Continuous flow photooxygenation with advanced rose bengal-anchored polymer colloids

Abstract

The development of sustainable photooxygenation processes is a key challenge in green chemical engineering, particularly for the efficient transformation of bio-based molecules under mild and environmentally friendly conditions. However, the implementation of efficient photosensitizers remains limited, with recyclability and process compatibility often being the major bottlenecks. This study addresses the engineering challenge of implementing advanced polymer colloids functionalized with rose bengal (RB) as robust heterogeneous photosensitizers that deliver both high photoreactivity and operational stability. We present an original continuous-flow approach using an LED-driven spiral-shaped millireactor and core–shell RB-functionalized colloids that are synthesized directly in a green solvent used for the selective photooxygenation of α-terpinene to ascaridole. Photoactive colloids were used under visible light irradiation and transported by the Taylor (slurry) flows using air as a sustainable reactant. The reactor configuration enabled fine control over irradiation conditions, residence time, and gas–liquid mass transfer, which were essential for consistent and efficient photoreactivity. Strikingly, the colloids retained their photooxygenation efficiency across different particle sizes and compositions, an unusual feature that underscores their robustness and sets them apart from most reported heterogeneous systems. Equally remarkable, their reactivity matched that of soluble RB, demonstrating that embedding the dye in a colloidal microenvironment does not compromise photochemical efficiency. As a result, all the tested colloidal systems showed very good performance and could be reused over multiple reaction cycles. To support process development and scale-up, a model was established to predict reaction rates as a function of operating parameters, providing valuable insights into the interplay between bubbly flow dynamics, light absorption, and photochemical kinetics. This work demonstrates a promising route for the implementation of recyclable heterogeneous photosensitizers in scalable continuous-flow photooxidation processes according to the principles of green chemical engineering.