Styrene, an aromatic hydrocarbon also known as vinyl benzene, is a colorless, volatile organic liquid. This compound serves as a fundamental building block in the global polymer industry, where its monomers are linked into long chains. The resulting polymers are used to manufacture synthetic materials, including polystyrene plastic, \(\text{ABS}\) resins, and \(\text{SBR}\) (styrene-butadiene rubber) used in tires and latexes. While recognized for its synthetic applications, styrene’s origins are diverse, encompassing minor natural occurrences and large-scale industrial production.
Natural Occurrence of Styrene
Styrene is present in trace quantities in various natural sources, making it a compound that humans encounter regularly outside of industrial settings. It occurs naturally in certain foods and beverages, including coffee beans, cinnamon, strawberries, nuts, and beef, contributing minor exposure through diet.
The compound was first isolated in the 19th century from storax balsam, a resin derived from trees in the Liquidambar genus, which is the origin of its name. Styrene is also a byproduct of the combustion or heating of organic materials, found in the exhaust from motor vehicles and as a component of tobacco smoke.
Industrial Synthesis: The Primary Manufacturing Process
The overwhelming majority of the world’s styrene is produced through a highly refined two-step industrial chemical process. The second, and most critical, step involves the catalytic dehydrogenation of ethylbenzene. This reaction accounts for approximately 80 to 85 percent of commercial styrene output globally.
Dehydrogenation is an endothermic reaction, meaning it requires a significant input of heat to proceed. The process occurs in the vapor phase, where ethylbenzene is mixed with superheated steam, often reaching temperatures up to \(600^\circ\text{C}\). Iron(III) oxide is the most common catalyst used to facilitate the removal of hydrogen from the ethylbenzene molecule, yielding styrene and hydrogen gas.
The steam plays multiple roles in this high-temperature environment, providing the necessary heat for the endothermic reaction and helping to prevent the buildup of carbon deposits that would deactivate the catalyst. Following the reaction, the crude product stream undergoes an intensive purification stage. Purification is achieved through a series of specialized distillation columns that separate the styrene monomer from the unreacted ethylbenzene, which is then recycled back into the process.
Necessary Chemical Precursors: Benzene and Ethylene
The industrial production of styrene fundamentally begins one step earlier, with the synthesis of the intermediate compound, ethylbenzene. Ethylbenzene is created by combining two simpler hydrocarbon feedstocks: benzene and ethylene. This initial reaction is known as alkylation, where an ethyl group is added to the benzene ring.
Benzene, an aromatic hydrocarbon, is primarily sourced from the complex processes of petroleum refining and, to a lesser extent, from coal tar. Ethylene is a light olefin typically derived from the cracking of natural gas liquids or petroleum naphtha. These two raw materials are reacted together in the presence of a catalyst, such as aluminum chloride or a zeolite, to form the ethylbenzene intermediate.
Pathways for Environmental and Human Exposure
Styrene moves from the factory into the environment and potential human contact through several distinct pathways. Emissions from manufacturing plants and facilities that use styrene to produce resins and polymers are a major source of environmental release, primarily into the air due to the chemical’s high volatility. Styrene also enters the environment through waste streams and from the finished products themselves.
Small amounts of the unreacted styrene monomer can leach from consumer products, such as polystyrene food containers and certain building materials. Within buildings, indoor air often contains higher levels of styrene than outdoor air, originating from products like:
- Carpet backing
- Insulation
- Photocopiers
- Laser printers
The primary route of human exposure for the general public is through inhalation of these vapors in both indoor and outdoor air. Ingestion is another exposure pathway, occurring through the consumption of foods that naturally contain styrene or through the transfer of trace amounts from styrene-based packaging materials. Although the chemical can be absorbed through the skin, inhalation remains the most significant route for absorbing styrene into the body.