PROTOTROPIC EQUILIBRIA IN AMMONIA ADSORPTION DESORPTION REACTIONS ON ALUMINUM-CONTAINING AND OR BORON-CONTAINING MFI-TYPE ZEOLITES

The temperature-induced desorption of ammonia from aluminum- and/or boron-containing MFI-type zeolites was studied with Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) experiments. The decrease in the N-H stretching band intensities and the contemporaneous growth of OH stretching bands intensities assigned to hydroxyl groups involved in H bonding interactions produced the formation of an isosbestic point in the IR spectra. The isosbestic point formation was revealed on aluminum and aluminum-boron-containing MFI-type zeolites but not on the boron-containing MFI-type zeolite. These findings were related to the formation of three charge transfer complexes involved in two consecutive proton transfer equilibria.