retracted: catalytic applications of ordered mesoporous magnesium oxide synthesized by mesoporous...
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al 338 (2008) 20–26
Applied Catalysis A: GenerCatalytic applications of ordered mesoporous magnesium
oxide synthesized by mesoporous carbon
Amit Dubey *, Braj Gopal Mishra, Divya Sachdev
Chemistry Group, Birla Institute of Technology and Science, Pilani, Rajasthan 333031, India
Received 8 August 2007; received in revised form 14 December 2007; accepted 16 December 2007
Available online 25 December 2007
D AbstractIn an attempt to introduce the intrinsic basicity properties, we synthesized ordered mesoporous magnesium oxide (MgO) using mesoporous
carbon (CMK-8) as the host material for more effective use for base catalysis. The catalytic activity studies for various base catalyzed condensation
reactions showed very high activity and selectivity. The knowledge obtained was extended for the aldol condensation reaction between
glyceraldehydes acetonide and acetone. Very high activity and selectivity of the desired product was observed compared to the values for other
mixed oxides reported in the literature.
# 2007 Elsevier B.V. All rights reserved.TE
Keywords: Mesoporous materials; Catalysts; Ordered mesoporous MgO
AC
1. IntroductionThe increasing amounts of industrial waste have a
significant and serious impact on the environment and force
the modern chemical industry towards clean processes
because of the increasing demand of chemicals in various
applications [1–6]. Heterogeneous systems is an alternative
approach over the wide use of concentrated acids, bases and
hazardous organic solvents. The nanostructured materials
with well-defined functionalities are receiving tremendous
attention due to their remarkable properties such as small and
uniform particle size and large surface areas, combined with
the tunable pore sizes, better dispersion of active centers and
their shape selectivity [1–3]. However, most of the porous
materials ranging from microporous materials (zeolites) to
the mesoporous materials (silica, aluminosilicates or carbon)
are either acidic or neutral. Hence, the basic functionalities
are introduced with post synthetic methods by mixing the
solutions of the host to the guest materials either on alkali
metal exchanged zeolites [7–9] or on mesoporous silica
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* Corresponding author. Fax: +91 1596 244183.
E-mail addresses: [email protected], [email protected]
(A. Dubey).
0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2007.12.015
[10,11]. The need of materials having intrinsic basicity is
strongly encouraged but few reports are available on the
synthesis towards this direction in the literature. One such
direction may be to synthesize ordered mesoporous oxides
with varying properties compared to the mixed oxides
synthesized using coprecipitaion, sol–gel, ultrasonic or
microwave assisted methods. Recently, the synthesis of
mesoporous MgO is reported [12] but no report is available
on the catalytic applications of ordered mesoporous
magnesium oxide (MgO) synthesized by exotemplating
methods, the term that is frequently used for mesoporous
carbon where mesoporous silica acts as an exotemplate [12].
Because the potential applications of the mixed metal oxides
derived from hydrotalcites and their hydrated forms [13–15]
are widely known, the demand of such ordered mesoporous
MgO catalyst is strongly sought for more improved and
advanced applications. With this view, we report the
synthesis of ordered mesoporous MgO by an exotemplating
technique using CMK-8 as the host material for many organic
transformations such as Knoevenagel condensation, Claisen
Schmidt condensation and Michael addition (Table 1). Many
catalysts, such as alkali-ion-exchanged zeolites, alkali-ion-
exchanged sepiolite, oxynitrides, t-BuOK supported on
xonotlite and reconstructed hydrotalcite are reported to give
quantitative yields for a variety of Knoevenagel condensa-
Table 1
Catalytic activity of different substrates with mesoporous MgO and MO-HT
S. no. Reactant (s) Product (s) Conversiona Selectivity (%)
1. 92 (80) 100 (85)
2. 84 (68) 100 (90)
3. 90 (70) 100 (85)
4. 80 (58) 90 (64) (2)b
Reaction conditions: substrate, 1 g; catalyst, 50 mg; temperature, 393 K; time, 6 h, without solvent.a Value in the bracket (–) corresponds to MO-HT.b Refers to the selectivity of product (2).
A. Dubey et al. / Applied Catalysis A: General 338 (2008) 20–26 21
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tions [16–18]. Furthermore, aluminophosphate oxynitrides
(ALPONS) are also found effective in similar kinds of
knoevenagel condensation reactions [19,20]. In addition,
mesoporous silica modified with amino groups was also
tested [21–23] for these conversions. The interesting
variation in the catalytic activity and selectivity results over
these substrates prompted us to test this catalyst for another
interesting liquid phase condensation reaction of glycer-
aldehyde acetonide with acetone for better conversion and
selectivity. The aldol condensation reaction between glycer-
aldehyde acetonide and acetone [24] is a very useful reaction
to synthesize a, b-unsaturated compounds that are otherwise
very difficult to synthesize. Their versatile uses as Michael
acceptor, in enantiomeric synthesis and as chirons in
methodological studies will be encouraged [17,25]. Gen-
erally, these products are synthesized via a Wittig reaction
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using phosphorous ylids or phosphoranes and suffer fromvarious disadvantages such as multi step synthesis and
triphenylphosphine as a major byproduct [26].
2. Experimental techniques
2.1. Synthesis of KIT-6 and CMK-8
The synthesis of KIT-6 and CMK-8 is done exactly in the
same way as reported by Ryoo and co-workers [27]. The large
mesoporous silica KIT-6 with cubic Ia3d symmetry is
prepared in aqueous solution using a 1:1 wt% mixture of
Pluronic P123 (EO20PO70EO20, MW = 5800, Aldrich) and
butanol in around 0.5 M HCl concentrations at 35 8C.
Tetraethoxysilane (TEOS) is used as a silica source. In a
typical synthesis batch with TEOS, 3 g of P123 was dissolved
A. Dubey et al. / Applied Catalysis A: General 338 (2008) 20–2622
A
in 100 g of distilled water and 5.9 g of conc. HCl (35%). To
this, 3 g of butanol (Aldrich, 99.4%) was added under stirring
at 35 8C. After 1 h stirring, 6.5 g of TEOS (ACROS, 98%)
was added at 35 8C. The mixture was left under stirring for
24 h at 35 8C, and subsequently heated for 24 h at 100 8Cunder static conditions in a closed polypropylene bottle. The
solid product obtained after hydrothermal treatment was
filtered and dried at 100 8C without washing. The template
was removed by extraction in an ethanol–HCl mixture,
followed by calcination at 550 8C.
2.2. Synthesis of CMK-8
The synthesis of CMK-8 is done in the same way as
reported by Ryoo et al. [28]. CMK-8 is synthesized using
KIT-6 as a template similar to the process reported earlier
[28]. Briefly, 0.5 g of KIT-6 was added to a solution obtained
by dissolving 0.6 g of sucrose and 0.1 g of H2SO4 in 4 g of
H2O. The mixture was placed in a drying oven for 6 h at
373 K, and subsequently the oven temperature was increased
to 433 K and maintained there for 6 h. The sample turned dark
brown or black during the treatment in the oven. The silica
sample, containing partially polymerized and carbonized
sucrose at the present step, was treated again at 373 and 433 K
using the same drying oven after the addition of 0.4 g of
sucrose, 0.05 g of H2SO4 and 2 g of H2O. The carbonization
was completed by pyrolysis with heating to typically 1173 K
under vacuum. The carbon–silica composite obtained after
pyrolysis was washed with 1 M NaOH solution (50 vol%
ethanol–50 vol% H2O) twice at 373 K or 5 wt% hydrofluoric
acid at room temperature, to remove the silica template. The
template-free carbon product thus obtained was filtered,
washed with ethanol, and dried at 393 K.
2.3. Synthesis of ordered mesoporous MgO
The typical procedure for the synthesis of ordered
mesoporous MgO involves the impregnation of the metal
nitrate solution into the pores of the mesoporous carbon, drying
in vacuum and finally calcining at high temperature. Typically,
15 ml solution of 0.25 M Mg (NO3)2�6H2O was mixed with
1.5 g of CMK-8 material. The solution was stirred for 4 h
followed by drying in vacuum at 630 K to convert the Mg
(NO3)3�6H2O to magnesium oxide. This procedure was
repeated three to four times so that all the mesopores of
CMK-8 are completely filled with Mg (NO3)2�6H2O solution.
The resulting solid was finally calcined at 1073 K in air to
remove the carbon and to obtain the ordered mesoporous MgO
with its increased thermal stability. In parallel, the mixed (Mg-
Al) oxide was also synthesized from hydrotalcites (by keeping
Mg/Al = 3) using a coprecipitation method followed by its
decomposition at 600 8C as reported earlier [20] to compare its
catalytic performance with ordered mesoporous MgO. The
material was named MO-HT. The samples thus synthesized
were characterized by powder X-ray diffraction method,
elemental analysis, N2 adsorption–desorption and TEM
analysis to confirm the ordered structure.
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2.4. Characterization
Powder X-ray diffraction (PXRD) patterns were recorded on
a Siemens D-500 diffractometer using Ni-filtered Cu Ka
radiation (l = 1.5418 A, Cu Ka). N2 adsorption isotherms were
measured at 77 K using a Quantachrome degas port of the
adsorption analyzer. Before the adsorption measurements,
samples were outgassed for 12 h at 353 K in the degas port of
the adsorption analyzer. The transmission electron micrographs
(TEM) were obtained with a JEOL-JEM 100SX microscope,
working at a 100 kV accelerating voltage. Samples for TEM
were prepared by dispersing the powdered sample in ethanol by
sonication and then drop drying on a copper grid (400 mesh)
coated with carbon film.
2.5. Catalysis
The catalytic activity studies were carried out in liquid phase
conditions without any solvent. Typically, the required amounts
of the reactants were mixed in glass reactors under nitrogen
atmosphere kept at 373 K, and freshly activated catalyst was
quickly added to the reaction mixture under stirring conditions.
Small amounts of the samples was periodically withdrawn from
the reaction mixture and analyzed by gas chromatography.
Specially designed stainless steel parr reactors were used for
the liquid phase condensation reaction of glyceraldehyde
acetonide and acetone. The synthesis of glyceraldehyde
acetonide was done exactly by the method reported earlier
[29]. The product analysis was done using gas chromatography
by taking the authentic samples after considering their response
factors. Mass balance of the reaction was calculated using n-
decane as an internal standard.
3. Results and discussions
The powder X-ray diffraction patterns of the MgO sample
(Fig. 1) showed the characteristic peaks at low angle similar to
KIT-6 and CMK-8 indicates the nature of the mesoporous MgO
catalyst [12,19]. PXRD pattern of MgAl-hydrotalcite (Mg/Al
atomic ratio 3) and its decomposition pattern that is shown in
Supplementary information, Fig. 1S showed the presence of
pure hydrotalcite and mixed oxide phase. PXRD pattern of
CMK-8 and ordered MgO at high angles are also shown in
Supplementary information, Fig. 2S. The presence of very
weak diffraction peaks around 2u 42.18 and 61.78 shows the
formation of the MgO. N2 adsorption–desorption isotherm
(Fig. 2) showed type-IV adsorption isotherm according to
IUPAC classification, with a sharp capillary condensation step
at relatively high pressure and with an H1 hysteresis loop
indicative of well-defined pores. Transmission electron micro-
scopy images (Fig. 3) further substantiate the long range
ordering of the MgO particles. All these results confirmed the
presence of the mesoporous nature and the long range ordering
of MgO catalysts. The structural parameters of the samples are
given in Table 3. Emphasis was mainly devoted to characterize
the MgO catalyst and was not devoted to the MO-HT reported
in Table 3.
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Fig. 2. N2 adsorption–desorption of (a) KIT-6, (b) CMK-8 and (c) ordered mesoporous MgO.
Fig. 3. TEM image of mesoporous MgO. Scale bar correspond to 70 nm.
Fig. 1. PXRD pattern of KIT-6, CMK-8 and ordered mesoporous MgO.
A. Dubey et al. / Applied Catalysis A: General 338 (2008) 20–26 23
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4. Catalytic activity
4.1. Effect of the catalyst on different substrates
All the substrates were chosen because of the importance of
their products.
The catalytic activity results in Table 1 showed very high
conversion of the reactants as well as the selectivity of different
products on ordered MgO catalysts. The high activity and
selectivity of these reactions clearly demonstrate the potential
use of this ordered MgO catalysts compared to the MO-HT. In
the present investigation, attempts were devoted to mainly seek
for better selectivity in the reactions where the self competitive
reactions (secondary products) are difficult to avoid. We believe
that the design of the catalysts having small size and ordered
arrangement may be helpful to solve the above cited difficulties
in many multi-product conversions. In particular, results
obtained on the activity and the selectivity (Table 1, entry-4)
Table 2
Possibility of different products in the liquid phase condensation of glyceraldehyde acetonide with acetone
S. no. Reactant (s) Product (s)
5.
(A) Formed from the aldol condensation between glyceraldehyde acetonide and acetone followed by the dehydration of the aldol condensation product (B) and (C).
Product C is obtained either from the aldol condensation between glyceraldehyde acetonide and diacetone alcohol, the latter being produced by the aldol self
condensation of acetone or by the aldol condensation between acetone and product (B). (D) Diacetone alcohol is formed by the self condensation of acetone or by the
aldol condensation between acetone and product (B).
A. Dubey et al. / Applied Catalysis A: General 338 (2008) 20–2624
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prompted us to explore the possibility of such ordered MgOcatalysts on the liquid phase condensation of glyceraldehyde
acetonide with acetone (Table 3) in order to seek better
selectivity.
4.2. Liquid phase condensation of glyceraldehyde
acetonide with acetone
The catalytic activity studies were checked in the liquid
phase aldol condensation of glyceraldehyde acetonide with
acetone without any solvent. The possibility of various products
(A–D) and the reaction conditions are mentioned in Table 2.
The results obtained were quite interesting (Table 3), as almost
85% conversion of the glyceraldehydes acetonide with 82.5%
selectivity for product (A) and only 11.9% of diacetone alcohol
(D) was observed. No conversion of the products (B) was
observed under our experimental conditions. Very high
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Table 3The structural parameters and observed catalytic activity of various catalysts
d[1 0 0] (nm) a (nm) SBET (m2 g�1) Vt (cm3 g
KIT-6 9.35 22.9 821 0.97
CMK-8 12.0 17.3 804 1.43
MgO 11.2 15.2 502 0.84
MO-HT – – 282 0.45
Reaction conditions, acetone/glyceraldehyde acetonide �30 (molar ratio); tempera
R
conversion of glyceraldehyde acetonide and the selectivity of
product (A) are quite different from the other mixed oxides or
rehydrated mixed oxides already reported on this reaction [24].
We believe that although very higher basicity is not required for
this type of aldol condensation, yet the desired activity and
selectivity can be achieved by tuning the morphology of the
catalysts. Therefore, very high specific surface area of the MgO
particles and the ordered structural arrangement in the present
investigation are responsible for the higher conversion of
glyceraldehyde acetonide and the high selectivity of the product
(A). This may be due to selective and uniform adsorption of the
reactant molecules on the surface of the catalyst thereby
reducing the self condensation of acetone to diacetone alcohol.
4.2.1. Effect of the reaction time
Careful examination of the time on stream studies in Fig. 4
will reveal very interesting observations on the initial kinetics
�1) wBJH ðnmÞ Conversion Selectivity (%)
A C D
8.3 0 0 0
7.4 0 0 0
6.9 85 82.5 5.6 11.9
– 50.6 39.2 10 50.8
ture, 373 K; catalyst, 25 mg; time, 24 h, without solvent.
Fig. 4. Variation of conversion of glyceraldehydes acetonide with actone over
MgO and MO-HT catalysts with time (conditions as in Table 3).
Table 4
Variation of conversion of glyceraldehydes acetonide with the weight of MgO
catalyst (conditions as in Table 3)
Catalyst weight (mg) Conversion (wt%) Product selectivity (%)
A C D
10 51.2 85 2.3 12.7
25 72.3 84.7 3.4 11.9
50 82.4 86.1 2.9 11.0
100 90.4 85.2 3.0 11.8
200 96.2 85.6 2.4 12
A. Dubey et al. / Applied Catalysis A: General 338 (2008) 20–26 25
A
of the reaction. The results showed that the reaction is almost
completed within 0.75 h of the reaction time using ordered
MgO catalyst compared to only 30% of the reaction in case of
MO-HT. No report is available in the literature that explains the
effect of the catalyst on the initial kinetics of this interesting
reaction. The sharp increase in the conversion may be due to the
very small size of the ordered MgO catalyst compared to the
size of MO-HT. These results have some advantages over some
of the CsX and CsX/CS exchanged zeolites (an X zeolite
exchanged with cesium and impregnated with cesium species)
[24] under similar reaction conditions. In order to see the
stability of the products, the reaction was allowed to proceed for
24 h. No inter conversion of the products and no formation of
secondary products was found under the reaction conditions,
indicating the promised use of ordered MgO catalyst to achieve
the high activity and selectivity.
4.3. Effect of the catalyst weight
Fig. 5 shows the effect of the catalyst weight on the
conversion of glyceraldehydes acetonide with time. Having
known the general principle of the catalysis that selectivity
R
Fig. 5. Variation of conversion of glyceraldehydes acetonide with different
weights of the catalyst with time over ordered MgO catalyst (conditions as in
Table 3).
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decreases with increase in conversions for multi-product
formation reactions, the endeavor to study the time-on stream
is to see the effect of ordered MgO particles on the initial
kinetics of the reaction on overall activity and selectivity. The
results clearly reveal that the conversion increases with the
increase in the weight of the catalyst but no difference in the
product selectivity was noted (Table 4), indicating that the
ordered arrangement of the catalyst particles is responsible in
bringing about this conversion.
4.4. Reusability of the catalyst
We wanted to check the reusability of mesoporous MgO
catalyst for all the substrates, thus the catalyst was centrifuged,
washed thoroughly with water and acetone and dried at 800 8Cto remove all the impurities and again used for fresh reaction.
The same activity and selectivity trends were observed up to
four cycles (we did not check after four cycles). These results
may have a significant impact on the industrial economy and
can be scaled up for higher reactant concentrations.
5. Conclusions
In conclusion, we have reported the synthesis of ordered
mesoporous MgO and its effective use in base catalyzed organic
transformations. The results obtained are quite significant and
may pave a way for other advanced catalytic applications.
Further efforts are currently underway in this direction.
Acknowledgements
A.D. and B.G.M. thank the Department of Science and
Technology (DST) for the financial help and the institute for
providing necessary help and support for this work.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.apcata.2007.12.015.
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