Abstract:
NaX-Zeolite was converted to Zn forms by replacing sodium ions by zinc ions, amounting to a replacement from 16.60 % to 62.90 %. Chemical, TGA and DTA analysis of the zeolite samples was conducted and data on the composition of the unit cell (u.c.), zeolitic water and thermal stability of the lattices were obtained. The percentage of exchange of Zn2+ cations at 60 °C for a particular period decreased with increasing the concentration of Zn2+ cations in the exchange solution. The zeolitic water contents of NaX and ZnNaX zeolites were investigated by TGA up to 1000 °C. The TGA data indicated that all the samples of zeolites after dehydration underwent a small change in weight, occurring higher temperature due to dehydroxylation. The water content per u.c. was found to be larger in ZnNaX than in NaX. The TG curves of lumpy nature indicated the formation of a series of intermediate structures during dehydration. The minimum endothermic peak of dehydration shifted toward high temperature for ZnNaX zeolites suggested H2O molecules to be more strongly bonded to Zn2+ cations than Na+ cation. The slight shift of exothermic peaks of DTA towards high temperature indicated that ZnNaX zeolites are thermally more stable than NaX zeolite. The DTA curves indicated that dehydrated NaX retained its crystal stability from 370 to 580 °C and NaX and ZnNaX zeotites from 370-440 °C to 800 - 840 oC. NaX amorphized at - 960 °C and ZnNaX zeolites amorphized at 920-990 °C depending on the % of exchange of Na+ ions. The results are discussed with respect to hydrated cations, possible sites they occupy in the zeolite framework, the effect of extra framework cations and Si / Al ratio on thermal stability.
Page(s):
5-11
DOI:
DOI not available
Published:
Journal: Journal of Chemical Society of Pakistan, Volume: 29, Issue: 1, Year: 2007