Structural and mechanistic investigation of rubidium ion exchange in potassium zirconium trisilicate
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abstract
Complete and intermediate incorporation of rubidium into the hydrothermally prepared mineral umbite has been accomplished by ion exchange. Potassium zirconium trisilicate, K2ZrSi3O9H 2O, is an open framework ion exchanger composed of polymeric chains of trisilicate groups linked by zirconium atoms. This arrangement creates a three-dimensional tunnel network. The exchangeable cations are found in two unique tunnels along the c-axis inside the framework. Site 1 is marked by a 12-membered ring and contains two cations. Site 2, a 16-membered ring, contains two cations and two water molecules. The parent compound, K2ZrSi 3O9H2O, was used to prepare compound 1, Rb2ZrSi3O9H2O, in which the original space group, P212121, is conserved. Cell dimensions of a = 10.4821(6) , b = 13.3869(5) , and c -7.2974(3) were found for compound 1. Analysis of Rb-O coordination shows average contact distance approaching the sum of the O2- and Rb+ ionic radii. Compound 2, K0.29H0.1Rb 161ZrSi3O9H2O, shows that Rb+ has completely substituted K+ in site 2 and only partially in site 1. Upon structural analysis of 50% incorporation of Rb+, K0.9H0.1RbZrSi3O 9H2O (3), and 22.5% incorporation, K 1.45H0.1-Rb0.45ZrSi3O 9H2O (4), it becomes apparent that the mechanism of exchange involves population of site 2 first. Further investigation of the mechanism of exchange through examination of the exchange kinetics has shown a plausible intermediate hydrolysis step occurs where K+ is replaced by H+ and subsequently H+ exchanges with Rb+. 2007 American Chemical Society.
