Conversion of levulinic acid to ethyl levulinate using tin modified silicotungstic acid supported on Ta2O5

Article abstract of DOI:10.1016/j.catcom.2019.105864

Tin modified silicotungstic acid (STA) deposited on Ta₂O₅ was explored as suitable catalyst for the conversion of levulinic acid to ethyl levulinate. The microwave synthesized catalysts of varying amounts of STA were characterised by N₂ adsorption, powder XRD, FT-IR, FE-SEM, XPS, pyridine-FT-IR, TGA-DTA and NH₃-TPD. Among the synthesized catalysts, 50% Sn₂STA/Ta₂O₅ catalyst exhibited excellent catalytic activity for the esterification of levulinic acid providing 78% of ethyl levulinate yield and stable recyclability up to three cycles. The rate of esterification was 6.6 × 10^?3 mol/g/min, which is significantly higher compared to reported values in the open literature. Mechanistic investigations revealed that the high catalytic activity was influenced by the presence of appropriate Br?nsted acid sites, surface area and porosity.

Full text of DOI:10.1016/j.catcom.2019.105864

CatalysisCommunications134(2020)105864
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Conversion of levulinic acid to ethyl levulinate using tin modified
silicotungstic acid supported on Ta2O5
Parameswaram Ganji, Sounak Roy⁎
Department of Chemistry, Birla Institute of Technology and Science, Pilani, Hyderabad Campus, Jawahar Nagar, Hyderabad 500078, Telangana, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tin modified silicotungstic acid (STA) deposited on Ta2O5 was explored as suitable catalyst for the conversion of
levulinic acid to ethyl levulinate. The microwave synthesized catalysts of varying amounts of STA were char-
acterised by N2 adsorption, powder XRD, FT-IR, FE-SEM, XPS, pyridine-FT-IR, TGA-DTA and NH3-TPD. Among
the synthesized catalysts, 50% Sn2STA/Ta2O5 catalyst exhibited excellent catalytic activity for the esterification
of levulinic acid providing 78% of ethyl levulinate yield and stable recyclability up to three cycles. The rate of
esterification was 6.6 × 10−3 mol/g/min, which is significantly higher compared to reported values in the open
literature. Mechanistic investigations revealed that the high catalytic activity was influenced by the presence of
appropriate Brønsted acid sites, surface area and porosity.
Silicotungstic acid
Tin
Ta2O5
Esterification
Levulinic acid
Ethyl levulinate
1. Introduction
[13,14,15]. Development of separable and eco-friendly recyclable solid
acid catalysts is therefore one of the key technologies to establish en-
Depletion of non-renewable fossil fuels, global warming and other
environmental hazards have resulted in a paradigm shift towards the
generation of fuels and chemicals from renewable materials. Against
this background, many researchers are relentlessly working in finding
varieties of biomass platform chemicals and synthesis routes to produce
useful key industrial chemicals and transportation fuels. According to
National Renewable Energy Laboratory reports (Denver, USA), the
lignocellulose biomass derived levulinic acid (LA) is one of the top-12
priority platform chemicals, since the presence of two functional moi-
eties in this γ-keto acid imparts it the potential to render a variety of
useful chemicals on subjection to different reactions [1,2]. The ester-
ification of LA with alcohol attracted much attention as these levuli-
nates or esters consist of versatile chemical feedstocks with numerous
potential applications in the flavouring and fragrance industry, or as a
blending component in biodiesel fuel [3–8]. Among them, ethyl levu-
linate (EL) is one of the most important ester as blending of 5 wt% of EL
in diesel leads to important modification in the fuel properties, such as
high lubricity, stability in flashpoint, reduced sulfur content, and im-
proved viscosity [6,9,10,11].
vironmentally sustainable approaches for the LA esterification pro-
cesses. In the category of solid acid catalysts, heteropoly acids have
gained significant attention because of their super strong Brønsted
acidity and special Keggin-type structural properties [7,9,16,17].
However, the poor chemical stability, high solubility in polar media and
small specific surface area have limited their industrial applications.
Dispersing the heteropoly acid on a porous support recently has at-
tracted much attention [9,11,18–20]. In spite of some minor improve-
ments, largely the catalysts are expensive, cumbersome to synthesize,
performances are poor, and, therefore, it would be a novel challenge to
exploit a fine catalyst to improve the synthetic efficiency of EL.
In this work we report for the first time a novel microwave-assisted
hydrothermal synthesis method of a tin (Sn) incorporated heteropoly-
acid supported on another water tolerant solid Brønsted acid catalyst,
like Ta2O5, Nb2O5 and Al2O3, and studied their performances towards
esterification of renewable LA with ethanol as an alkylating agent.
Among the synthesized catalysts, tin incorporated silicotungstic acid
over Ta2O5 was found to have excellent yield and very high initial rate
for the catalytic formation of EL from LA in ethanolic solvent, as
compared to the open literature. The performances of these synthesized
catalysts were correlated with the influence of surface acidity, porosity
and surface hydrophobicity.
EL can be efficiently produced by esterification of LA with ethanol
with traditional catalysts, like strong Brønsted liquid acids (such as
H2SO4, HCl, and polyphosphoric acid) [12,13]. Nevertheless, liquid
acid-catalysed homogeneous esterification processes suffer from severe
corrosion, waste and safety problems, handling and recycling issues
⁎ Corresponding author.
E-mail address: sounak.roy@hyderabad.bits-pilani.ac.in (S. Roy).
https://doi.org/10.1016/j.catcom.2019.105864
Received 4 September 2019; Received in revised form 22 October 2019; Accepted 24 October 2019
Availableonline31October2019
1566-7367/©2019ElsevierB.V.Allrightsreserved.

P. Ganji and S. Roy
CatalysisCommunications134(2020)105864
80
60
40
20
0
EL
AL
Sn2STA/Nb2O5
Sn2STA/Ta2O5
Sn2STA/Al2O3
Sn2STA
Different oxide catalysts
Fig. 1. %-Yield of EL and Al over unsupported and supported oxide catalysts after 3 h of esterification of LA at 70 °C; Inset: The reaction scheme with products
distribution.
2. Experimental
The weak and strong acidic sites on the surface of the catalysts were
evaluated by temperature-programed desorption of ammonia (NH3-
TPD), and FT-IR spectra of adsorbed pyridine over the as-prepared
materials, details of which were reported earlier [21,22]. The NH3-TPD
is one of the most conventional methods for characterizing the overall
acidity of the catalysts. The strong basicity and small molecular size of
ammonia facilitate identification of acidic sites present even on the
surface of the small pores in the solids. The NH3-TPD of catalytic
samples was carried out in a BELCAT II instrument (Japan). To screen
the as prepared catalysts for esterification, LA and ethanol were taken
in a round bottom flask connected with a reflux condenser. Typically,
the substrate LA (7 g) and ethanol were charged together into the flask
followed by the addition of the as prepared catalyst (0.05 g). Ester-
ification of LA was performed at 70 °C for 3 h using excess of ethanol
with a molar ratio of ethanol:LA = 5:1. Typically, the amount of
ethanol was 17.5 mL in a reaction. During reaction the aliquots were
periodically withdrawn and filtered prior to their analysis by gas
chromatography (GC). Reaction samples were analysed by GC (Agilent
Tin (II) chloride, silicotungstic acid (H4[W12SiO40], STA, 99.9%),
tantalum oxide, niobium oxide, alumina, levulinic acid (99.9%), Ethyl
Levulinate (99.9%) and ethanol were obtained from M/s E. Merck,
Mumbai (India). Tin incorporated STA on various solid acidic supports,
like Ta2O5, Nb2O5 and Al2O3 were prepared by a microwave assisted
hydrothermal method. The detailed synthetic procedure is reported in
previous publications [21,22]. In a typical process, calculated amount
of aqueous solution of SnCl2.2H2O was added drop wise to the required
amount of STA in de-ionised water, and the resulting mixture was
subjected to heating in a G30 vial at 70 °C for 2 h in the microwave
system (Anton Parr, Monowave 300, GmbH, Europe) in order to replace
the two protons of STA and produce the Sn2STA solid acid. After the
microwave treatment, the obtained partially soluble solid acid Sn2STA
was dried at 60 °C overnight in a hot air oven. For the Sn2STA/support
(support = Ta2O5, Nb2O5 and Al2O3) solids, the synthesized Sn2STA
was taken in deionised water with the oxidic support and subjected to
microwave heating in a similar method as stated above. The amount of
Sn2STA was varied from 15 to 60 wt% with respect to the oxide support.
For specific surface area (SSA, m2/g) and pore size distribution es-
timations of the synthesized catalysts, N2 physisorption isotherms were
measured at 77 K with a Microtrac BELSORP mini-II surface area ana-
lyser after using the BET method. Phase identification and degree of
crystallinity were recorded on a Rigaku ULTIMA-IV powder XRD op-
erated at 40 kV and 30 mA using CuKα radiation as an X-ray source. The
diffractograms were recorded with a step size of 0.02° and scan rate of
2° min−1. Thermo Scientific K-ALPHA surface analysis spectrometer
with X-ray source of Al Kα radiation (1486.6 eV) was used to record XPS
spectra of the synthesized catalysts. Binding energies are reported with
respect to C (1 s) at 284.8 eV. The FT-IR spectra (transmission mode) of
the synthesized materials were recorded by JASCO FT-IR-4200 instru-
ment at room temperature. The synthesized catalysts were mixed with
KBr, grounded with mortar and pestle, and a pellet was made after-
wards to record the FTIR spectrum in transmittance mode with a re-
solution of 4 cm−1. TGA was performed using a Shimadzu DTG-60 in-
strument in the temperature range of 30–700 °C and a heating rate of
10 °C min−1 under N2 gas atmosphere. The surface morphology of the
powders was evaluated by using FE-SEM (FEI, Apreo Model).
3900 Gas chromatograph) using
a capillary EB-WAX column
(30 m × 0.32 mm × 0.25 μm), nitrogen as a carrier gas within the
programmed temperature range of 50–210 °C and the use of a FID de-
tector. The %-yield of the reaction product was calculated by using the
following Eq. (1):
Actual Yield
Theoretical Yield
%Yield =
× 100%
(1)
The actual yield was calculated experimentally with the help of a
calibration curve of the EL concentration (ppm) vs area response under
the GC peak.
3. Results and discussion
As Nb2O5.nH2O, Ta2O5.nH2O and Al2O3 exhibit excellent Brønsted
acidity and are effective for esterification, olefin hydration, and alcohol
dehydration [23], these solid materials were chosen as support mate-
rials in the present investigation. Successful synthesis of 50% Sn2STA
over Ta2O5, Nb2O5 and Al2O3 was confirmed by powder diffraction
(Fig. S1, ESI). The esterification of LA to produce EL using these syn-
thesized catalysts was evaluated under the above-mentioned
2

P. Ganji and S. Roy
CatalysisCommunications134(2020)105864
(a)
(b)
50%Sn2STA-Al2O3
50%Sn STA/Al O
2 3
2
50%Sn2STA-Nb2O5
50%Sn2STA-Ta2O5
50%Sn STA/Nb O
2
2
5
50%Sn STA/Ta O
2
2
5
1800 1700 1600 1500 1400 1300 1200
Wavenumber (cm-1)
200 300 400 500 600 700 800
o
Temperature ( C)
Fig. 2. (a) FT-IR spectra of catalysts after adsorption of pyridine at room T. (b) NH3-TPD profiles of 50 wt% Sn2STA/Nb2O5, 50 wt% Sn2STA/Al2O3, 50 wt% Sn2STA/
Ta2O5. Blank data is provided in Fig. S3(a-b) of ESI.
experimental conditions, and the obtained results are presented in
Fig. 1. The GC profiles of formation of LA and AL are shown in Fig. S2
(ESI). Apparently, there is formation of two products, the primary
product EL and the by-product angelica lactone (AL). AL is formed due
to an acid catalysed dehydration followed by cyclization (see inset of
Fig. 1), which is also reported in the literature [24–26]. It is evident
from these results that Ta2O5 support outperforms the other two sup-
ports tested as well as the unsupported Sn2STA solid. The yield of EL
was found to be 78% over the 50% Sn2STA/Ta2O5 catalyst, whereas it
was only 33 and 23% over the 50% Sn2STA/Nb2O5 and 50% Sn2STA/
Al2O3, respectively. On the other hand, the unsupported Sn2STA
showed that the %-yield of EL is only 19%.
The selectivity of EL over AL was also found to be higher (96%) over
the 50% Sn2STA/Ta2O5 catalyst compared to the other two catalysts
(ca. 47% EL selectivity over 50% Sn2STA/Al2O3 and 76% EL selectivity
over 50% Sn2STA/Nb2O5). As the surface acidity plays a key role in
determining the catalytic performance for the present catalytic system,
to probe the higher efficiency of the Ta2O5 support, the surface acidity
of the synthesized catalysts was evaluated. Fig. 2a presents pyridine
adsorbed FT-IR spectra, and Fig. 2b NH3-TPD profiles of the three in-
vestigated catalysts. The major IR bands observed at 1441, 1482 and
1536 cm−1 correspond to surface Lewis acid sites, both Brønsted and
Lewis type acid sites, and Brønsted acid sites, respectively [21,22]. It is
evident from Fig. 2a that Ta2O5-supported catalyst possesses very
strong bands due to the presence of Brønsted acid sites, concluding the
highest Brønsted acidity over the 50% Sn2STA/Ta2O5 among the three
catalysts. Control experiments of adsorbed pyridine FT-IR spectra of the
three bare supports, only showed the 1441 cm−1 peak (Fig. S3a, ESI).
The NH3-TPD profiles shown in Fig. 2b also support the view that the
acidity is in the following order: Ta2O5 > Nb2O5 > Al2O3. The total
amount of NH3 desorbed equivalent with the total surface acidity, was
found to be 1.39, 0.78 and 0.55 mmol/g over the 50% Sn2STA/Ta2O5,
50% Sn2STA/Nb2O5 and 50% Sn2STA/Al2O3 catalyst, respectively.
Therefore, the highest yield of EL with high selectivity can be directly
correlated with the surface acidity exhibited by the 50% Sn2STA/Ta2O5
solid. It should be noted that the bare supports did not show any sig-
nificant NH3 desorption (Fig. S3b, ESI).
Fig. 3. XRD patterns of 15–60 wt% Sn2STA/Ta2O5, Ta2O5, and STA solid cat-
alysts. The shown (001), (100) and (101) planes correspond to Ta2O5 (JCPDS
19–1299).
appeared at 2θ = 10.3°, 25.6° and 34.7° and which can be assigned to
the diffraction lines of body centred cubic secondary structure of the
Keggin ion of STA. Ta2O5 showed major characteristic diffraction peaks
at 2θ = 22.75°, 28.38° and 36.79° corresponding to the (001), (100)
and (101) planes, respectively, which are due to the hexagonal phase of
Ta2O5 (JCPDS 19–1299). After using a lower loading of Sn2STA over
Ta2O5 (15 and 40 wt%), the STA peaks were obscured. However, with
increasing loading of Sn2STA the characteristic peaks were clearly ob-
served, indicating the retention of the Keggin structure in the Sn2STA/
Ta2O5 solid. The structural information was also corroborated from FT-
IR analyses (see Fig. S3c, ESI). All the synthesized catalysts exhibited
the characteristic FT-IR bands of SieO, terminal W]O, inter-octahedral
W–O–W, and intra-octahedral W–O–W at 1020, 980, 877 and
As the Ta2O5 support was found to have the best performance
compared to the other support, the next exploration was to correlate the
surface concentration of Sn2STA over the Ta2O5 support with catalytic
performance. The loading of Sn2STA was varied from 15 to 60 wt%, and
the corresponding powder diffraction profiles of the synthesized cata-
lysts are presented in Fig. 3. All the synthesized supported catalysts
showed good crystallinity as evident from the sharp diffraction peaks
recorded. In the top panel of Fig. 3, the three main diffraction peaks
3

P. Ganji and S. Roy
CatalysisCommunications134(2020)105864
778 cm−1, respectively, suggesting the intact Keggin structure after
exchange of protons with Sn in the secondary structure of STA [27,28].
Elemental concentration measured by XRF was found in accordance
with the experimental conditions applied (Table S1, ESI).
deposited on Ta2O5 were explored for the catalytic esterification up to
3 h of reaction, and results are shown in Fig. 4. With the lower loading
of 15 wt% Sn2STA the catalyst showed only 25% of EL yield and a
corresponding 93% selectivity. Thus, there is 7% selectivity for the by-
product AL. By further increasing Sn2STA loading to 40 wt%, the cat-
alyst did not show any significant change in the EL yield; however, the
selectivity towards EL was poor, and 15% of AL was obtained. Further
increase in loading of heteropoly acid sites on the Ta2O5 support surface
(50 wt% Sn2STA/Ta2O5) resulted in a 78% yield of EL with as high as
97% selectivity towards EL. With a further increase of Sn2STA loading
to 60%, the EL yield decreased to 37%, whereas the AL yield was in-
creased to 14%. It is apparent from these results that an optimum
loading is about 50% Sn2STA over the Ta2O5 support that shows ex-
cellent LA conversion to EL with higher selectivity under the experi-
mental conditions applied. Further time-dependent experiments on the
progress of reaction were conducted over the 50 wt% Sn2STA/Ta2O5,
and results are shown in the inset of Fig. 5. It is shown that the con-
centration of LA decreases with time as the % formation of EL is in-
creased, while AL shows a volcanic hump against time. The initial rate
of conversion of LA was calculated and found to be 6.6 × 10−3 mol/g/
min.
The thermal stability of prepared catalysts was evaluated by TGA
and DTA, and results are shown in Fig. S4 (ESI). The graph shows the
typical TG/DTA patterns for the Keggin-type heteropolyacid. The initial
major weight loss below 100 °C is related to the elimination of adsorbed
water, which is responsible for the crystallization of heteropolyanions
into a hydrate. The second weight loss just above 100 °C could be due to
the evolution of CO2 and decomposition of oxycarbonates to get a stable
catalyst. The final weight loss at elevated temperatures could be due to
the decomposition of heteropolyanion, with crystallization of oxides.
Apparently, the catalyst is thermally stable at the operating reaction
temperature. The oxidation state(s) of these surface active sites was
explored by XPS, and results are shown in Fig. S5 (ESI). The survey
spectra did show existence of Sn 3d and O 1 s XPS peaks. The core level
spectra of Sn in Sn2STA showed two peaks at 487.3 and 495.7 eV cor-
responding to 3d5/2 and 3d3/2 of Sn4+. It must be noted that the binding
energy of Sn4+ in STA is slightly higher than the reported binding
energies of Sn4+ in SnO2, indicating more ionic environment of Sn in
STA than in SnO2. There is also a minor shoulder at slightly lower
binding energy signifying existence of little amount of Sn2+ in the
sample.
In order to rationalize the catalytic performance dependence on the
surface acidity, pyridine adsorbed FT-IR and NH3-TPD experiments
were conducted and data are shown in Fig. 5. It is apparent from the
intensity of IR bands at 1545, 1445 and 1490 cm−1 in Fig. 5a, that the
Brønsted/Lewis site ratio increased with increasing Sn2STA loading up
to 50 wt% and with further increase of it the intensity of both IR bands
decreased. The uniform distribution of Sn2STA up to 50 wt% loading on
the surface of Ta2O5 (vide FE-SEM) might be responsible for the increase
of Brønsted and Lewis acid sites concentration, and with a higher
loading it might form multilayers that could block access to some acid
sites of Sn2STA from the catalyst support, thus deteriorating the cata-
lytic efficiency [9]. The desorbed NH3 against temperature traces of the
synthesized catalysts show very good surface acidic strengths for the
catalytic materials. For all the catalysts, there is a minor low-tem-
perature desorption peak, and a larger peak above 500 °C (Fig. 5b). The
amount of ammonia desorbed at a given temperature range is taken as a
measure of the acid site concentration, whereas the temperature ranges
at which most of the ammonia is desorbed indicates the acid strength
The surface textural features of the catalysts were probed by ni-
trogen adsorption−desorption. Interestingly, the specific surface area
increased with increasing Sn2STA loading on the Ta2O5 support, and a
highest surface area of 59.8 m2/g was achieved with 50 wt% Sn2STA
loading. Specific pore volume and mean pore diameter were also found
to be highest with 50 wt% Sn2STA loading. To understand the catalyst's
surface morphology, FE-SEM images of the as prepared Sn2STA/Ta2O5
catalysts were obtained and these are shown in Fig. S6 (ESI). The
average secondary particle size of Ta2O5 was found to be ~ 300 nm,
and it is evident from the micrographs that with the increase in Sn2STA
loading the surface of Ta2O5 is covered with more Sn2STA particles. The
Sn2STA particles are discrete up to 50 wt% of loading, and further in-
crease in loading makes them further agglomerated. The mean size of
Sn2STA secondary particles were ~ 50 nm.
The microwave synthesized catalysts with varied loading of Sn2STA
80
80
80
60
60
AL
EL
LA
EL
AL
40
40
60
20
0
20
0
40
60
80
100 120 140 160 180
Time (min)
40
20
0
O
5
O
O
6
a
T
/
A
O
a2
a2
O
a2
5
5
5
2
a
T
A/T
ST
Sn2
5
A/T
ST
Sn2
A/T
T
ST
Sn2
%
0
2
S
Sn2
%
5
1
%
%
0
5
0
4
Variation in Sn2STA loading over Ta2O5
Fig. 4. %-Yield of LA conversion to EL against varied amount of Sn2STA deposited on Ta2O5. Time dependent reaction profile with % conversion of LA and %
formation of El and AL over the 50 wt% Sn2STA/Ta2O5 catalyst is shown in inset graph.
4

P. Ganji and S. Roy
CatalysisCommunications134(2020)105864
Fig. 5. (a) Room temperature FT-IR spectra of pyr-
idine–adsorbed on 15–60 wt% Sn2STA/Ta2O5.
About 1 μL of pyridine was dropped on KBr pellets of
Sn2STA/Ta2O5 before the recording of spectra. (b)
NH3-TPD profiles of 15–60 wt% Sn2STA/Ta2O5 cat-
alysts. NH3 was passed over the catalysts at a flow
rate of 10 cc/min for 1 h before desorption. Blank
data is provided in ESI.
(b)
60%Sn STA/Ta
2
O
5
2
(a)
60%Sn2STA-Ta2O5
50%Sn STA/Ta
2
O
5
2
50%Sn2STA-Ta2O5
40%Sn STA/Ta
2
O
2
5
40%Sn2STA-Ta2O5
15%Sn2STA-Ta2O5
15%Sn STA/Ta
O
2
2
5
1800
1700
1600
1500
1400
1300
1200
100
200
300
400
500
600
700
Wavenumber (cm-1)
o
Temperature ( C)
distribution. The total amount of NH3 desorbed from the catalysts was
found to be 0.45, 0.95, 1.39, and 1.11 mmol/g for the 15, 40, 50 and
60 wt% Sn2STA/Ta2O5, respectively. It can be easily concluded from
pyridine adsorbed FT-IR and NH3-TPD studies that 50 wt% Sn2STA/
Ta2O5 has the optimum ratio of Brønsted to Lewis acid sites due to the
uniform distribution of Sn2STA over the Ta2O5 support, parameter that
makes this particular catalyst composition efficient for LA esterification
with high selectivity to EL.
yield and nearly complete selectivity, and with an initial conversion
rate of 6.6 × 10−3 mol/g/min. Additionally, the catalyst showed a
stable recyclability up to three cycles. The catalytic performance of the
50 wt% Sn2STA/Ta2O5 was strongly favourable when compared with
the literature. This work paves a path for alternative heterogeneous
catalytic materials to inorganic acids used for the synthesis of EL.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgements
To benchmark the performance of the synthesized catalyst, the re-
sults were compared with reported values in existing literature and
which are comprehensively presented in Table S2 (ESI). Literature
shows that a wide range of heteropoly acids like HPA-H4SiW12O40 and
H3PMo12O40 were used to modify the SiO2 surface and tested for the
catalytic esterification of LA [29]. However, after prolonged 6–10 h
reaction and high catalyst loading, the conversion was only 75%, and
the product yield was as low as 67%. The H/BEA zeolite modified with
NaOH was applied as catalyst for LA esterification with ethanol, where
the LA conversion was only 40% [11,30]. In another study, ZrO2/SBA-
15 catalysts were synthesized by the impregnation method with mod-
erate acidic sites [24]. The ZrO2 modified zeolite catalyst could only
convert 50% of LA at a very high temperature, ca. 250 °C. A significant
conversion of LA with high selectivity of EL was only achieved in a flow
reactor under vapour phase conditions over a WO3-incorporated SBA-
16 catalyst [31].
P. G. thanks SERB and DST, India, for the financial support in the
form of N-PDF grant [SERB/F/2788/20l8-2019, SERB/F/877/2019-
2020].
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.catcom.2019.105864.
References
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was used for the next cycle. The conversion of LA was found to be 81,
76 and 74%, and the percentage of EL yield was 78, 71 and 68% after
the 1st, 2nd and 3rd cycle, respectively.
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A novel microwave assisted hydrothermal method was employed to
prepare Sn- exchanged STA supported on Ta2O5. 50 wt% of Sn2STA
loading on the Ta2O5 support not only resulted in high surface area and
porosity, but also showed optimum ratio of Brønsted to Lewis acid sites
due to the uniform distribution of the active Sn-exchanged heteropoly
acid over the support oxide. The resulting 50 wt% Sn2STA supported
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