What Is Propionic Acid?
Propionic acid, also called propanoic acid, is a saturated carboxylic acid with the molecular formula C3H6O2 and the structural formula CH3CH2COOH. Its CAS number is 79-09-4 and its molecular weight is 74.08. The name comes from the Greek words protos, meaning first, and pion, meaning fat, because it is the smallest carboxylic acid that shows properties similar to fatty acids.
In its pure form propionic acid is a colorless oily liquid with a pungent, rancid odor. It is miscible with water and with most organic solvents. The compound occurs naturally and is produced by Propionibacterium bacteria during fermentation, which is why it is found in Swiss cheese and in the digestive systems of ruminants.
How Is Propionic Acid Made Industrially?
Industrial production of propionic acid follows several routes, depending on feedstock availability, catalyst technology and plant economics.
Hydrocarboxylation of Ethylene
The main industrial route is the hydrocarboxylation of ethylene, in which ethylene reacts with carbon monoxide and water in the presence of a nickel catalyst. The reaction is carried out at elevated temperature and pressure and produces propionic acid in high yield. This process is sometimes called Reppe-type carbonylation.
Oxidation of Propionaldehyde
Propionic acid can also be produced by liquid-phase oxidation of propionaldehyde with oxygen or air. Propionaldehyde itself is usually made by hydroformylation of ethylene, so this route is a two-step sequence from ethylene.
Byproduct and Fermentation Routes
Smaller quantities of propionic acid are recovered as a byproduct of certain oxidation processes, for example during the manufacture of acetic acid. In addition, a biotechnological route uses Propionibacterium or related strains to ferment sugars, lactate or whey into propionic acid. This route is being developed further because it uses renewable feedstocks.
Main Applications of Propionic Acid
Food and Feed Preservation
Propionic acid and its salts, including calcium, sodium and potassium propionate, inhibit the growth of molds and bacteria. The acid form is registered as food preservative E280 in the EU. These compounds are used to preserve baked goods and cheese products, and to prevent mold formation on grain and animal feed.
Chemical Intermediate
Propionic acid is a building block for herbicides, pharmaceuticals, rubber and plastic auxiliaries, cellulose esters and plasticizers. Its esters, such as ethyl propionate and benzyl propionate, are used as solvents and as flavor and fragrance ingredients.
Other Uses
Propionic acid salts have antifungal properties and have been used in topical preparations for fungal skin infections. The acid is also used as an antimicrobial agent in some industrial processes and as an intermediate in the production of other organic compounds.
Grades, Packaging and Buying Considerations
Propionic acid is supplied in food grade and technical grade. Food grade must meet the purity and impurity limits required by food additive regulations, including limits for heavy metals and residual solvents. Technical grade is suitable for chemical synthesis and industrial use. Typical packaging for bulk export includes 25 kg drums, IBC totes and ISO tank containers. Buyers should confirm the certificate of analysis, production date and transport classification with the supplier.
Safety and Handling
Propionic acid is corrosive and can cause burns to skin, eyes and the respiratory tract. It should be handled with gloves, goggles and adequate ventilation, and stored away from bases and oxidizers. Spills should be contained and neutralized carefully. The substance is classified as a corrosive liquid and requires proper dangerous goods documentation for transport.
Frequently Asked Questions
What is propionic acid? Propionic acid, also called propanoic acid, is a saturated carboxylic acid with the molecular formula C3H6O2 and the structural formula CH3CH2COOH.
How is propionic acid manufactured? Hydrocarboxylation of Ethylene The main industrial route is the hydrocarboxylation of ethylene, in which ethylene reacts with carbon monoxide and water in the presence of a nickel catalyst. Byproduct and Fermentation Routes Smaller quantities of propionic acid are recovered as a byproduct of certain oxidation processes, for example during the manufacture of acetic acid.
What are the main applications of propionic acid? These compounds are used to preserve baked goods and cheese products, and to prevent mold formation on grain and animal feed. Food and Feed Preservation Propionic acid and its salts, including calcium, sodium and potassium propionate, inhibit the growth of molds and bacteria.
What are the key properties of propionic acid? Propionic acid, also called propanoic acid, is a saturated carboxylic acid with the molecular formula C3H6O2 and the structural formula CH3CH2COOH. Propionic acid is corrosive and can cause burns to skin, eyes and the respiratory tract.
Does propionic acid occur naturally? The compound occurs naturally and is produced by Propionibacterium bacteria during fermentation, which is why it is found in Swiss cheese and in the digestive systems of ruminants. These compounds are used to preserve baked goods and cheese products, and to prevent mold formation on grain and animal feed.