Eco-friendly synthesis of bio-additive fuels from renewable glycerol using nanocrystalline SnO2-based solid acids

Article abstract of DOI:10.1039/c3cy00825h

The present work has been undertaken with an aim to synthesize valuable bio-additive fuels from glycerol acetalization using SnO₂-based solid acids. Various promoters, namely SO₄^2-, MoO₃ and WO₃ were incorporated to the SnO₂ using a wet-impregnation method. An extensive physicochemical characterization has been achieved by means of XRD, BET surface area, BJH analysis, FT-IR, pyridine adsorbed FT-IR, NH₃-TPD, ICP-OES and XPS techniques. The BET surface area of SnO₂ is significantly improved from 11 to 32, 56 and 41 m² g^-1 after the addition of the WO₃, MoO ₃, and SO₄^2- promoters, respectively. The XPS studies revealed that Sn is present in the +4 oxidation state, whereas Mo, W and S are in the +6 oxidation state in the prepared samples. In addition, the SO₄^2-/SnO₂ sample contained super acidic sites, along with strong- and medium-acidic sites. The amount of acidic sites was found to be 46.47, 61.81, 81.45 and 186.98 μmol g^-1 for the SnO₂, WO₃/SnO₂, MoO₃/SnO ₂, and SO₄^2-/SnO₂ samples, respectively. The pyridine adsorbed FT-IR studies revealed the existence of a superior quantity of Bronsted acidic sites than Lewis acidic sites in the synthesized catalysts. Promoted SnO₂ catalysts exhibited a promising catalytic performance for glycerol acetalization with acetone and furfural, and the activity of the catalysts was found to increase in the following order: SnO₂ < WO₃/SnO₂ < MoO₃/ SnO₂ < SO₄^2-/SnO₂. The outstanding performance of the SO₄^2-/SnO₂ catalyst is mainly due to the existence of a large amount of acidic sites associated with the super acidic sites. The achieved optimum glycerol conversions with acetone and furfural were ~98 and 99% over the SO ₄^2-/SnO₂ catalyst, respectively.

Full text of DOI:10.1039/c3cy00825h

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Eco-friendly synthesis of bio-additive fuels from
renewable glycerol using nanocrystalline
SnO2-based solid acids†
Cite this: Catal. Sci. Technol., 2014,
4, 803
*
Baithy Mallesham, Putla Sudarsanam and Benjaram M. Reddy
The present work has been undertaken with an aim to synthesize valuable bio-additive fuels from glycerol
acetalization using SnO2-based solid acids. Various promoters, namely SO42−, MoO3 and WO3 were incor-
porated to the SnO2 using a wet-impregnation method. An extensive physicochemical characterization
has been achieved by means of XRD, BET surface area, BJH analysis, FT-IR, pyridine adsorbed FT-IR, NH3-TPD,
ICP-OES and XPS techniques. The BET surface area of SnO2 is significantly improved from 11 to 32, 56 and
41 m2 g−1 after the addition of the WO3, MoO3, and SO42− promoters, respectively. The XPS studies revealed
that Sn is present in the +4 oxidation state, whereas Mo, W and S are in the +6 oxidation state in the prepared
samples. In addition, the SO42−/SnO2 sample contained super acidic sites, along with strong- and
medium-acidic sites. The amount of acidic sites was found to be 46.47, 61.81, 81.45 and 186.98 μmol g−1
for the SnO2, WO3/SnO2, MoO3/SnO2, and SO42−/SnO2 samples, respectively. The pyridine adsorbed FT-IR
studies revealed the existence of a superior quantity of Brønsted acidic sites than Lewis acidic sites in the
synthesized catalysts. Promoted SnO2 catalysts exhibited a promising catalytic performance for glycerol
acetalization with acetone and furfural, and the activity of the catalysts was found to increase in the
following order: SnO2 < WO3/SnO2 < MoO3/SnO2 < SO42−/SnO2. The outstanding performance of the
SO42−/SnO2 catalyst is mainly due to the existence of a large amount of acidic sites associated with the super
acidic sites. The achieved optimum glycerol conversions with acetone and furfural were ~98 and 99% over
the SO42−/SnO2 catalyst, respectively.
Received 20th October 2013,
Accepted 27th November 2013
DOI: 10.1039/c3cy00825h
www.rsc.org/catalysis
effective utilization of biomass products provides an excellent
opportunity to reduce the environmental impacts and the
1. Introduction
During the past few decades, most of the world's energy demand
has been met from carbon-based fossil fuels.1,2 Concurrently,
they emit high levels of carbon dioxide into the atmosphere,
which causes various detrimental environment and human
health problems.3 In addition, the fossil fuel energy sources
are nonrenewable and their reserves are becoming exhausted.
Thus, great research efforts have been undertaken to develop
potential alternatives to fossil resources.4,5 Biofuels, defined
as fuels from biomass, solid, liquid or gaseous, and composed
of renewable raw material from agricultural and silvicultural
products, show a great promise to partly replace fossil fuels.
Being proposed as an environmentally benign alternative, the
depletion of fossil resources.6
The chemical processes for producing fuels from biomass,
includes the transesterification of triglycerides to biodiesel,
the liquefaction and pyrolysis of biomass to bio-oils, the gasi-
fication of biomass to syngas, the hydrolysis of biomass to
aqueous phase sugars or lignin, etc.5 In addition, biodiesel is
one of the most attractive clean energy sources that show
numerous benefits when compared to petro-diesel fuel.6 For
example, biodiesel is a biodegradable and nontoxic diesel fuel
substitute that can be used in unmodified diesel engines.7–9
Particularly, it has shown a significant added value in comparison
to petro-diesel attributed to its higher lubricity, lack of sulfur and
aromatics as well as its carbon footprint benefits.6 Accordingly,
many countries, including United States, China, Canada, European
Union, Brazil, etc. have been developing a number of bio-refinery
processes to increase the biodiesel production.10,11 Nonetheless,
the commercialization of biodiesel production has been limited
to a great extent due to the high cost of the raw materials
(e.g., virgin oil). In this context, the successful utilization of
bio-glycerol (a major byproduct in the biodiesel production) to
Inorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical
Technology, Uppal Road, Hyderabad – 500 607, India. E-mail: bmreddy@iict.res.in,
mreddyb@yahoo.com; Fax: +91 40 2716 0921; Tel: +91 40 2719 3510
† Electronic supplementary information (ESI) available: pore size distribution,
FT-IR analysis, reusability, pyridine adsorbed FT-IR of spent catalyst. See DOI:
10.1039/c3cy00825h
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yield various industrially important chemicals could profitability
enhance biodiesel production.
(FTIR) spectroscopy, FTIR analysis of adsorbed pyridine and
NH3-temperature programmed desorption (NH3-TPD), and carbon,
hydrogen and nitrogen (CHN) analysis. The catalytic studies
of the SnO2-based samples for glycerol acetalization were
conducted at room temperature under solvent-free conditions.
The effect of various parameters, such as the time-on-stream,
the catalyst wt.% and the molar ratio of acetone or furfural to
glycerol as a function of time were examined to enhance the
glycerol conversion as well as the product selectivity. Great
efforts have been undertaken to know the key factors influenc-
ing the catalytic performance of the investigated catalysts. As
well, a special attention has been paid to understand the rea-
sons for the catalyst deactivation after reusability tests.
Many catalytic processes have been investigated to develop
a number of valuable products from glycerol, such as selective
oxidation, acetalization, hydrogenolysis, dehydration, pyrolysis,
esterification, etherification, carboxylation, carbonylation and
polymerization.12–16 Particularly, glycerol acetalization with
ketones or aldehydes is an efficient way to transform glycerol
into industrially useful chemicals.17 For example, glycerol acetals
and ketals are extensively used as effective fuel additives, bases,
protecting agents, scents, fragrance and flavors in several
industries.18–22 Moreover, a considerable reduction in harmful
emissions (e.g., hydrocarbons and carbon monoxide), unregulated
aldehydes and particulate matter can be attained by the addition
of glycerol acetals to the standard diesel fuel.23
2. Experimental
A wide variety of catalysts (both homogeneous- and
heterogeneous-catalysts) have been studied for glycerol
acetalization, such as inorganic acids, p-toluenesulfonic acid,
Lewis acids, ion-exchange resins, zeolites, silica-based mesoporous
materials, heteropoly acids, ionic liquids, promoted metal oxides,
clay minerals, etc.4,12,24–26 Usually, liquid acids and metal com-
plexes show a high catalytic activity for this type of reaction.17
However, homogeneous catalysts have difficulty in separation
and regeneration from the reaction system, along with a tedious
work-up procedure and the generation of large amounts of
waste. On the other hand, the heterogeneous catalysts are
emerged as the preferred alternatives for glycerol acetalization
because the products are easily separable, the catalysts are
reusable and the process is environmental friendly.15 Several
heterogeneous catalysts have been investigated in the acetalization
of glycerol to synthesize cyclic acetals and ketals. Among these
catalysts, promoted metal oxide catalysts are of great research
interest owing to various attractive features, for instance, a
facile preparation procedure, a high thermal stability, a large
amount of surface acidic sites and thereby, excellent catalytic
activity. In this context, tin dioxide (SnO2) is a versatile metal
oxide due to its unique beneficial characteristics: the variation
in the valence state and the existence of oxygen vacancy defects.27
Thus, SnO2 has been found to have widespread applications
in catalysis, antistatic coatings, solar energy conversion, electro-
chromic devices and transparent conductive electrodes.28
However, the application of pure SnO2 is greatly restricted in
2.1 Catalyst preparation
The pure SnO2 was prepared by a simple fusion method using
SnCl2·2H2O (Aldrich, AR grade) and NaNO3 (Aldrich, AR grade)
as the precursors. In a typical procedure, the required quanti-
ties of precursors (NaNO3: SnCl2·2H2O = 1 : 5) were dissolved
in double distilled water and vigorously stirred for 1 h at room
temperature. The subsequent evaporation of excess water
from the solution at 383 K resulted in a porous and foam-like
solid product. Afterward, the reaction temperature was increased
to ~523 K and continued at the same temperature until the
formation of a white powdered tin oxide. The obtained products
were cooled to room temperature, washed with deionized
water and oven dried at 373 K for 12 h. Some portions of the
sample were calcined at 923 K for 5 h in air with a heating
ramp of 5 K min−1.
The promoted SnO2 solid acids containing 10 wt.% of
metal oxides (MoO3 and WO3) were prepared by a wet-
impregnation method. In this method, the desired quantities
of (NH4)6Mo7O24·4H2O or (NH4)6H2W12O40·xH2O (Aldrich,
AR grade) were dissolved in doubly deionized water followed
by the addition of finely powdered tin oxide. The excess water
was evaporated on a hot plate under vigorous stirring conditions.
The obtained products were oven dried at 393 K for 12 h and
finally calcined at 923 K for 5 h in air with a heating ramp of
5 K min−1.
The sulfate promoted-SnO2 (SO42−/SnO2) sample was pre-
pared by a wet-impregnation method. In brief, the oven dried
tin oxide (1 g) was taken in a glass flask containing 15 ml of
catalysis due to its poor thermal stability.29 It is a well-known
2−
fact in the literature that the addition of promoters (SO4
,
2−
a 0.5 M H2SO4 aqueous solution (10 wt.% of SO4 with
MoO3 and WO3) to metal oxides (e.g., CeO2, TiO2, ZrO2, etc.)
enormously improves their thermal stability as well their cata-
lytic performance.30–32 Hence, it can be expected that the addi-
tion of the promoters to the SnO2 could enhance its thermal
stability, and hence give a superior catalytic performance. To
understand this, we have incorporated various promoters,
namely SO42−, MoO3 and WO3 to the SnO2 by means of a wet-
impregnation method. Several characterization techniques have
been used, namely, X-ray diffraction (XRD), BET surface area,
Barrett–Joyner–Halenda (BJH) pore size distribution, inductively
coupled plasma optical emission spectroscopy (ICP-OES), X-ray
photoelectron spectroscopy (XPS), Fourier transform infrared
respect to SnO2) to incorporate the sulfate ions. Then, the
excess water was evaporated on a hot plate and the resulting
sample was oven dried at 393 K for 12 h. The oven dried sample
was calcined at 923 K for 5 h in air with a heating ramp of
5 K min−1. For convenience, the prepared SnO2, MoO3/SnO2,
WO3/SnO2 and SO42−/SnO2 catalysts are referred to as Sn, MSn,
WSn, and SSn, respectively.
2.2 Characterization studies
X-ray diffraction measurements were carried out on a Rigaku
Multiflex instrument equipped with a nickel-filtered Cu Kα
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(0.15418 nm) radiation source and a scintillation counter
detector. The scattered intensities data were collected from
2θ values of 2 to 80° by scanning at 0.025° steps with a
counting time of 1 s at every step. The average crystallite size of
the synthesized catalysts was calculated by Scherrer equation.
N2-adsorption–desorption measurements were carried out on
a Micromeritics ASAP 2020 instrument to determine the surface
area using the BET method. The pore size distribution was
determined using the Barrett–Joyner–Halenda (BJH) method
applied to the desorption leg of the isotherms.
The surface species present in the prepared catalysts were
detected by X-ray photoelectron spectroscopy (Shimadzu
ESCA 3400). The spectrum was recorded with a Mg Kα line as
the excitation source (hν = 1253.6 eV). All binding energies
measured were within a precision of 0.2 eV. The binding
energy (BE) values were referenced to the C 1s peak of conta-
minant carbon at 284.6 eV. The elemental analysis of the pro-
moted SnO2 catalysts was investigated by the ICP-OES technique
(Thermo Jarrel Ash model IRIS Intrepid II XDL, USA) following
sample dissolution by acid digestion methods.
3. Results and discussion
3.1 Characterization studies
Fig. 1 shows the XRD profiles of the promoted SnO2 solid
acid catalysts. As shown in Fig. 1, the diffraction peaks of the
pure SnO2 obviously indicate the tetragonal structure of the
SnO2 (PDF #880287). Interestingly, the promoted SnO2 samples
also showed a typical tetragonal structured SnO2. However,
the XRD patterns of the promoted SnO2 samples are broad,
which reveals the reduction of the SnO2 crystallite size. To
understand this, we have determined the average crystallite
size of the promoted SnO2 samples by means of the Scherrer
equation and the obtained results are summarized in Table 1.
Clearly, the promoted SnO2 samples show smaller sized crys-
tallites in the nanoscale range compared to the pure SnO2,
indicating the beneficial effect of the promoters towards
the inhibition of the SnO2 crystallite growth against higher
thermal treatments. The average crystallite size of the Sn,
WSn, MSn and SSn samples are found to be ~13.47, 8.76, 6.06
and 7.91 nm, respectively.
The FT-IR spectra were recorded on a Nicolet 740 FT-IR
spectrometer at ambient conditions using KBr discs with a
nominal resolution of 4 cm−1 and averaging 100 scans for
every spectrum. For the FTIR analysis of adsorbed pyridine,
the sample was oven dried at 373 K for 1 h. The oven dried
sample (∼50 mg), in a sample cup, was contacted with pyri-
dine (∼0.1 cm3) directly. The sample cup was then kept in
a vacuum oven at 393 K for 1 h to remove the physisorbed
pyridine. After cooling to room temperature, the spectrum
was recorded with a nominal resolution of 4 cm−1 in the
spectral range of 1400–1900 cm−1 using a KBr disc.33
NH3-temperature programmed desorption experiments were
performed by using Auto-Chem 2910 instrument of Micro-
meritics. A quantity of 30 mg of the catalyst was placed in a
quartz tube and degassed up to 573 K under the flow of
helium. Then the ammonia gas was passed through the cata-
lyst surface for 30 min and subsequently flushed with helium
to remove the physisorbed gas. Then, the chemisorbed amount
of NH3 was determined in flowing He gas with a flow rate of
20 mL min−1 from 323 to 1073 K at a heating rate of 10 K min−1.
The N2 adsorption–desorption isotherms of the promoted
SnO2 catalysts are shown in Fig. 2. These isotherms can be
classified as type IV with H1-type hysteresis loop, which are
characteristic of the mesoporous nature of these materials.34
Although, no template was used in the present study, all of the
solid acids exhibited a remarkable porosity that pointed out
the significance of the preparation method. The obtained pore
volume and pore size of the pure SnO2 were 0.0739 cm3 g−1
and 9.035 nm, respectively (Table 1). The pore size distribution
profiles clearly revealed that the Sn, MSn and SSn catalysts
exhibit a unimodal pore size distribution, whereas the WSn
sample shows a bimodal pore size distribution (Fig. S1, ESI†).35
Among the prepared catalysts, the pure SnO2 exhibited a broad
pore size distribution. The Mo- and W-promoted SnO2 cata-
lysts showed a lower pore volume and smaller pore size when
2.3 Activity studies
The catalytic performance of the synthesized solid acid catalysts
was examined for the acetalization of glycerol with acetone,
furfural and its derivatives. We have performed the catalytic
experiments in a 50 mL round bottom flask with a 1 : 1 molar
ratio of glycerol to the carbonyl compounds under solvent-free and
room temperature conditions with constant stirring (~800 rpm).
The catalysts were pre-activated at 423 K for 2 h before cata-
lytic runs. At the end of the test, the catalyst was separated by
centrifugation and the assignment of the products was con-
firmed by GC-MS equipped with a DB-5 capillary column and
1
a flame ionization detector (FID), and also by H-NMR spec-
troscopy (solvent, CDCl3). The reaction mixture was taken sys-
tematically and analyzed by GC equipped with a BP-20 (Wax)
capillary column and a FID.
Fig. 1 The X-ray diffraction patterns of the pure SnO2 (Sn), WO3/SnO2
(WSn), MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) samples.
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Table 1 The surface area (S), crystallite size (D), acidic sites, pore volume (V) and pore size (P) of the SnO2 (Sn), WO3/SnO2 (WSn), MoO3/SnO2 (MSn)
and SO42−/SnO2 (SSn) samples
Catalyst
S (m2 g−1
)
Da (nm)
Acidic sitesb (μmol g−1
)
Vc (cm3 g−1
)
Pc (nm)
Sn
11
32
56
41
13.47
8.76
6.06
7.91
46.47
61.81
81.45
0.0739
0.0637
0.0679
0.1623
9.035
7.759
4.845
WSn
MSn
SSn
186.98
15.797
a
From the XRD spectra. b From the NH3-TPD studies. c From the BJH analysis.
Fig. 2 The N2 adsorption–desorption isotherms of the SnO2 (Sn),
WO3/SnO2 (WSn), MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) samples.
compared to the pure SnO2. Contrary, the SO42−/SnO2 sample
exhibited a higher pore volume and pore size than pure SnO2
catalyst. The estimated specific surface areas of the promoted
Fig. 3 The Mo 3d, W 4f and S 2p XP spectra of the MoO3/SnO2 (MSn),
WO3/SnO2 (WSn) and SO42−/SnO2 (SSn) samples, respectively.
SnO2 samples are presented in Table 1. It was found that the
addition of promoters considerably improves the specific sur-
face area of the SnO2, which is in good agreement with the
crystallite size decrease (Table 1). The obtained specific surface
areas of the Sn, WSn, MSn and SSn samples were 11, 32, 56 and
41 m2 g−1, respectively. The elemental analysis of the promoted
SnO2 catalysts was studied using the ICP-OES technique. The
obtained results (SO42− = 8.6, MoO3 = 9.5 and WO3 = 9.4) were
found to be close to the calculated values (actual loading of
the promoters is 10 wt.%).
The XPS studies were performed to know the oxidation state
of the elements present in the synthesized catalysts. Fig. 3
shows the S 2p, Mo 3d and W 4f XPS spectra of the SSn, MSn
and WSn catalysts, respectively. It can be found from Fig. 3
that the values of the binding energy for S 2p3/2, Mo 3d5/2 and
W 4f7/2 are consistent with the earlier reported values which
are 169.3, 232.7 and 35.6 eV, respectively.36–38 These values
clearly indicate the presence of S, Mo and W in the 6+ oxidation
state for SSn, MSn and WSn, respectively. Also, the S 2p3/2 was
fitted by a Gaussian function centred at 169.3 eV, which is in
good agreement with the binding energies of S 2p3/2 in the
SO42− group.36,39 The presence of the SO42− groups in the SSn cata-
lyst is also further confirmed by the FT-IR study (Fig. S2, ESI†).
Additionally, no peaks corresponding to other molybdenum
species, such as the Mo4+ (231.4 eV) and Mo5+ (237.5 eV) ions
were observed in the present investigation.15
Fig. 4 represents the Sn 3d core level XP spectra of the Sn,
WSn, MSn and SSn samples. As can be observed from the figure,
the peaks are symmetric and appeared as a spin–orbit doublet
at ~487.4 (3d5/2) and ~495.8 eV (3d3/2), indicating the presence
of the Sn4+ ions in the synthesized catalysts.15,40 Interestingly,
there is a small shift in the higher binding energy side of the
Sn 3d after the addition of promoters, which might be due to
the electron deficient nature of the Sn in the promoted SnO2
catalysts. This unusual observation can be explained by a shifting
of the electron density of the Sn towards the promoter species.
The obtained O 1s core level XP spectra of the promoted SnO2
samples are shown in Fig. 5. Interestingly, all samples exhibit
more than one type of oxygen ions. The lower binding energy
peak represents lattice oxygen and the higher binding energy
peak is most likely ascribed to the hydroxyl groups, surface
carbonates and surface oxygen ions caused by the change
of a static charge distribution within the samples.36 However,
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Fig. 4 The Sn 3d XP spectra of the SnO2 (Sn), WO3/SnO2 (WSn),
MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) samples.
Fig. 6 The NH3-TPD profiles of the SnO2 (Sn), WO3/SnO2 (WSn),
MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) catalysts.
the SO42−/SnO2 sample has super acidic sites, along with
strong- and medium-acidic sites, whereas the MoO3 and WO3
promoted SnO2 samples contain only strong- and medium-
acidic sites (Fig. 6). The total amount of acidic sites were
found to be ~46.47, 61.81, 81.45 and 186.98 μmol g−1 for the
Sn, WSn, MSn and SSn catalysts, respectively (Table 1). Although
the SSn catalyst has a low surface area compared to the MSn
sample, it exhibited a large number of acidic sites as well as
super acidic sites due to the presence of sulfate ions as evidenced
by FT-IR analysis (Fig. S2, ESI†).42 Interestingly, the total number
of acidic sites in Sn, WSn, MSn and SSn are well correlated
with their catalytic activity as discussed in the activity part.
As is well-known, NH3-TPD analysis does not provide any
information about the nature of the acidic sites present in
the solid acids (i.e., whether Brønsted or Lewis acid sites).
For this purpose, we have investigated pyridine adsorbed
FT-IR (Py-FTIR) analysis to distinguish between the Brönsted
and Lewis acidic sites present in the solid acid catalysts.43
Fig. 7 shows the Py-FTIR profiles of the Sn, WSn, MSn and
SSn catalysts. Several bands were found in the region of
1700–1300 cm−1. As can be seen from the figure, all of the
solid acids exhibit two prominent bands at around 1639
and 1550 cm−1, which are assigned to the pyridine adsorbed
on the Brönsted acidic sites. An additional band observed at
1455 cm−1 can be attributed to pyridine adsorbed on the
Lewis acidic sites. The noticed IR band at 1490 cm−1 indicates
the existence of both Brönsted and Lewis acidic sites.44,45 It is
obvious from the figure that all samples exhibit a higher
amount of Brønsted acid sites compared to Lewis acid sites.
The obtained FT-IR spectra (Fig. S2, ESI†) show several
bands in the region of 553–680 cm−1, corresponding to the
O–Sn–O and Sn–O–Sn vibrational modes.46,47 The appearance
of an IR band at ~947 cm−1 in the MSn sample demonstrates
the stretching modes of the terminal Mo–O bond with a certain
Fig. 5 The O 1s XP spectra of the pure SnO2 (Sn), WO3/SnO2 (WSn),
MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) samples.
for the SSn catalyst, the peak broadening was found on the
higher binding energy side, suggesting the existence of two
types of O species; one towards the oxides and another was
related to the sulfated species.36,37
The NH3-TPD profiles of the Sn, WSn, MSn and SSn cata-
lysts are shown in Fig. 6. The total number of acidic sites cal-
culated from the NH3-desorption peak areas are shown in
Table 1. The relative position of the peak maxima in the
Fig. 6 is an evidence of the magnitude of the ammonia
desorption activation energy, which indicates the relative
strength of the acidic sites.38,41 It can be noted from Fig. 6
and Table 1 that the addition of promoters to the SnO2 strongly
enhances its acidic strength as well as its concentration.
Among the promoters, more acidic sites were observed for
2−
the SO4 ion promoted SnO2 sample. To our surprise,
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Fig. 8 The acetalization of glycerol with acetone over SnO2 (Sn),
WO3/SnO2 (WSn), MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) catalysts.
Reaction conditions: molar ratio of acetone to glycerol = 1 : 1; reaction
time = 90 min; catalyst amount = 5 wt.% (w.r.t. glycerol).
Fig. 7 The pyridine adsorption FTIR spectra of the SnO2 (Sn), WO3/SnO2
(WSn), MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) samples. B – Brønsted
and L – Lewis acidic sites.
amount of double bond nature. As well, the existence of a band
at 860 cm−1 reveals the typical Sn–O–Mo modes.15 Several
absorption peaks were identified at ~1198, ~1138, ~1089 and
~1050 cm−1 for the SSn sample, which can be attributed to
the bidentate sulfate ion groups strongly coordinated to the
Sn4+ species.48–50 The existence of a strong interaction between
the sulfate ion groups and the SnO2 oxide might be the reason
for the super acidic nature of the SSn catalyst.51 The band at
1625 cm−1 can be assigned to the deformation vibration mode
of the adsorbed water. A strong and broad band was noticed
between 3600 and 3000 cm−1 due to the H-bonded hydroxyl
groups, which is more pronounced in the case of the promoted
SnO2 catalysts.23,52
Scheme
1 The acetalization of glycerol with acetone to yield
5-membered (solketal) and 6-membered ketals.
Table 1). Interestingly, a high selectivity of solketal (96%) was
obtained irrespective of the catalysts used. da Silva et al.
reported the acetalization of glycerol with acetone over the
Amberlyst-15 catalyst and achieved 95% of glycerol conversion
for 15 min of reaction time at 343 K.53 In the present study,
we obtained a reasonably better glycerol conversion (i.e., 70%)
for 90 min of reaction time even at room temperature conditions.
The influence of the catalyst wt.% on the glycerol conver-
sion and product selectivity was studied over a SSn catalyst
and the other reaction conditions were kept the same (Fig. 9).
Remarkably, even at low catalyst amounts a better glycerol
4. Catalytic activity studies
4.1 Glycerol acetalization with acetone
The catalytic performance of the SnO2-based solid acids was
investigated for the acetalization of glycerol with acetone and
the obtained results are shown in Fig. 8. Usually, the acetalization
of glycerol with acetone yields two types of cyclic acetals,
namely 5- (solketal) and 6-membered ketals as shown in
Scheme 1. The catalytic experiments were carried out at room
temperature under solvent-free conditions with a 1 : 1 molar
ratio of acetone to glycerol and 5 wt.% catalyst for 90 min of
reaction time with constant stirring of ~800 rpm. It was found
that the addition of promoters to the SnO2 significantly
enhances its catalytic activity. The obtained glycerol conversion
over the Sn, WSn, MSn and SSn catalysts was ~15, 55, 61 and
70%, respectively. The superior catalytic activity of the SSn
solid acid can be explained by the presence of a higher amount
of acidic sites associated with the super acidic sites (Fig. 6 and
Fig. 9 The influence of catalyst wt.% on the acetalization of glycerol
with acetone over the SO42−/SnO2 catalyst. Reaction conditions: molar
ratio of acetone to glycerol = 1 : 1; reaction time = 90 min.
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conversion was obtained, indicating the promising perfor-
mance of the SSn catalyst towards glycerol acetalization. The
glycerol conversion and selectivity of solketal at 1 wt.% of the
catalyst amount was 41 and 95%, respectively. With the
increase of the catalyst amount up to 7 wt.%, the glycerol con-
version was significantly increased, which could be due to the
availability of more acidic sites. Furthermore, no considerable
variation in the glycerol conversion was found with the increase
of the catalyst amount. The obtained glycerol conversion at
7, 9, and 10 wt.% was 82, 85, 86%, respectively. In contrast,
the product selectivity does not change much with the cata-
lyst amount.
Scheme 2 The catalytic acetalization of glycerol with furfural and its
derivatives to yield 5-membered and 6-membered acetals.
Fig. 10 shows the effect of the molar ratio of acetone
to glycerol (i.e., acetone : glycerol = 1 : 1 and 1.5 : 1) on the
glycerol acetalization over a SSn catalyst as a function of
time. These experiments were carried out at different time
intervals from 10 to 240 min. It is clear from the figure that
the glycerol conversion increases with increase of reaction
time for both molar ratios. However, the conversion of
glycerol was slightly increased with the increase of the molar
ratio of acetone to glycerol. The glycerol conversion at 10 min
of reaction time was 29 and 31% for 1 : 1 and 1.5 : 1 molar ratios
of acetone to glycerol, respectively. After 240 min of reaction
time, the conversion of glycerol reached 95 and 98% for 1 : 1
and 1.5 : 1 molar ratios of acetone to glycerol, respectively. In
contrast, a variation in the molar ratio of acetone to glycerol
does not influence the selectivity of solketal with reaction time.
Fig. 11 The acetalization of glycerol with furfural over the SnO2 (Sn),
WO3/SnO2 (WSn), MoO3/SnO2 (MSn) and SO42−/SnO2 (SSn) catalysts.
Reaction conditions: molar ratio of furfural to glycerol = 1 : 1; reaction
time = 30 min; catalyst amount = 5 wt.% (w.r.t. glycerol).
reaction time, 5 wt.% of catalyst, and 1 : 1 molar ratio of
furfural to glycerol. Fig. 11 shows the conversion of glycerol
and the selectivity of products over SnO2-based solid acids.
Here also, higher glycerol conversions were obtained over
the promoted SnO2 catalysts and 51, 67, 75 and 82% glycerol
conversions were obtained for the Sn, WSn, MSn and SSn
catalysts, respectively. As we stated in the above paragraphs,
the better catalytic activity of the SSn catalyst is due to the
presence of a large amount of surface acidic sites, along with
super acidic sites. Contrary to the glycerol acetalization with
acetone, here a significant variation in the products selectivity
was found for all the investigated catalysts. The selectivity
of 5- and 6-membered acetals was 67 and 33% over the SSn
catalyst, respectively.
4.2 Glycerol acetalization with furfural and its derivatives
We have also explored the catalytic efficacy of the promoted
SnO2 catalysts for the acetalization of glycerol with furfural
and its derivates. Scheme 2 shows the acetalization of glycerol
with furfural. For all catalysts, the reaction conditions were
fixed as follows: solvent-free, room temperature, 30 min of
Deutsch et al.14 suggested a possible reaction pathway for
the acetalization of glycerol with benzaldehyde through a
benzyl cation formation. Hence, a similar reaction path way
can be expected in the case of the acetalization of glycerol
with furfural as both molecules (i.e., benzaldehyde and furfural)
belong to same family. Initially, the positively charged carbonyl
carbon of the aldehyde can be formed due to the existence of
a strong interaction between the oxygen atom of the carbonyl
group and the solid acid catalyst. Afterward, the formed
carbocation will be attacked by the oxygen atom of the glycerol
giving the intermediate furfuryl cation (I) as shown in Scheme 3,
which eventually affords two isomeric cyclic acetals through
different routes. For a better understanding, the formation of
the 5-membered ring acetal (a) is presented in Scheme 3 and
Fig. 10 The effect of the molar ratio of acetone to glycerol as a
function of reaction time on the acetalization of glycerol with acetone
over the SO42−/SnO2 catalyst. Reaction conditions: catalyst amount =
5 wt.% (w.r.t. glycerol).
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catalyst that can be applied for the acetalization of glycerol
with the substituted furfural compounds.
Fig. 12 shows the effect of the reaction time on the
acetalization of glycerol with furfural over the SSn catalyst.
The catalytic experiments were examined at different time
periods up to 120 min at room temperature. It is evident
from the figure that the conversion of glycerol increases with
the increase of reaction time. It is interesting to note that the
SSn catalyst exhibits a glycerol conversion of ~51% even after
10 min of reaction time, and 82% glycerol conversion at
30 min of reaction time. Afterward, the enhancement in the
glycerol conversion was insignificant and 90, 95 and 99% of
glycerol conversions were noted at 60, 90 and 120 min of
reaction time, respectively. In the investigated time intervals,
a higher selectivity of the 5-membered acetal was obtained
than that of 6-membered acetal.
Scheme 3 The reaction pathway for the synthesis of the 5-membered
ring acetal (a) and the 6-membered ring acetal (b) over the solid acid-
catalyzed acetalization of glycerol with furfural.
the same pathway can be expected for the formation of a
6-membered ring acetal.
The condensation of glycerol with mono-substituted furfurals
was also examined over the SSn catalyst and the obtained results
are demonstrated in Table 2. It was found that all of the
substituted compounds show a lower glycerol conversion when
compared to unsubstituted furfural. This result could be due
to the effect of steric hindrance induced by the presence of
substitutes rather than the electronic effects of the substituent.15
In addition, 5-methylfurfural exhibited the highest glycerol
conversion (79%), whereas a low glycerol conversion was found
in the case of 5-chlorofurfural (65%). Interestingly, a high selec-
tivity of 5-membered acetals was obtained for all substituted fur-
fural compounds, and 5-nitrofurfural showed a selectivity of
84% for the 5-membered acetal. These interesting findings
reveal the versatile nature of the sulfate ion promoted SnO2
Fig. 12 The effect of the reaction time on the acetalization of glycerol
with furfural over the SO42−/SnO2 catalyst. Reaction conditions: molar
ratio of furfural to glycerol = 1 : 1; catalyst amount = 5 wt.% (w.r.t. glycerol).
Table 2 The acetalization of glycerol with monosubstituted furfural compounds over the SO42−/SnO2 catalyst. Reaction conditions: molar ratio of
furfural to glycerol = 1 : 1; reaction time = 30 min; catalyst amount = 5 wt.% (w.r.t. glycerol)
Glycerol
Substituted furfural
Gly. conv. (%)
82
Selectivity of products (%)
Chemical shifta (ppm)
5-mem : 6-mem
6.01 (s) : 5.89 (s)
65
71
79
68
5.89 (s) : 5.78 (s)
5.97 (s) : 5.81 (s)
6.12 (s) : 5.94 (s)
6.09 (s) : 5.87 (s)
a
Proton signal of OCH(C6H5)O.
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Fig. 13 The effect of the catalyst wt.% on the acetalization of glycerol
with furfural over the SO42−/SnO2 (SSn) catalyst. Reaction conditions:
molar ratio of glycerol to acetone = 1 : 1; reaction time = 30 min.
The effect of the catalyst wt.% on the glycerol acetalization
was studied over the SSn catalyst (Fig. 13). The catalytic
experiments were examined by varying the catalyst amount
from 1 to 7 wt.%. With the increase of the catalyst amount,
the glycerol conversion was increased and 44, 63, 82 and 91%
glycerol conversions were obtained at 1, 3, 5 and 7 wt.% of
the catalyst amount, respectively. However, with a further
increase of the catalyst amount, no considerable variation
in the glycerol conversion as well as the selectivity of the
products was observed. The selectivity of the 5- and 6-membered
acetals was found to be 67 and 33% at 5 wt.% of catalyst
amount, respectively.
Fig. 14 The powder X-ray diffraction patterns of the fresh and spent
SO42−/SnO2 (SSn) samples.
we have determined the average crystallite size of the fresh
and spent SSn catalysts by means of the Scherrer equation. The
obtained average crystallite size of the fresh and spent SSn
catalysts was 7.91 and 9.73 nm, respectively. Generally, an
increase in the crystallite size will result in a decrease in
the surface area. As we asserted, the BET surface area of
the SSn catalyst was decreased from 41 to 33 m2 g−1 after the
first run. These results clearly suggest the beneficial effect of
2−
the SO4 ions towards the textural properties of the SnO2
4.3 Reusability test
which decreased after the reusability test. We have deter-
mined by ICP-OES the sulfur content in the fresh and spent
SSn catalysts. The sulfur content of the SSn catalyst was
considerably decreased (~10%) after the first run, which indi-
cates the leaching of the sulfate species. Therefore, it can be
suggested that the increase in the crystallite size as well as the
decrease in the surface area, and thereby the catalyst deactiva-
tion is mostly due to the leaching of the sulfate species after the
reusability test. We have also performed the pyridine adsorbed
FT-IR of the fresh and spent SSn catalysts (Fig. S4, ESI†). It was
found that the intensity of the IR band at 1639 cm−1 related to
the Brönsted acidic sites was slightly decreased after the first run.
As well, the peaks at 1550 (Brönsted), 1490 (Brönsted & Lewis)
and 1455 (Lewis) almost disappeared after the reusability
of the SSn catalyst. These interesting observations can be
explained by the leaching of sulfate species after repeated use.
Therefore, it can be concluded that the leaching of sulfate
species shows an adverse effect on the textural as well as on
the acidic sites of the SSn catalyst, and thereby, decreases
the catalyst activity in the glycerol acetalization with furfural.
The formation of coking on the catalyst surface is obvious in
the vapour phase conditions due to the high temperature
operation conditions. The present study has been carried out
at room temperature. Hence, it can be expected that there
is no scope of the coke formation on the catalyst surface.
We have also tested the reusability of the SSn catalyst up to
5 cycles to understand its stability for the acetalization of
glycerol with furfural (Fig. S3, ESI†). The reaction conditions
remained same as showed in Fig. 11. After the completion of
the reaction, the catalyst was separated from the reaction
mixture by centrifugation and washed with methanol to
remove the adsorbed products, and pre-activated at 423 K for
2 h. It was found that the conversion of glycerol continuously
decreases after the repeated use of the catalyst. The glycerol
conversion at the 1st, 2nd, 3rd, 4th, and 5th cycles was found
to be ~82, 69, 58, 47 and 33%, respectively. On the other
hand, no considerable variation in the product selectivity was
observed. In all of the cycles, the selectivity of the 5- and
6-membered rings was found to be in the range of ~66 and
34%, respectively.
4.4 Reasons for the catalyst deactivation
A plausible reason for catalyst deactivation was discussed
by analyzing the spent catalyst with XRD, BET surface area,
ICP-OES, pyridine adsorbed FT-IR, etc. Fig. 14 shows the XRD
patterns of the fresh and spent SSn catalysts. A close examina-
tion of Fig. 14 reveals that an increase in the intensity of the
diffraction peaks of the SSn sample after the first run might
be due to the crystal growth of the SnO2. To understand this,
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However, to understand the coke formation on the catalyst sur-
face we have performed CHN analysis of the spent SO42−/SnO2
catalyst. No considerable coke formation was observed on the
catalyst surface after the repeated use. Nevertheless, these
results are not conclusive, further evidences are needed to
confirm the catalyst deactivation in the glycerol acetalization,
which are under investigation.
9 A. P. Vyas, J. L. Verma and N. Subrahmanyam, Fuel, 2010,
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Service, Washington, DC, 2010.
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5. Conclusions
In this work, we have prepared highly promising promoted
SnO2-based solid acids by means of a wet-impregnation method
and systematically characterized them by XRD, BET surface
area, BJH pore size distribution, ICP-OES, CHN, XPS, FT-IR,
pyridine adsorbed FT-IR and NH3-TPD techniques. The catalytic
performance of the SnO2-based solid acids was investigated
for the acetalization of glycerol with acetone, furfural and its
derivatives at room temperature and solvent-free conditions.
The activity results revealed that the promoted SnO2 catalysts
show an excellent catalytic performance in the acetalization
of glycerol in comparison to the pure SnO2. In addition, the
SO42−/SnO2 catalyst showed an outstanding catalytic activity
due to the presence of higher amounts of surface acidic sites
associated with super acidic sites. These interesting results
revealed the promising role of the SO42−/SnO2 catalyst for the
synthesis of valuable bio-additive fuels from renewable glycerol
under environmentally benign conditions. However, the activity
of the SO42−/SnO2 catalyst was found to continuously decrease
after repeated use and this might be due to the leaching of
the sulfate species.
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Acknowledgements
B.M. and P.S. thank the Council of Scientific and Industrial
Research (CSIR), New Delhi, for the award of research fellowships.
Financial support was received from Department of Science and
Technology, New Delhi, under SERB Scheme (SB/S1/PC-106/2012).
24 P. Sudarsanam, B. Mallesham, A. N. Prasad, P. S. Reddy and
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