Defining effective microporosity in carbon molecular sieves

Abstract

A pseudo equilibrium method is described for establishing the micropore distribution in carbon molecular sieves (CMS) having micropore sizes below 6 Å. The method uses a series of carefully selected molecular probes varying in their minimum Van der Waals diameter between 3.7 and 6.0 Å. 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 O 2-selective CMS useful for air separation. This method coupled with kinetic measurements of O 2 and N 2 uptake indicates that pores larger than 4 Å are not highly O 2-selective. Results from these molecular probe studies in combination with porosimetry and pycnometry measurements show that the O 2-selective carbons have an unusual bimodal pore distribution with no significant mesoporosity. We found obtaining pore distributions from equilibrium analysis of N 2 or O 2 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 O 2-selective.