Retro-analysis of silicate aggregation in pentasil zeolite formation

We present a retro-analysis of the zeolite assembly process by the decomposition of the frameworks of MFI and MEL type zeolites. We propose structures of the building units and the intermediates of the increasing complexity and the corresponding reactions that are necessary to accomplish the assembly process. The proposed intermediate structures are consistent with mechanistic synthesis proposals, especially that of the Leuven group. Based on Molecular Dynamics simulations of key elementary reaction steps we analyze the interaction between the structure directing agents (SDAs) and the silicate precursor species in MFI and MEL synthesis. A key result is the necessity of the alkyl chains of the alkylammonium ion templates to stabilize cavities in intermediate silicate oligomers as well as their role to assist the aggregation of such silicate clusters to form channel intersections. In the process the SDA cations move to positions different from their original position before aggregation. The differences between the stabilizing effects of the quaternary propylammonium cation in the formation of the MFI structure vs. that of the quaternary butylammonium used in the case of MEL crystallization suggest that this interaction plays an important role in the synthesis process.