Synthesis of Zeolites in the Absence of Strong Alkali

Zeolites are conventionally synthesized in the presence of strong alkalis, and a major role of these strong alkalis is to dissolve silica and aluminosilicate species in the synthetic systems. This feature normally leads to the formation of framework defects such as silanol, the production of wastes, and the hindrance of introducing more heteroatoms into the zeolites. To overcome these issues, new routes have been developed for the synthesis of zeolites in the absence of strong alkalis in recent years, giving opportunities for more sustainable synthesis and tailoring zeolite structures and compositions. These approaches can be broadly categorized into two strategies: fluoride-containing and fluoride-free systems. In the fluoride-containing media, F^- acts as an efficient mineralizer and charge-balancing anion, enabling the synthesis of pure-silica zeolites and novel framework topologies, yielding low-defect, large single crystals, and promoting heteroatom incorporation through fluoro-complex chemistry. Fluoride-free approaches mainly include (i) two-step crystallization driven by zeolite precursors and (ii) one-pot synthesis using urea as a mild mineralizer under near-neutral conditions. In this perspective, the synthesis of zeolites in the absence of strong alkalis for both fluoride-containing and fluoride-free systems is briefly summarized, highlighting principles for zeolite crystallization, defect control, and isomorphous metal substitution.