Adsorption of methane, ethane, ethylene, and carbon dioxide on silicalite-I

Abstract

The adsorption of methane, ethane, ethylene, and carbon dioxide on silicalite-I and the thermodynamics of adsorption have been investigated under similar conditions using a gravimetric adsorption apparatus. The order for the adsorption of the different adsorbates on silicalite-I is CH₄ < CO₂ < C₂H₄ ≤ C₂H₆. The Dubinin-Polanyi equation is found to fit the isotherm data for the adsorption of methane (305 K) and ethane (305-413 K); the Freundlich equation fits the adsorption data for methane (353 K), ethane (453 K), and carbon dioxide (353 K); and the adsorption of ethylene (305, 353 K) and carbon dioxide (305 K) follows the Langmuir isotherm model. The isosteric heat of adsorption for the adsorption of these gases on silicalite-I lies between 18 and 32 kJ . mol^-1, with ethane having the highest value. The heat of sorption is affected mildly by the surface coverage for all of the gases but methane. The heat of adsorption of methane decreases with an increase in its loading. Analysis of the entropy of adsorption reveals that ethane (at an adsorbate loading of <0.8 mmol . g^-1), methane, ethylene, and carbon dioxide (at all adsorbate loadings studied) are supermobile on the silicalite surface. The mobility of adsorbed methane increases, and that of the other adsorbates decreases with increasing the adsorbate loading.