Synthesis of KOH/SnO2 solid superbases for catalytic Knoevenagel condensation

Article abstract of DOI:10.1016/j.catcom.2015.03.008

KOH/SnO₂ solid superbases of specific morphology and uniform pore structure were synthesized. The SnO₂ support was prepared by reflux digestion using graphene oxide as template and employed for the loading of KOH by means of grinding

Full text of DOI:10.1016/j.catcom.2015.03.008

Catalysis Communications 66 (2015) 30–33
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Catalysis Communications
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Short communication
Synthesis of KOH/SnO2 solid superbases for catalytic
Knoevenagel condensation
⁎
Jun Xie, Lang Chen, Chak-Tong Au, Shuang-Feng Yin
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
KOH/SnO2 solid superbases of specific morphology and uniform pore structure were synthesized. The SnO2 sup-
port was prepared by reflux digestion using graphene oxide as template and employed for the loading of KOH by
means of grinding and thermal treatment. With base strength of 26.5 ≤ H− b 33.0 and superbasic sites of
1.362 mmol g−1, the 20 wt.%KOH/SnO2 catalyst exhibits excellent activity in Knoevenagel condensation under
mild conditions. The catalytic efficiency is closely related to the base strength and the amount of superbasic
sites. The findings disclose a new route for the synthesis of versatile solid superbases using SnO2 as supports.
© 2015 Elsevier B.V. All rights reserved.
Received 24 December 2014
Received in revised form 27 February 2015
Accepted 7 March 2015
Available online 10 March 2015
Keywords:
KOH/SnO2
Solid superbase
Graphene oxide
Knoevenagel condensation
1. Introduction
used SnO2 prepared using GO template as support for the synthesis
of superbases and investigated their catalytic activity towards the
Solid superbases are materials that possess basic sites with strength
(H−) higher than 26 [1]. In the past decades, there was rapid develop-
ment in the research of solid superbase catalysts due to their excellent
catalytic performance and environment-benign characteristics. Many
superbases (e.g., KNO3/γ-Al2O3 [2,3], Eu2O3/γ-Al2O3 [4], KNO3/ZrO2 [5,
6], KOH/ZrO2 [7], K/MgO [8], Na/NaOH/MgO [9], KNO3/KL [10] and
Ca(NO3)2/SBA-15 [11]) were reported. In these cases, the common
supports are γ-Al2O3, ZrO2, MgO and mesoporous molecular sieves.
Obviously, due to the limited use of support materials, the variety of
solid superbases is restricted. It is envisaged that the use of new support
materials is a way to enrich the diversity of solid superbase materials.
Previously, we synthesized a series of superbase materials using com-
posite oxides as supports and potassium salts as modifying agents
[12–14]. However, most of them show unsatisfactory catalytic perfor-
mance that can be related to the poor amount of superbasic sites.
SnO2 is not expensive. It has a fair amount of acidic sites, probably of
weak acid strength [15]. It is chemically stable and is widely used in gas
sensors [16], dye-based solar cells [17], transparent conducting elec-
trodes [18], and catalyst supports [19]. It shows physical properties
that are favorable for the formation of superbasic sites: high carrier mo-
bility, rich crystal defects and abundant vacant sites [20,21]. Moreover,
graphene oxide (GO) due to outstanding advantages such as low cost,
easy availability, abundant surface functional groups, nontoxic nature,
environmental friendliness and stability, has been recognized as the
most promising and friendly material [22,23]. In the present work, we
Knoevenagel condensation reaction under mild conditions.
2. Experimental
2.1. Catalyst preparation
SnO2 was prepared by using graphene oxide as template under
reflux digestion. SnCl4·5H2O (0.02 mol) was dissolved in deionized
water, and 1 mol L−1 KOH aqueous solution was added dropwise with
vigorous stirring until the pH value of the solution was 7. The solution
was stirred at 40 °C for 3 h, followed by centrifugation and washing
with water for 5 times to form a clear gel. The obtained gel was dis-
solved in water again, and mixed with an aqueous solution of 2 g L−1
GO synthesized by the modified Hummer's method [24]. The as-
resulted mixture was digested under reflux at ca. 100 °C for 24 h, and
the precipitate was filtered out. The obtained solid was dried at 110 °C
overnight and calcined at 550 °C for 3 h to remove the GO template
for the formation of SnO2.
The KOH-loaded SnO2 samples were prepared by grinding meth-
od, followed by thermal treatment at 550 °C under a flow of high-
purity N2 for 3 h. The as-prepared sample is denoted hereinafter as
“xwt.%KOH/SnO2”, where x stands for the weight percentage of
KOH in terms of the whole catalyst.
2.2. Catalyst characterization
XRD patterns were recorded on a Bruker D8 advance diffractometer
⁎
Corresponding author.
E-mail address: sf_yin@hnu.edu.cn (S.-F. Yin).
with monochromatized Cu Kα radiation (λ = 0.15406 nm) at a setting
http://dx.doi.org/10.1016/j.catcom.2015.03.008
1566-7367/© 2015 Elsevier B.V. All rights reserved.

J. Xie et al. / Catalysis Communications 66 (2015) 30–33
31
of 40 kV and 40 mA. SEM analyses were performed over a Hitachi
S-4800 electron microscope operating at 2.0 kV. The specific surface
areas of catalysts were determined by BET method based on nitrogen
adsorption–desorption isotherms (at −196 °C) collected over a
Beckman (SA 3100) surface area analyzer. The FT-IR spectra were deter-
mined using a Bruker vector 22 FT-IR spectrophotometer. The CO2-TPD
measurements were conducted on a Micromeritics 2920 apparatus
using thermal conductivity detector. The sample was activated at
550 °C for 1 h prior to the adsorption of CO2 at 100 °C. After purging at
100 °C to remove the physically adsorbed CO2, the sample was heated
to 800 °C at a rate of 10 °C min−1 and held at 800 °C for 40 min. The ba-
sicity of samples was measured using Hammett indicators after activa-
tion at 550 °C for 3 h as detailed in [25].
(Fig. S1a, b) show sizes of 13.2 and 12.6 nm, respectively. On the other
hand, the crystallite sizes of xwt.%KOH/SnO2 (Fig. S1c–f) are estimated
to be 10.0, 10.5, 11.1 and 9.9 nm at KOH loadings of 0, 10, 20 and
30 wt.%, respectively. The results demonstrate that the use of GO tem-
plate leads to a decrease of crystallinity and an increase in the amount
of defects [27].
3.1.2. SEM
Fig. 1 displays the SEM images of 20 wt.%KOH/SnO2 prepared with or
without the use of GO template. One can see that without the use of GO
template for the preparation of SnO2, the 20 wt.%KOH/SnO2 is irregular
in shape, and agglomeration is obvious. The use of GO as template for
the preparation of SnO2 leads to the formation of uniform nanoparticles
with diameter of ca. 25 nm. We deduce that the basic precursors are an-
chored covalently to GO through functional groups such as carboxyl and
hydroxyl [28]. Thus, the presence of GO template restrains agglomera-
tion of nanoparticles, and shows important effect on the morphology
of 20 wt.%KOH/SnO2.
2.3. Catalyst evaluation
Catalytic activity of the superbases was investigated in the
Knoevenagel condensation of aldehydes with active methylene com-
pounds (Scheme S1, see the supporting information, SI). The catalytic
reactions were conducted in a 25 mL round-bottomed flask at 25 °C
using aldehyde (2 mmol), active methylene reagent (2 mmol), DMF
(dimethylformamide, 1 mL), and catalyst (50 mg). After the reaction,
the catalyst was separated from the mixture by centrifugation, and
the reaction mixtures were analyzed using an Agilent Technologies
7820 gas chromatograph equipped with a flame ionization detector
and AB-FFAP capillary column (30 m × 0.25 mm × 0.25 μm). The con-
version of aldehydes and the yield of products were determined using
biphenyl as internal standard. All the products were analyzed over an
Agilent 6890-5973 MSD GC–MS equipment.
3.1.3. N2 adsorption/desorption isotherm
Depicted in Fig. 2 are the N2 adsorption/desorption isotherms and
pore-size distribution of samples. One can see type-IV isotherms, and
there are hysteresis loops that indicate mesoporosity of materials [3].
The BET surface area and pore volume of SnO2 prepared using GO
template are 39.1 m2 g−1 and 0.137 cm3 g−1, respectively. After the in-
troduction of 20 wt.% KOH, there is a decline of BET surface area and
pore volume (16.1 m2 g−1 and 0.051 cm3 g−1). Meanwhile, the pore-
size distribution suggests uniform pore structure. In addition, the sur-
face area and pore volume of SnO2 prepared without using GO template
are 21.7 m2 g−1 and 0.111 cm3 g−1, whereas those of 20 wt.%KOH/SnO2
with SnO2 prepared without using GO template are 10.0 m2 g−1 and
0.034 cm3 g−1, respectively. It is obvious that the use of GO template
leads to increase of BET surface area and pore volume. It is deduced
that GO promotes the dispersion while restraining the agglomeration
of nanoparticles.
3. Results and discussion
3.1. Characterization results
3.1.1. XRD
Fig. S1 (SI) shows the XRD patterns of samples. In Fig. S1c, the SnO2
sample prepared using GO template shows strong diffraction peaks at
2θ of 26.7, 34.1 and 51.7°, which are attributable to the SnO2 phase
(JCPDS no. 41-1445). There is no detection of additional peaks after
the introduction of KOH up to a loading of 20 wt.%, indicating high dis-
persion of KOH on SnO2. It is only at a KOH loading of 30 wt.% that there
is the detection of signals attributable to KOH (JCPDS no. 21-645). Ap-
parently, there exists a dispersion threshold of KOH on SnO2, similar
to the case of KNO3 on zirconia [6] and the case of K2CO3 on alumina
[26]. The spontaneous dispersion capacity of KOH on SnO2 is around
20 wt.%. When KOH loading exceeds 20 wt.%, there is blocking of active
sites and agglomeration of crystallites [12]. It is observed that the SnO2
and 20 wt.% KOH/SnO2 with SnO2 prepared without using GO template
3.1.4. FT-IR
Shown in Fig. S2 (SI) are the IR spectra of as-synthesized SnO2
and 20 wt.%KOH/SnO2. The SnO2 sample exhibits a strong band at
638 cm−1, which is attributable to the anti-symmetric Sn–O–Sn
stretching mode of surface-bridging oxide formed by condensation
of adjacent surface hydroxyl groups. The band at 3420 cm−1 indicates
the presence of hydroxyl groups, which is probably due to water ad-
sorption [29]. After the introduction of KOH, there is the detection of the
1410 cm−1 band attributable to CO23− vibration [30]. The presence of
the 1410 cm−1 band demonstrates that the introduction of KOH
promotes the formation of basic sites and the adsorption of CO2 on the
surface of SnO2. To confirm the functional group on GO, we collected
Fig. 1. SEM images of (a) 20 wt.%KOH/SnO2 with SnO2 prepared using GO template, and (b) 20 wt.%KOH/SnO2 with SnO2 prepared without using GO template.

32
J. Xie et al. / Catalysis Communications 66 (2015) 30–33
100
80
60
40
20
0
3.1.6. CO2-TPD
The surface basicity of the samples was further estimated by the
CO2-TPD technique (Fig. 3). Without KOH loading, the SnO2 sample
prepared using GO as template shows CO2 desorption peaks at ca. 126,
369, and 710 °C (Fig. 3c). With 20 wt.% KOH loading, desorption peaks
are observed at ca. 134 and 798 °C (Fig. 3a). It is apparent that the
KOH presence results in enhancement of CO2 desorption and rise of de-
sorption temperature. The desorption peaks at ca. 798 °C is a clear indi-
cation of superbasicity [30,32], and the amount of superbasic sites is
1.325 mmol g−1 calculated by the integral method based on the area
of the corresponding desorption peaks. In other words, the introduction
of KOH leads to obvious enhancement of basicity. As a control compar-
ison, the 20 wt.%KOH/SnO2 sample with SnO2 prepared without the use
of GO template shows CO2 desorption amount and desorption temper-
ature that are significantly lower (Fig. 3b). We deduce that the SnO2
prepared using GO template has much more oxygen vacancies on the
surface, which is favorable for the creation of superbasic sites. Despite
the formation mechanism of superbasic sites needs further investiga-
tion, it is apparent that the use of GO template can improve the surface
basicity of SnO2.
(d)
0.016
0.012
0.008
0.004
0.000
(c)
(b)
(a)
0
10
20
30
40
50
Pore Diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P )
o
Fig. 2. N2 adsorption/desorption isotherm of (a) SnO2 prepared using GO template,
(b) SnO2 prepared without using GO template, (c) 20 wt.%KOH/SnO2 with SnO2 prepared
using GO template, and (d) 20 wt.%KOH/SnO2 with SnO2 prepared without using GO
template.
3.2. Catalytic activity
the IR spectrum of GO and compared it with that of GO treated with
basic precursors (Fig. S3A, SI). The spectrum of GO showed absorption
at 1720 cm−1 ascribable to C = O vibration of COOH [31]. However,
this peak disappeared after GO was treated with basic precursors under
reflux at ca. 100 °C for 24 h (Fig. S3B, SI), indicating that the basic precur-
sors were anchored covalently to GO through the functional groups.
3.2.1. Effect of KOH loading
Fig. 4a shows the effects of KOH loading on catalytic activity in the
Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate.
The catalytic activity increases with increase of KOH loading, and
reaches a maximum at 20 wt.%. Further increase of the KOH loading
results in the decrease of the catalytic performance. It could be attribut-
ed to the agglomeration of catalyst particles and/or coverage of surface
active sites caused by excess amount of KOH. Based on the results, the
optimum KOH loading is 20 wt.%, in agreement with the results of
XRD and Hammett indicator characterization. Furthermore, the influence
of KOH loading on the basicity of KOH/SnO2 was studied and the results
are shown in Table S1 (SI) and Fig. 4b, it is obvious that the change of cat-
alytic activity is not only related to the base strength but also to the
amount of superbasic sites.
3.1.5. Hammett indicator method
The base strength (H−) and basic sites of the samples were mea-
sured using Hammett indicators, and the effect of the KOH loading on
the surface of SnO2 was investigated. As shown in Table S1 (SI), the
base strength of SnO2 is 15.0 ≤ H− b 18.4. From 0 to 20 wt.% KOH load-
ing, there is a gradual rise of base strength. Having a superbasic strength
of 26.5 ≤ H− b 33.0, the 20 wt.%KOH/SnO2 sample can be regarded as a
solid superbase [1], and it shows an amount of superbasic sites of
1.362 mmol g−1. With further increase of KOH loading, the amount of
superbasic sites decreases while the base strength of samples remains
at 26.5 ≤ H− b 33.0. Therefore, to reach the highest possible amount
of superbasic sites and to maintain excellence in base strength, a KOH
loading of 20 wt.% should be adopted.
3.2.2. Knoevenagel condensation of various aldehydes with active methylene
compounds
In order to test the applicability of 20 wt.%KOH/SnO2, we studied the
reaction of various aldehydes with methylene compounds having ethyl
cyano acetate or malononitrile active group (Table 1). One can see prod-
uct yields that are either comparable or superior to those reported in the
110
100
90
2.0
1.5
1.0
0.5
0.0
-0.5
(a)
-- -- (b)
800
a
600
400
200
0
80
b
70
60
c
50
-200
0
5
10
15
20
25
30
0
20
40
60
80
100
KOH loading (wt%)
Time (min)
Fig. 3. CO2-TPD profile of (a) 20 wt.%KOH/SnO2 with SnO2 prepared using GO template,
(b) 20 wt.%KOH/SnO2 with SnO2 prepared without using GO template, and (c) SnO2 pre-
pared using GO template.
Fig. 4. Effects of KOH loading on benzaldehyde conversion and basicity. (Reaction
conditions: benzaldehyde, 2 mmol; ethyl cyanoacetate, 2 mmol; DMF, 1 mL; catalyst,
50 mg; temperature, 25 °C; time, 15 min.)

J. Xie et al. / Catalysis Communications 66 (2015) 30–33
33
Table 1
Research Funds for the Central Universities. C.T. Au thanks the Hunan
University for an adjunct professorship.
Knoevenagel condensation of various aldehydes with active methylene compounds over
20 wt.%KOH/SnO2.
Entry
Aldehydes
Methylene compounds (R2)
Yield (%)
Appendix A. Supplementary data
1
2
3
4
5
6
7
8
9
10
Benzaldehyde
Benzaldehyde
–CN
–COOEt
–CN
–COOEt
–CN
–COOEt
–CN
–COOEt
–CN
–COOEt
97.4
98.1
90.2
92.2
96.1
94.5
98.2
95.9
92.8
90.4
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.03.008.
4-Nitro Benzaldehyde
4-Nitro benzaldehyde
4-Chloro benzaldehyde
4-Chloro benzaldehyde
4-Methyl benzaldehyde
4-Methyl benzaldehyde
4-Methoxy benzaldehyde
4-Methoxy benzaldehyde
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KOH/SnO2 solid superbases with specific morphology and uniform
pore structure were synthesized. We prepared the SnO2 support by
reflux digestion using graphene oxide as template. The use of GO tem-
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sites up to 1.362 mmol g−1, the 20 wt.%KOH/SnO2 catalyst shows excel-
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The project was financially supported by the National Natural
Science Foundation of China (Grant Nos. U1162109, 21273067,
21401054 and 21476065), Program for Changjiang Scholars and Inno-
vative Research Team in University (IRT1238), and the Fundamental