Palladium-catalyzed oxidation of geraniol in dense carbon dioxide

The performance and stability of supported Pd catalysts were investigated in the partial oxidation of geraniol with molecular oxygen in dense CO2. All catalysts deactivated to a certain extent in the continuous-flow reactor; the least stable among them was Pd/Al2O3. Beside oxidative dehydrogenation, isomerization and hydrogenation reactions were also catalyzed by Pd. The initial selectivity of Pd to citral formation was only 50–60% but improved up to 90% with time-on-stream. The extensive catalyst deactivation and the shift in product distribution are (partly) attributed to formation of geranic (neric) acid, and the acid-catalyzed dimerization and oligomerization of reactant and products leading to site blocking by the high molecular weight by-products. ATR-IR spectroscopy revealed that no CO was present on the Pd surface during geraniol oxidation though it appeared in the absence of oxygen. A shift from two-phase to single phase conditions diminished the reaction rate, presumably due to accumulation of water co-product on the catalyst surface. Despite of the initial deactivation of Pd/SiO2, an outstanding TOF of 169 h−1 was achieved on the stabilized catalyst under steady state conditions, at 80 °C and 150 bar.