Aluminum distribution in chabazite: An experimental and computational study

A set of synthetic and natural chabazite materials with compositions ranging from Si/Al = 2.6-7.6 have been obtained. Detailed studies using multinuclear MAS NMR have been used to analyze the distribution of aluminum within the crystalline framework. Comparison of the observed Si-29 NMR results to those predicted by various ordered and random models of the aluminum distribution indicates that, for the low-silica materials, the highest contribution from the ordered distribution to the spectrum originates from the configuration that has the least number of Al next nearest neighbor interactions. However, the results from all synthetic samples are also consistent with a random distribution, the one natural sample being less well described. Molecular modeling studies comparing the relative lattice energies of the different ordered distributions support these results.