AB-INITIO MOLECULAR-ORBITAL STUDY ON GAS-PHASE ADSORPTION ON SILICA-GEL

Adsorption of nine different gases and vapors on a silica gel was stud ied based on ab initio MO calculations. A single-type adsorption site constructed by one silanol group, and a paired-type adsorption site by two silanol groups were used as molecular models for adsorption sites on the silica gel. Adsorption structure and interaction energy were d etermined for the molecular models, and a decomposition of the energy was carried out. Adsorption equilibria of the nine gases and vapors on the silica gel were measured, and differential heats of adsorption we re determined by applying the Clausius-Clapeyron equation to the measu red isotherms. The interaction energy estimated by the MO calculation roughly corresponded to the measured heat of adsorption. The single-ty pe adsorption site is favorable for adsorption of ammonia. The interac tion energies of ethylene were almost the same on both adsorption site s. In the adsorption of alcohols, water, benzene and sulfur dioxide, t he paired-type adsorption site gave a more stable adsorption structure than the single-type site.