Comparison between XRF and EPMA applied to study the ionic exchange in zeolites

Zeolites are crystalline aluminosilicates consisting of SiO4 and AlO4 tetrahedral as primary units. One peculiar characteristic of zeolites is the ion exchange capacity defined as the capacity to locate specific cations in the framework of zeolites; it depends on the chemical composition and varies with the structure of the zeolite and with the cation nature. This work studies the exchange of the Na + monovalent cation of 5A and 13X synthetic zeolites by the Ca 2+ bivalent cation present in a CaCl2 solution. X-ray fluorescence (XRF) and electron probe microanalysis (EPMA) techniques were used to determine the cation exchange capacity (CEC). The efficiencies of the two X-ray detectors were compared and the minimum detection limits of the zeolite elements were calculated. Although both techniques differ in the sample excitation mode, the results obtained were compatible. The results showed that the CEC was higher for the 5A zeolite, in agreement with its lower SiO2/Al2O3 ratio and its greater BET area. It was also found that the amount of Na + ions exchanged by Ca 2+ ions was in complete agreement with the corresponding molar balance. The determination of the CEC using X-ray spectroscopy techniques can be considered a novelty as XRF and EPMA techniques permit to analyze the sample directly. Copyright c � 2009 John Wiley & Sons, Ltd.