Propylene, also known as propene, is an unsaturated organic compound with the chemical formula . It has one double bond, and is the second simplest member of the alkene class of . It is a colorless gas with a faint petroleum-like odor.
Propylene is a product of combustion from forest fires, cigarette smoke, and motor vehicle and aircraft exhaust.
Production
Steam cracking
The dominant technology for producing propylene is
steam cracking, using
propane as the
feedstock. Cracking propane yields a mixture of
ethylene, propylene,
methane,
hydrogen gas, and other related compounds. The yield of propylene is about 15%. The other principal feedstock is
naphtha, especially in the
Middle East and Asia.
[Ashford's Dictionary of Industrial Chemicals, Third edition, 2011, , pages 7766–9]
Propylene can be separated by fractional distillation from the hydrocarbon mixtures obtained from cracking and other refining processes; refinery-grade propene is about 50 to 70%.
In the United States,
shale gas is a major source of propane.
Olefin conversion technology
In the Phillips triolefin or olefin conversion technology, propylene is interconverted with
ethylene and 2-butenes.
Rhenium and
molybdenum catalysts are used:
- CH2=CH2{} + CH3CH=CHCH3 ->\text{Re, 2 CH2=CHCH3
The technology is founded on an olefin metathesis reaction discovered at Phillips Petroleum Company.
Related is the Methanol-to-Olefins/Methanol-to-Propene process. It converts synthesis gas (syngas) to methanol, and then converts the methanol to ethylene and/or propene. The process produces water as a by-product. Synthesis gas is produced from the reformation of natural gas or by the steam-induced reformation of petroleum products such as naphtha, or by gasification of coal or natural gas.
Fluid catalytic cracking
High severity fluid 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.
Other commercialized methods
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.
Market
Propene production has remained static at around 35 million
tonnes (Europe and North America only) from 2000 to 2008, but it has been increasing in East Asia, most notably Singapore and China.
Total world production of propene is currently about half that of ethylene.
Research
The use of engineered
has been explored but has not been commercialized.
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.
Uses
Propylene is the second most important starting product in the petrochemical industry after
ethylene. It is the raw material for a wide variety of products.
Polypropylene manufacturers consume nearly two thirds of global production.
Polypropylene end uses include films, fibers, containers, packaging, and caps and closures. Propene is also used for the production of chemicals such as
propylene oxide,
acrylonitrile,
cumene,
butyraldehyde, and
acrylic acid. In the year 2013 about 85 million tonnes of propylene were processed worldwide.
Propylene and benzene are converted to acetone and phenol via the cumene process.
Propylene is also used to produce isopropyl alcohol (propan-2-ol), acrylonitrile, propylene oxide, and epichlorohydrin.
In industry and workshops, propylene is used as an alternative fuel to acetylene in Oxy-fuel welding and cutting, brazing and heating of metal for the purpose of bending. It has become a standard in BernzOmatic products and others in MAPP substitutes,[ For example, "MAPP-Pro"] now that true MAPP gas is no longer available.
Reactions
Propylene resembles other alkenes in that it undergoes electrophilic 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:
Complexes of transition metals
Foundational to hydroformylation, alkene metathesis, and polymerization are metal-propylene complexes, which are intermediates in these processes. Propylene is
prochiral, meaning that binding of a reagent (such as a metal electrophile) to the C=C group yields one of two
.
Polymerization
The majority of propylene is used to form polypropylene, a very important commodity
thermoplastic, through chain-growth polymerization.
In the presence of a suitable catalyst (typically a Ziegler–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 a fluidized bed reactor.
Oligomerization
In the presence of
, propylene will form various short
. It can
to give 2,3-dimethyl-1-butene and/or 2,3-dimethyl-2-butene.
or trimerise to form
tripropylene.
Environmental safety
Propene is a product of combustion from forest fires, cigarette smoke, and motor vehicle and aircraft exhaust.
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.
[
]
In the United States and some European countries a threshold limit value of 500 parts per million (ppm) was established for occupational (8-hour time-weighted average) exposure. It is considered a volatile organic compound (VOC) and emissions are regulated by many governments, but it is not listed by the U.S. Environmental Protection Agency (EPA) as a hazardous air pollutant under the Clean Air Act. With a relatively short half-life, it is not expected to bioaccumulate.[
]
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.
Bio-propylene is the bio-based propylene.[ Bio-based drop-in, smart drop-in and dedicated chemicals][ Duurzame bioplastics op basis van hernieuwbare grondstoffen]
It has been examined, motivated by diverse interests such a carbon footprint. Production from glucose has been considered.
Storage and handling
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.[Encyclopedia of Chemical Technology, Fourth edition, 1996, (v.20), page 261]
Occurrence in nature
Propene is detected in the interstellar medium through microwave spectroscopy.
See also
-
Los Alfaques disaster
-
Inhalant abuse
-
2014 Kaohsiung gas explosions
-
2020 Houston explosion
-
Titan (moon)
