Room temperature acetalization of glycerol to cyclic acetals over anchored silicotungstates under solvent free conditions

Article abstract of DOI:10.1039/c4ra01851f

Heterogeneous catalysts comprising of parent Keggin type silicotungstate as well as monolacunary silicotungstate anchored to MCM-41 were synthesized and characterized by several physicochemical methods. A solvent free green route towards valorisation of glycerol via acetalization with benzaldehyde has been proposed. Both the catalysts showed very good activity as well as selectivity towards dioxolane derivatives within a short reaction time and at room temperature. The tuning of the acidity of the parent silicotungstate leads to an increase in the selectivity towards 1,3-dioxolane. The catalysts were also recycled up to four times without any significant loss in the conversion. The excellent performance of these mesoporous catalysts is attributed to their combination of acidity, wide pores and large specific surface area. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra01851f

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Room temperature acetalization of glycerol to
cyclic acetals over anchored silicotungstates
under solvent free conditions
Cite this: RSC Adv., 2014, 4, 19294
*
Nilesh Narkhede and Anjali Patel
Heterogeneous catalysts comprising of parent Keggin type silicotungstate as well as monolacunary
silicotungstate anchored to MCM-41 were synthesized and characterized by several physicochemical
methods. A solvent free green route towards valorisation of glycerol via acetalization with benzaldehyde
has been proposed. Both the catalysts showed very good activity as well as selectivity towards dioxolane
derivatives within a short reaction time and at room temperature. The tuning of the acidity of the parent
silicotungstate leads to an increase in the selectivity towards 1,3-dioxolane. The catalysts were also
recycled up to four times without any significant loss in the conversion. The excellent performance of
these mesoporous catalysts is attributed to their combination of acidity, wide pores and large specific
surface area.
Received 3rd March 2014
Accepted 14th April 2014
DOI: 10.1039/c4ra01851f
www.rsc.org/advances
multifunctional organic compounds.9 They also have direct
applications as fragrances, in lacquer industries, in cosmetics,
1. Introduction
pharmaceuticals, food and beverage additives, in detergents,
and as ignition accelerators and antiknock additives in
combustion engines10 and in port wine production.11 Glycerol
acetals are oen used as basis for surfactants.12
Traditionally, the acetalization of glycerol was carried out by
using mineral acids as homogeneous catalysts.5 However, the
effluent clearance leads to environmental snags and econom-
ical dilemmas. These hitches can be overcome by using
heterogeneous catalysts such as alumina,13 aluminosilicates,14
resins,15 transition metal complexes,16–18 and mixed metal
oxides.5,19,20
Glycerol is a side product (10 wt%) of biodiesel synthesis
leading to an increase of crude glycerol in the market. Produc-
tion of value-added products from glycerol is an attractive
substitute for the consumption of glycerol. Some latent uses of
glycerol include: hydrogen manufacture,1 glycerol acetate as a
fuel additive,2 citric acid production, composite additive,
cosmetic bonding agent and to propylene glycol synthesis,3
acrolein,4 ethanol and epichlorhydrin,5 and as anti-freezing
agent.6 Since glycerol is a massive biodiesel manufacturing by-
product, the development of new openings from waste glycerol
to value-added products is of great signicance to improve the
economic viability of biodiesel synthesis.
The different reaction procedures for conversion of glycerol
to value-added chemicals including oxidation, dehydration,
hydrogenolysis, pyrolysis, etherication, esterication, steam
reforming, acetalization, polymerization, oligomerization, and
others have been reported.7 Amongst, the acetalization of glyc-
erol is the most important methodology for the synthesis of
green and cost-effective bio-additive chemicals from glycerol.
Glycerol condense with simple carbonyl compounds to deliver
isomeric 1,3-dioxane and 1,3-dioxolane products as novel ne
chemical intermediates.
Even though acid catalysis by supported HPAs has been
greatly expanded from the viewpoint of their variety of struc-
tures and compositions, according to our knowledge, there is
only one report on acetalization of glycerol by using heteropoly
acids (HPAs).21 Ferreira, et al. have reported acetalization of
glycerol with acetone over silica immobilized HPAs. The
ꢀ
maximum conversion obtained was 97% at 70 C in 2 h with
glycerol: acetone mole ratio of 1 : 6.21 However reports on the
reactions involving glycerol, such as dehydration,22–25 esteri-
cation26 and dichloropropanol synthesis27 by supported HPAs
are available in the art.
We have successfully established the synthesis of parent
12-silicotungstic acid (SiW12) as well as mono lacunary silico-
tungstate (SiW11) anchored to MCM-41 and characterized by
different physicochemical techniques.28,29 Both SiW12 and
SIW11 based catalyst showed 90% and 81% conversions of oleic
acid esterication under mild conditions. In continuation of
our previous efforts to explore wider applicability of these
catalysts for acid catalysed organic transformations; green,
These cyclic acetals are potent precursors for the production
of green platform compounds 1,3-propanediol and 1,3-dihy-
droxyacetone.8 The acetalization reaction is also important for
protection of carbonyl groups during the manipulation of
Polyoxometalate and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The M. S. University of Baroda, Vadodara, 390002, India. E-mail:
aupatel_chem@yahoo.com; Tel: +91-265-2795552
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solvent-free and room temperature conversion of glycerol to
cyclic acetals have been carried out via acetalization with
various aldehydes. Based on the results the catalyst activity has
been correlated with the structural features and acidity of the
catalysts and the possible mechanism has also been proposed.
2.5. Characterization techniques
A detailed study on the characterization of synthesized mate-
rials can be found in our earlier publications.28,29 The main
characterization of the catalysts such as FT-Raman 29Si-MAS-
NMR, XRD and n-butyl amine acidity are given for readers'
convenience. The BET surface area measurements were per-
2. Experimental
2.1. Materials
formed in
a Micromeritics ASAP 2010 volumetric static
adsorption instrument with N2 adsorption at 77 K. The pore size
distributions were calculated by BJH adsorption–desorption
method. For FT-IR spectra, samples pressed with dried KBr into
discs were recorded by using a Perkin-Elmer spectrometer. The
Raman spectra were recorded on a FT-Raman Spectrophotom-
eter Model Bruker FRA 106. The magic-angle spinning (MAS)
solid state NMR study was carried out on a BRUKER NMR
spectrometer. The 29Si NMR spectra were recorded at 121.49
MHz using a with tetra methyl silane as an external standard.
All materials used were of A.R. (analytical) grade. 12-Tung-
stosilicic acid, benzaldehyde, tetraethylorthosilicate (TEOS),
glycerol, sodium tungstate, sodium silicate, acetonitrile,
n-butylamine and acetone were purchased from Merck.
2.2. Synthesis of the support (MCM-41)
MCM-41 was prepared using previously reported procedure.29
27.1 g surfactant (CTAB) was added to the dilute solution of
NaOH (2 M, 3.5 mL NaOH in 480 mL double distilled water) with
stirring at room temperature. Aer the solution became clear,
5 mL TEOS was added drop wise and the gel was aged for 2 h at
60 ꢀC. The resulting material was ltered, washed with distilled
water, dried in oven and calcined in air at 550 ꢀC for 5 h.
2.6. Determination of acidic strength (potentiometric
titration method)
A small quantity (0.1 mL) of 0.05 N, n-butylamine in acetonitrile
was added to a suspension of 0.5 g of the catalyst in 50 mL of
acetonitrile and the system was stirred at 25 ꢀC. Then, the
suspension was potentiometrically titrated against 0.05 N,
n-butylamine in acetonitrile. The electrode potential variation
was measured with a digital pH meter.
2.3. Synthesis of mono lacunary silicotungstate,
Na8SiW11O39$11H2O (SiW11
)
0.22 mol, 7.2 g sodium tungstate and 0.02 mol, 0.56 g sodium
silicate were dissolved in 150 mL double distilled water at 80 ꢀC.
The pH was then adjusted to 4.8 by dilute nitric acid. The
volume of the mixture was reduced to half and the resulting
solution was ltered to remove unreacted silicates. The lacunary
heteropoly anion was separated by liquid–liquid extraction with
acetone. The extraction was repeated until the acetone extract
showed the absence of nitrate ions. The extracted sodium salt of
mono lacunary silicotungstate was dried at room temperature
in air. The resulting material was designated as SiW11 (Na ¼ 6%;
W ¼ 63.8%; Si ¼ 0.89%).
The acidity of the catalyst measured by this technique allows
us to evaluate the total number of acid sites as well as their
acidic strength. In order to interpret the results, it is suggested
that the initial electrode potential (Ei) indicates the maximum
acid strength of the surface sites and the range where the
plateau is reached (meq. gꢁ1 solid) indicates the total number of
acid sites.21 The acidic strength of surface sites can be assigned
according to the following ranges: very strong site, Ei > 100 mV;
strong site, 0 < Ei < 100 mV; weak site, ꢁ100 < Ei < 0 mV and very
weak site, Ei < ꢁ100 mV.
2.7. Reaction procedure
2.4. Synthesis of the catalysts (SiW12/SiW11 anchored to
MCM-41)
A typical acetalization reaction of glycerol with aldehydes was
carried out in a 50 mL two-necked round bottom ask under
inert atmosphere. The ask was charged with glycerol
(10 mmol), benzaldehyde (12 mmol) and 100 mg of catalyst. The
reaction mixture was vigorously stirred at room temperature
(28–30 ꢀC) under N2 atmosphere for 60 min. The products were
analysed using Shimadzu 2014 GC equipped with RTX-5 capil-
lary column (internal diameter: 0.25 mm, length: 30 m). The
products were identied by GC-MS and 1H-NMR of the product
mixture.
A series of catalysts containing 10–40% of SiW12 anchored to
MCM-41 were synthesized by impregnating an aqueous solution
of SiW12 (0.1/10–0.4/40 g mLꢁ1 of double distilled water) with
MCM-41 (1 g) and dried at 100 ꢀC for 10 h. The resulting
materials were designated as 10% SiW12/MCM-41, 20% SiW12/
MCM-41, 30% SiW12/MCM-41 and 40% SiW12/MCM-41,
respectively.
Similarly, catalysts containing 10–40% of SiW11 anchored to
MCM-41 were synthesized by impregnation method. MCM-41
(1 g) was impregnated with an aqueous solution of SiW11 (0.1/
10–0.4/40 g mLꢁ1 of double distilled water) and dried at 100 ꢀC
2.8. Leaching test
for 10 h. The obtained materials were treated with 0.1 N HCl, A leaching of the active species from the support makes the
ltered, washed with double distilled water and dried at 100 ꢀC catalyst unattractive, and hence, it is necessary to study the
in order to convert the Na form of the catalyst in to the proton leaching of SiW12/SiW11 from the support. Polyoxometalates
form. The resulting materials were designated as 10% SiW11/ can be quantitatively characterized by the heteropoly blue
MCM-41, 20% SiW11/MCM-41, 30% SiW11/MCM-41 and 40% colour, which is observed when it reacted with a mild reducing
SiW11/MCM-41, respectively.
agent such as ascorbic acid. In the present study, this method
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was used for determining the leaching of SiW12/SiW11 from the
support. One gram of catalyst with 10 mL of conductivity water
was reuxed for 24 h. Then, 1 mL of the supernatant solution
was treated with 10% ascorbic acid. Development of blue colour
was not observed, indicating that there was no leaching. The
same procedure was repeated with alcohols and the ltrate of
the reaction mixture aer completion of reaction in order to
check the presence of any leached species. The absence of blue
colour indicates no leaching.
3. Results and discussion
3.1. Catalyst characterization
The elemental analyses performed on 30% SiW12/MCM-41
(theoretical: W ¼ 19%; Si ¼ 27%; observed: W ¼ 17.9%; Si ¼
27%) and 30% SiW11/MCM-41 (theoretical: W ¼ 15%; Si ¼ 28%;
observed: W ¼ 15.2%; Si ¼ 27.6%) were consistent with theo-
retical expected values.
Fig. 1 Raman spectra of (a) SiW12, (b) 30% SiW12/MCM-41, (c) SiW11
and (d) 30% SiW11/MCM-41.
nas(W–Od), nas(W–Ob–W) and ns(W–Oa), respectively (Fig. 1d).
The presence of these bands conrms the intact SiW11 species
in 30% SiW11/MCM-41.
Furthermore, the slight shi in the Raman bands for both
the catalysts indicates the chemical interaction of the active
species SiW11/SiW12 with the surface silanol groups of MCM-41.
The establishment of chemical interaction was further
conrmed by means of 29Si MAS NMR.
The BET surface area and pore diameter (BJH method) for
both the catalysts are presented in the Table 1. The incorpora-
tion of active species inside the channels of MCM-41 leads to
decrease in the total surface area of both the catalysts. The
overall decrease in surface area of both the catalysts with
respect to the support gives the rst indication of a chemical
interaction between SiW11/SiW12 and MCM-41. However, both
surface area and pore diameter of 30% SiW11/MCM-41 are
higher than those of 30% SiW12/MCM-41. This may be due to
the fact that the removal of W–O unit from the parent SiW12
results in decrease in the size of SiW11 species leading to
increase in the available space inside the channels of the
support.
Raman spectra of both the catalysts are shown in Fig. 1. The
Raman spectrum of SiW12 shows bands at 1054, 976, 888, 565,
and 208 corresponding to ns(W–Od), nas(W–Od), nas(W–Ob–W),
ns(W–Oc–W), and ns(W–Oa), respectively (Fig. 1a) where Oa, Ob,
Oc and Od corresponds to the oxygen atoms linked to silicon, to
oxygen atoms bridging two tungsten (from two different triads
for Ob and from the same triad for Oc) and to the terminal
oxygen W]O, respectively.30 The Raman spectrum of 30%
SiW12/MCM-41 remains almost the same, conrming the
retainment of the Keggin structure (Fig. 1b). The Raman spec-
trum of SiW11 shows typical bands at 971, 890, 814, 521 and 231
cmꢁ1 corresponding to ns(W–Od), nas(W–Od), nas(W–Ob–W),
ns(W–Oc–W), and ns(W–Oa), respectively (Fig. 1c). The presence
of these bands conrms the formation of lacunary SiW11
species. The catalyst 30% SiW11/MCM-41 showed Raman
bands at 969, 879, 793 and 224 cmꢁ1 with respect to ns(W–Od),
29Si MAS NMR is the most important method to study
chemical environment around the silicon nuclei in mesoporous
silica materials. Fig. 2 shows the 29Si MAS NMR spectra of MCM-
41, 30% SiW12/MCM-41 and 30% SiW11/MCM-41. The presence
of resonance originated from Q2 Si(OSi)2(OX)2, Q3 Si(OSi)3(OH)
and Q4 Si(OSi)4 in the catalysts indicates that MCM-41 retains
its structure in both the catalysts (Table 2). The spectra of the
catalysts are relatively broad and low in intensity when
compared to MCM-41. This is due to the strong hydrogen
bonding between SiW12/SiW11 and Q2 Si(OSi)2(OH)2 (surface
silanol groups) of MCM-41.
XRD patterns of MCM-41, 30% SiW12/MCM-41 and 30%
SiW11/MCM-41 are shown in Fig. 3. The XRD pattern of the
MCM-41 shows a sharp reection around 2q ¼ 2ꢀ corresponding
Table 1 BET surface area and pore diameter of support and the
catalysts
Material
Surface area (m2 gꢁ1
)
Pore diameter (nm)
MCM-41
30% SiW12/MCM-41
30% SiW11/MCM-41
659
349
536
4.79
2.92
3.96
Fig. 2 29Si-MAS NMR spectra of (a) MCM-41, (b) 30% SiW12/MCM-41
and (c) 30% SiW11/MCM-41.
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Table 2 29Si chemical shifts of both the catalysts
Material
Q2, ppm
Q3, ppm
Q4, ppm
30% SiW12/MCM-41
30% SiW11/MCM-41
ꢁ93
ꢁ103
ꢁ110
ꢁ102
ꢁ104.6
ꢁ110.5
to (100) plane indicating well-ordered hexagonal structure of
MCM-41. The comparison of the XRD patterns of MCM-41 and
the catalysts reveals that the mesoporous structure of MCM-41
is rather intact even aer anchoring of SiW12/SiW11 species.
Further the absence of characteristic peaks of crystalline phase
of SiW12 as well as SiW11 in the respective catalysts indicates
that the active species are highly dispersed inside the hexagonal
channels of MCM-41.
Fig. 4 Potentiometric titration curves of (a) MCM-41, (b) 30% SiW12
MCM-41 and (c) 30% SiW11/MCM-41.
/
The plots of the electrode potential as a function of meq.
amine per g of the catalysts are shown in Fig. 4. It is observed
that, both the catalysts contain very strong acid sites. The
strength of acidic sites in terms of initial electrode potential is
shown in Table 3. It is clear from the Table 3 that the incor-
poration of species SiW12/SiW11 increases the strength of the
acid sites of catalysts to a great extent. It is also interesting to
note that almost all values are similar in both the catalysts
except the acidic strength. The acidic strength of 30% SiW11/
MCM-41 is lower than that of 30% SiW12/MCM-41. The reason
being, the acidic character of polyoxometalates is mainly due to
the acidic addenda atoms i.e. tungsten in the present case and
removal of one tungsten–oxygen unit from the parent SiW12 is
expected to decrease the acidity of the SiW11. The obtained
value is in good agreement with the expected one.
Table 3 n-Butylamine acidity of support and the catalysts
Types of acid sites,
meq. gꢁ1
Acidic
strength, mV
Total acidic
Material
Very strong
Strong
sites, meq. gꢁ1
MCM-41
30% SiW12
MCM-41
30% SiW11
MCM-41
48
438
—
0.9
2.0
2.5
2.0
3.4
/
/
260
0.9
2.4
3.3
the strength of the strong acid sites is high in 30% SiW12
MCM41.
/
In order to conrm the distribution of acidic sites, NH3-TPD
acidity measurements were carried out. It is clear from the
Table 4 that the support is fairly acidic and both weak and
strong acid sites are gradually increased in both the catalysts.
Also the distribution of the acid sites is almost similar however;
3.2. Catalytic reaction
It is well known that lacunary silicotungstates, SiW11 have
commendable catalytic activity because of removal of a tung-
sten–oxygen octahedral moiety from a saturated SiW12 frame-
work leads to an increase and localization of the anionic charge,
the resulting lacunary anion becomes highly nucleophilic and
reacts easily with electrophilic groups. Hence, the acidic as well
as redox properties are altered from the parent saturated unit.
This study is focused on catalytic activity of both 30% SiW11
/
MCM-41 as well as 30% SiW12/MCM-41 for solvent free room
temperature acetalization of glycerol (G) with benzaldehyde (B).
Acetalization of glycerol with benzaldehyde produces two
main products; 1,3-dioxolane and 1,3-dioxane, whose relative
Table 4 NH3-TPD acidity of support and the catalysts
NH3 acidity,
mmol gꢁ1
Strength of acid
sites, ꢀ
C
Sample
Weak
Strong
Weak
Strong
MCM-41
30% SiW11/MCM41
30% SiW12/MCM41
0.23
0.25
0.29
0.63
0.71
0.93
190
210
225
410
440
650
Fig. 3 XRD patterns of (a) MCM-41, (b) 30% SiW11/MCM-41 and (c)
30% SiW12/MCM-41.
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formation depends on the acetalization position within the
glycerol molecule (Scheme 1). The glycerol acetalization reac-
tion favours the formation of kinetically favoured product, 1,3-
dioxolane.
The effect of % loading was studied by carrying out esteri-
cation reaction over 10% to 40% loaded catalysts for both the
systems (Fig. 5). Low conversions were obtained for low load-
ings of SiW12/SiW11. The optimum of 91% conversion with 74%
1,3-dioxolane selectivity was obtained by using 30% SiW12
/
MCM-41 and 85% conversion with 82% 1,3-dioxolane selectivity
was obtained by using 30% SiW11/MCM-41. The enhanced
activity can be assigned to the increase in SiW12/SiW11 content.
Further increase in loading beyond 30% loading no signicant
increase in the conversion was observed. As a result, 30%
loading was optimum and considered for the further studies.
The effect of glycerol to benzaldehyde mole ratio was studied
by varying ratio from 1 : 1 to 1 : 1.3 over both the catalysts
(Fig. 6). The glycerol conversion was found to be increasing with
increase in the mole ratio. However the selectivity remains
almost the same for both the catalysts. The optimum mole ratio
of glycerol: benzaldehyde was found to be 1 : 1.2 for both
catalysts.
Fig. 6 Effect of mole ratio on acetalization of glycerol over (a) 30%
SiW12/MCM-41 and (b) 30% SiW11/MCM-41. Reaction conditions: time:
60 min; temperature: 30 ꢀC; catalyst amount: 100 mg.
of the catalyst amount. Similar trend was observed for both the
catalysts. The increase in the conversion can be attributed to an
increase in the number of available catalytically active sites.
Hence, 100 mg of the catalyst was considered to be optimum for
the maximum conversion.
In order to examine the variation of glycerol conversion and
products selectivity with time, we have studied acetalization of
glycerol at different time intervals (reaction time varied from 15
to 75 min). The conversion of glycerol was increased with
increase in the reaction time. It was observed from the Fig. 8
that the kinetically favoured product dioxolane was formed
initially as major product and with increase in time the selec-
tivity towards thermodynamically more stable product, dioxane
increases slowly. At 60 minutes of the time 91% conversion with
74% selectivity to dioxolane was observed for 30% SiW12/MCM-
41 and 85% conversion of glycerol with 82% selectivity to
dioxolane was observed for 30% SiW11/MCM-41.
The effect of the amount of catalysts on glycerol conversion
was studied by varying catalyst amount in the range 50–150 mg.
As shown in Fig. 7, initial increase in the catalyst concentration
increases the conversion of glycerol with almost similar selec-
tivity to dioxolane and reaches saturation conversion at 100 mg
The optimized conditions for maximum conversion (91% for
30% SiW11/MCM-41 and 85% for 30% SiW11/MCM-41) are: mole
ratio G/B: 1/1.2; time: 60 min; temperature: 30 ꢀC; catalyst
Scheme 1 Acetalization of glycerol with benzaldehyde.
Fig. 5 Effect of loading on acetalization of glycerol over (a) SiW12
/
Fig. 7 Effect of catalyst amount on acetalization of glycerol over (a)
MCM-41 and (b) SiW11/MCM-41. Reaction conditions: mole ratio G/B: 30% SiW12/MCM-41 and (b) 30% SiW11/MCM-41. Reaction conditions:
1/1.2; time: 60 min; temperature: 30 ꢀC; catalyst amount: 100 mg.
mole ratio G/B: 1/1.2; time: 60 min; temperature: 30 ꢀC.
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same conversion of 23%. It has been reported by Sheldon et al.
that there are three categories for a catalyst to behave as a true
heterogeneous catalyst in context of leaching of metal from the
support (a) the metal leaches but is not active homogeneous
catalyst, (b) metal leaches to form an active homogeneous
catalyst and (c) the metal does not leach is a true heterogeneous
catalyst. The results indicate that the present catalyst fall into
category C.31 On the basis of these results, it can be concluded
that there is no any leaching of the SiW11 from the support and
the present catalysts are truly heterogeneous in nature.
3.5. Recycling study
The catalysts were recycled for four times in order to test their
activity in successive runs. The catalysts were separated from
Fig. 8 Effect of reaction time on acetalization of glycerol over (a) 30%
SiW12/MCM-41 and (b) 30% SiW11/MCM-41. Reaction conditions: mole the reaction mixture by simple centrifugation, washed with 5
ratio G/B: 1/1.2; temperature: 30 ꢀC; catalyst amount: 100 mg.
mL methanol and then with 5 mL double distilled water, dried
at 100 C in an oven for 10 h and the recovered catalysts were
ꢀ
charged for the further runs. The conversion of glycerol
amount: 100 mg. By looking at the industrial importance of the
observed for four successive runs over both the catalysts are:
dioxolane derivative, 30% SiW11/MCM-41 will be the choice of
85%, 84%, 85% and 82% for 30% SiW11/MCM-41 and 91%,
better catalyst. Further the catalyst 30% SiW11/MCM-41 was
89%, 88% and 89% for 30% SiW12/MCM-41. Hence, results
explored for acetalization of glycerol with various substituted
suggest that catalysts can be reused up to four cycles without
benzaldehydes.
signicant loss in the conversion.
Further the recycled catalysts were characterized by FT-IR
analysis and BET surface area in order to see any structural
3.3. Control experiments
The acetalization of glycerol was carried out without catalyst as
well as using MCM-41, and active species (SiW12 and SiW11). It is
clear from the Table 5 that the support MCM-41 is not much
active towards the acetalization and the activity of the species
SiW11/SiW12 has been retained in the catalysts. The activity of
30% SiW12/MCM-41 is higher than 30% SiW11/MCM-41. The
results are quite consistent with the acidity of both the catalyst.
However the selectivity for 1,3-dioxolane is lower in the case of
30% SiW12/MCM-41 as high acidic strength causes ring trans-
formation to 1,3-dioxane through benzyl cation.
change. The FT-IR spectra of recycled 30% SiW12/MCM-41
showed the retention of typical bands for SiW12, at 979 cmꢁ1
and 923 cmꢁ1 corresponding to W]Od and Si–Oa symmetric
stretching, respectively. The FT-IR spectra of the used catalyst
30% SiW11/MCM-41 (Fig. 9) shows retention of bands at 960
cmꢁ1 (W]Od), 900 cmꢁ1 (Si–Oa) suggesting that the structure of
SiW11 in regenerated catalyst is intact. The BET surface area
values of the recycled catalysts (520 for 30% SiW11/MCM-41 and
332 for 30% SiW12/MCM-41) are comparable with the fresh ones
(536 for 30% SiW11/MCM-41 and 349 for 30% SiW12/MCM-41).
3.4. Heterogeneity test
3.6. Comparison with the reported catalysts
For the rigorous proof of heterogeneity, a test was carried out by
ltering catalyst (30% SiW11/MCM-41) from the reaction
mixture aer 20 min and allowed the ltrate to react up to 1 h.
The reaction mixture of 20 min and ltrate was analysed by gas
chromatogram. Both 20 min and nal 60 min samples showed
It is observed from the Table 6 that all the reported catalysts
operate at higher temperatures, longer reaction times, low
Table
5 Control experiments for acetalization of glycerol with
benzaldehydea
Catalyst
% Conv.
% Sel.b
TON
No catalyst
MCM-41**
15
28
90
85
95
91
90
85
70
82
65
74
—
—
1048
989
1188
1139
SiW11
30% SiW11/MCM-41**
SiW12
*
*
30% SiW12/MCM-41**
a
Reaction conditions: mole ratio G/B: 1/1.2; time: 60 min; temperature:
30 ꢀC; catalyst amount: *23 mg/**100 mg. TON was calculated from the
b
Fig. 9 FT-IR spectra of fresh and recycled catalysts (a) 30% SiW12
/
formula, TON ¼ moles of product/moles of catalyst. Dioxolane
selectivity.
MCM-41 and 30% SiW11/MCM-41.
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conversion and use of toxic solvents.5,16,32 In contrast our cata-
There exists equilibrium between dioxolane and dioxane in
lyst prefers solvent free condition, requires very short time of 60 the presence of strong acid sites via key intermediate benzyl
min and most importantly both conversion and selectivity are cation. The catalyst 30% SiW12/MCM-41 possesses very strong
very high. The reaction occurred under solvent-free condition acid sites than 30% SiW11/MCM-41. The strength of the acid
and offered several advantages, such as environmental sites leads to the ring transformation in the case of 30% SiW12
/
compatibility, simplifying workup, formation of cleaner prod- MCM-41. Hence, relatively low selectivity was observed for 30%
ucts, reduction of byproducts.
SiW12/MCM-41.
4. Conclusions
3.7. The acetalization of glycerol with various aldehydes
The catalyst 30% SiW11/MCM-41 was further explored for
carrying out reaction using various substituted aldehydes (Table
7). Acetalization of glycerol with benzaldehyde produced 85%
conversion with 82% selectivity for dioxolane. It was found that
all substituted compounds exhibit relatively low glycerol
conversion than that of benzaldehyde indicating the inuence
of steric hindrance caused by the presence of substituents. All
over high conversions and selectivities reveal the potential of
the present catalyst for the acetalization of glycerol even with
the substituted benzaldehyde compounds.
The present catalytic systems show very high conversion and
selectivity for acetalization of glycerol with benzaldehyde. Very
short reaction time, room temperature and solvent free condi-
tions make the process environmentally benign. Tuning acidic
strength of the silicotungstate, we have achieved very high
conversion (85%) and selectivity (82%). The reusability of the
catalysts makes them ideal choice for acetalization of glycerol.
The catalyst 30% SiW11/MCM-41 is the better choice amongst
them in terms of industrial importance of dioxolane derivative.
We have revealed new, highly efficient heterogeneous catalyst for
the synthesis of value added product from a renewable feed-
stock, glycerol thereby opening new perspectives for sustainable
valorisation of this side-product of biodiesel production.
3.8. Proposed mechanism for acetalization
The rst step is the coordination and activation of the carbonyl
group of the benzaldehyde. Then, the carbon atom of the
carbonyl group can be attacked by the primary alcoholic group
of glycerol followed by the formation of a bond between the
carbonyl oxygen atom and the b-carbon of the glycerol. Finally,
the dehydration process leads to the formation of dioxolane.
Acknowledgements
We are thankful to UGC (39-837/2010 (SR)) for the nancial
support. One of the authors, Mr Nilesh Narkhede, is also
thankful to UGC, New Delhi, for the award of Research
Fellowship.
Table 6 Comparison of activity with reported catalysts for acetaliza-
tion of glycerol with benzaldehyde
Notes and references
1 A. T. Marshall and R. G. Haverkamp, Int. J. Hydrogen Energy,
2008, 33, 4649–4654.
Catalyst
Reaction conditionsa
% Conv. % Sel.b Ref.
RHABIm–HSO4
MoOx/TiO2–ZrO2 5 wt% : 1/1 : 60 : 30
50 : 1/2 : 120 : 120
61
54
82
48
40
74
32
16
5
This
work
This
work
2 J. A. Melero, R. V. Grieken, G. Morales and M. Paniagua,
Energy Fuels, 2007, 21, 1782–1791.
3 A. Nemeth and B. Sevella, Appl. Biochem. Biotechnol., 2007,
144, 47–58.
4 L. Ott, M. Bicker and H. Vogel, Green Chem., 2006, 8, 214–
220.
MoO3/SiO2
30% SiW12
MCM-41
30% SiW11
MCM-41
10 wt% : 1/1 : 100 : 480 72
/
/
100 : 1/1.2 : 30 : 60
100 : 1/1.2 : 30 : 60
91
85
82
a
5 S. B. Umbarkar, T. V. Kotbagi, A. V. Biradar, R. Pasricha,
J. Chanale, M. K. Dongare, et al., J. Mol. Catal. A: Chem.,
2009, 310, 150–158.
Reaction conditions ¼ amount of catalyst (mg): ratio of G/B: reaction
temperature ꢀC: reaction time (minutes). b Dioxolane selectivity.
6 P. H. R. Silva, V. L. C. Gonsalves and C. J. A. Mota, Bioresour.
Technol., 2010, 101, 6225–6229.
7 C.-H. Zhou, J. N. Beltramini, Y.-X. Fan and G. Q. Lu, Chem.
Soc. Rev., 2008, 37, 527–549.
Table 7 Acetalization of glycerol with various aldehydes over 30%
SiW11/MCM-41a
8 A. J. Showler and P. A. Darley, Chem. Rev., 1967, 67, 427–440.
9 T. W. Green and P. G. M. Wuts, Protective Groups in Organic
Synthesis, Wiley, New York, 2nd edn, 1991, vol. 4, p. 212.
10 M. J. Climent, A. Corma and A. Velty, Appl. Catal., A, 2004,
263, 155–161.
11 A. Cesar, D. S. Ferreira, J. C. Barbe, A. Bertrand and J. Agric,
Food Chem., 2002, 50, 2560–2564.
12 M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi and C. Della
Pina, Angew. Chem., Int. Ed., 2007, 46, 4434–4439.
Aldehyde
% Conv.
% Sel.b
Benzaldehyde
85
72
75
69
82
65
55
62
4-Bromobenzaldehyde
4-Methoxybenzaldehyde
4-Hydroxybenzaldehyde
a
Reaction conditions ¼ amount of catalyst (100 mg of 30% SiW11/MCM-
41): ratio of G/B (1 : 1.2): reaction temperature ꢀC (30 ꢀC): reaction time
(60 min). b Dioxolane selectivity.
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RSC Advances
13 T. Vu Moc and P. Maitte, Bull. Soc. Chim. Fr., 1975, 9, 2558– 24 L. Ning, Y. Ding, W. Chen, L. Gong, R. Lin and Y. Lu, Chin. J.
2560. Catal., 2008, 29, 212–214.
14 M. J. Climent, A. Corma, S. Iborra, M. C. Navarro and 25 S.-H. Chai, H.-P. Wang, Y. Liang and B.-Q. Xu, Appl. Catal., A,
J. Primo, J. Catal., 1996, 161, 783–789. 2009, 353, 213–222.
15 J. Deutsch, A. Martin and H. Lieske, J. Catal., 2007, 245, 428– 26 P. Ferreira, I. M. Fonseca, A. M. Ramos, J. Vital and
433. J. E. Castanheiro, Appl. Catal., B, 2009, 91, 416–422.
16 P. Sudarsanam, B. Mallesham, A. N. Prasad, P. S. Reddy and 27 S. H. Lee, D. R. Park and H. Kim, Catal. Commun., 2008, 9,
B. M. Reddy, Fuel Process. Technol., 2013, 106, 539–545. 1920–1923.
17 C. Crotti, E. Farnetti and N. Guidolin, Green Chem., 2010, 12, 28 V. Brahmkhatri and A. Patel, in Environmentally Benign
2225–2231.
Catalysts Based on Heteropolyacids for Clean Organic
Transformations, ed. A. Patel, Springer Inc., New York,
2013, pp. 189–208.
18 M. Cataldo, E. Nieddu, R. Gavagnin, F. Pinna and G. Strukul,
J. Mol. Catal. A: Chem., 1999, 142, 305–316.
19 B. Mallesham, P. Sudarsanam, G. Raju and B. M. Reddy, 29 A. Patel and N. Narkhede, Catal. Sci. Technol., 2013, 3, 3317–
Green Chem., 2013, 15, 478–489. 3325.
20 M. S. Khayoon and B. H. Hameed, Appl. Catal., A, 2013, 465, 30 N. Narkhede and A. Patel, Ind. Eng. Chem. Res., 2013, 52,
191–199. 13637–13644.
21 P. Ferreira, I. M. Fonseca, A. M. Ramos, J. Vital and 31 R. A. Sheldon, M. Walau, I. W. C. E. Arends and
J. E. Castanheiro, Appl. Catal., B, 2010, 98, 94–99. U. Schuchurdt, Acc. Chem. Res., 1998, 31, 485–493.
22 H. Atia, U. Armbruster and A. Martin, J. Catal., 2008, 258, 32 F. Adam, H. E. Hassan and K. Mohammed Hello, J. Taiwan
71–82.
Inst. Chem. Eng., 2012, 43, 619–630.
23 S.-H. Chai, H.-P. Wang, Y. Liang and B.-Q. Xu, Green Chem.,
2008, 10, 1087–1093.
This journal is © The Royal Society of Chemistry 2014
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