Adsorption of n-pentane and iso-octane for the evaluation of the porosity of dealuminated BEA zeolites

Abstract

Adsorption of n-pentane and 2,2,4-trimethylpentane at 298 K was carried out in order to characterize the porosity of a series of HBEA dealuminated zeolites. The HBEA samples were prepared from a calcined parent sample by steaming, by treatment with hydrochloric acid and by treatment with ammonium hexafluorosilicate (HFS) solutions. The comparison of the adsorption results with those previously obtained by nitrogen adsorption, show that the use of this latter adsorptive alone is insufficient for evaluating the changes provoked by dealumination. The narrow mesopores which were found in the parent sample show distinct behaviour with respect to each of the organic adsorptives. Treatment with HFS solutions and steaming causes the most significant changes in the microporosity, decreasing the micropore volumes of the samples. The former also causes a substantial loss of external area. Treatment with HCl solutions primarily removes extra-framework aluminium species formed during calcination; while this treatment has no significant effect on the microporosity, it causes an increase in the external areas of the samples. The removal of extra-framework aluminium species from the mesopores of the calcined sample contributes towards this increase. On the other hand, these mesopores are blocked during steaming and treatment with HFS solutions.