Sorption properties of crystalline molecular sieve AlPO₄-5

Abstract

The sorption capacity of a number of sorbates, viz. alcohols and hydrocarbons, and the isotherms of sorption of methanol, n-hexane, and benzene in AlPO₄-5 at 313 K were measured by gravimetric technique. The isotherms of sorption of n-hexane, cyclohexane, benzene, and pyridine in the AlPO₄-5, at higher temperatures (523-673 K) were determined by the GC peak maxima method. The isosteric heats of sorption of these sorbates at different sorbate loadings were obtained from the isotherm data. The heat of sorption near zero sorbate loading was determined separately by the GC pulse technique in the temperature range 523-673 K. The sorption capacities of AlPO₄-5 for the sorbates, differing widely in their dipole moments and critical molecular sizes, have been found to be nearly the same (about 0.14 cm³ . g^-1). The sorption of methanol, n-hexane, and benzene at 313 K occurs by the volume filling mechanism and the total volume of the sorption space in AlPO₄-5 has been found to be about 0.14 cm³ . g^-1. The sorption of n-hexane, cyclohexane, benzene, and pyridine at the higher temperatures follows the Freundlich isotherm. The influence of sorbate loading on the isosteric heat of sorption of benzene is very weak but the influence in the case of the other sorbates is very strong, particularly at the higher sorbate loadings. With the increase in the sorbate loading, the heat of sorption of n-hexane and pyridine increases and that of cyclohexane decreases. The heat of sorption of pyridine is much higher than that of the other sorbates. The values of the heat of sorption of n-hexane, cyclohexane, and benzene at near zero sorbate loading, obtained by the two methods, are very close to each other, and there is very good agreement between the heats of sorption obtained by the two methods. Analysis of the entropy change data for the high-temperature sorption reveals that the sorption of all the sorbates is mobile; the sorption of n-hexane at lower sorbate loading, in particular, and that of cyclohexane and benzene are "supermobile". However, in the case of sorption of n-hexane at higher sorbate loadings and pyridine, the sorbed molecules lose some of their rotational degrees of freedom.