Ag functionalized Carbon Molecular Sieves membranes for separating O 2 and N 2
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Ag functionalized Carbon Molecular Sieves membranes for separating O2 and N2 J.N. Barsema, J. Balster, N.F.A. Van der Vegt, G.H. Koops, V. Jordan1, M. Wessling Membrane Technology Group, Faculty of Chemical Technology, University of Twente, Enschede, The Netherlands. 1 Department of Chemical Engineering, Fachhochschule Münster - University of Applied Sciences, Steinfurt, Germany ABSTRACT In the last two decades substantial progress has been made in the preparation of Carbon Molecular Sieve (CMS) membranes for gas separation. Today, researchers actively study precursor materials and pyrolysis routes to fully explore the merits of CMS membranes. Successful separation of permanent gas mixtures in which the gaseous components posses only little to no affinity to adsorb onto the internal CMS surface relies highly on the exact tailoring of microsieving regions. Preparation of CMS structures, which are highly permselective towards one of such mixture components, is especially cumbersome if the molecular sizes differ only slightly (e.g. O2/N2). To facilitate the separation of O2 and N2 we have chosen to functionalize the carbon matrix. By introducing nano-sized (40 nm) metallic Ag-clusters, the affinity of the membrane matrix for O2 significantly increases. We have added a silver containing salt, AgNO3, to a solution of BMTA-TDI/MDI co-polyimide (P84, Lenzing) in NMP in the absence of light to obtain homogeneous flat film polymeric precursors. These precursors were pyrolysed, reducing Ag+ to Ag, at different temperatures (350, 500, 600, 700, and 800 °C) in a N2 atmosphere and characterized using Scanning Electron Microscopy, Atomic Force Microscopy, and gas permeation. Thermo Gravimetrical Analysis was used to follow the pyrolysis in detail. From this we observed an increase of the ideal separation factor of 1.6. Moreover, we observed an increase of the permeability to a maximum of 240 %. INTRODUCTION In the last two decades Carbon Molecular Sieve (CMS) membranes with pore sizes smaller than 50Å, have been developed for the separation of a broad range of gaseous mixtures. Generally these mixtures can be divided into two groups. Those containing a gaseous species that shows a high interaction with the carbon matrix, e.g. H2, CO2, C2H4, and C3H6, are separated mainly on the basis of preferential surface adsorption. The preferentially adsorbed species diffuses over the pore surface significantly faster than the non-adsorbed species. In extreme cases the adsorbed molecules will block the pore for any other molecule than the adsorbed one. When no highly interacting gases are present in the mixture, e.g. He, O2, N2, CH4, and C2H6, the separation will take place based on size exclusion. To obtain high selectivities by size exclusion the pore size of the microsieving region must be precisely tailored, as can be understood form the size difference of the O2/N2 gas mixture, which is just 0.24Å (O2: 3.4Å, N2: 3.64Å). Although several authors have reported [1,2] good results for the separation of O2 over N2, it is becoming in
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