SHORT-RANGE ORDER OF THE SI, AL DISTRIBUTION ON THE FAUJASITE FRAMEWORK

The Si, Al distribution over the tetrahedral network of faujasite is analyzed from earlier Si-29 NMR data by means of Monte Carlo simulations. The cation distribution over different structural units is considered, and special emphasis is laid upon the degree of dispersion of the Al cations over the framework. These cations are found to be more dispersed than required by the Al-O-Al avoidance, but the number of Al-O-Si-O-Al groups is actually not minimized. The experimental NMR results for synthetic samples can be interpreted by assuming an effective second-neighbor-cation interaction W2 of about 2k(B)T(e), where T(e) is the equilibrium temperature of the cation distribution. For dealuminated samples, the cation correlation further than nearest neighbors is less important, indicating that in this case the Si, Al distribution is more disordered than in as-synthesized specimens.