Petrochemical Synthesis

the synthesis of chemical products from petroleum and hydrocarbon gases. Hydrocarbons of petroleum, natural gases, casinghead gases, and petroleum refining gases are the main raw materials for the manufacture of the most important high-volume synthetic products—plastics, rubbers and fibers, nitrogen fertilizers, surface-active agents, detergents, lubricants and lubricant additives, plasticizers, fuels, solvents, and extractants. All these products are widely used in various branches of industry and in the household, and the development of many new areas of technology, such as space exploration and atomic power engineering, is related to them.

In the industrially developed countries, petrochemical synthesis has made possible the establishment of a large and rapidly expanding petrochemical industry. The hydrocarbons of petroleum and gases, which are readily available, cheaper, and more technologically efficient raw materials, are replacing other types of raw materials (coal, oil shale, vegetable oil, and animal fat) in almost all processes of organic synthesis.

Petrochemical synthesis is based on advances in organic chemistry, catalysis, physical chemistry, and chemical engineering and is related to the extensive study of the composition of petroleum and the properties of its components. Numerous reactions of organic chemistry, including pyrolysis, oxidation, alkylation, dehydrogenation and hydrogenation, halogenation, polymerization, nitration, and sulfonation, are the basis of the processes for converting hydrocarbon raw materials into finished products. Catalytic reactions are the most important of these reactions. The preparation of the hydrocarbon raw material and the production of primary initial hydrocarbons, including alkanes (paraffins), unsaturated hydrocarbons (olefins, dienes, and acetylene), aromatics, and naphthenes, are of great importance in the production of petrochemical products. Most of these primary products are converted into derivatives with functional groups containing oxygen, nitrogen, chlorine, fluorine, sulfur, and other elements.

Alkanes. Alkanes are important because of the large volume of their use in petrochemical synthesis. The lower gaseous hydrocarbons (methane, ethane, propane, butane, and the pentanes) and the liquid or solid paraffins (from C₆ to C₄₀) are used to make various chemical products. The lower paraffin hydrocarbons are isolated from natural and casinghead gases. Petroleum by-product gases and gases produced by petroleum stabilization contain C₂-C₅ alkanes in amounts from 83 to 97 percent by volume. The ethane-propane fraction, isobutane, N-butane, and pentane are isolated from the gases. Natural gas containing 96–97 percent methane is used as industrial methane, primarily for the production of ammonia, acetylene, methanol, chlorine derivatives, carbon disulfide, and hydrocyanic acid. Liquid and solid normal C₆-C₄₀ paraffins are obtained from products of petroleum refining (gasoline-kerosine, diesel, and oil distillates) by crystallization upon cooling, by carbamide deparaffination, and by means of molecular sieves. The refining of paraffin raw materials satisfies the ever-increasing demand of petrochemical synthesis for unsaturated hydrocarbons (olefins, dienes, and acetylene). The main method for the production of olefins (ethylene, propylene, and butylenes) is high-temperature pyrolysis of various raw materials, from ethane and natural gasoline to heavy petroleum fractions and crude petroleum. Olefins are also obtained as by-products in petroleum refining. Catalytic dehydrogenation is used for the conversion of butane into butadiene and of isopentane into isoprene. Butadiene and isoprene are the main monomers used in the production of synthetic rubbers.

The conversion of paraffin hydrocarbons into producer gas (a mixture of carbon monoxide and hydrogen) has great industrial importance. The raw material may be natural gases, casinghead gases, petroleum refining gases, or any petroleum fraction. Low-cost hydrogen, which is consumed in large quantities for the production of ammonia, for hydrorefining of petroleum products, and for hydrocracking, is obtained from producer gas. Ammonia is the raw material for the preparation of fertilizers (ammonium nitrate and urea) and hydrocyanic acid. The two-stage conversion of methane also yields concentrated carbon monoxide, which is used in many processes of petrochemical synthesis. Producer gas is widely used in the oxo process, which is based on the reactions of olefins with carbon monoxide and hydrogen. Methanol, which is the raw material for the production of formaldehyde, an important product in the manufacture of plastics, varnishes, and adhesives, is produced from carbon monoxide and hydrogen.

A wide variety of products is produced from paraffins by oxidation, halogenation, nitration, and sulfonation. Acetic acid is produced in large quantities by direct liquid-phase air oxidation of the light fractions of direct-distillation gasoline (boiling limits, 30°-90°C) at 150°-210°C and 4 meganewtons per sq m (40 technical atmospheres) in the presence of cobalt acetate or manganese acetate. Liquid-phase air oxidation of solid normal paraffins to higher fatty acids (C₁₀-C₂₀) is a large-scale process. The oxidation of N-paraffins (C₁₀-C₂₀) is used in industry for the production of higher alcohols, which are converted into surface-active agents and detergents of the alkyl sulfate type.

Halogen derivatives of paraffins are produced on an industrial scale. Methyl chloride, methylene chloride, chloroform, and carbon tetrachloride are produced from methane. Methylene chloride and carbon tetrachloride are good solvents. Chloroform is used in the synthesis of tetrachloroethylene, chlorofluorine derivatives, and the valuable monomer tetrafluoroethylene. Chlorination of ethane yields hexachloroethane and other chloro derivatives. Chloroparaffin-40 (about 40 percent CI), a product of chlorination of solid paraffins, is used as a plasticizer; chloroparaffin-70 (about 70 percent CI) is used for the impregnation of fire-resistant paper and fabrics. Products of the complete fluorination of narrow fractions of kerosine and gas oil are valuable lubricants and hydraulic fluids with high thermal and chemical stability. They can perform for extended periods at 250°-300°C in highly aggressive media. Freons—chlorofluorine derivatives of methane and ethane—are used as refrigerants. A mixture of nitroparaffins is obtained by nitration with nitric acid of propane and paraffins that boil above 160°-180°C. Such mixtures are used as solvents and intermediates in the production of nitro alcohols, amino alcohols, and explosives. Surface-active agents of the alkylsulfonate type are obtained by sulfochlorination and sulfoxidation of kerosine fractions and N-paraffins from C₁₂ to C₂₀.

Unsaturated hydrocarbons. Because of their great reactivity, unsaturated hydrocarbons are widely used in petrochemical synthesis. Many products are synthesized from olefins, diene hydrocarbons, and acetylene.

OLEFINS. Ethylene occupies first place among the olefins in terms of industrial use. Propylene and butenes are being used in increasing quantities. The most important higher olefins are straight-chain α-olefins obtained by thermal cracking of solid or soft paraffin at about 550°C and by catalytic oligomerization of ethylene using organoaluminum catalysts. Macromolecular substances such as polyethylene and polypropylene are obtained by polymerization of olefins. Polyethylene is the plastic produced in greatest quantities. Its production is increasing very rapidly, and it is widely used in all sectors of industry. The production of vinyl chloride by oxidative chlorination of ethylene or a mixture of ethylene with acetylene is expanding rapidly. Vinyl chloride is widely used for the production of many polymer materials. Polyvinyl chloride is used for making films and tubes.

Ethylene oxide and propylene oxide have acquired great importance in petrochemical synthesis; they are used for the production of glycols, surface-active agents, and ethanolamines. A considerable quantity of ethylene is consumed in the alkylation of benzene for the production of styrene and oxidation to acetaldehyde and acetic acid, and also for the production of vinyl acetate and ethyl alcohol. The oxo process is used for the preparation of alcohols and aldehydes. Many important solvents and insecticides are produced by chlorination of olefins. Alkyl sulfates and petroleum product additives are produced from higher olefins.

DIENES. The main monomers in the production of synthetic rubbers are 1, 3-butadiene and 2-methyl-l, 3-butadiene (isoprene). In industry, butadiene is obtained as a by-product of the pyrolysis of petroleum products and dehydrogenation of butane, the butylene fraction of the pyrolysis of petroleum raw material in the production of ethylene. The dehydrogenation of isoamy-lenes isolated from light cracked gasoline and the dehydrogenation of isopentane contained in casinghead gases and obtained by the isomerization of N-pentane are promising methods of producing isoprene. Part of the butadiene produced is used in making chloroprene and 1, 5, 9-cyclododecatriene, an intermediate in the production of polyamide fibers.

ACETYLENE. Large quantities of acetylene are produced from methane and other paraffin hydrocarbons by oxidizing pyrolysis, electrocracking, and pyrolysis of various petroleum raw materials in hydrogen plasma. Dimerization of acetylene in the presence of cuprous chloride yields vinylacetylene, which is mainly used for the production of chloroprene. Acrylonitrile, vinyl chloride, and acetaldehyde are also obtained from acetylene, but in these cases acetylene is gradually being replaced by cheaper ethylene and propylene.

AROMATIC HYDROCARBONS. Benzene, toluene, xylenes, trimethylbenzene and tetramethylbenzene, and naphthalene are valuable raw materials for the synthesis of many products. Aromatic hydrocarbons are formed by catalytic re-forming of gasoline and ligroin fractions. Such compounds are obtained in considerable quantities as by-products in the pyrolytic production of ethylene. Benzene and naphthalene are also obtained by dealkylation of their alkyl derivatives in the presence of hydrogen. For this method of producing benzene, alkyl aromatic hydrocarbons (toluene, xylenes, and higher alkyl derivatives) and pyrolytic gasolines are used. Heavy re-forming fractions and gas oil obtained by catalytic cracking are raw materials for the production of naphthalene. Alkylation of benzene by ethylene yields ethylbenzene, and alkylation by propylene yields isopropylbenzene; ethylbenzene and isopropylbenzene are converted by dehydrogenation into styrene and α-methylstyrene, which are valuable monomers for the production of rubber. Air oxidation of isopropylbenzene yields phenol and acetone in large quantities. Alkyl aromatic compounds are the basis for the synthesis of plasticizers, lubricants, lubricant additives, and surface-active agents. Oxidation of aromatic hydrocarbons yields terephthalic acid, which is used in the production of fibers (lavsan), and maleic and phthalic anhydrides, which are valuable plasticizers and components of heat-resistant plastics, such as polyimides. Chlorination and nitration of aromatic hydrocarbons are used to a lesser extent. Effective herbicides, solvents, and insulating oils for transformers are produced from chlorophenols and chloro-naphthalenes. Benzyl chloride is used in the production of a number of compounds containing the benzyl group (benzyl alcohol and its esters).

Naphthenes. Of the naphthenes, only cyclohexane has acquired great importance in petrochemical synthesis. Cyclohexane is isolated in small quantities by careful rectification of the gasoline fractions of petroleum (which contain 1–7 percent cyclohexane and 1–5 percent methylcyclopentane). Methylcyclo-pentane is converted into cyclohexane by isomerization with aluminum chloride. The industrial demand for cyclohexane is satisfied mainly by hydrogenation of benzene in the presence of a catalyst. Oxidation of cyclohexane by atmospheric oxygen yields cyclohexanone and adipic acid, which are used in the synthesis of synthetic polyamide fibers (kapron and nylon).

Adipic acid and the other dicarboxylic acids obtained in the oxidation of cyclohexane are used for the preparation of esters that serve as lubricants and plasticizers. Cyclohexanone is used as a solvent, and also as a camphor substitute.

A great deal of attention is being devoted to the development of microbiological synthesis using petroleum raw material. Concentrates containing protein and vitamins are produced from paraffin hydrocarbons and are used in feeding livestock.