MOLECULAR MECHANICS STUDIES OF TBA AND TPA IN MEL AND MFI

The interactions of the template ions TPA and TBA with the all-silica zeolite structure types MFI and MEL are studied by molecular mechanics calculations with complete structural relaxation of both the zeolite framework and the template ions. MFI as well as MEL can accommodate on ly two TBA ions per unit cell because overcrowding occurs when TEA ion s occupy adjacent channel intersections. With two TBA ions MEL is more stable than MFI. Both zeolite structures can accommodate four TPA ion s per unit cell. In this case MFI is the more stable structure. This r esult is explained by the fact that MEL has two different types of int ersections. One type can accommodate tetraalkylammonium ions in their most favorable conformation. Accommodation of such a template in the o ther type of intersection, however, introduces considerable strain in both the zeolite and the template ion. The preference of MEL for TEA i ons as template and of MFI for TPA ions as template agrees with experi mental experience. The empty, relaxed MEL structure exhibits Imm2 symm etry. This is in agreement with NMR data at 268 K.