PROBING EXTRA-FRAMEWORK CATIONS IN ALKALI-EARTH-METAL AND ALKALINE-EARTH-METAL LINDE TYPE-A ZEOLITES BY FOURIER-TRANSFORM FAR-INFRARED SPECTROSCOPY

The FT-far-IR spectra (350-30cm-1) of self-supporting wafers of alkali- and alkaline-earth-metal Linde type A (LTA) zeolites and some of their binary combinations are reported for the first time. The sensitivity of this region of the vibrational spectrum to metal cation type, location, coordination number, geometry, and population provides a convenient diagnostic for probing metal cation site distribution, the sequence of metal cation replacement resulting from selective ion exchange and metal cation mobility and reorganization during the removal of intrazeolitic water. Systematic trends in the far-IR spectra of the metal cations in conjunction with cation frequency, intensity, mass, charge, and bond length relationship provide a basis for an internally consistent set of vibrational signatures for different cations in specific sites of alkali-and alkaline-earth-metal LTA zeolites. Studies of this kind are likely to find utility in the identification of cations located in the zeolite pores, which will certainly assist in understanding the fine tuning of window sizes in LTA zeolites. This is important in their applications for gas separation and purification.