(152j) Microscopic Diffusion of Liquid Sorbates in Carbon Molecular Sieve Membranes By Pulsed Field Gradient NMR

Carbon molecular sieve (CMS) membranes are rigid and amorphous materials, which can be formed by the pyrolysis of polymeric precursors. The presence of controlled population of ultra-micropores and larger micropores along with the exceptional chemical and thermal stabilities make CMS membranes promising materials for liquid separations. CMS membranes can exhibit structural inhomogeneities. In particular, the transport and structural properties of the external membrane surface can be different from those in the bulk of the membrane. Hence, it is important to use microscopic techniques to quantify diffusion in CMS membranes, in addition to commonly used macroscopic measurements, such as permeation measurements. In this work, pulsed field gradient (PFG) NMR was employed to measure diffusion in CMS membranes for different liquid sorbates including methanol and water. The measurements were performed at different temperatures and for a broad range of root mean square displacements of sorbate molecules between a fraction of a micrometer and several micrometers. The results of these PFG NMR measurements will be compared with the corresponding data from macroscopic permeation measurements to shed light on possible transport barriers at the external membrane surface and other inhomogeneities in the transport properties inside the studied membrane.