Molecular modeling of adsorbed NDMA and water in MFI zeolites

• Adsorption of N-nitrosodimethylamine in MFI was modeled using density functional theory.
• H-ZSM5 shows stronger binding to N-nitrosodimethylamine than Na-ZSM5 and silaceous MFI.
• DFT results show electrostatic interactions key to adsorption.
• van der Waals forces are also a significant part of zeolite–adsorbate interactions.
• H-ZSM5 binds N-nitrosodimethylamine more strongly than water.

N-nitrosodimethylamine (NDMA), which is a carcinogenic and toxic N-nitrosamine, can be found in water resources associated with a multitude of processes in various industrial facilities or merely as a by-product of water or wastewater treatment. Therefore, the removal of NDMA from drinking water represents an important human safety and public health concern. The present paper presents a density functional theory study of NDMA adsorption in all-silica MFI, Na-ZSM-5 and H-ZSM-5 zeolites. The stability of NDMA inside the zeolite pores was investigated by calculating the amount of energy released during adsorption. Various configurations of adsorbed NDMA to the zeolites were investigated, predominantly at the intersection of straight and sinusoidal channels. The strength of the adsorption energies followed the order H-ZSM-5 > Na-ZSM-5 > all-silica MFI. NDMA has a dipole moment and the strongest binding of NDMA occurred through the interactions of the negatively charged O atom of the molecule to positive atoms of the zeolite. For comparison, similar calculations were performed for water adsorption in these three zeolites. The adsorption energy of water in these three structures followed the order Na-ZSM-5 > H-ZSM-5 > all-silica MFI. We also incorporated van der Waals corrections in the simulations, which had the effect of stabilizing NDMA within the zeolite channels, but did not significantly change the relative stability of the different adsorption geometries. It was concluded that H-ZSM-5 is a good candidate to remove NDMA. Finally, it should be pointed out that H-ZSM-5 has the largest difference in binding energy between NDMA and water of the studied zeolites, suggesting that separation of the two molecules is indeed feasible due to preferential interactions between zeolite and adsorbate molecule.

NDMA, a harmful pollutant, is adsorbed in zeolite H-ZSM5.Figure optionsDownload as PowerPoint slide