Propylene glycol is a viscous, colorless liquid that is nearly odorless but has a faintly sweet taste . It is a diol with the chemical formula C3H8O2.
The physicochemical properties of propylene glycol are similar to those of ethylene glycol. Its first recorded description was by Wurtz in 1859.
Industrial-scale production of 1,2-propanediol from propylene oxide and water started in the 1930s and is still in use today. The process simultaneously yields di- and tripropylene glycols.
The worldwide production capacity of 1,2-propanediol exceeds 2.56 million tons per year.
1,2-Propanediol has numerous applications, including use in unsaturated polyesters for thermoset composites, food chemistry, food processing equipment, cosmetics, pharmaceuticals, as well as deicers and automotive antifreeze components.
Table of Contents
1. Production of Propylene glycol
1.1. Production of Propylene glycol by Hydrolysis of Propylene Oxide
The dominant process for industrial production of 1,2-propanediol involves the direct hydrolysis of propylene oxide in the presence of water.
This process is known to generate dipropylene glycol and tripropylene glycol through the sequential addition of propylene oxide to 1,2-propanediol.
Therefore, the separation of all three products is carried out by distillation.
The manufacturing process of 1,2-propanediol is initiated by combining propylene oxide and water at a molar ratio of 1:15, at an initial temperature of 125°C and a pressure of about 2 MPa. The reactor effluent temperature increases to around 190°C due to the exothermic reaction.
The resulting mixture of 1,2-propanediol, dipropylene glycol, and tripropylene glycol is approximately in the ratio of 100:10:1, respectively, with this ratio achieved at the above ratio of water to oxide.
Increasing the water-to-oxide ratio can lead to higher propylene glycol ratios; however, this results in higher recycle rates, lower throughput, and increased energy costs.
The water-glycol mixture is withdrawn from the reactor zone at approximately 200°C and is subjected to water removal in dehydration columns. High-purity 1,2-propanediol, dipropylene glycol, and tripropylene glycol are separated by three successive vacuum distillations.
The residue comprises a combination of higher glycols that have limited commercial applications.
This process generates very low waste because of the low toxicity of 1,2-propanediol, dipropylene glycol, and tripropylene glycol, which poses minimal environmental hazards. All of these compounds are readily biodegradable.
a) Reactor; b) Evaporator (typically 2 -4 columns); c) 1,2-Propanediol distillation column; d) Dipropylene glycol distillation column; e) Tripropylene glycol distillation column
1.2. Production of Propylene glycol by Hydrogenolysis of Glycerol to 1,2-Propanediol
Glycerol, a byproduct of the transesterification of vegetable oils for biodiesel production, is an interesting starting material for the synthesis of 1,3-propanediol and 1,2-propanediol.
Approximately 10 kg of crude glycerol with a concentration of 50-55% can be obtained from the production of 100 kg of biodiesel.
Metallic catalysts and hydrogen are used to hydrogenate glycerol to 1,2-propanediol, which is commercially available as industrial and U.S.P. grades.
This reaction takes place in two steps through an acetol intermediate. The first step involves the conversion of glycerol to acetol in the presence of a metallic catalyst. In the second step, acetol is hydrogenated to 1,2-propanediol using a similar catalyst.
The choice of catalyst and its properties affect the selectivity of the reaction. Copper chromite was found to be the most effective catalyst.
1.3. Alternative Routes
A comprehensive study has been conducted on the process of acetoxidation of propene leading to the production of glycol acetates followed by hydrolysis to the glycol. This method is reported to offer a cost-effective alternative to the propylene oxide route, albeit with significantly higher fixed capital investment costs.
In addition, direct hydroxylation of propene to yield 1,2-propanediol has been reported, which employs the use of oxygen and osmium catalysts.
2. Chemical Reactions of Propylene Glycol
The chemical properties of 1,2-propanediol are primarily determined by its hydroxyl groups, and its reactions are characteristic of alcohols.
At elevated temperatures, 1,2-propanediol condenses with carboxylic acids to produce esters and water. It also readily reacts with isocyanates and acid chlorides to produce carbamates and esters, respectively.
Poly-condensation reactions of 1,2-propanediol with diacids result in the formation of polyesters. Notably, unsaturated polyesters are produced in significant amounts by reacting 1,2-propanediol with maleic anhydride and other diacid components.
1,2-propanediol reacts with propylene oxide to generate dipropylene glycol, tripropylene glycol, and polyether polyols. The polypropylene glycol ethers derived from this reaction serve as crucial industrial building blocks for polyurethane foams and elastomers.
Owing to its 1,2-diol structure, 1,2-propanediol can undergo various intriguing cyclization reactions. With aldehydes and ketones, cyclic acetals and ketals can be produced, respectively.
When subjected to elevated temperatures and acidic catalysts, 1,2-propanediol can undergo dehydration to yield cyclic ethers. These dehydration products are typically present at low levels in aqueous byproduct streams of poly-condensation reactions involving 1,2-propanediol.
3. Uses of Propylene Glycol
1,2-Propanediol finds its largest application in the production of unsaturated polyester resins. These resins are obtained by reacting 1,2-propanediol with saturated and unsaturated carboxylic diacids, such as maleic anhydride and iso-phthalic acid.
The resulting resins are dissolved in styrene or another polymerizable monomer and combined with fillers, chopped glass, peroxide polymerization initiators, and other additives to form a cross-linked, thermoset composite that is used in automotive plastics, construction, and fiberglass boats.
This application accounts for approximately 45% of the total production of 1,2-propanediol.
The FDA has designated 1,2-propanediol as Generally Regarded As Safe (GRAS), which has led to its extensive use in the food and pharmaceutical industries.
It serves as a humectant, solvent, and preservative in food and pet food products.
1,2-Propanediol is used as a lubricant for machinery, a solvent in food processing, in food wraps, and as an antifreeze agent in machinery cooling water.
It also acts as an emollient, softening agent, humectant, and carrier in cosmetics, skin-care products, and livestock medicinal products.
Due to its proven performance, low toxicity, biodegradability, and environmental acceptance, 1,2-propanediol is utilized in aircraft deicing and anti-icing fluids, offering freeze protection comparable to that of ethylene glycol, which is the main component of automotive antifreeze solutions.
In fact, 1,2-propanediol has replaced the more toxic ethylene glycol in automotive antifreeze compositions.
It is also used as a lubricant in combination with di- and tripropylene glycols, as a humectant for tobacco, and as a solvent for e-cigarettes.
1,2-Propanediol is employed in many latex paints as a freeze-thaw protector and to control evaporation in hot and dry environments.
It can react with fatty acids or long-chain carboxylic acids to produce ester lubricants, emulsifiers, and plasticizers.
Additionally, 1,2-propanediol is a precursor of numerous polyether polyols utilized in the manufacturing of urethane foam, elastomers, adhesives, and sealants.
Reference
- Propanediols, Ullmann’s Encyclopedia of Industrial Chemistry. – https://onlinelibrary.wiley.com/doi/10.1002/14356007.a22_163.pub2
