Direct synthesis of propylene oxide in the liquid phase under mild conditions

• The direct synthesis of propylene oxide (PO) in liquid phase is achieved in a flow micro reactor and under mild operating conditions.
• The direct synthesis of PO is compared, under the same conditions, with its intermediate step: hydrogen peroxide direct synthesis reaction.
• The catalyst design and optimization is achieved for minimizing the propylene hydrogenation and water formation.

Here, we study the direct synthesis of propylene oxide (PO) on Pd-based catalysts operating under mild conditions (40 °C, 5.5 bar) and in a continuous flow microreactor. We show that the PO yield can be improved by a factor of two, with respect to the values present in literature, by using a Pd–Pt/TS-1 catalyst in excess of oxygen. Moreover, we compare the PO reaction with the hydrogen peroxide (H2O2) synthesis, being H2O2 (or OOH species) the intermediate and rate limiting step of the direct PO formation. We found that the optimal conditions for the PO synthesis are not advantageous for the H2O2 productivity. In this respect, the presence of Pt, which improves the PO selectivity by lowering the hydrogenation of propylene, negatively affects the H2O2 productivity due to an acceleration of its side reactions. This effect is more pronounced for the Pd–Au/TS-1 catalyst, which shows a high performance for H2O2 production. However, the PO formation remains relatively poor due to a very fast hydrogenation of propylene to propane. We conclude that the optimization of the H2O2 synthesis is not sufficient to improve the direct PO formation. Indeed, the hydrogenation of propylene needs also to be considered.

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