Production of aromatic polyimide separating membranes

Claim 1

A process for producing an aromatic polyimide separating membrane, comprising the steps of: (A) preparing a dope solution containing an aromatic polyamic acid dissolved in a polar organic solvent at a temperature not above about 100° C., said polyamic acid having been prepared by polymerizing a carboxylic acid component consisting essentially of at least one aromatic tetracarboxylic acid compound selected from the group consisting of 3,3',4,4'- and 2,3,3',4'-biphenyl tetracarboxylic acids, 3,3',4,4'-benzophenone tetracarboxylic acid, 2,2-bis (3,4-dicarboxyphenyl) propane, bis (3,4-dicarboxyphenyl) methane, bis (3,4-dicarboxyphenyl) sulfone, bis (3,4-dicarboxyphenyl) ether, bis (3,4-dicarboxyphenyl) thioether, pyromellitic acid and acid anhydrides, salts and lower alcohol esters of the above acids, with a diamine component consisting of at least one aromatic diamine compound at a temperature of 120° C. or less in a polar organic solvent consisting essentially of at least one member selected from the group consisting of acetamide, formamide, N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, dimethylsulfoxide, diethylsulfoxide, hexamethylphosphoric amide, N-methyl-2-pyrrolidone, pyridine, dimethylsulfones, tetramethylenesulfone, dimethyltetramethylenesulfone, tetramethyl urea, tetraethyl urea and N-methyl caprolactam (B) forming a thin film of said aromatic polyamic acid dope solution; (C) bringing said thin film of said aromatic polyamic acid dope solution into contact with a coagulating liquid containing at least 60% by volume of at least one member selected from the group consisting of water and lower aliphatic alcohols having 1 to 5 carbon atoms at a temperature not higher than 60° C., to form a coagulated membrane of said aromatic polyamic acid; (D) drying said coagulated membrane of said aromatic polyamic acid; and (E) heating said dried aromatic polyamic acid membrane at a temperature of 100° C. to 300° C. whereby said aromatic polyamic acid membrane is converted to a separating porous aromatic polyimide membrane which exhibits a hydrogen gas-permeating rate (ρH 2 ) not lower than 5×10 -5 cm 3 /cm 2 sec cmHg and a ratio (ρH 2 /CO) of the hydrogen gas-permeating rate (ρH 2 ) to carbon monoxide gas-permeating rate (ρCO) in the range of from 2.0 to 10, determined at a temperature of 30° C. under a pressure of from 0.5 to 1 kg/cm 2 G.