A research team has successfully synthesized a novel polyoxometalate-based metal-organic complex (POMOC) and evaluated its catalytic performance in the oxidation reactions of various sulfides. The team, led by Zhong Zhang from the College of Chemistry and Materials Engineering at Bohai University, published their findings in the journal Polyoxometalates.
Selective oxidation of sulfides to sulfoxides has been extensively studied across different scientific disciplines due to the high value-added nature of sulfoxides in pharmaceuticals, agrochemistry, dyes, and as chiral auxiliaries. The conventional method of producing sulfoxides involves catalytic oxidation reactions of sulfides using homogeneous or heterogeneous catalysts and environmentally friendly oxidants under mild conditions. Researchers have been seeking to develop effective heterogeneous catalysts with excellent activity and easy separation for these oxidation reactions.
Polyoxometalates (POMs) are a class of nanomaterials known for their high negative charge, remarkable redox capabilities, and organic grafting potential. These unique properties have garnered significant interest in applications related to energy, catalysis, medicine, environmental protection, and hydrogen production. POM-based metal-organic complexes (POMOCs) have emerged as a promising branch of POMs, combining the outstanding features of POMs and metal-organic complexes while improving leaching, dissolution, and aggregation. POMOCs have found applications in various fields, with a particular focus on catalysis.
Despite the growing research in POMOCs, there are limited reports on POM-based metal-organic complexes with three-dimensional frameworks based on POM units and metal-organic complex bridges. The scarcity of such complexes prompted the research team to delve into this unexplored area. Zhong Zhang highlights the challenge of designing and synthesizing new POMOCs with three-dimensional frameworks based on POMs and copper-organic complexes, given the intriguing architectures, topologies, and applications of copper-organic complexes.
The team successfully obtained and characterized a copper-incorporated POMOC named CuW-EDDP, which belongs to the Keggin-type POMOCs. The Keggin-type refers to a specific structural form of POMs, alongside the Dawson-type. The CuW-EDDP was synthesized under hydrothermal conditions, involving three days of heating in a stainless steel reactor followed by cooling and drying. The synthesized complex was analyzed using infrared radiation spectra, elemental analysis, and thermogravimetry, revealing its distinctive three-dimensional structure. The team discovered that CuW-EDDP can serve as an effective heterogeneous catalyst for the selective oxidation of various sulfides.
Earlier studies have demonstrated the excellent catalytic capability of POMOCs containing copper, particularly in the selective oxidation of sulfides. POMs and copper centers act as catalytic active sites, promoting the oxidation process. The abundance of copper-active sites in CuW-EDDP enhances its efficiency as a catalyst.
To optimize the selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide, the team investigated various reaction conditions including temperature, CuW-EDDP dosages, tert-butyl hydroperoxide amounts, and solvent types. Control experiments were conducted to assess the catalytic activity of CuW-EDDP in the oxidation of other sulfide derivatives.
CuW-EDDP demonstrated exceptional catalytic performance as a heterogeneous catalyst in the selective oxidation of methyl phenyl sulfide, exhibiting high conversion (100%) and selectivity (98%) within just 30 minutes. The catalyst showcased excellent reusability, structural stability, and remarkable performance in the oxidation of other sulfide derivatives.
Moving forward, the successful synthesis of CuW-EDDP provides valuable guidance for the development of new POMOCs based on Keggin- or Dawson-type POMs and metal-organic complexes, offering novel topology and unique properties. The research team’s work opens up new possibilities for efficient and sustainable catalytic oxidation reactions and contributes to advances in the field of polyoxometalate-based materials.
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