DFT study of the acid strength of MCM-22 with double Si/Al substitutions in 12MR supercage

DFT calculations were performed on the acidity of MCM-22 zeolite with double Si/Al substitutions at NNN site in the 12 member-ring. Two different cluster models were used to investigate the effect of different model structures on the calculated results. All calculations were carried out by using BLYP functional and DNP basis set. The properties of Si/Al substitution energy, deprotonation energy, bridging hydroxyl stretching frequency and atomic charge on proton were calculated and used to measure Brönsted acid strength. The NH3 adsorption on the acid site including adsorption energy and adsorbed configuration was found to be more rational character to investigate the intrinsic acidity of zeolite. Through our calculations it was confirmed that the acidity in NNNN-substitution is stronger than NNN-substitution. For NNN-structure, HAl4-HAl4 has a slightly higher acidity than HAl1-HAl1. For NNNN-structure, if two protons are positioned outside of two Al atoms, the acidity is the strongest; in other cases, if the second hydrogen is positioned between two Al atoms, the acidity decreases. The calculated results are strongly dependent on the selected models. However, the adsorption energy of NH3 interacting with protonic zeolites shows minor dependency on the model structures.