A greener, pressure intensified propylene epoxidation process with facile product separation

A novel biphasic process concept for the synthesis of propylene oxide (PO) from propylene is presented, using the long known catalyst, methyl trioxorhenium and aqueous hydrogen peroxide as the oxidant. Propylene is fed as gas, which is transported to the liquid phase containing the oxidant, catalyst and methanol as a cosolvent that improves propylene solubility. The selective oxidation produces PO which is distilled easily from the liquid phase taking advantage of the relatively low normal boiling point of PO (∼34∘C) compared to those of methanol and water. The process satisfies the sustainability principles of waste minimization, use of benign reagents and process intensification at mild conditions. The process produces PO in yields exceeding 98%. It operates at essentially ambient temperature (20–35∘C) and moderate pressures (∼20bar), using easily recyclable, low hazard aqueous methanol, as solvent. The catalyst, methyl trioxorhenium, is robust under the operating conditions. A key aspect of the innovation is the use of nitrogen gas pressure to enhance propylene availability in the liquid phase increasing its conversion from ∼80%∼80% (without N2N2) to complete (with ∼20barN2N2) in a few hours. These findings pave the way for catalyst durability and recycle studies, aimed at demonstrating a continuous process that is economically and environmentally sustainable compared to existing processes.