Hybrid heteropolyacid-terpyridine heterogeneous catalysts for the epoxidation of olefins with H2O2

Epoxide compounds are widely used in the chemical industry to produce resins, paints, surfactants and as intermediates in organic synthesis. Nowadays, epoxides are commonly synthesized through methods that generate a vast amount of unwanted by-products and by using reactants which contain certain environmental risks. Therefore, the exploration of a new catalytic strategies is imperative in order to overcome these issues. Olefins epoxidation by heterogeneous catalysts has shown to be an attractive strategy with interesting advantages in terms of efficiency and catalyst reusability, when combined with hydrogen peroxide as a green epoxidizing agent. In this light, polyoxometalate (POM) compounds have proven to be interesting candidates for a wide variety of oxidation reactions thanks to their versatile acidic and redox properties. However, achieving high olefin conversions and epoxide selectivities while working in heterogeneous conditions still remains a challenge. This challenge could be circumvented by forming an appropriate polyoxometalate hybrid catalyst using hydrophobic organic ligands to optimize the catalytic performance and thus work in heterogeneous conditions. The organic ligands chosen for this study are terpyridines and phosphotungstic acid was chosen as POM. The nitrogen atoms of these terpyridines act as Lewis bases and form H-bonds with the POM terminal oxygens and effectively form solid organic-inorganic hybrids. The final objective of the studies surrounding this master’s thesis is to effectively develop hybrid catalysts based on polyoxometalates which present high activities and a significant selectivity towards epoxides in the epoxidation reactions of olefins in heterogeneous conditions. To do so, different terpyridine ligands were initially synthesized through a well-known synthesis route, with different functional groups attached to it. These terpyridines were then successfully linked to the Keggin structure of phosphotungstic acid H3PW12O40 to form the final hybrid catalyst. The catalysts were tested in the epoxidation reaction of cyclooctene using H2O2 as oxidizing agent in an acetonitrile solvent. However, even though the hybrid catalysts showed lower conversions than the homogeneous pristine II H3PW12O40, which has been investigated in previous studies, higher selectivities towards cyclooctane epoxide were found. Studies about the reaction mechanism of H3PW12O40 type heteropolyacids indicated that the active species in the epoxidation mechanism was the peroxo PW4O243- species. Therefore, in order to achieve better catalytic performances, H3PW12O40 was pre-activated with H2O2 before hybridization. This way, the new peroxo hybrid obtained presented much better conversion and selectivity numbers. This study leads to the conclusion that the newly synthesized peroxotungstate hybrid compound, is the responsible for the high catalytic activity in the epoxidation reaction. Hence, more research in this matter should be done to have a better understanding of the catalyst behaviour, and notably by performing reactions with different types of olefins. Recyclability has been tested and confirmed after 1 single run, but more runs has to be made in order to confirm the catalyst long-term recyclability and efficiency.