Microcalorimetric study of sorption of water and ethanol in zeolites 3A and 5A

Sorption of water and ethanol in zeolites 3A and 5A was investigated using gas flow-through microcalorimetry in view of finding differences in these materials performance as potential sorbents in the process of drying of ethanol. In microcalorimetric experiments, both zeolites show comparable properties for the sorption of water, but they differ profoundly in their sorption of ethanol, which was negligible small in zeolite 3A compared to zeolite 5A, in spite of the much larger enthalpy of sorption of ethanol in zeolite 3A. The difference can be explained in terms of a steric hindrance preventing the ethanol molecules from entering the narrow pores of the 3A structure, with the 5A structure remaining easily accessible for both the water and ethanol molecules alike. We propose to use a size selectivity index, defined as the ratio of sorption capacity of water and ethanol, to characterise the applicability of a zeolite as sorbent for drying of ethanol. The thermal effects of sorption of water in zeolite 3A was found to be a function of the nature of carrier gas, He or N2, used in microcalorimetric measurements.