The nature of the distribution of aluminum over the framework cation sites in aluminosilicate zeolites is known to influence the catalytic properties. Simulation is here used to probe how particular Al T-site substitution patterns are manifested in the framework geometry and unit cell dimensions of ZSM-5 (MFI-framework), arguably the most technologically important zeolite. Aluminum was substituted in turn onto each of the 12 crystallographically distinct T-sites in the MFI-framework. The corresponding anionic framework charge was compensated either by a nonframework tetrapropylammonium cation (TPA(+)) or by introducing a proton at each of the four apical oxygen atom positions in turn for each of these 12 T-site choices. Each of these 12 (TPA(+)) or 48 (H+) configurations was optimized at constant pressure to a zero-force, energy minimum configuration using a validated molecular mechanics force field. The manner in which the observed structural changes scale with the extent of Al substitution on each site was evaluated. Comparisons with available experimental data in the literature indicate that the Al distribution in real materials is, at least, moderately disordered. This is consistent with the similar Al site substitution energies found for the different T-sites in the present and previous simulation results.
Predicted Effects of Site-Specific Aluminum Substitution on the Framework Geometry and Unit Cell Dimensions of Zeolite ZSM-5 Materials†
RICCHIARDI, Gabriele;
1997-01-01
Abstract
The nature of the distribution of aluminum over the framework cation sites in aluminosilicate zeolites is known to influence the catalytic properties. Simulation is here used to probe how particular Al T-site substitution patterns are manifested in the framework geometry and unit cell dimensions of ZSM-5 (MFI-framework), arguably the most technologically important zeolite. Aluminum was substituted in turn onto each of the 12 crystallographically distinct T-sites in the MFI-framework. The corresponding anionic framework charge was compensated either by a nonframework tetrapropylammonium cation (TPA(+)) or by introducing a proton at each of the four apical oxygen atom positions in turn for each of these 12 T-site choices. Each of these 12 (TPA(+)) or 48 (H+) configurations was optimized at constant pressure to a zero-force, energy minimum configuration using a validated molecular mechanics force field. The manner in which the observed structural changes scale with the extent of Al substitution on each site was evaluated. Comparisons with available experimental data in the literature indicate that the Al distribution in real materials is, at least, moderately disordered. This is consistent with the similar Al site substitution energies found for the different T-sites in the present and previous simulation results.
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