M.J. Da Silva et al.
Molecular Catalysis 504 (2021) 111495
Alternatively, the protons exchange by cations with large radium
converting the HPAs to insoluble salts, making them attractive catalysts
to be used in acid-catalyzed reactions [27,28]. Tin heteropoly salts have
been active catalysts in various acid-catalyzed reactions, such as ben-
zylation, esterification and etherification of glycerol, esterification of
fatty acids, transesterification, and hydrolysis of vegetable oils [29–31].
Tin(II) silicotungstate was a highly effective catalyst in the one-pot
synthesis of alkyl levulinates from biomass carbohydrates [32]. How-
ever, these reactions required a high catalyst load (ca. 10 mol %) and
temperatures of 433 K, to achieve a maximum yield of 75 % starting
from fructose.
/ 1 h and kept under flow of N2 for the removal from an eventual
physiosorbed pyridine amount. FTIR Spectra were recorded in a Spec-
trum PerkinElmer FT-IR RXI equipment in the range of 1700ꢀ 1400
cmꢀ 1 (resolution of 2 cm-1 and 64 scans) and the discs were previously
prepared by mixing 10 mg of the catalyst with 80 mg of KBr and pressing
under vacuum for 3 min.
2.4. Catalytic runs
The catalytic reactions were carried in a glass reactor (50 mL) fitted
with a reflux condenser and sampling septum. The solutions were
magnetically stirred and heated in an oil bath. Typically, the solid acid
catalyst (ca. 0.6 mol %) was added to the solution of the alkyl alcohol
(ca. 144 mmol) and heated at the reaction temperature. Afterward, the
LA (ca. 8 mmol) was added, and the reaction was started, being moni-
tored by GC analyses of the samples collected at regular time intervals
(Shimadzu 2014 plus GC, FID, CP-WAX capillary chromatographic col-
In this work, the three main Keggin HPAs (i.e., H3PW12O40
,
H3PMo12O40, and H4SiW12O40) were converted to their Sn(II) heter-
opoly salts, which were spectroscopically characterized and evaluated
as catalysts in esterification reactions of the levulinic acid with alkyl
alcohols, providing alkyl levulinates and angelica lactone as the main
products. The impact of the main reaction variables (i.e., load catalyst,
temperature, nature of alcohol, the stoichiometry of the reactants) was
investigated. The reusability of the most active catalyst (i.e.,
Sn1.5PW12O40) was demonstrated.
umn (25 m ×0.32 mm x0.30 μm).
Dodecane was internal standard (ca. 0.1 mL). The plotting of GC peak
areas of the substrate and main products in their calibrating curves
allowed calculating the conversion and the checking the reaction mass
balance.
2. Material and methods
2.1. Chemicals
2.5. Catalyst recovery
All the chemicals and solvents were used without previous treat-
ment. Hydrated heteropolyacids (i.e., H3PW12O40, H3PMo12O40, and
H4SiW12O40; 99 wt. %) were purchased from Sigma-Aldrich. Levulinic
acid (97 wt. %), alkyl alcohols (i.e. methyl, ethyl, propyl, isopropyl and
butyl) having purity between equal or higher than 99.5 wt. %, were
acquired from Sigma-Aldrich. Sulfuric and p-toluene sulfonic acids (98
wt. %) were purchased from Vetec Química, Duque de Caxias, RJ, Brazil.
The catalyst was easily recovered by a liquid-liquid extraction pro-
cess. After to remove the excess of solvent, water was added to the
remaining liquid layer, and it was washed three times with ethyl acetate,
extracting the products and leaving the catalyst in the aqueous phase.
The water was evaporated under vacuum, providing the solid catalyst,
which was dried, weighted and reused in another catalytic run.
2.6. Products chromatographic identification
2.2. Synthesis of the Sn(II) heteropoly salt catalysts
The main reaction products were identified by GC/ MS analyses
(Shimadzu MS-QP 2010 ultra mass spectrometer instrument operating at
70 eV coupled Shimadzu 2010 GC), and co-injection with authentic
samples previously synthesized [15,19].
All the catalysts were synthesized in according to the literature [28,
30–32]. Typically, an alcoholic solution (ca. 5 mL) containing a stoi-
chiometric amount of the precursor (i.e., SnCl2) was gently dropped to
the solution of the adequate Keggin HPA (ca. 20 mL). This suspension
was stirred 3 h at 333 K temperature, evaporated at 383 K, and then
calcined at 573 K/ 5 h.
3. Results and discussion
3.1. Catalysts characterization
2.3. Characterization of the Sn(II) heteropoly salt catalysts
Fig. 1 shows the XRD patterns obtained from precursors (H3PW12O40
and SnCl2) and the Sn(II) heteropoly salt (i.e., Sn1.5PW12O40); the
characteristics peaks of the Keggin anions were highlighted [28,33,34].
The amount and the strength of the acid sites present in the Sn(II)
heteropoly salts were determined by potentiometric titration (BEL
potentiometer, model W3B, with glass electrode) with n-butylamine
solution (ca. 0.10 molLꢀ 1) [29]. To do it, the Sn(II) heteropoly salt (ca.
50 mg) was suspended in CH3CN (ca. 30 mL), and magnetically stirred
by 3 h. Posteriorly, the sample was titrated with a slow addition of
portions of n-butylamine until that electrode potential remain stable.
Powdered XRD patterns were recorded in a Bruker D8 Discovery,
with Cu radiation and Ni filter, 40 kV and 40 mA. The 2θ angle was
varied from 5 to 80 degrees (ca. 1◦ minꢀ 1). The surface properties of the
solid catalysts were determined by adsorption/ desorption isotherms of
nitrogen in a NOVA 1200 Quantachrome equipment. FT-IR spectra were
recorded in a Varian-660 with ATR accessory. Thermal analyses were
performed in a Simultaneous Thermal Analyzer (STA) 6000 da Perkin
Elmer, under nitrogen atmosphere, from 303 to 973 K.
While the SnCl2 belong to the monoclinic spatial group (P121; ICSD
–
31762), the crystalline structure of C3[PM12O40] (C = cation; M W or
–
Mo), can be described by a spherical approximation of heteropolyanion,
which is packed in a body-centred cubic (bcc) cell. The cations are placed
at the centre of each plane and edge of the unitary cell, giving 1:3 of
anion to cation proportion of 1:3 [33]. In a bcc cell, the {110} plane is
the most densely packed and the most stable [34]. On the other hand,
when comparing the XRD patterns of HPAs with those generated after
the protons exchange by large radium cations such as Sn(II), we realize
that the diffractogram has new diffraction lines [26,35] (Figs. 2 and 3).
The diffraction patterns of the peaks observed in XRD of Sn(II)
phosphomolybdate were enlarged, suggesting that there was a decrease
on the crystallite size and consequently in the crystallinity level of the
material. It can have been provoked by a distortion in the crystalline
network of the solid, which may amplify the diffraction peaks and
reduce the degree of crystallinity [28]. It is more common in molyb-
denum heteropoly salts.
Pyridine adsorbed FT-IR spectroscopy was used for the determina-
tion of the nature of acid sites. Typically, 10 mg of catalyst was weighed
in a small cup and placed into a quartz tube inside a tubular oven, which
was heated to 120 ◦C/ 2 h under a continuous flow of N2 for gas out the
samples. So, the oven temperature was reduced to 50 ◦C under a
continuous flow of pyridine/ 2 h for the chemisorption step. Once
completed the adsorption process, temperature was increased to 120 ◦C
Although less intense than those belonging to the precursors, the
diffraction lines present in the diffractogram of Sn(II) silicotungstate
were very similar to the parent acid [31].
2