Influence of Synthesis Method on LTA Time-Dependent Stability

Abstract

Time-stability of LTA zeolite formed by hydrothermal method with or without the action of ultrasonic irradiation was investigated by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The results show that 6 months after the synthesis by hydrothermal process with continuous sonication, LTA evolves into a more stable sodalite, whereas no differences are detected 12 months after LTA synthesis by conventional pre-fused hydrothermal process. These data confirm that using the two approaches, different mechanisms control both zeolite crystallization and time-stability of the newly-formed mineral at solid state. The results are particularly important in the light of the synthetic zeolite application.

Obsidian as a Raw Material for Eco-Friendly Synthesis of Magnetic Zeolites

“…In our hypothesis EMT and FAU formation by hydrothermal process is controlled by a double-step mechanism characterized by geopolymer precipitation from saturated solution followed by a slow zeolite growth involving the amorphous mass. This hypothesis is in accordance with the results obtained using both waste sources [ 35 ] and pure reagents [ 36 ]. It also explains the presence of a larger amount of geopolymers in HY4d and HY2h as well as the behavior of these samples over time.…”

“…Our previous data already demonstrated that obsidian can be successfully used as a precursor material for the organic-template-free EMT-type zeolite synthesis by a conventional hydrothermal process at 60 °C using a seawater solution [ 62 ]. Similarly, in other previous works we showed that different crystallization mechanisms control the zeolite synthesis according to the processes used for their formation [ 35 , 36 ]. In those papers, a waste material (fly ash) and Na 2 O-Al 2 O 3 -SiO 2 -H 2 O precursor system were used to perform the experiments.…”

“…In detail, 8 g of pre-fused OS powder was mixed with 42 mL of seawater. Finally, the sample was submitted to three different aging processes in separate experiments: (i) a conventional hydrothermal process at 60 °C for 4 days (HY4d) [ 29 , 30 ]; (ii) a hydrothermal water bath method for 2 h at 60 °C (HY2h) [ 35 , 36 ]; (iii) an ultrasonic water bath process (240 W, 35 kHz) for 2 h at 60 °C (US2h) [ 35 , 36 ]. After all the aging processes, the solids and solutions were separated by centrifugation.…”

“…As result, literature data have shown zeolite formation by different methods [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ], mainly using processes at low aging temperatures [ 29 , 30 , 31 , 32 , 33 , 34 ]. In our previous manuscripts [ 35 , 36 ], the zeolite formation by conventional hydrothermal process or ultrasonic was analysed. The experiments were performed using two different raw materials based on silica and aluminum.…”

“…In both cases, the results indicated that the hydrothermal process is responsible for slower geopolymer transformation into well-defined zeolite crystals, whereas the fast precipitation mechanism determines the formation of metastable zeolites by sonication. The data showed that the two different approaches also control the stability of the synthetic products over the years [ 35 , 36 ].…”