Distribution of Al and adsorption of NH3 in mordenite: A computational study

Dispersion corrected density functional theory (DFT-D2) were employed to investigate the distribution of Al in the framework of H-[Al]MOR and the strengths of Brönsted acid sites by NH3 adsorption. Thermodynamically, the most favorable site for distribution of Al is T2O5, followed by T4O2, T1O7 and T3O1, which are a little higher in energy when Al is incorporated. It was found that the energy differences for Al in different T sites are 0.03–0.07 eV, indicating that the Al atoms might distribute in all kinds of four non-equivalent crystallographic tetrahedral sites of MOR. Moreover, it is also suggested that the location of protons plays an important role in the stability of the Al substitution site. In addition, we also computed the adsorption energies for NH3 adsorbed at each crystallographic position of H-[Al]MOR by DFT and DFT-D2, respectively. By comparison, the DFT method always underestimates the substitution energy by 0.41 eV for the adsorption of NH3, indicating that the dispersion correction is necessary to calculate the adsorption of NH3 in H-MOR. The results show that the Brönsted acid site at T2O5 is stronger than the other acid sites, and the adsorption of NH3 on the Lewis acid sites is clearly weaker than on the Brönsted acid sites.