Analysis of the distribution of adsorbate molecules within zeolite crystals by thermokinetics

A Calvet type microcalorimeter maintained at 30°C was used to measure the differential heats of adsorption of NH3 over different zeolites. Experiments show that a fast consumption of adsorbate over the early stage of adsorption is followed by a slow redistribution of the guest molecules inside the zeolite pores. The position of the maximum on the kinetic curves depends on the chemical composition of zeolites and degree of filling of zeolite cavities by the molecules adsorbed. The slow liberation of the heat is associated with a redistribution of the adsorbed molecules. It is suggested that once fast adsorption of ammonia is accomplished, slow competition occurs among sorbed species for the strongest adsorption sites. The number of vacated strong sites decreases with coverage to suppress migration of NH3 molecules to these sites. A kinetic equation is derived which describes this redistribution process.