Influence of Alkali Cations (Me = Li, Na, K, Rb, Cs) on Physico-chemical Properties of the Liquid and Solid Phases of Me-aluminosilicate Hydrogels

Josip Bronić,^a,* Tatjana Antonić Jelić,^a Ivan Krznarić,^a Jasmina Kontrec,^a Boris Subotić^a and Gregor Mali^b

^a Ruđer Bošković Institute, Bijenička 54, P.O.Box 180, 10002 Zagreb, Croatia
^b National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia
* Corresponding author: E-mail: josip.bronic@irb.hr
Tel.: +385 145 60 991, Fax: +385 146 80 098

Abstract
Me-alumosilicate hydrogels having the batch molar oxide composition xMe₂O × Al₂O₃ × ySiO₂ × zH₂O (Me = Li, Na, K, Rb and Cs); x = 4.24–4.66, y = 2.49 and z = 334.53–344.48 of the hydrogels (series a), and x = 9.52–10.30, y = 7.91–7.95 and z = 1073.38–1097.29 of the hydrogels (series b), were prepared by mixing of the appropriate Me-silicate and Me-aluminate solutions at room temperature. After ageing for 48 h at 25 °C and solid-liquid separation, the chemical compositions (contents of Me₂O, Al₂O₃ and SiO₂) of the liquid and solid phases were determined by AAS. Solid samples were additionally analyzed by XRD, FTIR and DTG. Specific equilibrium distributions of Me₂O, Al₂O₃ and SiO₂ between the solid and liquid phases were discussed in the terms of: (a) influence of Me ions on distribution of different silicate species and their degrees of hydroxylation in the starting Me-silicate solutions and (b) influence of the degrees of hydroxylation of silicate species in the starting Me-silicate solutions on the polycondensation reactions with monomeric [Al(OH)₄]^– anions during the formation of the gel. The presence/absence of the formation of zeolite-like structures (“quasi-crystalline”) in the gel matrix was discussed in the terms of the “structure-forming” nature of Li⁺ and Na⁺ ions and the “structure-breaking” character of K⁺, Rb⁺ and Cs⁺ cations.

Keywords: Zeolite A, nucleation, hydrogels, “structure-breaking” cations, ageing