Styrene (ethenylbenzene) is a colorless, oily liquid hydrocarbon with the chemical formula C₈H₈. It is one of the most widely produced monomers globally, serving as the foundational building block for a vast array of materials. Its commercial value stems from its ability to undergo polymerization, linking small molecules into long chains. Styrene is the precursor to common plastics like polystyrene, synthetic rubbers such as styrene-butadiene rubber (SBR), and specialty plastics like acrylonitrile butadiene styrene (ABS). The primary industrial manufacturing method relies on a two-step process beginning with two common petrochemical feedstocks.
The Essential Starting Materials
The industrial synthesis of styrene requires two fundamental hydrocarbon ingredients: benzene and ethylene. Both compounds are derived from the petrochemical refining industry.
Styrene production is a major consumer of these raw materials; roughly half of the world’s entire benzene output is channeled into creating the styrene monomer. The two-step chemical process first combines these hydrocarbon molecules to construct the intermediate compound, ethylbenzene, before converting it into the final product.
Step One Creating Ethylbenzene
The first synthetic step is the alkylation of benzene with ethylene to form ethylbenzene. This reaction is highly exothermic, meaning it releases a significant amount of heat. The process can be conducted in either the liquid or vapor phase, but modern facilities favor liquid-phase alkylation due to its efficiency.
Liquid-phase processes operate at moderately high pressures, often around 20 atmospheres, but at relatively lower temperatures. The reaction relies on specialized catalysts to facilitate the joining of the two molecules. While older methods used Lewis acids like aluminum chloride, contemporary plants utilize solid acid catalysts, such as synthetic zeolites.
A challenge is ensuring high selectivity to minimize the formation of undesirable byproducts, primarily diethylbenzene (DEB), where a second ethylene molecule attaches to the benzene ring. To maximize the yield of ethylbenzene, the DEB byproduct is recycled back into a separate reactor. There, it undergoes a transalkylation reaction with fresh benzene to convert it into additional ethylbenzene, significantly improving feedstock efficiency.
Step Two The Dehydrogenation Process
The second step is the catalytic dehydrogenation of ethylbenzene to yield styrene monomer. This reaction involves the selective removal of two hydrogen atoms from the ethylbenzene molecule. The direct dehydrogenation process accounts for approximately 85% to 90% of the world’s styrene production.
The reaction is endothermic, requiring a continuous supply of heat to sustain the process. Industrial reactors operate under extremely high temperatures, typically between 600°C and 700°C, and at low pressures to encourage conversion. This environment necessitates the use of a robust catalyst, which is almost always based on iron oxide promoted with potassium carbonate.
A large amount of superheated steam is introduced along with the ethylbenzene feedstock. The steam serves multiple functions, including providing necessary heat energy for the endothermic reaction. More importantly, the steam acts as a diluent, lowering the partial pressure of the reactants. This reduction is thermodynamically favorable, shifting the reversible reaction equilibrium toward the formation of styrene and hydrogen gas. The steam also helps suppress the formation of carbon deposits, or coke, on the catalyst surface.
Final Separation and Alternative Methods
The mixture leaving the dehydrogenation reactor, known as the effluent, is a complex stream containing styrene, unreacted ethylbenzene, hydrogen gas, steam, and minor byproducts like benzene and toluene. Recovery first involves condensing the effluent to separate the liquid hydrocarbons and water from the non-condensable hydrogen and fuel gas stream.
The liquid hydrocarbon stream is then separated using a series of vacuum distillation columns. This separation is complex because styrene and ethylbenzene have similar boiling points, and the high temperatures required for distillation can cause the styrene to spontaneously polymerize. To prevent this unwanted reaction, chemical polymerization inhibitors are added throughout the purification train. Unreacted ethylbenzene is recovered and recycled back to the dehydrogenation reactor for further conversion.
While dehydrogenation is dominant, the Styrene Monomer/Propylene Oxide (SM/PO) process is an important alternative manufacturing route. This method co-produces styrene alongside propylene oxide, another high-value chemical. Although the SM/PO process is more complex, the profitability of simultaneously generating two products makes it an economically viable alternative for certain integrated chemical complexes.