Ionothermal Synthesis of Metal Oxide-Based Nanocatalysts and Their Application towards the Oxidative Desulfurization of Dibenzothiophene

Herein, different types of metal-containing ionic liquid (IL) complexes and various metal oxide-based nanocatalysts have been successfully prepared (from ionic liquids) and applied for the oxidative desulfurization (ODS) of dibenzothiophene (DBT). /e ILs complexes are comprised of N,N′-dialkylimidazolium salts of the type [RMIM-Cl]2[MCln], where [RMIM+]� 1 alkyl-3methylimidazolium and M�Mn(II)/Fe(II)/Ni(II)/Co(II). /ese complexes were prepared using an easy synthetic route by refluxing the methanolic solutions of imidazolium chloride and metal chlorides under facile conditions. /e as-prepared complexes were further used as precursors during the ionothermal and chemical synthesis of various metal oxide-based nanocatalysts./e resulting ILs salts andmetal oxides NPs have been characterized by FT-IR, TGA, XRD, SEM, and TEM analysis. /e results indicate that thermal and chemical treatment of ILs based precursor has produced different phases of metal oxide NPs. /e calcination produced α-Fe2O3, Mn3O4, and Co3O4, NPs, whereas the chemical treatment of the ILs salts have led to the production of Fe3O4, Mn2O3, and α-Co(OH)2. All the as-prepared salts andmetal oxide-based nanocatalysts were used as catalysts towards ODS of dibenzothiophene./e oxidation of dibenzothiophene was performed at atmospheric conditions using hydrogen peroxide as the oxygen donor. Among various catalysts, the thermally obtained metal oxide NPs such as α-Fe2O3, Mn3O4, and Co3O4, have demonstrated relatively superior catalytic activities compared to the other materials. For example, among these nanocatalysts, α-Fe2O3 has exhibited a maximum conversion (∼99%) of dibenzothiophene (DBT) to dibenzothiophene sulfone (DBTO2).


Introduction
e increasing demand for petroleum products has led to a significant enhancement in the environmental pollution, due to the emission of poisonous gases during combustion [1]. is has led to the formulations of stringent regulations for fuel specifications, which has tremendously increased the demand of deep desulfurization of transportation fuels. Particularly, the presence of sulfur-containing compounds, such as sulfides, disulfides, and thiophenes in transportation fuels, which typically produce SOx, is a major cause of air contamination [2]. To limit the emission of hazardous sulfur compounds, several countries have legislated stringent difficult to remove by HDS due to their steric hindrance [7,8].
ese compounds demand deep desulfurization, which is typically performed under the conditions of high temperature and pressure and also require highly active catalysts, leading to significant increase in process cost [9].
To overcome this, several deep desulfurization techniques have been developed, including extraction, oxidation, photooxidation, bioprocess, adsorption, and extraction by ionic liquids [10][11][12]. Among these techniques, oxidative desulfurization (ODS) effectively removes the aromatic sulfur compounds from fuels under mild condition and thus has gained significant prominence [13]. During the ODS, sulfur compounds are converted into their corresponding sulfones which are generally extracted using polar solvents. So far, several oxidants have been used during this process, such as, hydrogen peroxide (H 2 O 2 ), organic peroxides, molecular oxygen, and ozone [12,14]. Out of these oxidants, H 2 O 2 has been found to be more effective and produces only water as a side product [15]. e ODS can be performed by several methods which include solvent extraction, photocatalytic oxidation, microwave catalytic oxidation, and so on [16][17][18]. Apart from this, several other methods are being intensively studied to improve the current HDS technology by developing more effective catalysts and other materials used in this process [19].
In this regard, ionic liquid-(IL-) assisted ODS has also been used extensively due to the high efficiency of IL in the removal of sulfur compounds [20]. So far, ILs have demonstrated great potential in ODS due to their remarkable properties such as, good thermal stability, extremely low volatility, enhanced solubility, excellent ionic conductivity, and wide liquid temperature range [21]. Moreover, they can be used effectively both as catalysts and extractants (solvents) to replace volatile organic compounds which are flammable, hazardous, and are threat to the environment [22,23]. Currently, the process of catalytic ODS, in which ILs are applied as both homogeneous and/or heterogeneous catalysts have received greater attention when compared to the extraction of sulfur compounds with ILs [24]. Particularly, the trend of applying supported ILs as heterogeneous catalysts has become more popular, as being a solid catalyst it can offer greater advantage. For instance, IL-based solid catalysts demonstrate superior chemical properties such as, increased active sites and enhanced dispersion, and they are also easy to separate from the reaction mixture [25].
Recently, in several studies, metallic or metal oxide nanoparticles (NPs) together with ILs have been applied either as support or active catalyst for the catalytic conversion of sulfur compounds [26].
ese IL-based nanocatalysts exhibit both homogeneous and heterogeneous catalytic properties, which not only facilitate rapid and selective chemical transformations but also offer enhanced yield and easy separation and recovery of catalysts [27]. In several studies, ILs have demonstrated excellent potential for the synthesis of various inorganic metal and metal oxide NPs [28]. Particularly, the thermal synthesis of nanomaterials using ILs (ionothermal synthesis) has received considerable attention of researchers. However, the ionothermal synthesis of metallic or metal oxide NPs has been rarely studied. In our previous study, we have demonstrated the ionothermal synthesis of NiO NPs using N,N′-dialkylimidazolium salts of the type [RMIM-Cl] 2 [MCln], where [RMIM+] � 1-alkyl-3methylimidazolium and M � Ni(II) ionic liquid [29]. e asprepared IL was used a precursor, which was calcined at 500°C for several hours to produce Ni NPs. e study has revealed the significant effect of IL on the shape and morphology of resultant NPs Scheme 1.
For further continuation of our previous work, in this study, we demonstrate the preparation of transition metalcontaining IL-based complexes. e as-prepared complexes were used as precursors for the synthesis of different metal oxide NPs including Manganese (Mn), Iron (Fe), Cobalt (Co), and Nickel (Ni) NPs using thermal and chemical treatment methods. e IL-based precursors are made up imidazolium and N,N′-dialkylimidazolium salts of the type [RMIM-Cl] 2 [MCl n ], where [RMIM+] � 1-alkyl-3-methylimidazolium and M � Mn(II), Fe(II), Ni(II), and Co(II). e resultant complexes and metal oxide NPs have been characterized by powder X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Both the IL-based precursors and resultant NPs have been tested for their catalytic activity towards the oxidative desulfurization of dibenzothiophene (DBT). ), dibenzothiophene, and NaBH 4 were procured from Sigma-Aldrich. All chemicals and solvents were used without any further purification.

Preparation of [EMIM-Cl][MCl 2 ] Complexes.
[EMIM-Cl][MCl 2 ] complexes were prepared according to our previously reported method [29]. Briefly, equimolar quantities of metal salt (FeCl 3 ·4H 2 O, 0.009 mmol) and 1ethyl-3 methylimidazolium (0.009 mmol) were taken in 30 ml of methanol in a 250 mL three-neck flask. e mixture was stirred for several hours (4 hrs). After that, the reaction was stopped and cooled to room temperature. en the mixture was kept overnight in an acetone ice bath to obtain a dark blue liquid. Finally, the solid product was isolated on a rotary evaporator under reduced pressure and dried in an oven for 3 h at 120°C.

Chemical Treatment of [EMIM-Cl][MCl 2 ] Complexes.
e chemical treatment of as-prepared metal-containing ILbased complexes was performed as reported by our group earlier [29].

Calcination of [EMIM-Cl][MCl 2 ] Complexes.
e calcination of as-prepared metal-containing IL-based complexes such as [EMIM-Cl][MCl 2 ] was performed by a process reported by us earlier [29]. 2 Journal of Chemistry

Catalytic Oxidation of Dibenzothiophene (DBT).
e catalytic reactions were performed using a previously reported method by changing the catalyst [30]. A mixture of n-hexadecane and acetonitrile (50 mL each) was taken in 250 ml round bottom flask. To this mixture, small amount of catalyst (1 g·L −1 with respect to the solvent which is 100 ml) and DBT (2.932 g, with sulfur content of 500 ppm) was added, and the mixture was allowed to stir continuously. Subsequently, 50 mL of the H 2 O 2 /glacial acetic acid (1.5 M ratio, H 2 O 2 /DBT molar ratio 5 : 1) was slowly added in several steps to avoid the release of molecular oxygen. e reaction was continued for 60 min at atmospheric pressure. Samples were collected from the reaction mixture to analyze the oxidation of DBT and formation of DBTO 2 .

Characterization.
e thermal decomposition of the metal-containing ILs complexes ([EMIM-Cl][MCl 2 ]) was evaluated by simultaneous TGA/DTA analysis using the PerkinElmer ermogravimetric Analyzer 7 (PerkinElmer, Waltham, MA, USA). Fourier transform infrared spectroscopy (FT-IR) spectra were recorded as KBr pellets using a PerkinElmer 1000 FT-IR spectrophotometer (Perki-nElmer, Waltham, MA, USA). X-ray diffraction measurements were recorded using an Altima IV (Rigaku, Shibuyaku, Japan) X-ray diffractometer. SEM measurements were carried out using a JEOL SEM model JSM 6360A (JEOL Ltd., Akishima-shi, Japan), whereas, TEM analyses were performed on a JEOLTEM model JEM-1101 (JEOL Ltd., Akishima-shi, Japan). e conversion of the catalytic products was analyzed using GC, 7890A, Agilent Technologies Inc., equipped with a flame ionization detector (FID) and a 19019S-001 HP-PONA column. ] salts were determined in order to confirm the upper temperature limit (cf. Figure 1). For this purpose, the samples were heated over a temperature range of 25-800°C at a heating rate of 10°C·min −1 . Measurements demonstrated that all the samples melt around between 40 to 60°C and decomposed in several steps to form the corresponding metals. ] displayed similar prominent peaks above 3000 cm −1 , which corresponds to the different vibrational modes of the imidazolium ring. e two sharp and intense bands between 3100 and 3155 cm −1 correspond to antisymmetric and symmetric stretching vibrational modes of the C-H stretching of imidazolium ring, whereas the small band at the shoulder (∼2985 cm −1 ) corresponds to the aliphatic asymmetric (C-H) stretching due to the alkyl groups [31]. e broad peak above 3400 cm −1 in all the spectra indicates the formation of quaternary amine salt formation with chlorine Although all the ionic liquids complexes have similar spectra above 3000 cm −1 , however, they display slightly different pattern in the region between 500 to 2000 cm −1 due to the presence of different metallic species. For example, the [EMIM-Cl][FeCl 2 ] gave a characteristic peak at 1105 cm −1 , which corresponds to FeO 2 species, while the absorption at 622 cm −1 can be attributed to the coupling mode between Mn-O stretching modes in MnO 2 as observed in the FT-IR spectrum of [EMIM-Cl][MnCl 2 ] [32].

Results and Discussion
Once the formation of metal-containing IL complexes was confirmed, they were used as precursors for the preparation of respective metal NPs via two different methods, including a chemical and thermal decomposition method. In general, the chemical synthesis was performed in a methanolic solution containing IL complex and NaBH 4 . Stirring of mixture under aerobic conditions produced dark precipitates which were isolated by filtration. On the other hand, the thermal decomposition of the complexes was performed by calcination at 600°C for several hours at the rate of 10°C·min −1 , in static air. Notably, the complexes have produced different phases of NPs (which were identified by XRD) depending upon the method of decomposition. Typically, under chemical treatment methods, in the presence of reducing agents the initial reduction of metal cations occurs through a number of redox reactions. When the reductant is introduced into the aqueous solution of metal precursors, electrons are donated to the metal ions through a series of redox reactions, which is followed by the nucleation and growth of the metal atoms to form nanoparticles. In some cases, stable metal nanoparticles are formed. However, in many cases, when the metal NPs are not stable under certain circumstances, subsequent oxidation occurs to produce metal oxides nanoparticles. For instance, e.g., in our previous study, we have demonstrated that the chemical treatment of [EMIM-Cl][NiCl 2 ] produces stable Ni NPs in the hcp phase, whereas the thermal decomposition at high temperature has generated fcc phase of NiO NPs [29]. agreement with an earlier study, in which the chemical treatment of ferric chloride hexa-hydrate (FeCl 3 ·6H 2 O) with NaBH 4 has produced Fe 3 O 4 nanoparticles at ambient temperature [33]. As shown in Figure [37]. On the other hand, the chemically reduced sample has produced a mix phase of cobalt oxide NPs including alpha cobalt hydroxide (α-Co(OH) 2 belonging to JCPDS file no. 02-0925) and cubic phase of Co 3 O 4 [38].

TEM, SEM, and EDX Analysis.
e formation of metal oxide NPs via thermal and chemical decomposition is further confirmed by EDX analysis (data not show here). e EDX of all the samples obtained from both chemical and thermal methods indicated the formation of different types of metal oxide nanoparticles. e presence of oxygen and respective transition metals such as Fe, Mn, and Co in each sample confirms the formation of metal oxide NPs. Furthermore, morphology of as-prepared metal oxide NPs is determined by both SEM and TEM. Figure 6 shows the SEM images of all the metal oxide NPs obtained from both thermal and chemical treatments. e SEM images show cubic, spherical, and prismatic morphologies for Fe 3  Furthermore, the morphology of metal oxide nanoparticles obtained from the thermal and chemical treatment is determined by high-resolution TEM. e TEM images of α-Fe 2 O 3 , Mn 3 O 4 , and Co 3 O 4 obtained from calcination mainly exhibit smaller size spherical nanoparticles (5-25 nm) with slight agglomeration, while the samples obtained from chemical treatment such as, Fe 3 O 4 , Mn 2 O 3 , and α-Co(OH) 2 show agglomerated, irregular-shaped, largesize nanoparticles (cf. Figure 7).

Catalytic Activity of As-Prepared Metal Oxide
Nanoparticles. As-prepared metal oxide nanocatalysts obtained from both thermal and chemical methods were tested for their catalytic efficiency towards the oxidation of dibenzothiophene. Metal oxides belong to an important class of solid catalysts, which have been extensively applied in heterogeneous catalysis due to their unique outer electron configuration and excellent acid-base and redox properties [39]. ese materials are typically applied as active phase or as support to the existing catalysts. Most of the single component metal oxides crystallize with an isotropic      [40]. Particularly, the transition elements based metal oxides have been widely used due to their low cost of production, easy regeneration, and selective action for various organic transformations including oxidation [41]. Metal oxides have also played an active role in the catalytic oxidation of DBT.     ese dismal catalytic activities of chemically prepared nanocatalysts can be possibly attributed to the preparation conditions of the catalyst, as the chemical preparation of catalysts have produced different phases of metal oxide nanoparticles, when compared to the thermally produced methods. Moreover, the products obtained do not consist of single phases but were contaminated with other phases of metal oxides. Among chemically produced nanocatalysts, only the Ni-based catalyst has demonstrated excellent catalytic activity and has produced high conversion of DBT (∼95.5%), since, exceptionally, only the chemical treatment of Ni containing IL complex ([EMIM-Cl][NiCl 2 ]) has led to the production of Pure Ni NPs, as described in our earlier study [29]. Apart from this, all the as-obtained metal-containing IL complexes such as [

Conclusion
In summary, different types of metal-containing IL complexes and metal oxide-based nanocatalysts were successfully prepared and applied for the oxidation of DBT. e ILbased complexes were applied as precursors during the thermal and chemical synthesis of metal oxide based nanocatalysts. e calcination of the IL-based metal salts has produced high-quality, single-phase metal oxide NPs, whereas mixed phases of aggregated metal oxides NPs were obtained by the chemical treatment. Among all the asprepared materials, including IL-based complexes and metal oxide NPs, the thermally obtained metal oxides such as Data Availability e data for all results are included in the manuscript.

Conflicts of Interest
e authors declare no conflicts of interest.