Abstract
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). The ILs complexes are comprised of N,N′-dialkylimidazolium salts of the type [RMIM-Cl]2[MCln], where [RMIM+] = 1 alkyl-3-methylimidazolium and M = Mn(II)/Fe(II)/Ni(II)/Co(II). These complexes were prepared using an easy synthetic route by refluxing the methanolic solutions of imidazolium chloride and metal chlorides under facile conditions. The as-prepared complexes were further used as precursors during the ionothermal and chemical synthesis of various metal oxide-based nanocatalysts. The resulting ILs salts and metal oxides NPs have been characterized by FT-IR, TGA, XRD, SEM, and TEM analysis. The results indicate that thermal and chemical treatment of ILs based precursor has produced different phases of metal oxide NPs. The 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 and metal oxide-based nanocatalysts were used as catalysts towards ODS of dibenzothiophene. The 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).
Highlights
Several aromatic sulfur compounds, like benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyl dibenzothiophene (4,6-DMDBT), which are usually present in large proportion in diesel are Journal of Chemistry 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 require highly active catalysts, leading to significant increase in process cost [9]
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 ionic liquid- (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[MCln], 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)
Summary
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 been used extensively due to the high efficiency of IL in the removal of sulfur compounds [20]. 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] 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]. IL-based solid catalysts demonstrate superior chemical properties such as, increased active sites and enhanced dispersion, and they are easy to separate from the reaction mixture [25]
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