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A. Afzalinia et al. / Ultrasonics Sonochemistry 34 (2017) 713–720
Heteropoly acids as known a unique class of metal-oxide
2. Experimental
clusters, have many properties that make them candidates for
environmentally catalyst in green chemistry filed. Keggin-type
heteropoly acids have been extensively employed as the oxidative
desulfurization catalysts because of their high active and
selective indicated in ODS but these materials have low surface
area (1–10 m2/g) and high solubility in presence of solvents
[14,15]. In order to overcoming these disadvantages, heteropoly
acids immobilized or encapsulated in various porous materials
such as silica, activated carbon, and metal organic frameworks
(MOFs) [16–18].
MOFs are new class of nanoporous inorganic–organic hybrid
materials that as well as extensively potential applications such
as catalysis, separation, gas storage, carbon dioxide capture. Large
surface area, high porosity, and flexible pore size properties lead to
introduce new class of materials as a heterogeneous catalyst or a
catalyst carrier for chemical reactions [19]. When MOFs apply as
a carrier some functional groups such as amine on framework
surfaces can be help to immobilization the catalyst in pores.
Recently, Wang et al. employed the phosphotungstic acid that
encapsulated in the mesocages of amine-functionalized metal–or-
ganic frameworks (MIL-101(Cr)-NH2) as a catalyst for oxidative
desulfurization performance [20]. In 2013, Ribeiro et al. reported
an efficient heterogeneous catalyst for oxidative desulfurization
of model fuel [21]. Wan et al. reported application of a transition
metal complex and quaternary ammonium salts (QAS) phase
transfer agents with aqueous hydrogen peroxide as oxidant in
the UAOD process [22].
In this work for first time, TMU-17-NH2 that is a mixed-ligand
twofold interpenetrated metal-organic framework was prepared
with ultrasound irradiation. MOFs were traditionally prepared
by solvothermal processes which this method suffer from draw-
backs such high temperature using conventional electric heating.
The ultrasound technologies are quicker, more efficient and
greener alternatives to traditional synthesis methods. We were
also reported the encapsulation of phosphotungstic acid (PTA)
in TMU-17-NH2 that is an efficient heterogeneous catalyst for
successful ultrasound-assisted oxidative desulfurization of three
refractory sulfur-containing compounds (benzothiophene, BT;
dibenzothiophene, DBT; and 4,6-dimethyldibenzothiophene,
4,6-DMDBT) using H2O2 as the oxidant. Schematic representation
of the UAOD system is shown in Scheme 1.
2.1. Materials and method
In this study, all of the reagents used were analytical grade and
no further purification required. Elemental analyses (carbon,
hydrogen, and nitrogen) were carried out using an ECS4010 CHNSO
made in Costech. Italy. The powder X-ray diffraction patterns
(PXRD) were obtained by using a Philips X’pert diffractometer with
monochromated Cu-k radiation (k = 1.54056 Å). The Fourier
a
transform infrared spectra (FT-IR) were recorded by a Perkin-
Elmer system 2000 FT-IR spectrometer using the KBr disk tech-
nique at room temperature. Thermogravimetric analyses (TGA) of
the materials catalyst was obtained by a TGA-50 Shimadzu
thermo-balance. The gas chromatography-mass spectrometer
(GC–MS) (Agilent 7890/5975C-GC/MSD; HP-5 137 MS column,
30 m ꢀ 250
lm i.d ꢀ 0.25
lm) used to characterize the oxidized
sulfur-containing compounds. An Optima 8000 ICP-OES spectrom-
eter was used to determination of heteropoly acid amount encap-
sulated in MOF. Ultrasound waves generated by an ultrasound bath
(Sonic 6mx, 37 kHz with a maximum power output of 240 W,
Polsonic, Warsaw, Poland. S-4800 field-emission scanning electron
microscope (Hitachi, Japan) was employed to SEM images. Nitrogen
adsorption isotherms that performed with a Micromeritics ASAP
2000 over P/P0 = 0.0–1.0 .In order to monitoring the product of
desulfurization process, gas chromatography-flame photometric
detector (GC-FPD) was used. (Agilent 6890N equipped with a
capillary column (PONA, 50 m ꢀ 0.2 mm, id ꢀ 0.5 mm) and flame
photometric detector (FPD): Agilent H9261).
2.2. Preparation of PTA@TMU-17-NH2
Safarifard et al. solvothermally synthesized The TMU-17-NH2
that reported in the previous paper [23]. TMU-17-NH2 is an
amino-functionalized Zn-based MOF that has been synthesized
by using 1,4-bis(4-pyridyl)-2,3-diaza-2,3-butadiene (4-bpdb), and
linear 2-aminoterephthalic acid (NH2-BDC) as organic linker.
4-bpdb was prepared with same procedure that reported by
Ciurtin et al. [24]. In this work we prepared TMU-17-NH2 and
PTA/TMU-17-NH2 composite by ultrasound irradiation for first
time. In a typical process, 0.297 g (1 mmol) of Zn(NO3)2ꢁ6H2O,
0.210 g (1 mmol) of 4-bpdb, 0.181 g (1 mmol) of NH2-BDC were
dissolved in 15 ml of dimethylformamide (DMF). Then the reaction
flask transferred into ultrasonic bath in ambient temperature. The
mixture was exposed to ultrasound waves with 37 kHz frequency
and 240 W output power a predetermined time (i.e. 5 min, 10 or
15 min). The obtained material was filtered and washed with
DMF and then dried overnight at 60 °C under vacuum. The
PTA@TMU-17-NH2 was performed with same method in presence
of various amount of PTA (0.30, 0.60, 0.90 and 1.25 g). The proposed
strategy for synthesis of TMU-17-NH2 and PTA@TMU-17-NH2 was
indicated in Scheme 2.
2.3. Ultrasound-assisted oxidative desulfurization process (UAOD)
The UAOD generally carry out in biphasic system that consists
of a polar solvent and model fuel. The UAOD studies were
performed with model oil, with refractory sulfur compounds
commonly found in fuels (BT, DBT or 4,6-DMDBT), was prepared
by dissolving in n-octane (500 mg.Lꢂ1 for each compound). The
UAOD reactions were carried out in a biphasic medium that formed
by the model fuel and different polar solvent such as water,
acetonitrile (MeCN), isopropanol and dimethylformamide (DMF).
In a generic experiment, 15 mg of PTA@TMU-17-NH2 (containing
20 wt% of PTA) was placed in the vessel then a mixture of MeCN
Scheme 1. Schematic representation of the UAOD system.