CO(sub 2) diffusivity through hydrotalcite materials at 200 to 250 C was determined based upon the weight pick-up vs time. D/r(sup 2) (diffusivity/radius(sup 2)) for CO(sub 2) ranges from 3 x 10(sup -4) to 1 x 10(sup -3) depending upon the temperature. This range of diffusivity is consistent with the diffusivities through nanoporous materials, such as pillard clays and carbon molecular sieve, reported in the literature. Further the activation energy calculated based upon the diffusivity as a function of temperature is(approx)12 kcal/mole CO(sub 2), indicating activated diffusion for CO(sub 2) transport through the intracrystalline region of hydrotalcite. More importantly nitrogen diffusivity determined based upon the same methodology is negligible. This implies that the hydrotalcite materials have a strong affinity to CO(sub 2), but not nitrogen although the kinetic diameters for both molecules are similar. This result supports our proposed concept on the use of the hydrotalcite membrane for selective permeation of CO(sub 2). In the next quarter, we will conduct more calculation to determine the CO(sub 2) permeability of an ideal hydrotalcite membrane. This theoretical analysis will provide a quantitative basis for the design of a hydrotalcite membrane. Further, the theoretical diffusivity thus obtained can be used as a tool to (1) gauge the degree of defects of experimental membranes prepared, and (2) direct the future membrane synthesis and improvement.
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