(173i) Fabrication and Characterization of Silicalite Membranes Subject to Knudsen and Surface Diffusion Transportation Regimes

Inorganic membranes composed of zeolitic crystals inside ceramic membrane supports show particular promise for gas separation applications due their high permeability, chemical stability, and low operating costs. The fabrication of membranes of this type is however very challenging due to the significance of even a small number of defects on membrane performance. An elegant characterization model has therefore been developed which can be used to quantify the physical characteristics of defects in novel membranes based on a limited number of pure gas permeation tests, and evaluate novel membranes. Additional refinements to standard fabrication and testing conditions have also been developed which allow greater control of experimental variables, and are presented in this work.

To demonstrate this characterization model, several batches of silicalite membranes have been hydrothermally synthesized using ceramic membrane supports with active layers whose pore sizes are 0.45 and 0.8 µm. Pure gas permeation experiments were then conducted using these membranes with He, N₂, and CH₄ gases in turn. The transport of these gases through membranes of this type proceed according to the regimes of Knudsen diffusion through defects, and surface diffusion through silicalite crystals (excepting He, which is sufficiently small that it is predominantly transported by Knudsen diffusion through silicalite crystals at standard conditions). The results of these permeation experiments, in combination with previously reported diffusion data and adsorption isotherms have then been used to determine the effective silicalite channel path length, and defect size to length ratio that are unique to each membrane. Through the application of the proposed model, effective silicalite channel lengths and defect ratios for membranes that were fabricated in different batches at similar conditions have been found to vary up to 20% and 700% respectively.

Given these results, characteristic values such as permselectivity can be determined that are normalized to exclude the defect transport of gases, and are therefore expected to be more comparable. Furthermore, this model can be applied to many different inorganic type membranes, and enable more reliable simulations of scaled up membrane processes.