Molecular statistical calculation of the thermodynamic adsorption characteristics of zeolites using the atom–atom approximation. Part 3.—Adsorption of hydrocarbons
The molecular statistical calculation of the thermodynamic characteristics of adsorption of n-alkanes C1-C5, neopentane, cyclopropane, cyclopentane, cyclohexane, as well as of ethylene and benzene at low (zero) filling of X zeolites has been made, using the atom–ion and point quadrupole approximations. A small correction has been introduced into the values calculated for the parameters of the corresponding atom–ion potential functions of the intermolecular interaction of the atoms of alkane and non-strained cyclane molecules with the zeolite ions. This correction factor was determined by comparing the calculated values of Henry's constant with its experimental values for a few reference systems (ethane–zeolite NaX or NaY, propane–KX zeolite). The corrected atom–ion potentials were than used to calculate the thermodynamic characteristics of adsorption of alkanes C1-C3 on NaX and (partly) on KX and NaY zeolites, as well as those of adsorption of n-butane and n-pentane (with due regard for their rotational isomers), neopentane, cyclopropane, cyclopentane and cyclohexane on NaX zeolite. In all cases, except for cyclopropane, the calculated values are in agreement with experiment. The discrepancy observed in the case of cyclopropane adsorption is attributed to the specific structure of the strained cycle of this molecule. The thermodynamic characteristics of adsorption of ethylene and benzene have been calculated using the atom–ion approximation and the point quadrupole moment approximation for electrostatic orientation intermolecular interaction. The effective quadropole moment of cyclopropane has been estimated using chromatoscopy.