CHROMATOGRAPHIC STUDY OF ALKANES IN SILICALITE - TRANSPORT-PROPERTIES

Micropore diffusion coefficients for infinitely-dilute methane, ethane, propane, n-butane, and isobutane in silicalite were measured via concentration-pulse chromatography. Two different samples of large laboratory-synthesized crystals were investigated. Analysis of van Deemter plots clearly indicated the presence of a significant micropore mass-transfer resistance for isobutane. Diffusion coefficients evaluated from these plots were consistent with results measured by the membrane technique. The linear alkane data differed completely in that the micropore mass-transfer resistance for these adsorbates was very small, implying that the true diffusion coefficients were too large to be measured with this system. Thus, effective diffusion coefficients evaluated from these data represent a lower limit of the true values. Comparison of this limit with nuclear magnetic resonance data was favorable, since the latter are significantly larger in magnitude. Inconsistency was observed with diffusion coefficients obtained from other macroscopic techniques including the membrane method, zero-length and tracer-pulse chromatography, and some frequency response experiments.