Adsorption Properties from Pressure-Varying Langmuir Parameters: n-Butane and Isobutane on Activated Carbon

Isothermal adsorption data for n-butane and isobutane on BAX 1500 activated carbon reported by Whittaker et al. [Whittaker, P. B.; Wang, X.; Zimmermann, W.; Regenauer-Lieb, K.; Chua, H. T.Predicting the Integral Heat of Adsorption for Gas Physisorption on Microporous and Mesoporous Adsorbents. J. Phys. Chem. C 2014, 118 (16), 8350−8358, DOI: 10.1021/jp410873v] were modeled with pressure-varying Langmuir adsorption parameters using flexible least squares for pressure-varying linear regression. Coverage varies with pressure and at distinct transitions; when the ratio of uptake to capacity is 0.69 ± 0.04, monolayer coverage is achieved or micropore volume is filled. Monolayer transitions are observed for the 298, 323, and 348 K isotherms, while micropore volume transitions are only apparent for the 298 K isotherms. The resultant adsorbent surface area is 1335 ± 25 cm² g⁻¹, and the micropore volume is 0.48 ± 0.03 cm³ g⁻¹. Molecular areas, corresponding to excluded adsorbate areas, are dependent upon the temperature and range from 29.1 to 31.1 Ų for n-butane and from 31.8 to 32.7 Ų for isobutane for the 298–348 K isotherms. Average molecular areas, calculated from monolayer capacities, are 20.5 ± 0.4 Ų for n-butane and 21.9 ± 0.7 Ų for isobutane and correspond to minimum areas, excluding surface mobility and packing. Molecular volumes, calculated from micropore volume capacities, are 45 ± 2 ų for n-butane and 58 ± 2 ų for isobutane and are comparable to molecular volumes determined from Lennard–Jones 12:6 potentials. Entropies of adsorption increase from −1.06 ± 0.04 kJ K⁻¹ kg⁻¹ at 298 K to −0.671 ± 0.008 kJ K⁻¹ kg⁻¹ at 348 K for n-butane and from −0.948 ± 0.018 kJ K⁻¹ kg⁻¹ at 298 K to −0.682 ± 0.010 kJ K⁻¹ kg⁻¹ at 348 K for isobutane and indicate increased mobility at monolayer coverage.