Synthesis of zeolites Na-A and Na-X from tablet compressed and calcinated coal fly ash

Zeolites Na-A and Na-X are important synthetic zeolites widely used for separation and adsorption in industry. It is of great significance to develop energy-efficient routines that can synthesize zeolites Na-A and Na-X from low-cost raw materials. Coal fly ash (CFA) is the major residue from the combustion of coal and biomass containing more than 85% SiO₂ and Al₂O₃, which can readily replace the conventionally used sodium silicate and aluminate for zeolite synthesis. We used Na₂CO₃ to replace the expensive NaOH used for the calcination of CFA and showed that tablet compression can enhance the contact with Na₂CO₃ for the activation of CFA through calcination for the synthesis of zeolites Na-A and Na-X under mild conditions. We optimized the control variables for zeolite synthesis and showed that phase-pure zeolite Na-A can be synthesized with CFA at reactant molar ratio, hydrothermal reaction temperature and reaction time of 1.3Na₂O: 0.6Al₂O₃: 1SiO₂: 38H₂O at 80°C for 6 h, respectively, while phase-pure zeolite Na-X can be synthesized at 2.2Na₂O: 0.2Al₂O₃: 1SiO₂: 88H₂O at 100°C for 8 h, respectively. The composition, morphology, specific surface area, vibration spectrum and thermogravimetry of synthesized Na-A and Na-X were further characterized.