Heterogeneous Oxidation of Sulfides by Hexamethylphosphoramide/Polyoxometalate Hybrid Catalysts

Abstract

In this research, � -H4PW12O40 heteropoly acid (HPAs) is used as building blocks for designation functional materials novel organic–inorganic hybrid polyoxometalate (POM), H4PW12O40 · nHMPA, with hexamethylphosphoramide (HMPA) as the organic ligand. Different host-guest systems are then designed based on the prepared hybride POM using mesoporous silicas with different textural properties as hosts. MCM-48 and SBA-15 mesoporous silicas are synthesized hydrothermally and SBA3 is produced via sol–gel method. MCM-48 and SBA-15 are then functionalized with aminopropyl functional groups via grafting method. The parent Keggin HPA is also intercalated within SBA-15, as a model for other supports, to perform closer and more efficient investigation. The mesostructured organic–inorganic hybrid materials are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmition Electron Microscopy (TEM), Atomic absorption, BET-BJH, and CHN elemental analysis. Effects of physical (texture) and chemical (functional groups) properties of the immobilizing supports on immobilization pattern and quality are then taken into consideration. The newly designed hybrid catalysts are investigated for heterogeneous oxidation of sulfides. Reaction conditions are optimized by finding the most proper temperature, solvent, and catalyst amount. An interpretation of the differences in the catalytic activity of the precursors is put forward and their catalytic activity is compared with their HPA counterparts. Furthermore, effects of support texture, pore structure, and functionalities on catalyst activity, stability, and reusability are clarified. Tests are also performed to assure the heterogeneous nature of the systems. Results reveal that the designed mesostructured hybrid POM catalysts can selectively oxidize sulfides in presence of hydrogen peroxide as oxidant. The catalysts are shown to be heterogeneous and reusable without significant loss of activity in the proceeding rounds. Such kind of materials can be actively used as environmentally benign, green, and selective heterogeneous catalysts for oxidation of sulfides.