Stability of fly ash-based BEA-zeolite in hot liquid phase

Abstract

Coal fly ash (class F) was used as feedstock to extract silica for the indirect synthesis of BEA zeolite via a hydrothermal process. The extraction steps selectively retained major components of Si > Al > Na and eliminated the need for the addition of an extra silica source in the zeolite synthesis mixtures. The stability of BEA zeolite was studied in hot liquid phase at different exposure times (6, 12 and 24 h) and temperatures (150 and 200 °C). The structural changes in the framework of the treated zeolite was determined with X-ray diffraction, thermogravimetric analysis, N2 adsorption–desorption and 27Al and 29Si magic-angle spinning nuclear magnetic resonance. Structural degradation of the treated zeolite depended on prolonged exposure time and high temperature, reduced crystallinity and framework Si/Al ratio, the degree of desilication, and the presence of silanol defects. The relative crystallinity and changes in framework Si/Al ratio were shown to depend on the exposure time and temperature in hot liquid phase. With prolonged exposure time, the surface area and micropore volume decreased with an increase in the mesopore volume. The stability of BEA zeolite was influenced by the hydrolysis of Si-O-Si bonds (siloxane) through desilication which eventually caused the formation of structural defects at elevated treatment time and temperature, however, the crystal structure remained intact. Herein, the stability of fly ash based BEA zeolite in hot liquid shows its potential use as a catalyst for liquid phase reactions, considering the exposure time and temperature investigated.