MODEL STRUCTURES FOR MCM-41 MATERIALS - A MOLECULAR-DYNAMICS SIMULATION

Model structures for MCM-41 materials have been obtained using the cla ssical molecular dynamics simulation technique with effective interact ion potentials. A series of MCM-41 models with various lattice constan ts and wall thicknesses have been generated and analyzed. The density of T-sites, concentration of silanols, distribution of ring sizes, and the X-ray diffraction patterns were calculated for each structure wit h the latter compared to the experimental results. The results indicat e (i) the calculated X-ray diffraction pattern of the greater than or equal to 10 Angstrom wall amorphous silica models are in excellent agr eement with those observed experimentally, (ii) structures with wall t hicknesses less than 7 Angstrom are most likely unstable and give rise to X-ray diffraction patterns inconsistent with experiment, (iii) str uctures formed with smaller micelle templates require thicker walls to achieve thermodynamic stability, and (iv) models whose calculated X-r ay diffraction patterns are consistent with observed patterns have I7- 28% of the silicon as silanols, in good agreement with the 8-27% repor ted from NMR.