Most propylene glycol produced worldwide starts with propylene oxide, a petroleum-derived chemical that reacts with water in a process called hydration. This has been the dominant industrial method for decades, and it remains the source of the vast majority of propylene glycol on the market today.1Europe PMC. Insights on production mechanism and industrial applications of renewable propylene glycol But that petroleum-based route is no longer the only game in town. A growing number of producers are turning to renewable feedstocks like glycerol from biodiesel production and even engineered bacteria that ferment sugars directly into propylene glycol.
The Conventional Route Starts With Propylene Oxide
Propylene oxide is a small, reactive molecule shaped like a three-membered ring with an oxygen atom forming the bridge. That strained ring is what makes it so useful: it wants to open, and water is happy to help. When propylene oxide is mixed with a large excess of water and heated, the ring breaks open, and each molecule of propylene oxide picks up a water molecule to become propylene glycol. The chemistry was first demonstrated all the way back in 1859 by the French chemist Charles-Adolphe Wurtz, though some historical sources attribute early work to Pierre Berthelot around the same period.2International Journal of Pharmaceutical Sciences. The Versatility of Propylene Glycol: An Interdisciplinary Review
In modern plants, the reaction is carried out continuously. Water and propylene oxide are fed into a reactor at carefully controlled ratios and temperatures. Early studies of this reaction examined temperatures from roughly 38°C to 149°C (100°F to 300°F) and water-to-propylene-oxide ratios ranging from about 2.5 to 10 parts water per part propylene oxide by weight.3AIChE Journal. Kinetics of the catalyzed and uncatalyzed liquid‐phase hydration of propylene oxide The large excess of water is deliberate. It has everything to do with controlling what comes out on the other end.
Why So Much Water Matters
Propylene glycol itself has a hydroxyl group that can react with another molecule of propylene oxide. When that happens, you get dipropylene glycol instead, which is a larger molecule with different properties and a lower market value for most applications. If dipropylene glycol then reacts with yet another propylene oxide molecule, you get tripropylene glycol, and so on. Each of these “heavier” glycols is a byproduct that has to be separated from the desired monopropylene glycol.
Flooding the reactor with water tilts the odds. When there are far more water molecules around than propylene glycol molecules, the propylene oxide is more likely to react with water than with the glycol already formed. Research on zeolite catalysts has shown that the energy barrier for forming monopropylene glycol is lower than the barrier for forming dipropylene glycol, which helps too. On a ZSM-5 zeolite catalyst, one study calculated the rate-limiting activation energy for monopropylene glycol at about 27.4 kcal/mol via the dominant concerted mechanism, compared to about 30.8 kcal/mol for dipropylene glycol formation through the same type of mechanism.4The Journal of Physical Chemistry C. Reaction Mechanisms for the Formation of Mono-And Dipropylene Glycol from the Propylene Oxide Hydrolysis over ZSM‑5 Zeolite In other words, the chemistry already favors the product you want, and using excess water reinforces that advantage.
After the reaction, the mixture is a dilute soup of water, monopropylene glycol, dipropylene glycol, and trace amounts of heavier glycols. The water is boiled off and recycled, and the glycols are separated by distillation. Modern plants can achieve monopropylene glycol selectivity above 90 percent by keeping water ratios high and using efficient catalysts.
Catalysts and the Uncatalyzed Reaction
One of the interesting details about propylene oxide hydration is that it can proceed without any catalyst at all. Heat the mixture to a high enough temperature and the ring opens on its own. However, running the reaction without a catalyst means you need higher temperatures, which increases energy costs and can promote unwanted side reactions.
Acid catalysts, including sulfuric acid, have been used historically. Sulfuric acid speeds up the ring-opening reaction effectively, but it also corrodes equipment and creates waste disposal challenges.2International Journal of Pharmaceutical Sciences. The Versatility of Propylene Glycol: An Interdisciplinary Review That has pushed research toward solid acid catalysts that can be reused and do not dissolve into the product stream. Ion-exchange resins, for instance, have been studied as heterogeneous catalysts for this reaction in slurry reactors, where the solid catalyst beads are suspended in the liquid mixture. Research into these systems has shown that the heterogeneous catalyzed reaction follows somewhat different kinetics than the uncatalyzed one, and under certain conditions, the reaction rate is limited not by the chemistry itself but by how quickly propylene oxide can diffuse into the pores of the catalyst particles.5AIChE Journal. Hydration of propylene oxide using ion‐exchange resin catalyst in a slurry reactor Other solid catalysts that have attracted attention include sol-gel-prepared metal oxides such as sodium oxide dispersed in zirconia, which can work at lower water-to-propylene-oxide ratios, potentially simplifying the downstream water removal step.2International Journal of Pharmaceutical Sciences. The Versatility of Propylene Glycol: An Interdisciplinary Review
Making Propylene Glycol From Glycerol
The conventional route depends entirely on petroleum. Propylene oxide is made from propylene, which comes from oil refining or natural gas processing. That supply chain makes propylene glycol’s price and carbon footprint tied to fossil fuels. The biggest alternative that has emerged in recent years starts with glycerol, a thick, sweet liquid that is the main byproduct of biodiesel manufacturing. For every ton of biodiesel produced, roughly a tenth of a ton of crude glycerol comes along with it. The expansion of the biodiesel industry has created a surplus of glycerol, making it cheap and abundantly available.6PubMed Central. Heterogeneous Catalysts for Glycerol Biorefineries: Hydrogenolysis to 1,2-Propylene Glycol
The conversion is called hydrogenolysis. Glycerol is a three-carbon molecule with three hydroxyl groups. Under hydrogen pressure and with the right catalyst, one of those hydroxyl groups is selectively stripped away, leaving you with 1,2-propanediol, which is chemically identical to propylene glycol. The trick is doing this cleanly. Glycerol can also lose different hydroxyl groups to produce 1,3-propanediol, or it can break down further into ethylene glycol or smaller fragments. A well-designed catalyst pushes the reaction toward the desired 1,2-propanediol product and minimizes those competing pathways.
Copper-based catalysts, often combined with chromium, zinc, or aluminum oxides, have been the workhorses of glycerol hydrogenolysis research. Noble metal catalysts like ruthenium and palladium have also been explored. The reaction typically runs at temperatures between 180°C and 250°C under moderate hydrogen pressure. One persistent challenge is catalyst deactivation over time. The catalysts can lose activity through coking (carbon deposits building up on the active surface), leaching of the active metal into the liquid phase, and sintering (where metal particles clump together and lose surface area). Impurities in crude glycerol from biodiesel production, such as residual salts and methanol, can accelerate these deactivation processes.6PubMed Central. Heterogeneous Catalysts for Glycerol Biorefineries: Hydrogenolysis to 1,2-Propylene Glycol
Several companies already produce propylene glycol commercially from glycerol. The product is chemically indistinguishable from petroleum-derived propylene glycol, which means it can be used in the same applications without reformulation. The selling point is the renewable origin and, depending on the energy source used in the plant, a potentially lower carbon footprint.
The Fermentation Approach
Beyond chemical catalysis, there is a genuinely different route: using microorganisms to produce propylene glycol directly from sugars. Propylene glycol (specifically 1,2-propanediol) is a natural metabolic product of certain bacteria. Some species produce it as part of their normal sugar metabolism, and researchers have spent years engineering bacteria to ramp up production to industrially relevant levels.7PubMed Central. Microbial production and applications of 1,2-propanediol
The most intensively studied host is E. coli, the workhorse of metabolic engineering. Researchers have engineered strains that convert glucose to propylene glycol through an intermediate step involving lactic acid. In controlled fermenter conditions, one engineered strain produced the R-form of 1,2-propanediol at about 17.3 grams per liter with a molar yield of roughly 42% from glucose.8PubMed Central. Metabolic engineering of Escherichia coli for the de novo stereospecific biosynthesis of 1,2-propanediol through lactic acid Those numbers are impressive for a lab-scale fermentation but still a long way from the concentrations and throughput that conventional chemical plants achieve. The appeal of fermentation is that it runs at mild temperatures and atmospheric pressure, uses renewable sugar as feedstock, and can produce a specific mirror-image form of the molecule if that matters for the end use.
Why Mirror Images of Propylene Glycol Exist
Propylene glycol has a chiral center, meaning it exists in two mirror-image forms: R-1,2-propanediol and S-1,2-propanediol. Think of them like left and right hands: same components, same connectivity, but non-superimposable mirror images of each other. For most applications, including food additives, cosmetics, and antifreeze, nobody cares which form is present. The conventional petrochemical process produces a 50-50 mixture of both forms, and that is perfectly fine.
But in pharmaceutical synthesis and certain specialty chemical applications, one specific mirror image may be needed. This is where biological production methods have a genuine advantage. Enzymes are inherently selective about the three-dimensional shape of the molecules they produce. The engineered E. coli strains mentioned above can be tuned to produce predominantly R- or S-1,2-propanediol depending on which enzymes are expressed in the pathway.8PubMed Central. Metabolic engineering of Escherichia coli for the de novo stereospecific biosynthesis of 1,2-propanediol through lactic acid Another biological approach used the bacterium Ralstonia eutropha engineered with an alcohol dehydrogenase enzyme to produce R-1,2-propanediol at very high purity, reaching over 99.8% enantiomeric excess and a titer of 67.7 grams per liter.9PubMed Central. Hydrogen-driven asymmetric reduction of hydroxyacetone to (R)-1,2-propanediol by Ralstonia eutropha transformant expressing alcohol dehydrogenase from Kluyveromyces lactis Getting that level of purity through traditional chemistry would require extra separation steps that add cost.
Other Renewable Feedstocks Under Investigation
Glycerol and glucose are not the only renewable starting materials researchers have explored. Sorbitol, a sugar alcohol derived from corn starch or other plant materials, can be broken down catalytically into smaller molecules including propylene glycol. The chemistry involves using a metal catalyst (palladium, ruthenium, or nickel-based systems) under hydrogen pressure to crack the six-carbon sorbitol backbone into shorter-chain alcohols. In one study using a palladium-on-iron-oxide catalyst at 240°C and relatively low hydrogen pressure, sorbitol was fully converted, though ethanol rather than propylene glycol was the dominant liquid product under those particular conditions.10The Journal of Physical Chemistry C. Hydrogenolysis of sorbitol into valuable C3-C2 alcohols at low H2 pressure promoted by the heterogeneous Pd/Fe3O4 catalyst Getting high selectivity toward propylene glycol specifically, rather than a mix of small alcohols, remains the central challenge in sorbitol-based routes.
Cellulose and hemicellulose from wood, agricultural residues, and other lignocellulosic biomass have also attracted attention as feedstocks. The idea is to break down the complex sugars in plant cell walls and convert them to propylene glycol in one or two steps. The chemistry is more complex because the feedstock itself is harder to work with: cellulose must first be broken into soluble sugars before any glycol-forming reactions can happen. These routes are further from commercialization than the glycerol-based pathway but represent where a portion of the research community sees the long-term future.
How Petroleum-Grade and USP-Grade Differ
Regardless of the manufacturing route, the raw propylene glycol that comes out of a reactor or fermenter is not necessarily ready for all applications. The industry distinguishes between two broad grades. Industrial or technical-grade propylene glycol is used in applications like antifreeze, deicing fluids, and industrial heat-transfer systems. It needs to be reasonably pure but does not face the same scrutiny as a product destined for food or pharmaceuticals.
USP-grade propylene glycol (United States Pharmacopeia grade) meets stricter purity standards and is the form used in food, beverages, cosmetics, medications, and e-cigarette liquids. Achieving USP grade means additional purification, usually through careful fractional distillation, to remove trace byproducts like dipropylene glycol, residual catalyst components, and any other impurities. The final product must meet defined limits for water content, specific gravity, residue on ignition, and heavy metals, among other parameters. Whether the propylene glycol started from petroleum-based propylene oxide or from renewable glycerol, it must meet the same USP specification to be sold for food and pharmaceutical use.
Why the Shift Toward Renewable Production Is Slow
Given the variety of renewable routes available, you might wonder why petroleum-based propylene glycol still dominates. The answer comes down to economics and scale. Conventional propylene oxide hydration plants are enormous, well-optimized, and have had decades to push costs down. They produce propylene glycol at a price that renewable routes struggle to match, especially when oil prices are low.
Glycerol hydrogenolysis is the closest competitor. The feedstock is cheap, the chemistry is well understood, and several plants are operating commercially. But crude glycerol from biodiesel production contains impurities that can poison catalysts, which adds purification costs and shortens catalyst life.6PubMed Central. Heterogeneous Catalysts for Glycerol Biorefineries: Hydrogenolysis to 1,2-Propylene Glycol The hydrogen needed for the reaction also has to come from somewhere, and if it comes from natural gas reforming, the “renewable” label gets complicated.
Fermentation routes face even steeper scaling challenges. Biological processes are inherently slower than chemical ones, the product concentrations in the fermentation broth are low compared to a chemical reactor, and separating propylene glycol from water and metabolic byproducts at industrial scale adds cost. The niche where fermentation may find its best fit is in producing chirally pure propylene glycol for pharmaceutical intermediates, where the premium on purity justifies the higher production cost.
Propylene Glycol Versus Ethylene Glycol in Production
People often confuse propylene glycol with ethylene glycol because they have similar names and overlapping uses, particularly in antifreeze. The two molecules are produced by closely related chemistry: ethylene glycol comes from hydrating ethylene oxide, while propylene glycol comes from hydrating propylene oxide. The reactions follow the same pattern of ring-opening with water, and the same kinds of plants can produce both. The critical difference is in toxicity. Ethylene glycol is toxic to the kidneys even in relatively small amounts, while propylene glycol is considered safe enough to use in food and pharmaceuticals. That safety profile is why propylene glycol is specified for applications where human or animal contact is expected, from airplane deicing on runways near waterways to fog machines in theaters.
From a manufacturing standpoint, the push to develop renewable routes has been stronger for propylene glycol than for ethylene glycol, partly because glycerol hydrogenolysis naturally produces the three-carbon propylene glycol rather than the two-carbon ethylene glycol. The feedstock-to-product match is better. Turning glycerol into ethylene glycol requires breaking a carbon-carbon bond, which adds complexity and reduces selectivity.
What Happens to the Propylene Oxide Itself
To fully understand how propylene glycol is made, it helps to know where propylene oxide comes from, since that is the actual starting material in the dominant process. Propylene oxide is manufactured from propylene, a gas that comes from petroleum refining (fluid catalytic cracking) or from the steam cracking of natural gas liquids. There are several commercial processes for converting propylene to propylene oxide, and the choice of process affects the overall environmental footprint of the propylene glycol that eventually results.
The chlorohydrin process, one of the oldest methods, reacts propylene with chlorine and water to form propylene chlorohydrin, then treats it with a base to close the ring and form propylene oxide. It works but generates large volumes of chlorinated wastewater. Newer processes avoid chlorine entirely. The hydrogen peroxide to propylene oxide (HPPO) process uses hydrogen peroxide as the oxidant and a titanium silicalite catalyst, producing only water as a byproduct. It is considered one of the cleanest routes to propylene oxide currently available on a commercial scale. These upstream choices ripple through to the final propylene glycol product. A producer using the HPPO process for propylene oxide and then hydrating it to propylene glycol will have a meaningfully different environmental profile than one using the chlorohydrin route, even though the final propylene glycol molecule is chemically identical.