Anionic Tuning of Zeolite Crystallization

Zeolites are of great industrial relevance as catalysts, adsorbents, and ion-exchangers and typically synthesized under hydrothermal conditions. Rational regulation of their crystallization process is of great significance for zeolite production. In this work, we systematically investigate the role of anions in tuning zeolite crystallization via anion introduction including SO₄²⁻, F⁻, Cl⁻, Br⁻, I⁻, and SCN⁻ in the sodium form into the SiO₂–TPAOH–H₂O [tetrapropylammonium hydroxide (TPAOH)] synthetic system of silicalite-1 zeolite. The crystallization of silicalite-1 was accelerated by the sodium salts in the Hofmeister series based on the order of Na₂SO₄ > NaF > NaCl > NaBr > NaI > NaSCN. The liquid ¹H NMR and radial distribution function (RDF) analyses revealed that the anions were located around the template cation with a distance to the N atom of TPA⁺ in a sequence of SO₄²⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻ > SCN⁻. Combining the analyses on ¹H NMR and charge density of anions, we discovered that the anions affected the release of water molecules by activating the hydration spheres around the TPA⁺ cation in a different degree, thus regulating zeolitic crystallization. This work provides new insights into zeolite crystallization via anion additive introduction.