Investigating the role of sodium persulfate as a hydroxyl radical initiator in Catalyzing zeolite synthesis from fly ash

Sodium persulfate (Na₂S₂O₈) was employed as an initiator to enhance the depolymerization and dissolving of fly ash and to direct rearrangement of dissolved silicon-aluminum to construct high quality zeolite through the generation of hydroxyl radicals (•OH). The results demonstrated that Na₂S₂O₈ effectively generated •OH in the alkaline fly ash synthesis system, with the amount of •OH produced increasing proportionally to the concentration of Na₂S₂O₈. Furthermore, the introduction of •OH reduced the synthesis time by 12 h and accelerated the formation of pore fractals during the zeolite synthesis from fly ash by an additional 12 h. The average size of the synthesized NaP zeolite decreased from 283.81 to 164.95 nm, the interlayer distance decreased from 0.55 nm to 0.34 nm, the surface area increased from 34.23 to 41.77 m²/g, and the silicon-to-aluminium ratio increased from 1.08 to 1.51, yielding a higher-quality zeolite product within a shorter synthesis period. In addition, theoretical calculations indicated that the reaction pathway for •OH-catalyzed depolymerization of inert silicon-aluminium is similar to that of hydroxide ions (OH⁻). The energy barrier for the depolymerization reaction with the participation of •OH was 3.29 kcal/mol, which is lower than the 3.43 kcal/mol required for OH⁻. Consequently, •OH demonstrated a more pronounced catalytic effect on the depolymerization of inert silicon-aluminium species compared to OH⁻, and •OH significantly reduced the required alkalinity for the synthesis system, thereby optimizing the conditions for zeolitization of fly ash. This study will provide valuable insights for the future design and synthesis of nanomaterial from fly ash.