Regulating Fe3+ Migration and AlO4-Associated Hydroxyl Evolution via Solid-State Ion Exchange for Enhanced Glycerol Ketalization over Y Zeolite

Abstract

Solid-state ion exchange (SSIE) method generates minimal wastewater and provides high ion-exchange rates, making it an emerging strategy for tuning zeolitic structures and properties. However, the mechanistic understanding involving ionic diffusion in zeolites remains unclear. In this work, we used Y zeolite as the proof-of-concept case, demonstrating that FeY catalysts prepared via the SSIE method exhibit superior catalytic activity for ketalization of glycerol and cyclohexanone. As one of the key findings, SSIE induces significantly less structural damage to the zeolite framework than ion exchange in liquid medium, where Fe³⁺ unusually migrates from supercages to β cages in the solid state. Na⁺ species associated with AlO₄ tetrahedra are replaced by Fe³⁺, forming isolated Fe³⁺ and oligomeric Fe_xO_y species that correspond to [Fe(OH)₂]⁺ and [HO-Fe-O-Fe-OH]²⁺. Consequently, the introduction of Fe³⁺ also modulates Lewis acidity, more importantly, generating additional Brønsted acid sites. Isolated Fe³⁺ species are favorably formulated during solid-state diffusion. Computational studies further reveal that the acidic hydroxyl groups associated with isolated Fe³⁺ species are the intrinsic active sites for glycerol ketalization. The mechanistic insights provide both experimental and theoretical foundations for designing next-generation zeolite catalysts for the green synthesis of bio-based specialty chemicals.