Low temperature synthesis of bio-fuel additives via valorisation of glycerol with benzaldehyde as well as furfural over a novel sustainable catalyst, 12-tungstosilicic acid anchored to ordered cubic nano-porous MCM-48

Article abstract of DOI:10.1016/j.apcata.2020.117729

The present article demonstrates designing of novel catalyst, 12-tungstosilicic acid (TSA) anchored to ordered nano-porous MCM-48 (nMCM-48); TSA/nMCM-48, characterization and evaluation for synthesis of bio-fuel additives via glycerol valorisation with aromatic aldehydes. The nanopores of support were confirmed by BET and TEM while the interaction between TSA and nMCM-48 was confirmed by decrease in the surface area and pore volume of the catalysts. Assessment of vital reaction parameters (% loading of active species, mole ratio of reactants, catalyst amount, temperature and time) were performed to achieve maximum conversion of glycerol. The catalyst showed noteworthy performance at 30 °C towards conversion (>85 %) and thermodynamically stable dioxane derivative (>60 %) with remarkable TON (5945 for benzaldehyde and 7355 for furfural). The catalyst was regenerated and used for successive four catalytic runs with almost same activity. The superiority of novel catalyst is because of its geometry and nano porosity.

Full text of DOI:10.1016/j.apcata.2020.117729

Journal Pre-proof
Low temperature synthesis of bio-fuel additives via valorisation of glycerol
with benzaldehyde as well as furfural over a novel sustainable catalyst,
12-tungstosilicic acid anchored to ordered cubic nano-porous MCM-48
Anjali Patel (Conceptualization) (Writing - review and editing)
(Supervision), Dhruvi Pithadia (Methodology)
(Software)<ce:contributor-role>Writing-original draft) (Writing -
review and editing)
PII:
S0926-860X(20)30322-7
DOI:
https://doi.org/10.1016/j.apcata.2020.117729
APCATA 117729
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
19 April 2020
30 June 2020
2 July 2020
Please cite this article as: Patel A, Pithadia D, Low temperature synthesis of bio-fuel additives
via valorisation of glycerol with benzaldehyde as well as furfural over a novel sustainable
catalyst, 12-tungstosilicic acid anchored to ordered cubic nano-porous MCM-48, Applied
Catalysis A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117729

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Low temperature synthesis of bio-fuel additives via valorisation of glycerol with
benzaldehyde as well as furfural over a novel sustainable catalyst, 12-tungstosilicic acid
anchored to ordered cubic nano-porous MCM-48
Anjali Patel* and Dhruvi Pithadia
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science.
The Maharaja Sayajirao University of Baroda. Vadodara-390002. Gujarat. India.
*E-mail: anjali.patel-chem@msubaroda.ac.in
ORCID: 0000-0003-0177-3956*
Graphical abstract
Highlights
 New heterogeneous catalyst comprising nano-porous MCM-48 & 12-tungstosilicic acid
 Solvent-free acetalization of glycerol with benzaldehyde & furfural at ambient reaction
conditions
 Synthesis of dioxane derivatives with > 85 % conversion and > 60 % selectivity
with high TON
1

Abstract
The present article demonstrates designing of novel catalyst, 12-tungstosilicic acid (TSA)
anchored to ordered nano-porous MCM-48 (nMCM-48); TSA/nMCM-48, characterization and
evaluation for synthesis of bio-fuel additives via glycerol valorisation with aromatic aldehydes.
The nanopores of support were confirmed by BET and TEM while the interaction between
TSA and nMCM-48 was confirmed by decrease in the surface area and pore volume of the
catalysts. Assessment of vital reaction parameters (% loading of active species, mole ratio of
reactants, catalyst amount, temperature and time) were performed to achieve maximum
conversion of glycerol. The catalyst showed noteworthy performance at 30 ℃ towards
conversion (>85 %) and thermodynamically stable dioxane derivative (>60 %) with remarkable
TON (5945 for benzaldehyde and 7355 for furfural). The catalyst was regenerated and used for
successive four catalytic runs with almost same activity. The superiority of novel catalyst is
because of its geometry and nano porosity.
Keywords
Bio-fuel additives; Glycerol; Valorisation; Acetalization; Heterogeneous catalyst; High TON.
1. Introduction
The never-ending demand of energy supplements has geared the sudden urge of replacing non-
renewable fossil fuels by renewable ones. As a result of this, the breakthrough comes from the
utilization of renewable feedstock, converting into valuable chemicals, bio-additives, biodiesel
and biofuels. Hence, from last two decades, there has been an unparalleled revolution in
biodiesel industries producing surplus glycerol as by-product [1,2]. Collectively, up to date,
these industries are unable to exploit all the glycerol formed from biodiesel manufacturing
2

units. Henceforth in present situation it is important to highlight the economic utilization of
spare glycerol by other alternatives. In a view of upgrading glycerol at marketed level,
valorisation is the best way that can be carried out via dehydration [3], hydrogenation [4],
etherification [5], hydrogenolysis [6] and others [7], especially by reacting three hydroxyl
groups through acetalization reaction.
Glyceryl acetals and ketals, potential biodiesel additives/blends [8,9], can be formed by
condensation of glycerol with simple aldehydes & ketone using acid catalysts. Acetalization of
glycerol with benzaldehyde yields isomeric products, 1,3-dioxolane (5-membered) and 1,3-
dioxane (6-membered). They are important precursors to produce green platform chemicals,
1,3-propanediol and 1,3-dihydroxyacetone [10]. They have tremendous contribution as
antiknock additives, excellent antioxidants and accelerators for ignition fuel, as fragrances, as
food additives, in beverage, lacquer industries and pharmaceuticals [9].
According to the recent forecast data reported by International Energy Agency (Iea), in 2019,
biofuel production for transport has extended 6% reaching to 96 Mtoe (161 billion litres) year-
on-year and over the next five years 3% annual production growth is expected. (Figure 1) [11].
Thus, from the viewpoint of economic demands for clearing environmental sewage and adverse
ecological impact along with traditional disadvantages of homogeneous catalysts, the new
generation acetalization reactions via heterogeneous catalytic approach has gained tremendous
attention for maximum conversion of glycerol to selective acetals/ketals.
3

Figure 1 Global biofuel production 2010-2025 compared to consumption in the Sustainable
development Scenario [IEA (2019), Tracking Transport, IEA, Paris
https://www.iea.org/reports/tracking-transport-2019]
Several solid acid catalysts such as, metal-based catalysts [12,13], montmorillonite [14],
activated carbon [15], ion exchange resins [16], mesoporous silicates [17-19] and heteropoly
acids [2,20-29] have been applied for the same. Amongst all, the contribution from heteropoly
acids (HPAs) based catalysts, especially Keggin type, are found to be excellent from the
viewpoint of their compositions and varied structural properties. Keggin type of HPAs are
Bronsted acids, having metal-oxygen octahedra with general formula [XM12O40] n-, where X is
the heteroatom (P or Si) and M is the addenda atom (Mo, W, V etc). Sufficient illustrations for
acetalization of glycerol using 12-tungstophosphoric acid are available in an art [19, 21, 22,
24, 25, 28]. Literature survey shows that, even though, TSA is the second most acidic in the
HPAs series, only two reports are available, [23, 27] for the same. It is also observed that, no
reports are available for acetalization of glycerol with furfural, another important industrial
reaction, using anchored TSA.
Keeping in mind the importance of TSA as well as glycerol valorisation, we came with a novel
heterogeneous catalyst comprising of TSA and nMCM-48. Both, support (nMCM-48) and
catalyst (TSA/nMCM-48) were characterized by physicochemical/spectral techniques and
evaluated for valorisation of glycerol. Essential reaction parameters (mole ratio of glycerol:
benzaldehyde/furfural, catalyst amount, reaction temperature and reaction time) were screened
and optimised to obtain maximum conversion and selectivity towards desired product. The
catalyst was recovered by simple centrifugation and reused up to 3 cycles. The regenerated
catalyst was characterized by acidity, FT-IR and BET surface area measurements to confirm
its sustainability. A comparison with the reported catalysts was also carried out and correlated
with nano porosity as well as geometry.
2. Experimental
2.1. Materials
All chemicals used were of A.R. grade. 12-tungstosilicic acid, liquor ammonia, TEOS
(tetraethylorthosilicate), benzaldehyde, furfural, ethanol and 2-propanol were used as received
from Merck. CTAB (Cetyltrimethylammonium bromide) was purchased from Loba chemie
and glycerol was acquired from Suvidhinath Laboratories (SULAB; Vadodara, Gujarat).
2.2. Synthesis of the catalyst
The synthesis of TSA anchored to nMCM-48 was carried out in two steps.
Step-I: Synthesis of nMCM-48
4

The synthesis of nMCM-48 was carried out, by following the reported method with some
modification [30]. To 50 g of distilled water, CTAB surfactant (2.4 g, 6.6 mmol) was added
and allowed to dissolve completely at 35 ˚C. To this solution, ethanol (50 ml, 0.87 mmol) and
25 % wt liquor ammonia (~15.4 ml, 0.225 mol) was added and stirred for 15-20 mins, following
dropwise addition of TEOS (3.4 g, 16 mmol). The resulting white suspension was aged for 2
h, filtered and washed with distilled water, dried at room temperature and calcined at 550 ˚C
for 6 h. The material obtained was designated as nMCM-48.
Step-II: Synthesis of the TSA anchored to nMCM-48
A series of catalysts containing 10-40 % of TSA anchored to nMCM-48 were synthesized by
wet impregnation method. 1 g of nMCM-48 was impregnated with an aqueous solution of TSA
(0.1 g/ 10 mL – 0.4 g/ 40 mL of double distilled water) and dried at 100 ˚C for 10 h. The
obtained catalysts were designated as TSA1/nMCM-48, TSA2/nMCM-48, TSA3/nMCM-48,
and TSA4/nMCM-48, respectively.
2.3. Characterization
2.3.1. Determination of total acidity, acidic strength and sites
Acidic properties of synthesized support and catalysts were determined by n-butylamine
titration, potentiometric titration method and temperature programmed desorption of ammonia
(NH3-TPD).
n-butylamine titration. For determining total acidity, n-butylamine titration method was
performed. 0.25 g of support/catalyst was suspended for 24 h in 25 ml of 0.025 N n-butylamine
solution in toluene. The excess base was then titrated against 0.025 N trichloroacetic acid
solution in toluene using neutral red indicator to obtain total acidity of the material.
Potentiometric titration. To a suspension of 0.25 g of the support/catalyst in 25 mL of
acetonitrile, 0.1 mL of 0.05 N, n-butylamine in acetonitrile was added and stirred at 25 ˚C for
3 h. Then, the suspension was titrated potentiometrically against 0.05 N solution of n-
butylamine in acetonitrile. Variation in electrode potential was measured with a digital pH
meter.
NH3-TPD. The temperature-programmed desorption (TPD) of NH3 was measured on
BELCAT-II (Japan) instrument. 0.05 g of dry material was loaded and allowed to pre-treat at
300 °C for 1 h in presence of pure He gas (99.9%, 30 mL per min). Ammonia (10% NH3 in
He gas) was adsorbed on the surface of material for 1 h and then the physisorbed ammonia was
removed by flushing with pure He gas. Finally, the temperature programmed was performed
from 100 to 600 °C with a ramp rate of 10 °C/min. Desorbed NH3 was monitored by using
TCD (thermal conductivity detector) of the apparatus.
5

2.3.2. Physicochemical techniques
Elemental analysis was carried out using JSM-7100F EDX-SEM analyser. Thermogravimetric
analysis (TGA) of synthesized support/catalyst was performed using a Mettler Toledo Star SW
7.01 instrument under nitrogen atmosphere in the temperature range of 50–800 °C with flow
rate of 2 mL min−1 and with heating rate of 10 °C min−1. For FT-IR spectra, the samples were
compelled with dry KBr into discs and recorded in the range of wave numbers 4000-400 cm-1
by using Shimadzu instrument (IRAffinity-1S). The BET surface area measurements were
performed 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. Scanning electron microscope (SEM) images were recorded on JSM-7100F scanning
electron microscope. Transmission electron microscopy (TEM) was performed on JEOL TEM
instrument (model-JEM 2100) by applying acceleration voltage of 200 kV using carbon coated
200 mesh Cu grid. The samples were dispersed by ultrasonication and coated on grid, leaving
overnight for drying in air. The X-ray powder diffraction (XRD) patterns of the support and
the catalyst were measured using a D8 FOCUS X-ray Diffractometer from Bruker in the 2θ
29
range of 0-80⁰ using CuKα radiation (λ = 1.54056 Å). Si MAS NMR was recorded using
JOEL ECX 400 MHz High resolution multinuclear FT-NMR spectrometer for solids.
2.4. Catalytic evaluation
The acetalization of glycerol was carried out in 50 mL 2 neck glass reactor under nitrogen
atmosphere. The reactor was charged with glycerol (0.01 mol), benzaldehyde (0.01 mol) /
furfural (0.01 mol) and catalyst. The reaction mixture was stirred vigorously at 30 ˚C under N2
atmosphere, followed by diluting the contents with 2-propanol and analysed using Shimadzu
2014 GC equipped with RTX-5 capillary column (internal diameter: 0.25 mm, length: 30 m).
The products were identified by GC-MS (Gas chromatography-mass spectrometry- Thermo
scientific DSQ-II; Capillary column- Thermo TR-MS).
Turnover number (TON) refers to the moles of product formed per mole of catalyst. It was
calculated by using the following formula:
number of moles of product
TON =
number of moles of catalyst
Number of moles of catalyst was calculated by taking the amount of TSA incorporated.
2.5. Leaching test
Escape of any active species from the support makes the catalyst unappealing and hence it is
mandatory to analyse leaching of TSA from nMCM-48. Heteropoly acids can be determined
6

qualitatively via heteropoly blue colour, when treated with mild reducing agent like ascorbic
acid. To check the leaching of any TSA from the support, suspension of 1 g of catalyst with 10
ml conductivity water was allowed to reflux for 24 h. The resulting supernatant solution was
treated with 10 % ascorbic acid solution and the absence of blue colour indicates no leaching
of TSA. The similar test was performed for the residue obtained after reaction completion and
absence of blue colour indicates no leaching of TSA.
3. Results and discussion
3.1. Catalyst Characterization
The effective TSA loading in all catalysts was calculated by the following equation.
Loading amount (%) =
Weight of TSA
× 100
(Weight of TSA) + (Weight of nMCM-48)
According to the given formula, for the series of catalysts, TSA1/nMCM-48, TSA2/nMCM-48,
TSA3/nMCM-48 and TSA4/nMCM-48, the subscript 1, 2, 3 and 4 indicates loading of TSA,
which is 9, 16.66, 23.07 and 28.5 wt%, respectively.
The values of elemental analysis by EDX (Table 1) for all catalysts were in good agreement
with theoretical calculated value of tungsten (W) except for TSA4/nMCM-48 indicating
effective loading of TSA. However, it should be noted that the variation in case TSA4/nMCM-
48, may be due to insufficient loading. The EDX elemental mapping of TSA3/nMCM-48 is
shown in figure 2 while others are included in supplementary material (Figure S1).
Table 1. Elemental analysis of catalysts
W (wt%)
Material
Si (wt%) O (wt%)
By EDX Theoretical
TSA1/nMCM-48
TSA2/nMCM-48
TSA3/nMCM-48
TSA4/nMCM-48
41.34
35.96
23.73
30.59
52.19
51.46
58.93
49.28
6.48
6.96
12.7
17.7
21.8
12.58
17.34
20.13
7

Figure 2 EDX elemental mapping of TSA3/nMCM-48 (colour)
It is seen from Table 2 that as the amount of TSA loading increases, total acidity also increases.
This may be due to enhancement in the concentration of Bronsted acidity of TSA inside
nMCM-48, which is in good agreement with results of elemental analysis. However, on
increasing the amount of TSA from 30 % to 40 %, no significant change in total acidity was
observed. The acidic strength and total number of acidic sites of synthesized catalysts including
support were determined by potentiometric titration. The acid strength of surface sites can be
assigned according to the following scale: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV
(strong sites), -100 < Ei < 0 mV (weak sites) and Ei < -100 mV (very weak sites) [28]. The
plots of the electrode potential as a function of meq. n-butylamine per g of the material are
shown in supplementary figure S2. From the data presented in Table 2, it is well distinct that
as the amount of TSA loading on nMCM-48 increases from 10 – 30 %, the acidic strength
increases significantly, while negligible change observed from 30 – 40 %. The increase in
number of very strong and strong acidic sites is directly related to the amount of TSA which is
attributed to enhanced Bronsted acid sites.
Table 2. Acidity measurements of support and catalysts
n-butylamine
Potentiometric titration
titration
Material
Types of acidic sites
(meq g-1)
Total No. of
acidic sites
Acidic
strength
(mV)
Total acidity
(mmol g-1)
Very strong Strong (mmol g-1)
0.48
0.52
0.66
0.98
1.04
nMCM-48
168
0.1
0.3
0.6
2.3
2.3
2.7
1.04
1.28
1.32
TSA1/nMCM-48 646
TSA2/nMCM-48 676
TSA3/nMCM-48 700
TSA4/nMCM-48 706
1.4
1.3
3.5
3.6
1.44
1.46
8

Noted that only a slight increase in the acidic strength as well as the total number of acidic sites
was observed from 30 % to 40 % loading of TSA, which is in good agreement with the value
of total acidity. This may be due to the blocking of the sites as well as insufficient impregnation,
which was also observed in the EDX analysis. Hence TSA3/nMCM-48 was considered for
detailed characterization and re-coded as TSA/nMCM-48.
To confirm the types of acidic sites, the support and catalyst were further characterized by NH3-
TPD analysis (Figure 3). The profile of support and the catalyst exhibits combined desorption
peak for weak as well as moderate acid sites. Desorbed amount of ammonia can be directly
related to the amount of the acid sites on the sample and also the strength of the acid sites can
be determined by desorption temperature. NH3 desorbed below 200 ℃ is due to weak acidic
site. Around 250-400 ℃, desorption occurs due to medium/moderate acidic sites and above 500
℃ is due to strong acid site. It is well distinct from the figure 1 that for nMCM-48 negligible
amount of ammonia was desorbed indicating presence of few weak and moderate acidic sites.
After introduction of TSA, elevation of peak area from 100 -300 ℃was observed corresponding
to the increase in weak and moderate Bronsted acid sites [31]. Thus, the overall enhancement
in the acidic sites of material was obtained due to incorporation of TSA inside the pores of
nMCM-48, which is in good agreement with reported one [23].
Figure 3 NH3-TPD profile of nMCM-48 and TSA/nMCM-48 (colour)
TGA for nMCM-48 and TSA/nMCM-48 are shown in supplementary figure S3. From the plots
it is seen that the weight loss occurs mainly in two stages. For nMCM-48, the initial weight
loss of 8.9 % up to 110 ℃may be due to the desorption of physically absorbed water molecules.
Final weight loss of <1 % up to 400 ℃is attributed to the condensation of silanol groups present
in the nMCM-48 [32]. After that, no weight loss was observed, which indicates the stability up
to 550 ℃. While TSA/nMCM-48, shows initial weight loss of 8.1 % up to 115 ℃corresponding
9

to adsorbed water. The second weight loss of 0.67 % up to 200 ℃can be attributed to crystalline
water molecules present in Keggin unit [23]. After that, no gradual weight loss up to 550 ℃,
shows the stability of the catalyst.
FT-IR spectrum (Figure 4) of pure nMCM-48 shows a broad band around 1100 cm-1 and 1250
cm-1 which corresponds to Si-O-Si asymmetric stretching. It also shows bands at 578 and 462
cm-1, which can be attributed to the symmetric stretching of Si-O-Si and bending vibration of
Si-O respectively. Symmetric stretching vibration of Si-O-Si and bending vibration of Si-O are
depicted by the bands around 578 and 462 cm-1 respectively. The broad band at 3448 cm-1 is
due to the absorption of hydroxyl group from presence of large amount of H2O. The obtained
bands are in good agreement with the reported one [33]. FT-IR of TSA/nMCM-48 shows two
of the characteristic bands for the Keggin unit TSA at 972 and 925 cm-1 corresponding to
asymmetric vibrations of W-Od (terminal oxygen linked to a lone tungsten atom) and (Si-Oa)
respectively. Presence of these fingerprint bands directs that the primary structure of TSA is
completely unchanged even after impregnation on the support.
Figure 4 FT-IR spectra of a) TSA b) nMCM-48 c) TSA/nMCM-48
The nonappearance of vibrational bands around 880 cm-1(Vas(W-Ob-W)) and 785 cm-1 (Vas (W-
Oc-W)) of TSA may be due to superimposition with the bands of nMCM-48 [34]. Further, FT-
IR spectra of TSA/nMCM-48 also shows an additional peak at 1635 cm-1, may be due to the
bending vibration of bridging hydroxyl groups, resulting from the hydrogen bond between
terminal oxygen of TSA and hydrogen of silanol groups of nMCM-48.
10

Textural properties of support as well as catalysts are shown in Table 3. Relatively small pore
diameter of nMCM-48 gives the first indication that the support is nano-porous in nature with
high specific surface area. Significant decrease in both, surface area as well as pore volume
clearly indicate the incorporation of TSA inside the porous network of nMCM-48. It is very
interesting to note down a decrease in surface area and increase in the pore value for
TSA4/nMCM-48, indicating blocking of the sites because of the multilayer adsorption of TSA.
Looking at the values of pore volume and pore diameter, once again, TSA3/nMCM-48 was
selected for the further study.
Table 3. Textural properties of support and catalysts
Material
Surface area (m2/g) Pore volume (cm3/g) Pore diameter (Å)
nMCM-48
1307
0.63
0.24
0.25
0.21
0.23
20.8
20.1
19.2
18.3
17.8
TSA1/nMCM-48 712
TSA2/nMCM-48 637
TSA3/nMCM-48 588
TSA4/nMCM-48 333
*TSA3/nMCM-48 = TSA/nMCM-48
The N2 adsorption-desorption isotherms of nMCM-48 and TSA/nMCM-48 (Figure 5) shows
typical nature of type IV(b), conforming the formation of mesoporous structure. According to
the IUPAC technical report, capillary condensation for the pores having smaller width are
completely reversible with the absence of a hysteresis loop. In the present case, the obtained
completely reversible isotherms are without hysteresis loop indicating the smaller width and
confirming the nano-porous structure. The observation is in good agreement with the reported
one [35]. The uniformity in the size of nanopores is well distinct from the sharpness of the step
and position of the inflection point which is related to the diameter of pores (20.8 Å). For
nMCM-48, adsorption in the mesopores is well distinct in the range of relative pressure
between 0.2 - 0.3 which disappears for TSA/nMCM-48 indicating the effective filling of
mesopores by TSA [36].
11

Figure 5 Nitrogen (N2) adsorption-desorption isotherms (colour)
SEM images (supplementary figure S4) shows spherical morphology with a uniform particle
size of nMCM-48. SEM micrograph of TSA/nMCM-48 indicates the homogeneous
distribution of spheres and there is no noticeable change observed in the morphology even after
the introduction of TSA species, indicating its fine dispersion into three dimensional pores,
further confirmed by XRD.
TEM images (Figure 6) of nMCM-48 and TSA/nMCM-48 were recorded at various
magnifications. Figure 6a & 6b of nMCM-48 shows well-ordered pore networks with uniform
particle diameter, confirming the nano-porous structure, which is in good agreement with the
BET surface area analysis. Opaque spheres were observed in TSA/nMCM-48 (Figure 3c &
3d), where the maximum pores are uniformly filled with TSA. This is further confirmed by
XRD.
Figure 6 TEM images of a, b) nMCM-48 c, d) TSA/nMCM-48
12

The Low angle powder XRD analysis (Figure 7a) shows a typical pattern of nMCM-48
exhibiting characteristic diffraction peaks at 3.0⁰ and 3.36⁰ 2θ corresponding to the plane 211
and 220, respectively. In the range of 4 – 5⁰ 2θ, numerous peaks correspond to the reflection
planes 400, 321 and 420 of MCM-48 having Ia3d cubic symmetry [37]. The XRD pattern of
TSA/nMCM-48 depicts the decrease in intensity of characteristic peak with slight broadening
which is in good agreement with the reported one [38]. Wide angle XRD pattern (Figure 7b)
of anchored MCM-48 shows the absence of characteristic peaks of the crystalline phase of
TSA, signifying very fine dispersion of it into the pores of nMCM-48 [39]. Collectively, it can
be inferred from the XRD results, that the structure of nMCM-48 remains intact even after
impregnation of TSA.
Figure 7 a) Low angle Powder XRD patterns b) Wide angle Powder XRD patterns (colour)
29
To identify the interactions between of nMCM-48 and TSA, solid state Si MAS-NMR was
performed. Typical broad peak for nMCM-48 was observed between -95 and -120 ppm
corresponding to the three main components Q2, Q3 and Q4 (Figure 8). The notation Qx
describes the number of x siloxane linkages to a silicon atom. For instance, the Q2 signal
corresponds to the disilanol linkage Si-(O-Si)2 (-O-X)2, where X is H or TSA. Similarly,
resonating peaks for (O-X)-Si-(O-Si)3 and Si-(O-Si)4 corresponds to Q3 and Q4 respectively.
The chemical shift values for all components are mentioned in table 3. The observed values of
chemical shifts for nMCM-48 are in good agreement with the reported literature [40]. The
observed minor downfield shift in the value of chemical shift for Q2, Q3 and Q4 as well as
broadening in the spectrum of TSA/nMCM-48 as compared to n MCM-48 may be due to the
presence of TSA. Also, a slight change in the intensities of Q2 and Q3 peaks were observed,
indicating the strong bonding of TSA to proton of silanol groups of nMCM-48. Evidently, the
signal for TSA appears in the range of -81 to -90 ppm, which is depicted in the spectrum at -
87.97 ppm, showcasing the unchanged Keggin structure of TSA. [41].
13

Table 4. 29Si chemical shifts of nMCM-48 and TSA/nMCM-48
Material
Si nuclei ppm (TSA)
Q2 ppm
-95.51
-96.92
Q3 ppm
-103.97
-104.78
Q4 ppm
-106.53
-107.51
nMCM-48
TSA/nMCM-48
-
-87.97
Figure 8 29Si MAS NMR spectra of a) nMCM-48 b) TSA/nMCM-48
Thus, acidity measurements show that the total acidity as well as acidic strength increases after
the introduction of TSA into nMCM-48. BET and FT-IR studies indicates the inclusion of
undegraded TSA into the nanopores of nMCM-48 via hydrogen bonding between terminal
oxygen and hydrogen of silanol groups of nMCM-48. TEM, XRD and 29Si MAS NMR confirm
the meso structure with nanopores, fine dispersion of TSA inside nMCM-48 and strong
interaction of silanol groups to TSA inside pores of nMCM-48, respectively.
3.2. Catalytic evaluation
3.2.1 Acetalization of glycerol with benzaldehyde
Glycerol acetalization with benzaldehyde yields two cyclic products, 1,3-dioxolane (5-
membered) and 1,3 dioxane (6- membered) (Scheme-1), amongst which 1,3-dioxane is
thermodynamically favoured. To study the effect of % loading of TSA, reactions were carried
using 10, 20, 30 and 40 % TSA loaded nMCM-48. From figure 9a, it is seen that as the % of
14

TSA increases, % conversion also increases from 67 to 98 %. However, the trend does not
show a significant change in conversion from 30 to 40 % loading. Hence, considering the
equivalent effect in terms of acidity measurements as well as % conversion, the catalyst with
30 % TSA loading, TSA/nMCM-48 was selected for the detailed study.
Scheme 1 Acetalization of glycerol with benzaldehyde
15

Figure 9. (a) Effect of % loading TSA. Mole ratio: 1:1; Catalyst amount: 50 mg; Temperature: 30
℃; Time: 60 min; (b)Effect of mole ratio. Catalyst amount: 50 mg; Temperature: 30 ℃; Time: 60
min; (c) Effect of catalyst amount. Mole ratio: 1:1; Temperature: 30 ℃; Time: 60 min; (d) Effect of
reaction temperature. Mole ratio: 1:1; Catalyst amount: 20 mg; Time: 60 min; (e) Effect of reaction
time. Mole ratio: 1:1; Catalyst amount: 20 mg; Temperature: 30 ℃(colour)
The effect of mole ratio of glycerol to benzaldehyde was studied by varying from 1:1 to 1:2
(Figure 9b). 98 % glycerol conversion with 65 % and 35 % selectivity of 1,3-dioxane and 1,3-
dioxolane respectively, was obtained with 1:1 mole ratio. By increasing the ratio to 1: 1.5 and
1: 2, there was no significant change in the conversion of glycerol and selectivity. As a result,
1:1 mole ratio was considered optimum for further catalytic evaluation. The initial feasibility
of the catalyst indicates the selective formation of thermodynamically stable 1,3- dioxane (six
membered acetal) and hence further parameters were optimised by focusing the selectivity for
the same.
16

The effect of catalyst amount was studied by screening the range 10 - 50 mg (Figure 9c).
Initially, from 10 mg to 20 mg marginal increase in the % conversion was observed. Further
increase in catalyst amount did not show any significant effect in terms of conversion as well
as selectivity. The observed behaviour is may be due to the blocking of sites/ saturation of the
number of active sites available vs. reactant amount. Hence, 20 mg was considered to be
optimum amount.
The most important parameter which governs the equilibrium of reaction is the reaction
temperature. It is well-known that acetalization reaction has low equilibrium constant and is
exothermic in nature [16]. Screening of reaction temperature ranging from 20 to 40 ℃ over
TSA/MCM-48 was carried out. As depicted by the plot in figure 9d, there was no major rise in
the % conversion and selectivity of desired thermodynamically stable product, hence, an easily
accessible temperature, 30 ℃was optimized for the said conversion.
From figure 9e it is seen that % conversion increased from 30 to 40 min, with no change in the
selectivity of products. Despite increasing the reaction time from 40 to 60 min, very negligible
rise of 2 % in glycerol conversion was observed which indicates the establishment of
equilibrium beyond 40 min. Thus, 40 min was considered to be optimum time for reaction.
The optimised reaction conditions for the maximum conversion of glycerol (95 %) and
selectivity of dioxane (65 %) are: glycerol: benzaldehyde w/w ratio 1: 1; catalyst amount 20
mg; active amount of TSA 4.6 mg; substrate: catalyst ratio (mol: mol) 6257:1; reaction
temperature 30 ℃; reaction time 40 min with TON 5945.
3.2.2. Acetalization of glycerol with furfural
In order to evaluate efficiency and versatility of catalyst, reaction with heterocyclic aldehyde-
furfural (Scheme 2) was also studied by varying the same parameters as mentioned for
benzaldehyde and the obtained results are presented in Figure 10.
Scheme 2 Acetalization of glycerol with furfural
17

Figure 10 (a) Effect of % loading TSA. Catalyst amount: 20 mg; Temperature: 30 ℃; Time: 40 min; (b)
Effect of mole ratio. Catalyst amount: 20 mg; Temperature: 30 ℃; Time: 40 min; (c) Effect of catalyst
amount. Mole ratio: 1:1; Temperature: 30 ℃; Time: 40 min; (d) Effect of reaction temperature. Mole
ratio: 1:1; Catalyst amount: 15 mg; Time: 40 min (e) Effect of reaction time. Mole ratio: 1:1; Catalyst
amount: 15 mg; Temperature: 30 ℃(Colour)
The optimum conditions for glycerol acetalization with furfural for maximum conversion (89
%) and selectivity (69 %) towards dioxane are: glycerol: furfural w/w ratio 1:1; catalyst amount
15 mg; active amount of TSA 3.5 mg; substrate: catalyst ratio (mol: mol) 8223:1; reaction
temperature 30 ℃; time 40 min with TON 7355. Requirement of low catalyst amount (15 mg)
can be due to the presence of active furan ring in furfural.
18

3.3. Kinetics: Determination of activation energy
For determining the activation energy, both the reactions were subjected to different time
intervals at 20, 30 and 40 ℃, keeping the other parameters same as in optimised reaction
condition and the obtained rate constants (k) are presented in table 5. Using these data, the
value of activation energy (Ea) was calculated form the slope of graph (ln k vs 1/T, K-1) (Figure
11) by Arrhenius equation.
Table 5. Kinetic parameter (k min-1) at different temperatures and activation energy (Ea)
Glycerol with Benzaldehyde
Glycerol with Furfural
Temperature
(K)
Rate constant Activation energy Rate constant Activation energy
k (min-1)
Ea (kJ/mol)
43
k (min-1)
Ea (kJ/mol)
36
293
303
313
1.512 x 10-2
4.032 x 10-2
4.680 x 10-2
1.85 x 10-2
2.55 x 10-2
4.73 x 10-2
Figure 11 Plot for determination of activation energy (ln k vs 1/T, K-1) a) Glycerol acetalization with
benzaldehyde b) Glycerol acetalization with furfural
It is reported in the literature that for the reactions following diffusion regime, the activation
energy is as low as 10-15 kJ/mol, while for the reactions governed by truly chemical step, the
activation energy is greater than 25 kJ/mol [42]. Here, in both the cases, the activation energy
is exceeding 25 kJ/mole, which clearly suggests that the reactions are governed truly by
chemical step.
It is interesting to note that the activation energy for the acetalization of glycerol with furfural
(36 kJ/mol) is lower as compared to that with benzaldehyde (43 kJ/mol). The values of
activation energy clearly indicate that the lower activation energy for furfural can be attributed
19

to the presence of active furan ring which requires comparatively less prompt effect for the
reaction to proceed in forward direction.
3.4. Control experiments
The control experiments for acetalization of glycerol with benzaldehyde and furfural were
performed in their respective optimised reaction conditions. From the results demonstrated in
Table 6, it was observed that the support alone is not active for the desired transformations and
almost similar activity was seen for TSA and TSA/nMCM-48. This confirms that the reactivity
is only due to the active keggin unit, TSA, present inside the porous framework and it can be
attributed to successful anchoring of TSA. Hence, we have fruitfully synthesized a catalyst
which is truly heterogeneous in nature.
Table 6. Control experiments
Glycerol with Benzaldehydea
Glycerol with Furfuralb
Catalysts
% selectivity of
% Conversion
% selectivity of
% Conversion
Dioxane
Dioxane
52
nMCM-48
TSA
1.5
97
95
49
62
65
3.1
91
89
58
TSA/nMCM-48
69
Reaction conditions: Mole ratio 1:1, reaction temperature, reaction time: 40 min, amount of nMCM-48
a15.4 mg, b11.5 mg, amount of TSA a4.6 mg, b3.5 mg, catalyst amount a20 mg, b 15 mg
3.5. Recycling studies
To reduce the overall cost of production, feasibility of heterogeneous catalyst can be
determined by its recycling. For recycling studies, the catalyst was collected by centrifugation
after reaction completion and simply treated with methanol to remove the substrate and product
remains from the surface, followed by water wash. The catalyst was dried at 100 ℃and reused
for the next catalytic run. The result (Figure 12) shows that the catalyst is stable without any
leaching of TSA during the reaction.
20

Figure 12 (a) Reaction conditions: Mole ratio (Glycerol: Benzaldehyde) 1:1; Catalyst amount: 20
mg; Temperature: 30 ℃; Time: 40 min
(b) Reaction conditions: Mole ratio (Glycerol: Furfural) 1:1; Catalyst amount: 15 mg; Temperature:
30 ℃; Time: 40 min (colour)
3.6. Characterization of regenerated catalyst
In order to check sustainability, the regenerated catalyst was characterized by n-butyl amine
titration, BET surface area and FT-IR analysis. Total acidity obtained by n-butyl amine titration
method for regenerated catalyst (1.43 mmol g-1) was in good agreement with that of fresh one
(1.44 mmol g-1), indicating the unaltered acidic property of catalyst. The N2 adsorption-
desorption isotherms (Supplementary figure S5a) of fresh and regenerated catalyst shows
identical curves with no change in BET surface area, 588 & 580 m2/g respectively. This also
suggests that there is no alteration in the structure of catalyst after reaction. In FT-IR analysis
(Supplementary figure S5b), presence of all the characteristic bands in spectra with no shifting
of values, confirms that there is no leaching and change in the structure of keggin unit.
However, a little bit of decrease in intensity of FT-IR bands was observed due to the
regeneration process.
3.7. Effect of nature of substrates
To study the relative reactivity of benzaldehyde and furfural in presence of synthesized
catalyst, reactions were carried in same experimental conditions and obtained results are
presented in table 7. It is interesting to note from the results that simple aromatic aldehyde,
benzaldehyde requires more catalyst amount than heterocyclic aldehyde, furfural for maximum
conversion and selectivity. This can be attributed to the presence of active furan ring in furfural
which proceeds to form acetals more dominantly than benzaldehyde. This observation is in
good agreement with the obtained values of activation energy, where formation of acetals of
21

furfural shows comparatively lower activation energy than that for acetals of benzaldehyde.
Also, in case of furfural increase in catalyst amount (20 mg) decreases the conversion (80 %)
as well as selectivity (60 %) (Figure 10c) which may be due to the mass density increase and
less available surface sites for catalytic reaction. Hence, it can be inferred that heterocyclic
aldehydes requires a less trigger effect from the acid catalyst to readily form acetals at ambient
reaction conditions.
Table 7. Effect of nature of substrates
Substrate
% Conversion % selectivity of Dioxane TON
Benzaldehyde
Furfural
88
89
60
69
7272
7355
Reaction conditions: mole ratio 1:1, reaction temperature 30 ℃; reaction time 40 min; catalyst amount:
15 mg; active amount of TSA 3.5 mg
3.8. Comparison with the reported catalysts
For the present catalyst, catalytic activities for both the reactions are compared with the
reported ones, considering the conversion based on glycerol and focusing on selectivity of
dioxane derivative.
As seen in table 8, in case of benzaldehyde, the advantage of present catalyst lies in the best
glycerol conversion (95 %) as well as selectivity towards dioxane compared to the reported
catalysts. Similarly, for furfural, highest conversion (89 %) as well as double the dioxane
selectivity (69 %) is obtained compared to the reported ones. It is interesting to note that while
the reported systems have been carried out under comparatively harsher conditions, the present
catalyst is highly effective and greener in terms of very less catalyst amount, lower temperature,
less reaction time and solvent free condition. The superior activity of the present catalyst can
be attributed to the unique acidity of TSA as well as 3D cubic geometry and nano porosity of
support which enables the formation of thermodynamically stable derivative at ambient
conditions.
22

Table 8. Comparison with reported catalysts
Glycerol with benzaldehyde
Reaction conditions
Catalyst
%
Conversion %
Selectivity
Mole ratio/cat amt /temp/time
Glycerol
Dioxane
1.1:1/10 wt% /100 ℃/480 min
(solvent: toluene)
43
1% MoO3/SiO2
37
43
72
74
95
63
1.1:1/10 wt % /100 ℃/480 min
(solvent: toluene)
43
10% MoO3/SiO2
62.5
60
1.1:1/10 wt% /100 ℃/480 min
(solvent: toluene)
43
20 %MoO3/SiO2
10%
ZrO2
MoO3/TiO2-
1:1/5 wt % /100 ℃/30 min
1:1/ 2.17 wt %/30 ℃/40 min
51
44
TSA/nMCM-48
65
(Present work)
Glycerol with furfural
45
SnO2
1:1/5 wt %/20℃/30 min
1:1/5 wt %/20℃/30 min
1:1/5 wt %/20℃/30 min
51
67
75
38
37
36
45
WO3/SnO2
45
MoO3/SnO2
TSA/nMCM-48
(Present work)
1:1/ 1.63 wt%/30℃/40 min
89
69
3.9. Reaction mechanism
The probable reaction mechanism for acid catalysed acetalization of glycerol with
benzaldehyde and furfural are shown in figure 13 For the reaction of glycerol with
benzaldehyde, we have proposed the same mechanism as reported earlier by our group [23].
Similarly, for the acetalization of glycerol with furfural, the first step is initialized by the
Bronsted acidity, proton present in TSA, which activates the carbonyl carbon of aldehyde. In
the second step, the primary hydroxyl group of glycerol attacks the carbonyl carbon of aldehyde
and forms hemiacetal. The subsequent formation of carbocation and removal of water molecule
gives the dioxane derivative.
23

Figure 13 Plausible reaction mechanism for acetalization of glycerol with benzaldehyde and furfural
4. Conclusion
In a nutshell, first time nanoporous MCM-48 was successfully synthesized and used as a
support for anchoring 12-tungstosilic acid. XRD, N2- sorption analysis and TEM confirms the
3D cubic geometry and nanoporosity of nMCM-48. Further, the interaction between TSA and
nMCM-48 was established by BET as well as 29Si MAS NMR. The synthesized catalyst was
used for the synthesis of bio-fuel additives via catalytic valorisation of glycerol and the
superiority of the catalyst lies in the inspiring results obtained at ambient reaction conditions.
The obtained values of activation energy confirmed that both the reactions were truly governed
by a chemical step. As compared to the reported ones, nMCM-48 proves to be better support
due to its 3D geometry and nanoporosity which is responsible for higher selectivity of
thermodynamically stable dioxane obtained for both the aldehydes. The catalyst was also
recycled for consecutive four catalytic cycles and showed consistent conversion rate, thereby
contributing effectively in the cost reduction of process as well as making the overall procedure
sustainable and environmentally benign.
24

Associated content
The Supporting Information is available.
Credit author statement
Anjali Patel: Conceptualization, Reviewing and editing, Supervision
Dhruvi Pithadia: Methodology, Software, Writing-original draft preparation, Reviewing and editing.
Conflict of Interest Disclosure
There are no conflicts to declare.
Acknowledgement
We are thankful to Dr. N Lingaiah for NH3-TPD analysis at CSIR-IICT, Hyderabad. We are
also thankful to Department of Chemistry, The Maharaja Sayajirao University of Baroda for
BET surface area analysis and Sud-chemie India Pvt. Ltd., Nandesari, Vadodara for Powder
XRD analysis.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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