High performance nanoporous carbon membranes for air separation Anna Merritt a , Ramakrishnan Rajagopalan b , Henry C. Foley a,b,c, * a Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA b The Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA c Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA Received 24 October 2006; accepted 17 January 2007 Available online 13 February 2007 Abstract The preparation of porous stainless steel supports was found to have a significant impact on the properties of nanoporous carbon membranes fabricated upon them. Nanofillers were incorporated into porous stainless steel supports to modify the pore structure by reducing the average pore size and porosity. Carbon membrane properties were examined as a function of support variables such as filler content, shape, size and nature of the particles. Optimum performances, in terms of the ideal selectivity ratio for oxygen to nitro- gen permeances ðS O 2 =N 2  3–6Þ and the oxygen permeance (10 8 mol m 2 s 1 Pa 1 ), were obtained when the filler completely saturated the support. This represents about a two order of magnitude improvement in oxygen permeance when compared to carbon mem- branes prepared on unmodified porous stainless steel supports. The origin of the improvement in the permeance is due to the forma- tion of carbon membranes which are on average two orders of magnitude thinner than those formed on unmodified supports, i.e., the carbon membranes exists as very thin layers around and between the silica nanoparticles. A simple geometric model based on the packing of silica particles inside the porous stainless steel support is proposed to visualize and quantify this effect. The generality of the support modification concept is also demonstrated by the ability to employ different types of nanofillers and support geometries to obtain carbon membranes with high flux. Air separation experiments show that these membranes can produce both oxygen rich streams enriched to as much as 48% by volume and nitrogen rich streams enriched to over 90% by volume at reasonable operating conditions. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Size selective carbon membranes have begun to emerge as a potentially viable and promising technology for effi- cient and environmentally sound gas separations. Carbon membranes are readily prepared by the pyrolysis of a suit- able precursor material, which has been pre-formed into membrane geometry, usually by some type of spraying, casting or spin coating process. Some polymeric materials are a source of nanoporous carbons (NPC) and generally yield a narrow distribution of sub-nanometer sized pores upon pyrolysis. Considerable research has been invested into enhancing the performance of NPC membranes through variation of precursor chemistries, pyrolysis condi- tions and pre/post-treatments of membranes; a recently published review paper provides a comprehensive summary of these works [1]. However, despite the large amount of work on such variables, there has been less focus on phys- ical and mechanical aspects of membrane fabrication. Coating methodology as well as support selection can play significant roles in the performance and durability of the resultant membrane. NPC is a glassy, brittle material; thus many carbon membranes with acceptable permeances and mechanical stability are fabricated on porous supports. Although car- bon membranes have been synthesized on a variety of sup- port types, NPC membranes supported on porous metal 0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.01.022 * Corresponding author. Address: Department of Chemical Engineer- ing, The Pennsylvania State University, University Park, PA 16802, USA. Fax: +1 814 863 9659. E-mail address: hcf2@psu.edu (H.C. Foley). www.elsevier.com/locate/carbon Carbon 45 (2007) 1267–1278