Influence of the alkalinity and [formula omitted]/Cl − anionic composition on the synthesis of the cancrinite–sodalite system

Abstract

The formation conditions of nitrate–cancrinite and chlorosodalite-type zeolites were studied by investigating the influence of the pH and NO 3 - /Cl − ratios on the synthesis of these compounds. The pH was varied by using different NaOH concentrations i.e., 3, 8 and 16 mol/L (aqueous solution) and the ratio of NO 3 - /Cl − ions was defined by different weight fractions of NaNO 3/(NaNO 3 + NaCl) i.e., 0, 25, 50, 75 and 100%. In the case of 3 M NaOH one additional experiment with 2% NaNO 3 was carried out in order to investigate that area of the phase system in more detail. All of the reactions were carried out at 80 °C and under autogenous pressure for 40 h. Zeolite X was used as silicon and aluminum source. The obtained solids were characterized by powder X-ray diffraction (XRD), FTIR-spectroscopy, scanning electron microscopy (SEM) including elemental analyses using the EDX method and BET surface area measurements. The study shows that pure nitrate–cancrinite was obtained with all NO 3 - /Cl − compositions with NO 3 - contents from 2% to 100% and independently of the NaOH concentration. With 3 M NaOH and 0% NO 3 - (=100% Cl −) the reaction lead to a mixture of chlorosodalite and the starting material zeolite X. Pure chlorosodalite could only be synthesized if the reaction gel was formulated with 0% NO 3 - and at higher NaOH concentrations (8 and 16 M). These results allowed to deduce a dependency of the formation of the different phases in the studied zeolite system of the NaOH and the NO 3 - concentration.