Current efforts to design more efficient gas separation membranes are hampered by an incomplete understanding of material structure and of the microscopic features that control gas diffusion in membranes. In this study we seek to develop techniques to characterize these features, and to study the effects of thermal treatment on the properties of microporous silica membranes. We make the important distinction between traditional characterization techniques, which characterize void space, and size exclusion experiments that probe the small subset of features that actually control diffusion. The results of size exclusion, gas adsorption, 129Xe NMR, and transport experiments are combined to propose a model of the amorphous microporous silica structure. Thermal treatment is found to have almost no effect on void space, while profoundly affecting transport rates. We attribute the transport changes to the removal of water from the system, which increases pore interconnectivity thereby allowing alternate diffusion pathways.

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