1,3-Propanediol Production

1,3-Propanediol, trimethylene glycol, HOCH2CH2CH2OH, is produced on a much smaller scale than its isomer, propylene glycol. In spite of some interesting areas of application, total production remains relatively small. The difficulties of manufacturing the product result in a poorly competitive price structure relative to other diols.

Chemical properties

1,3-Propanediol is a clear, colorless, odorless liquid that is miscible with water, alcohols, ethers, and formamide. It is sparingly soluble in benzene and chloroform. The chemical properties of 1,3-propanediol are typical of alcohols. Like 1,2-propanediol, 1,3-propanediol condenses with carboxylic acids at elevated temperature to yield esters. It also reacts with isocyanates and acid chlorides to yield urethanes and esters, respectively. Unlike 1,2-propanediol, 1,3-propanediol has two primary hydroxyl groups with equivalent reactivity.

Production

1,3-Propanediol is prepared by a two-step process involving the hydrolysis of acrolein to 3-hydroxypropanal followed by hydrogenation : 

Hydrolysis is carried out under weakly acidic conditions in water containing initially ca. 20 % acrolein. Higher concentrations of acrolein tend to lead to greater amounts of undesired byproducts as a result of reaction between acrolein and hydroxypropanal. 

3-Hydroxypropanal can be hydrogenated in the aqueous phase directly; however, the preferred technique is to extract the aldehyde into an organic solvent – particularly 2-methylpropanol – and then hydrogenate the aldehyde to yield the diol. Hydrogenation is conducted with Raney nickel under pressure in the aqueous phase and with nickel-supported catalysts at 2 – 4 MPa and 110 – 150 ◦C in the organic phase. The diol is subsequently separated from solvent and water by distillation. 

The yield of desired product by this route is relatively low – approximately 45 %. Alternative techniques for synthesizing the diol have appeared in the patent literature. Hydroformylation of ethylene oxide followed by hydrogenation yields 1,3-propanediol in good yield (92 %), but a high catalyst concentration and a very large excess of solvent render the process uneconomical. More recently, hydroformylation of ethylene oxide directly to 1,3-propanediol with a rhodium – phosphine catalyst system has been disclosed. The reaction of ethylene with formaldehyde and carboxylic acids has also not been commericialized because of low selectivity.