Insights into the Wastewater-Free Synthesis of Zeolites

Conventional synthesis of zeolites produces a large amount of alkaline wastewater containing organic substances and dissolved silica species from hydrothermal crystallization, washing, and ion-exchange procedures, which are environmentally unfriendly and costly. To avoid these drawbacks, a near-neutral and solvent-free synthetic route has been developed, which simplifies the zeolite synthesis and post-treatment procedures, containing only grinding, crystallization, and calcination, achieving zero emission of wastewater. Furthermore, the reactor in this process has been scaled up to 300 L, which is an industrial scale for zeolite synthesis. Because of the solvent-free and near-neutral conditions, <i>ex situ</i> ²⁹Si MAS NMR and <i>in situ</i> ²⁹Si, ¹³C, and ¹H MAS NMR were successfully employed for the characterization of zeolite crystallization, showing that the silica species basically remain as a large amount of Si(4Si) species during zeolite crystallization, a typical feature of the solid-state phase transformation mechanism. Furthermore, 2D ¹H-²⁹Si HETCOR MAS NMR analyses reveal the interaction between Si(4Si) species in the zeolite framework and the methyl and methylene groups of the TPA⁺ template, suggesting a potential templating contribution. This work not only presents an environmentally friendly strategy suitable for industrial-scale zeolite production but also offers fundamental insights into the solid-state phase transformation mechanism.