Variation of the Orientations of Organic Structure-Directing Agents inside the Channels of SCM-14 and SCM-15 Germanosilicates Obtained by Ab Initio Molecular Dynamic Simulations

TL;DR

This paper uses simulations to study how a molecule orients itself inside channels of two types of germanosilicates.

Contribution

The study reveals that the molecule adopts a stable perpendicular orientation in SCM-14 channels through dynamic simulations.

Findings

OSDA positions itself almost perpendicular to the large channels of SCM-14 after simulations.

Simulated structures are more stable than those from initial geometry optimization.

N-H stretching frequencies in IR spectra do not indicate molecule orientation.

Abstract

We report ab initio molecular dynamic simulations of the organic structure-directing agent (OSDA) in the channels of SCM-14 and SCM-15 germanosilicates for models with different germanium distribution. Since OSDA was free to move inside the channels, independent of its initial orientation after the simulations in all structures the OSDA, protonated 4-pyrrolidinopyridine, is positioned almost perpendicular to the large channels of SCM-14. The structures obtained from the dynamic simulation are more stable by 157 to 331 kJ/mol than the structures obtained by initial geometry optimization. After simulations, the average distance between the N atom of the pyridine moiety of the OSDA and O from Ge-O-Ge is shorter by 0.2 Å than the same distance obtained from initial optimization. The stretching N-H frequencies in the IR spectra of the OSDA and other calculated vibrational frequencies are not characteristic of the orientation of the molecule and cannot be used to detect it.

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