Method for the preparation of 1,2-propanediol

A method for the preparation of 1,2-propanediol, comprising:

• a) continuously reacting propene with hydrogen peroxide in the presence of a catalyst mixture, comprising a quaternary ammonium salt and a polytungstophosphate, in a liquid reaction mixture comprising an aqueous phase with a maximum apparent pH of 6 and an organic phase comprising an alkylaromatic hydrocarbon solvent;
• b) withdrawing the liquid reaction mixture from a) and adding a water-soluble sulfate salt or sulfuric acid to the liquid reaction mixture, to obtain a mixture comprising from 500 to 10,000 mg/kg of sulfate ions in an aqueous phase;
• c) separating the mixture obtained in b) into an aqueous phase (Pa) comprising 1,2 propanediol and an organic phase (Po);
• d) recycling at least a part of the organic phase (Po) to the reaction a);
• and
• e) recovering the 1,2-propanediol from the aqueous phase (Pa),
• wherein apparent pH is a value determined by measurement with a glass electrode employing a commercial pH meter calibrated with aqueous buffer solutions of known pH for measuring dilute aqueous solutions.