DEFINING EFFECTIVE MICROPOROSITY IN CARBON MOLECULAR-SIEVES

A pseudo equilibrium method is described for establishing the micropore distribution in carbon molecular sieves (CMS) having micropore sizes below 6 angstrom. The method uses a series of carefully selected molecular probes varying in their minimum Van der Waals diameter between 3.7 and 6.0 angstrom. Various small-pore zeolites were used to validate the method. Over the critical micropore size range these probes can establish the ''effective'' microporosity which can be modified by post-treatment to produce an O2-selective CMS useful for air separation. This method coupled with kinetic measurements of O2 and N2 uptake indicates that pores larger than 4 angstrom are not highly O2-selective. Results from these molecular probe studies in combination with porosimetry and pycnometry measurements show that the O2-selective carbons have an unusual bimodal pore distribution with no significant mesoporosity. We found obtaining pore distributions from equilibrium analysis of N2 or O2 isotherms at cryogenic conditions was inappropriate on these carbons. The study clearly demonstrates the importance of the method which monitors the size and relative amounts of micropores which are O2-selective.