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Chemically controlled particulate properties of zeolites: towards the face-less particles of zeolite A. 2. Influence of aluminosilicate batch concentration and alkalinity of the reaction mixture (hydrogel) on the size and shape of zeolite A crystals

Influence of the simultaneous change of batch alkalinity (A = [Na2O/H2O]b) and total batch silica concentration (tsc = [SiO2/H2O]b) of the reaction mixture, at two characteristic batch molar ratios, y  [SiO2/Al2O3]b (1.3 and 2.0), on the duration of crystallization, reaction yield and the particulate properties (size and shape) of the crystalline end product (zeolite A) was investigated. Sharp decrease of the duration of crystallization process with simultaneous increase of A and tsc is caused by simultaneous increase of the number of nuclei formed in gel matrix, rate of their release from the gel matrix and their growth with the increase of the alkalinity of the reaction mixture. Although the reaction yield, YR, is proportional to the total batch silica concentration, tsc, the ratio YR/tsc slightly decreases with increasing alkalinity as a consequence of the increasing solubility of zeolite A with the increase of the alkalinity. Specific number of crystals (number of crystals per unit mass of the crystalline end products), NS increases with simultaneous increase of A and tsc as a consequence of the increase of the number of nuclei formed in the gel matrix with increase of alkalinity of the reaction mixture. On the other hand, a complex dependence of the crystal size distribution on A, tsc and y, is a consequence of influence of the mentioned chemical factors not only on the number of nuclei, but also on their distribution in the gel matrix. Four different morphological forms of zeolite A, namely, regular cubic crystals with “sharp” edges and apexes, cubic crystals with truncated edges and (iii) cubic crystals with “rounded” edges and apexes and (iv) pseudo-spherical, face-less particles are obtained during hydrothermal treatment of investigated hydrogels. Influence of the chemical factors on the formation of particular morphological forms of zeolite A are discussed and the possible mechanisms of their formation are considered.

aluminosilicate hydrogels ; crystallization ; zeolite A ; crystal morphology ; crystal size

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