Interpretation of net and excess adsorption isotherms in microporous adsorbents

• Supercritical CO2CO2 adsorption data on 13X zeolite crystals and pellets are presented.
• Net and excess adsorption isotherms are evaluated through a graphical method.
• The adsorbent’s volume is overestimated when Helium is used as a probing gas.
• CO2CO2 adsorbs as a dense liquid and entirely fills the micropores.
• The statistical adsorption isotherm model provides a good fit to the measured data.

Adsorption data are routinely reported as net or excess amounts adsorbed; although measuring techniques are nowadays well established, the interpretation and further use of these two measures is limited by the uncertainty on the estimated internal pore volume of the material and, accordingly, the volume (or density) of the adsorbed fluid. In this study, adsorption data are presented that have been measured with CO2 on 13X zeolite in both crystal and pellet forms at 50 °C and in the pressure range 0.02–14 MPa by using a magnetic suspension balance. The adsorbents’ structural parameters have been obtained through a combination of independent measuring techniques, including low-pressure adsorption and mercury intrusion porosimetry, and a methodology is presented where both net and excess adsorption isotherms are simultaneously evaluated through a graphical method. While providing additional insights on the meaning of these two different frameworks, the application of such an integrated approach allows for a more consistent interpretation of the obtained adsorption data. It is shown that for both materials the adsorption process is entirely controlled by the filling of the micropores and that the adsorbent’s volume is overestimated when helium is used as a probing gas. The statistical adsorption isotherm model proposed by Ruthven is applied to describe the measured adsorption isotherms and provides a much better fit than the Langmuir model. For both crystals and pellets, and over the whole pressure range, CO2 adsorbs as a dense liquid with density values starting from the critical density of the fluid and reaching ∼∼27 mol/L (i.e. 15 molecules/cage) at the highest pressure of the experiment.

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