THERMAL DECOMPOSITION OF NH4-ANALCIME

Abstract

The thermal decomposition of ammonium-exchanged natural analcime is characterized by gas chromatography, IR spectroscopy and X-ray diffraction. The de-ammoniation and dehydroxylation proceed in parallel throughout the decomposition, which evidences the instability of the protonated analcime framework. The mechanism of degassing of NH 4-analcime changes throughout its decomposition. At the initial step, the mechanism of de-ammoniation consists in thermal dissociation of NH 4 + molecule onto NH 3 and proton (framework OH group) and diffusion of NH 3 out of the structure. Subsequent decomposition and removal of the OH groups lead to a progressive loss of crystallinity. At this step, an apparent activation energy for NH 3 desorption is estimated to be 145(±13) kJ mol -1. This value is within the upper limit of the activation energy characteristic for the NH 3 desorption from proton centres in large-pore zeolites. At the final step, the adsorption of NH 3 and protons onto the defect centres in the amorphosed aluminosilicate framework results in a significant increase of an apparent activation energy for the de-ammoniation and dehydroxylation up to 270(±20) kJ mol -1.