Efficient Photooxygenation Process of Biosourced α-Terpinene by Combining Controlled LED-Driven Flow Photochemistry and Rose Bengal-Anchored Polymer Colloids
Résumé
This work studies the reactivity of poly(N-vinylcaprolactam-co-vinyl acetate-co-vinylbenzyl Rose Bengal) microgels (VBRB@MG) as heterogeneous photosensitizers in a continuous-flow process for sustainable singlet oxygen sensitized photooxygenation of a bio-based molecule. Experiments were carried out in a LED-driven spiral-shaped microreactor in which slurry Taylor flows were generated, allowing accurate control of irradiation, light absorption and gas-liquid flow conditions. The benchmark photooxygenation of -terpinene was implemented in ethanol to provide a green solvent using air as a safe supply of oxygen. Swollen RB-grafted colloids formed an efficient substrate for converting -terpinene into ascaridole, providing up to high conversion with high selectivity under continuous-flow conditions, and within short residence times of a few minutes. The supported RB exhibited a reactivity similar to that of the free RB. The reactivity of the supported photosensitizer was maintained for several cycles with a reproducible level after 8 months of storage. Under experimental conditions favouring photobleaching of RB, the photobleaching level of RB was lower with the VBRB@MG colloids than with free RB, suggesting that grafting RB molecules onto the colloid can prevent their photodegradation. KEYWORDS Flow photochemistry. Singlet oxygen. Photoactive polymer colloids. Green conditions. Rose Bengal. Alpha-terpinene.
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