Method for the preparation of 1,2-propanediol, dipropylene glycol and tripropylene glycol

A method for the preparation of 1,2-propanediol, dipropylene glycol, and tripropylene glycol, the method comprising:

• a reacting propene with hydrogen peroxide containing nitrate in the presence of a catalyst mixture, comprising a phase transfer catalyst and a heteropolytungstate, in a liquid reaction mixture comprising an aqueous phase with a maximum apparent pH of 6 and an organic phase, 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 forming a liquid reaction mixture comprising an aqueous phase comprising 1,2-propanediol, dipropylene glycol, tripropylene glycol, and nitrate and an organic phase;
• b) separating the liquid reaction mixture into a separated aqueous phase (Pa) comprising 1,2 propanediol, dipropylene glycol, tripropylene glycol, and nitrate, and a separated organic phase (Po);
• c) recycling at least a part of the separated organic phase (Po) to a);
• d) hydrogenating the separated aqueous phase (Pa) with a heterogeneous hydrogenation catalyst thereby converting nitrate and nitric acid present in said aqueous phase (Pa) to molecular nitrogen and volatile nitrogen oxides, to provide a hydrogenated aqueous phase;
• and
• e) recovering the 1,2-propanediol, the dipropylene glycol, and the tripropylene glycol from the hydrogenated aqueous phase by a sequential multiple step distillation, comprising
• a first water distillation step and optionally further water distillation steps, each providing an overhead product comprising water and a bottoms product which is passed to a subsequent first glycols distillation step,
• the subsequent first glycols distillation step, providing a first overhead product comprising the 1,2 propanediol and a first bottoms product which is passed to a subsequent second glycols distillation step,
• the subsequent second glycols distillation step, providing a second overhead product comprising the dipropylene glycol and a second bottoms product which is passed to a subsequent third glycols distillation step, and
• the subsequent third glycols distillation step, providing a third overhead product comprising the tripropylene glycol and a residuals bottoms product.