From its discovery as a compound in apples to its use in industrially synthesized products, maleic acid has journeyed across a spectrum of applications, evolving from its modest origin. This organic compound, with two carboxylic acid groups, is integral to various industries, from water treatment to cosmetics. Dive into the multifaceted world of maleic acid, exploring its production, properties, and paramount uses in the modern industrial landscape.
What is Maleic Acid?
Maleic acid is a colorless to white crystalline organic compound with the chemical formula C₄H₄O₄. Due to its first isolation from apple, its name is derived from the Latin word “malum,” meaning apple. Maleic acid is classified as a dicarboxylic acid because it contains two carboxylic acid (-COOH) groups in its molecular structure. It is an isomer of fumaric acid, meaning it has the same molecular formula but a different chemical structure.
Source: Wikipedia
Found in small quantities in various plants and as a byproduct in specific microbial metabolic processes, maleic acid is a contributor to the tartness in some unripe fruits. Beyond its natural occurrence, maleic acid has a wide range of industrial applications. It plays a pivotal role in producing polymers and resins, serves as a pH regulator in cosmetics and personal care items, and is employed as a scale and corrosion inhibitor in water treatment processes.
How is Maleic Acid Produced?
Maleic acid is a naturally occurring organic acid found in various fruits, including apples and cherries. Due to its prevalence in apples, it is often called “apple acid.” It is also present in other fruits, particularly in unripe or sour varieties, and contributes to their tartness or acidity. Furthermore, malic acid is an essential intermediate in the metabolic cycles of plants, humans, and animals. It plays a vital role in the Krebs cycle (also known as the citric acid cycle) as an intermediate in energy production.
Maleic acid is primarily produced through industrial processes using various chemical methods.
Here are some of the standard methods used for producing maleic acid:
Catalytic Oxidation of Benzene
One of the most common methods involves the catalytic oxidation of benzene or other hydrocarbons containing benzene rings. This process typically uses a catalyst in air or oxygen, such as vanadium pentoxide (V₂O₅).
The oxidation of benzene or its derivatives produces maleic anhydride (C₄H₂O₃), which can then be hydrolyzed to produce maleic acid. Maleic anhydride is a precursor to maleic acid.
Fumaric Acid Isomerization
Fumaric acid can be isomerized to maleic acid through a chemical reaction. This process involves heating fumaric acid in the presence of a suitable catalyst, such as a mineral acid.
The cis-trans isomerization converts fumaric acid into maleic acid by shifting the double bond from the trans-configuration to the cis-configuration.
Hydrolysis of Maleic Anhydride
Maleic anhydride, readily available from the oxidation of hydrocarbons, can be hydrolyzed to produce maleic acid. This process involves adding water to maleic anhydride to break the anhydride ring, yielding maleic acid.
Biofermentation
Some microbial strains have been engineered to produce maleic acid through fermentation processes. Microorganisms like bacteria or yeast are cultured in suitable conditions, and their metabolic pathways are modified to produce maleic acid as a byproduct.
Industrial Uses and Applications of Maleic Acid
Maleic acid and its derivatives play pivotal roles in various industries. They serve as scale and corrosion inhibitors in water treatment. They enhance textile dye uptake and adhesion, formulate adhesives in the construction and automotive sectors, and regulate pH in cosmetics and food products. Additionally, they are employed in synthesizing polymers, resins, alkyd paints and also function as chemical intermediates, facilitating enhanced oil recovery techniques.
Water Treatment
| Scale Inhibition | Maleic acid and its derivatives, such as maleic anhydride and copolymers, are effective scale inhibitors. In cooling water systems, especially in industrial processes and power plants, the water can contain high concentrations of dissolved minerals like calcium and magnesium. When this water is heated or evaporated, these minerals can precipitate and form scale deposits on heat exchange surfaces, pipes, and equipment. Scale can reduce heat transfer efficiency and lead to equipment damage and increased energy consumption. Maleic acid-based inhibitors can prevent the formation of these scale deposits by sequestering the mineral ions and keeping them in solution. |
| Corrosion Inhibition | Maleic acid and its derivatives are corrosion inhibitors in cooling water treatment. Corrosion can occur when metals in the cooling system come into contact with water, leading to the deterioration of equipment and pipes. Maleic acid and its derivatives create a protective film on metal surfaces, which acts as a barrier between the metal and corrosive substances in the water. This film helps prevent corrosion and extends the lifespan of the equipment. |
Polymers and Resins
Maleic acid can be employed as one of the components in the synthesis of alkyd resins. Alkyd resins are widely used to produce paints, coatings, and varnishes. They offer excellent adhesion, durability, and weather resistance when applied to various surfaces. In the case of polyester resins, maleic acid or its anhydride form (maleic anhydride) is often utilized in combination with glycols (e.g., propylene glycol, ethylene glycol) to produce polyester resins. These polyester resins can be modified with cross-linking agents and other additives to meet specific performance requirements.
Food and Beverage Industry
| Acidulant | Maleic acid is used as an acidulant in the food industry to add tartness or acidity to various food and beverage products. It can enhance the flavor profile of these products by providing a sour or tangy taste. Maleic acid is used in different food items, including beverages, confectionery, bakery products, and canned and processed foods. |
| pH Control Agent | Maleic acid is a pH control agent in various food and beverage products. It helps food manufacturers maintain the desired pH levels, which is critical for ensuring food safety and stability. By controlling pH, maleic acid can extend the shelf life of certain products and prevent spoilage. |
Textile Finishing
| Improved Dye Uptake | Maleic acid can be used to improve the absorption and penetration of dyes into textile fibers. By modifying the surface characteristics of the fabric, maleic acid helps to ensure that dyes are evenly distributed and adhere effectively to the fibers during the dyeing process. This results in more vibrant and uniform coloration of textiles. |
| Adhesion of Dyes | Maleic acid also promotes the adhesion of dyes to textile fibers. This adhesion is fundamental to prevent dye bleeding and fading during subsequent washing or exposure to environmental conditions. Improved dye adhesion helps maintain the colorfastness and durability of dyed fabrics. |
| Cross-Linking | In some cases, maleic acid or its derivatives are used in textile finishing to facilitate cross-linking reactions. Cross-linking involves creating covalent bonds between individual fibers or within the fibers themselves. This process enhances the strength, durability, and wrinkle resistance of textiles. |
Adhesive Production
| Construction Industry | Maleic acid-based adhesives are used in the construction industry for bonding a wide range of building materials, including wood, concrete, metals, and plastics. These adhesives are valued for their strong bond strength and durability, making them suitable for structural applications. |
| Automotive Industry | In the automotive sector, maleic acid produces adhesives and sealants for bonding vehicle components and provides a secure, durable seal. These adhesives are used in various automotive applications, such as panel bonding, glass installation, and sealing joints, to prevent leaks and reduce noise. |
Cosmetics and Personal Care Products
| pH Regulation | Maleic acid is added to cosmetics and personal care products to raise or lower the pH to the desired level. This helps ensure that the product is within the pH range that is most suitable for the skin or hair. For example, skincare products like facial cleansers and toners may contain maleic acid to maintain the optimal pH for skin health. |
| Buffering Agent | Maleic acid can act as a buffering agent, helping maintain the stability of a product’s pH when exposed to external factors, such as air or changes in temperature. This stability is essential for preserving the integrity of the product over time. |
Oil Recovery
Enhanced oil recovery (EOR) is a set of techniques used in the oil and gas industry to increase the amount of oil that can be extracted from reservoirs beyond what can be achieved through primary and secondary recovery methods. Maleic acid and its derivatives are utilized as chemical additives in specific EOR processes, such as polymer flooding.
Chemical Intermediates
Maleic acid is often used as a starting material in synthesizing other chemicals, such as fumaric acid, maleic anhydride, and various esters. Maleic acid can be converted into fumaric acid through a simple isomerization reaction, and maleic acid can be dehydrated to produce maleic anhydride.
Properties of Maleic Acid
| Chemical Formula | C4H4O4 |
| Molecular Weight (g/mol) | 116.1 |
| Odor | Faint acid odor |
| Appearance | Colorless, odorless crystals or a white crystalline powder. |
| Melting Point | 135°C (275°F; 408 K) |
| Boiling Point | 355.00 to 356.00°C. (760.00 mm Hg) |
| Density (g/cm³) | 1.59 |
| Vapor Pressure (mm Hg at 25 °C ) | 1.34×10-5 |
| Solubility | Maleic acid is highly soluble in water due to its polar nature and the presence of carboxylic acid functional groups, which can form hydrogen bonds with water molecules. The solubility of maleic acid in water increases with temperature, meaning it becomes more soluble as the temperature rises. |
| Stability | Maleic acid is generally stable under normal storage conditions. It does not readily decompose or undergo chemical reactions without specific reactants or conditions. However, it is crucial to protect it from exposure to potent oxidizing agents or high temperatures, which could lead to degradation. |
Maleic Acid Derivatives
Due to its versatile chemical structure, maleic acid can synthesize various derivatives with unique properties and applications. Some common maleic acid derivatives include:
| Maleic Anhydride | Maleic anhydride is a crucial derivative produced by dehydrating maleic acid. It is widely used for resins, polymers, coatings, and agricultural chemicals. It is a precursor for various compounds, including alkyd resins, fumaric acid, and maleic acid esters. |
| Fumaric Acid | Fumaric acid is the geometric isomer of maleic acid, and it is produced by isomerizing maleic acid. It has applications as an acidulant in the food industry and in the production of resins, polymers, and various pharmaceuticals. |
| Maleic Acid Esters | Maleic acid can react with alcohols to form esters, such as dimethyl maleate, diethyl maleate, and dibutyl maleate. These esters have applications in producing plasticizers, adhesives, and coatings. |
| Maleic Acid Copolymers | Maleic acid can be copolymerized with other monomers to produce maleic acid copolymers. These copolymers find applications as dispersants, scale inhibitors, and corrosion inhibitors in various industries, including water treatment, agriculture, and oil production. |
| Maleic Acid Resins | Maleic acid can be polymerized to form maleic acid resins. These resins are used as adhesives, sealants, and ion exchange resins. |
Safety and Regulatory Considerations
Maleic acid has various safety considerations, including its potential to irritate the skin, eyes, and respiratory system, and requires careful handling and storage. Regulatory guidelines oversee its production, usage, and disposal in sectors like food and cosmetics, emphasizing environmental protection and consumer safety.
| FDA Information | 21 CFR 582.1069 |
| Workplace Exposure Limits | No exposure limits have been established for Maleic Acid. This does not mean that this substance is not harmful. Safe work practices should always followed.* It should be recognized that Maleic Acid can be absorbed through your skin, thereby increasing your exposure. |
| Other Regulatory Information | Maleic acid is on the Hazardous Substance list because it is cited by DOT and EPA. |
Safety & Toxicity of Maleic Acid
Maleic acid has a relatively low acute toxicity when handled and appropriately used. However, it can still pose health risks if not handled with care. Here is some toxicity information related to maleic acid:
Acute Toxicity
| Skin and Eye Irritation | Acute exposure to maleic acid can lead to feelings of dizziness and lightheadedness. These symptoms may result from the irritant effects on the respiratory system and the body’s response to exposure. |
| Respiratory Irritation | Inhaling maleic acid vapor or aerosols can irritate the nose, throat, and lungs. Inhalation can cause symptoms such as coughing, wheezing, and shortness of breath. The severity of these symptoms may depend on the concentration and duration of exposure. |
| Dizziness and Lightheadedness | Acute exposure to maleic acid can lead to feelings of dizziness and lightheadedness. These symptoms may be a result of the irritant effects on the respiratory system and the body’s response to exposure. |
| Neurological Effects | Exposure to high levels of maleic acid, primarily through inhalation, can have more severe neurological effects. Exposure may include tremors, seizures, and, in extreme cases, coma. It’s essential to seek immediate medical attention if these symptoms occur. |
Chronic Toxicity
As previously mentioned, there is limited evidence of chronic toxicity associated with maleic acid. Based on current available data, it is not considered a carcinogen, and its reproductive effects have not been extensively studied.
It’s essential to follow safety guidelines and best practices when working with maleic acid, including using appropriate personal protective equipment (PPE) such as gloves, goggles, and lab coats, working in a well-ventilated area, and having access to emergency eyewash stations and safety showers. Additionally, always store and handle maleic acid per safety regulations and guidelines provided by regulatory authorities and chemical manufacturers.
Identification Numbers
| CAS Number | 110-16-7 |
| Chemical Name: | 2-Butencdioie Acid |
| Other Names: | Toxilic Acid; Malenic Acid |
| EC Number | 203-742-5 |
Fun Facts About Maleic Acid
- Geometric Isomerism: Maleic acid has a geometric isomer called fumaric acid. These two compounds have the same chemical formula (C4H4O4) but different spatial arrangements of atoms. Maleic acid has cis-configuration, while fumaric acid has a trans-configuration. This difference in structure leads to distinct chemical and physical properties.
- Alternative Energy Storage: Researchers have explored the potential of maleic acid derivatives in alternative energy storage systems, such as redox flow batteries, for their potential to store and release electrical energy.
Additional Resources
- Wikipedia: Maleic acid
- PubChem: Compound 444266
- [Analytical Profiles of Drug Substances and Excipients: Harry G. Brittain, 2001](Harry G. Brittain, in Analytical Profiles of Drug Substances and Excipients, 2001)
- ECHA Substance Information: Substance Info 100.003.403
- New Jersey Department of Health: Fact Sheet 1151
