Commercial production of propylene glycol is by hydration of propylene oxide. Di- and tripropylene glycols, as well as small quantities of higher glycols, are also produced in the reaction.
The reaction between propylene oxide and water takes place at a temperature of 200oC and 12 bar pressure. The amount of water is controlled to favour MPG production. The reaction mixture is dehydrated by evaporation and the various glycols separated by distillation.
There is a lot of interest in producing PG from renewable resources such as glycerine. Glycerine is a byproduct in biodiesel manufacture and with the boom in biodiesel projects, much effort is being made in finding new uses for glycerine. For example, a process for converting natural glycerine to PG has been developed by the University of Missouri. The conversion rate is claimed to be about 75% efficient.
A French biochemical company, Metabolic Explorer, has been awarded a French patent for a fermentation process based on renewable resources. Archer Daniels Midland (ADM) plans to build a plant using agricultural raw materials to produce pharmaceutical and industrial grade PG.
A carbohydrate-based route to PG could result from work at Michigan State University. It has demonstrated that crude aqueous (10%) lactic acid produced by fermentation of corn starch, dextrose or other carbohydrates can be readily hydrogenated to PG in 86% selectivity at 98% conversion.