Molecular statistical calculation of the thermodynamic adsorption characteristics of zeolites using the atom–atom approximation. Part 1.—Adsorption of methane by zeolite NaX

Abstract

The calculation of the thermodynamic parameters for the adsorption of methane by zeolite NaX was carried out using the atom–atom approximation, in which the potential energy of the intermolecular interaction of a molecule with the zeolite lattice can be represented as the sum of the interaction energies of the constituent atoms of the molecule with all the oxygen ions and cations of the lattice. The configuration integrals were calculated numerically, different positions and orientations of the molecule in the zeolite cavity being taken into account. To simplify the calculations the large cavity of the zeolite was divided into small cells. The energies of interaction, at the centre of each cell, of C and H atoms with the lattice were calculated by computer. At any point inside a given cell the corresponding contribution to the energy of interaction was assumed to be equal to that for the centre of the cell. For very small (zero) filling of the zeolite cavity the thermodynamic adsorption characteristics (Henry's constants, changes in differential internal energy, isosteric heats of adsorption and heat capacities of the adsorbed substance) were calculated. By making a reasonable choice of the molecular parameters the atom–atom approximation gives thermodynamic quantities which are in satisfactory agreement with corresponding experimental values.