Ethylene oxide is made by passing ethylene gas and oxygen over a silver-based catalyst at high temperature and pressure inside large tubular reactors. This single chemical step, called epoxidation, has been the dominant industrial route for decades, and silver remains the only metal that does the job well enough for commercial production. But behind that simple description sits a surprisingly intricate system of catalyst chemistry, promoter additives, safety engineering, and heat management that determines whether a plant runs profitably or dangerously.
The Core Reaction
At its heart, ethylene oxide production is a partial oxidation. You take ethylene, the simplest alkene and one of the most widely produced organic chemicals in the world, and react it with oxygen to slip a single oxygen atom across its carbon-carbon double bond. That creates a strained three-membered ring: the epoxide. The challenge is stopping there. The reaction wants to keep going, burning the ethylene all the way to carbon dioxide and water, which releases far more energy but produces nothing useful. Getting the oxygen atom to insert neatly into the double bond without over-oxidizing the molecule is the central engineering problem of the entire process.
Modern plants feed high-purity ethylene and oxygen (rather than air, as older plants once did) into reactors operating at roughly 200–300 °C and pressures around 10–30 bar. The gas mixture passes over catalyst pellets packed into thousands of narrow tubes, and the ethylene oxide that forms is swept downstream for recovery. Even under optimized conditions, some ethylene inevitably burns to CO₂, so achieving high “selectivity” toward ethylene oxide rather than total combustion is the metric that makes or breaks a catalyst’s commercial value.
Why Silver and Nothing Else
Silver is essentially the only metal that works for this reaction at an industrial scale, and the reasons are rooted in how oxygen behaves on different metal surfaces. Researchers have studied gold and copper oxides as potential alternatives, and both fail for distinct reasons. On copper oxide, the overall epoxidation reaction is thermodynamically uphill, and even when ethylene oxide does form, the ring opens back up on the copper surface and rearranges into acetaldehyde rather than releasing as product. Gold oxide cannot regenerate the surface oxygen it needs to keep the reaction going. Silver oxide, by contrast, allows ethylene to react with surface oxygen and release ethylene oxide without an energy barrier blocking the way.
1Journal of Catalysis. Why silver is the unique catalyst for ethylene epoxidationIn practice, the catalyst consists of small silver particles deposited on a porous alpha-alumina support, a ceramic material that provides structural strength and surface area. The silver particles themselves are dynamic under reaction conditions. Studies using electron microscopy have shown that under industrially relevant pressures and temperatures, silver particles change shape and size during operation. Smaller particles below about 30 nanometers tend to shrink, while very large particles above 100 nanometers can break apart into smaller ones, particularly when chlorine-containing compounds are present in the feed.
2Applied Catalysis B: Environmental. Dynamics of silver particles during ethylene epoxidationThe Promoter System That Makes It Practical
Pure silver on alumina gets you ethylene oxide, but not enough of it. The selectivity of unpromoted silver catalysts tops out well below what is commercially viable. The real breakthrough came from discovering that adding tiny amounts of other elements to the catalyst dramatically shifts the balance away from total combustion and toward the desired epoxide product. Modern high-performance catalysts use a cocktail of promoters, each playing a distinct role.
Chlorine was one of the earliest promoters adopted. Traces of chlorine-containing compounds, typically organic chlorides like vinyl chloride or ethyl chloride, are fed into the reactor gas stream. On the silver surface, chlorine atoms park themselves on vacant oxygen sites. Those vacant sites are precisely the spots where ethylene would otherwise adsorb and form an intermediate that rearranges into acetaldehyde instead of ethylene oxide. By blocking those sites, chlorine suppresses the unwanted side pathway.
1Journal of Catalysis. Why silver is the unique catalyst for ethylene epoxidationCesium, an alkali metal, is deposited directly onto the catalyst during manufacturing. Research using surface spectroscopy and computational modeling has shown that cesium weakens the grip of oxygen atoms on the silver surface. This matters because once ethylene oxide forms as a precursor on the surface, it needs to detach and float away as a gas-phase product before it can rearrange into something else. Cesium shifts the electronic properties of silver in a way that makes that desorption step easier, pushing selectivity higher.
3Molecular Catalysis. Origin of enhanced ethylene oxide selectivity by Cs-promoted silver catalystRhenium, added at remarkably low concentrations (on the order of tens of parts per million), handles a different step in the mechanism. Where cesium helps the product leave the surface, rhenium adjusts the electronic environment to make the initial oxygen attack on ethylene’s double bond more selective. Studies on catalysts containing both cesium and rhenium have shown that the two promoters act on sequential stages: rhenium enhances the formation step, and cesium enhances the release step. Together, they boosted selectivity in one study from roughly 79% to about 83%.
4Journal of Catalysis. An investigation on the role of Re as a promoter in Ag–Cs–Re/α-Al2O3 high-selectivity, ethylene epoxidation catalystsThat jump from 79% to 83% may sound modest, but at the scale of a world-class ethylene oxide plant producing hundreds of thousands of tons per year, every percentage point of selectivity translates into enormous savings in raw ethylene and reductions in CO₂ byproduct. The economics of the industry hinge on squeezing out these incremental gains.
Inside the Reactor
The reactors used for ethylene oxide production are large shell-and-tube heat exchangers, sometimes containing tens of thousands of narrow tubes packed with catalyst pellets. Each tube is typically just a few centimeters in diameter and several meters long. The ethylene-oxygen mixture flows through the tubes while a coolant, usually a heat-transfer fluid or pressurized water, circulates on the shell side to carry away heat.
This design exists because of the reaction’s thermal personality. The desired epoxidation is exothermic, and the competing total combustion is far more exothermic still. If heat is not removed quickly enough, local hot spots develop, which accelerate the combustion pathway and can trigger a runaway reaction. Modeling work on industrial-scale multitubular packed-bed reactors has focused on simultaneously maximizing ethylene oxide output and selectivity while maintaining safe operating margins around the oxygen content in the reactor gas.
5Chemical Engineering & Technology. Heterogeneous Reactor Modeling of an Industrial Multitubular Packed‐Bed Ethylene Oxide ReactorOxygen concentration is carefully controlled for safety reasons. Ethylene oxide itself is flammable and can decompose explosively under the right conditions. Research on its explosive decomposition at elevated temperatures and pressures has shown that turbulence significantly amplifies the severity of an explosion, particularly the rate of pressure rise, which contradicts some earlier assumptions.
6PubMed. Explosive decomposition of ethylene oxide at elevated condition: effect of ignition energy, nitrogen dilution, and turbulenceTo manage these risks, plants operate with gas compositions well below the explosive limits and use nitrogen or methane as ballast gas to dilute the mixture. The reactor feed is also recirculated: unreacted ethylene and oxygen are separated from the product stream and sent back through, which improves overall conversion without requiring dangerously high single-pass oxygen levels. Heat integration across the plant is extensive, with one optimization study identifying 16 streams available for heat recovery in a typical ethylene-to-ethylene-oxide process, split between hot streams that need cooling and cold streams that need heating.
7Chemical Engineering Research and Design. Simultaneous process optimization and heat integration for ethylene-to-ethylene oxide process: A surrogate model-based approachWhat Happens After the Reactor
The gas leaving the reactor contains a modest concentration of ethylene oxide mixed with unreacted ethylene, oxygen, CO₂, and ballast gas. The first recovery step is typically an absorber column where the gas contacts water, which preferentially dissolves the ethylene oxide out of the stream. The remaining gases loop back to the reactor. The ethylene oxide is then stripped out of the water in a separate column and purified.
Most ethylene oxide never reaches the market as ethylene oxide. The majority is immediately converted into derivatives on-site or at nearby facilities. The largest downstream product by far is monoethylene glycol (MEG), which is used to make polyester fibers, PET plastic bottles, and automotive antifreeze. MEG is made by reacting ethylene oxide with water, but there are two competing approaches: direct hydration, which simply adds water, and indirect hydration, which first converts ethylene oxide into ethylene carbonate and then hydrolyzes that intermediate to MEG.
A comparative study of these two routes found that direct hydration consumes substantially more energy in utilities, roughly 279 megawatts more than the indirect route for a comparable plant. The environmental gap is even wider: greenhouse gas emissions from the direct hydration process were about three times greater than from the indirect route using ethylene carbonate.
8PubMed Central. A comparative study of mono ethylene glycol economic production via different techniquesBeyond MEG, ethylene oxide is the starting material for ethoxylates (used in detergents and emulsifiers), polyethylene glycols (pharmaceuticals, cosmetics, industrial lubricants), ethanolamines (gas treating, personal care products), and glycol ethers (solvents and coatings). Its versatility as a chemical building block is why global production runs into the tens of millions of tons annually.
How Chlorine Reshapes the Catalyst Over Time
Chlorine’s role as a gas-phase promoter comes with a trade-off that affects how long a catalyst charge lasts. While chlorine improves selectivity by blocking unwanted adsorption sites, it also drives dramatic physical changes in the silver particles. Electron microscopy studies tracking the same catalyst locations before and after 60 hours of operation found that without chlorine, silver particles were only slightly affected. With chlorine in the feed, particles smaller than 30 nanometers rapidly disappeared, and particles larger than 100 nanometers broke apart through a chlorine-induced redispersion process.
2Applied Catalysis B: Environmental. Dynamics of silver particles during ethylene epoxidationOver months and years of operation, this reshaping contributes to gradual catalyst deactivation. Industrial catalyst charges typically last several years before they must be replaced, and the economic decision to swap a catalyst load involves weighing the declining selectivity against the cost of purchasing and installing a new charge. Spent catalysts are valuable for silver recovery.
Worker Safety and Exposure
Ethylene oxide is not only explosive but also a known human carcinogen, which makes worker exposure a serious concern at production facilities and wherever EO is used as a sterilant. Ethylene oxide sterilization is the standard method for medical devices that cannot withstand steam autoclaving, and this downstream use actually generates some of the highest worker exposures in the supply chain.
Studies in medical-supply manufacturing have found that the unloading step after sterilization is a critical exposure point. Short-term exposures during unloading dropped significantly when facilities increased the number of post-sterilization purge cycles, which flush residual EO from the chamber before workers open it. Even after implementing controls, average exposure levels of about 3.4 ppm were still measured, and respirator use was recommended until levels could be brought below permissible limits.
9PubMed. Ethylene oxide sterilization in the medical-supply manufacturing industry: assessment and control of worker exposureOver the past four decades, engineering controls and workplace management practices have substantially reduced EO exposures across all industries that handle it, including chemical manufacturing, industrial sterilization, and healthcare. More stringent environmental regulations have particularly targeted industrial sterilization facilities, where exposures have historically been highest.
10PubMed Central. Ethylene Oxide Measurements From OSHA Workplace Investigations: Patterns in Exposure by Industry, Occupation, and Over TimeEmerging Routes to Ethylene Oxide
The conventional process depends on fossil-derived ethylene and molecular oxygen, but researchers are exploring alternatives that could change one or both of those inputs.
One approach starts from bioethanol instead of petroleum-derived ethylene. In a demonstration process, ethanol was first dehydrated to ethylene over a zeolite or alumina catalyst, and the resulting ethylene was then epoxidized using lattice oxygen from a solid oxide material (a strontium iron oxide) rather than gaseous oxygen. This “chemical looping” approach avoids mixing ethylene and oxygen gas directly, which simplifies safety engineering. The process achieved 57% selectivity to ethylene oxide at 15% ethylene conversion, exceeding what a conventional unpromoted silver catalyst delivers with molecular oxygen.
11Chemical Engineering Journal. Demonstrating on-demand production of bio-ethylene oxide in a two-step dehydration-epoxidation process with chemical looping operationsElectrochemical epoxidation is another frontier. Instead of thermally activating the reaction over a hot silver catalyst, this method uses renewable electricity to drive the oxidation of ethylene through a halide-mediated pathway. Recent work using a ruthenium-based electrode achieved a Faradaic efficiency for ethylene oxide of up to about 85% and demonstrated stable operation for over 120 hours.
12PubMed. Boosting Electrochemical Ethylene Epoxidation via Ruthenium Valence State StabilizationThe appeal of the electrochemical route goes beyond novelty. A techno-economic assessment found that electro-oxidation of ethylene powered by renewable electricity could cut CO₂ emissions roughly in half compared to conventional thermal plants.
13Journal of Cleaner Production. Investigation of the sustainable production of ethylene oxide by electrochemical conversion: Techno-economic assessment and CO2 emissionsNeither the bio-based nor the electrochemical route is close to displacing the conventional silver-catalyzed process, which benefits from decades of optimization and enormous installed capacity. But both represent serious research directions rather than academic curiosities, and the pressure to decarbonize heavy chemicals will keep pushing them forward.
Transporting a Difficult Molecule
Getting ethylene oxide from the plant to downstream users is logistically tricky. It is flammable, toxic, and capable of explosive decomposition even in the absence of air if contaminated or overheated. Most ethylene oxide moves by pipeline between adjacent chemical facilities, which minimizes handling. When rail transport is necessary, it travels in specially designed tank cars subject to stringent safety standards, including reinforced head and shell designs and top fittings protection systems engineered to withstand rollover impacts.
14Federal Register. Hazardous Materials: Improving the Safety of Railroad Tank Car Transportation of Hazardous MaterialsThermal management during storage and transport matters, too. Reviews of ethylene oxide’s thermal reactivity have emphasized that contamination with common substances, including rust, acids, and alkalis, can catalyze dangerous decomposition reactions at temperatures well below what pure EO would tolerate. Facilities handling the compound maintain rigorous cleanliness standards for all vessels and piping, and storage tanks are typically kept under inert nitrogen blankets to prevent contact with air and moisture. The entire supply chain, from reactor outlet to final consumer, reflects the tension between ethylene oxide’s extraordinary usefulness as a chemical intermediate and its equally extraordinary hazard profile.