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Understanding Isomerization and Direct Aminaton of Isohexides over Heterogeneous Ruthenium Catalysts

jeu 23 sep 2021



Soutenance de thèse de M. HU HANG sous la Direction de M. MARC PERA-TITUS

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Description générale

In this thesis, we investigated heterogeneously catalytic transformations of bio-based isohexides (isomannide, isosorbide, isoidide) targeting the synthesis of added value products. First we investigated the isomerization of isohexides over heterogeneous catalysts, with emphasis on Ru/C. Although the reaction is known to proceed at high H2 pressure, the mechanistic role of H2 in the reaction is today not fully understood. To this aim, we conducted a systematic study on isosorbide isomerization over Ru/C, optimized the reaction conditions and identified the different by-products. Supported by DFT simulations, the presence of hydride species on Ru was found to be key to promote the reaction rate, these being very sensitive to the H2 concentration in the reaction medium. The reaction rate was enhanced at lower H2 pressure under an inert gas (N2, He, Ar), promoting the H2 concentration and in turn mass-transfer, as confirmed by 1H NMR. Moreover, the use of isopropanol as (co)solvent with water promote isosorbide isomerization under an inert gas without external H2. Next, we investigated the direct amination of isohexides with NH3 over Ru/C targeting the synthesis of isohexide diamines. We studied the relationship between isomerization and amination, and it was found that NH3 strongly inhibits the isomerization of isomannide due to competitive adsorption between NH3 and H2 on Ru. The H2 pressure promotes the reaction by favoring the rehydrogenation of imine intermediates. However, beyond a threshold H2 pressure, hydrogenolysis and ring-opening side-reactions of isosorbide lead to a decline of the carbon balance. Besides Ru/C catalyst, a series of Ru catalysts was prepared over different supports to reduce the H2 pressure in the amination of isomannide.

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