N. Pasha et al.
acid (LA) has been under special concern, since LA is one
of the top biomass-derived platform molecules that can be
2 Experimental
synthesised by the acid treatment of C -membered sugar
2.1 Preparation of Zirconium Salt
of Phosphotungstic Acid
6
carbohydrates from renewable biomass for instance starch
or lignocelluloses [8]. Conventionally, levulinate esters
were prepared by esteriꢀcation of LA using strong Brøn-
sted liquid acids such as H SO , HCl, polyphosphoric and
The Zirconium-exchanged phosphotungstic acid catalyst
2
4
series (Zr TPA) was synthesized as per reported literature
x
p-toluenesulfonic acid. These homogeneous catalyst exhibit
high catalytic activity and selectivity to the esteriꢀcation
of LA because of strong Brønsted acidity. However, these
are not suitable as they are soluble in the reaction medium.
Esteriꢀcation process by homogeneous mineral acid cata-
lysts serves problems from corrosion and safety. Substitute
of these homogeneous catalysts with the heterogeneous ones
is tremendously desirable [9]. Design of stable solid acid
catalysts is desirable to improve the ability of levulinate ester
production.
[28]. In a typical procedure, calculated amount of TPA
(M/s. Sigma Aldrich, USA) dissolved in distilled water
and to this solution the required amount of ZrOCl·8H O
2
(M/s. Sigma Aldrich, USA) dispersed in smallest amount
of water was added with vigorous stirring at room tem-
perature. The resulting precipitate was stirred for 2 h and
the excess water was evaporated on a hot plate at low
temperature. The obtained samples was hot air oven dried
at 100 °C for overnight and calcined at 300 °C for 2 h.
The samples were denoted as Zr TPA, where x designates
x
Heteropolyacid HPAs for example Keggin heteropoly-
tungstate are environmentally friendly and economically
viable substitute to conventional acid catalysts since their
strong Brønsted acidity, stability, and they exhibit tremen-
dous catalytic behaviour in acid and oxidation catalysis [10,
the number of Zirconium ions (x = 0.25, 0.5 and 0.75,
respectively).
Powder X-ray diꢃraction patterns of the samples were
analyzed by using a Ultima-IV (M/s. Rigaku Corporation,
Japan) XRD item operated at 40 kV and 40 mA provided
with nickel-ꢀltered Cu Kα(λ = 1.54056 Å) radiation.
1
1]. However, the easy dissolving nature in polar solvents,
less thermally stability and small surface area control its
application of solid acid catalyst. These disadvantages can
be overcome by supporting them on appropriate supports
The Temperature-programmed desorption (TPD) of NH
3
was measured on BELCAT-II (Japan) instrument. In a typi-
cal procedure, 0.05 g of dry sample loaded and pre-treated
at 300 °C for 1 h in presence of pure He gas (99.9%, 30 mL
[
12–14] or exchanging the protons of HPAs with metal
4
+
cations [15–17]. It is anticipated that Zr can be used to
substitute the protons of TPA to inꢁuence the catalytic prop-
erties related with Zr along with TPA. Shmizu et al. [18]
per min). After pre-treatment, ammonia (10% NH in He
3
gas) was adsorbed on the catalyst surface for 1 h. For remov-
ing physisorbed ammonia, sample was ꢁushed with pure He
gas. Finally, the temperature programmed was performed
from 100 to 800 °C with a temperature ramping of 10 °C/
+
+
3+
3+
4+
4+
4+
4+
prepared Ag , Cs , Al , Y , Ti , Zr , Hf , Sn salts of
tungstophosphoric acid and catalytic ability was examined
for alkylation of toluene with cyclohexene. We believe that,
detailed study has not been reported on zirconium exchanged
tungstophosphoric acid for the ethyl levulate synthesis from
min. Desorbed NH was monitored by using TCD (thermal
3
conductivity detector) of the apparatus.
Surface area (BET) was determined from the N adsorp-
2
LA. Zirconia (ZrO ) has received extensive attention due
tion–desorption method at liquid Nitrogen temperature
using BEL Sorb-II apparatus, Japan. Before the analyses
the samples were degassed at 200 °C for 2 h.
2
to it has semiconducting nature, thermally stable, anti cor-
rosion property, reducing nature and acid–base catalytic
activity [19, 20]. ZrO alone possess both Brønsted and
Fourier transform IR spectra were recorded on DIGILAB
(USA) IR spectrometer. For calculating acidic sites (Brøn-
sted and Lewis) of the catalysts, pyridine adsorbed FT-IR
spectra was adapted. Diꢃuse reꢁectance infrared Fourier
Transform (DRIFT) method was used for recording the spec-
tra. Before adsorption of pyridine the sample was degassed
in the vacuum at 200 °C for 2 h. After that, dry pyridine was
suspended into the sample. Then, the sample was heated
at 120 °C for 1 h. FT-IR spectra of the pyridine-adsorbed
samples were recorded at room temperature.
2
Lewis acidity [21, 22]. The modiꢀcation of Keggin units
by exchange of protons with Zr would enhance the acidic
features through strong W–O–Zr covalent bindings between
H PW O clusters and the ZrO [23–27]. Meanwhile, the
3
12 40
2
strong relation between H PW O and ZrO can eꢂciently
3
12 40
2
inhibit H PW O leaching. The objective of the work is
3
12 40
to exchange the phosphotungstic acid proton with varying
content of Zr and characterize by X-ray diꢃraction, BET
surface area, FT-IR, NH -TPD and pyridine FT-IR. The
3
catalytic performance of prepared catalysts was evaluated
for the conversion of LA to Ethyl levulinate. The inꢁuence
of diꢃerent reaction parameters such as weight of catalyst,
eꢃect of reaction temperature, eꢃect of reaction time and
solvent molar ratio were also optimized.
Raman spectra were recorded using a Horiba Jobin–Yvon
Lab Ram HR spectrometer with an excitation wavelength
of 632.8 nm. The powdered samples were spread on a glass
plate below the microscope for measurements.
1
3