Covalent bonding of crown ethers to gamma-zirconium phosphate - New layered ion exchangers showing selective recognition

The topotactic exchange reactions of gamma-zirconium phosphate and gamma-zirconium phosphate/methylphosphonate with phosphonic acids derived from aza-crown ethers of different sizes was examined. Prior intercalation of hexylamine was necessary in most cases to attain the maximum exchange level predicted by molecular modeling. Materials with imbricated or nonimbricated layers were obtained depending on crown size and exchange level, Exchange of methylphosphonates in gamma-zirconium phosphate/methylphosphonate was only observed when hexylamine had previously been intercalated. The exchange with the bisphosphonic acid derived from 1,10-diaza[18]crown-6 gave a material with an interlayer distance compatible with pillaring of the gamma-phase, The materials obtained from the exchange reactions of gamma-zirconium phosphate with phosphonic acids derived from [12]crown-4 and [18]crown-6 showed thermodynamic selectivities towards Na+ and K+, respectively, that were much higher than those observed for the simple crowns in solution, This observation suggests that the inorganic molecular framework enhances molecular recognition by minimization of solvation effects.