Structure Stability of Georgian Natural Heulandite

Abstract

Zeolites have a unique set of molecular - sieve, sorption, ion exchange and catalytic properties due to their framework microporous structure, and structural stability is an important characteristic and often a decisive factor in the application and performance of natural zeolites. The aim of our work was to study the processes occurring under the influence of heat, which determine the thermal stability of the zeolite - containing tuff of the Tedzami - Dzegvi deposit, with zeolite phase content up to 90%, consisting of heulandite and chabazite in a ratio of 8:1, and chemical composition |Na 0.25 K 0.06 Ca 0.19 Mg 0.15 |[AlSi 3.6 O 9.2 ] . 3H 2 O. It was found that as a result of exposure to heat, a slight dealumination of the surface of the calcined ( 400 - 500 °C ) samples occurs, as well as dehydration and amorphization of the crystal structure are observed. Sample dehydration occurs in several stages: (i) most of the water ( ≈60% of the total water content ) is continuously lost at temperatures below ≈250 °C, (ii) the part of the remainder ( ≈24% ) is slowly dehydrated up to 650 °C, (iii) complete dehydration of the sample is achieved at ≈800 o C. Amorphization of the heulandite phase begins at temperatures above 200 °C, the transition to the metastable heulandite B phase at ≈340 o C is not fixed, but at high temperatures wairakite or another mineral of the 9.GB.05 group and quartz are formed; t he chabazite phase is stable up to ≈1000 o C, and at temperatures above 1100 o C , leucite (K,Na)AlSi 2 O 6 and cristobalite SiO 2 are formed. The adsorption of water vapor and benzene on heat - treated samples decreases monotonically with an increase in the calcination temperature, following amorphization. Nitrogen adsorption - desorption isotherms show slight decrease of the absorbent surface area with an increase in the calcination temperature and nonmonotonic changes in average mesopore diameters. It is also shown that heat treatment improves the acid resistance of heulandite by reducing dealumination after sample treatment with hydrochloric acid.