Synthesis of Highly Porous Micro- and Nanocrystalline Zeolites from Aluminosilicate By-Products

Abstract

The present study is aimed to the preparation of synthetic zeolites of a defined Na-X type (Faujasite) from the fly ash (FA) generated by the incineration of Bulgarian lignite coals. FA zeolitization was carried out by three different methods: classical alkaline conversion, two-stage fusion-hydrothermal synthesis, and atmospheric aging, all with sodium hydroxide (NaOH) as an alkaline activator. The nature, morphology and chemistry of the zeolites were studied by X-ray diffraction, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses. The results from the classical alkaline conversion show the formation of an aluminosilicate hydrogel on the surface of the FA particles but a crystallization of any zeolitic phase does not take place. Zeolites with a dominant Na-X phase were obtained under by a two-stage synthesis under the following conditions: fusion at 550 °C, hydrothermal treatment at 90 °C for 2 h of duration, and NaOH/FA ratios of 2.0–2.4. Zeolite Na-X is also obtained by atmospheric aging for 1 year at a NaOH/FA ratio of 1.