Method for preparing vitamin d2 using a microchannel reactor

A method for preparing vitamin D2 using a microchannel reactor, comprising:

• (a) dissolving ergosterol in a first solvent to form a first reaction solution;
• (b) introducing the first reaction solution into the microchannel reactor defined with a first photoreaction zone irradiated by a first light source and a second photoreaction zone located downstream of the first photoreaction zone and irradiated by a second light source, the first reaction solution being irradiated with the first light source and the second light source in sequence as the first reaction solution passes through the first photoreaction zone and the second photoreaction zone, wherein the first light source has a wavelength of 260 nm to 300 nm, the second light source has a wavelength of 345 nm to 385 nm, and the first reaction solution is irradiated by the first light source and the second light source in the first photoreaction zone and the second photoreaction zone at an irradiation flux ratio from 0.5:1 to 25:1;
• (c) collecting and concentrating the first reaction solution irradiated by the second light source;
• (d) optionally, mixing the concentrated first reaction solution of step (c) with a second solvent to form a second reaction solution;
• (e) heating the first reaction solution of step (c) or the second reaction solution of step (d) to obtain the vitamin D2.