Framework aluminum distribution in ZSM-5 zeolite directed by organic structure-directing agents: a theoretical investigation

Abstract

The distribution of framework Al atoms in zeolite catalysts plays a significant role on dedicating the catalytic activity and product selectivity. Here, the relationship between framework Al distribution and organic structure-directing agents (OSDA) in ZSM-5 zeolite was investigated by the periodic density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. It was found that OSDAs with alkyl chains, i.e., tetrapropylammonium (TPA+) and tetraethylammonium (TEA+) cations would lead to the Al atoms mainly distributed at the intersection. While OSDA with pyrazolium ring, i.e., 1,2,4-trimethylpyrazolium (124TMP+), would preferably direct the Al distributed at the sinusoidal channel of ZSM-5. Further energy analysis and electronic structure analysis showed that the Al distribution was not only determined by the Al intrinsic substitution ability of each T sites, but also dominated by the electrostatic interaction between OSDA and zeolite framework. These results indicated that the choice of OSDA used in the hydrothermal synthesis could strongly regulate the Al distribution in zeolite frameworks and further effectively determine the catalytic performance. Therefore, our work would provide a theoretical guidance for the precise design of zeolite catalysis.