Deep eutectic solvent‐assisted synthesis of highly efficient nanocatalyst (n‐TiO₂@TDI@DES (ZnCl₂:urea)) for chemoselective oxidation of sulfides to sulfoxides

Abstract

This study proposed a straightforward process to synthesize 2,4-toluene diisocyanate (TDI)-functionalized TiO 2 nanoparticles in which a cost-effective linker (TDI) with high reactivity was employed to couple nano-TiO 2 through covalent bonding to a deep eutectic solvent (DES). By this method, DES was successfully immobilized on the TiO 2 @TDI surface as an adsorbent and stabilizer. The structural, morphological, and physicochemical characteristics of the synthesized nanocatalysts were evaluated using various analytical methods including Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDX), and elemental analysis. The heterogeneity of the catalyst was also examined by a hot filtration test. The obtained TiO 2 @TDI@DES nanoparticles offered superior catalytic behavior and excellent yield as well as recyclability for the chemoselective oxidation of sulfide into sulfoxide using a green oxidant (hydrogen peroxide). This catalyst exhibited excellent reusability as it can be recovered for six successive cycles with no significant leach or reduction of catalytic efficiency.

Recent Advances in the Synthesis of Inorganic Materials Using Environmentally Friendly Media

“…These DES include chloride salts, which have been employed in three different preparations of titania-based nanomaterials: the nano-photocatalyst n-TiO 2 -P25@TDI@DES, which is highly recyclable and selective, was obtained by combining TiO 2 -P25 powder (70% anatase, 30% rutile) with a ZnCl 2 :urea 1:4 mixture using 2,4-toluene diisocyanate (TDI) as a bifunctional covalent linker. The nanocatalyst was employed successfully in the oxidation of benzyl alcohols to aldehydes and sulfides to sulfoxides [ 132 , 133 ]; the synergy between LADES and NPs was also exploited in the coupling between “Hierarchical TiO 2 ” (H-TiO 2 ) microspheres and FeCl 2 /CuCl 2 urea in 1:4 mixtures to increase the reaction yields in the preparation of pyrrolidyn-2-one heterocycles [ 134 ] and by grafting the ZnCl 2 :urea 1:4 mixture onto magnetic ferrite nanoparticles to generate a DES@MNP homogeneous catalyst (which was found to be “magnetically recyclable” at the end of one-pot multicomponent syntheses [ 135 ]). Coming back to the synthesis of the nanoparticles themselves, some lanthanide-based type IV DES (Ln-DES) containing hydrated nitrates were prepared [ 136 ].…”

Novel Synthesis of Zinc Oxide Nanoparticles from Type IV Deep Eutectic Solvents

“…By using LADES, the metal source necessary for the synthesis is introduced in the system directly as a component of the DES mixture, at variance with the more common procedures, where the metal salt is added to a previously formed liquid system, as it occurs, for instance, when metal oxides are solubilized in the protypical DES reline (the mixture choline chloride:urea at 1:2 M ratio, a type III DES) to perform, for instance, the electrodeposition of the metal or oxide [40][41][42], to synthesize composite systems [43][44][45] or directly nanoparticles [46][47][48]. Among the few examples of LADES reported, the majority of the studies deal with chloride salts, like ChCl:2ZnCl 2 , ChCl:2CrCl 3 •6H 2 O and ZnCl 2 :urea melts [49][50][51]. The last type of mixtures have been reported to produce homogeneous and transparent liquid phases in a wide range of ZnCl 2 :urea compositions (ranging from 2.5 to 4.5:10) and temperatures, and have glass to liquid transition temperatures (measured by DSC experiments) as low as −20 °C [52], though in Abbott et al work dedicated to metal-containing eutectics the lowest freezing temperature (-9°C) was observed for 1:3.5 mixtures [53].…”

KuQuinone as a Highly Stable and Reusable Organic Photocatalyst in Selective Oxidation of Thioethers to Sulfoxides

“…More recently, the use of green oxidants, i.e., H 2 O 2 or O 2 , as easily available and safe reagents has been preferred. Thus, the oxidation of thioethers with H 2 O 2 , in the presence of metal catalysts, − metal-free supramolecular catalysts, biocatalysts, and deep eutectic solvents , has been proposed. Analogously, promising results have been achieved in thioether oxidation with O 2 promoted by suitable photocatalysts that, after excitation, are able to activate molecular oxygen through an energy transfer or an electron transfer mechanism, respectively forming singlet oxygen or superoxide anion as oxidizing species.…”