Molecular mechanics studies of TBA and TPA in MEL and MFI

Abstract 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 only two TBA ions per unit cell because overcrowding occurs when TBA ions occupy adjacent channel intersections. With two TBA ions MEL is more stable than MFI. Both zeolite structures can accommodate four TPA ions per unit cell. In this case MFI is the more stable structure. This result is explained by the fact that MEL has two different types of intersections. One type can accommodate tetraalkylammonium ions in their most favorable conformation. Accommodation of such a template in the other type of intersection, however, introduces considerable strain in both the zeolite and the template ion. The preference of MEL for TBA ions as template and of MFI for TPA ions as template agrees with experimental experience. The empty, relaxed MEL structure exhibits Imm2 symmetry. This is in agreement with NMR data at 268 K.