Synthesis of mazzite-type zeolites in the presence of organic solvents: study of the structure directing role of p-dioxane

Syntheses were performed in the system 10SiO₂:1Al₂O₃:xNa₂O:[y wt.%organic compound + (100-y)wt.%H₂O] at 150°C. In the presence of dioxane, for Na₂O contents between 2 and 3, primarily mazzite-type products were obtained with Si/Al ratios ranging from 3.3 to 4.1. The dioxane content (y=5–85) influences both the availability of the dioxane-Na⁺ complex, which is essential for MAZ crystallization, and the effective alkalinity. As such, well-crystallized MAZ samples with high and low Si/Al ratios could be obtained at low (y=5–10) and high dioxane content (y=85), respectively, while at intermediate content (y=50), denser phases crystallized. For Na₂O contents lower than 2, sodalite phases were formed with the neutral dioxane molecule acting as a templating agent, while above 3, mainly gismondine and analcime were produced. For syntheses with high dioxane content, the cyclic ether was partially replaced by other organic solvents (ethers and alcohols) to study the effect of the solvent properties on zeolite crystallization. The solubility of the organic compound in the aqueous phase and its solvating properties toward anions and cations are of key importance with respect to the rate and the type of zeolite synthesis in organic medium.