Sustainable valorisation of glycerol via acetalization as well as carboxylation reactions over silicotungstates anchored to zeolite Hβ

Article abstract of DOI:10.1016/j.apcata.2016.02.010

A simple, green and effective pathway towards valorisation of glycerol to value added products has been demonstrated. In this context, catalysts comprising parent as well as lacunary silicotungstates anchored to large pore zeolite Hβ have been synthesized and characterized by various physicochemical methods. Parent silicotungstic acid based catalyst proved to be better catalyst in terms of conversion of glycerol showing 73% conversion for carboxylation reaction and 97% conversion of glycerol towards acetalization reaction. The better activity of the catalyst was correlated with its strong acidic character. It was observed that by tuning the acidity of parent silicotungstate by formation of lacunary silicotungstate leads to the increase in the selectivity of 5-membered dioxolane from 68% to 78% and selectivity of glycerol carbonate from 72% to 75%. Both the catalysts were reusable up to four cycles under the investigated reaction conditions. The probable mechanisms for both the reactions are also discussed.

Full text of DOI:10.1016/j.apcata.2016.02.010

Accepted Manuscript
Title: Sustainable Valorisation of Glycerol via Acetalization as
well as Carboxylation Reactions over Silicotungstates
Anchored to Zeolite H␤
Author: Nilesh Narkhede Anjali Patel
PII:
S0926-860X(16)30065-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.02.010
APCATA 15769
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-10-2015
3-2-2016
6-2-2016
Please cite this article as: Nilesh Narkhede, Anjali Patel, Sustainable Valorisation
of Glycerol via Acetalization as well as Carboxylation Reactions over
Silicotungstates Anchored to Zeolite Hrmbeta, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.02.010
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Sustainable Valorisation of Glycerol via Acetalization as well as
Carboxylation Reactions over Silicotungstates Anchored to
Zeolite Hβ
Nilesh Narkhede, Anjali Patel* aupatel_chem@yahoo.com
Polyoxometalate and Catalysis Laboratory, Department of Chemistry, Faculty of Science,
The M. S. University of Baroda, Vadodara, 390002, India.
*Corresponding author: Tel: +91-265-2795552
1

Graphical Abstract
Synopsis
Environmentally benign route towards valorisation of glycerol via acetalization as well as
carboxylation has been demonstrated over silicotungstates anchored to zeolite Hβ.
2

Highlights





Valorization of glycerol over silicotungstates anchored to zeolite Hβ.
Effect of creating a lacuna on the parent silicotungstate has been discussed.
Highly efficient catalytic systems in terms of activity and selectivity.
Mild reaction conditions make the system environment friendly.
Catalysts can be recycled up to 4 times without significant loss in the activity.
3

Abstract
A simple, green and effective pathway towards valorisation of glycerol to value added
products has been demonstrated. In this context, catalysts comprising parent as well as
lacunary silicotungstates anchored to large pore zeolite Hβ have been synthesized and
characterized by various physicochemical methods. Parent silicotungstic acid based catalyst
proved to be better catalyst in terms of conversion of glycerol showing 73% conversion for
carboxylation reaction and 97% conversion of glycerol towards acetalization reaction. The
better activity of the catalyst was correlated with its strong acidic character. It was observed
that by tuning the acidity of parent silicotungstate by formation of lacunary silicotungstate
leads to the increase in the selectivity of 5-membered dioxolane from 68% to 78% and
selectivity of glycerol carbonate from 72% to 75%. Both the catalysts were reusable up to
four cycles under the investigated reaction conditions. The probable mechanisms for both the
reactions are also discussed.
Keywords: Silicotungstates; Zeolite Hβ; Glycerol; Acetalization; Carboxylation;
Heterogeneous catalysis
4

1. Introduction
Biodiesel is nontoxic, simple to use, biodegradable, and essentially free of sulfur and
aromatics. It is a great alternative to fossil fuels as little or no modifications are required to
the existing diesel engines. Further oxygen content in biodiesel improves combustion
efficiency. Glycerol is formed as a by-product (~10 wt.%) during biodiesel production
leading to an increase of crude glycerol in the market. The high manufacturing cost of
biodiesel raises the need to study the uses of glycerol as a renewable feedstock for synthesis
of value added fine chemicals [1].
The cyclization of glycerol via acetalization reaction is one of the most important procedures
for the synthesis of green and cost‐effective bio‐additive chemicals from glycerol. Glycerol
reacts with simple carbonyl compounds to provide isomeric 1,3‐dioxane and dioxolane
products as novel fine chemical intermediates. These cyclic acetals are potential precursors
for the manufacture of green platform chemicals 1,3-dihydroxyacetone and 1,3-propanediol
[2]. The acetalization reaction is many times necessary for protection of carbonyl groups
during the manipulation of multifunctional organic molecules [3] as well as it has direct
applications as fragrances, in cosmetics, food and beverage additives, pharmaceuticals, in
detergents, in lacquer industries and as ignition accelerators and antiknock additives in
combustion engines [4] and in port wine production [5]. Glycerol acetals can also be used as
the basis for surfactants [6].
Carboxylation of glycerol is also important class of reaction to produce glycerol carbonate.
Glycerol carbonate has excellent properties such as low toxicity, good biodegradability and a
high boiling point which make it a very attractive chemical for a variety of applications such
as a high boiling polar solvent, an intermediate in organic synthesis [7] and in the synthesis of
polycarbonates [8], polyurethanes [9] and in cosmetic and medical institutes as they have low
5

toxicity, volatility, combustibility and good moisturizing ability [10]. Besides, they also play
a major role as a component of surfactants, paints, coatings and gas-separation membranes.
Additionally, there is a general consensus that the potential of glycerol carbonate as an anti-
explosive additive for gasoline and diesel will result in an unprecedented growth in the
coming years [11-12].
Previous work has shown a great potential for the establishment of a variety of new solid acid
catalyzed processes for the valorisation of glycerol mainly via acetalization and carboxylation
reaction. An increasing number of studies dealing with the chemo-selective acid catalyzed
acetalization of glycerol have been found, mainly using alumina [13], ion-exchange resins
[14], mesoporous aluminosilicates [15], mixed metal oxides [16-18] and transition metal
complexes [19-21]. Further literature survey shows that Lewis acids such as metal salts [22-
23], HTc-Zn derived from hydrotalcite [24] metal oxides [25-26] and gold supported metal
oxides [27-28] have been employed successfully for the production of glycerol carbonate
(GlyC) via glycerolysis of urea.
Even though, supported polyoxometalates (POMs) are excellent solid acid catalysts, there are
only four reports in the literature on glycerol valorisation via acetalization or carboxylation
reactions. Castanheiro et al. have reported the synthesis and characterization of silica
immobilized tungstophosphoric, molybdophosphoric, tungstosilicic and molybdosilicic acids
for acetalization of glycerol with acetone [29]. All catalysts tested for acetalization of
glycerol showed high selectivity towards solketal (97%) with near complete conversion.
Further recycling studies showed that after the fourth cycle, the catalysts lost 10-13% of its
initial activity. Lingaiah et al., have synthesized Sm3+ and Zn2+ exchanged tungstophosphoric
acid (TPA) for glycerolysis of urea [30-31]. It was reported that TPA retains its Keggin
structure even after exchange with metal ions. The catalytic studies showed that Zn-TPA
comes out to be better catalyst amongst the two. Both the catalysts were recycled successfully
6

up to three times. To the best of our knowledge there is only one report available in the
literature on anchored silicotungstates for the valorisation of glycerol and that too by our
group [32]. Solid acid catalysts consisting of parent Keggin type silicotungstate as well as
monovacant silicotungstate and MCM-41 were synthesized by impregnation method and
characterized by different physicochemical methods and catalytic activity was evaluated for
utilization of glycerol via acetalization reaction with benzaldehyde. It was observed that
altering the acidity of parent silicotungstate increases the selectivity towards 1,3-dioxolane to
a great extent.
Recently, we have also reported the synthesis and characterization of parent silicotungstate
(SiW12) [33] as well as monolacunary silicotungstate (SiW11) [34] anchored to zeolite Hβ for
biodiesel synthesis by esterification and transesterification reactions. By keeping in mind the
recent developments on valorisation of glycerol into value-added chemicals, as an extension
of our previous work here we describe the sustainable route towards solvent free valorisations
of glycerol via carboxylation and acetalization reactions over anchored silicotungstates.
In the present work, two series of catalysts comprising 10-40 wt.% of SiW12/SiW11 and
zeolite Hβ were synthesized by impregnation method and the 30% anchored catalysts were
characterized by different physicochemical approaches. The catalytic activities were
evaluated for the acetalization of glycerol with benzaldehyde as well as carboxylation of
glycerol with urea. The effects of various reaction parameters like mole ratio, catalyst amount
as well as reaction time and temperature were studied to optimize the reaction conditions for
maximum conversion. Also, the catalysts were recovered after simple workup procedure and
successfully recycled up to four cycles.
7

2. Experimental section
2.1. Materials
All the chemicals used were of A.R. grade. Glycerol, urea, benzaldehyde, sodium tungstate,
toluene, sodium silicate, n-butylamine, 12-tungstosilicic acid and methanol were purchased
from Merck. Benzaldehyde was distilled prior to use. The sodium form of zeolite b (Si/Al
ratio 10) was used as received and was purchased from zeolites and Allied Products,
Bombay, India.
2.2. Preparation of the support (Hβ)
The sodium form of zeolite, Naβ was converted in to ammonium form by conventional ion
exchange procedure using 10 wt. %, 1M NH4Cl aqueous solution [34]. The obtained
ammonium type zeolite was converted to proton type by calcination in air for 6 h at 823 K.
2.3. Synthesis of monolacunary silicotungstate, Na8SiW11O39*11H2O (SiW11)
Monolacunary silicotungstate was synthesized by following the reported method from the
individual salts sodium silicate (0.02 mol, 0.56 g) and sodium tungstate (0.22 mol, 7.2 g) at
pH 4.8 [34]. The isolated compound was dried in air and resulting material was named as
SiW11. (Na = 6%; Si = 0.89%; W = 63.8%).
2.4. Synthesis of the catalysts (30% SiW12/SiW11 anchored to Hβ)
A catalyst containing 30% of SiW12 anchored to Hβ was prepared by impregnation method.
One gram Hβ was impregnated with an aqueous solution of SiW12 (0.3 g in 30 mL distilled
water) and dried at 373 K for 10 h. The resulting material was named as Hβ2.
Similarly, catalyst containing 30% of SiW11 anchored to Hβ was synthesized by following
the same method. 1 g Hβ was impregnated with an aqueous solution of SiW11 (0.3g in 30 mL
8

distilled water) and dried at 373 K for 10 h. The obtained material was treated with 10%, 0.1
N HCl, filtered, washed with distilled water and dried at 373 K. The resulting material was
named as Hβ1.
2.5. Catalyst Characterization
A detailed study on the characterization of synthesized materials can be found in our earlier
29
publications [33-34]. Additional new characterizations such as FT-Raman, Si-MAS-NMR,
TEM and n-butyl amine acidity are included for readers' convenience. EDX analysis was
carried out by using JSM 5910 LV joint with an INCA instrument for EDX-SEM. The BET
surface area measurements were performed on Micromeritics ASAP-2020 volumetric static
adsorption instrument with N2 adsorption at 77 K. The Raman analysis was performed on a
29
FT-Raman Spectrophotometer Model Bruker FRA 106. The solid state Si MAS-NMR was
done on a BRUKER NMR spectrometer. The 29Si NMR spectra were recorded using a 7 mM
rotor probe using TMS as standard at 121.49 MHz. TEM images were taken on a JEOL
(Japan) TEM instrument (model-JEM 100CX II) with an accelerating voltage of 220 kV. NH3
-TPD chemisorption studies were carried out using the Micromeritics Pulse Chemisorb-2705.
2.6. Determination of acidic strength (potentiometric titration method)
A very small quantity (0.1 mL) of 0.05 N, n-butylamine in acetonitrile was added to a
suspension of 0.5 g of catalyst in 50 mL acetonitrile and the system was agitated at 298 K for
3 h. 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.
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
9

sites and the range where the plateau is reached indicates the total number of acid sites [29].
The acidic strength of surface sites can be assigned according to the following ranges: very
strong acid site, Ei > 100 mV; strong acid site, 0 < Ei < 100 mV; weak acid site, -100 < Ei <
0 mV and very weak acid site, Ei < -100 mV.
2.7. Reaction Procedure
A typical acetalization reaction of glycerol with aldehydes was carried out in a 50 mL two-
necked round bottom flask under inert atmosphere in order to prevent air oxidation of
benzaldehyde. The flask was charged with glycerol (10 mmol), freshly distilled benzaldehyde
(10 mmol) and 50 mg of catalyst. The reaction mixture was vigorously stirred at room
temperature (303 K) under N2 atmosphere for 60 min.
In a typical carboxylation reaction, glycerol (10 mmol), urea (10 mmol) and the catalyst (100
mg) were added in a 50 mL round bottom flask. The reaction was performed at 413 K and N2
was purged in order to knock off the ammonia, formed during the reaction. After completion
of the reaction, methanol was added and the catalyst was separated by centrifugation. The
products were examined using Shimadzu 2014 Gas Chromatography instrument equipped
with RTX-5 capillary column (internal diameter: 0.25 mm, length: 30 m). The products were
recognized by comparison with the standard samples. The conversion and selectivity were
calculated by using the following equations,
( )
( )
2.8 Leaching test
10

In the present study leaching test (ascorbic acid test) was carried out with Water, alcohols and
the filtrate of the reaction mixture in order to check the presence of any leached species
following the method described in the literature [33]. The absence of blue colour indicates no
leaching.
3. Results and Discussion
3.1. Catalyst Characterization
Elemental analysis performed on Hβ1 was consistent with theoretical expected values.
(Theoretical: W = 16.7%; Si = 26.9%; Al = 2.9, observed: W = 16.5%; Si = 27.6%; Al= 2.2).
The analytically calculated percentages of elements for Hβ2 are as follows: W= 17.66%; Si=
26%; Al= 2.5%. Values obtained from EDX are as follows: W= 18.5%; Si= 26.6%; Al=
2.2%. The EDX results were comparable with the expected ones.
TGA-DTA profile of Hβ, Hβ2 and Hβ1 are shown in supplementary material, SM 1. A
unique weight loss of 13-15 % was observed up to 523 K for Hβ, which is attributed to
desorption of physically adsorbed water. No further weight loss was observed indicating
zeolite Hβ retains its framework structure up to 873 K. For the catalyst Hβ2, similar weight
loss of 10-12 % up to 473 K assigned to adsorbed water was detected. Second gradual weight
loss of 1.2-1.5 % was observed between 473 K-573 K due to the loss of water of
crystallization of Keggin anion. TGA analysis suggests that Hβ2 is stable up to 753 K. The
TGA of Hβ1 shows initial weight of 6% up to 443 K may be due to the removal of adsorbed
water molecules. Second weight loss of 2% has been observed up to 513 K which is due to
loss of water of crystallization. No notable loss up to 683 K indicates the stability of the
catalyst up to 683 K.
11

The TGA data shows that anchoring of SiW12/SiW11 to Hβ increases the thermal stability
indicating the presence strong interaction between SiW12/SiW11 and Hβ.
Table 1 displays specific surface area and pore diameter of Hβ, Hβ1 and Hβ2. The specific
surface area and pore diameter of Hβ was decreased after inclusion of SiW12/SiW11 (Table 1).
The incorporation of SiW11 into the framework of the support is apparent here, as it leads to
25% reduction in surface area as well as 31% reduction in pore width. This indicates that
SiW11 occupies sites inside the zeolite framework. The decrease in pore diameter of the
catalyst is due to the presence of SiW11 inside the zeolite framework, as anticipated. Similar
trend was observed in the case of Hβ2. However, both surface area and pore diameter of Hβ1
are higher than those of Hβ2. 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 the increase in the
available space inside the channels of the support.
FT-Raman of SiW11 (Fig.1) shows bands at 970, 890, 814, 521 and 231 cm-1 which are
attributed to νs(W-Od), νas(W-Od), νas(W-Ob-W), νs(W-Oc-W), and νs (W-Oa), respectively
[35]. The FT-Raman spectrum of Hβ1 shows the retention of 973 cm-1 (νs (W-Od)), 891 cm-1
(νas (W-Od)), 813 cm-1 (νs(W-Ob-W)), 519 cm-1 (νs(W-Oc-W)) and 223 cm-1 (νs (W-Oa),)
bands which indicates that the framework of SiW11 has been retained after anchoring to the
support. FT-Raman of SiW12 shows bands at 1054, 976, 888, 565 and 208 cm-1 which are
attributed to νs(W=Od), νas(W=Od), νas(W-Ob-W), νs(W-Oc-W), and νs(W-Oa), respectively
[36]. The Raman spectrum of Hβ2 shows the retention of all the typical bands of SiW12 at
1020, 973, 911, 521 and 231 cm-1 due to νs(W=Od), νas(W=Od), νas(W-Ob-W), νs(W-Oc-W),
and νs(W-Oa), respectively. Remarkable shifts in the Raman bands for the catalysts confirm
the strong interaction of SiW12/SiW11 with the hydrogen of silanol group (Si-OH) inside the
channels of the support.
12

29
Fig. 2 shows Si-MAS NMR spectra of Hβ, Hβ1 and Hβ2. Typical characteristic peaks for
SiW12 (-83.2) and SiW11 (-80.8) were observed for Hβ2 and Hβ1, respectively which are in
good agreement with the reported ones [37]. This confirms that SiW12/SiW11 remains intact
even after anchoring to the support. Further, two different types of silicon nuclei Si(OAl) (at-
109.6 for Hβ2 and -109.4 for Hβ1) and Si(1Al) ) (at-101.9 for Hβ2 and -101.8 for Hβ1) were
observed in agreement with the previously reported ones [38]. These two peaks are identical
to those observed for support, Si(OAl) at -111.9 and Si(1Al) at -104.53. The shift as well as
broadening of these peaks in the catalysts confirms the strong interaction between terminal
oxygens of SiW12/SiW11 and silanol groups of the support.
The BET, FT-Raman and 29Si-NMR analyses confirm the presence of strong hydrogen
bonding between intact SiW12/SiW11 and H of the Si-OH group of the zeolite framework. To
study the dispersion of active species inside the zeolite framework, the catalyst was further
characterized by TEM analysis.
The TEM image of Hβ (supplementary material, SM 2) at 50 nm resolution displays the
porous surface morphology with uniform pore sizes. The TEM images of the catalysts show
uniform porous morphology over very large scales very identical to the support. The absence
of aggregates of SiW12/SiW11 indicates that the active species are well dispersed inside the
micro pores of the zeolite support with retention of uniform porous structure (inset Fig SM2).
TEM analysis confirms the retention of uniform zeolite framework as well as uniform
dispersion of the active species.
The plot of the potential as a function of milliequivalent of n-butylamine per gram of the
catalysts and support is shown in Fig. 3a. It was observed that, the support and catalysts
contain very strong acid sites. The strength of acidic sites in terms of initial electrode
potential is shown in Table 1. It is clear from the table that the incorporation of species
13

SiW12/SiW11 increases the strength of the acid sites of catalysts to a great extent. It is also
interesting to note that there is a drastic decrease in the acidic strength of Hβ1 as compared to
Hβ2. 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.
NH3-TPD is a useful tool for determining the surface acid sites of the solid acid catalyst. In
order to confirm the types of cidic sites the catalysts were further characterized by NH3-TPD
analysis. The NH3-TPD profiles of the catalysts and the support are illustrated in Fig. 3b. The
NH3-TPD profile of support and the catalysts exhibits weak as well as medium strength acid
sites. Acidic amount determined by the amount of desorbed ammonia and acidic strength was
determined by desorption peak position (supplementary material, SM 3). Stronger acidic sites
show higher desorption temperatures. Further, a new strong acidic character was introduced
in the catalysts after incorporation of SiW12/SiW11 at 801 K for Hβ2 and at 786 K for Hβ1.
NH3-TPD acidity values indicate an increase in total acidity of the catalysts with respect to
the support and both total acidity as well as strength of the acidic sites for Hβ2 is greater than
Hβ1. The results are in good agreement with the n-butylamine potentiometric method.
3.2 Catalytic reactions
This study is focused on i) carboxylation of glycerol with urea and ii) solvent free room
temperature acetalization of glycerol with benzaldehyde (supplementary material, SM 4).
Acetalization of glycerol with benzaldehyde produces two main products, 1,3-dioxolane and
1,3-dioxane, whose relative formation depends on the acetalization position within the
glycerol molecule. The glycerol acetalization reaction favours the formation of kinetically
favoured product, 1,3-dioxolane. In the carboxylation of glycerol with urea, nitrogen was
14

bubbled throughout the experiment in order to avoid the NH3 generated during the reaction.
The main reaction products observed in our case were GlyC, GlyU and (2-oxo-1,3-dioxolan-
4-yl)methyl carbamate.
3.2.1 Carboxylation of glycerol with urea over Hβ1.
The activity of synthesized catalyst was assessed for the preparation of GlyC via glycerolysis
of urea. The effects of various reaction parameters on the conversion as well as selectivity for
GlyC were evaluated.
The effect of % loading of SiW11 was studied by varying the loadings from 10-40% (Fig. 4a).
It is clear that the increase in the % loading of SiW11 linearly increases the conversion of
glycerol up to 30% loading. The increase in the concentration of SiW11 ultimately increases
the concentration of strong acid sites of the catalyst which helps in the formation of
carbocation on the carbonyl carbon. The optimum conversion of 60% with 78% selectivity of
GlyC was achieved with 30% loading. The higher loadings of SiW11 up to 40% did not result
in the increase in the activity. It is well known that increasing % loading via impregnation
technique leads to poorer dispersion, which results in the development of larger aggregates of
the active species at the expense of more active smaller clusters [28]. As a result, 30% loaded
Hβ1 was selected for the further studies.
Fig. 4b shows the influence of glycerol to urea mole ratio on the conversion of glycerol and
selectivity of the GlyC. It can be noted from the Fig 4b that with G: U molar ratio of 1:1, the
conversion of glycerol was 60% with 78% selectivity towards GlyC. However, further
increase in the mole ratio lead to the decrease in the selectivity due to the formation of (2-
oxo-1,3-dioxolan-4-yl) methyl carbamate via reaction of free hydroxyl group present in the
GlyC with excess urea. This is in good agreement with the previous reports [39-40]. As a
15

result 1:1 ratio was considered to be optimum for maximum conversion and selectivity of
GlyC.
The effect of catalyst amount on the conversion and selectivity was studied by varying the
amount of catalyst in the range of 50-150 mg (Fig 4c). Initially, with increase in the amount
of the catalyst, conversion of glycerol as well as selectivity of GlyC was also increased. The
major by-product found here was glycerol urethane (GlyU). On increasing the catalyst
amount, the catalytically active acidic site also increases and hence the intermediate product,
GlyU further loses NH3 to give desired product, GlyC. The optimum conversion was
achieved for catalyst amount of 100 mg.
Reaction time is an important parameter and long reaction times were adopted usually for the
carbonylation of glycerol to obtain reasonable conversion. The effect of reaction time was
studied over Hβ1 and the results are shown in Fig. 4d. It is interesting to note down that
initially the selectivity towards GlyU was high and with increase in the reaction hours, the
selectivity towards GlyC was increased. This indicates that GlyU is quickly formed
intermediate which is then slowly converted in to the GlyC, which is further confirmed by
FT-IR of the reaction mixture at different reaction hours (Fig 5). After 8 h of the reaction
time 60% conversion with 78% selectivity of GlyC was observed.
The effect of reaction temperature on conversion of glycerol and the selectivity to GlyC are
shown in Fig. 6. The temperature variation study was carried out by varying the temperature
in the range of 373 K to 423 K. The conversion of glycerol was very low (~34%) when
reaction was carried out below 393 K. However, a drastic increase in the conversion of
glycerol up to 60% was observed when reaction temperature increased to 413 K. The
maximum conversion of glycerol (conversion of 68% and selectivity of 75% for GlyC) was
16

achieved at 423 K which was considered to be optimum temperature for the present reaction.
Further increase in the temperature showed coke formation of the reaction mixtures.
Similarly, the optimization of reaction conditions was carried out over Hβ2 (supplementary
material, SM 5-SM 9). The optimized conditions for both the catalysts are as follows, for
Hβ1 maximum conversion, 68% and selectivity of 75% of GlyC and for Hβ2 maximum
conversion of 73 % and selectivity of 72% was achieved under optimized conditions of, mole
ratio G:U, 1:1, temperature: 423 K, catalyst amount: 100 mg, reaction time: 8 h.
3.2.2 Control experiments
The carboxylation of glycerol was carried out without catalyst as well as using support, and
active species (SiW12 and SiW11). It is clear from the Table 2 that the support was not much
active towards the carboxylation and the activities of the species SiW12/SiW11 have been
retained in the catalysts. The conversion of glycerol was higher for Hβ2 than Hβ1 irrespective
of the surface area of both the catalysts. Further, the results are quite consistent with the
acidity of both the catalyst (supplementary material, SM 3). Both strength and number of
acidic sites are higher in Hβ2 as compared to Hβ1. As a result the both acidity and activity of
the catalysts follow the same trend. Hence it can be concluded that the present catalytic
system is not a surface type in which the catalytic activity is proportional to the surface area
but it is a pseudo-liquid type (I) heterogeneous system where activity is directly proportional
to the acidity of the catalyst [41].
3.2.3 Heterogeneity test
For adequate proof of heterogeneity, a test was carried out by removing the catalysts from the
reaction mixture after 4 h by centrifugation and allowed the substrates to react up to 8 h. The
reaction mixture of 4 h and filtrate was analysed by Gas Chromatography. Both samples
17

showed same conversion of 42% (Fig 7a) for Hβ2. Similar trend was observed for Hβ1 (Fig
7b). In another experiment after 4 h of the reaction time, the catalysts were filtered by
centrifugation and new reactant, glycerol (10 mmol) and urea (10 mmol) was added to the
filtrate and reactions were further continued for 4 h. The conversion of final samples was
similar to that of the 4 h sample. According to Sheldon et al., there are three categories for a
catalyst to act 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 [42]. The results show that the present catalytic systems fall into category c. On the
basis of these results, it can be concluded that there is no leaching of SiW12/SiW11 from the
support and the present catalysts are truly heterogeneous in nature.
3.2.4 Recycling study
The catalysts were separated from the reaction mixture by simple centrifugation, washed with
5 mL methanol and then with 5 mL distilled water, dried at 373 K in an oven for 10 h and the
recovered catalysts were characterized for EDX elemental analysis, acidic strength, FTIR and
BET. The values obtained from EDX elemental analysis of recycled Hβ1 (W = 16.4%; Si =
27.5%; Al= 2.3) and Hβ2 (W= 18.3%; Si= 26.7%; Al= 2.4%) are quite comparable with the
fresh catalysts. The acidic strength of recycled catalysts was comparable to that of the fresh
one (for Hβ2: fresh= 657 mV, recycled 650 mV; for Hβ1: fresh= 390 mV, recycled 386 mV).
Further in order to confirm the retention of the SiW12/SiW11 species on to the support, FT-IR
analysis was carried out for the reused catalysts (supplementary material, SM 10). The FT-IR
spectrum of recycled Hβ2 showed the retention of typical bands for SiW12, at 920 cm-1
corresponding to symmetric Si-Oa stretching. The FT-IR spectrum of the used catalyst Hβ1
shows retention of bands at 942 cm-1 (Si-Oa), 860 cm-1 (W-Ob-W) and 715 cm-1 (W-Oc-W)
suggesting that the structure of SiW11 is intact in the regenerated catalyst. The BET surface
18

area values of the recycled catalysts (424 m2/g for Hβ1 and 412 m2/g for Hβ2) are
comparable with the fresh ones (439.8 m2/g for Hβ1 and 419.2 m2/g Hβ2).
The conversion of glycerol observed for four successive runs over both the catalysts are:
68%, 67%, 65%, and 64% for Hβ1 and 73%, 71%, 70%, and 68% for Hβ2. Hence, results
suggest that catalysts can be reused up to four cycles without significant loss in the
conversion.
3.2.5 Comparison with the reported catalysts for carboxylation of glycerol with urea.
Hammond and co-workers reported application of different gold based catalysts for synthesis
of GlyC [27]. The catalysts showed high conversions under relatively mild conditions.
However, low selectivities and use of costly metal are demerits. Wang, et al., reported use of
solid base catalysts for the glycerolysis of urea. La2O3-600 showed better activities amongst
different bases used with 69% conversion and 98% selectivity of GlyC [43]. Kim and co-
workers reported polystyrene-supported metal containing imidazolium salt as an efficient
catalyst for glycerolysis of urea [44]. The catalyst, PS-(Im)2ZnCl2 showed 72% glycerol
conversion with 84% selectivity of GlyC. However, the catalyst recycling studies showed
about 15% decrease in the conversion after 3rd cycle. In contrast our catalysts gives high
conversion as well as selectivity with low G:U ratio. In addition it can be recycled without
any significant change in the conversion and selectivity.
3.2.6 Acetalization of glycerol with benzaldehyde over Hβ1.
The reaction of glycerol with benzaldehyde (supplementary material, SM 4) produces two
main products; 1,3-dioxolane and 1,3-dioxane, whose relative formation relies on the
acetalization position within the glycerol molecule. In the present case, glycerol acetalization
reaction favours the formation of kinetically favoured product, 1,3-dioxolane.
19

The effect of % loading was studied by varying the SiW11 loading from 10% to 40% (Fig.
8a). Low conversions were obtained for low loadings of SiW11. With increase in the loading,
% conversion was increased however selectivity of the dioxolane was decreased. The reason
being increased catalytically active sites which favour the acid catalysed transformation of
dioxolane to dioxane. This is in good agreement with the literature result [20]. The optimum
of 95% conversion of glycerol with 78% selectivity of 1,3-dioxolane was obtained. Further
increase in the loading beyond 30% produces no significant change in the conversion. As a
result, 30% loading was considered as optimum and used for the further studies.
The effect of glycerol to benzaldehyde mole ratio was studied by varying ratio from 1:1 to
1:1.4 (Fig. 8b). Surprisingly, the conversion of glycerol was not affected much on variation of
the mole ratio rather the selectivity of dioxolane was decreased slightly due to the formation
of six membered dioxane derivatives. Hence the optimum mole ratio of was considered as
1:1.
The effect of the amount of catalysts on glycerol conversion was studied by varying catalyst
amount in the range 25-100 mg. As shown in Fig. 8c, initial increase in the catalyst amount
increases the conversion of glycerol and reaches saturation conversion of 95% at 50 mg of
the catalyst amount. The increase in the conversion can be attributed to an increase in the
number of available catalytically active sites. Further increase in the catalyst amount from 50-
100 mg shows saturation in the conversion however selectivity decreases. The excess strong
acidic sites of the catalyst are responsible for ring transformation from five membered
dioxolane to six membered dioxane derivatives. Hence, 50 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 carried out acetalization reaction by varying reaction time from 15 to 75 min. It was
20

found that the conversion of glycerol was increased with increase in the reaction time. It was
observed from the Fig. 8d that the kinetically favoured product dioxolane was formed
initially as major product and with increase in time the selectivity towards
thermodynamically more stable product, dioxane increases slowly. The evolution of
dioxolane to dioxane at longer reaction times was also reported by Gonzalez-Arellano et al
[45]. At 60 minutes of the time 95% conversion with 78% selectivity to dioxolane was
observed.
Similarly, the optimization of reaction conditions was carried out over Hβ2 (supplementary
material, SM 11- SM 14). The optimized conditions for both the catalysts are as follows, For
Hβ1 maximum conversion 95% and selectivity of 78% of dioxolane and for Hβ2 maximum
conversion of 97% and selectivity of 68% was achieved under following optimized
conditions: mole ratio G:B, 1:1; temperature, 303 K; catalyst amount, 50 mg; reaction time,
60 min.
Both the catalysts were recovered after simple workup as described earlier and charged for
the next run. The activity of the reused catalysts is shown in supplementary material, SM 15.
The results show that there is no significant loss in the activity as well as selectivity up to
four times.
3.2.7 Comparison with the reported catalysts for acetalization of glycerol with
benzaldehyde.
The catalyst, 20% MoO3/SiO2 showed 72% conversion with 40% selectivity of dioxolane
[18]. Here, 60% selectivity of six membered 1,3-dioxane was observed. The higher reaction
time and temperature may have facilitated the ring transformation to six membered 1,3-
dioxane. Very similarly, MoOx/TiO2-ZrO2 showed 74% conversion with 49% selectivity at
373 K. The reusability of the catalyst has not been carried out [20]. In other hand, RHABIm-
21

HSO4 showed 75% glycerol conversion with 82% selectivity of 1,3-dioxolane at very low
catalyst concentration [46]. However, the catalyst shows progressive decrease in the
conversion from first reuse (75% conversion) to third reuse (58% conversion). The advantage
of present catalytic systems lies in obtaining very high conversion as well as selectivity under
relatively mild conditions such as solvent free condition, very short reaction time of 60 min
and room temperature.
Among both the catalysts, Hβ1 showed better selectivity of dioxolane than Hβ2 with almost
similar conversion. There exists equilibrium between dioxolane and dioxane in the presence
of strong acid sites via key intermediate benzyl cation (supplementary material, SM 16). The
catalyst Hβ2 possesses very strong acid sites than Hβ1 (Table 1). The strength of the acid
sites leads to the ring transformation in the case of Hβ2 leading to the relatively low
selectivity.
3.3 Reaction mechanism
The probable mechanism for acetalization as well as carboxylation of glycerol is shown in
Scheme 1. Very high surface area and pore dimension of the present catalyst plays an
important role in diffusion of large substrate molecules on to the surface of the catalyst. For
glycerolysis of urea, a tetrahedral intermediate is formed by nucleophilic attack of hydroxyl
group of glycerol to the carbonyl carbon of urea. The subsequent release of two ammonia
molecules leads to the formation of GlyC [30]. The leaving off of the product molecule is
again a steric effect in the heterogeneous catalysis and the present catalyst contains large pore
diameter that facilitates the escape of product molecule.
In the case of glycerol acetalization, the first step is the activation of the carbonyl group of
the benzaldehyde via coordination. The carbon atom of the carbonyl group is attacked by the
primary alcoholic group of glycerol followed by bond formation between the carbonyl
22

oxygen and the β-carbon of the glycerol. Finally, the dehydration leads to the formation of
dioxolane.
4. Conclusion
A highly stable, recyclable and efficient catalysts, parent as well as lacunary silicotungstates
anchored to Hβ were utilized as a heterogeneous catalyst for the synthesis of two industrially
important chemicals. The catalyst, Hβ2 was found to be superior showing 73% glycerol
conversion for carboxylation reaction and 97% conversion for acetalisation reaction.
However, the selectivity was better for Hβ1 for glycerol acetalization reaction because of ring
transformation of dioxolane to dioxane in the presence of strong acidic Hβ2. The trend in the
activity of both the catalysts was in good agreement with their acidic strength. The unique
catalytic activity of the synthesized catalyst is due to the combination of well-defined ordered
channel framework, homogeneous dispersion of the active sites, strong acidity, making it
promising environmentally benign solid acid catalyst.
Acknowledgements
Mr. Nilesh Narkhede is thankful to CSIR, New Delhi, for the award of Senior Research
Fellowship (09/114/197/2014-EMR-I).
23

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26

Figure Captions
Fig. 1 FT-Raman Spectra of a) SiW12, b) Hβ2, c) SiW11 and d) Hβ1.
27

Fig. 2 29Si-MAS NMR spectra of a) Hβ, b) Hβ2 and c) Hβ1 showing retention of
SiW12/SiW11.
28

750
600
450
300
150
0
a)
b)
H
H1
H2
H1
H2
H
-150
400 500 600 700 800 900
0
1
2
3
4
5
6
Milliequivalent per gram
Temperature, K
Fig. 3 a) Potentiometric titration curves and b) NH3-TPD profiles of Hβ, Hβ2 and Hβ1.
29