The requirements in lower carboxylic acids: acetic, formic and propionic were given. The mechanism of liquid-phase oxidation of n-pentane was proposed. The main directions of the formation of the oxidation products of n-pentane were illustrated. Based on a study of the reaction mechanism and the influence of the main process parameters on the yield of lower carboxylic acids, was developed a new method of their obtaining by the liquid-phase oxidation of light hydrocarbon fractions. This method is highly competitive since it is based on the oxidation of cheap raw materials and allows obtaining the lower carboxylic acids with a total selectivity of their formation about 50 %. The firm «Proksinfo» is interested in business partners for joint industrial implementation of proposed process. You can contact us by e-mail: prouv11@yahoo.com.
Lower carboxylic acids: acetic, formic and propionic are widely used in industry and in home. Acetic acid is used in the food industry and household cooking for preserving food, obtaining medicinal and aromatic substances, solvents. It is used in printing and dyeing, as the raw material for obtaining substances like acetic anhydride and acetates. The main methods for producing acetic acid are: oxidation of acetaldehyde, butane, and light hydrocarbons C5 – C7, the catalytic carbonylation of methanol, acetone hydration and biochemical oxidation of ethanol. World production of acetic acid is 14 – 18 million tons per year. However, only 10 % of acetic acid is produced by the biochemical method. This acid is used to produce food vinegar. Formic acid is used in the dyeing and trimming of textile and paper, leather processing, as preservative in the ensilage of green mass, disinfection. As a raw material, it is used to produce drugs, pesticides, solvents, salts, and ethers. The main industrial method of formic acid production is the dehydration of H2 and CO2, the hydration of CO in the presence of alkali, hydrolysis of formamide, oxidation of methanol and byproducts of the production of acetic acid, butane and light hydrocarbon fractions. World production of formic acid is 300 – 350 thousand tons per year. Propionic acid is a valuable raw material for obtaining of many substances, for the manufacture of herbicides, pharmaceuticals, fragrances, plastics, solvents and surfactants. The main industrial method of production of propionic acid is the carbonylation of ethylene, oxidation of propionic aldehyde and by-product of the production of acetic acid during the oxidation of light hydrocarbon fractions. World production of propionic acid is 250 – 300 thousand tons per year. It should be noted that each year the need for these acids increases by a few percent. Each of mentioned methods of producing acetic, formic and propionic acids have own advantages and disadvantages. One of the cheapest ways to produce acetic, formic and propionic acid is a liquidphase oxidation of light hydrocarbon fractions C5 – C7, because of its cheap precursors. The disadvantage of this method is the small selectivity of formation of these acids. The total selectivity of acetic, formic and propionic acids is 30-35 %. It means that apart from these acids during the oxidation of hydrocarbons a lot of by-products (ketones, alcohols and esters) are formed. Therefore, there is a problem of the use of oxidation by-products. A large amount of by-products during the oxidation process of light hydrocarbons is due to a difficult radical-branched mechanism of hydrocarbons oxidation. An idea of the mechanism of branched radical oxidation of light hydrocarbons can be explained on the oxidation of n-pentane. During the oxidation of n-pentane, only at the initiation of branched-radical oxidation process near one of the secondary carbon atoms, the next conversion of the intermediate products of the oxidation occurs.
Mechanism of the oxidation of n-pentane
According to the mechanism of oxidation oxygen eliminates hydrogen atom at the secondary carbon atom with the formation of hydrocarbon radical. Formed radical attaches the molecule of oxygen and forms the peroxide radical:
Peroxide radical may transform by three main ways: 1. Thermal dissociation of the peroxide radical 2. Transformation of the peroxide radical into hydroperoxide by the elimination of hydrogen atom from another molecule of the n-pentane (intermolecular transformation) 3. Transformation of the peroxide radical into hydroperoxide by the elimination of hydrogen atom from the secondary carbon atom inside the molecule of n-pentane (intramolecular transformation)
Thermal dissociation of the peroxide radical is carried out by four directions: The first direction is breaking the bond between 2 and 3 carbon atoms in the molecule of n-pentane. This produces an acetic and propionic acid. The second direction is breaking the bond between 3 and 4 carbon atoms in the molecule of n-pentane. By this way acetic acid and acetone are produced. The third direction is breaking the bond between 4 and 5 carbon atoms in the molecule of n-pentane with the formation of methylethylketone and formic acid. The fourth direction is breaking the bond between 1 and 2 carbon atom in the molecule of n-pentane which produces formic and butyrate acid.
Mainly, the process of thermal dissociation of n-pentane occurs by the first and second directions.
Thermal dissociation of the hydroperoxide of n-pentane
Thermal dissociation of the hydroperoxide occurs with the dissociation of the hydroperoxide group and formation of oxyradical. Last one transforms into methylethylketone and iso-amyl alcohol. Mainly the methylethylketone is formed which oxidizes by three directions:
The first direction is breaking the bond between 2 and 3 carbon atoms in the molecule of methylethylketone. This produces an acetic and propionic acid. The second direction is breaking the bond between 1 and 2 carbon atom in the molecule of methylethylketone. By this way propionic and formic acid are produced. The third direction is breaking the bond between 3 and 4 carbon atom in the molecule of methylethylketone with the formation of pyruvic and acetic acid.
Intramolecular transformation of the peroxide radical in the molecule of npentane
Intramolecular transformation of peroxide radical in the molecule of n-pentane occurs due to elimination of hydrogen atom from 4 carbon atoms in the molecule of n-pentane with the formation of hydroperoxide with alkyl radical. By alkyl radical is occurred attaching of oxygen molecule with the formation of hydroperoxide which has peroxide radical. This compound is unstable and immediately undergoes thermal dissociation with the formation of acetone, formic and propionic acid. The presented mechanism is one of the main directions of obtaining oxidation products of n-pentane. It should be noted that pentane molecule has five carbon atoms (in addition to the given direction of the oxidation of n-pentane exist also other directions of generation of branched-radical process near the other carbon atoms in the molecule of npentane). Besides this way of transformation of n-pentane, there are also other ways of generation of radical oxidation near other carbon atoms. Intermediates such as acids, ketones, aldehydes and alcohols actively participate during oxidation. Of all the lower carboxylic acids, acetic is the most stable. It changes slightly during the oxidation. Formic acid is unstable, undergoes thermal dissociation and can be easily oxidized to carbon oxides. Propionic acid is also oxidized with the formation of acetic acid. Therefore, control of branched-radical oxidation process is very difficult. Even the use of catalysts in the liquid-phase oxidation of light hydrocarbons not significantly increases the total selectivity of the formation of acetic, formic and propionic acids. The most significant impact on the selectivity of the formation of lower carboxylic acids has Mn-catalyst. It increases the total selectivity of the formation of lower carboxylic acids by several percent due to the substantial increase the yield of formic acid and decrease of propionic. Mn-catalyst slightly affects the yield of acetic acid. In connection to this there is a need to find other methods for increasing the selectivity of the formation of lower carboxylic acids. Based on the mechanism of oxidation of light hydrocarbons it can be seen that it is necessary to reduce the speed of branched-radical oxidation process and promote those directions of the oxidation process, which increases the formation of acetic, formic and propionic acids. For this purpose was studied the effect of temperature, pressure, reaction time, concentration of light hydrocarbons and oxygen in the reaction mixture on yield of acetic, formic and propionic acids. It was studied the influence of water addition to the reaction mixture and the withdrawal of water-acid phase of the oxidate from the oxidation zone on the yield of lower carboxylic acids. It was studied the oxidation of main intermediates: acids, ketones, aldehydes and alcohols. Based on a thorough study of the mechanism of oxidation and the influence of the basic technological parameters on the liquid-phase oxidation of light hydrocarbons were selected optimal parameters, which allow to increase the yield of lower carboxylic acids. Herewith the total selectivity of the formation of acetic, formic and propionic acids increases more than 10 %. This enabled the firm «Proksinfo» to develop a new process of producing acetic, formic and propionic acids by liquid-phase oxidation of light hydrocarbon fractions. This process is based on cheap raw materials with the formation of lower carboxylic acids with good selectivity about 50 %. Thus, the firm «Proksinfo» proposes a new competitive method of producing acetic, formic and propionic acids. The firm «Proksinfo» has no opportunity to implement this method in the industry, therefore, is looking for business partners for joint implementation. Anyone who is interested in this way of the obtaining acetic, formic and propionic acids, please contact us by e-mail: proksinfos@gmail.com.