Bio-ethylene and bio-ethylene oxide

Ethylene is the highest-volume organic molecule in industry: tens of millions of tonnes a year going into polyethylene, ethylene oxide, ethylene glycol, styrene and vinyl chloride. It is made by steam cracking naphtha or ethane — a process of enormous scale, deep maturity and thin margin. That is what any biological route competes with.

Ethanol dehydration: it works, and the arithmetic is known

The chemistry is simple. Ethanol loses water over an acid catalyst — alumina or a zeolite — at 300-500 °C, giving ethylene at above 99 percent selectivity. The reaction is endothermic and needs heat, but otherwise behaves impeccably, and the technology is about a century old: it predates cracking and returns wherever oil is expensive and sugar is cheap.

The constraint is not selectivity but mass. Ethanol weighs 46, ethylene 28; the limiting mass yield is 60.9 percent, so a tonne of ethylene needs at least 1.64 tonnes of ethanol and, with losses, closer to 1.7. That equation decides everything: bio-ethylene competes exactly where ethanol is cheap relative to naphtha. The largest operating plant was built in Brazil on sugarcane ethanol and runs for a specific set of buyers willing to pay for renewable carbon in polyethylene. It is a real industrial asset, but its existence is explained by cheap feedstock, a provenance premium and policy — not by dehydration being technically superior to cracking.

Direct fermentation of ethylene: why not

An ethylene-forming enzyme does exist in nature; in Pseudomonas syringae it is a dioxygenase using ferrous iron and 2-oxoglutarate. The problem is the stoichiometry. The enzyme consumes a five-carbon TCA-cycle intermediate and releases a two-carbon gas, with the remaining three carbons leaving as CO2. More carbon is lost per molecule of product than ends up in it, and that is written into the mechanism rather than into an unoptimised strain. Add a low turnover number and self-inactivation by the oxidative cycle, and the picture closes. Direct fermentative ethylene remains a research subject; it does not survive comparison with a route that is 61 percent efficient on mass through ethanol.

Ethylene oxide

Ethylene oxide is made by direct epoxidation of ethylene with oxygen over a silver catalyst. That reaction has a ceiling of its own, unrelated to feedstock origin: some of the ethylene burns to carbon dioxide and water, and industrial selectivity sits around 80-90 percent. Biology cannot raise it — it is a property of silver surface chemistry.

Which leads to something worth knowing when reading a label. “Bio-ethylene oxide” is chemically indistinguishable from the ordinary kind: the same oxide, from the same silver catalysis, made from ethylene of biological origin. The claim is about the carbon in the feedstock, and it is substantiated either by isotopic measurement of biogenic carbon content or by a mass-balance scheme across the supply chain. That is a legitimate and auditable statement — but a statement about provenance, not about process, and the two should not be conflated.