Novel and versatile solid superbases derived from magnesium-zirconium composite oxide and their catalytic applications

Article abstract of DOI:10.1039/c3ra46559d

Versatile solid superbases were derived from magnesium-zirconium composite oxide and KOH through thermal treatment in a N₂ flow. The magnesium-zirconium composite oxide was prepared by a modified co-precipitation technique with reflux-digestion in basic solution and then mixed with KOH through grinding. The as-prepared solid superbases were characterized by low-temperature N₂ physisorption, powder X-ray diffraction, and Fourier transformation infrared spectrophotometry. The superbasic sites were characterized by the use of Hammett indicators and CO₂ temperature-programmed desorption methods. We found that there are ample superbasic sites (0.590 mmol g^-1) on the surface with strength in the 26.5 ≤ H_- < 33.0 range. The as-prepared solid superbase catalyst was found to show excellent catalytic activity towards Knoevenagel condensation, Michael addition and transesterification reactions. The findings open up a new route for the synthesis of new functional superbases using composite oxides prepared by reflux-digestion in basic solution as supports.

Full text of DOI:10.1039/c3ra46559d

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Novel and versatile solid superbases derived from
magnesium–zirconium composite oxide and their
Cite this: RSC Adv., 2014, 4, 6159
catalytic applications
a
Jin Zhao,a Jun Xie,a Chak-Tong Auab and Shuang-Feng Yin
*
Versatile solid superbases were derived from magnesium–zirconium composite oxide and KOH through
thermal treatment in a N2 flow. The magnesium–zirconium composite oxide was prepared by a modified
co-precipitation technique with reflux-digestion in basic solution and then mixed with KOH through
grinding. The as-prepared solid superbases were characterized by low-temperature N2 physisorption,
powder X-ray diffraction, and Fourier transformation infrared spectrophotometry. The superbasic sites
were characterized by the use of Hammett indicators and CO2 temperature-programmed desorption
methods. We found that there are ample superbasic sites (0.590 mmol gꢀ1) on the surface with strength
Received 13th November 2013
Accepted 13th December 2013
in the 26.5 # H < 33.0 range. The as-prepared solid superbase catalyst was found to show excellent
ꢀ
catalytic activity towards Knoevenagel condensation, Michael addition and transesterification reactions.
The findings open up a new route for the synthesis of new functional superbases using composite
oxides prepared by reflux-digestion in basic solution as supports.
DOI: 10.1039/c3ra46559d
www.rsc.org/advances
a basic support could result in the generation of stronger basic
sites.22 Previously, we synthesized a KOH/La2O3–MgO solid
superbase catalyst by co-precipitation.23 In the present work, we
Introduction
Solid superbases are materials that possess basic sites with
strength (H ) higher than 26 (H corresponding to the pKa value
prepared MgO–ZrO2 support large (165.9 m2 gꢀ1) in surface area
by reux-digestion in a basic solution. We describe herein the
synthesis of a series of solid superbases using the magnesium–
zirconium composite oxide as support and KOH as modier.
We characterized the structure and surface properties of the
KOH/MgO–ZrO2 samples. The as-prepared solid superbase were
tested as catalysts for a number of important organic base-
catalyzed reactions, viz. Knoevenagel condensation, Michael
addition and transesterication.
ꢀ
ꢀ
of the indicator).1 The use of solid superbases has attracted
much attention because they catalyze a number of organic
reactions under mild conditions, resulting in a signicant
reduction in pollutant generation.1–3 In the past decades, there
were reports on solid superbase catalysts such as KNO3/g-
Al2O3,4,5 KNO3/ZrO2,6,7 KF/g-Al2O3,8 KOH/ZrO2,9 K2CO3/g-
Al2O3 10 and Ca(NO3)2/SBA-15.11 It is noted that only a few types
of materials were employed as supports for the preparation of
superbases, viz. g-Al2O3,4,5,8 ZrO2
6,7,9
and SBA-15 mesoporous
molecular sieve.11 Therefore, the selection and/or synthesis of
suitable materials as supports for the synthesis of superbases
should be considered essential for future development of
superbases.
In recent years, MgO–ZrO2 has been widely used as solid
catalysts or basic supports in various chemical reactions (e.g.
O-tert-Boc protection of alcohols and phenols,12 alkylation,13
aldol condensation,14–17 Knoevenagel condensation18 and
transesterication19–21). However, there is no report on magne-
sium–zirconium composite oxides with sites that are super-
basic. In a study of Clacens et al., it was observed that the use of
Results and discussion
Displayed in Fig. 1 are the XRD patterns of MgO–ZrO2 and KOH/
MgO–ZrO2 samples. We detect MgO and magnesium–zirco-
nium composite oxide (Mg2Zr5O12 JCPDS le 80-967) but not
ZrO2 in the MgO–ZrO2 sample (Fig. 1a). It is induced that the
octahedral vacant sites on the surface of ZrO2 prompt the Mg2+
ions to occupy the position of octa-coordinated Zr4+, leading to
the strong interaction of MgO with ZrO2,21 consequently facili-
tating the formation of superbase sites. Similar phenomenon
was also observed in previous reports.6,7 With the loading of 7 or
14 wt% of KOH on MgO–ZrO2 (Fig. 1b and c), the obtained
patterns are similar to that of MgO–ZrO2. Nonetheless extremely
weak signals of KOH are observed when KOH loading is 21 wt%
(JCPDS le 21-645) (Fig. 1d), and these signals become evident
at KOH loading of 28 wt% (Fig. 1e). It is clear that there exists a
dispersion threshold of KOH on MgO–ZrO2, a kind similar to
aState Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry
and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China.
E-mail: sf_yin@hnu.edu.cn; Fax: +86-731-88821171; Tel: +86-731-88821171
bDepartment of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong,
China
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Fig. 1 XRD patterns of (a) MgO–ZrO2 (b) 7% KOH/MgO–ZrO2 (c) 14%
KOH/MgO–ZrO2 (d) 21% KOH/MgO–ZrO2 (e) 28% KOH/MgO–ZrO2. *:
Mg2Zr5O12, A: MgO, C: KOH.
Fig. 2 FT-IR spectra of (a) MgO–ZrO2 and (b) 21% KOH/MgO–ZrO2.
optimize the generation of superbasic sites, potassium loading
around 21% is the most appropriate, consistent with the results
of XRD investigation.
the dispersion of KNO3 on zirconia7 or K2CO3 on alumina.10
Based on the results, we deduce that the highest spontaneous
dispersion capacity of KOH on MgO–ZrO2 is ca. 21%.
The basicity of samples was further investigated by CO2-TPD.
As depicted in Fig. 3, the MgO–ZrO2 sample shows main
desorption peaks at ca. 143 and 568 ꢁC. Aer the introduction of
KOH, desorption peaks are observed at ca. 144, 286,ꢁ588 and 653
ꢁC. The emergence of desorption peaks at ca. 653 C is a clear
indication of superbasicity.27,29 The amounts of superbasic sites
on the 7, 14, 21, 28 wt% KOH/MgO–ZrO2 samples as calculated
by the integral method based on the area of the corresponding
desorption peaks are 0.284, 0.446, 0.553 and 0.318 mmol gꢀ1
respectively (Table 1). According to the reports of Zhu and
co-workers, there exits an overlapping structure of basic species
formed on their samples.4,7 With such an overlapping structure,
basic species covered by the upper layers were not exposed to
CO2, but would react with an acidic agent if the top layers were
dissolved in an aqueous titration process. As a result, the CO2-
TPD data only reveal the amount of basic sites exposed at the
top of the overlapped structure while the titration data repre-
sent the total basicity of the composite, with the former smaller
than the latter. It is hence considered that the CO2-TPD results
are in good agreement with those obtained by the Hammett-
indicator method (Table 1).
The BET surface area and pore volume of MgO–ZrO2 and
KOH/MgO–ZrO2 catalysts of different KOH loadings are given in
Table 1. The BET surface area of MgO–ZrO2 is 165.9 m2 gꢀ1
.
With increase of KOH content, there is decrease of specic
surface area as well as pore volume. The results suggest that
there is the diffusion of potassium species into the pores of
MgO–ZrO2.
Fig. 2 gives the IR spectra of as-synthesized MgO–ZrO2 and
21 wt% KOH/MgO–ZrO2. For the two samples, the broad band at
around 3450 cmꢀ1 is assigned to OH stretching vibration of the
hydroxyl groups attached to Mg. The bands in the 400–700 cmꢀ1
region are ascribable to the vibration of ZrO2 24 whereas those in
the 1300–1450 cmꢀ1 region are attributable to the vibration of
2ꢀ
CO3 , with the latter being stronger than the former.25–27 The
results demonstrate that with the introduction of KOH, there is
the creation of basic sites and promotion of CO2 adsorption.
The base strength (H ) and amount of basic sites of samples
ꢀ
were measured using Hammett indicators (Table 1). One can
see that the basic strength of MgO–ZrO2 is in the 15.0 # H
<
ꢀ
18.4 range. With the introduction of KOH, basic strength of
The Knoevenagel condensation reaction is important for the
formation of C]C double bond between carbonyl and active
KOH/MgO–ZrO2 samples are in the 26.5 # H < 33.0 range. It is
ꢀ
clear that the KOH/MgO–ZrO2 materials can be regarded as
solid superbases.1 The KOH/MgO–ZrO2 samples are different
in the amount of superbasic sites, showing a maximum
(0.59 mmol gꢀ1) at KOH loading of 21%. In other words, to
Table 1 Physicochemical characteristics of MgO–ZrO2 and KOH/
MgO–ZrO2 samples
a
b
KOH loading SBET
(wt%)
Vr
Base
strength (H )
Superbasic sites
(m2 gꢀ1
)
(cm3 gꢀ1
)
(mmol gꢀ1
)
ꢀ
0
7
14
21
28
165.9
139.5
91.1
50.7
34.6
0.536
0.454
0.361
0.240
0.163
15.0 # H < 18.4
0
ꢀ
26.5 # H < 33.0 0.297c, 0.284d
ꢀ
ꢀ
ꢀ
ꢀ
26.5 # H < 33.0 0.468c, 0.446d
26.5 # H < 33.0 0.590c, 0.553d
26.5 # H < 33.0 0.361c, 0.318d
a
b
c
Fig. 3 CO2-TPD profiles of (a) MgO–ZrO2 and (b) 21% KOH/MgO–
ZrO2.
SBET: BET specic surface area. Vr: pore volume. Measured by
Hammett indicators method. d Measured by CO2-TPD.
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methylene compounds.18,28–30 Depicted in Table 2 are the results on 21 wt% KOH/MgO–ZrO2 are suitable for this kind of base-
of Knoevenagel condensation of various aldehydes with active catalyzed processes.
methylene compounds over 21 wt% KOH/MgO–ZrO2. In all
Transesterication of ester with alcohol is important in the
cases the products are generated in good to excellent yield, synthesis of ne chemicals, and is commonly catalyzed by basic
comparable or superior to those of solid bases such as materials.26,36,37 We used 21 wt% KOH/MgO–ZrO2 to catalyze the
CexZr1ꢀxO2, In/AlMCM-41, MgO–ZrO2 and LDH-F.18,29–31 transesterication of propylene carbonate (PC) with methanol to
Namely, the 21 wt% KOH/MgO–ZrO2 catalyst can be regarded as produce DMC and propylene glycol (PG). The results are shown
an excellent catalyst for Knoevenagel condensation reactions.
in Fig. 4. It is worth noting that the yield of DMC reaches 53.3%
Michael addition is widely used for C–C bond formation in at a reaction time of 1 h and increases to 68.2% at 3 h under
organic synthesis.30–34 We report here the catalytic properties of atmospheric pressure, higher than those recorded in the litera-
21 wt% KOH/MgO–ZrO2 for different Michael additions (Table tures.26,36,37 It is observed that the DMC yield remains almost
3). A comparison of substrates with different pKa not only allows constant even the reaction is prolonged to 5 h, suggesting that
us to estimate the reactivity of the catalyst but also enable us to the reaction is limited by thermodynamic equilibrium.38 For
evaluate the basic strength of the catalyst surface.35 The fast comparison, the typical solid base MgO–ZrO2 and MgO were
reactions at RT demonstrate that 21 wt% KOH/MgO–ZrO2 has employed to catalyze the transesterication reaction. Under the
sites of high basic strength able to abstract a proton from same reaction conditions, only 12.7% and 10.4% of DMC was
reactants of high pKa. The high yield and short reaction time at produced at 3 h respectively. This demonstrates the excellent
RT and atmospheric pressure conrm that the strong basic sites catalytic performance of the 21 wt% KOH/MgO–ZrO2 catalyst.
Table 2 Knoevenagel condensation catalyzed by 21 wt% KOH/MgO–ZrO2 and some other catalysts reported in the literaturea
Entry
1
R
R1
Catalyst
Time/min
Yield (%)
Ref.
C6H5
CN
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
15
98.2
82b
93c
95d
96.2
78b
92f
50b
20c
25d
60
28
17
29
In/AlMCM-41
2
3
4
C6H5
COOEt
CN
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
LDH-F
75b
120e
90c
15
28
30
17
MgO–ZrO2
80c
97
4-NO2C6H4
4-NO2C6H4
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
45b
20c
45d
60
90b
67c
89d
98.7
85b
88c
79d
93.6
81b
65c
94.3
78b
86 f
65c
97.8
78b
82c
91d
91.9
72b
84c
82d
91.4
68c
88d
87
28
17
29
In/AlMCM-41
COOEt
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
55b
90c
150d
15
28
17
29
In/AlMCM-41
5
6
4-ClC6H4
4-ClC6H4
CN
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
LDH-F
60b
20c
60
28
17
COOEt
60b
120e
90c
15
28
30
17
MgO–ZrO2
7
8
9
4-CH3C6H4
4-CH3C6H4
CN
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
60b
20c
30d
60
28
17
29
In/AlMCM-41
COOEt
21 wt% KOH/MgO–ZrO2
CexZr1ꢀxO2
MgO–ZrO2
In/AlMCM-41
21 wt% KOH/MgO–ZrO2
MgO–ZrO2
In/AlMCM-41
21 wt% KOH/MgO–ZrO2
In/AlMCM-41
90b
90c
110d
15
28
17
29
4-CH3OC6H4
4-CH3OC6H4
CN
20c
40d
60
17
29
10
COOEt
145d
78d
29
a
b
Reaction conditions: aldehyde (2 mmol), active methylene compound (2 mmol), catalyst (0.035 g), DMF (1 ml), RT. Aldehyde (1 mmol), active
methylene compound (1.1 mmol), catalyst (10 wt%), ethanol, 80 ꢁC. c Aldehyde (10 mmol), active methylene reagents (10 mmol), catalyst (20 wt%),
d
e
60 ꢁC. Aldehyde (10 mmol), active methylene compound (12 mmol), catalyst (0.1 g), ethanol reuxed. Aldehyde (1 mmol), active methylene
reagents (1.1 mmol), catalyst (0.035 g), DMF (5 ml). f Aldehyde (1 mmol), active methylene reagents (1.1 mmol), catalyst (0.035g), DMF or MeCN (5 ml).
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Table 3 Michael addition results over 21 wt% KOH/MgO–ZrO2 and some other catalysts reported in the literaturea
Entry
Donor
pKa in DMSO
Catalyst
Reaction time (h)
Yield (%)
Ref.
21 wt% KOH/MgO–ZrO2
Modied Mg–Al hydrotalcite
MgO
Mg–La
21 wt% KOH/MgO–ZrO2
MgO
Mg–La
21 wt% KOH/MgO–ZrO2
Mg–Al–O-t-Bu
Mg–La
21 wt% KOH/MgO–ZrO2
Mg–Al–O-t-Bu
0.15
2b
99.2
88b
88c
99d
91.7
88c
97d
93
31
32
34
1
11.1
1.5c
0.16d
0.5
0.5c
1.5d
1
2
3
13.1
14.2
32
31
0.16e
1d
86e
82d
94.8
95e
96b
93c
95f
33
34
1
2e
33
31
32
30
4
16.4
Modied Mg–Al hydrotalcite
MgO
LDH-F
6b
2c
2f
21 wt% KOH/MgO–ZrO2
Mg–Al–O-t-Bu
1
95.2
1e
92e
33
5
6
16.4
17.2
CH3NO2
21 wt% KOH/MgO–ZrO2
Mg–Al–O-t-Bu
Mg–La
0.5
0.16e
2d
97.4
93e
98d
90f
33
34
30
LDH-F
2f
a
b
Reaction conditions: acceptor (1 mmol), donor (1.1 mmol), catalyst (0.05 g), DMF (2 ml). Acceptor (2 mmol), donor (2 mmol), catalyst (0.2 g),
c
d
methanol (10 ml). Acceptor (2 mmol), donor (2 mmol), catalyst (0.05 g), DMF (2 ml). Acceptor (2 mmol), donor (2.2 mmol), catalyst (0.1 g),
DMF (4 ml). Acceptor (2 mmol), donor (2 mmol), catalyst (0.1 g), methanol (10 ml). Acceptor (1 mmol), donor (1 mmol), catalyst (0.1 g),
MeCN (5 ml).
e
f
Fig.
5 Recyclability profile of 21% KOH/MgO–ZrO2 in trans-
Fig. 4 DMC yield vs. time in the transesterification of PC with meth-
anol over 21% KOH/MgO–ZrO2.
esterification reaction.
We also tested the recyclability of the catalyst for the trans-
esterication of propylene carbonate (PC) with methanol
(Fig. 5). Aer each run, the catalyst was separated from the
Experimental
Catalysts preparation
product by centrifugation, and was immediately reused (with Magnesium–zirconium composite oxide (Mg/Zr molar ratio ¼ 2)
the addition of fresh reactants) without any treatment. We was prepared by co-precipitation method. Required amounts of
observed that there is an obvious decrease of product yield Mg(NO3)2$6H2O and ZrO(NO3)2$5H2O were dissolved in
across the ve runs. The deactivation of catalyst is likely due to distilled water separately to form transparent solutions, which
the blocking of basic and superbasic sites by organic entities. were then mixed together. 1 M KOH aqueous solution was
We nd that once the organic entities are removed through the added dropwise to the mixture under stirring until a pH of 10–
adopted calcination process, the spent catalyst recovers in 12. The mixture was digested under reux at 366 K for 24 h, and
activity (see number 6 and 7 in Fig. 5). Therefore, the superbase then subject to ltration and washing with water until the pH
catalyst is highly recyclable and reusable.
value of ltrate was near 7. The as-obtained solid substance was
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dried at 383 K overnight and calcined at 823 K for 4 h to get
magnesium–zirconium composite oxide.
The KOH-modied MgO–ZrO2 samples were prepared by
grinding method, followed by thermal treatment under a ow of
N2 (purity: 99.999%) at 500 ꢁC for 2 h. The as-prepared sample is
denoted hereinaer as “x wt% KOH/MgO–ZrO2”, where x stands
for the weight percentage of KOH in terms of the whole catalyst.
Scheme 2 Michael addition of chalcone and methylene compounds.
Catalyst characterization
XRD patterns were recorded on a Brucker D8 advance diffrac-
tometer with monochromatized Cu Ka radiation (l ¼ 0.15406 nm)
at a setting of 40 kV and 40 mA. The specic surface areas of
catalysts were determined by BET method based on nitrogen
adsorption–desorption isotherms (at 77 K) collected over a Beck-
man (SA 3100) surface area analyzer. The FTIR spectra were
determined using a Bruker vector 22 FTIR spectrophotometer
(using KBr tablets). CO2-TPD measurements were conducted on a
Micromeritics 2920 apparatus using thermal conductivity
Scheme 3 Transesterification of cyclic carbonate with methanol to
produce DMC.
Agilent Technologies 7820 GC equipped with a FID and HP-5
capillary column (30 m ꢂ 0.32 mm ꢂ 0.25 mm). The conversion
of chalcone was determined using biphenyl as internal
standard.
Transesterication of propylene carbonate with methanol to
produce dimethyl carbonate (DMC) was carried out at 353 K
using 0.01 mol propylene carbonate, 0.10 mol of methanol, and
100 mg catalyst in a glass batch reactor equipped with
condenser and a magnetic stirrer (Scheme 3). The analysis of
products was similar to that of Michael addition.
ꢁ
detector. The sample was activated at 400 C for 2 h prior to the
adsorption of CO2 at 100 ꢁC. Aer purging the physically adsorbed
CO2 at 100 ꢁC, the sample was heated to 850 ꢁC at a rate of 8 ꢁC
minꢀ1. The base strength (H ) of the sample was also determined
ꢀ
by Hammett indicators.39 To measure the total basicity, the acti-
vated samples was immersed in 0.05 mol lꢀ1 HCl (20 ml) with
stirring, and the remaining acid was then titrated against stan-
dard NaOH solution (0.02 mol lꢀ1).
Conclusions
Catalyst evaluation
A versatile solid superbase material has been obtained by
loading KOH onto magnesium–zirconium composite oxide of
high surface area, followed by thermal treatment at 500 ꢁC
under a ow of high-purity N2. There are ample superbasic sites
Knoevenagel condensation of aldehydes and methylene
compounds was performed at RT in a glass reactor (Scheme 1).
In a typical run, aldehyde (2 mmol), active methylene reagent
(2 mmol), catalyst (20 mg), and DMF (1 ml) were mixed in a
25 ml round-bottomed ask. The reaction was monitored by
thin layer chromatography (TLC). Aer the reaction, the mixture
was subject to centrifugation for catalyst recovery. The reaction
mixtures were analyzed using an Agilent Technologies 7820 gas
chromatograph equipped with a ame ionization detector (FID)
and AB-FFAP capillary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm).
The conversion of aldehyde was determined using biphenyl as
internal standard.
up to 0.590 mmol gꢀ1 with strength in the 26.5 # H < 33.0
ꢀ
range. The materials show high catalytic activity and selectivity
in Knoevenagel condensation, Michael addition and trans-
esterication reactions. The one with 21 wt% KOH loading is
the best, showing the highest superbasicity and catalytic
activity. It is envisaged that this versatile catalyst is also useful
for other important organic reactions and has a wide scope in
catalysis technology.
Michael addition of chalcone and methylene compounds
was performed in a glass reactor (Scheme 2). Chalcone
(1 mmol), methylene compounds (1 mmol), catalyst (0.05 g) and
DMF (2 ml) were added and the mixture was stirred at RT for an
appropriate period of time. Aer reaction completion (moni-
tored by TLC), the mixture was subject to centrifugation for
catalyst recovery. The reaction mixture was analyzed using an
Acknowledgements
The project was nancially supported by the National Natural
Science Foundation of China (Grant no. U1162109), the
program for New Century Excellent Talents in Universities
(NCET-10-0371), Program for Changjiang Scholars and Innova-
tive Research Team in University (IRT1238), and the Funda-
mental Research Funds for the Central Universities. C. T. Au
thanks the Hunan University for an adjunct professorship.
Notes and references
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p. 3.
Scheme
methylene compounds.
1 Knoevenagel condensation of aldehydes with active
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