Scheme 1. Schematic Drawings of Influence of Local Geometry on Charge Localization around Ion Exchange Sites.

(A) shows a Si–O–Al bridge compressed from both ends, whose negative charge is delocalized, strongly acidic (prefers NH₄⁺ than H⁺), and prefers Cs⁺ than Na⁺. (B) shows a Si–O–Al bridge stretched from both ends, whose negative charge is localized to an O atom, weakly acidic (prefers H⁺ than NH₄⁺), and prefers Na⁺ than Cs⁺.