Molecular dynamics study on the influence of additives on the high-temperature structural and acidic properties of ZSM-5 zeolite

The ReaxFF reactive force field method was used to study the effect of various dopants on the thermal stability and melting process of the zeolites. The force field parameters were fitted against quantum mechanical data for Si/Al/Fe/O/H interactions. Molecular dynamics (MD) simulations were performed using an isothermal-isobaric (NPT) ensemble to investigate the thermal stability of different zeolite frameworks. The zeolite frameworks simulated consisted of silicalite, Al-doped HZSM-5, Fe-doped ZSM5, hydrated ZSM5, and Al-and Fe-doped hydrated HZSM-5. The Lindemann index was used to characterize the melting process. MD results indicate that most of the zeolite frameworks remain stable until 3500 K on the nanosecond time scales accessible to MD simulations. Above 3500 K, the frameworks start to collapse inward, indicating the loss of porous channels. The melting process is initiated due to the disruption of the Si-O-Si network. The presence of water weakens this network due to the hydrogen transfer from water to bridging oxygen, whereas in the presence of Al and Fe, disruption is mainly caused by the presence of protons and by diffusion of Al and Fe atoms. The heating rate has a significant effect on the predicted melting point, and a progressively lower melting point, approaching experimental numbers, can be obtained by employing a lower heating rate. These results show how reactive MD simulations can be utilized to study the relative thermal stability of complex, multicomponent materials.