Intracrystalline sorption of sulphur and phosphorus by some porous crystals

Abstract

The uptake of sulphur and phosphorus has been investigated in Ca–A, Na near-faujasite (Na–X), and (Ca, Na)-chabazite, for the former guest species and in Ca–A and Na–X for the latter. The measurements were made at 250–325°. Sulphur gives reversible and remarkably rectangular isotherms for these high temperatures, with differential heats near saturation of the host crystals of about –25 and –31·5 kcal. per mole for Ca–A and Na–X respectively. Kinetic studies made on the sulphur–chabazite system show a slow penetration into this crystal, by an activated diffusion for which the activation energy is 27·3 kcal. per mole. Evidence is presented that the sulphur diffuses as chains.

Phosphorus sorption was slow even in Na–X which is fully open enough to permit rapid diffusion of P₄ molecules. In Ca–A the sorption kinetics conformed approximately to zero order for uptake at constant pressure and temperature and to first order for desorption. The sorption kinetics in Ca–A can be explained in terms of the dissociation P₄⇌ 2P₂ as rate-determining in an externally adsorbed layer giving a Type III isotherm. The desorption kinetics can be explained in terms of a corresponding intracrystalline dissociation. In each case only the P₂ is mobile within Ca–A. In Na–X reversible Type IV isotherms were obtained with a differential heat rising at first rapidly and then very slowly to a value of –29·6 kcal. per mole as the amount sorbed increased. The isotherms were described and interpreted using Fowler's approximation to localised sorption with interaction.