Selective Matrimid Membranes Containing Mesoporous Molecular Sieves
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Selective Matrimid Membranes Containing Mesoporous Molecular Sieves Kenneth J. Balkus, Jr., Kyle Cattanach, Inga H. Musselman, John P. Ferraris University of Texas at Dallas, Department of Chemistry and the UTD NanoTech Institute, Richardson, TX 75083-0688 ABSTRACT We have employed mesoporous molecular sieves in polymer membranes in an effort to enhance the permselectivity. The principal advantage of these materials is that the polymer chains can penetrate the pores reducing the nonselective voids that are often observed with inorganic additives. In this study, we have prepared Matrimid® membranes with various loadings of the all silica molecular sieve DAM-1 (Dallas Amorphous Material) as well as DAM-1 functionalized with amines in the channel wall, to enhance the gas permeability characteristics of a high performance polymer. For all gases tested (N2, O2, CO2, CH4), the permeability increased in proportion to the wt % of the amine DAM-1 present in the membrane. The addition of the amine DAM-1 resulted in modest ideal O2/N2 permselectivity, while the ideal CO2/CH4 permselectivity values were >100, depending upon the moisture content of the feed. The ideal CO2/CH4 permselectivity values are among the highest for this type of composite membrane. Details of membrane fabrication as well as permeability and permselectivity results will be presented for a range of Matrimid®/molecular sieve composites. INTRODUCTION A promising strategy for enhancing the permselectivity of polymer membranes is the incorporation of nanoporous materials such as zeolites and related molecular sieves [1-10]. Size and shape selectivity is a zeolite property that might be exploited in a mixed matrix membrane. The range of pore architectures available among the molecular sieves creates many possibilities for controlling the diffusion of small molecules. Additionally, the compositional variance that is possible for these materials, as well as the ease in functionalization, might be used to increase the membrane affinity for certain target gases. A problem that has arisen with zeolite/polymer composite membranes is the formation of non-selective voids at the polymer/zeolite interface [3,8,11,12]. If the zeolite is not adequately dispersed and/or the polymer does not effectively wet the crystals surface, then Knudsen flow will occur through the resulting gaps. A variety of chemical methods have been suggested for improving the contact between the zeolite particles and polymer chains. However, we reasoned that if the polymer chains could simply penetrate the molecular sieve pores, then one might achieve a more intimate contact between the organic and inorganic components as shown in Figure 1. We initially tested this idea using the all silica mesoporous molecular sieve MCM-41 and polysulfone [5,6]. It was found that the polysulfone indeed penetrated the 4 nm pores of MCM-41 resulting in an increase in permeability for all gases tested with no loss in selectivity. We have now extended this work to include the all silica molecular sieve DAM-1 (
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