The dominant technology for producing propylene issteam cracking, usingpropane as thefeedstock. Cracking propane yields a mixture ofethylene, propylene,methane,hydrogen gas, and other related compounds. The yield of propylene is about 15%. The other principal feedstock isnaphtha, especially in theMiddle East and Asia.[8] Propylene can be separated byfractional distillation from the hydrocarbon mixtures obtained from cracking and other refining processes; refinery-grade propene is about 50 to 70%.[9] In the United States,shale gas is a major source of propane.
High severityfluid catalytic cracking (FCC) uses traditional FCC technology under severe conditions (higher catalyst-to-oil ratios, higher steam injection rates, higher temperatures, etc.) in order to maximize the amount of propene and other light products. A high severity FCC unit is usually fed with gas oils (paraffins) and residues, and produces about 20–25% (by mass) of propene on feedstock together with greater volumes of motor gasoline and distillate byproducts. These high temperature processes are expensive and have a high carbon footprint. For these reasons, alternative routes to propylene continue to attract attention.[13]
On-purpose propylene production technologies were developed throughout the twentieth century. Of these, propane dehydrogenation technologies such as the CATOFIN and OLEFLEX processes have become common, although they still make up a minority of the market, with most of the olefin being sourced from the above mentioned cracking technologies. Platinum, chromia, and vanadium catalysts are common in propane dehydrogenation processes.
Propene production has remained static at around 35 milliontonnes (Europe and North America only) from 2000 to 2008, but it has been increasing in East Asia, most notably Singapore and China.[14] Total world production of propene is currently about half that of ethylene.
The use of engineeredenzymes has been explored but has not been commercialized.[15]
There is ongoing research into the use of oxygen carrier catalysts for the oxidative dehydrogenation of propane. This poses several advantages, as this reaction mechanism can occur at lower temperatures than conventional dehydrogenation, and may not be equilibrium-limited because oxygen is used to combust the hydrogen by-product.[16]
Propylene is the second most important starting product in thepetrochemical industry afterethylene. It is the raw material for a wide variety of products.Polypropylene manufacturers consume nearly two thirds of global production.[17] Polypropylene end uses include films, fibers, containers, packaging, and caps and closures. Propene is also used for the production of chemicals such aspropylene oxide,acrylonitrile,cumene,butyraldehyde, andacrylic acid. In the year 2013 about 85 million tonnes of propylene were processed worldwide.[17]
In industry and workshops, propylene is used as an alternative fuel to acetylene inOxy-fuel welding and cutting, brazing and heating of metal for the purpose of bending. It has become a standard inBernzOmatic products and others in MAPP substitutes,[20] now that trueMAPP gas is no longer available.
Propylene resembles other alkenes in that it undergoeselectrophilic addition reactions relatively easily at room temperature. The relative weakness of its double bond explains its tendency to react with substances that can achieve this transformation. Alkene reactions include:
Foundational to hydroformylation, alkene metathesis, and polymerization aremetal-propylene complexes, which are intermediates in these processes. Propylene isprochiral, meaning that binding of a reagent (such as a metal electrophile) to the C=C group yields one of twoenantiomers.
The majority of propylene is used to form polypropylene, a very important commoditythermoplastic, throughchain-growth polymerization.[17] In the presence of a suitable catalyst (typically aZiegler–Natta catalyst), propylene will polymerize. There are multiple ways to achieve this, such as using high pressures to suspending the catalyst in a solution of liquid propylene, or running gaseous propylene through afluidized bed reactor.[21]
Propene is a product of combustion from forest fires, cigarette smoke, and motor vehicle and aircraft exhaust.[5] It is an impurity in some heating gases. Observed concentrations have been in the range of 0.1–4.8 parts per billion (ppb) in rural air, 4–10.5 ppb in urban air, and 7–260 ppb in industrial air samples.[9]
In the United States and some European countries athreshold limit value of 500 parts per million (ppm) was established for occupational (8-hourtime-weighted average) exposure. It is considered avolatile organic compound (VOC) and emissions are regulated by many governments, but it is not listed by the U.S. Environmental Protection Agency (EPA) as ahazardous air pollutant under theClean Air Act. With a relatively short half-life, it is not expected to bioaccumulate.[9]
Propene has low acute toxicity from inhalation and is not considered to be carcinogenic. Chronic toxicity studies in mice did not yield significant evidence suggesting adverse effects. Humans briefly exposed to 4,000 ppm did not experience any noticeable effects.[23] Propene is dangerous from its potential to displace oxygen as anasphyxiant gas, and from its high flammability/explosion risk.
Bio-propylene is thebio-based propylene.[24][25]It has been examined, motivated by diverse interests such acarbon footprint. Production fromglucose has been considered.[26] More advanced ways of addressing such issues focus on electrification alternatives tosteam cracking.
Propene is flammable. Propene is usually stored as liquid under pressure, although it is also possible to store it safely as gas at ambient temperature in approved containers.[27]
Propene is detected in theinterstellar medium through microwave spectroscopy.[28] On September 30, 2013,NASA announced the detection of small amounts of naturally occurring propene in the atmosphere ofTitan using infrared spectroscopy.[29][30][31] The detection was made by a team led byNASA GSFC scientistConor Nixon using data from theCIRS instrument[32][33] on the Cassini orbiter spacecraft, part of theCassini-Huygens mission. Its confirmation solved a 32-year old mystery by filling a predicted gap in Titan's detectedhydrocarbons, adding the C3H6 species (propene) to the already-detected C3H4 (propyne) and C3H8 (propane).[34]
^Rasmussen, Seth C. (2018), Rasmussen, Seth C. (ed.),"Introduction",Acetylene and Its Polymers: 150+ Years of History, SpringerBriefs in Molecular Science, Cham: Springer International Publishing, pp. 1–19,doi:10.1007/978-3-319-95489-9_1,ISBN978-3-319-95489-9, retrieved2023-12-30
^Ashford's Dictionary of Industrial Chemicals, Third edition, 2011,ISBN978-0-9522674-3-0, pages 7766-9
^Ghashghaee, Mohammad (2018). "Heterogeneous catalysts for gas-phase conversion of ethylene to higher olefins".Rev. Chem. Eng.34 (5):595–655.doi:10.1515/revce-2017-0003.S2CID103664623.
^Banks, R. L.; Bailey, G. C. (1964). "Olefin Disproportionation. A New Catalytic Process".Industrial & Engineering Chemistry Product Research and Development.3 (3):170–173.doi:10.1021/i360011a002.
