(350a) Liquid Transport in Carbon Molecular Sieve Membranes By Diffusion NMR and Permeation Measurements

Carbon molecular sieve (CMS) membranes are rigid, high-performance membranes that offer a combination of high selectivity and permeability for molecular separations of solvents. CMS membranes are typically formed by a controlled pyrolysis of a polymer precursor where they develop a slit-like structure consisting of micropores and ultramicropores. Controlled pore size distributions of CMS membranes allow for optimized transport rates of guest molecules within the microporous environments of these membranes. Transport properties of the external surface of CMS membranes can be different from those of the bulk of the membranes. However, no direct quantification of such differences has been reported until now. This gap is addressed in the current work by performing microscopic diffusion measurements by pulsed field gradient (PFG) NMR combined with macroscopic permeation studies for the same membranes at the same or similar measurement conditions. While PFG NMR measurements provide intramembrane diffusion data on different microscopic length scales outside the external membrane surface, permeation results are affected by both diffusion in the membrane bulk and through its external surface. Hence, a model-based comparison of the self-diffusivities from PFG NMR and corrected diffusivities from permeation data can be used to quantify the permeance of the membrane's external surface (viz. skin layer). Such quantification will be demonstrated for single-component liquid sorbates with different molecular sizes including methanol and xylene isomers. All diffusion data will be discussed in detail for different temperatures and sorbate concentrations.