Clean process for preparing chloroformyl-substituted benzene

A process for preparing a chloroformyl-substituted benzene, comprising

• (1) reacting a methyl aromatic hydrocarbon of formula (X)aC6H6-a-b(CH3)b or a pendant alkyl chloride thereof with chlorine gas under illumination conditions to prepare a trichloromethyl-substituted benzene and obtain hydrogen chloride as a byproduct, wherein X is a chlorine, bromine, or fluorine atom, a is an integer of 0, 1, 2, 3, 4, or 5, b is an integer of 1, 2, 3, or 4, and a+b≤6, and the pendant alkyl chloride is a compound where hydrogen atoms of the pendant alkyl group in the methyl aromatic compound are not completely substituted by chlorine atoms;
• (2) reacting the trichloromethyl-substituted benzene with water or a corresponding aromatic acid of formula (X)aC6H6-a-b(COOH)b in presence of a catalyst to prepare the chloroformyl-substituted benzene and to obtain hydrogen chloride as a byproduct, wherein the corresponding aromatic acid (X)aC6H6-a-b(COOH)b has the same substituent (X)a that is located at the same substitution position as the substituent (X)a of the methyl aromatic hydrocarbon (X)aC6H6-a-b(CH3)b or the pendant alkyl chloride thereof;
• (3) subjecting a gas stream containing the hydrogen chloride as the byproducts from steps (1) and (2) to catalytic oxidation in one or more reactors to form a product gas stream, wherein the catalytic oxidation is conducted by
• providing a first reactor and one or more downstream reactors, each of the reactors being filled with a catalyst and connected in series or in parallel,
• providing a gas stream containing hydrogen chloride and a gas stream containing oxygen to the first reactor and providing a gas stream containing hydrogen chloride, a gas stream containing oxygen, or both, to the downstream reactor for the catalytic oxidation of hydrogen chloride to obtain a product gas stream;
• directly returning a part of the product gas stream from the last reactor to any of the reactors without separation of products;
• and
• providing a remainder of the product gas stream from the last reactor for separation of a product;
• (4) separating the remainder of the product gas stream to obtain a gas stream containing chlorine, a gas stream containing oxygen, and optionally, a gas stream containing hydrogen chloride;
• and
• (5) introducing the gas stream containing chlorine as raw material into chlorination reaction in step (1).