Statistical Thermodynamics Models for Polyatomic Adsorbates: Application to Adsorption of n-Paraffins in 5A Zeolite

F. ROMÁ; J. L. RICCARDO; A. J. RAMIREZ-PASTOR

LANGMUIR

American Chemical Society

Año: 2005 vol. 21 p. 2454 - 2459

0743-7463

Experimental adsorption isotherms of five n-paraffins (ethane,propane, butane, pentane, and hexane) in 5A zeolite were describedby means of a statistical thermodynamics model for linearadsorbates (MLA) developed by Ramirez-Pastor et al. (1999) andcompared with the well-known multisite Langmuir model (MSL) ofNitta et al. (1984). The experimental data, obtained by differentauthors in a wide range of temperatures and pressures, werecorrelated by using an algorithm of multiple fitting. Two mainconclusions were drawn from the analysis of experimental data: (i)for small molecules (ethane, propane), MLA is the more accuratemodel, validating the hypothesis of the linear rigid character ofthe adsorbate and reinforcing previous results obtained from theanalysis of computational experiments developed for dimers andlinear trimers; (ii) for large molecules (n-butane, n-pentane,n-hexane), the better performance of the MSL model suggests thatthe admolecules adsorb in a nonlinear structure. The isostericheat of adsorption dependence on the number of carbons obtainedfrom our study, ranging between 23.84 kJ/mol for ethane and 59.26kJ/molforhexane, showed a very good agreement with previousresults reported in the literature, confirming the consistency ofour analysis.