A STUDY OF NATURAL ZEOLITE OUTGASSING BY SIMULTANEOUS THERMAL ANALYSIS/QUADRUPOLE MASS SPECTROMETRY

Although there is a plethora of work on the dehydration behavior of zeolites, few studies have simultaneously determined the structural and evolved gas properties of these minerals during dynamic heating. We present a study of natural zeolites by Simultaneous Thermal Analysis/Quadrupole Mass Spectrometry (STA/QMS). This method combines differential scanning calorimetry, thermogravimetry, and evolved gas analysis allowing for the identification of phase changes in zeolites by observing mass losses, exothermic and endothermic reactions, and the release of volatiles. Seven different zeolites representing the three thermal categories of Alberti and Vezzalini were studied: 1) chabazite, mordenite 2) laumontite, yugawaralite 3) heulandite, stilbite, phillipsite and clinoptilolite that belongs to either category 1 or 2. Zeolites of category 1 are the least affected structurally by thermal treatment and category 3 the most affected.

Zeolites were heated at a constant rate from 25 to >900 ºC in helium. Water was the only volatile observed. Samples experienced mass losses of 12-18 wt% due to dehydration. Every sample exhibits a first H₂O release at <170 ºC. The wide range in maximum release temperatures (~90 to 170 ºC) may reflect variation in binding energies for the channel water. For chabazite, clinoptilolite, mordenite, and stilbite only the first water release is observed. A second H₂O release occurred between 210 and 280 ºC for yugawaralite, heulandite, laumontite, and phillipsite, which may correspond to H₂O released from channels. Yugawaralite, laumontite, and phillipsite also show a third H₂O release at high temperatures (at ~430, 350, and 435 ºC, respectively). In yugawaralite and laumontite this peak corresponds to release of water bonded to Ca atoms. In the case of laumontite this release leads to irreversible framework collapse. Endothermic reactions that do not correspond to any mass loss or volatile release are observed in mordenite (at 480 ºC) and stilbite (at 540 ºC), which are probably related to structural transformations. The H₂O release temperatures and total mass losses observed in this study agree with those determined in other dehydration studies using other methods. The thermal behavior of our clinoptilolite is consistent with category 1 thermal behavior suggesting a K-rich variety.