ZnO–TiO2 Nanoparticles Coated by the Dioxomolybdenum (VI) bis-Schiff Base Complex for Catalytic Oxidation of Sulfides

Abstract

The oxygenation processes for the organic compounds have great applications in fine organic fabrications. To speed up progress in the industrialization of such oxidation systems, a monometallic dioxomolybdenum (VI) bis-Schiff base complex (MoO2L2) was synthesized through coordination of easily accessible 4-hydroxyiminophenol ligand (HL) with cis-MoO22+ ion in an octahedral geometry. The structural conformation of MoO2L2 and its coordinated ligand (HL) was determined using modern spectrophotometric tools. A heterogeneous catalyst (MoO2L2@ZnO–TiO2) was successfully prepared by immobilization of MoO2L2 on the surface of ZnO–TiO2 nanostructured particles through the deprotonation of 4-OH group in its coordinated ligand. Various analytical techniques were used to examine the surface morphology and internal structure features of ZnO–TiO2 and MoO2L2@ZnO–TiO2 nanocomposites. To study the catalytic efficiency of MoO2L2 and MoO2L2@ZnO–TiO2-based catalysts, they were employed in the oxygenation process of Ph2S (diphenyl sulfide) and MePhS (methylphenyl sulfide) using the most environmentally friendly oxidant (an aqueous H2O2) under an aerobic environment. A high degree of selectivity was observed for the productivity of diphenyl sulfoxide (Ph2SO) and methylphenyl sulfoxide (MePhSO), with some contamination of overoxidation side products (Ph2SO2 and MePhSO2). The catalytic oxidative ability was not observably superior for the heterogeneous coated catalyst by ZnO–TiO2 over its homogeneous one (MoO2L2) due to the less redox activity of ZnO and TiO2 nanoparticles in the catalytic cycles for the oxygen-transfer mechanism. Both catalysts were successfully recycled, in which the homogeneous catalyst sustained its high catalytic potential for up to three cycles, while the heterogeneous catalyst was able to be reused up to six times.