Preparation of supported asymmetric carbon molecular sieve membranes

Asymmetric carbon membranes were made by casting a solution of 13 wt% polyamic acid in N-methylpyrrolydone (NMP) upon a macroporous carbon support. The polymeric solution was coagulated in a bath of isopropyr alcohol and dried at room temperature and at 150 degrees C in air. The resulting polymer was heat treated under vacuum involving two steps: (1) imidization at 380 degrees C during 1 h(heating rate: 1 degrees C/min) and (ii) carbonization at 550 degrees C for 1 h (heating rate: 0.5 degrees C/min). The carbon membrane obtained in only one casting step shows an asymmetric structure formed by a dense skin layer with a thickness of around 1 mu m and a porous substrate (approximate to 6 mu m thickness) of the same material. The gas permeation results indicate that the gas transport through the membrane occurs according to an activated mechanism (molecular sieving). The selectivity and permeation rate measured at 25 degrees C for the O-2/N-2, He/N-2, and CO2/CH4 systems were respectively: alpha(O-2/N-2)=5.3, P(O-2)= 1.14x 10(-9) mol/m(2) s Pa; alpha(He/N-2)=26.5, P(He)=5.7 x 10(-9) mol/m(2) s Pa; alpha(CO2/CH4)=37.3, P(CO2)=4.0x 10(-9) mol/m(2) s Pa. (C) 1998 Elsevier Science B.V.