Industrial production of propylene oxide starts from propylene.Two general approaches are employed, one involving hydrochlorination and the other involving oxidation. In 2005, about half of the world production was through chlorohydrin technology and one half via oxidation routes. The latter approach is growing in importance.
Hydrochlorination route
The traditional route proceeds via the conversion of propene to propylene chlorohydrin according to the following simplified scheme: The mixture of 1-chloro-2-propanol and 2-chloro-1-propanol is then dehydrochlorinated. For example: Lime is often used to absorb the HCl.
Oxidation of propylene
The other general route to propylene oxide involves oxidation of propylene with an organic peroxide. The reaction follows this stoichiometry: The process is practiced with four hydroperoxides:
t-Butyl hydroperoxide derived from oxygenation of isobutane, which affords t-butanol. This coproduct can be dehydrated to isobutene, converted to MTBE, an additive for gasoline.
Ethylbenzene hydroperoxide, derived from oxygenation of ethylbenzene, which affords 1-phenylethanol. This coproduct can be dehydrated to give styrene, a useful monomer.
Cumene hydroperoxide derived from oxygenation of cumene, which affords cumyl alcohol. Via dehydration and hydrogenation this coproduct can be recycled back to cumene. This technology was commercialized by Sumitomo Chemical.
Hydrogen peroxide is the oxidant in the hydrogen peroxide to propylene oxide process, catalyzed by a titanium-doped silicalite:
:C3H6 + H2O2 → C3H6O + H2O
In principle, this process produces only water was a side product. In practice, some ring-opened derivatives of PO are generated.
Reactions
Like other epoxides, PO undergoes ring-opening reactions. With water, propylene glycol is produced. With alcohols, reactions, called hydroxylpropylation, analogous to ethoxylation occur. Grignard reagents add to propylene oxide to give secondary alcohols. Some other reactions of propylene oxide include:
Between 60 and 70% of all propylene oxide is converted to polyether polyols by the process called alkoxylation. These polyols are building blocks in the production of polyurethane plastics. About 20% of propylene oxide is hydrolyzed into propylene glycol, via a process which is accelerated by acid or base catalysis. Other major products are polypropylene glycol, propylene glycol ethers, and propylene carbonate.
Niche uses
Fumigant
The United States Food and Drug Administration has approved the use of propylene oxide to pasteurize raw almonds beginning on September 1, 2007, in response to two incidents of contamination by Salmonella in commercial orchards, one incident occurring in Canada and one in the United States. Pistachio nuts can also be subjected to propylene oxide to control Salmonella.
Microscopy
Propylene oxide is commonly used in the preparation of biological samples for electron microscopy, to remove residual ethanol previously used for dehydration. In a typical procedure, the sample is first immersed in a mixture of equal volumes of ethanol and propylene oxide for 5 minutes, and then four times in pure oxide, 10 minutes each.
In 2016 it was reported that propylene oxide was detected in Sagittarius B2, a cloud of gas in the Milky Way weighing three million solar masses. It is the first chiral molecule to be detected in space.
