Improving Tandem Epoxidation Efficiency via Hydrogel Confinement Effect towards Photoelectrochemical Propylene Oxide Synthesis.

Abstract

Propylene oxide (PO) is among the world's most abundantly produced commodity chemicals, but it suffers from an energy-intensive and highly polluting industrial production route. In this work, we present a tandem photoelectrochemical (PEC) PO production system involving water oxidation to H2O2 via a BiVO4 photoanode and subsequent propylene epoxidation by titanium silicalite-1 (TS-1) catalyst-loaded hydrogel using in situ-generated H2O2. The hydrogel encapsulated on the BiVO4 photoanode surface provides a confined space to enhance H2O2 enrichment, as well as propylene transport, thereby reinforcing the epoxidation kinetics with conversion efficiencies of 94.06% for H2O2 and 75.55% for propylene. With the assistance of solar energy, the PO productivity per unit of electricity can reach 6.10 mol·cm-2·kWh-1, which is the lowest electricity consumption for existing alkene epoxidation technology. For industrial scalability, a multi-pass light absorption configuration is designed for decimeter-sized reactor to address the issue of a plunge in solar to chemical (STC) efficiency arising from the scale-up of the photoanode, achieving the optimum light harvesting efficiency of 98.21% and STC efficiency of 5.57% which is comparable to its 1 cm2 counterpart (86% retention). The continuous PO productivity in flowing electrolyte can reach 1.74 mmol·h-1 with a steady selectivity of 91.05% under AM 1.5G illumination. Finally, a techno-economic analysis is provided to offer targets that need to be met for economically compelling industrial implementation.