Use of Steam Activation as a Post-treatment Technique in the Preparation of Carbon Molecular Sieve Membranes Hui-Chun Lee, † Majid Monji, † Doug Parsley, § Muhammad Sahimi, † Paul Liu, § Fokion Egolfopoulos, ‡ and Theodore Tsotsis* ,† † Mork Family Department of Chemical Engineering and Materials Science and ‡ Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California 90089, United States § Media and Process Technology, Inc., Pittsburgh, Pennsylvania 15236, United States * S Supporting Information ABSTRACT: Carbon molecular sieve (CMS) membranes have been studied in the past few years as an alternative to both inorganic and polymeric membranes for gas separation under high temperature and pressure conditions. These membranes are made by the pyrolysis of polymeric precursors, and control of their pore size and separation characteristics is accomplished conventionally mainly by choosing the appropriate precursor and by varying the conditions, such as atmosphere, temperature, and duration, of the carbonization procedure. Often, however, the technique does not succeed to consistently provide the tight pore size control required for the separation of important gas pairs, and thus, an additional post-treatment step is needed. In this investigation steam activation was studied as a post-treatment technique in the preparation of CMS membranes. The goal was to adjust the structural characteristics in order to further improve the membrane properties. The impact on separation performance was evaluated based on gas permeation measurements with test gases, such as He, Ar, H 2 , CO 2 , and CH 4 , and via nitrogen adsorption to determine the membrane pore volume and internal surface area before and after steam treatment. Steam activation was shown to be an effective technique to improve membrane throughput without adversely impacting selectivity. The application of the post-treatment technique for the preparation of membranes with “reverse selectivity” appropriate for the removal of chemical warfare agents from contaminated air streams is briefly discussed as well. 1.0. INTRODUCTION The field of nanoporous membranes for gas separations has experienced good progress during the last two decades. Currently, most of the commercial membrane-based gas separation applications use polymeric membranes. These are easily fabricated in various configurations (e.g., flat sheet, hollow fiber, etc.) and have a modest cost. On the other hand, polymeric membranes have, in general, modest separation characteristics (dictated by a permeability vs selectivity trade-off relationship 1 known as the Robeson plot) and are not, typically, intended for use under high temperatures and pressures. Carbon molecular sieve (CMS) membranes, formed by the carbonization of polymeric precursors in a controlled atmosphere (e.g., vacuum or inert gas), exhibit separation performance which often lies above the Robeson plot of competitive polymeric membranes and, in addition, show resistance to high temperatures and pressures. During pyrolysis to prepare CMS membranes, most of the heteroatoms present in the precursor polymeric macro- molecules are progressively removed, with only a cross-linked and stiff, primarily carbon skeleton structure remaining. 2 The pore structure of the CMS membranes is thought to be nonhomogeneous and to consist 3 of both larger-pore regions, with sizes in the range of 10-20 Å (which explains the large gas permeation rates of such materials), separated by nanoporous constrictions (a few angstrom in size) that are, principally, responsible for their molecular sieving characteristics. 4 It is such a hybrid pore structure that explains the ability of CMS membranes to perform molecular sieving type separations while still maintaining the high-flux character of carbon materials. This group and others have produced high-quality CMS membranes in the past few years. Ismail and David 5 reviewed some of the earlier work, and Table A in the Supporting Information section lists some of the key studies. The development of CMS membranes with the appropriate pore size characteristics has been accomplished mainly by choosing the appropriate polymeric precursor and by varying the carbonization conditions (e.g., the atmosphere, temperature and duration of pyrolysis). Often, however, the technique does not succeed 5 to consistently provide the tight pore size control required for the separation of important gas pairs (e.g., O 2 /N 2 and CO 2 /CH 4 ), and thus, an additional post-treatment step is needed to fine-tune the pore structure of the CMS membranes. A generic “platform” post-treatment technique employed by this team is steam activation, and a systematic investigation of the approach and its application to the preparation of CMS membranes is provided here. Though steam activation is a novel post-treatment step for membrane preparation, it is a technique used often for preparing activated carbons (AC) and other microporous materials with desired pore structures, see Table B in the Special Issue: Baker Festschrift Received: January 29, 2012 Revised: April 5, 2012 Accepted: April 5, 2012 Published: April 5, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 1122 dx.doi.org/10.1021/ie300261r | Ind. Eng. Chem. Res. 2013, 52, 1122-1132