Mesoporous tin oxide: An efficient catalyst with versatile applications in acid and oxidation catalysis

Article abstract of DOI:10.1016/j.cattod.2017.10.009

Mesoporous tin oxide was prepared by template assisted and template-free methods. As-prepared materials were calcined at various temperatures to generate different nature (Br?nsted and Lewis), amount and strength of acidic sites. The physico-chemical properties of the catalysts were studied by XRD, N₂ sorption, pyridine-FTIR, NH₃-TPD, DRS UV-vis, TGA, SEM, TEM, ¹H MAS and ¹¹⁹Sn MAS NMR analyses. The catalytic behavior of mesoporous tin oxide catalysts was evaluated for acetalization and ketalization of glycerol with benzaldehyde and acetone respectively under solvent free conditions. The catalytic performance of mesoporous tin oxide was compared with that of other conventional solid acid catalysts namely H-ZSM-5, H-mordenite, H-beta, Al-MCM-41, Al-SBA-15 and Al-TUD-1. The efficiency of mesoporous tin oxide was also tested for cyclohexene epoxidation reaction. The catalyst prepared by template assisted method showed excellent catalytic performance compared to other catalysts due the difference in nature and amount of acidic sites in the catalyst. Meso-SnO₂-T-350 was stable and reusable catalyst for four cycles without any appreciable loss in activity, and therefore it offers a good catalyst for potentially wide applications.

Full text of DOI:10.1016/j.cattod.2017.10.009

Accepted Manuscript
Title: Mesoporous tin oxide: An efficient catalyst with
versatile applications in acid and oxidation catalysis
Authors: Pandian Manjunathan, Vijaykumar S. Marakatti,
Prakash Chandra, Atul B. Kulal, Shubhangi B. Umbarkar,
Raman Ravishankar, Ganapati V. Shanbhag
PII:
S0920-5861(17)30686-7
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2017.10.009
CATTOD 11076
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
4-4-2017
21-9-2017
4-10-2017
Please cite this article as: Pandian Manjunathan, Vijaykumar S.Marakatti,
Prakash Chandra, Atul B.Kulal, Shubhangi B.Umbarkar, Raman Ravishankar,
Ganapati V.Shanbhag, Mesoporous tin oxide: An efficient catalyst
with versatile applications in acid and oxidation catalysis, Catalysis
Today https://doi.org/10.1016/j.cattod.2017.10.009
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Mesoporous tin oxide: An efficient catalyst with versatile applications in acid
and oxidation catalysis
a,b
a
a
c
Pandian Manjunathan, Vijaykumar S. Marakatti, Prakash Chandra, Atul B. Kulal,
c
d
a
Shubhangi B. Umbarkar, Raman Ravishankar and Ganapati V. Shanbhag*
aMaterials Science Department, Poornaprajna Institute of Scientific Research (PPISR),
Bidalur Post, Devanahalli, Bengaluru-562164, India
bGraduate Studies, Manipal University, Manipal-576104, Karnataka, India
cCatalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune -
4
11008, India
dHindustan Petroleum Green Research & Development Centre, Hindustan Petroleum
Corporation Ltd. (HPCL),KIADB Industrial Area, Tarabahalli, Hoskote Taluk, Bengaluru -
5
60067, Karnataka, India
Corresponding author: shanbhag@poornaprajna.org, gvshanbhag@gmail.com
1

Graphical abstract





Catalyst
Acetalization
Ketalization
Epoxidation
Glycerol
Highlights




Nature, strength and amount of acidity in meso-SnO
2
were tuned by calcination
Meso-SnO showed high catalytic activity compared to other solid catalysts
2
Brønsted acidic sites were found to be active sites for the studied reactions
Direct correlation of amount of acidity with catalyst performance in meso-SnO
2
ABSTRACT
Mesoporous tin oxide was prepared by template assisted and template-free methods.
As-prepared materials were calcined at various temperatures to generate different nature
(Brønsted and Lewis), amount and strength of acidic sites. The physico-chemical properties of
the catalysts were studied by XRD, N
2
sorption, pyridine-FTIR, NH -TPD, DRS UV-vis, TGA,
3
SEM, TEM, H MAS and 119Sn MAS NMR analyses. The catalytic behavior of mesoporous
1
2

tin oxide catalysts was evaluated for acetalization and ketalization of glycerol with
benzaldehyde and acetone respectively under solvent free conditions. The catalytic
performance of mesoporous tin oxide was compared with that of other conventional solid acid
catalysts namely H-ZSM-5, H-mordenite, H-beta, Al-MCM-41, Al-SBA-15 and Al-TUD-1.
The efficiency of mesoporous tin oxide was also tested for cyclohexene epoxidation reaction.
The catalyst prepared by template assisted method showed excellent catalytic performance
compared to other catalysts due the difference in nature and amount of acidic sites in the
catalyst. Meso-SnO -T-350 was stable and reusable catalyst for four cycles without any
2
appreciable loss in activity, and therefore it offers a good catalyst for potentially wide
applications.
Keywords: Mesoporous tin oxide
3

1
. Introduction
Discoveries of ordered mesoporous silicas and other mesoporous supports like oxides,
phosphates, carbons, polymers and organic–inorganic hybrid periodic mesoporous
organosilicas through supramolecular templating pathways helped in developing high
performance catalysts for the synthesis of fine chemicals [1–6]. Interest in employing wide
range of mesoporous materials in catalysis has dramatically increased so far and their
environmental aspects for clean chemical synthesis have been well established. Due to their
interesting properties like high surface area, tunable pore size and their diverse frameworks,
mesoporous materials have attracted immense interest in green catalytic processes. The
presence of mesopores (pore size ranging from 2 – 50 nm) in catalysts can overcome diffusion
constraints which are seen in microporous catalysts (pore size < 2nm), and the presence of large
surface area facilitates ease accessibility of active sites to the reactant molecules leading to a
better catalytic performance. In addition to their wide range of applications in organic and fine
chemicals synthesis, mesoporous materials have also been used in adsorption, sensors, lithium-
ion batteries, drug delivery, and nanodevices [7].
Metal oxides are found to be interesting materials owing to their acid–base and redox
properties. The surface of metal oxides may terminate with M–OH, M–O–M, M=O or M– (O-
vacant) functionalities [8]. Additionally, metal oxides possessing mesoporosity can be
excellent catalysts since they endow properties of mesoporous materials and metal oxides. The
nature of active sites in these metal oxides can be modulated by adopting different synthetic
strategies and activation steps (calcination temperature). This leads to generation of active sites
with a combination of Brønsted and Lewis characters which can be tuned by changing synthesis
conditions.
Several tin based heterogeneous catalysts have contributed significantly in liquid phase
organic transformations and biomass conversion. Tin containing zeolites have been used for
4

isomerisation of glucose [9], 1,3-dihydroxyacetone conversion into ethyl lactate [10], Sn-MFI
as Lewis acid catalysts for isomerisation of cellulosic sugar [11], Sn-beta zeolite for the
synthesis of 5-hydroxymethylfurfural from carbohydrates [12]. Zeolite Sn-MWW was used for
the conversion of sugar into methyl lactate and lactic acid [13]. Sn-silicates were used for
hydroxylation of phenol [14,15], whereas Sn-silicate with MFI structure was employed for
ethylbenzene oxidation reaction [16]. Among other Sn based catalysts, Sn-MCM-41, Sn-SBA-
1
5 and Sn(OH)Cl have been used for the synthesis of nopol [17–19]. Also, Sn(IV) grafted on
MCM-41 gave good performance for Baeyer-Villiger oxidation reaction [20]. Hence, several
tin based heterogeneous catalysts have been investigated and have proven to be promising
candidates for different organic transformations. However, tin dioxide (SnO ) has not been
2
exploited much as a catalyst (not as mere support) in spite of having variable valence state and
oxygen vacancy defects as its unique characteristics [21]. Recently, our group has reported the
application of mesoporous SnO as catalyst for few organic transformation reactions [22].
2
Glycerol is one of the most abundant platform chemicals obtained as a by-product
during biodiesel synthesis. As surplus amount of glycerol is produced, it triggered the
researchers to transform glycerol into value-added products viz., chemicals and fuel additives.
Glycerol can be transformed into various important products like acetins [23], acrolein [24],
tert–butyl glycerol ethers [25], glycerol carbonate [26,27], 1,3-propanediol [28], 1,3-
dihydroxyacetone [29], solketal [30] and more [31]. Glycerol undergoes acetalization (with
aldehyde) and ketalization (with ketone) which results into the formation of isomeric six- and
five-membered cyclic acetals respectively. The six-membered cyclic acetals are potential
precursors to produce green platform chemicals 1,3-dihydroxyacetone and 1,3-propanediol
[
32]. The five-membered cyclic acetal (solketal) obtained from glycerol ketalization with
acetone can be used as a cold flow improver to enhance cold weather performance of diesel
fuel, which is also used as additive in ointments in the chemical industry [30,33].
5

Epoxidation of olefins is another class of reaction used for the synthesis of epoxide
which acts as precursor for commodity chemicals such as drug intermediates, agrochemicals
and food additives [34,35]. The choice of hydrogen peroxide as oxidant in olefin epoxidation
reaction is advantageous from the view of environmental concern and green chemistry, since
it generates water as the side-product. Moreover, it is cheaper and safer to use than organic
peroxides or peracids [36].
Our earlier results inspired us to further explore the properties of mesoporous SnO
its application as a catalyst in important organic transformations. In this study, we report
mesoporous SnO as an acid catalyst for the synthesis of 5-hydroxy-2-phenyl-1,3-dioxane and
solketal from acetalization and ketalization of glycerol respectively, and cyclohexeneoxide
from epoxidation of cyclohexene. The mesoporous SnO was synthesized by supramolecular
template and template-free approach and the active sites were modulated by varying calcination
temperature. The catalytic performance of mesoporous SnO was evaluated by comparing with
2
and
2
2
2
other well-known mesoporous (Al-MCM-41, Al-SBA-15 and Al-TUD-1) and microporous (H-
ZSM-5, H-mordenite and H-beta zeolite) catalysts. A clear correlation of B/L ratio and acidic
sites with catalytic performance was obtained.
2
. Experimental
2
.1. Chemicals and Materials
Cetyltrimethylammonium bromide (CTAB) and tin chloride pentahydrate were
purchased from Loba Chemie Pvt. Ltd, India. Ammonium hydroxide (25 wt%), acetone,
benzaldehyde, cyclohexene, glycerol, hydrogen peroxide (50%) and methanol were purchased
from Merck India Pvt. Ltd. Tetraethyl orthosilicate, tetraethyleneglycol, cyclohexeneoxide and
solketal were purchased from Sigma Aldrich Pvt. Ltd, India. NH -Beta zeolite (SAR = 25) was
4
obtained from Nankai, China. NH
4
-ZSM-5 zeolite (SAR = 30) was purchased from Zeolyst
6

International. H-Mordenite zeolite (SAR = 16) was kindly donated by Sud-Chemie India Pvt.
Ltd, India.
2
.2. Catalyst preparation
Synthesis of mesoporous tin oxide by template assisted and template-free method
The mesoporous tin oxide was synthesized at room temperature using SnCl
4
·5H O as
2
the tin precursor and cetyltrimethylammonium bromide (CTAB) as the template [22,37,38]. In
a typical procedure, 18 g of CTAB was dissolved in 150 ml of distilled water under stirring to
get a homogeneous solution. To this solution, 12 ml of NH
of distilled H O was added under stirring. Further, aq. SnCl
50 ml distilled water) was added drop wise to this solution under continuous stirring to obtain
4
OH (25 wt%) dissolved in 48 ml
2
4
(15 g of SnCl ·5H O dissolved in
4
2
1
a white slurry. It was stirred for 3 h followed by aging at room temperature for 48 h. The
obtained product was filtered, washed with distilled water and dried at 120 °C. The as-prepared
white solid was calcined at different temperatures of 300, 350, 400 and 500 °C under the flow
−1
of air with a heating rate of 2 °C min for 2 h. The obtained yellow solid samples are designated
as meso-SnO -T-x (where T = template assisted synthesis, x = calcination temperature).
2
The template-free synthesis of tin oxide was carried out by adopting the aforementioned
procedure in the absence of CTAB. The obtained product was filtered, washed, dried and
calcined at respective temperatures and are designated as meso-SnO -TF-x. Other mesoporous
2
catalysts namely Al-MCM-41 (SAR = 25), Al-SBA-15 (SAR = 30) and Al-TUD-1 (SAR = 30)
were prepared from the reported literature [39–41]. The ammonium forms of zeolites were
-1
calcined at 540 °C under static air for 4 h with the ramp rate of 5 °C min to convert ammonium
form into protonic form. The obtained samples were labelled as H-beta, H-ZSM-5 and H-
mordenite.
2
.3. Catalyst characterization
7

The powder X-ray diffraction of the catalysts was recorded with Bruker D2 phaser X-
ray diffractometer using CuKα radiation (λ =1.5418 Å) with high resolution Lynxeye detector.
The mean crystallite sizes of the samples were calculated according to the Scherrer equation
using the full width at half maximum intensity (FWHM) of the (110) plane.
kλ
Scherrer equation, D =
p
βcosθ
D
p
is the mean crystallite size, k= 0.94, λ= X-ray wavelength; β is the full width at half
maximum intensity (FWHM) of the (110) plane, θ is the Bragg angle (in degrees).
Nitrogen sorption measurements of catalysts were performed at 77 K using
Quantachrome instrument. The specific surface areas of the samples were calculated by using
the Brunauer–Emmett–Teller (BET) method in relative pressure (P/P ) range of 0.05 to 0.25.
0
The pore-size distributions of the samples were determined by Barrett–Joyner–Halenda (BJH)
method. The total pore volume of each catalyst was accumulated at a relative pressure of
P/P
0
=0.99.
The Brønsted and Lewis acidic sites in catalysts were investigated by pyridine
adsorption study using FT-IR (Bruker α-T model). The self supported wafers of the catalysts
were prepared by a pellet press instrument. The wafer was then calcined at the respective
calcination temperatures of the catalysts for 1 h, later cooled to 250 °C and placed in a
desiccator to maintain moisture free condition. Then the samples were saturated with pyridine
and heated at 150 °C for 1 h to remove physisorbed pyridine. FTIR spectra were recorded in
–1
absorbance mode in the wavelength range from 1400 to 1600 cm . The spectrum obtained
after pyridine treatment was subtracted with spectrum of pyridine untreated sample to get peaks
only due to pyridine–acid interaction. The B/L ratio was calculated from the relative peak
-1
intensity obtained at the frequency 1545 and 1450 cm [30].
8

The amount of acidity present in the catalysts were determined by temperature
programmed desorption instrument equipped with thermal conductivity detector using
ammonia as a probe molecule. In all the experiments, the sample (200 mg) was calcined at the
–
1
desired temperature for 1 h in helium gas flow (25 mL min ) and then cooled to 50 °C. At 50
°
C, the sample was saturated with 10% of ammonia in a helium stream for 30 min. After the
saturation of probe molecule, the sample was flushed with helium for 1 h at 50 °C to remove
the physisorbed probe molecule, and then desorption of ammonia was performed in the
temperature range from 50 to 600 °C (till its calcination temperature) with a heating rate of 8
-1
°
C min [30].
The UV–vis DRS spectra of tin oxide samples were recorded on a Perkin Elmer
Lambda 650 UV/Vis spectrometer equipped with an integration sphere diffused reflectance
attachment. The spectra were obtained in the range of 200 – 800 nm against the BaSO as
4
reference.
The H and 119Sn MAS NMR spectra of the samples were recorded on Bruker
1
instrument. The resonance frequencies for H and 119Sn were 399.78 and 149.08 MHz
1
1
respectively. H MAS NMR analysis was performed with the operation condition at spinning
rate of 8KHz with 5 s relaxation delay for 200 scans. 119Sn MAS NMR analysis was performed
with the condition of spinning rate of 8KHz with 10 s relaxation delay for 6000 scans. The
chemical shifts were referenced to tetramethylsilane (TMS) for H and tin oxide for 119Sn
respectively [42].
1
TGA curves were recorded using a Universal V 2.4F-TA analyzer by heating the
−
1
sample from room temperature to 800 °C at the rate of 10 °C min in flowing of nitrogen gas.
Scanning electron microscopy (SEM) images of tin oxide catalysts were recorded in
Zeiss microscope to investigate the particle size and morphology. The transmission electron
microscopy (TEM) images, selected-area electron diffraction (SAED) patterns, and high-
9

resolution transmission electron microscope (HRTEM) images were recorded on a TEM-
JEOL-2010 instrument.
2
.4. Catalytic activity studies
General procedure for catalytic experiments
The glycerol acetalization reaction with benzaldehyde was performed in a 50 mL 2-
necked glass batch reactor equipped with a reflux condenser. In a typical experiment, the
required amounts of glycerol and benzaldehyde were taken along with preactivated catalyst
(wt% referred to glycerol weight). The reaction mixture was flushed with nitrogen gas in order
to remove air to avoid aerobic oxidation of benzaldehyde. Then, reaction mixture was
magnetically stirred at required temperature under nitrogen atmosphere.
The glycerol ketalization reaction with acetone was performed in a 25 mL glass batch
reactor equipped with a reflux condenser. In a typical experiment, the required amounts of
glycerol and acetone were taken along with preactivated catalyst (wt% referred to glycerol
weight) and reaction mixture was magnetically stirred at required temperature.
Cyclohexene epoxidation was performed using a 25 mL glass batch reactor equipped
with a reflux condenser. In a typical experiment, 10 mmol cyclohexene, 40 mmol H
2
2
O and 8
mL acetonitrile (as solvent) were taken along with 10 wt% (referred to weight of cyclohexene)
of pretreated catalyst, and magentically stirred at 80 °C.
Analysis procedure for glycerol reactions: After the desired time, the reaction mixture was
taken in methanol and centrifuged to separate out the catalyst from liquid phase. The obtained
liquid was analyzed by gas chromatography (Agilent 7820A) equipped with a capillary column
(0.25 mm I.D and 60 m length, HP-INNOWAX) and flame ionization detector. The glycerol
conversion and product yield were calculated using the following formulae:
(
moles of glycerol taken−moles of unreacted glycerol)
moles of glycerol taken
Glycerol conversion (mol%) =
X 100
moles of glycerol converted (%) ꢀ desired product selectivity (%)
Product yield (mol%) =
ꢁꢂꢂ (%)
1
0

Analysis procedure for cyclohexene epoxidation reaction: After a desired time, the reaction
mixture was centrifuged in order to separate out the catalyst from the liquid phase. The obtained
liquid was analyzed by gas chromatography (Agilent 7820A) equipped with a capillary column
(0.25 mm I.D and 30 m length, HP-5) and flame ionization detector. The cyclohexene
conversion and product selectivity were calculated using the following formulae:
(
moles of cyclohexene taken−moles of unreacted cyclohexene )
Cyclohexene conversion (mol%) =
X 100
moles of cyclohexene taken
moles of desired product
Product selectivity (mol%) =
X 100
moles of cyclohexene converted
1
1

3
. Results and discussion
3
.1 Characterization
X-ray diffraction: The low angle XRD patterns of the meso-SnO
mesoporous tin oxide) calcined at different temperatures are shown in Fig. 1a. The mesoporous
tin oxide calcined at 300 °C (meso-SnO -T-300) shows the presence of diffraction peak which
2
-T (template assisted
2
can be indexed as (100) reflection due to two-dimensional hexagonal structure. However,
calcination of mesoporous tin oxide at ≥ 350 °C resulted in the disappearance of low angle
diffraction peak which is attributed to the loss of orderedness in the mesoporous structure due
to sintering at high temperatures [43–45].
The wide angle XRD patterns of calcined tin oxide catalysts are depicted in Fig. 1b
and it shows diffraction peaks which can be indexed to (110), (101), (200) and (211) reflections
assigned to a tetragonal rutile crystal structure with space group of P42/mnm. XRD patterns of
tin oxide showed narrowing of diffraction peaks by increasing the calcination temperatures
from 300 – 500 °C, suggesting the gradual increase of crystallinity due to the agglomeration of
crystallites. The crystallite size of tin oxide determined by Scherrer equation confirms the
increase of crystallite size from 4.4 to 13.8 nm with increase in calcination temperatures in the
range of 300
̶
500 °C (Table S1). Interestingly, template-free tin oxide (meso-SnO
2
-TF-350)
-T-350).
showed a similar crystallite size compared to template assisted tin oxide (meso-SnO
2
Nitrogen sorption: The textural properties of the catalysts were investigated by N sorption
2
measurements and tabulated in Table 1. Fig. 2 (a and b) shows the adsorption and desorption
curves of nitrogen which makes it evident that tin oxide has a typical type IV isotherm which
is characteristic of mesoporous materials. The meso-SnO
2
-T-300 and meso-SnO -T-350 show
2
H2-type hysteresis loop, indicative of cavitation effect in ink bottle type pores which could be
attributed to the formation of spherical pores along with the cylindrical pores present in the
materials. However, meso-SnO
2
-T-400 and meso-SnO -T-500 possess H1-type hysteresis
2
1
2

suggesting a well-defined cylindrical-like pore channels present in the porous system [46–48].
Apparently, the pore diameter of tin oxide increased systematically from 3.4 to 6.7 nm with an
increase in calcination temperatures confirming the presence of mesoporosity. The surface
area, pore volume and pore size of tin oxide and other catalysts are given in Table 1. The meso-
2
−1
SnO
2
-T-300 exhibited a highest surface area of 160 m g among all the tin oxide samples.
However, further increase of calcination temperature > 300 °C results in significant decrease
2
−1
of surface area from 160 to 51 m g . The decrease in surface area and pore volume of tin oxide
could be ascribed to the sintering and destruction of pores at higher calcination temperatures
[
22,37,38]. Interestingly, the template-free meso-SnO
a H2-type hysteresis loop and the presence of mesoporosity with the pore size of 5.3 nm.
Although the meso-SnO -TF-350 exhibited mesoporsity, it showed lower surface area and pore
2
-TF-350 showed a type IV isotherm with
2
volume compared to meso-SnO
2
-T-350 which suggests the role of template during the
synthesis.
Pyridine-FTIR: Pyridine-FTIR studies were performed to distinguish the nature of acidic sites
in the catalysts (Fig. S1). The relative ratios of Brønsted to Lewis acidic sites of catalysts were
+
−1
calculated from the intensity of PyH and PyL peaks (1545 and 1450 cm respectively). These
peaks arise due to the interaction of pyridine with Brønsted and Lewis acid sites present on the
catalyst surface and are listed in Table 1. The meso-SnO -T exhibits both Brønsted and Lewis
2
acidic sites with a systematic decline in B/L ratio with increase in the calcination temperature
due to dehydroxylation at higher temperatures. The presence of Brønsted acidity in the catalyst
could be attributed to the surface Sn–OH or hydrogen bonded Sn–OH groups, whereas Lewis
4
+
acidity arises due to the framework Sn sites [22]. The py-FTIR of other catalysts namely Al-
MCM-41, Al-SBA-15, Al-TUD-1, H-beta, H-ZSM-5 and H-mordenite also showed a
combination of Brønsted and Lewis acidity with varying B/L values.
1
3

NH
temperature programmed desorption using ammonia as a probe molecule (NH
and Table 1). The NH –TPD measurements of mesoporous tin oxide were carried out up to its
calcination temperature since the material undergoes dehydroxylation beyond its calcination
temperature. The total amount of acidic sites in the meso-SnO -T catalyst reduced significantly
3
-TPD: The amounts of acidity in mesoporous tin oxide catalysts were determined by
3
-TPD) (Fig. 3
3
2
with increasing of calcination temperatures > 350 °C. The decrease in total acidity could be
attributed to the decrease of surface area and dehydroxylation of surface active sites at higher
temperatures. Meso-SnO
acidity than meso-SnO -T-350 (template assisted) which could be due to its lower surface area.
In general, mesoporous SnO samples contained a lower amount of acidic sites compared to
Al-MCM-41, Al-SBA-15, Al-TUD-1, H-beta, H-ZSM-5 and H-mordenite catalysts (Table 1).
UV-vis DRS: The UV-vis DRS studies were performed for template-free (meso-SnO -TF) and
template assisted tin oxide (meso-SnO -T) and depicted in Fig. 4. The spectra of the sample
2
-TF-350 (template-free tin oxide) catalyst showed a lower amount of
2
2
2
2
showed three absorption bands in the range of 200 – 400 nm suggesting the presence of Sn in
different coordination environment. The absorbance around 207 nm corresponded to the
4+
tetrahedrally coordinated Sn , which often assumed to be Lewis acidic sites, while the
4
+
absorbance band around 224 nm was attributed to the Sn in octahedral coordination which
was assumed to be Brønsted acidic sites. Additionally, the absorbance around 280 nm was due
to a hexa co-ordinated polymeric Sn–O–Sn type species in SnO
2
. Notably, the absence of band
around 280 nm in the uncalcined sample (meso-SnO -TF-120) suggested that the formation of
2
Sn–O–Sn species in calcined samples was due to the polymerization of Sn species [49–51].
1H MAS NMR: It is an important technique which gives information about the relative strength
of acidic sites in the material from the change in chemical shift due to bond polarization. As
the calcination temperature of meso-SnO
2
-T increased from 300 to 350 °C, the chemical shift
showed an increment from 6.43 to 6.63 ppm (Fig. 5a). The low-field shift indicated an increase
1
4

1
in acidic strength of the catalyst. The change in the chemical shift of H could be caused by
weak hydrogen bonding in SnO–H. Higher the chemical shift towards low-field, greater is the
strength of acidic sites due to a greater extent of bond polarization of SnO–H. Weaker bond
strength of SnO–H corresponds to higher “intrinsic” acidic strength [42,19]. The chemical shift
of meso-SnO
indicating that strength of acidic sites slightly decreases upon increasing of calcination
temperature above 350 °C. The chemical shift of meso-SnO -T-350 at low-field was attributed
2
-T calcined above 350 °C shifts towards high-field from 6.63 to 6.51 ppm
2
to the presence of relatively stronger acidic sites compared with other meso-SnO
2
-T catalysts.
1H MAS NMR spectra of meso-SnO
-T and meso-SnO -TF are shown in Fig. 5b. Both the
2
2
samples exhibit peaks at identical chemical shift of 6.63 ppm which indicates the presence of
similar strength of acidic sites in them.
119Sn MAS NMR: The UV−Vis DRS analysis results revealed that Sn species in meso SnO
2
samples were coordinated in tetrahedral and octahedral environment leading to a further study
in this regard. 119Sn MAS NMR spectroscopy provides the coordination states of Sn species
and the results are shown in Fig. 5c. Meso-SnO
2
-T-350 and meso-SnO -TF-350 gave NMR
2
peaks at -491 ppm, -547 ppm, -604 ppm, -656 ppm and -711 ppm. In the literature, the main
resonance signal at −604 ppm is reported to be characteristic of octahedrally co-ordinated Sn
species [52]. These results suggest that the SnO prepared by template and template-free routes
2
possessed Sn species in similar coordination environment.
Scanning electron microscopic analysis: The morphology and particle size of meso-SnO
2
calcined at different temperatures were determined by SEM analysis and the images are shown
in Fig 6. All the meso-SnO -T calcined at different temperatures possessed non-uniform
particles with rough-edged morphology of varying average particle size from 3.1 – 5.8 μm.
Meso-SnO -T catalysts calcined at 300 and 350 °C showed similar particle size of 3.1μm.
Further increase in calcination temperature from 350 to 500 increased particle size from 3.1 to
2
2
1
5

5
.8 μm which could be due to agglomeration of particles caused by sintering of particles at
elevated temperatures. Meso-SnO -TF-350 (template-free) shows irregular morphology with
average particle size of 1.6 μm which is smaller compared to meso-SnO -T-350 catalyst. The
larger particle size in Meso-SnO -T-350 could be attributed to the influence of structure
2
2
2
directing agent during the synthesis which results in the growth of particle size.
Transmission electron microscopic analysis: TEM, HRTEM, fast Fourier transform (FFT)
pattern and SAED images of the meso-SnO
image of meso-SnO -T-350 demonstrates that the mesostructured SnO
nanocrystalline domains and it agrees with the results obtained from Scherrer calculation. TEM
images also clearly confirmed the presence of disordered mesoporosity in meso-SnO -T-350.
2
-T-350 are shown in the Fig.7 (a–d). The TEM
2
2
is composed of
2
The SAED pattern exhibited the concentric rings indicating the presence of small crystallites
with well-defined (110), (101), (200), (211) and (301) lattice planes. The HRTEM of meso-
SnO
2
2
-T-350 reveals well-defined lattice fringes corresponding to tetragonal rutile phase of
with d-spacings of 0.33 and 0.26 nm for (110) and (101) planes respectively. The FFT
SnO
pattern of the meso-SnO
spots could be indexed to tetragonal SnO
4].
Thermogravimetric analysis: The TGA profile of tin oxide catalysts exhibited a weight loss in
three steps (Fig. S2). As-synthesized meso-SnO -T-120 catalyst showed an initial weight loss
2
-T-350 further confirms low crystallinity of the sample and diffraction
2
corresponding to (110) and (101) planes [44, 53,
5
2
of 5.6% below 120 °C due to a loss of water molecules. The second weight loss in the region
of 120 – 400 °C was ascribed to the removal of the surfactant. The final small weight loss
above 400 °C could be due to dehydroxylation. The meso-SnO catalyst calcined at different
2
temperatures showed a negligible weight loss as compared to as-synthesized catalyst indicating
a complete removal of the surfactant, and the weight loss was attributed to the dehydroxylation
of hydroxyl group in Sn–OH to form Sn–O–Sn [22].
1
6

3
.2. Catalytic activity studies
The catalytic behavior of mesoporous tin oxide was evaluated by performing three
reactions, namely acetalization of glycerol with benzaldehyde, ketalization of glycerol with
acetone and oxidation of cyclohexene in the presence of hydrogen peroxide.
3
.2.1. Acetalization of glycerol with benzaldehyde
Glycerol acetalization with aldehydes results into the formation of isomeric five- and
six-membered cyclic acetals (Scheme 1). The six-membered cyclic acetal [5-hyroxy-2-phenyl-
,3-dioxane (HPD)] is a potential precursor to produce green platform chemicals viz., 1,3-
dihydroxyacetone and 1,3-propanediol via hydrogenation and oxidation processes respectively
32]. Direct synthesis of 1,3-propanediol by glycerol hydrogenolysis reaction is limited to lab
1
[
scale due to a high temperature and pressure conditions. However, 1,3-propanediol can be
selectively produced from glycerol by a new approach described by Wang et al involving
selective transformation of secondary hydroxyl group of glycerol into a tosyloxy group and
then removing the transformed group by catalytic hydrogenolysis [55,56].
A variety of solid acid catalysts have been investigated for acetalization of glycerol
with benzaldehyde. MoO
3
/SiO
2
[57], ZrO
2
, TiO
2
–ZrO
2
, MoO
x
/ZrO
2
, and MoO
x
/TiO
2
–ZrO
2
[
58] amberlyst-36, H-beta, nafion and montmorillonite K-10 [59] catalysts were studied for
this reaction in presence of a solvent. The use of solvent in the reaction medium has certain
practical difficulties in the purification of products. In order to overcome these problems,
reactions under solvent free conditions are most desirable. Catalysts viz. SO
4
2-/SnO
,
2
MoO /SnO , WO /SnO [60], SiW/MCM-41 [61], Al-SBA-15 [62], Fe/Al-SBA-15 [63] are
3
2
3
2
reported under solvent free conditions. H-beta and amberlyst-36 are reported to be best
catalysts in the literature. They gave around 94% conversion with 60% selectivity to HPD.
3
.2.1.1. Catalyst screening studies
1
7

The catalytic behavior of meso-SnO -T-x as well as conventional microporous and
2
mesoporous catalysts were evaluated for glycerol acetalization with benzaldehyde under
solvent free condition (Table 2).
Prior to catalytic reaction, a blank run without catalyst was performed. The reaction
without a catalyst resulted in only 21% of glycerol conversion with ~46% HPD selectivity.
Among the catalysts screened in this study, mesoporous tin oxide (meso-SnO -T-350)
2
exhibited a greater catalytic performance even though it contains lower amount of acidic sites
compared to microporous and mesoporous catalysts. The greater catalytic activity of meso-
SnO
predominately catalyzed by Brønsted acidic sites. The catalytic activity of meso-SnO
template-free tin oxide) was lower than meso-SnO -T-350 which could be attributed to the
2
-T-350 can be attributed to the presence of higher B/L ratio (3.2), since the reaction is
2
-TF-350
(
2
presence of lower amount of acidity and surface area. Among the zeolites, with glycerol
conversion of 60.4%, H-beta exhibited a higher catalytic activity than H-mordenite and H-
ZSM-5. The higher performance of H-beta zeolite could be attributed to the presence of large
pore size. Mesoporous solid acid catalysts viz. Al-MCM-41, Al-SBA-15 and Al-TUD-1 were
also investigated to study the behaviour of porosity towards the reaction. Mesoporous acid
catalysts showed lower glycerol conversion of < 50% with similar (~ 48%) HPD selectivity as
observed for H-beta zeolite. The lower conversion using mesoporous catalysts could be due to
lower amount of acidic sites in the catalysts. In order to measure the efficiency of catalysts, the
turn over frequency (TOF) was calculated. Mesoporous catalysts showed higher TOF
compared to microporous zeolites which suggests the role of mesoporosity towards the
reaction. The TOF of catalysts decreased in the following trend (refer Table 2): Meso-SnO
50 > Al-SBA-15 > Al-MCM-41 > Al-TUD-1 > H-beta (large pore) > H-ZSM-5 (medium
pore) > H-mordenite (1D dual pore). It should be noted that the TOF of H-beta zeolite was 3
times lower than meso-SnO -T-350. Further, H-beta zeolite with identical amount of acidic
2
-T-
3
2
1
8

sites as in meso-SnO
with 47% HPD selectivity lower than meso-SnO
meso-SnO -T-350 could be attributed to a combination of mesoporosity and the nature of acidic
2
-T-350 was taken in the reaction. It showed 42.1% glycerol conversion
2
-T-350. The greater catalytic performance of
2
sites which offers better distribution of active sites leading to easy accessibility to the reactant
molecules and facilitates easy diffusivity of molecules over microporous catalysts. It is evident
from the study that the mesoporous tin oxide exhibited superior performance over microporous
and mesoporous catalysts even though it contains lower amount of acidic sites. These results
show that the performance of catalysts mainly depends upon the amount and nature of acidic
sites.
3
.2.1.2. Catalytic behavior of meso-SnO
A series of meso-SnO -T-x catalyst calcined at temperatures ranging from 300 to 500
C bearing different amount of acidic sites were used to study the effect of acidity towards the
glycerol acetalization with benzaldehye (Fig. 8). The glycerol conversion increased
appreciably from 57.3 to 60.2% upon increasing the calcination temperature of meso-SnO -T-
x from 300 to 350 °C. However, further increasing of calcination temperature from 350 to 500
C resulted in decrease of the conversion considerably (from 60.2 to 51.8%) with similar HPD
selectivity (~50%).
In order to understand the catalytic behavior of the catalysts calcined at different
2
-T-x calcined at different temperatures:
2
°
2
°
temperatures, the physico-chemical properties and their relationship with catalytic performance
were studied (Table 1). It is observed that the total amount of acidic sites and B/L ratio
decreased with increase in calcination temperatures from 350 to 500 °C. The decrease in active
sites upon increasing calcination temperature could be attributed to the decrease of surface area
and increase in particle size due to agglomeration of particles resulting in less surface metal-
oxygen pairs which is also evident from nitrogen sorption studies [64,26]. Relationship
between the amount of acidic sites in meso-SnO -T-x catalysts and their catalytic activity on
2
1
9

glycerol conversion is depicted in Fig. 8. The enhanced catalytic perfromance of meso-SnO
2
-
T-350 was mainly attributed to the presence of greater amount of acidic sites compared to other
catalysts. Therefore, meso-SnO -T-350 catalyst was selected as the best catalyst for further
2
studies.
3
.2.1.3. Influence of reaction conditions
The effect of reaction temperature, reactant mole ratio and catalyst amount on the catalytic
activity was studied.
The influence of reaction temperature was studied using meso-SnO
from 80 to 110 °C with glycerol: benzaldehyde mole ratio of 1:1 using 0.1 g of catalyst (10
wt% referred to glycerol mass) under N atmosphere. The glycerol conversion found to
2
-T-350 catalyst
2
increase marginally (49.5 to 60.2%) with increase in reaction temperature from 80 to 100 °C
and the results are shown in Fig. 9a. However, the glycerol conversion remains almost the same
with further increase of reaction temperature to 110 °C. Glycerol being more polar compared
to benzaldehyde, it preferentially adsorbs onto the active sites. Therefore, higher concentration
of carbonyl compound could be necessary to achieve greater conversion. There is no
considerable change in HPD selectivity with increase in reaction temperature. Hence, the
reaction temperature of 100 °C was selected for further studies.
The effect of mole ratio of benzaldehyde to glycerol on catalytic activity was studied at
1
00 °C using 10 wt% of meso-SnO
2
-T-350 catalyst (wrt glycerol weight) under N atmosphere.
2
The mole ratio of benzaldehyde to glycerol was varied from 1 to 2.25 as shown in Fig. 9b. The
glycerol conversion increased from 60.2 to 92.3% with increase in mole ratio from 1 to 2 due
to an increase in availability of benzaldehyde to glycerol. It is observed that there is no
improvement in glycerol conversion with further increase in mole ratio to 2.25, whereas
selectivity to HPD decreased marginally from 50 to 48%. Initially the reaction proceeds at
higher rate due to higher concentration of reactants in the medium. Once the product
2
0

concentration increases in the reaction medium, the concentration of the reactants decreases
leading to a less accessibility to the active sites. Therefore, there might be higher adsorption of
product molecules on the active sites resulting in not much improvement in the activity once
the conversion reached > 90%. Based on the above results, benzaldehyde to glycerol mole ratio
of 2 was found to be optimum condition for the catalytic transformation of glycerol to HPD.
The effect of catalyst amount for glycerol acetalization with benzaldehyde was studied
at 100 °C with glycerol: benzaldehyde mole ratio of 1:2 under N
2
atmosphere using meso-
SnO -T-350 catalyst (5 to 20 wt%) as shown in Fig. 9c. With the increase of catalyst amount
2
from 5 to 10 wt%, the glycerol conversion significantly increased from 78.1 to 92.3% due to
an increase in the number of accessible active sites to the reactant molecules. Further increasing
the catalyst amount > 10wt% showed no significant improvement in the conversion and
selectivity to HPD remained constant (48%). Hence, 10 wt% of catalyst with respect to glycerol
was taken as an optimum catalyst weight required for this reaction.
The influence of reaction time for the synthesis of HPD was studied under optimized
reaction conditions. The glycerol conversion and selectivity for HPD increased with increase
of reaction time as shown in Fig. 9d. The glycerol conversion was 71.2% after 5 min and then
reached to 92.3% after 30 min of reaction time. It shows that glycerol conversion marginally
increased to 93.6 at higher reaction time at 120 min with almost similar selectivity to HPD.
Initially, kinetically favoured product, 5-membered cyclic acetal forms at a higher rate than the
6
-membered HPD (thermodynamically favoured product) [59]. However, with the progress of
reaction, glycerol conversion increases with increase in selectivity for HPD due to the increase
in thermodynamically favoured product to reach equilibrium between the two products. This
increases the selectivity of 6-membered acetal with increase in reaction time.
3
.2.2. Ketalization of glycerol with acetone:
2
1

Glycerol undergoes ketalization with acetone to produce solketal (5- membered) and 6-
membered compounds (Scheme 2). Solketal can be used as a cold flow improver to enhance
cold weather performance of diesel fuel and also reduces its viscosity. Solketal blended with
regular gasoline in 1, 3 or 5 vol% decreases the gum formation and also it enhances the octane
number up to 2.5 points [65–67]. It is used in cosmetics industry as an additive in ointments in
the chemical industry and as an additive in low temperature transfer fluids [33]. A variety of
acid catalysts namely SnCl
2
[68], SnF
2
[69], heteropoly acids [70], Cs2.5H0.5PW12O40 [30],
+
heteropoly acids immobilized over silica [71], (C
3
H
7
)
4
N /PWA [33], MoO
3
/SiO
2
[30],
MoO
3
/SnO
2
, WO
3
/SnO
2
, SO
4
2−/SnO 2−/ZrO
2
[48], SO
4
2
[72], TiO
2
-SiO
2
[73], titanate
nanotubes [74], Nb
77], nanoporous hydroxyapatite [78], Brønsted acid ionic liquids (BAILs) [79], sulfonated
hollow sphere carbon [80], sulfonic acid-functionalized mesoporous polymer (MP-SO H) [81],
acid functionalized carbon–silica-meso composite [82], acid functionalized activated carbons
83], montmorillonite K-10, amberlyst-15, zeolites [30,84], Ar-SBA-15 [85], aluminium
triflate grafted MCM-41 [86], Sn-SBA-15 (S-Sn-OH) [87], Sn-TUD-1 [88], Fe/Al-SBA-15
63], molybdenum phosphate supported on SBA-15 [89], MOF [90] have been employed for
condensation reaction of glycerol and acetone.
2
O
5
[75], niobium aluminium mixed oxides [76], niobium oxyhydroxide
[
3
[
[
3
.2.2.1. Catalytic activity studies:
The catalytic efficiency of meso-SnO -T-350 in the synthesis of solketal via
2
ketalization of glycerol with acetone was studied. The activity of the screened catalysts on its
performance after 30 min reaction time was compared with other catalysts (Table 3). Prior to
catalytic activity studies, blank run was carried out without a catalyst, which resulted in
negligible glycerol conversion (0.1%) signifying that the reaction is truly catalytic.
Mesoporous catalysts namely Al-MCM-41, Al-SBA-15 and Al-TUD-1 gave 35.5, 26.1 and
1
0.2% with the solketal selectivity of 94.2, 96.3 and 89.0% respectively. The greater activity
2
2

of Al-MCM-41 (TOF =154) compared to Al-TUD-1 (TOF = 45) could be attributed to its 2-
dimensional ordered pore structure. Microporous catalysts, namely H-ZSM-5, H-mordenite
and H-beta resulted in glycerol conversion of 34.1, 16.2 and 47.4% respectively. The higher
activity of H-beta could be attributed to easy diffusivity of molecules in its large pores. The
meso-SnO
compared to other catalysts investigated in this study. Higher activity of meso-SnO
could be attributed to the presence of mesoporosity and high B/L ratio of 3.2 since this reaction
is predominantly catalyzed by Brønsted acidity. Meso-SnO -T-350 (template assisted) catalyst
showed significantly greater glycerol conversion than meso-SnO -TF-350 (template-free)
owing to the presence of higher amount of acidity and larger surface area. H-Beta zeolite with
identical amount of acidic sites as in meso-SnO -T-350 was taken and performed the reaction.
It showed 38.8% glycerol conversion with 97.6% selectivity to solketal. Overall, meso-SnO
T-350 exhibited a higher turnover frequency compared to other catalysts which signifies the
importance of B/L ratio and mesoporosity towards the reaction. Thus, meso-SnO -T-350 is an
2
-T-350 showed higher glycerol conversion of 51.3% with 98.0% solketal selectivity
2
-T-350
2
2
2
2
-
2
efficient acid catalyst for the synthesis of solketal better than other conventional solid acid
catalysts tested in this study.
3
.2.2.2. Catalytic behavior of meso-SnO
Meso-SnO -T-x catalysts calcined at different temperatures ranging from 300 to 500
C were investigated for the synthesis of solketal using the reaction conditions. A clear
correlation was obtained for glycerol conversion with amount of acidic sites as presented in
Fig. 10. Meso-SnO -T calcined at 300 °C gave 47.6% glycerol conversion with 97.5% solketal
selectivity and further increase in calcination temperature of meso-SnO -T to 350 °C increased
the conversion to 51.3%. The enhanced catalytic performance is attributed to increase in the
presence of total acidity in meso-SnO -T-350. However, further increase of calcination
tempearture ≥ 400 °C resulted in decline in glycerol conversion (48.7 to 42.4%) due to a
2
-T-x calcined at different temperatures:
2
°
2
2
2
2
3

decrease in total acidity and increase in particle size. The superior catalytic behavior of meso-
SnO -T-350 was due to the presence of higher amount of acidity than other meso-SnO
catalysts.
.2.2.3. Influence of reaction conditions
The influence of reaction parameters viz. reaction temperature, reactants mole ratio and
2
2
3
catalyst amount on the catalytic activity using meso-SnO
2
-T-350 catalyst was investigated for
glycerol acetalization with acetone.
The effect of reaction temperature was studied at different temperatures ranging from
0 to 70 °C using glycerol to acetone mole ratio of 1:1 for 30 min. The glycerol conversion
3
and solketal selectivity was found to be low at 30 °C (Fig. 11a). The glycerol conversion
increased significantly (from 33.5 to 51.3%) with increase in reaction temperature from 30 to
6
0 °C which also resulted in increase of solketal selectivity from 96 to 98.5%. Further increase
of reaction temperature to 70 °C (reflux condition) resulted in 45.4% glycerol conversion which
is marginally lower compared to 60 °C. Thus, 60 °C reaction temperature was found to be
optimum for solketal synthesis, and hence it was taken for further studies.
The effect of different concentrations of reactants was studied with acetone to glycerol
mole ratios varying from 1 to 6 at 60 °C using 5 wt% catalyst. Glycerol conversion increased
significantly from 51.3 to 90.1% with increase in the mole ratio from 1 to 5 which could be
due to increase in availability of acetone for the reaction (Fig. 11b). Nonetheless, further
increase of mole ratio to 6 resulted in similar glycerol conversion and selectivity to solketal.
Hence, acetone to glycerol mole ratio of 5 was used for further studies.
The influence of catalyst amount was studied by increasing the amount of catalyst from
1
to 7wt% and shown in Fig. 11c. As a result, with increase of the catalyst amount from 1 to 5
wt%, the glycerol conversion and solketal selectivity increased marginally from 78.7 to 90.1%
and 95 to 98.5% respectively. This increase in activity was attributed to the increase of the
2
4

number of available acidic sites to the reactant molecules. Further addition of catalyst amount
to 7wt% resulted in marginal decline in activity. Hence, 5wt% of catalyst was found to be
optimum and used for further studies.
The effect of reaction time on solketal synthesis was studied under the optimized
reaction conditions (Fig. 11d). Initially it gave 73% conversion after 5 min, which further
increased to 90% at 30 min with 98.1 % solketal selectivity. Further increase in reaction time
showed a marginal decrease in glycerol conversion (87%) with 98.1% solketal selectivity. A
marginal decrease in glycerol conversion after 30 minutes indicates the formation of glycerol
by the hydrolysis of ketals due to a reverse reaction. Similar results were obtained for other
catalysts also in previous reports [33, 91, 92].
3
.2.3. Effect of carbonyl substrates on glycerol reaction
The effect of substrate on catalytic activity was studied using glycerol and carbonyl
compounds present in different structural environment like acetone, benzaldehyde and
furfuraldehyde in order to understand the variations in product selectivity (Table 4). It is
observed that for acetone as a reactant, selectivity to 5-membered ring molecule (98%) was
favored, while for furfuraldehyde, more of 5-membered (68%) than 6-membered (32%) acetal
was obtained. On the other hand, benzaldehyde favors almost equal formation of 5- and 6-
membered acetals. It is well-known in the literature that 5-membered ring is kinetically
favored, whereas 6-membered ring is thermodynamically favored product [59]. It is clear from
the results that ketalization of glycerol with acetone occurs at a lower temperature favouring
kinetically preferred 5-membered ketal. For furfuraldehyde and benzaldehyde reactions which
occur at higher temperatures, formation of thermodynamically favoured acetal increases.
During the first step, 5-membered ring forms at a higher rate than 6-membered ring. In the
second step, equilibrium between the two products is reached with increase in degree of
conversion for the 6-membered cyclic acetal depending on the thermodynamic conditions.
2
5

3
.2.4. Reaction mechanism for acetalization and ketalization reactions:
It is found that syntheses of 5 or 6-membered rings from glycerol and carbonyl
compounds are catalyzed by Brønsted/Lewis acid sites. However, it is evident from the
literature and this study that Brønsted acidic sites are particularly more active than Lewis sites
for this reaction. Based on the results obtained, a plausible mechanism is proposed for meso-
SnO catalyzed reaction of glycerol with acetone / benzaldehyde as shown in Scheme 3. In the
2
first step, the carbonyl group in acetone/ benzaldehyde was activated by acid sites of the
catalyst. In the second step, –OH group of glycerol attacks carbonyl carbon and forms an
intermediate called hemiacetal. This intermediate undergoes cyclization by the attack of lone
pair of electrons present on either the adjacent or terminal hydroxyl group on tertiary carbon
atom leading to the formation of 5 or 6-membered cyclic acetal with the elimination of water
molecule in the last step.
3
.2.5. Epoxidation of cyclohexene with hydrogen peroxide
Epoxidation of olefins has been extensively studied because epoxides act as precursors
for commodity chemicals such as drug intermediates, agrochemicals and food additives
respectively [34,35]. Various homogeneous transition metal complexes and heterogeneous
catalysts are known for this reaction [93]. Schuchardt et al reported γ-alumina as an active
catalyst for epoxidation reaction, in which they discovered that the presence of Al–OH species
with weak to moderate Brønsted acidic sites could be responsible for the reaction because they
can easily replaced on alumina surface by hydrogen peroxide because of their high mobility
thereby creating Al–OOH (hydroperoxy groups) [94,95]. This phenomenon arises due to the
polarizing effect of Al (III) ions that can activate the O–O bond facilitating distal oxygen
transfer to the nucleophilic olefin. However, the involvement of strong acid sites in the reaction
can lead to the catalytic decomposition of H
2
2
O causing an undesirable decrease in oxidant
selectivity. Alumina catalyst, though a good example to study this reaction, is not highly active
2
6

for converting cyclohexene (35% conversion). Chandra et al reported silica microspheres
containing surface hydroxyl groups as efficient epoxidation catalysts [96]. They also confirmed
that tertiary silanols groups act as catalytic active sites for olefin epoxidation reaction by DFT
calculations. Recently Dalai et al have investigated the application of sulphated SnO as
2
catalyst for olefin epoxidation reaction [97]. Tin incorporated periodic mesoporous
organosilicas (Sn–PMOs) have also been effectively used for olefin epoxidation reaction [98].
Anion-resin supported peroxo phosphotungstic acid has been reported to be an efficient catalyst
for the synthesis of cyclohexene oxide with 98.1% selectivity using cyclohexene and hydrogen
peroxide (cyclohexene conversion is 92.4%) [99]. Epoxidation of cyclohexene with hydrogen
peroxide produces cyclohexeneoxide which may undergo hydrolysis by acidic sites resulting
in 1,2-cyclohexanediol as a side product (Scheme 4). It is considered to be one of the reasons
for low yield of the epoxide in most of the catalysts. On the other hand, side products namely
2
-cyclohexen-1-ol and 2-cyclohexen-1-one, are formed through the cyclohexenyl
hydroperoxide radical [100]. Hence, it requires a suitable catalyst which selectively catalyzes
the reaction to produce cyclohexeneoxide. There are only few reports on Sn catalyzed olefin
epoxidation available in the literature [97,98]. Therefore, mesoporous SnO
catalyst for epoxidation of cyclohexene using hydrogen peroxide, since it possesses Sn metal
centre to activate H as well as acid sites for activation of olefin.
.2.5.1. Catalytic activity studies
Meso SnO catalyst was evaluated for cyclohexene epoxidation using aqueous 50%
as green oxidant and acetonitrile as solvent at 80 °C, and results are summarized in Table
2
was explored as
2
O
2
3
2
H
2
O
2
5
. Firstly, the epoxidation reaction was carried without using a catalyst. It gave 5% cyclohexene
conversion in 10 h with 2-cyclohexen-1-one as a major product with 49% selectivity.
Formation of 2-cyclohexen-1-one was due to an auto catalysis by thermal reaction, and
therefore it requires a suitable catalyst to produce cyclohexeneoxide. When meso-SnO -T-350
2
2
7

was used as a catalyst, it showed significant increase in cyclohexene conversion (77%) with
high selectivity towards cyclohexeneoxide (92.5%). Further increasing the recation time to 16
h resulted in increase of cyclohexene conversion (94%) with marginal decrease in
cyclohexeneoxide selectivity (89%). Notably, meso-SnO
2
-TF-350 catalyst exhibited lower
cyclohexene conversion (61%) with 82.5% cyclohexeneoxide selectivity. The remarkable
catalytic activity of meso-SnO
2
-T-350 is attributed to a unique combination of larger surface
area and weak acidic sites.
The calcination of meso-SnO
2
-T-x at different temperatures affected the cyclohexene
-T at
00 °C converted 75% of cylcohexene into product with 92.7% selectivty to cyclohexeneoxide.
conversion and cyclohexeneoxide selectivity (Table 5). Lower calcination of meso-SnO
2
3
Meso-SnO -T-350 gave similar cyclohexene conversion (77%) with 92.5% cyclohexeneoxide
2
selectivity. However, the conversion of cyclohexene significantly decreased from 77 to 30%
with marginal decrease in cyclohexeneoxide selectivity (92.5 to 88%) by increasing the
calcination temperature >350 °C. The higher catalytic activity of meso-SnO -T-350 compared
2
to other tin oxide catalysts is attributed to greater amount of acidity and smaller particle size of
the catalyst. Furthermore, the decrease of acidic sites suppresses the conversion of cyclohexene
which indicates that the amount and nature of acidic sites greatly influence the epoxidation of
cyclohexene.
3
.2.6. Catalyst reusability studies
The most active meso-SnO -T-350 catalyst for acetalization, ketalization and
2
epoxidation reactions was investigated for its reusability and the results are presented in Fig.
2a-c. It was performed under optimized reaction conditions for four consecutive cycles (Fresh,
1
R-1, R-2 and R-3). After each run, the catalyst was filtered and washed with methanol to
remove the adsorbed reactants on the catalyst surface. The catalyst was dried and calcined at
3
50 °C for 2 h. For acetalization and ketalization reactions, the catalyst showed good
2
8

recyclability with a marginal decrease in glycerol conversion (~3%) with similar selectivity to
the respective products compared to the fresh catalyst for both the reactions for four
consecutive runs (Fig. 12a,b). In case of epoxidation reaction, the conversion of cyclohexene
varied marginally after the first cycle with a slight variation in cyclohexeneoxide selectivity
(Fig. 12c). However, the catalyst retained its catalytic activity till fourth cycle with marginal
loss in cyclohexene conversion. The structural integrity of the recycled catalyst was
investigated by X-ray diffraction as shown in Fig. 12d. The XRD pattern of the spent meso-
SnO
fresh catalyst indicating no change in structure of the catalyst even after four consecutive
cycles. These results suggest that meso-SnO -T-350 catalyst is stable and reusable without any
2
-T-350 catalyst for all the three reactions matched well with the characteristic peaks of
2
appreciable loss in activity, and therefore it offers good opportunity for potentially wide
applications in catalysis.
4
. Conclusions
Mesoporous tin oxide catalysts were prepared by two different synthetic approaches
viz. template assisted and template-free methods and calcination temperature was varied to
generate acidic sites of different nature, strength and amounts. The materials were well-
characterized by various techniques and catalytic efficiency was evaluated for acetalization and
ketalization of glycerol with benzaldehyde and acetone respectively. The chemical shift of
1
meso-SnO
2
-T-350 from H MAS NMR indicated the presence of relatively stronger acidic sites
than that of other meso-SnO
2
catalysts. Catalytic performance of meso-SnO
2
was compared
catalyst
with other well-known conventional solid acid catalysts. Template assisted meso-SnO
2
exhibited excellent catalytic performance as compared to other catalysts. The superior catalytic
performance of mesoporous tin oxide catalyst was mainly attributed to the presence of its
mesoporosity and higher B/L ratio. Template-free mesoporous tin oxide gave lower catalytic
activity compared to template assisted method due to the presence lower surface area and lower
2
9