Novel MgO-SnO2 solid superbase as a high-efficiency catalyst for one-pot solvent-free synthesis of polyfunctionalized 4H-pyran derivatives

Article abstract of DOI:10.1007/s10562-012-0805-5

We report for the first time the hydrothermal synthesis of MgO-SnO ₂ solid superbase using P123 as template. The basicity of the materials was determined by two approaches of Hammett indicators method and temperature- programmed desorption using CO₂ as adsorbate (CO ₂-TPD). It was found that Mg/Sn molar ratio has an effect on MgO-SnO₂ basicity, and superbasicity was observed only at Mg/Sn molar ratio of 1. With variation of Mg/Sn molar ratio, superbase strength (H-) was in the 26.5-33.0 range, showing superbasic value up to 0.939 mmol/g. The structure and texture of the as-prepared materials were studied by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2 physioadsorption methods. We detected particles of spherical morphology having diameter of ca. 150 nm. N2 adsorption- desorption results suggested that the materials are of mesoporous structure, having specific surface area of 115.2 m2/g and average pore diameter of 6 nm. The superbase was found to exhibit excellent catalytic activity towards the one-pot synthesis of polyfunctionalized 4Hpyrans through the condensation of aldehydes, malononitrile, and an active methylene compound. Its excellent catalytic efficiency is related to its superbasicity of the MgO-SnO₂. The results provide a new route for the design and preparation of composite oxide superbases. Furthermore, the solid superbases will facilitate a strategy for high-efficiency synthesis of polyfunctionalized 4H-pyrans.

Full text of DOI:10.1007/s10562-012-0805-5

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Catal Lett (2012) 142:608–614
DOI 10.1007/s10562-012-0805-5
Novel MgO–SnO2 Solid Superbase as a High-Efficiency Catalyst
for One-Pot Solvent-Free Synthesis of Polyfunctionalized
4H-pyran Derivatives
•
Shu-Guo Zhang Shuang-Feng Yin
•
•
Yu-Dan Wei Sheng-Lian Luo Chak-Tong Au
•
Received: 19 January 2012 / Accepted: 10 March 2012 / Published online: 27 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract We report for the first time the hydrothermal
synthesis of MgO–SnO2 solid superbase using P123 as
template. The basicity of the materials was determined by
two approaches of Hammett indicators method and tem-
perature-programmed desorption using CO2 as adsorbate
(CO2-TPD). It was found that Mg/Sn molar ratio has an
effect on MgO–SnO2 basicity, and superbasicity was
observed only at Mg/Sn molar ratio of 1. With variation
of Mg/Sn molar ratio, superbase strength (H-) was in
the 26.5–33.0 range, showing superbasic value up to
0.939 mmol/g. The structure and texture of the as-prepared
materials were studied by powder X-ray diffraction (XRD),
scanning electron microscopy (SEM), and N2 physio-
adsorption methods. We detected particles of spherical
morphology having diameter of ca. 150 nm. N2 adsorp-
tion–desorption results suggested that the materials are of
mesoporous structure, having specific surface area of
115.2 m2/g and average pore diameter of 6 nm. The
superbase was found to exhibit excellent catalytic activity
towards the one-pot synthesis of polyfunctionalized 4H-
pyrans through the condensation of aldehydes, malononit-
rile, and an active methylene compound. Its excellent
catalytic efficiency is related to its superbasicity of the
MgO–SnO2. The results provide a new route for the design
and preparation of composite oxide superbases. Further-
more, the solid superbases will facilitate a strategy for
high-efficiency synthesis of polyfunctionalized 4H-pyrans.
Keywords Solid superbase Á MgO–SnO2 Á
Condensation reaction Á 4H-pyrans Á Catalyst
1 Introduction
Solid superbases are materials possessing basic sites with
strength (H-) higher than 26 [1]. In the development of
environment-benign processes, they are used as catalysts so
that reactions can be conducted under mild conditions and
without generation of much waste [1–3]. In the past two
decades, there were many reports on solid superbases such
as NaN3/c-Al2O3 [4], Na/NaOH/c-Al2O3 [5], K/MgO [6],
Eu2O3/c-Al2O3 [7], KNO3/c-Al2O3 [8], KNO3/ZrO2 [9],
KOH/ZrO2 [10], KF/c-Al2O3 [11], and Ca(NO3)2/SBA-15
[12]. Recently, we found that the solid superbases Na2SnO3
[13, 14] and KOH/La2O3–MgO [15] showed excellent
catalytic efficiency towards some organic reactions at room
temperature (RT). However, industrial application of solid
superbases is limited because most of them are sensitive to
O2 and/or CO2, hence are difficult to prepare [1, 2]. In
addition, conventional solid superbases have low surface
area and superbasic sites. It is, therefore, highly desirable
to develop high-surface-area solid superbases with more
superbasic sites as well as simple preparation and easy
storage.
After a systematic survey, we found that the presence of
an alkali metal component is rather common among the
reported solid superbases [1–3], and reports on composite
oxide superbases free of alkali metal component are few. It
was regarded that crystal defects and/or vacant sites are
needed for the formation of superbasic sites over some
superbases [9, 16, 17]. Knowing that SnO2 is rich in crystal
defects [18, 19] and special MgO shows superbasicity [1],
S.-G. Zhang Á S.-F. Yin (&) Á Y.-D. Wei Á S.-L. Luo Á C.-T. Au
College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, Hunan, China
e-mail: sf_yin@hnu.edu.cn
C.-T. Au
Department of Chemistry, Hong Kong Baptist University,
Kowloon Tong, Hong Kong
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Novel MgO–SnO2
609
we synthesized MgO–SnO2 composite superbases, and
investigated their potentials in catalytic application.
The synthesis of polyfunctionlized 4H-pyrans group has
attracted much attention since it is a constituent of various
natural products [20–30]. The 4H-pyran derivatives can be
used as anti-coagulants, anticancer agents, anti-anaphy-
lactics, photoactive materials, etc. [21, 22]. Many catalysts
have been employed to synthesize the 4H-pyran unit, e.g.,
amino-functionalized ionic liquid [23], tetrabutylammo-
nium bromide [25], tetra-methyl ammonium hydroxide
[26], [BMIm]BF4 [27], SiO2 [28], piperidine [29], and Mg/
La oxide [30]. All of these catalysts have merits, while
some have disadvantages or limitations such as harsh
reaction conditions, low yields, poor selectivity, tedious
work-ups, use of organic solvent, and/or poor recyclability.
Thus, the development of a suitable solid catalyst for
efficient synthesis of 4H-pyrans in a single step is attrac-
tive. We describe herein the synthesis of a novel MgO–
SnO2 solid superbase. We employed various methods to
characterize its structure and surface property, and exam-
ined its catalytic activity towards one-pot synthesis of
polyfunctionalized 4H-pyrans through the condensation
of aldehydes, malononitrile and an active methylene
compound.
2 Experimental
2.1 Chemicals and Reagents
Most of the reagents and chemicals (analytic grade unless
stated otherwise) were purchased from Kemiou Chemical
Reagent Company and Tianjin Damao Chemical Reagent
˚
Co. Ltd. (Tianjin, China). They were dehydrated using 3 A
molecular sieves at RT. Ethyl cyanoacetate, acetylacetone
and malononitrile were purchased from Aladdin Chemistry
Co. Ltd. (Shanghai, China). Cyclohexane was dried by
sodium under reflux. Aldehydes were purified by reduced
pressure distillation (to remove acid) prior to use.
2.2 Catalysts Preparation
Magnesium-tin composite oxides (denoted hereinafter as
MgO–SnO2) were prepared as shown in Scheme 1. First tin
tetrachloride (SnCl4) was dissolved in water to prepare a
solution of 0.5 M; then 0.1 M KOH was added to the
aqueous SnCl4 solution and white precipitate appeared in a
pH range of 5–7. After being washed and filtered out, the
white precipitate was transferred to a flask, and 0.5 M
KOH solution was added to it to prepare ‘‘solution A’’ with
Scheme 1 Flow chart of MgO–
SnO2 preparation
SnCl4 Solution
Adding KOH Solution
(pH=5-7)
Precipitation
filtering
Mg(NO3)2 Template Solution
Adding KOH Solution
(pH=12-14)
Adding drops by drops
Stirring violently
Solution
Hydrothermal treatment
Calcination
MgO-SnO2
Precipitation
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610
S.-G. Zhang et al.
pH of 12–14. Under vigorous stirring, ‘‘solution A’’ was
added dropwise to 0.5 M aqueous solution of magnesium
nitrate and P123 template. After the mixed solution was
stirred at RT for 12 h, it was transferred to an autoclave
and kept at 180 °C for 24 h. The crystallized precipitate
was filtered out, washed and dried for 12 h to form a
precursor. The MgO–SnO2 was prepared by calcining the
precursor at 550 °C for 2 h (5 °C/min for temperature rise)
under high-purity N2.
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 Hammett Indicators Method
We investigated the effect of Sn/Mg molar ratio on the
basicity of MgO–SnO2. As shown in Table 1, at Sn/Mg
molar ratio of 4/1 and 2/1, the base strength of the as-
synthesized materials is in the range of 15.0 B H- \ 18.4;
while at Sn/Mg molar ratio of 1/2 and 1/4, the base strength
is in the range of 22.5 B H- \ 26.5. The MgO–SnO2
shows superbasicity only at Sn/Mg molar ratio of 1. The
superbasic strength is 26.5 B H- \ 33.0, and the total
superbasic sites is up to 0.833 mmol/g. Therefore, in this
study, only MgO–SnO2 with equimolar ratio of Sn/Mg was
investigated.
2.3 Characterization
X-ray diffraction (XRD) patterns were recorded on a
Brucker D8 advance diffractometer with monochromatized
Cu Ka radiation (k = 0.15406 nm) at a setting of 40 kV
and 40 mA. scanning electron microscopy (SEM) analyses
were performed over a Hitachi S-4800 electron microscope
operating at 2.0 kV. CO2-TPD measurements for the esti-
mation of surface basicity were conducted on a Microm-
eritics 2920 apparatus using thermal conductivity detector
(TCD). The basicity of the as-prepared materials was also
measured by the Hammett indicators method as described
elsewhere [13, 31–33]. N2 adsorption and desorption
isotherms were measured using a Quantachrome Autosorb-
1C/TCD Automatic Chemisorption & Physisorption Ana-
lyzer. The samples were degassed at 473 K under vacuum
for 3 h prior to measurement. The Brunauer-Emmett-Teller
specific surface area was calculated using adsorption data
in the relative pressure range of 0.04–0.2. The total pore
volume was determined from the amount of adsorbed N2 at
a relative pressure of about 0.95.
3.1.2 CO2-TPD
The CO2-TPD result of MgO–SnO2 (molar ratio of Mg/
Sn = 1) is depicted in Fig. 1. In CO2-TPD investigation,
desorbed peaks at 423, 723 and 973 K indicate weak,
Table 1 Effect of Sn/Mg molar ratio on MgO–SnO2 basicity
Sn/Mg Basic
molar ratio strength (H-)
Superbasic Total basic
sites (mmol/g) sites (mmol/g)
4/1
2/1
1/1
1/2
1/4
15.0 B H- \ 18.4
15.0 B H- \ 18.4
0
0
0.317
0.761
1.515
1.308
1.022
26.5 B H- \ 33.0 0.833
22.5 B H- \ 26.5
22.5 B H- \ 26.5
0
0
2.4 Catalyst Evaluation
The basicity was measured by Hammett indicator method
Catalytic activity of the prepared materials was evaluated
for the one-pot synthesis of polyfunctionalized 4H-pyrans
through the condensation of aldehydes, malononitrile and
an active methylene compound. The catalytic reactions
were conducted in a 25 ml round-bottomed flask at RT
using 1 mmol aldehyde, 1.1 mol malononitrile and
1.1 mmol acetoacetate derivatives without use of any sol-
vent. The catalyst loading in all the cases was 0.035 g.
After the reaction, the catalyst was separated from the
mixture by centrifugation. The reaction mixtures were
analyzed using an Agilent Technologies 7820 gas chro-
matograph equipped with FID and AB-FFAP capillary
column (30 m 9 0.25 mm 9 0.25 lm). The conversion of
benzaldehyde was determined using biphenyl as internal
standard. All products were analyzed over an Agilent
6890-5973 MSD GC–MS equipment, and confirmed by 1H
0.939 mmol/g
0.576 mmol/g
0.113 mmol/g
100
200
300
400
500
600
700
800
1
NMR. The H NMR spectra were recorded at 25 °C over
an INOVA-400 M (Varian) instrument calibrated using
Temperature (°C)
Fig. 1 CO2-TPD profile of MgO–SnO2
tetramethyl silane as internal standard.
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Novel MgO–SnO2
611
250
200
150
100
50
strong and superbasic sites, respectively [34, 35]. The
amounts of these basic sites were calculated by the integral
method based on the area of the corresponding desorption
peaks; they are 0.576, 0.113 and 0.939 mmol/g, respec-
tively. It is apparent that the distribution of base strength
obtained from the results of Hammett indicators and CO2-
TPD methods are in good agreement with each other
(Table 1; Fig. 1).
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0
10 20 30 40 50 60
Pore diameter (nm)
3.1.3 XRD
Figure 2 shows the XRD pattern of MgO–SnO2. Compared
to standard card of JCPDS 24-0723, it can be deduced that
the weak diffraction peak is due to Mg2SnO4. There is no
peak related to compounds such as MgO and SnO2. It is
plausible that both MgO and SnO2 in the composite are
amorphous. Since Mg2? and Sn4? are almost the same in
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 3 N2 adsorption/desorption isotherm and pore diameter distri-
bution of MgO–SnO2
4?
2?
diameter (Mg2?: 0.66 A, Sn : 0.69 A), it is easy for Mg
˚
˚
introduction of SnO2 into MgO not only improves super-
basicty, but also increases the specific surface area.
to intercalate into the defects of SnO2 [36], hindering the
crystallization of SnO2 and enhancing the formation of
superbasic sites.
3.1.5 SEM
3.1.4 N2 Adsorption/Desorption Isotherm
Figure 4 shows the SEM images of MgO–SnO2 prepared
with and without the use of P123 template. It is obvious
that the use of P123 template led to the formation of uni-
form spheres with diameter of ca. 150 nm. The SEM result
is in accord with the N2 adsorption/desorption isotherm.
However, without the use of template, the MgO–SnO2 is
irregular in shape. Therefore, the presence of template
shows important effect on the morphology of MgO–SnO2.
As shown in Fig. 3, the N2 adsorption/desorption isotherm
of MgO–SnO2 is typical of type-IV isotherm. According to
the hystersis loop, MgO-SnO2 is mesoporous in nature and
possesses narrow pore diameter distribution [8], and the
channels of such a dimension are favorable for the diffu-
sion of reactants into the interior part of the catalyst. The
specific surface area and pore volume of MgO–SnO2 are
115.2 m2/g and 0.348 ml/g, respectively. Generally, the
specific surface area of MgO prepared by conventional
method is not higher than 70 m2/g [37]. Therefore, the
3.2 Catalytic Performance
In order to evaluate the catalytic performance of MgO–
SnO2, one-pot synthesis of 4H-pyrans from benzaldehyde,
malononitrile and ethyl acetoacetate was used as a model
reaction, and MgO, SnO2, CaO, KF/Al2O3 and MgO–
Al2O3 were employed as reference catalysts. One can see
from Table 2 that MgO–SnO2 shows high catalytic activity
towards this reaction, and the 4H-pyrans yield is up to
87.1 %, much higher than that of MgO (49.7 %), SnO2
(12.5 %), CaO (50.6 %), KF/Al2O3 (57.2 %) and MgO–
Al2O3 (53.8 %). It is noted that compared to the use of
amino-functionalized ionic liquid [23], tetrabutylammo-
nium bromide [25], tetra-methyl ammonium hydroxide
[26], (BMIm)BF4 [27], SiO2 [28], piperidine [29], and Mg/
La oxide [30], the use of the superbases results in much
better catalytic efficiency; also the process is simpler and
the reaction conditions milder. By correlation of the cata-
lytic performance and the basicity of these catalysts in
Table 2, one can deduce that strong basicity is beneficial
for this reaction, which is similar to the results in the
ref. [30].
Mg2SnO
4
10
20
30
40
2θ(o)
50
60
70
80
Fig. 2 XRD pattern of MgO–SnO2
123