Propylene oxide

Production

Industrial production of propylene oxide starts from propylene. Two general approaches are employed, one involving chlorohydrin formation 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.

Chlorohydrin route

The traditional route proceeds via the conversion of propylene to propylene chlorohydrin according to the following simplified scheme:

:[[File:Propylenoxid Darstellung 1.svg|450px]]

The mixture of 1-chloro-2-propanol and 2-chloro-1-propanol then undergoes internal cyclization. For example:

:[[File:Propylenoxid Darstellung 2.svg|330px]]

Lime (calcium hydroxide) 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:

:CH3CH=CH2 + RO2H → CH3CHCH2O + ROH

The process is practiced with four hydroperoxides:

• In the Halcon process, 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 (isopropylbenzene), 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 (HPPO) process, catalyzed by a titanium-doped silicalite:
• :C3H6 + H2O2 → C3H6O + H2O

In principle, this process produces only water as a side product. In practice, some ring-opened derivatives of PO are generated.

Propylene oxide is chiral building block that is commercially available in either enantiomeric form ((R)-(+) and (S)-(–)). The separated enantiomers can be obtained through a Co(III)-salen-catalyzed hydrolytic kinetic resolution of the racemic material.

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:

• Reaction with aluminium oxide at 250–260 °C leads to propionaldehyde and a little acetone.
• Reaction with silver(I) oxide leads to acetic acid.
• Reaction with sodium–mercury amalgam and water leads to isopropanol.

Uses

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.

Munition

Propylene oxide is sometimes used in thermobaric munitions as the fuel in fuel–air explosives. In addition to the explosive damage from the blast wave, unexploded propylene oxide can cause additional effects from direct toxicity.

Safety

Propylene oxide is both acutely toxic and carcinogenic. Acute exposure causes respiratory tract irritation, eventually leading to death. Signs of toxicity after acute exposure include salivation, lacrimation, nasal discharge, gasping, lethargy and hypoactivity, weakness, and incoordination. Propylene oxide is also neurotoxic in rats, and presumably in humans. Propylene oxide alkylates DNA and is considered a mutagen for both animals and humans. Pregnant rats exposed to 500ppm of propylene oxide for less than 8 hours gave birth to litters with significant deformities and weight deficiencies. Similar exposure has also shown to reduce animal fertility. As such, it is a known animal carcinogen and potential human carcinogen, and is included into the List of IARC Group 2B carcinogens.

Propylene oxide is an extremely flammable liquid, and its vapors can form explosive mixtures with air at concentrations as low as 2.3% (Lower Explosive Limit). Propylene oxide vapor is twice as dense as air. When exposed to an open atmosphere, the vapor can accumulate in low-lying areas while spreading out over long distances and reach ignition source, causing flashback or an explosion. When heated, propylene oxide can rapidly self-polymerize and decompose producing other toxic gases such as carbon monoxide and various free radicals. Propylene oxide fires are especially dangerous and difficult for firefighters to extinguish. In a fire, sealed tanks of propylene oxide should be cooled with fire hoses to prevent explosion from self-polymerization. When burning in open air however, water can transport propylene oxide outside of the fire zone which can reignite upon floating to the surface. Additional firefighting measures should be taken to prevent propylene oxide from washing out to nearby drains and sewers contaminating the surrounding environment.

Natural occurrence

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, albeit with no enantiomeric excess.

References

Cited sources

References

1. {{harvnb. Haynes. 2011
2. {{harvnb. Haynes. 2011
3. {{harvnb. Haynes. 2011
4. [http://www.otrain.com/OTI_MSDS(NFPA)PMAP.html "NFPA DIAMOND"]. ''www.otrain.com''.
5. GOV, NOAA Office of Response and Restoration, US. [http://cameochemicals.noaa.gov/chemical/5159 "PROPYLENE OXIDE. CAMEO Chemicals. NOAA"]. ''cameochemicals.noaa.gov''.
6. {{PGCH. 0538
7. {{IDLH. 75569. Propylene oxide
8. (2006). "The Production of Propene Oxide: Catalytic Processes and Recent Developments". Industrial & Engineering Chemistry Research.
9. (2002). "Propylene Oxide".
10. (2006). "The Production of Propene Oxide: Catalytic Processes and Recent Developments". Industrial & Engineering Chemistry Research.
11. "Summary of Sumitomo process from Nexant Reports".
12. (2013). "Chemical and Technical Aspects of Propene Oxide Production via Hydrogen Peroxide (HPPO Process)". Industrial & Engineering Chemistry Research.
13. (2002-02-01). "Highly Selective Hydrolytic Kinetic Resolution of Terminal Epoxides Catalyzed by Chiral (salen)Co III Complexes. Practical Synthesis of Enantioenriched Terminal Epoxides and 1,2-Diols". Journal of the American Chemical Society.
14. (1953). "Dictionary of Organic Compounds". Oxford University Press.
15. (2005). "Polyurethanes".
16. "Usage of proplyene oxide". Dow Chemical.
17. (June 2009). "Guidance for Industry: Measures to Address the Risk for Contamination by Salmonella Species in Food Containing a Pistachio-Derived Product As An Ingredient; Draft Guidance".
18. Agricultural Marketing Service, USDA. (30 March 2007). "Almonds Grown in California; Outgoing Quality Control Requirements". Federal Register.
19. (February 1, 2000). "Backgrounder on Russian Fuel Air Explosives ("Vacuum Bombs") | Human Rights Watch". Hrw.org.
20. National Research Council (US) Committee on Acute Exposure Guideline Levels. (2010). "Acute Exposure Guideline Levels for Selected Airborne Chemicals: Volume 9". National Academies Press.
21. (February 1993). "Polyneuropathy due to ethylene oxide, propylene oxide, and butylene oxide". Environmental Research.
22. (February 1972). "Alkylation by propylene oxide of deoxyribonucleic acid, adenine, guanosine and deoxyguanylic acid". The Biochemical Journal.
23. Albertini, Richard J.. (April 2003). "Correspondence re: Czene et al., Analysis of DNA and hemoglobin adducts and sister chromatid exchanges in a human population occupationally exposed to propylene oxide: a pilot study. Cancer Epidemiol. Biomark. Prev., 11: 315-318, 2002". Cancer Epidemiology, Biomarkers & Prevention.
24. (1981). "Mutagenicity Study of Workers Exposed to Alkylene Oxides (Ethylene Oxide/Propylene Oxide) and Derivatives". Journal of Occupational Medicine.
25. Fishersci. (1 July 1999). "Material Safety Data Sheet Propylene Oxide".
26. (May 2014). "E-cigarettes: a scientific review". Circulation.
27. (2023-01-01). "Explosion behaviors of vapor–liquid propylene oxide/air mixture under high-temperature source ignition". Fuel.
28. HARDWICK, T. (March 8, 1968). "Thermal decomposition of propylene oxide". Canadian Journal of Chemistry.
29. "Emergency Response Guide No. 127P for FLAMMABLE LIQUIDS (Water-Miscible) – HazMat Tool".
30. (2016-06-15). "Scientists just detected this life-forming molecule in interstellar space for the first time". Science Alert.
31. (September 1982). "Epoxy resins are mutagenic: implications for electron microscopists". Journal of Ultrastructure Research.