Chabazite structures with Li+, Na+, Ag+, K+, NH4+, Rb+ and Cs+ as extra-framework cations

Abstract

Chabazites with Li+, Na+, Ag+, K+, NH4+, Rb+, Cs+ as monovalent extra-framework cations have been prepared from natural chabazite (ORI-CHA, Ca1.6Na0.5Si8.4Al3.6O24·14.3H2O) and characterized using a combination of Rietveld analyses of high-resolution synchrotron X-ray powder diffraction data and synchrotron infrared (IR) spectroscopy. All monovalent cation-exchanged chabazites crystallize in the rhombohedral space group R3¯m; Li-CHA (Li2.9Si8.6Al3.4O24·13.2H2O), a = 9.4068(6), α = 94.9156(9); Na-CHA (Na3.4Si8.6Al3.4O24·11.4H2O), a = 9.4216(3), α = 94.0773(6); Ag-CHA (Ag3.5Si8.5Al3.5O24·15.9H2O), a = 9.4402(4), α = 94.1354(9); K-CHA (K3.2Si8.7Al3.3O24·10.7H2O), a = 9.4420(1), α = 94.2540(3); NH4-CHA ((NH4)2.1Si8.4Al3.6O24·11.6H2O), a = 9.4378(9), α = 94.6101(1); Rb-CHA (Rb4.1Si7.9Al4.1O24·6.5H2O), a = 9.4318(4), α = 94.533(1); Cs-CHA (Cs3.4Si8.6Al3.4O24·6.4H2O), a = 9.4447(4), α = 94.2604(8). We establish correlations between the unit-cell volume, extra-framework cation distribution, and cation selectivity in chabazites. We establish that the unit cell volume of monovalent-cation exchanged chabazites decreases in the order of decreasing ion selectivity, based on the standard free hydration energies of exchange i.e., Cs+ > K+ > Ag+ > Rb+ > Na+ > Li+ and not the cation size.