Solvent Regeneration in the Synthesis of Vitamin E

Abstract

For most drugs, development of the optimum production technologies is determined by the correct choice of solvents and solvent regeneration methods. In particular, this is valid for the final step of vitamin E production: the stage of -tocopherol synthesis and acetylation with acetic anhydride. During the more than half a century that passed since Karrer and Fritzsche [1] reported on the synthesis of vitamin E, a number of solvents were tried in both commercial and laboratory practice. These include aromatic hydrocarbons (benzene, toluene, xylene), aliphatic hydrocarbons (heptane, octane, nonane), alkane derivatives (chloro, nitro, etc.), organic acids, ketones, and esters. Among the special requirements to the solvents employed in this particular process, important points are a not too high boiling temperature (about 100°C), large solvating capacity, ability to dissolve all initial reagents (including catalyst in a homogeneous catalytic process), ability of water removal from the reaction zone in the form of an azeotrope, possibility of washing the catalyst, and small solvent losses in the course of regeneration. Acetic acid esters meet all these requirements to a maximum extent. However, the butyl acetate employed at present in the commercial processes boils at a relatively high temperature (125°C), which favors the formation of side products contaminating vitamin E under the commercial synthesis conditions. A highly promising solvent for the commercial synthesis of vitamin E is ethyl acetate (EA). Indeed, EA possesses a very good solvating capacity with respect to the reagent (trimethylhydroquinone) and allows the process temperature to be reduced to 80°C, which would certainly increase the product (vitamin E) quality. However, there are some problems related to the regeneration of ethyl acetate: in comparison with butyl acetate, EA is more soluble in water, more volatile, and more subject to hydrolysis under the conditions of synthesis. Previously [2], we suggested a wasteless scheme for the regeneration of EA, which allowed this solvent to be multiply reused in the synthesis of vitamin E. The scheme involves four rectification columns and one reaction-rectification column characterized by high reflux numbers. However, a considerable increase in the cost of energy has posed a problem of developing an energy-saving EA regeneration technology. Here we propose an improved regeneration scheme, which is based on the chemical interaction of acetic anhydride in the reaction-rectification column with ethanol and water present in the used EA. This technological scheme provi