Microwave synthesis of UiO-66 with encapsulated caffeine: Liquor mother reuse and release from polyamide composites

In this study, UiO-66 was in-situ synthesized with encapsulated caffeine (CAF@UiO-66) following a one-pot DMF-free microwave-assisted method. The microwave-assisted approach allowed for an efficient and fast encapsulation of caffeine within the metal-organic framework in a single step thus achieving a more environmentally friendly process. It was shown that the CAF@UiO-66 solid was capable of gradually dosing caffeine into aqueous media over a long period. Moreover, the recovered synthesis solution containing residual caffeine and solvent was successfully reused in several encapsulation cycles, giving capsules that showed good caffeine release properties. In addition, CAF@UiO-66 solids were incorporated into polyamide 6 (PA6) fibers through an extrusion process without altering their physicochemical properties. These composites showed a gradual and sustained release of caffeine, extending the dosage time and shelf life of the additive which made it potentially useful for its application in functional textile products. Finally, the release of caffeine from the capsules was fitted to a Fickian model while that from the polymeric composites was adjusted to the semi-empirical Korsmeyer-Peppas model. • CAF@UiO-66 is successfully synthesized using a one-pot microwave method with acetone. • Release experiments show a gradual caffeine delivery over time, denoting a good encapsulation. • Reusing the synthesis medium enables the formation of capsules with good properties. • Solvent and additive waste is reduced by 70 % through the sustainable reuse strategy. • The incorporation of capsules into PA6 composites allows a controlled caffeine release, increasing their useful life.