Heteropolyacid (H3PW4O24) hybridized with terpyridine (C15 H11N3) as heterogeneous catalyst for epoxydation of olefins

Details

Supervisors

Gaigneaux, Eric M.

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries

Abstract

In the field of catalysis, epoxides production has seen many diverse types of catalysts being developed over the years. The uses of oxiranes compounds are broad, they are either employed to produce paints, resins, adhesives or even as intermediates in organic synthesis. Several processes have been exploited yet nowadays, the direct oxidation by air of alkenes on silver-based catalyst is the most widespread one. However, the latter is a major carbon emitter of the industry. Consequently, industries are looking for new eco-friendly catalytic processes to obtain a sustainable production of epoxides. Among the potential candidates to accomplish this reaction, polyoxometalates (POMs) have demonstrated their property of being a good oxidation catalyst for a wide variety of reactions. These large anionic molecules have many properties such as a strong acidity when protonated, thermal stability and eventually their ability to accept and release electrons without decomposing or changing their structures. One of them, {PO4[W(O)(O2)2]4}3- (=PW4) was reported to be very active and selective towards epoxidation of olefins in homogeneous conditions. Nevertheless, industries prefer having heterogeneous catalytic processes, especially when large quantities are produced. Changing the nature of this homogeneous catalyst to a heterogeneous one remains though a challenge. The hybridization of this POM species with organic ligands have already been reported being successful using bipyridine. PW4 was chosen to be hybridized with organic terpyridines ligands containing different functionalization. The main objective is to assess the influence of the catalyst polarity, by hybridizing a hydrophilic peroxo-tungstate acid (H_3 〖PW〗_4 O_24) with hydrophobic terpyridine ligands, for the heterogeneous catalytic epoxidation of cyclooctene by hydrogen peroxide. To do so, trials to synthesize PW4 and then hybridize it with the organic ligands were performed. The idea is to use 6 distinct ligands, giving 6 hybrids to obtain the catalyst with the highest affinity with the reactants. The situation is that PW4 itself is very hydrophilic compared to an alkene, but the hydrophobicity brought by the terpyridine ligand is supposed to increase the affinity between reactants and catalyst. These hybrids were synthesized even if, there is no certainty about the exact content in their inorganic part. Then, the catalytic activity of these hybrids was tested in the epoxidation of cyclooctene using hydrogen peroxide as oxidant and acetonitrile as solvent. Large differences between the hybrids was observed. Nonetheless for the best one of them, results similar to the Venturello anion in homogeneous phase were obtained. Eventually, leaching tests to judge the catalyst heterogeneity were operated. These have exposed that the hybrids catalysts are prone to leaching. These tests were followed by other ones demonstrating that the active species of the hybrids are the leached compounds. To conclude, this study has revealed that the hybridization of peroxotungstates with functionalized terpyridines yields an active catalyst that leaches. The nature of the leached species was not established, further works still has to be carried out. Additional investigation is needed concerning the exact identification of the inorganic part of the hybrids which seems to be composed of a mix of PW4, PW3 and PW2 species.