Laser-Mediated Photo-High-p,T Intensification of Vitamin D3 Synthesis in Continuous Flow

A new reactor design for photosynthesis in microflow is presented based on spatial overlap of a laser beam through a capillary tube. A conical Archimedean spiral facilitates exposure of the entire volume of the reaction solution to the irradiation beam. In this fashion, photo-high-p,T intensification of the photolysis of 7-dehydrocholesterol is achieved using either a pulsed ultraviolet (UV) laser excitation (in the order of femtoseconds) alone or in tandem with a conventional UV lamp. The goal is to control the kinetics of the side photoreactions under pulsed photolysis, as well as to reduce the reaction time and enhance the vitamin D-3 yield. Three kinds of effects are targeted: a) Utilizing a very large and spatially homogeneous photon flux inside the flowing solution; b) the novel process window of combining photo- with thermal intensification to promote the final, irreversible thermal step to vitamin D-3; and c) the use of sub-ps laser pulses to supply excitation energy on a timescale shorter than the formation of side photoproducts. Finally, photo-processing with the laser alone is compared to the tandem irradiation by the laser-UV lamp.