Sonohydrothermal synthesis of zeolite A and its phase transformation into sodalite

The sonohydrothermal (SHT) treatment is an innovative technique allowing the simultaneous coupling of low frequency ultrasound and hydrothermal conditions for the synthesis of materials. The aim of the present work was to investigate, for the first time, the synthesis of zeolite A and its formation mechanism under SHT conditions. The zeolite synthesis was carried out under sonohydrothermal conditions using a specially designed reactor that allows the application of ultrasonic irradiation at 20 kHz in an autoclave-type reactor heated up to 200 °C under autogenous pressure. The conversion kinetics of the amorphous hydrogel to zeolite A and its further conversion to sodalite were studied. Syntheses were performed in the SHT reactor at 80 and 100 °C, varying the synthesis time from 15 minutes to several hours. The required time to obtain fully crystalline zeolite A under sonohydrothermal conditions was only 25 minutes, highlighting a significantly improved crystallization rate compared to silent conditions (a 9.6-fold kinetic gain). In addition, the resulting zeolite A has smaller particles and a more homogeneous particle size distribution than the zeolite synthesized by hydrothermal treatment. These results can be explained by the sonofragmentation of the amorphous gel and the concomitant enhanced mass transfer of the building units at the interface between the crystallite surface and the solution resulting from the acoustic cavitation activity under SHT conditions. Compared to classical hydrothermal heating, a drastic kinetic increase of the transformation of zeolite A into the more stable sodalite phase was also observed under sonohydrothermal conditions.