Combined Application of Proton and Carbon-13 Pulsed Field Gradient NMR for Studies of Gas Diffusion In Carbon Molecular Sieve Membranes

Carbon molecular sieve (CMS) membranes are promising materials for separations of mixtures of small gases such as carbon dioxide and methane. CMS membranes are almost pure carbon materials that can be formed by thermal treatment of polymers. In contrast to zeolites and zeolite-like porous solids, CMS materials are amorphous. As a result, CMS membranes exhibit a distribution over sizes and shapes of pores making understanding and prediction of their transport properties a very challenging task.  Here we report results of microscopic studies of self-diffusion of carbon dioxide and methane in selected samples of CMS membranes. The diffusion data were obtained by pulsed field gradient (PFG) NMR method under conditions of application of large (up to 30 T/m) gradients and high (17.6 T) magnetic field. Combined application of proton and carbon-13 PFG NMR for diffusion studies of the same types of guest molecules allowed us to obtain reliable diffusion data even under the conditions when traditional proton PFG NMR measurements suffer from short T2 NMR relaxation times. The PFG NMR diffusion data will be discussed in relation to the membrane selectivities and permeabilities, which were previously measured for the same gases in the same membranes.