Kinetics of ion exchange in quasi-crystalline aluminosilicate zeolite precursors

The kinetics of binary ion exchange of Ca2+ and Mg2+ for Na+ in quasi-crystalline and crystalline zeolite A has been investigated. Diffusion of either divalent cation into crystalline zeolite NaA is relatively fast, while the exchanges are slower in the quasi-crystalline precursor materials. The diffusion coefficient for Mg2+ exchange (D-(Mg)) into crystalline zeolite A is an order of magnitude lower than that of Ca2+ (D-(Ca)), due to the larger hydration sphere of the Mg2+ ion. However, the ratio (D-(ca):D-(Mg)) was found to scale with the degree of crystallinity of the sample. Calcium and magnesium ions diffuse into the quasi-crystalline materials at comparable rates; this phenomenon is ascribed to the presence of larger pores in the quasi-crystalline structure, as compared to the crystalline material. Low impact mechanical treatment of quasi-crystallinge zeolite precursors further decreases the difference between D-(Ca) and D-(Mg), while the crystallinity of the material is not noticeably decreased. (c) 2005 Elsevier Inc. All rights reserved.