Microstructure of carbon molecular sieve membranes and their application to separation of aqueous bioethanol

Pyrolytic carbons were fabricated from a Kapton precursor. The microstructure of the carbon molecular sieve membranes (CMSMs) was examined by means of sorption tests using pure liquid sorbates of different molar volumes and with the application of positron annihilation lifetime spectroscopy (PALS). The pore sizes of the CMSMs were calculated by PALS and our recently proposed equation relating positron lifetime with free volume radius. The mean pore radius in the CMSMs was demonstrated to be smaller than 3 Å. Their ability to separate water from bioethanol was determined. Results of aqueous bioethanol sorption showed evident molecular sieving behavior of the CMSMs despite their hydrophobic surface. This behavior became significant at a high pyrolysis end temperature of 700 °C, at which the carbon content was high. The c700 membrane delivered a high permeation flux of 1199 g/m2 h and a high separation factor of 683. This performance suggests that CMSMs have more potential to separate water from bioethanol through pervaporation or vapor permeation than organic membranes.