Green production of diosgenin from alcoholysis of dioscorea zingiberensis C. H. wright by a magnetic solid acid

A new method of the catalytic alcoholysis of Dioscorea zingiberensis C. H. Wright into diosgenin by magnetic solid acid in a Teflon-lined stainless-steel autoclave for the first time. The magnetic solid acid, Fe₃O₄@SiO₂–SO₃H, was prepared by the connecting the –SO₃H groups on the surface of SiO₂ encapsulated Fe₃O₄ microspheres. The synthesized catalysts were systematically analyzed by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analyzer (TGA), X-ray diffraction (XRD), vibrating sample magnetometry (VSM), temperature programmed desorption of ammonia (NH₃-TPD), scanning electron microscope (SEM), transmission electron microscope (TEM) and transmission electron microscope-mapping(TEM-mapping). To obtain the best conditions for the alcoholysis of Dioscorea zingiberensis C. H. Wright, single factor test was first utilized to determine the range of optimal conditions and optimized by response surface method. A three-level-three-factor Box-Behnken design was adopted to illustrate the effect of solid acid mass, solvent volume, reaction time and temperature on the yield of diosgenin. The optimum conditions obtained by response surface methodology were: 0.42 g Fe₃O₄@SiO₂–SO₃H, 4.38 mL solvent of ethanol, the reaction temperature at 100 °C and 6.09 h reaction time. At the optimal conditions, this method can reach the highest yield 2.64% from Dioscorea zingiberensis C. H. Wright. The yield of the new method was 21.66% higher than conventional sulfuric acid hydrolysis based on the analysis results of UV and HPLC. Importantly, the catalysts and solvents used in the alcoholysis process by the solid acid can be recovered, which is green and environmentally friendly, indicating that the catalytic alcoholysis with magnetic solid acid under hydrothermal conditions had a broad application prospect in diosgenin production.