A petrochemical is any chemical derived from crude oil or natural gas that serves as a raw material for manufacturing everyday products. Rather than being burned as fuel, these chemicals are extracted and transformed into plastics, synthetic fibers, medicines, detergents, and thousands of other goods. The petrochemical industry is the largest industrial consumer of fossil fuels and accounts for roughly 1.3 gigatonnes of direct carbon dioxide emissions per year, about 3.6% of the global total.
Where Petrochemicals Come From
Crude oil is a complex mixture of hydrocarbons. When it arrives at a refinery, it goes through fractional distillation, a process that separates it into different components based on their boiling points. The lighter fractions collected near the top of the distillation column include gases like propane and butane. Heavier fractions collected lower down include diesel, fuel oil, and bitumen used for road surfaces. Somewhere in the middle sits naphtha, the fraction most important to the petrochemical industry.
Naphtha and other light fractions are the primary feedstocks, meaning they are the starting materials that get chemically broken down and reassembled into new substances. Natural gas also contributes key feedstocks, particularly ethane and propane. So while most of a barrel of crude oil ends up as fuel for cars, planes, and ships, a meaningful share is diverted into chemical manufacturing.
The Core Building Blocks
The petrochemical industry ultimately rests on a small number of foundational molecules. These fall into two main families: olefins and aromatics.
Olefins
Olefins are hydrocarbons that contain at least one double bond between carbon atoms, which makes them chemically reactive and useful as starting points for building larger molecules. The three most important olefins are ethylene, propylene, and butylene. Ethylene is the single highest-volume petrochemical in the world. It is the raw material for polyethylene, the plastic found in grocery bags, bottles, and food packaging. Propylene becomes polypropylene, used in everything from car bumpers to yogurt containers. Butylene feeds into synthetic rubber production.
Aromatics
The second family, aromatics, centers on three molecules often grouped together as BTX: benzene, toluene, and xylene. These ring-shaped hydrocarbons serve different roles. Para-xylene is the most commercially significant of the xylene variants because it is the source of terephthalic acid, one of the two ingredients in PET plastic (the material in most clear drink bottles and polyester clothing). Ortho-xylene goes into resins and paints. Benzene is a precursor for nylon, certain plastics, and detergents. Toluene has more limited direct chemical applications, though it is widely used as an industrial solvent.
How Feedstocks Become Petrochemicals
The key industrial process that converts oil and gas fractions into usable petrochemicals is called steam cracking. In a steam cracking furnace, hydrocarbon feedstocks like naphtha or ethane are mixed with steam and heated to extreme temperatures, typically 800 to 900 °C, inside metal coils. At these temperatures, the large hydrocarbon molecules break apart into smaller, more reactive ones. The process is endothermic, meaning it requires enormous energy input to sustain those temperatures.
The output of a steam cracker is a mixture of different molecules. Ethylene, propylene, butylene, and benzene all come out together and then need to be separated through further distillation and processing steps. The specific mix of products depends on the feedstock: cracking ethane from natural gas yields mostly ethylene, while cracking naphtha from crude oil produces a broader range of olefins and aromatics.
Products You Use Every Day
The gap between “petrochemical” and “finished product” is bridged by intermediate chemicals. Ethylene becomes ethylene glycol, one of the two building blocks of PET polyester. That same ethylene glycol shows up in antifreeze. Propylene is converted into polypropylene for rigid packaging but also into acrylonitrile for acrylic fabrics. The chain from raw feedstock to store shelf typically involves two or three chemical transformations.
Synthetic fibers illustrate this well. Polyester (PET) is built from ethylene glycol and terephthalic acid, both petrochemical derivatives. Nylon 6-6, commonly found in carpets and clothing, comes from two six-carbon chains originally sourced from benzene. Kevlar, the fiber used in body armor and protective gear, is also an aromatic polyamide tracing back to petrochemical aromatics.
Petrochemicals also reach into medicine. Aspirin, antihistamines, cortisone, rubbing alcohol, glycerin, and petroleum jelly all rely on petroleum-derived precursors. Soft contact lenses and the gelatin capsules around vitamins are petrochemical products too. Even hospital supplies like IV bags, syringes, and surgical gloves depend on plastics made from ethylene and propylene.
Scale of the Industry
Ethylene and propylene are the two highest-volume intermediate chemicals produced globally, and demand continues to grow. Propylene has become the second most important petrochemical after ethylene, driven largely by expanding polypropylene use in packaging and automotive parts. Both are currently manufactured almost entirely from fossil sources, with naphtha and propane as the dominant feedstocks.
The environmental footprint is significant. The chemical sector produces about 1.3 gigatonnes of direct CO₂ emissions annually. Within the industry, primary chemicals (the base molecules like ethylene and propylene) are the most carbon-intensive to produce. According to a 2025 assessment by the Transition Pathway Initiative at the London School of Economics, the emissions intensity of primary chemical production benchmarks at roughly 1.73 tonnes of CO₂ equivalent per $1,000 of revenue, about three times higher than non-primary chemical manufacturing.
Alternatives on the Horizon
Because conventional petrochemical production depends on fossil fuels and generates substantial greenhouse gas emissions, researchers are working on ways to produce the same molecules from renewable sources. Bio-based routes use plant-derived feedstocks instead of petroleum. For propylene, one of the more promising approaches is bio-propane dehydrogenation, which starts with propane sourced from biological rather than fossil processes. Specialized catalysts, particularly a type of zeolite called ZSM-5, have shown strong selectivity for producing propylene from these alternative feedstocks.
These sustainable pathways are still in earlier stages compared to the mature, economically optimized fossil-based processes that have been refined over decades. The transition involves not just developing the chemistry but proving that bio-based routes can match the cost and scale of conventional production, a challenge that life cycle assessments and techno-economic analyses are actively evaluating.