Flow chemistry chlorination
Chlorination refers to the chemical reaction of introducing chlorine atoms into a compound molecule. The technologies involved in the chlorination process are called chlorination technologies, mainly including various types such as substitution chlorination, addition chlorination, and oxidative chlorination. As a fundamental reaction type in chemical production, chlorination plays a crucial role in numerous manufacturing fields. The following details the applications of chlorination in different industrial sectors:
Chemical Industry: In the chemical industry, chlorination is a core process in the production of alkanes, chlorinated hydrocarbons, aluminum chloride, and other basic chemicals. Furthermore, it is widely used in the production of organic solvents, dyes, and pesticides. These chemicals have widespread applications in daily life and industrial production, and advancements in chlorination technology directly drive the technological development of the chemical industry.
Petroleum Industry: The petroleum industry also heavily relies on chlorination technology. Through chlorination treatment, crude oil can be purified and its quality improved. At the same time, chlorination can adjust the properties of petroleum products to meet the needs of different application fields. This technology plays a vital role in petroleum processing and refining.
Pharmaceutical Industry: In the pharmaceutical industry, chlorination is used in the synthesis of drug molecules, especially in the synthesis of chlorine-containing groups. Introducing these chlorine-containing groups can alter the biological activity and pharmacological properties of drug molecules, thus developing new drugs with therapeutic effects. Therefore, chlorination occupies an irreplaceable position in the pharmaceutical field.
Textile and Dyeing Industry: The textile and dyeing industry also widely uses chlorination technology. Through chlorination treatment, dyes and auxiliaries required for dyeing and printing can be prepared. These dyes and auxiliaries have excellent dyeing performance and printing effects, providing strong support for the diversified development of textiles.
Pesticide and Fertilizer Production: Chlorination also plays a crucial role in the production of pesticides and fertilizers. Through chlorination reactions, compounds with insecticidal, fungicidal, or growth-promoting functions can be synthesized. These compounds play a key role in agricultural production, helping to increase crop yields and improve crop quality.
Chlorination reactions play a significant role in organic synthesis, and chlorinated organic compounds have widespread applications in pharmaceuticals, pesticides, dyes, and materials science. However, traditional chlorination reactions suffer from numerous problems, such as vigorous reactions, numerous side reactions, and low safety. These issues limit the application of chlorination reactions in large-scale industrial production. In recent years, the emergence of microreactor continuous flow technology has brought new opportunities for chlorination reactions. Microreactor continuous flow technology offers advantages such as fast reaction rates, high selectivity, and good safety, effectively addressing the problems associated with traditional chlorination reactions.
Chlorination reactions are commonly used in the synthesis of organic compounds such as dyes, pesticides, pharmaceuticals, and fragrances. However, chlorine gas is highly toxic, and leaks pose a significant hazard. Chlorination reactions are highly exothermic, and traditional processes often result in high levels of side products due to unstable temperature control. Furthermore, traditional processes often use a large excess of chlorine gas to achieve higher conversion rates. Microchannel reactors can effectively control the amount of chlorine gas used, ensuring precise material ratios, reducing the content of polychlorinated byproducts, intensifying the reaction process, improving conversion and yield, and enhancing reaction safety.
Chlorination reactions using chlorine gas are of great importance in the chemical industry, primarily due to the high reactivity of chlorine gas. High reactivity means fast reaction rates with substrates, but also poor reaction selectivity, easily leading to over-chlorination and oxidation. Therefore, chlorination reactions are often difficult to control or even uncontrollable in conventional reactors. The direct consequences are numerous side reactions, environmental pollution, poor product quality, and explosion hazards. Chlorination reaction processes are among the hazardous chemical processes under strict national regulation.
The free radical substitution reaction of chlorine gas with active hydrogen in the reaction raw materials is an important method for carbon-hydrogen bond functionalization. For example, chlorination at the benzylic position can yield monochlorinated and polychlorinated intermediates, which can then be converted into other intermediates; the chlorination of the active methylene group of ethyl acetoacetate is an effective way to obtain various heterocycles.
Traditional chlorination processes typically involve batch bubbling of chlorine gas, resulting in low chlorine utilization and difficult selectivity control. Although falling film reactors can reduce backmixing, they still require the recycling and recovery of a large excess of chlorine gas.
Continuous flow microchannel reactors offer another continuous option for chlorination. Chlorine gas and raw materials enter the reactor in a co-current flow, allowing for precise control of the reactant stoichiometry. Ideal plug flow operation is achieved at a specific temperature, thereby improving reaction selectivity.
The chlorination reaction requires precise control of the chlorine gas flow rate; therefore, the design and performance of the chlorine gas feeding system are crucial factors for success.
I. Hazardous Characteristics of the Chlorination Reaction Process
(1) The chlorination reaction is an exothermic process, especially at higher temperatures, where the reaction is more vigorous, faster, and releases a larger amount of heat;
(2) Most of the raw materials used are flammable and explosive;
(3) Chlorine gas, the commonly used chlorinating agent, is a highly toxic chemical with strong oxidizing properties and is stored under high pressure. Most chlorination processes using liquid chlorine involve vaporization before chlorination, posing a significant risk in case of leakage;
(4) Impurities in chlorine gas, such as water, hydrogen, oxygen, and nitrogen trichloride, can easily cause hazards during use, especially the accumulation of nitrogen trichloride, which can easily lead to explosion;
(5) The generated hydrogen chloride gas is highly corrosive when it comes into contact with water;
(6) The tail gas from the chlorination reaction may form an explosive mixture.
Therefore, in the production of organochlorine products, effective control of the organochlorine reaction is essential, both for achieving clean production processes and for ensuring product quality and production safety. Microreactor technology addresses the problems of inefficient mass transfer and heat transfer in traditional stirred tank reactors and enables linear scale-up from research scale to production scale without scale-up effects. At the same time, microreactors transform the traditional batch operation method into a continuous feed and discharge method, demonstrating superior advantages in automated control, quality control, and safety control of production.
Microchannel reactors can significantly improve heat and mass transfer, enhancing reaction and heat exchange efficiency. Furthermore, because they allow for precise control of material ratios and reaction temperatures, chemical reactions can be carried out with precise stoichiometry, strict temperature control, high selectivity, and in a green and safe manner, providing a practical solution to many problems in the traditional production process of chlorinated products. Chlorination reactions primarily refer to substitution reactions using chlorine gas as a raw material. Due to the hazardous nature of chlorine gas, microchannel reactors are used to ensure a more reasonable molar equivalent of chlorine and better safety control. Since chlorination reactions often produce HCl, which is highly corrosive to traditional metal equipment, material selection is a critical issue for the safe production of chlorinated products. Silicon carbide and glass microreactors are designed for highly corrosive reaction conditions involving strong acids, providing reliable safety for chlorination reactions. Glass microreactors not only have excellent corrosion resistance but can also be used for photochemical chlorination reactions under light irradiation.
Compared with traditional batch reactors, the use of microreactors significantly shortens the chlorination reaction time, improves reaction yield and selectivity, enhances product quality, and reduces solvent usage, thereby reducing waste and mitigating environmental pressure.
Introduction to Chlorination Reactions: A reaction that introduces one or more chlorine atoms into an organic compound molecule is called a chlorination reaction.
Types of Chlorination Reactions: Chlorination of organic compounds includes substitution chlorination, addition chlorination, and oxychlorination.
Chlorination Reaction Mechanisms: Chlorination reactions can be broadly divided into two types of mechanisms: free radical chain reaction mechanisms and ionic reaction mechanisms.
Free Radical Chain Reaction Mechanism: This mechanism includes thermal chlorination and photochemical chlorination. The reaction involves three stages: chain initiation, chain propagation (chain transfer), and chain termination, forming a chain reaction process.
Ionic Reaction Mechanism: Catalytic chlorination mostly belongs to the ionic reaction mechanism, commonly using non-protonic acid catalysts such as FeCl₃ and AlCl₃. Addition to double and triple bonds of hydrocarbons, chlorohydrination (hypochlorination) of olefins, hydrochlorination (chlorination reaction with HCl as the chlorinating agent), and catalytic chlorination of hydrogen atoms on the benzene ring are all examples of this type of reaction mechanism.
II. Classification of Chlorination Reactions in Continuous Flow Reactors Based on Reactant Properties and Reaction Conditions
Gas-Liquid Phase Chlorination Reactions
Gas-liquid phase chlorination reactions are a common type of chlorination reaction, usually involving the reaction of chlorine gas (Cl₂) with organic compounds. In a microreactor continuous flow system, chlorine gas can be precisely controlled via a gas delivery system and thoroughly mixed with liquid reactants in microchannels, thus achieving efficient chlorination reactions. Photochemical chlorination is a special gas-liquid phase chlorination reaction that requires irradiation with ultraviolet or visible light. Photochemical chlorination in a microreactor continuous flow system offers significant advantages because the small size of the microchannels allows for uniform illumination, thereby improving reaction selectivity and efficiency.
Liquid-Liquid Phase Chlorination Reactions
Liquid-liquid phase chlorination reactions involve the mixing and reaction of two liquid reactants. In a microreactor continuous flow system, efficient liquid-liquid phase chlorination reactions can be achieved by precisely controlling the flow rates and mixing methods of the two liquids.
Solid-Liquid Phase Chlorination Reactions
Solid-liquid phase chlorination reactions involve the reaction of a solid reactant with a liquid chlorinating agent. In a microreactor continuous flow system, the solid reactant can be contacted with the liquid chlorinating agent through suspension or a fixed bed, thereby achieving efficient solid-liquid phase chlorination reactions.
III. Typical Processes
Substitution Chlorination: Occurs between chlorine atoms and hydrogen atoms of organic compounds. Typical processes include: chlorine substitution of hydrogen atoms in alkanes to produce chloroalkanes; chlorine substitution of hydrogen atoms in naphthalene to produce polychlorinated naphthalene; reaction of methanol with chlorine to produce chloromethane, etc.
Addition Chlorination: Occurs between chlorine atoms and unsaturated hydrocarbons. Typical processes include: addition chlorination of ethylene with chlorine to produce 1,2-dichloroethane; addition chlorination of acetylene with chlorine to produce 1,2-dichloroethylene; addition of acetylene and hydrogen chloride to produce vinyl chloride, etc.
Oxychlorination: Chlorination in the presence of chloride ions and oxygen atoms, producing chlorine-containing compounds. Typical processes include: oxychlorination of ethylene to produce dichloroethane; oxychlorination of propylene to produce 1,2-dichloropropane; oxychlorination of methane to produce methane chlorides, etc.
Other Processes: Reaction of sulfur with chlorine to produce sulfur monochloride; preparation of titanium tetrachloride; reaction of yellow phosphorus with chlorine gas to produce phosphorus trichloride, phosphorus pentachloride, etc. IV. Case Studies of Chlorination Reactions in Microreactor Technology
Benzyl Chlorination: Microreactor application results: reaction time 9.3 seconds, conversion rate 50%, selectivity 81%, while batch reactor conversion rate was 44%, selectivity 65%.
Fatty Acid Chlorination: Microreactor application results: yield above 90%, with only 0.01% dichloro byproduct, while the batch reactor produced 3.5% dichloro byproduct.
Chlorination of Pyridine Compounds.
Chlorination of Aromatic Compounds: Chlorination of aromatic compounds is a common type of chlorination reaction, widely used in the pharmaceutical and pesticide fields. In a microreactor continuous flow system, the chlorination of aromatic compounds can be achieved through gas-liquid or liquid-liquid phase reactions. For example, Jähnisch et al. used a microreactor to achieve the chlorination of toluene under UV irradiation, obtaining chlorinated products with high selectivity and high yield.
Chlorination of Aliphatic Compounds: Chlorination of aliphatic compounds also has important applications in materials science and organic synthesis. In a microreactor continuous flow system, the chlorination of aliphatic compounds can be achieved through gas-liquid or liquid-liquid phase reactions. For example, Ryu et al. used a microreactor to achieve the chlorination of cycloalkanes, obtaining high reaction efficiency and selectivity.
Photochemical Chlorination Reactions: Photochemical chlorination is a special type of chlorination reaction that requires light irradiation. Photochemical chlorination reactions in microreactor continuous flow systems have significant advantages because the small size of the microchannels allows for uniform light irradiation, thereby improving the selectivity and efficiency of the reaction. For example, Hessel et al. used a microreactor to achieve the chlorination of benzene under UV irradiation, obtaining chlorinated products with high selectivity and high yield.
