Preparation and characterisation of gas selective microporous carbon membranes

Tubular, composite asymmetric carbon membranes, consisting of a macroporous substrate and a microporous thin layer, have been prepared from commercial phenolic based resin precursors and were studied using static (adsorption) and dynamic (permeability/selectivity) techniques. Based on sorption measurements, it was concluded that asymmetric activation occurs mainly due to the pre-formulated rigid shape of the composite membrane. In the case of skin samples, the observed low-pressure hysteresis was attributed to the existence of constrictions, which hinder the access of gas molecules to certain micropores. On the other hand the similarity of pore size distributions of all samples, pointed out that the pore microstructure, derived from similar phenol–formaldehyde resins, is not affected by the powder size of the precursor. According to high temperature (T = 308 K) CO2 adsorption isotherms, enhanced adsorption capacity was observed, even at supercritical conditions. Integral permeability experiments performed on the developed membranes revealed defect free behaviour with molecular sieving properties. The gas flux followed the activated diffusion mechanism and activation energies were calculated. The membranes exhibited high CO2 selectivity (about 15) for equimolar mixture of CO2/N2. This property, which can be attributed to micropore blocking caused by the preferential adsorption of CO2, enables the use of the produced membranes in numerous industrial processes involving CO2 separation. Furthermore, both single gas and gas mixture permeation techniques revealed that the separation mechanism is a combination of molecular sieving and selective adsorption mechanisms.