The influence of microwa®e radiation on adsorption selecti®ity was studied using the adsorption and desorption of cyclohexane and methanol on high-silica zeolites.
Moment analysis of capillary column inverse gas chromatography (IGC) data provides accurate measurement of polymer-solute diffusion coefficients. The technique is ideal for studying interactions of volatile materials with molten or rubbery polymers, at conditions approaching infinite dilution of the volatile component. The analysis presented by Pawlisch et al.,1 however, can provide flawed results if the geometry of the column coating is not uniform or if the polymer/solute system exhibits elution curves with significant skewing. In this paper, we present two improvements that overcome these limitations: (1) a model that accounts for a nonuniform polymer film and (2) a method for parameter estimation in the Fourier domain. The usefulness of the improvements is demonstrated by measuring the diffusivity and activity of benzene, toluene, and ethylbenzene in polystyrene, between 110 and 140 °C, the system investigated in part 1, and of methanol, methyl acetate, and methyl methacrylate in poly(methyl methacrylate) at temperatures above the glass transition temperature. In addition, the IGC technique is demonstrated to be applicable at temperatures below the glass transition temperature, as successful sub-Tg measurements were made for methanol in poly(methyl methacrylate). Finally, criteria are presented for the experimental modifications necessary to reduce the skewing of elution curves, which can reduce the reliability of results.
The diffusion of cyclohexane and several alkylcyclohexanes in
silicalite was studied using a simultaneous
thermal analyzer. This apparatus allowed for the determination of
heats of sorption and diffusion coefficients.
Zero length chromatography was also used to determine
diffusivities of some of the adsorbates. The trend
in diffusion rates observed was
trans-1,4-dimethylcyclohexane ≫ methylcyclohexane >
cyclohexane >
ethylcyclohexane, cis-1,4-dimethylcyclohexane. This is
in qualitative agreement with the previous results of
Chon and Park. A comprehensive interpretation of the experimental
results was developed, using the basic
qualitative arguments of transition state theory. The fast
diffusion of trans-1,4-dimethylcyclohexane is
associated with a relatively low energy barrier for channel crossing.
The transition entropy for diffusion
appears to be the reason for the more rapid diffusion of
methylcyclohexane relative to cyclohexane.
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