Textural properties vs. CEC and EGME retention of Na–X zeolite prepared from fly ash at room temperature

Abstract

Zeolite Na–X (FAU type) was synthesized from F-class fly ash in simple and economical way. Several months of storage of fly ash in NaOH solution, at room temperature, without any prior treatment, results in the production of material with ca. 50% content of Na–X zeolite. The most efficient reaction takes place at (OH) − activity of 0.1–0.15 mol (OH) −/g of ash and at S/L ratio of 33 to 67 g/dm 3. Higher S/L value causes an accelerating dissolution–crystallization equilibrium attainment. Cl − ion acts as the inhibitor of this reaction. Na–X crystals show Si/Al ratios of framework in the range of 1.14 to 1.45. Zeolite content controls the micropore volume and the surface area of the samples, limiting the ethylene glycol monoethyl ether (EGME) retention and the cation exchange capacity (CEC) values. The temperature of pre-heating is a crucial factor for the amount of retained EGME. Insufficient dehydration at 250 °C (as in conventional procedure for clays) does not allow EGME molecules to enter all micropores. Pre-heating at 400 °C causes EGME adsorption on all available surfaces. EGME molecule occupation area is ca. 41 Å 2, using BET algorithm and ca. 52 Å 2 for Langmuir equation, or package ratio 16 Å 3/1 EGME molecule if calculating adsorption as mechanism of micropores infilling. Total CEC value (including sodalite cages) of materials rich in Na–X zeolite, measured using Ba 2+ and Mg 2+ cations is ca. 2.5 meq/g. Exchange positions available for [Co(NH 3) 6] 3+ cation occur only inside 12-ring space and loops of FAU framework (secondary building unit — SBU). They produce CEC which attains almost 1.2 meq/g, following directly the development of microporous texture. Produced materials reach maximum N 2-BET surface area 344 m 2/g from 15.3 m 2/g of raw fly ash. Simple procedure of EGME retention can be successfully used to determine microporous texture of X zeolite available to organic molecules.