Dynamics of water molecules in VPI-5 and AlPO_4-5 studied by deuterium NMR spectroscopy

Abstract

The dynamic properties of water and ammonia within the channels of VPI-5 and AlPO_4-5 were studied by ^2H NMR spectroscopy over a wide range of temperatures. The results were correlated with the corresponding ^(27)Al MAS NMR spectra. In both materials two distinct types of water molecules were detected, bound molecules and physisorbed molecules undergoing isotropic reorientation within the channels. The bound molecules were assigned to molecules coordinated to framework Al undergoing some local motion. In AlPO_4-5 the line shape changes were reproduced by using a dynamic model of a two-site exchange where one site corresponds to the bound water and the second to the free water molecules. The relative populations of the two sites were found to be temperature dependent. In VPI-5 the water exhibits a higher degree of order and the two-site jump between free and bound molecules is associated with an additional 3-fold-site jump, resulting in a 6-site system. In this case the relative populations are temperature independent within the temperature range of 0-60 °C. Three types of ND_3 molecules were distinguished in VPI-5 adsorbed with ammonia: physisorbed molecules, bound molecules undergoing a rotation about the N-A1 axis, and rigid molecules. No exchange takes place between these three states within the NMR time scale.

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