Process intensification and kinetic investigation of solvent-free photoisomerization of norbornadiene to quadricyclane using photomicroreactors

Photochemical transformations under solvent-free conditions provide a gateway to sustainable and green chemistry. In this work, we established a continuous-flow photochemical system with the capability to automatically switch between different capillary microreactors and synthesized quadricyclane from norbornadiene under solvent-free conditions at near-unity conversion and yield in short residence times and low photosensitizer loading. Various parameters, such as photocatalyst loading, capillary size, geometry and configuration, were investigated to optimize the quadricyclane yield and selectivity. We developed an updated kinetic model for this solvent-free reaction system and validated its zero-order kinetics by varying light intensity, initial NBD concentration and photosensitizers. This kinetic model was also validated under double-sided irradiation conditions that involved the presence of a reflecting mirror. Computational fluid dynamics (CFD) simulations were performed to characterize light intensity distribution, and the shape and size of the capillary microreactor were integrated into the reaction rate equation as an auxiliary variable of light intensity to define the effective specific surface area. Moreover, the scope of linear correlations between the effective surface area and apparent rate constants was extended for chip-based glass microreactors. Finally, based on the reaction rate equation, we designed and tested a reactor with high production capacity that can achieve a daily output of 3 kg of quadricyclane.