Mobility of cations and guest compounds in cesium-exchanged and impregnated zeolites Y and X investigated by high-temperature MAS NMR spectroscopy

Through the application of high-temperature Na-23 and Cs-133 magic-angle spinning (MAS) NMR spectroscopy, the mobility of sodium and cesium ions in dehydrated zeolites Y and X in the sodium form, after exchange with cesium cations and after impregnation with cesium hydroxide, has been investigated. At temperatures typical for heterogeneous catalysis on basic zeolites Y and X, sodium and cesium cations exhibit a high mobility and are not rigidly located at their cation positions. Starting at a temperature of 570 K, a chemical exchange of sodium cations in supercages and sodalite cages occurs. At 770 K, sodium cations located at SI positions in the hexagonal prisms are involved in a chemical exchange with sodium cations in sodalite units. Cs-133 MAS NMR spectroscopy of zeolites CsNaY and CsNaX indicates that the cesium exchange starts at 380-420 K. Starting at 380 K, a chemical exchange of the cesium atoms contributing to guest compounds with cesium cations located at cation positions in the zeolite cages was observed. For the cesium exchange processes in zeolites CsNaY/CsOH and CsNaX/CsOH occurring at 420-770 K, activation energies of E-a = 85 +/- 10 and 115 +/- 10 kJ/mol, respectively, were determined.