
gation of particles of the used catalyst that may block the
active sites present on the catalyst [59].
ÂÀ
Á
Ã
Proposed mechanism
S=N Ratio ½dBꢁ ¼ ꢂ10 log y21 þ y22 þ y32 þ ꢀ ꢀ ꢀ =n
ð2Þ
A proposed mechanism for the reaction forming 2-aryl-
methylidienemalononitrile in the presence of NP-SnO2
catalyst is shown in Scheme 2.
The characteristic property is shown by yi and n is the
number of the experiment. Table 6 shows the S/N ratio for
particle size calculated according to the equation 2.
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J IRAN CHEM SOC (2013) 10:141–149
147
Scheme 2 The proposed
mechanism
O-2
Sn+4
CH(CN)2
H
O
H CH(CN)2
Ar
H
SnO2
H
CN
CN
(CN)2
C
C
Ar
Ar
O
H
O
H
NC CN
Ar
H
Table 5 Comparison of activities of benzaldehyde/malononitrile
over different catalysts
Table 6 Experimental measured values and S/N ratio for particles
size of SnO2 (Taguchi orthogonal array table of L9)
Entry Condition
Time
(min)
Yields
(%)
Exp. no.
A
B
C
Average particle size
Raw data (nm)
S/N ratio (dB)
1
2-Hydroxy ethylammonium formate
(2 mL)/rt [56]
300
88
1
2
3
4
5
6
7
8
9
1
1
1
2
2
2
3
3
3
1
2
3
1
2
3
1
2
3
1
2
3
2
3
1
3
1
2
2.90
24.50
39.25
2.80
4.46
4.20
2.80
6.50
3.70
-9.24
-27.8
-31.8
-8.9
2
3
Znb (0.006 g)/140 °C [61]
Diethylaminemodified PEG 600
(2 mol%)/rt [55]
360
1
16.1
91
4
Zwitterionic liquid (HEAPS)
(10 mol%)/rt/H2O [62]
10
95
-12.9
-12.4
-8.9
5
6
I2 (3 mol%)/K2CO3/rt/H2O [60, 63]
12
20
86
92
[2-aemim][PF6] (0.16 mol%)/rt/H2O
[64]
-16.2
-11.4
7
TiO2 (1 mmol)/80 °C [43]
SnO2 (10 mol%)/rt
420
420
420
5
95
30
92
95
8
9
SnO2 (10 mol%)/80 °C
NP-SnO2 (10 mol%)/rt
10
level 1. As shown in Fig. 6, the data (max.–min.) for A are
at the highest value. Therefore, it can be found that molar
concentration ratio of [NaNO3]/[SnCl4] is the significant
parameter, influencing the particle size during crystal
formation.
The mean S/N ratio for each level of the parameters
is summarized and the S/N response graph for particle size
is shown in Fig. 6. The greater the S/N ratio, the smaller is
the variance of crystalline size around the desired value.
Therefore, the optimum parameters are A2, B1 and C1. In
other words, based on the S/N ratio, the optimal conditions
for crystalline size are the A at level 2, B at level 1 and C at
In another attempt, by the optimally parameterized
method of this work, SnO2 nanoparticles were prepared
using the reactants molar ratio of 2/1, calcination temper-
ature of 200 °C and calcination time of 2 h.
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Conclusions
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The resultant nanoparticles were efficient catalysts in the
KC reaction. We have demonstrated a very simple and
highly efficient method using NP-SnO2 under solvent-less
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