Study of the influence of the basicity and chromate-chloride anionic composition in the synthesis of the cancrinite–sodalite system

Abstract

The influence of basicity and anionic composition (CrO ²⁻₄ and Cl⁻) was studied in the synthesis of chromate-cancrinite and chlorosodalite zeolites. Zeolite X was used as starting material by using different NaOH concentrations (3, 8 and 16 M) and variable weight ratio (NaCl/(NaCl + K₂CrO₄)) equal to: 0.00, 0.25, 0.50, 0.75, and 1.00 in such a way that the total grams were 1.5 g. The syntheses were carried out at 80 °C and autogenous pressure during 40 h. The crystals obtained were characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, and scanning electron microscopy (SEM) coupled with an energy-disperse X-ray detector (EDX). Depending on the basicity employed and anionic composition (CrO ²⁻₄ and Cl⁻), the chromate-cancrinite or chlorosodalite zeolite was obtained. Owing to a low 3 M NaOH concentration and 100% Cl⁻, chlorosodalite mixed with zeolite X was observed in the XRD pattern. Owing to another Cl⁻–CrO ²⁻₄ proportions, inclusive to 100% CrO ²₄ , the starting material were not transformed (3 M NaOH). Pure chromate-cancrinite was produced when 8 and 16 M NaOH and 0% Cl⁻ concentrations were used. Chloride anions showed a strong structure directing effect for sodalite framework structure in comparison with chromate anions which favoring the cancrinite structure. Therefore, we determined pure chlorosodalite, when it was used until 75%Cl⁻ (8 M NaOH) and 25% (16 M NaOH). Results showed that cancrinite or sodalite zeolites are separately formed.