Proton MAS NMR studies of hydroxyl groups in alkaline earth cation-exchanged zeolite Y

Abstract

The hydrolysis of divalent cations in alkaline earth cation-exchanged zeolite Y is studied by proton magic-angle-spinning ( 1H MAS) NMR. Ca 2+ and Mg 2+ dissociate about one water molecule per cation giving rise to a Brønsted proton and a non-acidic MOH + cation (M = Ca or Mg); two 1H MAS NMR signals appear in the spectra. The chemical shift of the bridging OH group is 4.0±0.2 ppm while the signal of the metal-bound OH groups lies between −0.5 and 3.0 ppm. The concentration of OH groups decreases with increasing temperature of activation. As a result of the so-called Plank mechanism the samples contain less than 0.1 bridging hydroxyl group per cavity after activation at 400°C.